Jaagsiekte Sheep Retrovirus

Overview and Taxonomy of Jaagsiekte Sheep Retrovirus

Jaagsiekte sheep retrovirus (JSRV) occupies a singular and formidable position within the Retroviridae family, representing the etiological agent of ovine pulmonary adenocarcinoma (OPA), a contagious and universally fatal neoplastic lung disease of sheep and, less frequently, goats [5, 18]. The virus is classified within the genus Betaretrovirus, a group characterized by type B/D retroviral morphology with a distinctive condensed, eccentric core, and a complex genomic organization that includes, crucially for JSRV, an overlapping open reading frame (orf-x) within the env gene that encodes the Rej regulatory protein [18]. The betaretrovirus genus encompasses not only the pathogenic exogenous JSRV (exJSRV) but also the closely related ovine enzootic nasal tumor virus types 1 and 2 (ENTV-1, ENTV-2), which induce nasal adenocarcinomas, and, most significantly, a vast array of endogenous JSRV (enJSRV) proviral copies stably integrated into the host sheep genome [5, 12].

From a taxonomic and functional standpoint, the dichotomy between the exogenous, horizontally transmissible form (exJSRV) and the inherited, endogenous elements (enJSRVs) is the most critical defining feature of JSRV biology. The enJSRVs, of which at least 27 loci have been characterized in the sheep (Ovis aries) genome, represent the remnants of ancestral retroviral infections of the germline that have been fixed in the host population over millions of years [6]. These elements are highly homologous to exJSRV but are generally presumed to be replication-defective due to mutations and deletions, though some retain open reading frames and biological activity. Indeed, a landmark comparative study employing fluorescent in situ hybridization (FISH) mapped 23 distinct enJSRV loci in both domestic sheep (2n=54) and river buffalo (Bubalus bubalis, 2n=50), revealing a remarkable conservation of chromosomal localization between these two species, which diverged within the Bovidae family [6]. This conservation indicates that the integration of these ancestral betaretroviruses into the germline of a common ancestor occurred prior to the speciation of sheep and buffalo, providing a powerful tool for tracing host and viral evolution [6].

The proviral genome of JSRV, regardless of its exogenous or endogenous origin, exhibits a canonical retroviral structure of approximately 7.5 to 8 kilobase pairs, flanked by long terminal repeats (LTRs) and containing the typical gag, pro, pol, and env genes [1, 18]. However, the genetic distinctions between exJSRV and enJSRV are concentrated in specific hypervariable regions that act as molecular signatures of pathogenicity. The LTR-U3 region, which houses the promoter and enhancer elements critical for viral transcription and tissue-specific expression, is a primary region of divergence [1, 2]. The env gene, encoding the viral surface (SU) and transmembrane (TM) glycoproteins, harbors variable region 3 (VR3), which is essential for the unique oncogenic properties of exJSRV [1, 17]. The gag gene, encoding the structural capsid and matrix proteins, also contains variable regions (VR1 and VR2) that differentiate exogenous from endogenous strains [1]. A comprehensive analysis of CpG island distribution across 42 full-length JSRV genomes revealed further molecular differences: while both forms have CpG islands predominantly in the LTR, gag, and env genes, the distribution and density of these epigenetic regulatory motifs differ significantly, with enJSRVs showing a higher prevalence of these sequences compared to exJSRVs [2]. The biological significance of these epigenetic differences in regulating viral latency, expression, or host immune evasion remains a critical area for future investigation.

Phylogenetically, exJSRV isolates from around the world cluster into distinct clades that reflect both geographic origin and likely viral evolution within specific sheep populations. The recent characterization of the complete genome of the Inner Mongolia exJSRV strain (NMJS12) demonstrated a 98.8% nucleotide identity with the Chinese exJSRV-C1 isolate, placing these strains firmly within a unique Asian clade [1]. In contrast, they share only 93-96% identity with foreign isolates, such as the well-characterized UK strain (AF105220.1) and the South African strain (M80216), underscoring significant intercontinental divergence [1, 7, 16]. The identification of exJSRV type 2 (exJSRV2) in Romanian sheep further refines this phylogenetic landscape, showing high similarity to the UK strain and demonstrating the circulatory presence of distinct viral subtypes within European populations [7]. Iraqi Awassi sheep harbor yet another genetically divergent local isolate, suggesting independent evolutionary trajectories in geographically isolated flocks [9]. This phylogenetic diversity has implications for diagnostic sensitivity, as PCR and serological assays designed against one clade may fail to detect divergent strains, and for future vaccine development, which may need to be region-specific or multivalent to cover the existing viral diversity [4, 9].

A defining and unique characteristic of JSRV biology is the direct oncogenic capacity of its envelope (Env) glycoprotein. Unlike essentially all other known retroviruses, which induce tumors through insertional mutagenesis or the transduction of cellular proto-oncogenes, the JSRV Env protein itself functions as a potent oncogene capable of transforming cells both in vitro and in vivo [14, 17, 18]. The cytoplasmic tail (CT) of the TM protein is the primary determinant of this transforming activity, acting through the constitutive activation of signaling cascades, particularly the phosphatidylinositol 3-kinase (PI3K)/Akt/mTOR and the Ras/Raf/MEK/ERK1/2 pathways [14, 17]. However, transformation is a multifaceted process requiring the coordinated function of multiple Env domains. Chimeric studies between JSRV Env and the non-oncogenic Env of enJSRV have revealed that the membrane-spanning region (MSR), the N-terminal region of the SU protein, and even specific di-amino acid residues in the TM ectodomain all contribute to the full transforming phenotype [17]. This structural complexity suggests that Env-mediated oncogenesis involves not only intracellular signaling but also proper protein trimerization, membrane fusion, and potentially, interactions with the cellular receptor, hyaluronoglucosaminidase 2 (Hyal2) [12, 17].

The tissue tropism of JSRV is tightly restricted to the respiratory tract, specifically type II alveolar epithelial cells (ATII) and non-ciliated bronchiolar epithelial (Clara) cells in the lung, resulting in the classic OPA lesion [5, 8, 10, 14]. This remarkable specificity is a product of the interplay between the viral Env protein, the host receptor Hyal2, and the promoter/enhancer activity within the U3 region of the LTR. The Hyal2 receptor is expressed on a wide variety of cells, including human cells, so receptor availability alone does not explain the strict lung tropism [12, 13]. Elegant studies using chimeric JSRV-ENTV viruses have demonstrated that the LTR is the primary driver of tissue-specific expression, with the JSRV LTR preferentially directing transcription in lung epithelial cells, while the ENTV-1 LTR directs transcription in nasal turbinate epithelium [12]. This indicates that the cellular transcription factors available within the unique microenvironment of the lung are critical for enabling the viral replication cycle and subsequent Env-driven transformation. Furthermore, the env gene itself contributes to this specificity; while both JSRV and ENTV utilize Hyal2 for entry, ENTV entry appears more dependent on high levels of Hyal2 expression, as seen in nasal turbinate and cartilage, whereas JSRV entry is less constrained by this factor, allowing it to infect and transform the Hyal2-expressing ATII cells of the deep lung [12]. The confirmed ability of JSRV to also induce nasal adenocarcinomas in sheep and, strikingly, in a juvenile goat with no sheep contact, reveals that this tropism is not absolute and can expand under specific conditions, likely dictated by subtle variations in the host genetic background or the viral strain involved [3, 11]. This phenomenon may be related to the presence of exJSRV2 or other variants with altered LTR or Env properties that permit infection of the nasal mucosa [3, 7, 11]. The host range is largely confined to Caprinae (sheep and goats), and despite in vitro evidence that JSRV can infect human cells via the conserved Hyal2 receptor, extensive serological and molecular surveys have failed to provide robust evidence for a role of JSRV in human lung adenocarcinoma, a disease it closely resembles histologically [13, 15]. As such, while OPA is recognized by the World Organisation for Animal Health (WOAH) as an important transmissible disease of small ruminants, it is not considered a zoonotic threat [5, 13].

Genomic Structure and Phylogenetic Characterization of Exogenous and Endogenous JSRV

The genomic architecture of Jaagsiekte sheep retrovirus presents a remarkable dichotomy between its exogenous, pathogenic form (exJSRV) and the multiple endogenous copies (enJSRV) that have become fixed within the ovine and caprine germlines over evolutionary time. This duality not only underpins the unique biology of JSRV-induced oncogenesis but also provides an exceptional model for understanding retroviral endogenization, host-virus coevolution, and the molecular determinants of tissue tropism and pathogenicity. A comprehensive dissection of the genomic structure, from the long terminal repeats (LTRs) through the canonical retroviral genes gag, pro, pol, and env, alongside the auxiliary rej open reading frame, reveals the precise nucleotide-level distinctions that separate a harmless Mendelian element from a potent, horizontally transmissible oncovirus.

Comparative Genomic Architecture of exJSRV and enJSRV

The proviral genome of JSRV, whether exogenous or endogenous, conforms to the archetypal betaretrovirus organization of approximately 7.5–7.9 kb, flanked by LTRs that contain the U3, R, and U5 regions. However, the critical functional divergence between exJSRV and enJSRV is encoded within subtle but consistent sequence variations distributed across the genome. Whole-genome characterization of the Inner Mongolian exJSRV strain NMJS12 revealed a genome length of 7,462 nucleotides (excluding the 3′ polyadenylation tract), exhibiting 98.8% identity with the Chinese exJSRV-C1 isolate, yet only 88.73–92.26% identity with enJSRV reference sequences [1]. This degree of divergence, approximately 7–11% at the nucleotide level between the exogenous and endogenous forms, is non-randomly distributed and clusters within discrete hypervariable regions (VRs) that serve as definitive molecular signatures for distinguishing the two classes.

The most diagnostically robust and functionally consequential differences reside in three genomic compartments: the U3 region of the LTR, the central region of gag (encompassing VR1 and VR2), and the env gene, particularly VR3 within the surface (SU) subunit. These regions are highly conserved among exJSRV isolates globally but differ markedly from their enJSRV counterparts, a feature that has been exploited for differential PCR-based diagnostics [11, 25]. The U3 region, in particular, harbors promoter and enhancer elements that govern cell-type-specific transcription; exJSRV U3 sequences contain binding sites for transcription factors that are preferentially active in type II pneumocytes and Clara cells, whereas enJSRV U3 sequences often carry mutations or deletions that attenuate promoter strength or alter tissue specificity [12]. This distinction is not merely academic, it directly underpins the ability of exJSRV to drive high-level viral gene expression in the distal respiratory epithelium, whereas most enJSRV loci are transcriptionally silent or expressed only under specific physiological conditions, such as placental morphogenesis [6, 21].

The Long Terminal Repeat: A Master Regulator of Tropism and Pathogenicity

The LTRs of JSRV, spanning approximately 500–550 bp each, are the primary determinants of transcriptional control and, consequently, of viral tissue tropism. The U3 region of exJSRV contains a series of cis-acting elements, including a TATA box, CAAT box, and multiple binding sites for transcription factors such as Sp1, NF-κB, and members of the C/EBP family. Critically, the exJSRV U3 also harbors a unique 21–24 bp sequence that is either absent or substantially divergent in enJSRV LTRs. This region has been directly implicated in the preferential transcription of exJSRV in lung epithelial cells. Chimeric virus studies swapping the LTRs of exJSRV with those of the closely related enzootic nasal tumor virus (ENTV) demonstrated that the U3 promoter, not the envelope protein, is the dominant determinant of anatomical tropism between the lung and the nasal turbinates [12]. When the exJSRV LTR was replaced with the ENTV LTR, the resultant chimeric virus lost its ability to efficiently transcribe in lung tissue slices, instead gaining activity in nasal epithelial cells. This elegantly demonstrates that the LTR acts as a molecular zip code, dictating which cell types can support productive infection.

From a phylogenetic perspective, the U3 region is also the most rapidly evolving portion of the JSRV genome, accumulating mutations that allow the virus to adapt to host transcriptional environments. Phylogenetic analysis of U3 sequences from Indian exJSRV isolates revealed that these sequences formed two distinct clusters, with 96–100% identity to the UK reference strain AF105220.1 but only 88–93% identity to the South African strain M80216 [16]. This clustering pattern suggests that geographical isolation has driven independent evolutionary trajectories within the U3, potentially reflecting adaptation to different sheep breeds or management systems. The U3 region’s plasticity is further underscored by the fact that it contains the primary determinants of CpG island distribution, which differ significantly between exJSRV and enJSRV. A comprehensive analysis of 42 full-length JSRV genomes revealed that 66.66% of exJSRV strains and 84.84% of enJSRV strains contain at least one CpG island within the LTR [2]. This differential methylation potential may contribute to the epigenetic silencing of enJSRV loci in somatic tissues, as CpG islands within promoters are classical targets for DNA methyltransferase-mediated repression, providing a host defense mechanism to prevent reactivation of endogenous retroviruses.

The gag Gene: Structural Protein and Diagnostic Divergence

The gag gene of JSRV encodes the matrix (MA), capsid (CA), and nucleocapsid (NC) proteins, which are essential for virion assembly and particle morphology. While the overall organization of gag is conserved between exJSRV and enJSRV, two hypervariable regions, VR1 and VR2, located within the CA-coding sequence, exhibit substantial sequence divergence [1]. These regions are of particular interest because they are not under the same functional constraints as the rest of gag; the CA protein must maintain structural integrity for capsid formation, but VR1 and VR2 appear to tolerate considerable amino acid variation. This makes them ideal targets for diagnostic PCR, as primers designed to anneal within VR1 and VR2 can discriminate between exJSRV and enJSRV with high specificity [7, 20, 25].

Phylogenetic analysis of gag sequences from concurrent JSRV and maedi-visna virus (MVV) infections in China revealed that the JSRV gag amplicon shared highest homology with the Inner Mongolian strain JQ837489 [20]. This finding demonstrates that gag sequences can serve not only for diagnostic discrimination but also for molecular epidemiology, tracking viral spread across geographic regions. The gag gene also exhibits differences in CpG island content between viral classes, with 33.33% of exJSRV and 57.57% of enJSRV sequences containing at least one CpG island within this gene [2]. The biological significance of this disparity warrants further investigation, as CpG methylation within gag could influence the efficiency of translation or the stability of unintegrated viral DNA.

The env Gene and rej: Oncoprotein and Regulatory Mastery

The env gene of JSRV is arguably the most intensively studied segment of the genome, and for compelling reasons: it encodes both the surface (SU) and transmembrane (TM) subunits of the envelope glycoprotein, which serves the dual function of receptor binding and membrane fusion, and, uniquely among retroviruses, also functions as a potent oncogene [17, 18]. The exJSRV Env protein binds to the host receptor hyaluronoglucosaminidase 2 (Hyal2), initiating entry into target cells. However, the oncogenic signal is transduced through the cytoplasmic tail (CT) of the TM subunit, which activates the PI3K/Akt/mTOR and Ras/Raf/MEK/MAPK signaling cascades in a ligand-independent manner [10, 14, 17, 22]. The CT of exJSRV Env contains a YXXM motif (tyrosine-x-x-methionine) that, upon phosphorylation, recruits the p85 regulatory subunit of PI3K, thereby activating Akt. In contrast, most enJSRV Env proteins carry a phenylalanine at the critical tyrosine position (Y590F in some strains), which abrogates PI3K binding and renders them non-oncogenic [17, 18].

The hypervariable region VR3, located within the SU subunit, is a major determinant of antigenic variation and is also a target for discrimination between exJSRV and enJSRV. Sequence analysis of the env gene from the Inner Mongolian NMJS12 strain confirmed the presence of exJSRV-specific residues within VR3, aligning it unequivocally with the exogenous lineage [1]. Phylogenetic analysis of env sequences has revealed that exJSRV isolates from diverse geographic regions, including Iraq, Romania, India, and China, cluster together in a monophyletic group, distinct from enJSRV sequences [9, 16, 23, 24]. Notably, the Romanian exJSRV type 2 strain (MT809678.1) showed high similarity to the UK strain AF105220.1, suggesting a common ancestral origin and global dissemination, likely through historical sheep trade routes [7, 23]. Conversely, the Iraqi isolate exhibited some genetic divergence from other global isolates, clustering most closely with strains KT279066.1 and KT279065.1 from the NCBI database, indicating that regional variants may exist that have not yet been extensively characterized [9].

Embedded within the env gene is a second open reading frame that encodes the Rej (regulator of expression of JSRV) protein [18]. Rej is a trans-acting factor that facilitates the nuclear export of unspliced genomic RNA, thereby enabling Gag protein synthesis and virion assembly. This function is analogous to the Rev protein of HIV-1 or the Rex protein of human T-cell leukemia virus. The rej coding sequence is absent or non-functional in many enJSRV loci, providing another layer of regulation that prevents enJSRV from completing a full replication cycle. The precise mapping of rej within the env transcript and its interaction with host RNA export machinery remain active areas of investigation, with implications for understanding why exJSRV can achieve productive replication while enJSRV remains trapped in a defective state.

Phylogenetic Characterization: Global Lineages and Evolutionary Dynamics

The phylogenetic landscape of JSRV is characterized by a clear bifurcation between exJSRV and enJSRV, with enJSRV sequences forming a diverse array of lineages that have been integrated at multiple distinct loci within the ovine genome. Comparative fluorescence in situ hybridization (FISH) mapping has localized 23 of the 27 known enJSRV copies to specific ovine chromosomes, demonstrating that these integrations occurred at different evolutionary time points and that some loci are insertionally polymorphic, present only in certain individuals or breeds [6]. The conservation of enJSRV loci in homologous chromosomal bands of river buffalo (Bubalus bubalis) provides compelling evidence that these integrations predate the divergence of Caprinae and Bovinae, pushing the minimum age of the oldest enJSRV loci to at least 15–20 million years [6]. This ancient origin stands in stark contrast to the recent emergence or introduction of pathogenic exJSRV strains, which appear to have circulated globally only within the last several centuries, likely facilitated by the intensification of sheep husbandry.

Phylogenetic analysis based on whole-genome sequences, or on individual genes such as env or gag, consistently recovers exJSRV as a monophyletic clade nested within the broader enJSRV diversity. This topology strongly suggests that exJSRV arose from a single ancestral enJSRV locus that regained infectivity and horizontal transmissibility through a series of mutations or recombination events. The NMJS12 strain from Inner Mongolia provides a representative example: it occupies the same evolutionary clade as the Chinese exJSRV-C1 and is clearly separated from enJSRV reference sequences [1]. However, within the exJSRV clade, there is substructuring into at least two subtypes: exJSRV type 1 and exJSRV type 2. The Romanian studies identified the presence of exJSRV type 2, which clusters with the UK strain and is distinct from type 1 strains found in other parts of Europe and North America [7, 23]. This subtype classification may have implications for virulence, diagnostic sensitivity, and vaccine development, although the functional correlates of these phylogenetic differences have not yet been fully elucidated.

The role of geographical isolation in shaping JSRV phylogeny is evident from comparisons between isolates from different continents. The Indian exJSRV U3 sequences, while closely related to the UK strain, formed two distinct subclusters, suggesting independent evolutionary trajectories since their introduction [16]. Similarly, the detection of enJSRV-related sequences in blood-fed mosquitoes from Senegal via xenosurveillance revealed that the amplified env and 3′ UTR sequences clustered with known enJSRV references rather than with exJSRV [19]. This finding not only validated the xenosurveillance approach but also highlighted the ubiquity of enJSRV sequences in sheep populations globally, even in regions where OPA has not been extensively documented. The lack of exJSRV detection in these samples suggests that either exJSRV was not present in the sampled flocks or that the viral load in blood was below the threshold of detection, consistent with the predominantly intratumoral replication of exJSRV.

CpG Island Landscapes and Epigenetic Regulation

The distribution of CpG islands across the JSRV genome provides a fascinating lens through which to view the evolutionary forces acting on exogenous versus endogenous forms. CpG islands are regions of DNA with a high frequency of CpG dinucleotides, and they are typically associated with promoter regions where they can regulate gene expression through methylation. The comprehensive survey by Liu et al. revealed that while all exJSRV genomes contain at least one CpG island in the env gene (100%), this figure is slightly lower for enJSRV (96.96%) [2]. Conversely, the LTR CpG island prevalence is higher in enJSRV (84.84%) than in exJSRV (66.66%). The differential distribution suggests that during the endogenization process, selection has favored CpG islands in the LTR, presumably to facilitate the methylation-dependent silencing that prevents enJSRV expression in somatic tissues. For exJSRV, the high CpG content in env may reflect selective pressures to maintain specific coding sequences required for oncogenicity and receptor interaction, even at the cost of increased vulnerability to methylation-based host restriction. The elucidation of the biological significance of these CpG islands remains an important goal, as they may represent a target for epigenetic therapies aimed at reactivating silenced enJSRV proviruses or, conversely, at silencing exJSRV transcription in infected sheep.

Genomic Consequences of enJSRV Insertional Mutagenesis

Beyond their role as molecular fossils, enJSRV elements are active agents of host genome evolution. The integration of a 7.9 kb enJSRV sequence into the first intron

Molecular Pathogenesis of Ovine Pulmonary Adenocarcinoma

The molecular pathogenesis of ovine pulmonary adenocarcinoma (OPA) represents a paradigm of viral oncogenesis wherein the etiologic agent, Jaagsiekte sheep retrovirus (JSRV), employs a unique and direct mechanism of cellular transformation that is fundamentally distinct from most other oncogenic retroviruses. Unlike classical acutely transforming retroviruses that carry captured cellular oncogenes, or slow-transforming retroviruses that activate proto-oncogenes via insertional mutagenesis, JSRV encodes its oncogenic determinant within the envelope (Env) glycoprotein itself [17, 18]. This singular feature places the viral Env protein at the epicenter of OPA pathogenesis, acting as both the mediator of viral entry and the primary driver of neoplastic transformation in susceptible pulmonary epithelial cells.

The Envelope Glycoprotein as a Direct Oncogene: Structural Determinants of Transformation

The oncogenic capacity of the JSRV Env protein has been definitively established through a series of gain- and loss-of-function experiments. Expression of the exogenous JSRV (exJSRV) Env alone is sufficient to induce cellular transformation in vitro and tumor formation in vivo, a property not shared by the closely related endogenous JSRV (enJSRV) Env proteins, which are non-oncogenic [17, 18]. This functional divergence, despite high sequence homology, has allowed for the precise mapping of transformation determinants through the construction of chimeric Env proteins. The cytoplasmic tail (CT) of the transmembrane (TM) subunit has been identified as a critical domain for transformation. Specifically, a YXXM motif within the CT, which serves as a docking site for the p85 regulatory subunit of phosphatidylinositol 3-kinase (PI3K), is essential for the activation of the PI3K/Akt/mTOR signaling cascade [17, 18]. Mutation of key tyrosine residues within this motif, particularly Y590, abrogates transformation, underscoring the reliance of JSRV Env on this pathway for its oncogenic activity.

However, the CT is not the sole determinant. Elegant studies using chimeras between the oncogenic exJSRV Env and the non-oncogenic enJSRV Env (specifically the 5F16 clone) have revealed that the membrane-spanning region (MSR) also plays a significant, albeit context-dependent, role in transformation [17]. Substitution of the exJSRV MSR with that of enJSRV substantially reduces transforming efficiency, and the residual transformation by such chimeras becomes highly dependent on both the Ras/Raf/MEK/MAPK and PI3K/Akt/mTOR pathways, suggesting a complex interplay between these signaling axes [17]. Furthermore, the ectodomain of the TM protein, including a leucine-rich putative trimerization domain, is also implicated. While trimerization of Env is not absolutely required for all aspects of transformation, as demonstrated by complementation assays between a trimerization-deficient mutant and a CT mutant, it is necessary for the full oncogenic potential of the protein [17]. The surface unit (SU) of Env also contributes, with the amino-terminal region of SU being more critical for transformation than the carboxy-terminal portion [17]. This multi-domain requirement highlights that the transforming signal is not a simple linear motif but rather a complex, conformationally dependent output of the entire Env complex.

Activation of Oncogenic Signaling Cascades: From Env to Cellular Proliferation

The binding of the JSRV Env protein to its cellular receptor, hyaluronoglucosaminidase 2 (Hyal2), initiates a cascade of intracellular signaling events that culminate in uncontrolled proliferation and resistance to apoptosis. Hyal2, a glycosylphosphatidylinositol (GPI)-anchored protein, is expressed on the surface of type II pneumocytes and Clara cells, the target cells for JSRV infection and transformation [12, 13]. The interaction of Env with Hyal2 is thought to trigger receptor-mediated signaling, although the exact mechanism by which this interaction activates downstream pathways remains an area of active investigation. It is hypothesized that Env binding may sequester Hyal2 away from its natural ligand or signaling partners, or that Env itself may mimic a ligand, leading to the activation of associated kinases.

The most well-characterized downstream effectors of JSRV Env are the PI3K/Akt/mTOR and Ras/Raf/MEK/MAPK pathways. Transcriptomic analyses of naturally occurring OPA cases have consistently demonstrated the upregulation of genes within these pathways [10, 22]. Immunohistochemical analysis of OPA tumor cells reveals robust phosphorylation of Akt and ERK1/2, confirming the activation of these kinases in situ [10, 14]. The PI3K/Akt pathway is a central regulator of cell survival and metabolism, and its constitutive activation in OPA provides a potent anti-apoptotic signal. Concurrently, the MAPK pathway drives cell cycle progression and proliferation. The convergence of these two pathways creates a powerful oncogenic synergy.

More recent transcriptome profiling has unveiled the involvement of the Hippo signaling pathway in OPA pathogenesis, a finding that was previously unreported in naturally infected cases [22]. RNA sequencing of lung tissues from sheep with naturally occurring OPA identified 366 differentially expressed genes (DEGs), with significant enrichment in the Hippo pathway, in addition to the canonical PI3K/Akt/mTOR and MAPK pathways [22]. The Hippo pathway, a key regulator of organ size and cell contact inhibition, is frequently dysregulated in human cancers. In OPA, JSRV Env was shown to increase the protein expression of key Hippo pathway components, and functional assays using JSRV-Env-transduced sheep trophoblast cells (STCs) confirmed that Env expression promotes malignant transformation, including increased cell viability, migration, invasion, and colony formation, in a Hippo-dependent manner [22]. This suggests a more complex signaling network than previously appreciated, where JSRV Env simultaneously engages multiple oncogenic pathways to drive tumorigenesis.

Another critical pathway activated in OPA involves the AGR2 (anterior gradient 2)/YAP1 (yes-associated protein 1)/AREG (amphiregulin) axis. Transcriptional profiling of JSRV-infected lungs identified AGR2 as one of the most highly upregulated genes [10]. AGR2 is a protein disulfide isomerase that promotes cell migration and metastasis in various cancers. Its upregulation in OPA tumor cells is associated with the activation of YAP1, a transcriptional co-activator and downstream effector of the Hippo pathway, and the subsequent expression of its target gene, amphiregulin (AREG), a ligand for the epidermal growth factor receptor (EGFR) [10]. This AGR2/YAP1/AREG axis provides a mechanistic link between JSRV Env expression and the activation of EGFR signaling, further amplifying the proliferative drive within the tumor.

The Role of the Long Terminal Repeat (LTR) and Tissue Tropism

While the Env protein is the primary oncogene, the viral long terminal repeat (LTR) plays an indispensable role in determining the tissue tropism of JSRV and, consequently, the anatomical location of OPA. The U3 region of the LTR contains the viral promoter and enhancer elements that govern the efficiency and cell-type specificity of viral transcription [1, 12]. Comparative studies between JSRV and the closely related enzootic nasal tumor virus (ENTV), which causes nasal adenocarcinomas, have been particularly illuminating. Despite sharing high sequence identity and utilizing the same Hyal2 receptor, JSRV induces tumors in the deep lung, while ENTV targets the nasal turbinates [12]. Using chimeric JSRV-ENTV viruses, it was demonstrated that the LTR is the primary determinant of this tissue specificity. The JSRV LTR drives robust transcription in lung epithelial cells, whereas the ENTV LTR is more active in nasal epithelial cells [12]. This differential promoter activity ensures that the oncogenic Env protein is expressed in the appropriate target cell population, dictating the site of tumor formation.

The LTR also contributes to the distinction between pathogenic exJSRV and non-pathogenic enJSRV. The U3 region of exJSRV contains specific sequence motifs and CpG island distributions that differ from those of enJSRV [1, 2]. These differences likely influence the transcriptional activity and epigenetic regulation of the provirus. For instance, a higher proportion of enJSRV sequences contain CpG islands in the LTR compared to exJSRV, which may render endogenous copies more susceptible to silencing via DNA methylation, a potential host defense mechanism against retrotransposition [2]. In contrast, the exJSRV LTR may be more resistant to such silencing, allowing for sustained expression of the oncogenic Env.

Host Transcriptome Dysregulation and the MicroRNA Landscape

JSRV infection induces profound changes in the host cell transcriptome, extending well beyond the direct activation of signaling pathways by Env. A comprehensive RNA sequencing study identified 1,971 ovine genes that are differentially expressed in JSRV-infected lung tissue compared to uninfected controls [10]. These DEGs are enriched for functions in carcinogenesis, cell cycle control, and immunomodulation. Notably, there was a marked upregulation of genes associated with innate immunity, including cytokines, chemokines, and complement system proteins, while genes involved in T-cell-mediated adaptive immunity showed little evidence of activation [10]. This suggests that JSRV may actively evade or suppress the adaptive immune response, contributing to the progressive, unhindered growth of the tumor. The transcriptome also revealed a significant increase in genes related to macrophage function, reflecting the abundant infiltration of these cells into OPA lesions [10].

The dysregulation of microRNAs (miRNAs) represents another layer of molecular pathogenesis. Small RNA sequencing of JSRV-infected lungs identified 32 differentially expressed miRNAs compared to mock-infected controls [27]. Among the most highly upregulated were miR-182, miR-183, miR-96, and miR-135b, all of which have established roles in human lung cancer [27]. These miRNAs function as oncogenes or tumor suppressors by post-transcriptionally regulating the expression of multiple target genes. Network analysis of the predicted targets of these deregulated miRNAs revealed their involvement in pathways known to be dysregulated in OPA, including PI3K/Akt and MAPK signaling [27]. Importantly, no JSRV-encoded miRNAs were identified, indicating that the virus relies entirely on manipulating the host miRNA machinery to fine-tune the cellular environment for its replication and oncogenic program [27].

The Role of Endogenous Retroviruses and Co-Infections

The ovine genome harbors at least 27 copies of endogenous JSRV (enJSRV) elements, which are remnants of ancient germline infections [6]. These enJSRVs are generally non-infectious and non-oncogenic, but they play a critical role in host physiology, particularly in placental development. The enJSRV Env protein promotes trophoblast cell fusion during placentation by activating the PKA/MEK/ERK1/2 signaling pathway [21]. This physiological function of enJSRV Env stands in stark contrast to the oncogenic activity of exJSRV Env, highlighting how subtle sequence differences can lead to dramatically different biological outcomes. The presence of enJSRV sequences in the host genome can also complicate molecular diagnostics, as PCR assays must be carefully designed to distinguish between exogenous and endogenous copies [1, 19]. Furthermore, the insertion of an enJSRV element into the BCO2 gene has been shown to cause yellow fat in Norwegian Spælsau sheep, demonstrating that these endogenous elements can have significant phenotypic effects beyond their role in placentation [8].

Co-infection with other ovine pathogens can modulate the pathogenesis of OPA. Concurrent infection with Maedi-Visna virus (MVV), a lentivirus that causes interstitial pneumonia, is a frequent finding in OPA-affected flocks [20, 25]. In a study of OPA cases in Romania, a co-infection rate of 47.6% was observed [25]. The presence of MVV co-infection was associated with more severe perineoplastic pathology, including interstitial lymphoplasmacytic infiltrates, lymphoid hyperplasia, and fibromuscular hyperplasia [25]. This suggests that MVV-induced inflammation and immune dysregulation may create a more permissive microenvironment for JSRV-induced tumor progression. Similarly, the detection of JSRV provirus in milk macrophages, but not in lymphocytes or mammary epithelial cells, points to a specific cellular route for lactogenic transmission and highlights the importance of understanding viral dissemination within the host [26].

Comparative and Zoonotic Considerations

The striking histological and molecular similarities between OPA and human lung adenocarcinoma, particularly the lepidic-predominant subtype (formerly bronchioloalveolar carcinoma), have positioned OPA as a valuable large animal model for human lung cancer [10, 18, 27]. The shared activation of pathways such as PI3K/Akt, MAPK, and Hippo, as well as the overlapping patterns of miRNA dysregulation, underscore the relevance of this model for studying human disease [10, 22, 27]. However, the question of whether JSRV itself plays a role in human lung cancer remains contentious. While some studies have reported detection of JSRV-like sequences or antigens in human lung tumor tissue [15], more comprehensive investigations using highly specific monoclonal antibodies and sensitive PCR assays have failed to find evidence of JSRV infection in human lung cancers, even in patients with high occupational exposure to sheep [13]. The consensus, supported by the World Organisation for Animal Health (WOAH) and other international bodies, is that JSRV is not a zoonotic agent and does not contribute to human lung carcinogenesis [13]. The detection of JSRV-related sequences in mosquitoes feeding on sheep, as demonstrated by xenosurveillance studies, is a reflection of the host's blood meal rather than viral replication in the vector, further confirming the lack of an arthropod-borne transmission cycle for JSRV [19].

Epidemiology and Transmission Dynamics of JSRV

Global Distribution and Prevalence of Ovine Pulmonary Adenocarcinoma

Jaagsiekte sheep retrovirus (JSRV) is the etiological agent of ovine pulmonary adenocarcinoma (OPA), a contagious and fatal neoplastic disease of sheep that has been documented across virtually all sheep-rearing regions of the world. The global distribution of JSRV is remarkably broad, with confirmed cases reported from Europe, Asia, Africa, the Americas, and Oceania, reflecting the virus’s ability to persist in diverse ecological and management systems [5, 18]. The World Organisation for Animal Health (WOAH) recognizes OPA as a significant transmissible disease of small ruminants, though its insidious nature and prolonged incubation period often lead to substantial underreporting. Prevalence estimates vary dramatically depending on diagnostic methodology, population sampled, and geographic region, but abattoir-based surveys consistently reveal infection rates ranging from 0.5% to over 20% in endemic flocks.

In Europe, comprehensive surveillance efforts have provided some of the most robust prevalence data. A landmark study in Ireland examined lungs from 1,911 adult sheep at slaughter and confirmed JSRV infection in 1.6% of animals by RT-PCR and immunohistochemistry, with an OPA prevalence of 0.5% [31]. Notably, JSRV-positive sheep exhibited clear geographic clustering within specific counties, Donegal, Kerry, Kilkenny, Offaly, Tipperary, Waterford, and Wicklow, suggesting that localized transmission dynamics and flock-level management practices profoundly influence infection risk [31]. A subsequent Irish study refined diagnostic algorithms, demonstrating that macroscopic and histological examination combined in parallel provides the highest positive agreement (0.38) with RT-PCR, while negative agreement remains exceptionally high (0.95–0.96), indicating that animals without gross or microscopic lesions are unlikely to harbor detectable virus [30]. In Romania, a large-scale survey of 2,693 adult ewes slaughtered between 2017 and 2019 revealed an OPA prevalence of 1.26%, with classical OPA accounting for 88.24% of cases and atypical forms comprising 11.76% [7, 23]. Phylogenetic analysis of Romanian isolates identified exogenous JSRV type 2 (exJSRV2) with high similarity to the UK strain AF105220.1, confirming transboundary viral circulation [7]. Critically, a retrospective study in Transylvania found that 47.6% of OPA cases were coinfected with Maedi-Visna virus (MVV), highlighting the frequency of retroviral coinfections in endemic regions and their potential to exacerbate pulmonary pathology [25].

In Asia, JSRV epidemiology is increasingly well-characterized. A study in Inner Mongolia, China, involving 319 Dorper rams identified concurrent JSRV and MVV infection in 0.94% of animals, with gross pathology revealing diffuse lung enlargement, greyish-white miliary nodules, and characteristic white foamy tracheal fluid [20]. The complete genome of an Inner Mongolian JSRV strain (NMJS12) was recently elucidated, showing 98.8% nucleotide identity to the Chinese JSRV-C1 strain but only 93.05–95.84% identity to foreign isolates, indicating regional viral evolution [1]. In India, a patho-epidemiological survey across three states (Delhi, Andhra Pradesh, and Uttar Pradesh) examined 1,350 lung samples and found an overall JSRV prevalence of 2.29%, with sheep positivity at 3.49% and goats at 0.00% [24]. Prevalence was highest in Gannavaram (20.00%), followed by Delhi (2.10%), with no positive cases detected in Bareilly [24]. Age-specific analysis revealed significantly higher infection in sheep over three years of age compared to younger cohorts, and female sheep exhibited higher infection rates than males, potentially reflecting management-related exposure patterns [24]. Seasonal variation was also noted, with monsoon season showing the highest prevalence (6.00%), followed by winter (3.34%) and summer [24]. In Iraq, RT-PCR screening of 50 Awassi sheep with chronic respiratory distress detected JSRV in 44% of lung secretion samples, and phylogenetic analysis revealed that the Iraqi isolate is genetically divergent from most global isolates, clustering closely with KT279066.1 and KT279065.1 [9].

In Africa, JSRV has been confirmed in Algeria, where a six-month-old Ouled Djellal lamb presented with classic clinical signs, profuse whitish foamy nasal discharge upon lowering the head, and post-mortem examination revealed enlarged, edematous lungs with diffuse reddish-white foci [29]. Immunohistochemistry confirmed JSRV envelope protein expression exclusively in tumor cells [29]. In Senegal, a novel xenosurveillance approach detected JSRV-related sequences in blood-fed mosquitoes collected in the Barkedji region, with phylogenetic analysis revealing strong similarity to endogenous JSRV (enJSRV) sequences integrated in the sheep genome [19]. This finding, while not indicative of biological vector competence, demonstrates that mosquitoes can serve as environmental sentinels for livestock-associated retroviruses, expanding the toolkit for JSRV surveillance in resource-limited settings [19].

Host Range and Species Susceptibility

The canonical host range of JSRV has traditionally been considered limited to domestic sheep (Ovis aries), with goats (Capra hircus) considered less susceptible. However, accumulating evidence challenges this paradigm. A landmark case report from Turkey documented the first confirmed JSRV-associated nasal adenocarcinoma in a 12-month-old goat, with histopathological examination revealing a mixed glandular adenocarcinoma arising from surface and glandular epithelium with turbinate bone invasion [3]. PCR confirmed JSRV proviral DNA (385 bp) in tumor tissue, while tests for enzootic nasal tumor virus types 1 and 2 (ENTV-1, ENTV-2) were negative. Phylogenetic analysis classified the detected strain within the exogenous JSRV group, and critically, the infected goat originated from a herd with no documented contact with sheep, suggesting either cryptic transmission within goat populations or a previously unrecognized reservoir [3]. This case expands the known host and tissue tropism of JSRV and raises important questions about viral envelope-receptor interactions and promoter elements that govern species-specific susceptibility [3].

In sheep, JSRV infects and transforms type II alveolar epithelial cells (AECII) and non-ciliated bronchiolar epithelial (Clara) cells, leading to the development of multifocal or diffuse pulmonary adenocarcinomas [14, 29]. The virus utilizes hyaluronoglucosaminidase 2 (Hyal2) as its cellular receptor, a molecule conserved across mammalian species, including humans [13]. Indeed, JSRV is fully capable of infecting human cells in vitro, as measured by reverse transcription and persistence of proviral DNA [13]. This has prompted investigations into a potential zoonotic role for JSRV in human lung adenocarcinoma, particularly bronchioloalveolar carcinoma (now classified as lepidic-predominant adenocarcinoma), which shares striking histological similarities with OPA [13, 15]. However, comprehensive serological and molecular studies have failed to establish a causative link. Testing of 128 human lung cancers, including 73 cases of bronchioloalveolar carcinoma, using highly specific monoclonal antibodies against JSRV Env, yielded no detectable viral protein expression [13]. Furthermore, neutralizing antibodies were absent in sera from 138 Peruvians living in JSRV-endemic sheep-farming areas, 24 of whom had direct occupational exposure [13]. While some studies have reported amplification of JSRV-like env and gag sequences from human lung tumor tissue arrays, these findings have been inconsistent and likely represent low-level contamination or cross-reactivity rather than genuine infection [15]. The weight of evidence currently indicates that JSRV plays little, if any, role in human lung carcinogenesis, though the virus’s ability to infect human cells in culture underscores the importance of continued surveillance [13].

Transmission Mechanisms and Routes of Spread

Understanding the transmission dynamics of JSRV is critical for developing effective control strategies, yet many aspects remain incompletely characterized due to the virus’s long incubation period and the difficulty of detecting subclinical infections. The primary route of JSRV transmission is horizontal, via aerosolized respiratory secretions from infected animals. Clinically affected sheep produce copious amounts of frothy, virus-laden fluid that accumulates in the trachea and bronchi, and this fluid is expelled during coughing or when the animal’s head is lowered [18, 29]. Experimental studies have demonstrated that bronchoscopic instillation of JSRV into the segmental bronchus of the right cardiac lung lobe consistently induces OPA, with 89% of instilled animals developing tumors across low, intermediate, and high viral dose groups [28]. This experimental model revealed that tumor volume doubling times, calculated using thoracic CT 3D reconstructions, average 14.8 ± 2.1 days, providing quantitative insight into the rapidity of neoplastic progression once infection is established [28].

Lactogenic transmission represents a second, epidemiologically significant route. JSRV provirus has been detected in milk somatic cells from naturally infected ewes, and experimental evidence supports the ability of infected colostrum or milk to transmit infection to lambs [26]. A detailed investigation of milk cell populations from 12 subclinically infected, PCR-positive ewes revealed that JSRV provirus was present exclusively in adherent cells (macrophages and monocytes), with no detection in CD4+ or CD8+ T lymphocytes or B cells [26]. Importantly, immunohistochemical analysis of mammary gland tissue from these ewes was uniformly negative for JSRV surface protein, indicating that the mammary gland epithelium does not support viral replication and that milk-borne transmission is mediated by infected macrophages rather than free viral particles [26]. This finding has practical implications for management, as it suggests that pasteurization of colostrum or milk, or the use of artificial rearing systems, could interrupt this transmission route.

The role of endogenous JSRV (enJSRV) elements in transmission dynamics is a subject of ongoing investigation. The sheep genome harbors at least 27 copies of enJSRV, endogenous retroviruses highly related to the pathogenic exogenous JSRV [6]. Some of these loci are insertionally polymorphic, present only in certain individuals or populations, and have provided invaluable insights into host and viral evolution [6]. Comparative fluorescence in situ hybridization (FISH) mapping has revealed that enJSRV loci are conserved in homologous chromosomes of sheep and river buffalo, indicating that these integrations occurred in the common ancestor of Bovidae before species divergence [6]. While enJSRV elements are generally considered replication-defective and non-pathogenic, they can influence host biology in unexpected ways. For example, insertion of a 7.9 kb enJSRV sequence into the first intron of the beta-carotene oxygenase 2 (BCO2) gene in Norwegian Spælsau sheep abolishes BCO2 function, leading to yellow discoloration of adipose tissue due to carotenoid accumulation [8]. More significantly, the enJSRV envelope protein (enJSRV-Env) plays a physiological role in placental morphogenesis by promoting trophoblast cell fusion through activation of the PKA/MEK/ERK1/2 signaling pathway [21]. This endogenous expression does not, however, confer protective immunity against exogenous JSRV infection, and enJSRV sequences can confound molecular diagnostic assays if primers are not carefully designed to distinguish between endogenous and exogenous genomes [1, 2].

Viral Shedding, Environmental Persistence, and Risk Factors

The efficiency of JSRV transmission is influenced by viral shedding dynamics, environmental stability, and host-level risk factors. Experimentally infected lambs exhibit hematological changes as early as one month post-infection, including decreased red blood cell counts, hematocrit, and platelet values, alongside increased hemoglobin and white blood cell counts, though these changes are often not statistically significant until the terminal stages of disease [33]. Blood gas analysis reveals progressive respiratory compromise, with elevated pCO2, bicarbonate, and sodium/potassium concentrations, and decreased oxygen saturation, reflecting the gradual replacement of functional lung parenchyma by tumor tissue [33]. These prodromal changes occur months before clinical signs become apparent, highlighting the potential for subclinical shedders to perpetuate transmission within flocks.

Environmental persistence of JSRV is poorly characterized, but the virus is enveloped and likely susceptible to desiccation, ultraviolet light, and common disinfectants. However, the high density of animals in intensive sheep production systems, combined with prolonged housing during winter months, creates conditions conducive to aerosol transmission. Risk factor analyses from India identified significantly higher prevalence in sheep over three years of age, in females compared to males, and during the monsoon season, suggesting that management practices such as confinement during inclement weather and the stress of lactation may increase exposure risk [24]. The clustering of JSRV-positive animals within specific flocks and geographic regions, as observed in Ireland and Romania, indicates that once introduced, the virus can become endemic within a flock, with between-flock transmission occurring through the movement of infected but clinically normal animals [7, 23, 31].

Coinfection Dynamics and Implications for Transmission

The epidemiological landscape of JSRV is further complicated by frequent coinfections with other ovine respiratory pathogens, particularly Maedi-Visna virus (MVV). In a Romanian study of 82 exJSRV-positive OPA cases, 47.6% were coinfected with MVV, as determined by PCR and histopathological examination [25]. Coinfected animals exhibited more severe pulmonary pathology, including interstitial lymphoplasmacytic infiltrates in all cases, lymphoid hyperplasia in 60.6%, and fibromuscular hyperplasia in 63.7% [25]. Similarly, in Inner Mongolia, 3 of 319 Dorper rams (0.94%) showed concurrent JSRV and MVV infection, with ultrastructural examination revealing abundant virions, autophagosomes, and severely damaged mitochondria in lung tissue, indicative of enhanced cellular stress and mitophagy [20]. The gag gene sequences from these coinfected animals showed highest homology with Inner Mongolian MVV strains (MW248464), suggesting local co-circulation of these retroviruses [20]. These findings have significant implications for transmission dynamics, as MVV-induced immunosuppression may increase susceptibility to JSRV infection or enhance viral shedding from coinfected animals, thereby amplifying within-flock transmission rates.

Molecular Epidemiology and Phylogenetic Insights

The molecular epidemiology of JSRV has been substantially advanced by whole-genome sequencing and phylogenetic analyses of isolates from diverse geographic regions. The complete genome of the Inner Mongolian strain NMJS12 was recently characterized, revealing 98.8% identity to Chinese JSRV-C1 but only 88.73–92.26% identity to enJSRV sequences, confirming its exogenous origin [1]. Phylogenetic analysis placed NMJS12 and JSRV-C1 in the same evolutionary clade, distinct from foreign isolates, indicating regional viral diversification [1]. Comparative analysis of CpG island distribution between exogenous and endogenous JSRV strains has revealed significant differences: 66.66% of exJSRV sequences contain at least one CpG island in the LTR, compared to 84.84% of enJSRV sequences, while 100% of exJSRV and 96.96% of enJSRV sequences have CpG islands in the env gene [2]. These epigenetic differences may contribute to differential gene expression and host immune evasion, though their biological significance remains to be fully elucidated [2].

Phylogenetic studies from India identified six JSRV isolates with 96–100% homology to the UK strain AF105220.1 but only 88–93% identity to the South African strain M80216, with segregation into two distinct clusters [16]. Iraqi isolates similarly showed genetic divergence from global strains, clustering with KT279066.1 and KT279065.1 [9]. These data suggest that JSRV evolves independently in geographically separated sheep populations, likely driven by founder effects, genetic drift, and adaptation to local host genetics. The U3 region of the LTR, which contains promoter and enhancer elements, shows particularly high variability between strains and is a key determinant of tissue tropism, working in concert with the envelope glycoprotein-receptor interactions to target either lung or nasal epithelium [12]. Chimeric virus studies have demonstrated that the LTR promoters are primarily responsible for tissue-specificity, while Hyal2 expression levels influence entry efficiency [12]. This molecular plasticity has implications for transmission dynamics, as strains with altered tropism or replicative fitness may emerge in different ecological niches.

Surveillance Challenges and Diagnostic Considerations

Accurate estimation of JSRV prevalence and transmission rates is hampered by the limitations of current diagnostic tools. The virus has a prolonged incubation period, often exceeding one to two years, during which infected animals appear clinically normal but may shed virus [32]. Real-time TaqMan PCR of whole blood samples from clinically healthy sheep has revealed proviral DNA in 18.75% of animals, while histopathological examination of corresponding lung tissue detected OPA lesions in only 13.75%, indicating that molecular methods are more sensitive for identifying subclinical infections [32]. However, the presence of proviral DNA in blood does not necessarily correlate with active viral shedding or transmission competence, complicating interpretation of prevalence data.

The high degree of sequence similarity between exogenous and endogenous JSRV poses additional diagnostic challenges. Exogenous JSRV shares 88.73–92.26% nucleotide identity with enJSRV, with the main variation regions being the LTR, gag, and env genes [1, 2]. Diagnostic PCR assays must target regions of maximum divergence, such as the U3 region of the LTR or specific variable regions (VR1, VR2, VR3) in gag and env, to avoid amplification of endogenous sequences [1, 2]. The development of U3 hemi-nested PCR combined with density gradient centrifugation for virion purification has improved the specificity of exogenous JSRV detection [1]. Furthermore, the identification of JSRV in mosquitoes via xenosurveillance represents a novel, non-invasive approach for monitoring viral circulation in livestock populations, though this method detects host-derived DNA from blood meals rather than replicating virus [19].

In conclusion, the epidemiology and transmission dynamics of JSRV are characterized by global distribution, species-specific but expanding host tropism, multiple transmission routes including aerosol and lactogenic pathways, frequent coinfections with other ovine retroviruses, and significant genetic diversity that complicates both diagnosis and control. The virus’s ability to establish subclinical infections with prolonged shedding periods, combined with the lack of effective vaccines or treatments, makes OPA a persistent challenge for sheep producers worldwide. Continued surveillance, improved molecular diagnostics, and a deeper understanding of host-virus interactions are essential for developing evidence-based control strategies and mitigating the economic and welfare impacts of this unique retroviral disease.

Diagnostic Approaches and Xenosurveillance for JSRV Detection

The detection of Jaagsiekte sheep retrovirus (JSRV) and its associated pathology, ovine pulmonary adenocarcinoma (OPA), presents a formidable challenge to veterinary medicine, owing to the virus's complex interplay with highly related endogenous retroviruses (enJSRVs) and the protracted, often subclinical, course of disease. A diagnostic paradigm for JSRV must therefore integrate multiple, complementary modalities, ranging from gross pathology and histology to advanced molecular virology and, most recently, novel xenosurveillance strategies, to achieve both sensitivity and specificity. This section provides an exhaustive analysis of the current diagnostic arsenal, emphasizing the biological underpinnings that necessitate such a multi-layered approach, and explores the emergent field of arthropod-based surveillance as a sentinel for viral circulation.

The Foundational Triad: Gross Pathology, Histopathology, and Immunohistochemistry

Traditional post-mortem examination remains the cornerstone of OPA diagnosis in abattoir-based surveillance and diagnostic investigations. Grossly, classical OPA is characterized by firm, greyish-white, coalescing nodules, predominantly in the diaphragmatic lobes, often accompanied by the pathognomonic accumulation of copious white foamy fluid within the trachea and bronchi [7, 20]. However, reliance on macroscopic examination alone is fraught with limitations. Lee et al. [30] demonstrated that the positive percent agreement between macroscopic examination and reverse-transcriptase PCR (RT-PCR) was poor (0.38), indicating that a significant proportion of visually suspicious lesions are not JSRV-associated. Conversely, histopathological examination reveals the definitive diagnosis: a lepidic growth of neoplastic type II pneumocytes (alveolar type II cells) and non-ciliated bronchiolar epithelial (Clara) cells, forming acinar, papillary, or mixed patterns [7, 31]. This microscopic hallmark is often accompanied by the infiltration of macrophages and a lack of significant stromal desmoplasia in early lesions.

To enhance specificity, immunohistochemistry (IHC) for JSRV capsid (CA) protein (e.g., targeting the matrix protein MA) or the envelope (Env) surface (SU) protein has become a confirmatory gold standard. Toma et al. [7] achieved 94.11% positivity for JSRV-MA in OPA tumor cells, including both epithelial and mesenchymal components of myxoid growths. IHC is particularly critical for differentiating OPA from other pulmonary neoplasms, including those induced by the closely related enzootic nasal tumor virus (ENTV-1) in cases of nasal adenocarcinoma. Jahns and Cousens [11] reported a seminal case where a nasal adenocarcinoma in a sheep was JSRV-positive by PCR and IHC, necessitating the use of differential PCR to rule out ENTV-1, a virus absent in many regions including the British Isles. Thus, while IHC provides spatial resolution of viral antigen expression, it must be paired with molecular confirmation to distinguish between exogenous (exJSRV) and endogenous (enJSRV) sequences, a distinction that histology alone cannot provide.

Molecular Diagnostics: The Critical Distinction Between Exogenous and Endogenous JSRV

The ovine genome harbors at least 27 copies of enJSRV, which share up to 92% identity with exJSRV [6]. This high homology creates a significant diagnostic hurdle; PCR primers must be meticulously designed to target regions of maximal divergence to avoid amplifying endogenous proviral DNA. The long terminal repeat (LTR), particularly the U3 region, and the variable regions (VR1, VR2, and VR3) of the gag and env genes contain the greatest sequence disparity between exJSRV and enJSRV [1, 2].

Conventional and Hemi-nested PCR

The most widely adopted molecular approach involves hemi-nested PCR (hn-PCR) targeting the U3 region. Zhang et al. [1] successfully utilized U3 hn-PCR combined with density gradient centrifugation to purify virion particles, enabling the first whole-genome characterization of an Inner Mongolian exJSRV strain. This technique exploits the low-identity region of the U3 to specifically amplify exogenous sequences, a strategy also employed by Devi et al. [16] for sequencing Indian isolates. For routine diagnostics, a single-round PCR or hn-PCR amplifying a 382-bp fragment of the env gene is frequently employed. Al-Husseiny et al. [9] used this method to detect JSRV in 44% of nasal secretion samples from symptomatic Iraqi Awassi sheep.

However, conventional PCR suffers from lower sensitivity compared to real-time methods, particularly in preclinical cases where proviral load is minimal. A landmark study by Bahari et al. [32] demonstrated the superior sensitivity of real-time TaqMan PCR over histopathology in apparently healthy sheep. They detected JSRV proviral DNA in 18.75% of blood samples, whereas only 13.75% of corresponding lung tissues showed microscopic OPA lesions. This finding highlights a critical biological reality: JSRV infection is significantly more prevalent than overt disease, and latent, subclinical carriers serve as reservoirs for viral transmission. The use of quantitative PCR (qPCR) targeting the env or gag genes with specific probes, as validated by Ndione et al. [19] for confirmation of mosquito-borne sequences, offers the ability to quantify proviral burden, which may correlate with disease progression or transmission potential.

RT-PCR and Whole-Genome Sequencing

Reverse-transcriptase PCR (RT-PCR) is essential for detecting the replicating, RNA-based viral genome, as opposed to the integrated proviral DNA. Lee et al. [30, 31] applied RT-PCR to lung tissue homogenates from abattoir samples, confirming the first molecular detection of JSRV in Ireland. The use of RT-PCR is particularly important for confirming active infection in cases where only formalin-fixed tissue is available or when attempting to amplify the full-length genome. Zhang et al. [1] advanced this by combining long-fragment RT-PCR with 3′RACE technology, circumventing the instability of JSRV RNA and enabling the cloning of the complete 7.4 kb genome.

Next-generation sequencing (NGS) and phylogenetic analysis now play a critical role in molecular epidemiology. The Inner Mongolia strain NMJS12 (98.8% identity to JSRV-C1) [1] and the Romanian exJSRV type 2 (98% identity to UK strain AF105220.1) [7] demonstrate geographic clustering, but also continuous genomic drift. Phylogenetic analysis of the env and gag genes, as performed in co-infection studies with Maedi-Visna virus (MVV) [20, 25], is vital for tracking viral evolution, emergence of novel strains, and understanding cross-species transmission events, such as the first report of JSRV-induced nasal adenocarcinoma in a goat [3].

Advanced Diagnostic and Monitoring Platforms: CT Imaging and Biomarker Discovery

The development of an experimentally induced OPA model by Cousens et al. [28] has revolutionized our ability to monitor disease progression in vivo. Using monthly computed tomography (CT) imaging and thoracic ultrasound, the team tracked tumor development following bronchoscopic JSRV instillation. CT 3D reconstructions allowed calculation of tumor volume doubling times (14.8 ± 2.1 days), revealing the aggressive proliferative capacity of JSRV-transformed cells. Ultrasonography showed a strong linear correlation with CT measurements (R² = 0.799), providing a more accessible, field-deployable method for longitudinal studies.

Transcriptomic and proteomic analyses have further expanded the diagnostic landscape. RNA sequencing of OPA-affected lung tissue has identified 1,971 differentially expressed genes, including activation of the Hippo signaling pathway (via YAP1) and anterior gradient 2 (AGR2), as well as PI3K/Akt/mTOR and MAPK pathways [10, 22]. These dysregulated pathways provide potential molecular biomarkers that could be detected in bronchoalveolar lavage fluid or serum. Similarly, microRNA (miRNA) profiling by Garcia et al. [27] revealed upregulation of miR-182, miR-183, miR-96, and miR-135b, miRNAs also associated with human lung adenocarcinoma. While host-derived miRNAs are promising diagnostic targets, no viral miRNAs have been identified, limiting the utility of this approach for direct JSRV detection.

Xenosurveillance: Expanding the Detection Frontier

A paradigm-shifting diagnostic approach has emerged from the field of vector ecology: xenosurveillance, which leverages blood-feeding arthropods as passive samplers of host and pathogen genetic material. Ndione et al. [19] reported the first identification of JSRV-related sequences in blood-fed mosquitoes (primarily Culex and Aedes species) collected in Senegal. RT-qPCR targeting conserved regions of the env gene and 3′ untranslated region (UTR) confirmed the presence of enJSRV DNA in a mosquito pool, and phylogenetic analysis revealed strong similarity to endogenous JSRV sequences integrated in the sheep genome.

This finding has profound implications for surveillance ecology. Mosquitoes do not serve as biological vectors for JSRV (retroviral replication is not supported in arthropod cells); rather, they act as mechanical carriers, reflecting the host's bloodmeal content. This means that detection of JSRV in a mosquito indicates recent, direct contact with an infected sheep. The utility of this approach is twofold: (i) it enables non-invasive sampling of livestock populations without the need for animal handling, blood collection, or abattoir access, and (ii) it captures a broad ecological snapshot, potentially identifying infected herds or flocks in remote, resource-limited settings where routine veterinary surveillance is absent. As Ndione et al. [19] emphasize, mosquitoes can simultaneously host DNA and RNA from multiple viruses, including arboviruses and non-arthropod-borne agents like JSRV, making them integrated sentinels for multi-pathogen surveillance.

The potential for xenosurveillance to inform OPA epidemiology is substantial. Given that OPA is a chronic disease with a long incubation period, and that subclinical shedders are primary transmission sources, mosquitoes feeding on these asymptomatic animals could reveal hidden viral hotspots long before clinical signs emerge. Future studies should focus on optimizing mosquito collection protocols (e.g., selecting for freshly blood-fed individuals using colorimetric or photonic screening), developing multiplex qPCR assays to simultaneously detect exJSRV and enJSRV in mosquito homogenates, and integrating this data with geospatial mapping to predict areas of elevated transmission risk. Such an approach aligns with the World Organisation for Animal Health (WOAH) guidelines for emerging disease surveillance, advocating for innovative, ecosystem-based detection strategies to complement traditional veterinary diagnostic networks.

Integrated Diagnostic Algorithms and Future Perspectives

No single diagnostic test provides complete sensitivity and specificity for JSRV. The current best practice, as evidenced by the Irish abattoir survey [30, 31] and the Romanian prevalence study [7], involves a parallel testing algorithm: IHC or PCR should be performed on all macroscopically suspicious lungs, while negligible added value is gained from testing macroscopically normal lungs. For live-animal diagnosis, qPCR on blood or nasal swabs remains the most sensitive ante-mortem tool, but its positive predictive value is limited by the presence of enJSRV sequences. However, the use of U3 hn-PCR or sequencing of the variable regions (VR1-VR3) can reliably discriminate exJSRV [1, 2].

The detection of JSRV in mosquitoes [19] opens a new diagnostic paradigm, but rigorous validation is required. Standardization of protocols for RNA extraction from insect vectors, sequencing depth requirements, and bioinformatic pipelines for distinguishing between host-derived endogenous retroviruses and replicating exogenous virus must be established. Additionally, the stability of JSRV RNA in the mosquito midgut over time post-bloodmeal needs characterization to define the temporal window of detection.

In conclusion, the diagnostic landscape for JSRV is evolving from a purely pathological discipline to a multi-modal, integrative science. The convergence of high-resolution imaging, transcriptomic biomarker discovery, and xenosurveillance has the potential to transform our ability to detect, track, and ultimately control OPA. The unique challenge of differentiating exogenous from endogenous sequences will continue to drive innovation in PCR primer design and sequencing technology, while the use of mosquitoes as sentinel species may provide a cost-effective, scalable tool for global surveillance of this insidious pathogen.

Viral Particle Assembly and In Vitro Packaging Systems

The assembly of infectious Jaagsiekte sheep retrovirus (JSRV) virions represents a complex, multi-step process that is intrinsically linked to the unique biology of this betaretrovirus. Understanding the molecular choreography of particle assembly, from the synthesis of genomic RNA and viral proteins to the budding of mature virions, is not merely an academic exercise; it is a prerequisite for developing effective antiviral strategies, constructing reliable diagnostic reagents, and establishing the in vitro systems necessary for dissecting the virus's oncogenic mechanisms. The inherent difficulties in propagating JSRV in standard continuous cell lines have historically hampered progress, but recent advances in molecular cloning and packaging systems are beginning to illuminate the critical determinants of virion morphogenesis and provide the essential tools for experimental manipulation.

The Molecular Choreography of JSRV Particle Assembly

JSRV, as a member of the Betaretrovirus genus, follows a general retroviral assembly pathway, yet it possesses distinctive features that are vital for its life cycle and pathogenicity. The virus is enveloped, with a dense, centrally-located, and presumably spherical core. The process begins with the translation of the unspliced full-length viral RNA, which serves a dual role as both the genomic RNA for encapsidation and the mRNA for the Gag and Gag-Pol polyproteins. A critical, and somewhat unique, aspect of JSRV gene expression is its reliance on a trans-acting factor, Rej, which is encoded within the env gene. As described by Hofacre and Fan, Rej is essential for the cytoplasmic accumulation and translation of unspliced viral RNA, thereby enabling Gag synthesis [18]. Without Rej, the viral RNA is retained in the nucleus, and the structural proteins required for assembly are not produced, establishing a sophisticated post-transcriptional regulatory checkpoint.

The Gag polyprotein is the primary driver of particle assembly. It contains the matrix (MA), capsid (CA), and nucleocapsid (NC) domains, along with a late domain that recruits cellular machinery, such as the endosomal sorting complexes required for transport (ESCRT), to facilitate membrane budding. The JSRV Gag protein directs the assembly of immature, non-infectious particles at the plasma membrane. During or immediately after budding, the viral protease (PR) cleaves Gag into its constituent domains, a process known as maturation. This proteolytic cleavage triggers a dramatic structural rearrangement, leading to the condensation of the core into the characteristic mature, infectious morphology. The Env glycoprotein, which is synthesized from a spliced subgenomic mRNA, is trafficked to the plasma membrane independently. Here, the surface subunit (SU) and the transmembrane (TM) subunits, linked by disulfide bonds, are incorporated into the nascent viral envelope. The incorporation of Env is a highly regulated process, and the Env protein itself is the primary determinant of JSRV’s potent oncogenic potential [17, 18]. Notably, the interaction between the Env cytoplasmic tail and the Gag matrix domain is a critical step for specific Env incorporation into budding virions.

The Landmark Achievement: An In Vitro Packaging System for JSRV

For decades, the study of JSRV assembly was severely constrained by the inability to produce the virus efficiently in controlled culture systems. This limitation was a major roadblock, as it prevented researchers from performing standard virological assays such as infectivity titrations, mutagenesis of assembly domains, and the production of high-titer viral stocks for experimental infections. A major breakthrough, however, was reported by Zhang et al. in their 2025 study on the Inner Mongolia strain (NMJS12). This work represents the first successful construction of a whole-genome eukaryotic expression plasmid for JSRV and, critically, the subsequent achievement of viral particle packaging in a heterologous system, human embryonic kidney (293T) cells [1].

This accomplishment is a paradigm shift for the field. The authors cloned the complete exogenous JSRV genome (NMJS12) into a plasmid under the control of a strong, constitutively active eukaryotic promoter (such as CMV). When this plasmid was transfected into 293T cells, the cellular transcription machinery produced the full-length viral RNA, leading to the expression of all viral proteins (Gag, Pol, Env, and Rej). The resulting assembly of particles was then confirmed and characterized [1]. This system proves that the JSRV genome contains all the cis-acting elements necessary for transcription, RNA export, translation, and assembly, even in a non-ovine cell line [1]. This cross-species compatibility is a testament to the conserved nature of the fundamental cellular machinery involved in retroviral assembly.

The implications of this system are profound. It provides a robust platform for producing defined, clonal viral stocks. Such stocks are crucial for standardizing experimental infections, such as the bronchoscopic instillation model developed by Cousens et al. [28], allowing for precise control of viral dose and genetic makeup in pathogenesis studies. Furthermore, this packaging system opens the door to high-throughput screening for antiviral compounds that specifically block JSRV assembly or release. By using a reporter gene (e.g., GFP or luciferase) in place of a viral gene, a packaging cell line could be engineered to screen libraries of small molecules. The system also allows for the systematic mutagenesis of viral proteins to map domains essential for particle assembly. For instance, one could precisely delete or mutate the Rej-responsive element or the Gag late domain within the proviral plasmid and directly assess the effect on particle production in 293T cells.

Alternative Models and the Importance of Cellular Context

While the 293T-based packaging system is a monumental step forward, it is not a complete substitute for studying assembly in the natural target cell, the type II pneumocyte. The host cell environment profoundly influences virus assembly efficiency, particle composition, and even morphology. This was elegantly demonstrated by Johnson and Fan in their work with the JS-7 cell line, an OPA-derived line that had lost its differentiated type II pneumocyte characteristics when grown in standard two-dimensional (2D) monolayer culture. Under these conditions, JS-7 cells ceased to produce JSRV particles. However, when transferred to a three-dimensional (3D) culture system using Matrigel, a basement membrane matrix, these cells reorganized into polarized spheres, re-expressed surfactant proteins, and, remarkably, re-initiated the production of infectious JSRV particles [34]. This phenomenon underscores a critical principle: JSRV assembly is exquisitely sensitive to the differentiation state and the cellular architecture of the host cell. The 3D environment presumably restores the intracellular signaling and trafficking pathways that are essential for proper Gag and Env transport and assembly, which are lost in 2D culture.

Further insights into the complexities of JSRV assembly and morphology come from studies of concurrent infections. Transmission electron microscopy (TEM) analysis of lung tissues from sheep co-infected with JSRV and Maedi-Visna virus (MVV) revealed the presence of numerous virions and autophagosomes, along with severely damaged mitochondria and evidence of induced mitophagy [20]. This observation suggests that JSRV assembly does not occur in isolation but can be modulated by the cellular stress response and interactions with other viruses. The presence of autophagosomes in infected cells raises the intriguing possibility that autophagy, a cellular degradation pathway, might be co-opted or subverted during JSRV assembly, potentially influencing particle release or turnover. This interaction between assembly, cellular stress, and co-infection is a rich area for future investigation.

The World Organisation for Animal Health (WOAH, formerly OIE) recognizes OPA as a significant disease, and the ability to produce JSRV in cell culture is critical for developing sensitive and specific diagnostic assays mandated for trade and disease surveillance. While the 293T system produces particles, a key question remains regarding the authenticity of these particles compared to those produced in sheep lung. The particles from 293T cells are likely to have a different lipid composition and possibly different glycan structures on the Env protein. Therefore, while the 293T system is ideal for molecular virology, the ex vivo lung slice culture model, established by Cousens et al. [14], may remain the gold standard for studying assembly in a more physiologically relevant context, as it retains the complex three-dimensional architecture of the lung and the differentiated state of the target cells.

In summary, the field of JSRV assembly has transitioned from a descriptive phase, reliant on TEM of tumor tissue, to a functional and mechanistic era enabled by the development of a robust in vitro packaging system. The 293T-based system provides the necessary tractability for genetic and pharmacological manipulation, while the 3D culture models and ex vivo lung slices provide the necessary biological context. The future challenge lies in integrating these approaches: using the packaging system to identify assembly inhibitors, validating them in the lung slice model, and ultimately understanding how the unique cellular environment of the ovine type II pneumocyte orchestrates the final steps of virion morphogenesis to create the infectious agent that drives OPA pathogenesis. The interplay between the viral Rej protein, the Gag-Env interaction, and the host cell's ESCRT machinery and differentiation state will be central to this ongoing investigation, with direct relevance to the development of novel control strategies for this devastating disease.

Evolutionary and Host Interactions Between Exogenous and Endogenous JSRV

The relationship between exogenous Jaagsiekte sheep retrovirus (exJSRV) and its endogenous counterparts (enJSRV) represents one of the most fascinating and complex evolutionary narratives in retrovirology. This interplay, forged over millions of years of coevolution, has fundamentally shaped both the pathogenic potential of the exogenous virus and the physiological landscape of its ovine host. To fully apprehend the nature of exJSRV-induced disease, one must first appreciate the deep evolutionary history etched into the sheep genome by enJSRVs, and the ongoing molecular dialogue between these two viral entities.

The Paleovirological Record: Endogenization and Genomic Colonization

Endogenous retroviruses (ERVs) are the fossilized remnants of ancient germline infections, representing viral genetic material that has been vertically transmitted for generations. The genome of domestic sheep (Ovis aries) harbors at least 27 distinct copies of enJSRVs, a remarkable testament to repeated waves of retroviral invasion and fixation [6]. These loci are not merely inert passengers; they are active participants in host biology, providing a unique window into the evolutionary arms race between virus and host. Comparative fluorescence in situ hybridization (FISH) mapping has demonstrated that enJSRV loci are conserved in homologous chromosomes and bands of sheep and river buffalo (Bubalus bubalis), strongly suggesting that these integrations occurred in the common ancestor of the Bovidae family before the divergence of these two species [6]. This deep evolutionary conservation underscores that the relationship between the host and these endogenous elements is ancient and, in many respects, symbiotic.

The distinction between exogenous and endogenous JSRV is critically important for understanding pathogenesis. While exJSRV is a horizontally transmitted, pathogenic virus that causes ovine pulmonary adenocarcinoma (OPA), enJSRVs are vertically inherited, replication-defective, and generally non-pathogenic. However, the proviral genomes of exJSRV and enJSRV share an extraordinarily high degree of nucleotide sequence identity, ranging from 88.73% to 92.26% depending on the specific isolate and genomic region analyzed [1]. This genetic proximity indicates that the endogenous elements are derived from ancient infections by an ancestral exogenous betaretrovirus. The key genomic regions that distinguish the two are concentrated in the long terminal repeat (LTR)-U3 region, the gag variable regions (VR1 and VR2), and the env variable region 3 (VR3) [1]. These discrete zones of divergence are precisely where the determinants of pathogenicity, host range, and replication capacity are encoded.

Genomic Divergence and the Evolutionary Arms Race: CpG Islands and Viral Restriction

The battle between host and virus is often waged at the epigenetic level, and this is vividly illustrated by the differential distribution of CpG islands between exJSRV and enJSRV. CpG dinucleotides are hotspots for DNA methylation, a key epigenetic modification that can silence gene expression. A landmark analysis of 42 full-length JSRV genomic sequences revealed striking differences in CpG island distribution between exJSRV and enJSRV strains [2]. In exJSRVs, 100% of sequences presented at least one CpG island in the env gene, while only 66.66% and 33.33% presented CpG islands in the LTR and gag genes, respectively. In contrast, enJSRVs showed a higher prevalence of CpG islands in the LTR (84.84%) and gag (57.57%) genes, with a slightly lower prevalence in the env gene (96.96%) [2]. This differential distribution suggests that the host has applied selective pressure on the integrated endogenous elements through methylation, potentially as a mechanism to suppress their expression and prevent their reactivation. The fact that exJSRV, particularly in its env gene, retains a CpG island profile distinct from its endogenous relatives may reflect an evolutionary adaptation to evade this host restriction, or conversely, it may indicate that the exogenous virus has not yet been subjected to the same long-term epigenetic silencing forces that have shaped the enJSRV loci over millennia.

Functional Domestication of enJSRV: The Envelope Protein and Placental Morphogenesis

One of the most profound host interactions involves the co-option of enJSRV-encoded proteins for physiological function. The concept of viral domestication is a recurring theme in evolutionary biology, and JSRV provides an exemplary case. The endogenous envelope protein (enJSRV-Env) has been domesticated by the sheep for a critical role in placental development. Specifically, enJSRV-Env promotes trophoblast cell fusion, a process essential for the formation of the syncytiotrophoblast layer that mediates nutrient and gas exchange at the maternal-fetal interface [21]. The molecular mechanism underlying this fusogenic activity has been elegantly elucidated: enJSRV-Env activates the PKA/MEK/ERK1/2 signaling cascade. Co-immunoprecipitation studies revealed a physical interaction between enJSRV-Env and PKA, leading to the downstream activation of MEK and ERK1/2. Crucially, pharmacological inhibition of either PKA or ERK1/2 completely abolished enJSRV-Env-induced multinucleated cell formation in coculture systems of endometrial luminal epithelial cells and sheep trophoblast cells [21]. This functional domestication is a powerful example of how a once-pathogenic viral protein has been repurposed to serve a vital host function, a process that likely exerted strong selective pressure to maintain these enJSRV loci in the sheep genome.

Genetic Determinants of Tropism and Pathogenicity

The distinct tissue tropism of JSRV, targeting the lung, and in rare cases the nasal cavity [11] or even the mammary gland [26], is governed by a complex interaction between the viral envelope protein and the host receptor, as well as the transcriptional activity of the viral LTR. The envelope (Env) protein of exJSRV is not only the attachment and entry protein but also the primary oncogene. This dual functionality is unique among retroviruses. Studies using chimeric viruses between JSRV and the related enzootic nasal tumor virus (ENTV) have been instrumental in dissecting the genetic determinants of tropism. While both JSRV and ENTV use the same cellular receptor, hyaluronoglucosaminidase 2 (Hyal2), for entry, they target distinct regions of the respiratory tract, the lung and the nose, respectively. By constructing JSRV-ENTV chimeric lentivectors, researchers demonstrated that Hyal2 expression levels strongly influence ENTV entry, but that the LTR promoters are likely responsible for the ultimate tissue specificity of these viruses [12]. This indicates that transcriptional regulation, driven by the U3 region of the LTR, is a primary determinant of where the virus establishes infection and induces transformation.

The envelope protein itself is the central player in oncogenesis. A mechanistic dissection of JSRV Env domains revealed that the cytoplasmic tail (CT) of the transmembrane (TM) protein is critical for transformation, but other regions are also essential. Chimeric analysis between JSRV Env and the non-oncogenic enJSRV Env (clone 5F16) showed that the membrane-spanning region (MSR) plays a significant role, as substitution with the enJSRV MSR substantially reduced transformation capacity. Furthermore, the amino-terminal region of the surface unit (SU) also contributes to transformation, while the C-terminal part appears dispensable [17]. These findings highlight that the evolution of oncogenicity in exJSRV involved the acquisition of specific sequences in multiple domains of the Env protein, differentiating it from its harmless endogenous cousins. The trimerization of the Env protein is also essential, as mutation of a leucine-rich trimerization domain abolished transformation, although complementation experiments revealed that oligomerization is not necessary for all aspects of Env function [17].

Coinfection, Host Microenvironment, and the Role of Endogenous Elements in Disease

The host-pathogen interaction is further complicated by the frequent coinfection of sheep with JSRV and other pathogens, particularly maedi-visna virus (MVV). This concurrent infection is not a rare event; retrospective studies in Transylvania (Romania) have identified coinfection rates of 47.6% in exJSRV-positive OPA cases [25]. In another study in Inner Mongolia, China, coinfection with JSRV and MVV was confirmed in 3 out of 319 slaughtered rams, with histopathological evidence of both OPA and MVV-associated lesions in the same lung tissue [20]. The pathological implications are significant: coinfected animals exhibit pronounced interstitial lymphoplasmacytic infiltrates, lymphoid hyperplasia, and fibromuscular hyperplasia in perineoplastic areas [25]. This suggests that MVV-induced immunosuppression or inflammatory signaling may exacerbate OPA progression, or conversely, that the tumor microenvironment created by JSRV may predispose to secondary viral infections.

At the cellular level, JSRV infection induces profound changes in the host transcriptome. RNA-seq analysis of naturally infected OPA lung tissues identified 366 differentially expressed genes (DEGs), with 154 up-regulated and 212 down-regulated compared to healthy controls. These DEGs were significantly enriched in pathways governing cell proliferation, differentiation, apoptosis, and migration, including the PI3K/Akt/mTOR, MAPK, and Hippo signaling pathways [22]. Notably, the Hippo signaling pathway, which had never been previously implicated in natural OPA cases, was found to be activated in both lung tumor tissues and in sheep trophoblast cells transformed with JSRV Env-expressing lentivirus. This activation was confirmed by immunohistochemistry and western blot, and functional assays demonstrated that JSRV Env promotes malignant transformation through this pathway [22]. The transcriptional response also includes the activation of anterior gradient 2 (AGR2), yes-associated protein 1 (YAP1), and amphiregulin (AREG), indicating a role for this oncogenic axis in OPA [10]. Conversely, there was little evidence for upregulation of T-cell immunity genes, suggesting that JSRV effectively evades adaptive immune responses, a strategy that may be partially attributed to the tolerizing influence of enJSRV expression during development.

The Zoonotic Frontier: JSRV and Human Lung Cancer

The evolutionary and host interaction story takes a controversial turn when considering the potential for JSRV to infect humans. Given the striking histological similarity between OPA and human lung adenocarcinoma, particularly adenocarcinoma in situ (formerly bronchioloalveolar carcinoma), there has been sustained interest in whether JSRV might play a role in human disease. Early studies detected JSRV Env-reactive antigens and amplified JSRV-like env and gag sequences from human lung cancer tissue arrays, albeit inefficiently, suggesting that a JSRV-like virus might infect humans [15]. However, more definitive studies using highly specific mouse monoclonal antibodies and rabbit polyclonal antisera against JSRV Env found no detectable Env expression in lung cancers from 128 human subjects, including 73 cases of adenocarcinoma with lepidic components. Importantly, the 73 cases included 8 JSRV DNA-positive samples from Sardinia, Italy, an area with high sheep density and endemic JSRV. Furthermore, no neutralizing antibodies were found in sera from 138 Peruvians living in a sheep-farming region, 24 of whom had direct sheep exposure [13]. The consensus, supported by the World Organisation for Animal Health (WOAH) and the scientific community, is that JSRV is not a significant cause of human lung cancer. While JSRV can infect human cells in vitro, as shown by reverse transcription and DNA persistence in cultured cells, the absence of serological or immunohistochemical evidence in large patient cohorts argues strongly against a zoonotic role [13].

Xenosurveillance and the Expanding Host Range

A novel dimension to the host interactions of JSRV has emerged from the field of xenosurveillance. Mosquitoes, long studied as vectors for arthropod-borne viruses, have been shown to act as passive carriers of non-arthropod-borne viruses, including retroviruses. In a study conducted in Senegal, JSRV-related sequences were detected for the first time in a blood-fed mosquito pool collected in the Barkedji region. Phylogenetic analysis revealed that the detected sequence had strong similarity to known endogenous JSRV (enJSRV) sequences integrated in the sheep genome, indicating that the material originated from host DNA ingested during blood feeding [19]. This finding expands the concept of host-vector contact and demonstrates that mosquitoes can serve as environmental sentinels for monitoring livestock-associated viruses, even those that are not biologically vectored. This technique offers a powerful, non-invasive method for surveying the prevalence and distribution of both exogenous and endogenous JSRV in complex ecosystems.

The host range of JSRV has also expanded in unexpected directions. A recent report documented the first confirmed case of JSRV-associated nasal adenocarcinoma in a 12-month-old goat from a herd with no documented contact with sheep [3]. Histopathological examination confirmed a mixed glandular adenocarcinoma with turbinate bone invasion, and PCR analysis confirmed the presence of JSRV proviral DNA, while tests for the related enzootic nasal tumor viruses (ENTV-1 and ENTV-2) were negative. Phylogenetic analysis classified the viral strain within the exogenous JSRV group [3]. This case indicates a potential expansion of the virus’s host and tissue tropism, raising questions about the envelope-receptor interactions and promoter elements that govern these changes. Similarly, nasal adenocarcinomas associated with JSRV have been reported in sheep in Ireland, a country previously thought to be free of ENTV-1 [11].

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