# HAVCR1 (TIM-1): Hepatitis A Virus Attachment, Phosphatidylserine Receptor Function, and Viral Entry


## Key Takeaways

- HAVCR1 (TIM-1) is a type I transmembrane glycoprotein that functions as the primary high-affinity receptor for Hepatitis A Virus (HAV), mediating viral entry through specific interactions with the viral capsid and phosphatidylserine (PS) on the viral quasi-envelope.
- The protein's N-terminal immunoglobulin variable (IgV)-like domain contains a metal-ion-dependent ligand binding site (MILIBS) crucial for recognizing externalized PS on apoptotic cells and on the lipid envelopes of enveloped viruses like Ebola, Dengue, and Zika.
- Beyond viral interactions, HAVCR1 acts as a potent costimulatory molecule on activated CD4+ T helper 2 (Th2) cells, amplifying cytokine production and influencing allergic and autoimmune responses, with specific germline polymorphisms (e.g., 6-amino acid insertion) linked to asthma susceptibility.
- Aberrant HAVCR1 expression is observed in multiple solid tumors, particularly renal cell carcinoma and hepatocellular carcinoma, where it promotes epithelial-mesenchymal transition, immune evasion, and chemoresistance, making it a target for antibody-drug conjugates (ADCs) and monoclonal antibodies.
- HAVCR1 is a validated urinary biomarker for acute kidney injury (AKI), with elevated levels indicating proximal tubular damage, and its genetic variations can influence susceptibility to various viral infections and responses to targeted therapies.

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## Executive Summary & Key Metadata

HAVCR1 (Hepatitis A Virus Cellular Receptor 1), also widely designated TIM-1 (T-cell Immunoglobulin and Mucin domain-containing protein 1), is a type I transmembrane glycoprotein that operates at the interface of host immunity, apoptotic cell clearance, and viral pathogenesis. The gene product is a canonical member of the TIM family of phosphatidylserine (PS) receptors, characterized by an N-terminal immunoglobulin variable (IgV)-like domain that contains a metal-ion-dependent ligand binding pocket (MILIBS) specific for PS. This structural feature enables HAVCR1 to function as a pattern recognition receptor for apoptotic membranes and as an attachment factor for enveloped viruses that display PS on their lipid envelopes, including Ebola virus, Dengue virus, and Zika virus. Most notably, HAVCR1 serves as the primary high-affinity receptor for Hepatitis A Virus (HAV), a positive-sense single-stranded RNA picornavirus that causes acute hepatitis in humans. Beyond its role in viral entry, HAVCR1 is a potent costimulatory molecule on activated CD4+ T helper 2 (Th2) cells, where it amplifies cytokine production and modulates allergic and autoimmune responses. In oncology, HAVCR1 expression is aberrantly upregulated in multiple solid tumors, particularly renal cell carcinoma and hepatocellular carcinoma, where it promotes epithelial-mesenchymal transition, immune evasion, and chemoresistance. The protein is also a validated urinary biomarker for acute kidney injury. This reference manual provides a comprehensive, biophysically grounded analysis of HAVCR1, spanning its genomic architecture, structural biology, signaling networks, pathogenic mutations, viral interactions, and therapeutic targeting.

| **Attribute** | **Value** |
|:---|:---|
| **HGNC Symbol** | HAVCR1 |
| **UniProt Accession** | Q96D42 |
| **Representative PDB ID** | 5FGR |
| **Chromosomal Locus** | 5q33.3 (GRCh38: chr5:157,029,000–157,058,000) |
| **Primary Molecular Function** | Phosphatidylserine receptor; viral attachment factor; T-cell costimulation |
| **Disease & Pathology Associations** | Hepatitis A infection; acute kidney injury; renal cell carcinoma; hepatocellular carcinoma; asthma/allergy susceptibility; atherosclerosis |

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The *HAVCR1* gene is located on the long arm of human chromosome 5 at cytogenetic band 5q33.3. This region is a well-established immunoregulatory locus, containing a cluster of TIM family genes including *TIMD4* (TIM-4), *HAVCR2* (TIM-3), and *TIMD1* (TIM-1). The genomic span of *HAVCR1* is approximately 29 kilobases (kb) on the plus strand of chromosome 5. The gene comprises 9 exons and 8 introns, with the coding sequence distributed across exons 2 through 9. Exon 1 is entirely untranslated (5' UTR) and is separated from the translation initiation codon in exon 2 by a large intron of approximately 12 kb. The mature mRNA transcript is approximately 3.5 kb in length, including a 3' UTR of roughly 1.2 kb that contains multiple AU-rich elements (AREs) implicated in post-transcriptional regulation by RNA-binding proteins such as HuR and TTP.

The promoter region of *HAVCR1* lacks a canonical TATA box but contains a high-density CpG island spanning approximately 1.5 kb upstream of the transcription start site (TSS) and extending into exon 1. This CpG island is subject to dynamic DNA methylation, and its hypomethylation correlates with transcriptional activation in Th2-polarized T cells and in malignant tissues. Multiple Sp1 binding sites and a conserved GATA-3 response element have been identified within the proximal promoter, linking *HAVCR1* transcription directly to Th2 lineage commitment. Additionally, a functional NF-κB binding site at position −320 relative to the TSS mediates inducible expression in response to inflammatory cytokines such as TNF-α and IL-1β. Chromatin immunoprecipitation (ChIP) studies in primary human T cells have demonstrated that the *HAVCR1* promoter is occupied by histone H3 lysine 4 trimethylation (H3K4me3) marks at the TSS and H3K27ac marks at a distal enhancer located approximately 8 kb upstream, indicating active enhancer-promoter looping in Th2 cells.

### 1.2 Alternative Splicing and Isoform Diversity

Alternative splicing of the *HAVCR1* pre-mRNA generates multiple transcript variants. The canonical full-length isoform (isoform 1, UniProt Q96D42-1) encodes a 364-amino acid protein. A second major isoform (isoform 2) arises from the use of an alternative 3' splice acceptor site in intron 6, resulting in an in-frame deletion of 18 amino acids within the mucin domain. This shorter mucin domain variant exhibits altered O-glycosylation patterns and reduced PS-binding avidity when expressed on cell surfaces. A third isoform, generated by exon 4 skipping, produces a truncated protein lacking the entire mucin domain and the transmembrane region; this soluble form is secreted into the extracellular milieu and can act as a decoy receptor, sequestering PS-bearing ligands and modulating immune responses. Notably, a naturally occurring splice variant that introduces a premature stop codon in exon 7 encodes a C-terminally truncated protein that retains the IgV domain but lacks the cytoplasmic tail; this variant exerts a dominant-negative effect on full-length HAVCR1 signaling by sequestering ligands without transducing intracellular signals.

Quantitative RT-PCR analyses across human tissues reveal that *HAVCR1* mRNA is most abundant in kidney proximal tubule epithelial cells, followed by testis, liver, and peripheral blood leukocytes. In the immune compartment, expression is restricted to activated CD4+ T cells (particularly Th2 and Th17 subsets), regulatory T cells (Tregs), and a subset of innate lymphoid cells. Resting naïve T cells express negligible levels, indicating that *HAVCR1* is an activation-induced gene.

## 2. 3D Protein Domain Architecture & Structural Biology

### 2.1 Primary Structure and Domain Organization

The HAVCR1 protein is a 364-amino acid type I transmembrane glycoprotein with a theoretical molecular weight of approximately 39.8 kDa for the unmodified polypeptide; extensive N- and O-linked glycosylation increases the apparent molecular weight to 70–85 kDa on SDS-PAGE. The protein is organized into five distinct structural regions from the N-terminus to the C-terminus:

1. **Signal peptide** (residues 1–20): Cleaved co-translationally by signal peptidase in the endoplasmic reticulum.
2. **Immunoglobulin variable (IgV)-like domain** (residues 21–129): The functional core of the protein, containing the PS-binding pocket and the viral attachment interface.
3. **Mucin-like domain** (residues 130–250): A heavily O-glycosylated, extended rod-like structure rich in threonine, serine, and proline residues. This domain projects the IgV domain away from the plasma membrane and contributes to cell-cell adhesion.
4. **Transmembrane domain** (residues 251–273): A hydrophobic α-helix anchoring the protein in the lipid bilayer.
5. **Cytoplasmic tail** (residues 274–364): Contains multiple tyrosine and serine phosphorylation sites and a conserved immunoreceptor tyrosine-based switch motif (ITSM) at residues 310–315 (Tyr-Glu-Asp-Val-Ile-Tyr).

### 2.2 Three-Dimensional Structure of the IgV Domain

The high-resolution crystal structure of the HAVCR1 IgV domain has been solved by X-ray crystallography (PDB: 5FGR) at 2.1 Å resolution. The domain adopts a classical immunoglobulin fold composed of nine β-strands (A, B, C, C', D, E, F, G, and A') arranged into two antiparallel β-sheets. The structure is stabilized by a conserved disulfide bond between Cys58 (in the B strand) and Cys119 (in the F strand), which bridges the two β-sheets. A second disulfide bond links Cys21 (A strand) to Cys44 (C' strand), further rigidifying the N-terminal region.

The most functionally critical structural feature is the **metal-ion-dependent ligand binding site (MILIBS)**, located at the membrane-distal face of the IgV domain. The MILIBS is a shallow, solvent-exposed cleft formed by the C'C'' and FG loops. The pocket coordinates a single calcium ion (Ca²⁺) through the side-chain carboxylates of Glu87 and Asp89, the backbone carbonyl of Leu91, and two water molecules. The Ca²⁺ ion serves as a bridging ligand that coordinates the phosphate group of PS. The hydrophobic side chains of Phe66, Trp72, and Leu93 form a hydrophobic ridge adjacent to the Ca²⁺ binding site, which accommodates the two fatty acyl chains of PS. Mutagenesis studies have demonstrated that substitution of Glu87 or Asp89 with alanine abolishes PS binding and viral attachment, confirming the absolute requirement of the MILIBS for ligand recognition.

The FG loop (residues 96–108) is a structurally dynamic region that adopts distinct conformations in the apo and ligand-bound states. In the apo state, the FG loop is partially disordered; upon PS binding, it undergoes an induced-fit conformational change that stabilizes the closed, high-affinity state. This conformational plasticity is critical for the broad ligand specificity of HAVCR1, allowing it to recognize PS on apoptotic cell membranes, viral envelopes, and oxidized lipoproteins.

### 2.3 Mucin Domain and O-Glycosylation

The mucin domain is a structurally disordered, extended polypeptide that is heavily decorated with O-linked glycans. Mass spectrometry-based glycoproteomics has identified over 30 distinct O-glycosylation sites within this domain, predominantly occupied by core 1 and core 2 O-glycans (Galβ1-3GalNAc and GlcNAcβ1-6GalNAc, respectively). The dense glycosylation confers a rigid, extended conformation that functions as a spacer, elevating the IgV domain approximately 20–30 nm above the plasma membrane. This spatial separation is essential for efficient engagement of ligands on opposing cells or viral particles. The mucin domain also contains multiple cleavage sites for matrix metalloproteinases (MMPs), particularly MMP-1 and MMP-9, which can shed the ectodomain from the cell surface, generating soluble HAVCR1 (sHAVCR1) that is detectable in serum and urine.

### 2.4 Cytoplasmic Tail and Signaling Motifs

The cytoplasmic tail of HAVCR1 contains several functionally important motifs. The ITSM (Tyr310-Ile-Tyr315) is a docking site for the SH2 domain-containing protein tyrosine phosphatases SHP-1 and SHP-2, as well as the p85 subunit of PI3K. Phosphorylation of Tyr310 and Tyr315 by Src-family kinases (e.g., Lck) is required for recruitment of these downstream effectors. Additionally, the tail contains a conserved serine residue (Ser322) that is phosphorylated by protein kinase C (PKC) following T-cell receptor engagement, modulating the strength and duration of HAVCR1-mediated costimulation. The C-terminal 20 residues (344–364) constitute a PDZ-binding motif (Thr-Ser-Ala-Val) that interacts with the PDZ domain-containing scaffold protein GOPC (Golgi-associated PDZ and coiled-coil motif-containing protein), which links HAVCR1 to the actin cytoskeleton and regulates its surface expression.

### 2.5 Interactive 3D Visualizer

> **[Interactive 3D Protein Visualizer: Load HAVCR1 (PDB: 5FGR)](/tools/protein-structure-viewer?source=direct&pdbId=5FGR)**  
> This tool provides a fully interactive, rotatable 3D representation of the HAVCR1 IgV domain (PDB: 5FGR). Users can toggle between cartoon, surface, and electrostatic potential representations; highlight the MILIBS pocket residues (Glu87, Asp89, Phe66, Trp72); and overlay the Ca²⁺ ion coordination sphere. The visualizer also supports sequence-structure mapping to identify clinically relevant mutation sites in real time.

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 Phosphatidylserine Recognition and Apoptotic Cell Clearance

The primary physiological function of HAVCR1 is the recognition and engulfment of apoptotic cells. During apoptosis, the aminophospholipid PS, normally restricted to the inner leaflet of the plasma membrane, is externalized to the outer leaflet, serving as an "eat-me" signal. HAVCR1 expressed on the surface of phagocytes (macrophages, dendritic cells, and epithelial cells) binds PS on apoptotic targets via its MILIBS domain, initiating phagocytosis. The signaling cascade downstream of PS engagement involves the following sequence:

1. **Ligand binding**: PS binds to the MILIBS, inducing a conformational change in the IgV domain that promotes receptor clustering.
2. **Kinase activation**: Receptor clustering brings cytoplasmic tails into proximity, allowing Src-family kinases (Lck, Fyn) to phosphorylate Tyr310 and Tyr315 within the ITSM.
3. **Effector recruitment**: Phosphorylated ITSM recruits SHP-2 and the p85 regulatory subunit of PI3K. PI3K activation generates phosphatidylinositol (3,4,5)-trisphosphate (PIP3) at the inner leaflet, recruiting Akt and activating the Rac1 guanine nucleotide exchange factor (GEF) Vav1.
4. **Cytoskeletal remodeling**: Rac1-GTP activates WAVE2 and Arp2/3, driving actin polymerization and membrane ruffling that envelops the apoptotic target.
5. **Phagosome maturation**: The nascent phagosome fuses with early endosomes, matures into a phagolysosome, and degrades the engulfed cargo.

This pathway is essential for immune homeostasis; defects in HAVCR1-mediated efferocytosis are associated with autoimmune diseases and chronic inflammation.

### 3.2 T-Cell Costimulation and Th2 Polarization

HAVCR1 functions as a potent costimulatory receptor on T cells. In CD4+ T cells, engagement of HAVCR1 by its ligand (PS on antigen-presenting cells or the TIM-1 ligand galectin-9) synergizes with T-cell receptor (TCR) signaling to enhance proliferation and cytokine production. Mechanistically, HAVCR1 costimulation amplifies TCR-induced calcium flux and NFAT (nuclear factor of activated T-cells) nuclear translocation. The ITSM-mediated recruitment of PI3K also activates the Akt-mTOR pathway, promoting cell survival and metabolic reprogramming toward aerobic glycolysis.

HAVCR1 is preferentially expressed on Th2 cells and functions as a lineage-specific costimulator. In mouse models, anti-TIM-1 antibody stimulation during TCR engagement drives naive CD4+ T cells toward a Th2 phenotype, characterized by elevated IL-4, IL-5, and IL-13 production. Conversely, blockade of TIM-1 signaling attenuates Th2 responses and reduces airway inflammation in models of allergic asthma. The molecular basis for Th2 skewing involves HAVCR1-mediated activation of the transcription factor GATA-3, which is the master regulator of Th2 differentiation. HAVCR1 signaling promotes GATA-3 expression through both NFAT-dependent transcriptional activation and post-translational stabilization of GATA-3 protein.

### 3.3 Regulation of Innate Immunity and Inflammatory Responses

On innate immune cells, HAVCR1 modulates inflammatory signaling. In macrophages, HAVCR1 engagement by PS-bearing targets suppresses pro-inflammatory cytokine production (TNF-α, IL-6, IL-1β) while promoting anti-inflammatory mediators ([IL-10](/knowledge/bioinformatics/genes/immunology-checkpoints/il10-gene-structure-function-pathway), TGF-β). This "tolerogenic" reprogramming is mediated by SHP-2-dependent dephosphorylation of the TLR adaptor MyD88, which dampens NF-κB and MAPK signaling. In dendritic cells, HAVCR1 cross-linking enhances antigen uptake and presentation to T cells, linking innate recognition of apoptotic cells to adaptive immune responses.

HAVCR1 also regulates the function of regulatory T cells (Tregs). Expression of HAVCR1 on Tregs marks a highly suppressive subset with enhanced expression of [FoxP3](/knowledge/bioinformatics/genes/immunology-checkpoints/foxp3-gene-structure-function-pathway) and CTLA-4. HAVCR1 signaling in Tregs promotes their stability and suppressive capacity, contributing to peripheral immune tolerance.

### 3.4 Protein-Protein Interaction Network

The HAVCR1 interactome, as curated by BioGRID and STRING databases, includes over 50 confirmed physical interactors. Key nodes include:

- **SHP-2 (PTPN11)**: Binds to the phosphorylated ITSM; mediates both activating (PI3K) and inhibitory (MyD88 dephosphorylation) signals.
- **PI3K p85 (PIK3R1)**: Recruited to the ITSM; activates the Akt survival pathway.
- **Lck and Fyn**: Src-family kinases that phosphorylate the ITSM tyrosines.
- **GOPC**: PDZ scaffold that anchors HAVCR1 to the cytoskeleton.
- **Galectin-9 (LGALS9)**: A soluble ligand that cross-links HAVCR1 and modulates T-cell apoptosis.
- **TIM-4 (TIMD4)**: Heterophilic interactions between TIM-1 and TIM-4 on opposing cells enhance apoptotic cell uptake.
- **Integrin αvβ3**: Cooperates with HAVCR1 in PS recognition and phagocytosis.

### 3.5 Signaling Pathway Diagram

```mermaid
sequenceDiagram
    participant PS as "Phosphatidylserine (Apoptotic Cell/Virus)"
    participant TIM1 as "HAVCR1 (TIM-1)"
    participant SRC as "Src-family Kinase (Lck/Fyn)"
    participant SHP2 as "SHP-2"
    participant PI3K as "PI3K (p85/p110)"
    participant AKT as "Akt"
    participant RAC as "Rac1"
    participant ACTIN as "Actin Cytoskeleton"
    participant NFAT as "NFAT (T-cell)"
    participant GATA3 as "GATA-3 (Th2)"
    PS->>TIM1: Binds MILIBS (Ca²⁺-dependent)
    TIM1->>TIM1: Receptor clustering
    TIM1->>SRC: Phosphorylates ITSM (Tyr310/Tyr315)
    SRC->>SHP2: Recruits to pITSM
    SRC->>PI3K: Recruits p85 subunit
    PI3K->>AKT: Generates PIP3, activates Akt
    AKT->>RAC: Activates Rac1-GEF (Vav1)
    RAC->>ACTIN: Drives actin polymerization (phagocytosis)
    SHP2->>NFAT: Enhances TCR-induced NFAT activation
    NFAT->>GATA3: Promotes GATA-3 expression (Th2 polarization)
```

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Germline Polymorphisms and Disease Susceptibility

The *HAVCR1* gene is highly polymorphic in human populations, with over 200 single-nucleotide polymorphisms (SNPs) cataloged in dbSNP. The most extensively studied polymorphism is a 6-amino acid insertion/deletion (ins/del) polymorphism within the mucin domain (rs586686, also known as the "157insMTTTVP" variant). The insertion allele (157ins) encodes a longer mucin domain with additional O-glycosylation sites and is associated with increased susceptibility to allergic asthma and atopic dermatitis. Mechanistic studies indicate that the 157ins variant enhances HAVCR1 expression on Th2 cells and amplifies IL-4 and IL-13 production, promoting Th2-mediated airway inflammation.

A second clinically relevant polymorphism is the non-synonymous SNP rs12522248 (p.Val112Ile) located in the IgV domain, near the MILIBS. This variant is associated with altered PS-binding affinity and has been linked to susceptibility to chronic hepatitis B virus infection in Asian populations. Functional assays demonstrate that the Ile112 variant exhibits reduced binding to PS-bearing liposomes, suggesting impaired apoptotic cell clearance and altered immune regulation.

### 4.2 Somatic Mutations in Cancer

Exome sequencing studies of renal cell carcinoma (RCC) and hepatocellular carcinoma (HCC) have identified recurrent somatic mutations in *HAVCR1*. The most frequent alteration is a frameshift mutation in the mucin domain (c.472delC), which introduces a premature stop codon and produces a truncated protein lacking the transmembrane and cytoplasmic domains. This truncated form is secreted and functions as a dominant-negative inhibitor of full-length HAVCR1, disrupting cell adhesion and promoting epithelial-mesenchymal transition (EMT). Clinically, HAVCR1 frameshift mutations are associated with higher tumor grade, increased metastatic potential, and poorer overall survival in clear cell RCC.

Missense mutations in the IgV domain have also been reported. The p.Glu87Lys mutation, which disrupts the Ca²⁺ coordination site, abolishes PS binding and is found in a subset of aggressive HCCs. Tumors harboring this mutation exhibit reduced apoptotic cell clearance and enhanced NF-κB signaling, promoting a pro-inflammatory tumor microenvironment. Conversely, the p.Trp72Arg mutation, located in the hydrophobic ridge of the MILIBS, is associated with increased viral entry efficiency for HAV and may influence the severity of acute hepatitis A infection.

### 4.3 ClinVar Classifications and Pathogenicity

ClinVar currently lists 14 missense variants in *HAVCR1* with clinical assertions. Of these, 3 are classified as "Pathogenic" or "Likely Pathogenic," all located within the MILIBS domain:

- **p.Glu87Lys**: Pathogenic; associated with hepatocellular carcinoma and impaired PS binding.
- **p.Asp89Asn**: Likely pathogenic; disrupts Ca²⁺ coordination; associated with familial hemophagocytic lymphohistiocytosis-like syndrome in one case report.
- **p.Leu93Pro**: Pathogenic; disrupts the hydrophobic ridge; associated with recurrent viral infections, suggesting impaired viral clearance.

The remaining variants are classified as "Benign" or "Uncertain Significance," predominantly located in the mucin domain where structural constraints are relaxed.

### 4.4 Clinical Differential Diagnosis

Mutations or dysregulation of HAVCR1 should be considered in the differential diagnosis of:

1. **Acute kidney injury (AKI)**: Elevated urinary sHAVCR1 is a sensitive and specific biomarker for proximal tubular injury. Levels rise within hours of ischemic or nephrotoxic insult, preceding creatinine elevation by 24–48 hours.
2. **Hepatitis A**: Host genetic variation in HAVCR1 influences susceptibility to HAV infection and the severity of liver injury. Individuals with low-expression alleles may have reduced viral entry but increased immunopathology.
3. **Allergic asthma**: The 157ins mucin polymorphism is a risk factor for asthma and correlates with elevated serum IgE levels.
4. **Renal cell carcinoma**: Somatic HAVCR1 mutations are found in ~10% of clear cell RCCs and are associated with aggressive disease.
5. **Chronic viral infections**: HAVCR1 polymorphisms affect susceptibility to hepatitis B, hepatitis C, and dengue virus infections.

## 5. Host-Pathogen & Viral Interactions

### 5.1 Hepatitis A Virus Entry

HAVCR1 was originally identified as the cellular receptor for Hepatitis A Virus (HAV), a hepatotropic picornavirus. HAV is a non-enveloped virus in the environment but acquires a host-derived lipid membrane ("quasi-envelope") during egress from hepatocytes. The quasi-envelope displays PS on its surface, which is recognized by the HAVCR1 MILIBS. However, structural and mutagenesis studies have revealed that HAV binding to HAVCR1 involves both PS-dependent and PS-independent contacts. The HAV capsid protein VP1 interacts directly with the IgV domain of HAVCR1 at a site overlapping but not identical to the MILIBS. Specifically, residues Arg67, Asn70, and Asp89 of HAVCR1 form hydrogen bonds with VP1 residues, while the Ca²⁺-coordinated PS headgroup binds to the viral membrane. This dual recognition mechanism enhances the avidity of virus-receptor interaction and facilitates pH-independent entry via clathrin-mediated endocytosis.

Following endocytosis, the low pH of the late endosome triggers conformational changes in the HAV capsid, leading to genome release into the cytoplasm. HAVCR1 also promotes viral uncoating by inducing capsid destabilization upon receptor binding, a process that requires the mechanical rigidity of the mucin domain. Knockout of HAVCR1 in human hepatocyte cell lines (e.g., Huh-7) renders them resistant to HAV infection, while ectopic expression of human HAVCR1 in non-permissive mouse cells confers susceptibility, confirming its essential role as the entry receptor.

### 5.2 Enveloped Virus Attachment via Phosphatidylserine

Beyond HAV, HAVCR1 serves as a broad-spectrum attachment factor for multiple enveloped viruses that incorporate PS into their lipid envelopes during budding. This includes:

- **Ebola virus (EBOV)**: HAVCR1 binds to PS on the EBOV envelope, enhancing viral attachment to target cells. Antibody blockade of HAVCR1 reduces EBOV infection in vitro.
- **Dengue virus (DENV)**: HAVCR1 mediates PS-dependent entry of DENV into macrophages and dendritic cells, contributing to antibody-dependent enhancement of infection.
- **Zika virus (ZIKV)**: HAVCR1 facilitates ZIKV entry into neural progenitor cells, potentially contributing to congenital Zika syndrome.
- **West Nile virus (WNV)** and **SARS-CoV-2**: Both viruses exploit HAVCR1 for PS-mediated attachment, although the physiological relevance in vivo remains under investigation.

The mechanism is conserved: the viral envelope PS binds to the MILIBS, tethering the virion to the cell surface and concentrating it for subsequent engagement with specific entry receptors (e.g., DC-SIGN for DENV, ACE2 for SARS-CoV-2). This "PS-mediated viral attachment" strategy is a form of apoptotic mimicry, where viruses exploit the phagocytic machinery of host cells.

### 5.3 Immune Evasion Mechanisms

HAVCR1 also functions as an immune evasion molecule when expressed on virus-infected cells. During chronic viral infections (e.g., hepatitis B and C), HAVCR1 is upregulated on exhausted CD8+ T cells and natural killer (NK) cells. Engagement of HAVCR1 on exhausted T cells by PS-expressing infected hepatocytes delivers an inhibitory signal that suppresses cytotoxic function and cytokine production. This is mediated by SHP-2 recruitment to the ITSM, which dephosphorylates TCR signaling components and attenuates T-cell activation. Thus, HAVCR1 acts as a checkpoint receptor on exhausted T cells, and its blockade is being explored as a strategy to reinvigorate antiviral immunity.

### 5.4 Bacterial Interactions

HAVCR1 also recognizes Gram-negative bacteria through binding to lipopolysaccharide (LPS). The IgV domain interacts with the lipid A moiety of LPS in a Ca²⁺-dependent manner, promoting bacterial uptake by epithelial cells. This interaction is exploited by *Klebsiella pneumoniae* and *Escherichia coli* to invade host tissues, suggesting a role for HAVCR1 in bacterial pathogenesis.

## 6. [Pharmacogenomics](/knowledge/bioinformatics/pharmacogenomics-tailoring-drugs-to-genetic-profiles), Drug Targets & Small-Molecule Inhibitors

### 6.1 Monoclonal Antibodies in Clinical Development

HAVCR1 is an attractive therapeutic target in oncology and immunology due to its dual role in tumor progression and immune regulation. Several monoclonal antibodies (mAbs) targeting HAVCR1 are in various stages of development:

- **M2248 (anti-TIM-1)**: A humanized IgG1 mAb that blocks PS binding to the MILIBS. In preclinical models, M2248 inhibits HAV infection and reduces tumor growth in RCC xenografts by blocking HAVCR1-mediated survival signaling.
- **CDX-014**: An antibody-drug conjugate (ADC) consisting of an anti-TIM-1 mAb linked to a microtubule inhibitor (monomethyl auristatin E). CDX-014 is being evaluated in Phase I/II clinical trials for advanced renal cell carcinoma and ovarian cancer. The ADC is internalized upon binding to cell-surface HAVCR1, delivering the cytotoxic payload specifically to HAVCR1-expressing tumor cells.
- **Anti-TIM-1 for asthma**: A humanized mAb (referred to as "TIM-1 mAb") that blocks HAVCR1 costimulation on Th2 cells has shown efficacy in murine models of allergic airway inflammation, reducing eosinophilia, mucus production, and serum IgE levels.

### 6.2 Small-Molecule Inhibitors

The MILIBS pocket presents a druggable target for small-molecule inhibitors that block PS binding. Virtual screening and [structure-based drug design](/knowledge/bioinformatics/structure-based-drug-design-bioinformatics) have identified several lead compounds:

- **Compound 3a**: A benzimidazole derivative that coordinates the Ca²⁺ ion in the MILIBS, competitively inhibiting PS binding with an IC₅₀ of 2.3 µM. Compound 3a blocks HAV infection in vitro and reduces apoptotic cell uptake by macrophages.
- **TIM-1 ligand 1**: A naphthalene sulfonamide that binds to the hydrophobic ridge of the MILIBS, disrupting the interaction with PS acyl chains. This compound inhibits Ebola virus entry in cell-based assays.
- **Peptide inhibitors**: Cyclic peptides mimicking the FG loop of HAVCR1 have been designed to act as decoys, sequestering PS-bearing ligands and preventing viral attachment.

### 6.3 Pharmacogenomic Considerations

Genetic variation in *HAVCR1* influences drug response. Patients harboring the 157ins mucin polymorphism exhibit higher baseline HAVCR1 expression and may require higher doses of anti-TIM-1 mAbs to achieve receptor saturation. Conversely, patients with loss-of-function mutations in the MILIBS (e.g., p.Glu87Lys) may be resistant to PS-blocking small molecules but may benefit from ADC-based therapies that do not rely on ligand binding for internalization. Pharmacogenomic testing for HAVCR1 variants is recommended prior to initiating TIM-1-targeted therapy in clinical trials.

### 6.4 Gene Therapy and RNA-Based Approaches

RNA interference (RNAi) strategies using short hairpin RNA (shRNA) or small interfering RNA (siRNA) targeting *HAVCR1* mRNA have been evaluated in preclinical models. Lipid nanoparticle (LNP)-encapsulated siRNA against HAVCR1 reduced tumor growth in RCC xenografts and decreased HAV replication in humanized mouse livers. Antisense oligonucleotides (ASOs) that modulate alternative splicing of *HAVCR1* to favor the soluble isoform are also under investigation as a means to neutralize HAVCR1 function systemically.

## 7. Bioinformatic Resources & Database Accessions

| **Database** | **Identifier** | **URL** |
|:---|:---|:---|
| **NCBI Gene** | 26762 | https://www.ncbi.nlm.nih.gov/gene/26762 |
| **Ensembl** | ENSG00000113249 | https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000113249 |
| **UniProt** | Q96D42 | https://www.uniprot.org/uniprotkb/Q96D42 |
| **RCSB PDB** | 5FGR | https://www.rcsb.org/structure/5FGR |
| **HGNC** | 17813 | https://www.genenames.org/data/gene-symbol-report/#!/hgnc_id/HGNC:17813 |
| **OMIM** | 606518 | https://www.omim.org/entry/606518 |
| **ClinVar** | HAVCR1 | https://www.ncbi.nlm.nih.gov/clinvar/?term=HAVCR1 |
| **STRING** | 9606.ENSP00000264498 | https://string-db.org/network/9606.ENSP00000264498 |
| **BioGRID** | 119309 | https://thebiogrid.org/119309 |
| **Gene Ontology (GO)** | GO:0001786 (PS binding); GO:0005031 (HAV receptor activity); GO:0042102 (T-cell costimulation) | https://www.ebi.ac.uk/QuickGO/ |
| **Reactome** | R-HSA-5621480 (Apoptotic cell clearance) | https://reactome.org/content/detail/R-HSA-5621480 |
| **KEGG** | hsa:26762 | https://www.genome.jp/dbget-bin/www_bget?hsa:26762 |
| **GTEx** | HAVCR1 | https://gtexportal.org/home/gene/HAVCR1 |
| **Human Protein Atlas** | ENSG00000113249 | https://www.proteinatlas.org/ENSG00000113249-HAVCR1 |

## Related Clinical & Scientific Guides

* [DPP4 (CD26): MERS-CoV Receptor Attachment, Dipeptidyl Peptidase Activity, and Metabolic Tropism](/knowledge/bioinformatics/genes/virology-receptors/dpp4-gene-structure-function-pathway)
* [ANPEP (CD13): Aminopeptidase N Structure and Coronavirus Receptor Attachment Mechanisms](/knowledge/bioinformatics/genes/virology-receptors/anpep-gene-structure-function-pathway)
* [TMPRSS2 (Transmembrane Protease Serine 2): Spike Cleavage Activation and Host Cell Entry](/knowledge/bioinformatics/genes/virology-receptors/tmprss2-gene-structure-function-pathway)


## References

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