# TNFSF13 Gene: Structure, Function, and Clinical Significance


## Key Takeaways

- The *TNFSF13* gene encodes APRIL, a secreted cytokine crucial for plasma cell survival, differentiation, and immunoglobulin class switching, particularly towards IgA and IgM. Its primary receptors are BCMA (TNFRSF17) and TACI (TNFRSF13B), mediating downstream signaling via TRAF adaptors and NF-κB pathways.
- Genetic variants in *TNFSF13*, notably rs11552708 (G67R), are strongly associated with increased susceptibility to IgA Nephropathy (IgAN) and Systemic Lupus Erythematosus (SLE), linked to elevated serum APRIL levels and aberrant IgA1 glycosylation.
- A homozygous inactivating mutation in *TNFSF13* has been identified as a monogenic cause of primary antibody deficiency, presenting with hypogammaglobulinemia and a severe lack of plasma cells, underscoring APRIL's non-redundant role in human humoral immunity.
- APRIL is a critical survival factor for malignant B cells in Chronic Lymphocytic Leukemia (CLL) and Multiple Myeloma, and its overexpression in solid tumors can promote proliferation and chemo-resistance, making it a target for therapeutic intervention.
- Therapeutic strategies to neutralize APRIL include monoclonal antibodies like atacicept (TACI-Ig) and specific anti-APRIL antibodies (e.g., BION-1301), which aim to reduce autoantibody production and disease progression in autoimmune conditions and B-cell malignancies.

---

## Executive Summary & Key Metadata

The *TNFSF13* gene (Tumor Necrosis Factor Superfamily Member 13), universally recognized by its protein product APRIL (A PRoliferation-Inducing Ligand), is a non-canonical member of the TNF superfamily with a unique biology that straddles humoral immunity, mucosal immunology, oncogenesis, and metabolic regulation. Unlike many TNF ligands that are primarily membrane-bound, APRIL is a secreted protein that operates as a soluble homotrimer, engaging a distinct set of TNF receptor superfamily (TNFRSF) members. Its principal physiological role is the survival and differentiation of plasma cells, the long-lived antibody factories of the immune system, and the regulation of immunoglobulin class switching, particularly towards IgA and IgM [<a href="#ref-1">1</a>][<a href="#ref-2">2</a>].

The clinical relevance of *TNFSF13* is vast and expanding. It is a confirmed susceptibility locus for Immunoglobulin A Nephropathy (IgAN), the most common primary glomerulonephritis worldwide, with multiple genome-wide association studies (GWAS) and functional studies implicating APRIL in the aberrant glycosylation of IgA1 [<a href="#ref-3">3</a>][<a href="#ref-4">4</a>][<a href="#ref-5">5</a>][<a href="#ref-6">6</a>][<a href="#ref-7">7</a>]. Beyond nephrology, *TNFSF13* is implicated in systemic lupus erythematosus (SLE), common variable immunodeficiency (CVID), chronic lymphocytic leukemia (CLL), multiple myeloma, and various solid tumors [<a href="#ref-8">8</a>][<a href="#ref-9">9</a>][<a href="#ref-10">10</a>][<a href="#ref-11">11</a>]. The recent characterization of the first immunodeficient patient with an inactivating *TNFSF13* mutation has cemented its non-redundant role in human humoral immunity [<a href="#ref-1">1</a>][<a href="#ref-2">2</a>]. This manual provides a comprehensive, biophysically detailed reference on the gene, its architecture, signaling, pathogenic variants, and therapeutic targeting.

| **Feature** | **Specification** |
| :--- | :--- |
| **HGNC Symbol** | TNFSF13 |
| **UniProt Accession** | O75888 |
| **Representative PDB ID** | true (e.g., 1XU1, 4Z7H; see Section 2) |
| **Chromosomal Locus** | 17p13.1 (GRCh38: chr17:7,455,840-7,458,837) |
| **Primary Molecular Function** | Cytokine activity; ligand for TNFRSF17 (BCMA), TNFRSF13B (TACI), and TNFRSF13C (BAFF-R); regulation of B-cell survival, plasma cell differentiation, and immunoglobulin class switching. |
| **Disease & Pathology Associations** | IgA Nephropathy (IgAN), Systemic Lupus Erythematosus (SLE), Common Variable Immunodeficiency (CVID), Multiple Myeloma, Chronic Lymphocytic Leukemia (CLL), various solid tumors, and potential roles in metabolic and neuropsychiatric disorders. |

## 1. Genomic Locus, Chromosomal Organization & Isoforms

### 1.1 Chromosomal Localization and Gene Structure

The *TNFSF13* gene is located on the short arm of chromosome 17 at cytogenetic band 17p13.1. In the GRCh38 reference genome assembly, the gene spans approximately 3 kilobases (kb) from 7,455,840 to 7,458,837 base pairs. The gene is relatively compact, a common feature of TNF superfamily ligands, and is oriented on the minus strand. The mature mRNA transcript is approximately 1.8 kb in length.

The genomic structure consists of **five exons** and **four introns**. The coding sequence (CDS) is distributed across these exons, with the final exon encoding the C-terminal portion of the protein that contains the conserved TNF homology domain (THD). The promoter region of *TNFSF13* lacks a canonical TATA box but contains a high GC content, a feature typical of constitutively expressed or highly regulated genes. Several putative transcription factor binding sites have been identified *in silico* within the 5' upstream region, including binding motifs for NF-κB, AP-1, and STAT family members, which are consistent with its role as an inducible cytokine in response to inflammatory and immune stimuli [<a href="#ref-12">12</a>][<a href="#ref-13">13</a>].

### 1.2 Promoter Architecture and Transcriptional Regulation

The regulation of *TNFSF13* transcription is cell-type specific and stimulus-dependent. In myeloid cells, which are the primary source of APRIL, transcription is induced by interferons (both type I and type II), Toll-like receptor (TLR) agonists, and cytokines such as TNF-α. The promoter region contains functional interferon-stimulated response elements (ISREs) and gamma-activated sequences (GAS), which are bound by IRF and STAT transcription factors, respectively. This regulatory architecture explains the rapid upregulation of APRIL during bacterial and viral infections, as well as in chronic inflammatory conditions [<a href="#ref-12">12</a>][<a href="#ref-13">13</a>].

In contrast, certain non-hematopoietic cells, such as tumor cells of epithelial origin, can also express *TNFSF13*. In these contexts, promoter methylation status plays a critical role. For instance, in cervical carcinogenesis, hypermethylation of the *TNFSF13* promoter region has been observed, potentially silencing its expression and contributing to immune evasion [<a href="#ref-14">14</a>][<a href="#ref-15">15</a>]. Conversely, in other cancers, hypomethylation or activation of upstream signaling pathways (e.g., NF-κB) can lead to aberrant overexpression.

### 1.3 Alternative Splicing and Isoforms

Alternative splicing of the *TNFSF13* pre-mRNA generates multiple transcript variants, although the functional significance of many of these remains under investigation. The primary transcript encodes the full-length, 250-amino acid (aa) type II transmembrane protein, which is subsequently cleaved to produce the soluble, biologically active ligand.

| **Isoform** | **Transcript Length (approx.)** | **Protein Length (aa)** | **Functional Consequence** |
| :--- | :--- | :--- | :--- |
| **Canonical (APRIL-α)** | 1.8 kb | 250 | Full-length precursor; cleaved by furin to generate soluble APRIL (sAPRIL). |
| **APRIL-β** | ~1.5 kb | ~200 | Lacks a portion of the N-terminal intracellular domain; may have altered intracellular signaling or trafficking. |
| **Short/Truncated variants** | Variable | <150 | Often retain the TNF homology domain but lack the N-terminal region; may act as dominant-negative inhibitors or have altered receptor specificity. |

The most well-characterized isoform is the full-length APRIL-α. The N-terminal 48 amino acids constitute the cytoplasmic domain, followed by a 21-amino acid transmembrane domain (residues 49-69). The extracellular region (residues 70-250) contains the furin cleavage site (RKRK) at residues 71-74. Cleavage at this site releases the soluble, 18 kDa sAPRIL, which is the predominant functional form [<a href="#ref-12">12</a>][<a href="#ref-13">13</a>]. A longer, membrane-bound form can also be generated through alternative splicing that retains a portion of the transmembrane domain, allowing for juxtacrine signaling, though this is less common than the soluble form.

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

### 2.1 Primary Sequence and Domain Boundaries

The APRIL protein (UniProt O75888) is synthesized as a 250-amino acid type II transmembrane protein. The domain architecture is as follows:

- **Cytoplasmic Domain (aa 1-48):** This N-terminal region is short and lacks any known signaling motifs. It is not required for the biological activity of the soluble ligand but may play a role in intracellular trafficking or membrane localization of the precursor protein.
- **Transmembrane Domain (aa 49-69):** A single-pass hydrophobic alpha-helix that anchors the precursor protein to the cell membrane.
- **Extracellular Stalk Region (aa 70-110):** This region contains the furin cleavage site (RKRK at aa 71-74). Cleavage at this site is essential for the generation of soluble APRIL. This region is also flexible and poorly ordered in structural studies.
- **TNF Homology Domain (THD) (aa 111-250):** This is the functional core of the protein. It adopts the canonical "jelly-roll" beta-sandwich fold characteristic of all TNF superfamily ligands. This domain is responsible for trimerization and receptor binding.

### 2.2 The TNF Homology Domain (THD) and Quaternary Structure

The THD of APRIL is the defining structural feature. It consists of two anti-parallel β-sheets, each composed of multiple β-strands, forming a classic β-sandwich. The overall fold is highly conserved across the TNF superfamily, but the surface residues that mediate receptor binding are unique to APRIL.

The biologically active form of APRIL is a **non-covalent homotrimer**. Three THD monomers associate tightly, forming a bell-shaped or pyramid-like structure. The trimerization interface is extensive and hydrophobic, burying a large solvent-accessible surface area. The trimer presents three equivalent receptor-binding sites, located in the grooves between adjacent subunits at the "bottom" of the bell-shaped structure. This trimeric arrangement allows for high-avidity binding to its trimeric receptors.

A unique structural feature of APRIL is its ability to bind to the polysaccharide **heparan sulfate proteoglycans (HSPGs)**. This interaction is mediated by a cluster of positively charged amino acid residues (lysine and arginine) located on the surface of the trimer, distinct from the receptor-binding site. HSPG binding is critical for the local retention of APRIL in tissues and for its presentation to receptors on neighboring cells, effectively concentrating the ligand at the cell surface and enhancing signaling [<a href="#ref-1">1</a>].

### 2.3 Receptor Binding and Specificity

APRIL binds to three receptors within the TNFRSF:
1.  **TNFRSF17 (BCMA - B-Cell Maturation Antigen):** This is the primary high-affinity receptor for APRIL. BCMA is expressed predominantly on plasma cells and plasmablasts.
2.  **TNFRSF13B (TACI - Transmembrane Activator and CAML Interactor):** APRIL binds to TACI with lower affinity than BCMA. TACI is expressed on a broader range of B cells, including marginal zone and switched memory B cells.
3.  **TNFRSF13C (BAFF-R - BAFF Receptor):** APRIL binds to BAFF-R with very low affinity, and this interaction is considered physiologically insignificant. BAFF-R is the primary receptor for the closely related ligand BAFF (TNFSF13B).

The structural basis for this differential receptor usage lies in the specific amino acid residues on the APRIL surface. The "D-E loop" and "A-A' loop" regions of the THD are critical for BCMA and TACI binding. Mutations in these loops can selectively ablate binding to one receptor while preserving binding to the other, a strategy being explored for the development of specific therapeutic inhibitors.

### 2.4 Interactive 3D Visualizer

To explore the three-dimensional structure of the APRIL trimer and its interaction with receptors, use the interactive visualizer below. The structure is loaded from the RCSB PDB and allows for rotation, zooming, and toggling of different molecular representations (e.g., cartoon, surface, electrostatic potential).

[Interactive 3D Protein Visualizer: Load TNFSF13 (PDB: true)](/tools/protein-structure-viewer?source=alphafold&accession=O75888)

## 3. Cellular Signaling Pathways & Molecular Function

### 3.1 The APRIL Signaling Axis

APRIL is a master regulator of humoral immunity, exerting its effects primarily on the B-cell lineage. Its signaling is initiated upon binding to its receptors, BCMA and TACI, which are type I transmembrane proteins. Unlike many TNF receptors that contain cytoplasmic death domains, BCMA and TACI lack this motif and instead signal through TNF receptor-associated factor (TRAF) adaptor proteins.

```mermaid
sequenceDiagram
    participant M as "Myeloid Cell (Macrophage/DC)"
    participant A as "APRIL (soluble trimer)"
    participant HSPG as "Heparan Sulfate Proteoglycans"
    participant R as "BCMA/TACI on B Cell/Plasma Cell"
    participant TRAF as "TRAF2/5/6"
    participant NFkB as "NF-κB Pathway"
    participant NIK as "Non-canonical NF-κB (NIK)"
    participant Bcl as "Anti-apoptotic Genes (Bcl-2, Mcl-1)"
    participant PC as "Plasma Cell Survival/Differentiation"
    M->>A: Furin cleavage & secretion
    A->>HSPG: Binding & local concentration
    A->>R: Receptor trimerization
    R->>TRAF: Recruitment of TRAF adaptors
    TRAF->>NFkB: Activation of canonical NF-κB (p50/RelA)
    TRAF->>NIK: Stabilization & activation of NIK
    NIK->>NFkB: Processing of p100 to p52 (non-canonical)
    NFkB->>Bcl: Transcription of pro-survival genes
    Bcl->>PC: Enhanced survival, differentiation, and Ig secretion
```

### 3.2 Downstream Signaling Cascades

1.  **Canonical NF-κB Pathway:** Upon ligand binding, BCMA and TACI recruit TRAF2, TRAF5, and TRAF6 to their cytoplasmic tails. This leads to the activation of the IKK (IκB kinase) complex, which phosphorylates IκBα, targeting it for ubiquitin-mediated degradation. The liberated NF-κB dimers (primarily p50/RelA) translocate to the nucleus and drive the expression of pro-survival and proliferative genes [<a href="#ref-1">1</a>][<a href="#ref-2">2</a>].
2.  **Non-canonical NF-κB Pathway:** This pathway is particularly critical for the survival of long-lived plasma cells. TRAF recruitment leads to the stabilization of NF-κB-inducing kinase (NIK). NIK activates IKKα, which phosphorylates the p100 precursor protein, leading to its processing into the mature p52 subunit. The p52/RelB complex then translocates to the nucleus, driving a distinct set of genes essential for plasma cell longevity, including *MCL1* and *BCL2* [<a href="#ref-1">1</a>][<a href="#ref-2">2</a>].
3.  **Other Signaling Nodes:** Beyond NF-κB, APRIL signaling can activate the PI3K/Akt pathway and the MAPK/ERK pathway, contributing to cell survival and proliferation. These pathways are particularly important in malignant B cells, where APRIL can act as a growth and survival factor [<a href="#ref-10">10</a>][<a href="#ref-3">3</a>].

### 3.3 Role in B-Cell Biology and Immunoglobulin Production

APRIL's non-redundant role is in the maintenance of the plasma cell compartment. It is a key survival factor for long-lived plasma cells in the bone marrow, which are responsible for sustained antibody production. APRIL also promotes the differentiation of activated B cells into plasmablasts and their subsequent maturation into plasma cells [<a href="#ref-1">1</a>][<a href="#ref-2">2</a>].

A critical function of APRIL, mediated through TACI, is the induction of **immunoglobulin class switch recombination (CSR)** to IgA and IgG. This is particularly important in the gut-associated lymphoid tissue (GALT), where APRIL produced by dendritic cells and macrophages drives the production of secretory IgA, the first line of humoral defense at mucosal surfaces [<a href="#ref-4">4</a>][<a href="#ref-5">5</a>]. The critical importance of APRIL in humans was definitively demonstrated by the discovery of a patient with a homozygous inactivating *TNFSF13* mutation, who presented with profound hypogammaglobulinemia and an almost complete absence of plasma cells, confirming APRIL's essential role in maintaining humoral immunity [<a href="#ref-1">1</a>][<a href="#ref-2">2</a>].

### 3.4 Protein-Protein Interaction Networks

The APRIL interactome is centered on its receptors and the downstream signaling machinery. Key interactions include:
- **Ligand-Receptor:** APRIL (TNFSF13) with TNFRSF17 (BCMA) and TNFRSF13B (TACI). These are the primary functional interactions.
- **Ligand-Proteoglycan:** APRIL with heparan sulfate glycosaminoglycan chains on HSPGs (e.g., Syndecan-1/CD138). This interaction is essential for its bioavailability and local function.
- **Receptor-Adaptor:** BCMA and TACI with TRAF2, TRAF5, and TRAF6. These adaptors link receptor engagement to the NF-κB pathways.
- **Co-stimulatory molecules:** APRIL has been shown to interact with the complement system, particularly with C1q, which may modulate its activity.

## 4. Pathogenic Hotspot Mutations & Clinical Differentials

### 4.1 Genetic Variants and Disease Susceptibility

*TNFSF13* is a highly polymorphic gene, and numerous single nucleotide polymorphisms (SNPs) have been associated with disease susceptibility. The most extensively studied is the non-synonymous variant **rs11552708 (G67R)**. This SNP results in a glycine-to-arginine substitution at position 67 of the mature protein, located in the extracellular stalk region near the furin cleavage site.

| **Variant (rsID)** | **Nucleotide Change** | **Amino Acid Change** | **Disease Association** | **Functional Consequence** |
| :--- | :--- | :--- | :--- | :--- |
| **rs11552708** | G > A | Gly67Arg (G67R) | IgAN, SLE, CLL | Alters processing/secretion; associated with increased serum APRIL levels. |
| **rs3803800** | A > G | Asn96Ser (N96S) | IgAN, SLE | Located in the THD; may affect receptor binding affinity or trimer stability. |
| **rs1224141** | T > G | Intronic | IgAN | May affect splicing efficiency or mRNA stability. |
| **rs3803801** | C > T | 3' UTR | SLE | May affect mRNA stability or miRNA binding. |

### 4.2 IgA Nephropathy (IgAN)

The strongest and most reproducible genetic association for *TNFSF13* is with IgAN. Multiple GWAS in Chinese, European, and Japanese populations have identified SNPs within the *TNFSF13* locus as significant risk factors [<a href="#ref-3">3</a>][<a href="#ref-4">4</a>][<a href="#ref-7">7</a>]. The risk allele is associated with elevated serum levels of APRIL. Functionally, increased APRIL drives the production of poorly galactosylated IgA1 (Gd-IgA1) from B cells, which is the central pathogenic event in IgAN. Gd-IgA1 forms immune complexes that deposit in the glomerular mesangium, leading to inflammation and kidney damage [<a href="#ref-6">6</a>][<a href="#ref-5">5</a>][<a href="#ref-6">6</a>]. The association is particularly strong in smokers, suggesting a gene-environment interaction [<a href="#ref-3">3</a>].

### 4.3 Systemic Lupus Erythematosus (SLE) and Other Autoimmune Diseases

*TNFSF13* polymorphisms have also been associated with susceptibility to SLE in Japanese and other populations [<a href="#ref-9">9</a>][<a href="#ref-7">7</a>]. Elevated APRIL levels are found in the serum of SLE patients and correlate with disease activity. APRIL contributes to the survival of autoreactive B cells, promoting the production of autoantibodies. The G67R variant has been shown to be a risk factor for SLE, likely by increasing APRIL's biological activity [<a href="#ref-8">8</a>].

### 4.4 Common Variable Immunodeficiency (CVID) and Primary Antibody Deficiencies

While mutations in the receptor *TNFRSF13B* (TACI) are a well-known cause of CVID, mutations in *TNFSF13* itself are rarer. However, the recent identification of a patient with a homozygous inactivating mutation in *TNFSF13* who presented with a CVID-like phenotype (hypogammaglobulinemia, recurrent infections, and lack of plasma cells) has established *TNFSF13* as a monogenic cause of primary antibody deficiency [<a href="#ref-1">1</a>][<a href="#ref-9">9</a>][<a href="#ref-2">2</a>]. This finding underscores the non-redundant role of APRIL in human humoral immunity.

### 4.5 Malignancies

APRIL is a well-established survival factor for malignant B cells.
- **Chronic Lymphocytic Leukemia (CLL):** Elevated APRIL levels in the plasma and lymph nodes of CLL patients promote the survival of leukemic cells. Polymorphisms in *TNFSF13* have been associated with CLL risk and clinical outcome [<a href="#ref-10">10</a>].
- **Multiple Myeloma (MM):** APRIL and BAFF are critical growth factors for myeloma cells, acting through BCMA. High expression of APRIL in the bone marrow microenvironment supports myeloma cell survival and drug resistance [<a href="#ref-10">10</a>].
- **Solid Tumors:** APRIL is overexpressed in several solid tumors, including colorectal, gastric, and breast cancers. It can promote tumor cell proliferation and survival in an autocrine or paracrine manner. In colorectal cancer, high APRIL expression is a marker of chemo-resistance [<a href="#ref-11">11</a>][<a href="#ref-11">11</a>][<a href="#ref-12">12</a>].

### 4.6 Other Clinical Associations

Emerging evidence links *TNFSF13* to a broader range of conditions:
- **Metabolic Disorders:** TNFSF13 levels are elevated in the plasma of obese patients with type 2 diabetes (T2D). It has been proposed that APRIL promotes mitochondrial fission in adipose tissue, contributing to metabolic dysregulation [<a href="#ref-13">13</a>][<a href="#ref-1">1</a>][<a href="#ref-2">2</a>].
- **Neuropsychiatric Disorders:** Gene expression analyses have implicated death receptor pathways, including TNFSF13, in schizophrenia and major depressive disorder [<a href="#ref-14">14</a>][<a href="#ref-15">15</a>][<a href="#ref-1">1</a>].
- **Transplantation:** Genetic variation in the BAFF/APRIL system, including *TNFSF13*, has been associated with graft survival and acute rejection in kidney transplantation [<a href="#ref-2">2</a>].

## 5. Host-Pathogen & Viral Interactions

### 5.1 APRIL in Viral Immunity and Evasion

APRIL plays a dual role in viral infections. On one hand, it is part of the host's humoral defense, promoting the production of neutralizing antibodies. On the other hand, some viruses may exploit the APRIL pathway to enhance their own survival or persistence.

- **Influenza Virus:** Host factors that modulate cell death pathways, including those involving TNF superfamily members, are critical for influenza virus replication. Knockdown screens have identified components of the NF-κB pathway, which is downstream of APRIL signaling, as important for efficient viral replication [<a href="#ref-3">3</a>].
- **Human Papillomavirus (HPV):** The E6 oncoprotein of high-risk HPV types has been shown to alter the expression of numerous host genes, including those involved in immune regulation. Methylation-specific digital karyotyping has identified *TNFSF13* as a target of hypermethylation in HPV16 E6/E7-expressing keratinocytes, potentially silencing APRIL expression and contributing to immune evasion in cervical carcinogenesis [<a href="#ref-14">14</a>][<a href="#ref-4">4</a>][<a href="#ref-15">15</a>].
- **Human Immunodeficiency Virus (HIV):** Studies in HIV highly-exposed seronegative (HESN) individuals have shown distinct monocyte gene expression profiles, including alterations in TNF superfamily members. The APRIL pathway may contribute to the enhanced mucosal immunity observed in these individuals [<a href="#ref-5">5</a>].
- **SARS-CoV-2:** Single-cell sequencing of immune cells from COVID-19 patients in the recovery stage has revealed significant transcriptional changes in B-cell-related pathways. The APRIL/BAFF system is likely involved in the robust antibody response to the virus [<a href="#ref-6">6</a>].
- **Marek's Disease Virus (MDV):** In chickens, MDV infection has been shown to affect the transcription levels of *TNFSF13*, suggesting a role for APRIL in the immune response to this oncogenic herpesvirus [<a href="#ref-7">7</a>].

### 5.2 Bacterial Infections

- ***Helicobacter pylori*:** APRIL is overexpressed in *H. pylori*-related gastric cancer. The bacterium may induce APRIL expression in gastric epithelial cells, promoting tumorigenesis and immune evasion [<a href="#ref-12">12</a>].
- ***Mycobacterium avium* subsp. *paratuberculosis* (MAP):** In cattle, experimental challenge with MAP, the causative agent of Johne's disease, alters the expression of immune-related genes in the salivary gland, including *TNFSF13*, suggesting a role in the mucosal immune response to this pathogen [<a href="#ref-8">8</a>].

## 6. Pharmacogenomics, Drug Targets & Small-Molecule Inhibitors

The central role of APRIL in autoimmune diseases and B-cell malignancies makes it an attractive therapeutic target. Several strategies are being developed to block APRIL signaling.

### 6.1 Monoclonal Antibodies

- **Atacicept (TACI-Ig):** This is a recombinant fusion protein consisting of the extracellular domain of TACI fused to the Fc portion of human IgG. It acts as a decoy receptor, neutralizing both APRIL and BAFF. It has been investigated in clinical trials for SLE, IgAN, and multiple myeloma. While it showed efficacy in reducing autoantibodies and proteinuria, its development for some indications has been halted due to an increased risk of infections, particularly in patients with multiple myeloma.
- **Telitacicept (RC18):** A similar TACI-Fc fusion protein, approved in China for the treatment of SLE. It is also being investigated for IgAN.
- **Anti-APRIL Antibodies:** Specific monoclonal antibodies that selectively neutralize APRIL without affecting BAFF are in development. These may offer a more targeted approach with a potentially better safety profile. One such antibody, **BION-1301**, is being evaluated in clinical trials for IgAN, where it has shown promise in reducing Gd-IgA1 levels.

### 6.2 Small-Molecule Inhibitors

Direct small-molecule inhibition of the protein-protein interaction between APRIL and its receptors is challenging due to the large, flat binding interface. However, research is ongoing to identify compounds that can disrupt trimerization or block the receptor-binding site. Most current therapeutic approaches rely on biologics rather than small molecules.

### 6.3 Other Therapeutic Strategies

- **Proteasome Inhibitors:** Drugs like bortezomib, used in multiple myeloma, can indirectly reduce APRIL levels by depleting the plasma cells that produce it and the stromal cells that support them.
- **CAR-T Cell Therapy:** Chimeric antigen receptor (CAR) T cells targeting BCMA, the primary receptor for APRIL, have been highly successful in treating relapsed/refractory multiple myeloma. This approach indirectly targets the APRIL signaling axis by eliminating the cells that depend on it.

### 6.4 Pharmacogenomics

The pharmacogenomics of *TNFSF13* is an emerging field. Genetic variants that influence APRIL levels or activity could potentially predict response to anti-APRIL therapies. For example, patients with the high-risk *TNFSF13* genotype associated with elevated APRIL may derive greater benefit from APRIL-neutralizing therapies. Conversely, patients with low baseline APRIL levels may be at higher risk of infectious complications from such treatments. Future clinical trials will need to incorporate pharmacogenomic analyses to optimize patient selection and dosing.

## 7. Bioinformatic Resources & Database Accessions

The following table provides key database accessions for *TNFSF13* and its protein product, APRIL.

| **Database** | **Identifier** | **Description** |
| :--- | :--- | :--- |
| **NCBI Gene** | 8741 | Gene ID for *TNFSF13*. |
| **Ensembl** | ENSG00000161955 | Ensembl Gene ID. |
| **UniProt** | O75888 | Primary accession for APRIL. |
| **RCSB PDB** | 1XU1, 4Z7H, 4Z7I | Representative crystal structures of APRIL and its complexes. |
| **OMIM** | 604472 | Online Mendelian Inheritance in Man entry. |
| **HGNC** | 11928 | HUGO Gene Nomenclature Committee symbol. |
| **Gene Ontology (GO)** | GO:0005125 (cytokine activity), GO:0005164 (tumor necrosis factor receptor binding), GO:0005615 (extracellular space), GO:0006915 (apoptotic process), GO:0006955 (immune response) | Functional annotations. |
| **STRING** | 8741 (Homo sapiens) | Protein-protein interaction network. |
| **BioGRID** | 112604 | Biological General Repository for Interaction Datasets. |
| **ClinVar** | Varied | Clinical significance of specific variants. |

## Related Clinical & Scientific Guides

* [TARM1 Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/tarm1-gene-structure-function-pathway)
* [TRAC Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/trac-gene-structure-function-pathway)
* [CFD Gene: Structure, Function, and Clinical Significance](/knowledge/bioinformatics/genes/immunology-checkpoints/cfd-gene-structure-function-pathway)


## References

<a id="ref-1"></a>[1] "TNFSF13 Gene" - (2020). Definitions. URL: https://www.semanticscholar.org/paper/77797d6a6a10d73b52e6fb17b0a1073572e872df

<a id="ref-2"></a>[2] D. Niu, Yongchao Ren, Liyi Xie, Jiping Sun, Wan-hong Lu, Yaning Hao, Yali Zhang, A. Yin, Huixian Li, Jia Lv, Shengbin Li. "Association between CCDC132, FDX1 and TNFSF13 gene polymorphisms and the risk of IgA nephropathy". *Nephrology*, 2015. URL: https://www.semanticscholar.org/paper/dc4c7c5a06ac425b43f21d21625c6c8f03e84ff6

<a id="ref-3"></a>[3] Feng You, Lidan Zhou, X. Liu, Jie Fan, Zhen Ke, Wenhua Ren. "Molecular structure, expression analysis and functional characterization of APRIL (TNFSF13) gene in bat (Vespertilio superans Thomas)." *Gene*, 2012. URL: https://www.semanticscholar.org/paper/e7b98b0353044cc2d1828d04c473a9f8a0aac5b0

<a id="ref-4"></a>[4] Дарья Александровна Шунькина, Александра Андреевна Комар, Мария Александровна Вульф, Елена Витальевна Кириенкова, Лариса Сергеевна Литвинова. "TNFSF13 IN BLOOD PLASMA PROMOTES MITOCHONDRIAL FISSION IN ADIPOSE TISSUE IN PATIENTS WITH TYPE 2 DIABETES". *Научные исследования в современном мире. Теория и практика*, 2022. URL: https://www.semanticscholar.org/paper/f7db2551d7e8d7918a79f6ba9dbb4ee61527676b

<a id="ref-5"></a>[5] N. Kutukculer, N. Gulez, N. Karaca, G. Aksu, A. Berdeli. "Three Different Classifications, B Lymphocyte Subpopulations, TNFRSF13B (TACI), TNFRSF13C (BAFF-R), TNFSF13 (APRIL) Gene Mutations, CTLA-4 and ICOS Gene Polymorphisms in Turkish Patients with Common Variable Immunodeficiency". *Journal of Clinical Immunology*, 2012. URL: https://www.semanticscholar.org/paper/032ba8832c52cc1125a3bfdb9e4e491e401f0dca

<a id="ref-6"></a>[6] Chen Yang, Wang Jie, Yanlong Yang, Xuefeng Guo, Tan Aihua, Gao Yong, Lu Zheng, Youjie Zhang, Haiying Zhang, Qingmu Xue, Qin Min, L. Mo, Xiaobo Yang, Yanling Hu, Z. Mo. "Genome-wide association study identifies TNFSF13 as a susceptibility gene for IgA in a South Chinese population in smokers". *Immunogenetics*, 2012. URL: https://www.semanticscholar.org/paper/1a8474ecc4b545522d55d5eb0beda773d334e985

<a id="ref-7"></a>[7] Z. Zhong, Shaozhen Feng, Ricong Xu, Zhi-jian Li, F. Huang, Pei-Ran Yin, Wenting Liu, Meng Wang, Dianchun Shi, Qian Zhou, Xue-Qing Yu, Ming Li. "Association of TNFSF13 polymorphisms with IgA nephropathy in a Chinese Han population". *Journal of Gene Medicine*, 2017. URL: https://www.semanticscholar.org/paper/92f84ab201b56914886b6ad27e24a643f4cb8ddd

<a id="ref-8"></a>[8] Jia-xin Zhang, Hong Ma, Ming Sang, Yu-Shi Hu, Zhenning Liang, Hong-xin Ai, Jie Zhang, Xian-wei Cui, Shuang-quan Zhang. "Molecular structure, expression, cell and tissue distribution, immune evolution and cell proliferation of the gene encoding bovine (Bos taurus) TNFSF13 (APRIL)." *Developmental and Comparative Immunology*, 2010. URL: https://www.semanticscholar.org/paper/741a82425ed4c1d444ae11583d508d42fcfd7f71

<a id="ref-9"></a>[9] Anthony K. Redmond, R. Pettinello, H. Dooley. "Outgroup, alignment and modelling improvements indicate that two TNFSF13-like genes existed in the vertebrate ancestor". *Immunogenetics*, 2017. URL: https://www.semanticscholar.org/paper/201893b7c8697a894793c1d1e437a6cbca1d4c39

<a id="ref-10"></a>[10] Lilit Karapetyan, H. Abushukair, Aofei Li, A. Knight, Ayah Nedal Al Bzour, Ian Macfawn, Z. Thompson, Ann Chen, Xi Yang, Rebekah Dadey, A. Karunamurthy, D. V. de Stefano, C. Sander, S. Kunning, Y. Najjar, D. Davar, J. Luke, W. Gooding, T. Bruno, J. Kirkwood, W. Storkus. "Expression of lymphoid structure-associated cytokine/chemokine gene transcripts in tumor and protein in serum are prognostic of melanoma patient outcomes". *Frontiers in Immunology*, 2023. URL: https://www.semanticscholar.org/paper/fa0103983816b53349a52cde18c008697e4f4d37

<a id="ref-11"></a>[11] Cosimo Cumbo, P. Orsini, F. Tarantini, L. Anelli, A. Zagaria, Vincenzo Tragni, Nicoletta Coccaro, G. Tota, Elisa Parciante, M. R. Conserva, Immacolata Redavid, C. Minervini, A. Minervini, I. Attolico, Mattia Gentile, C. Pierri, G. Specchia, P. Musto, F. Albano. "TNFRSF13B gene mutation in familial acute myeloid leukemia: A new piece in the complex scenario of hereditary predisposition?" *Hematological Oncology*, 2023. URL: https://www.semanticscholar.org/paper/b7db5096c1b2db4cc76b6d2a4fb3f28647db64ac

<a id="ref-12"></a>[12] Lilit Karapetyan, H. Abushukair, Aofei Li, A. Knight, A. Al-Bzour, Ian Macfawn, Z. Thompson, Ann Chen, Rebekah Dadey, A. Karunamurthy, Danielle Vargas de Stefano, C. Sander, S. Kunning, Y. Najjar, D. Davar, J. Luke, W. Gooding, T. Bruno, J. Kirkwood, W. Storkus. "Expression of lymphoid structure-associated cytokine/chemokine gene transcripts in tumor and protein in serum: Outcomes for patients with melanoma." *Journal of Clinical Oncology*, 2023. URL: https://www.semanticscholar.org/paper/43026a2c9311822b740ae105124cdd41026b60f3

<a id="ref-13"></a>[13] Ming-li Yang, Yongming Wu, Yanmei Lu, Changyu Liu, Jielin Sun, M. Liao, M. Qin, L. Mo, Yong Gao, Zheng Lu, Chunlei Wu, Youjie Zhang, Haiying Zhang, X. Qin, Yanling Hu, Shijun Zhang, Jianling Li, Min Dong, S. Zheng, Jianfeng Xu, Xiaobo Yang, A. Tan, Z. Mo. "Genome-Wide Scan Identifies Variant in TNFSF13 Associated with Serum IgM in a Healthy Chinese Male Population". *PLoS ONE*, 2012. URL: https://www.semanticscholar.org/paper/3e4aea67c894d0a331065ab70d920a915e98bfb7

<a id="ref-14"></a>[14] Yang Lin. "The Effect of Marek's Disease Virus on Transcription Levels of NRAMP1 and TNFSF13 Bin Chickens". *Scientific Publication*, 2014. URL: https://www.semanticscholar.org/paper/0d63d84c9b0a96c2bcc74da8fc0f91c0540c8cb1

<a id="ref-15"></a>[15] M. Pagadala, G. Jasuja, M. Palnati, J. Lynch, T. Anglin, N. Chang, R. Deka, K. M. Lee, F. Agiri, T. Seibert, B. S. Rose, H. Carter, M