| Property | Value |
|---|---|
| Gene Symbol | TNFRSF13B |
| Full Name | TNF Receptor Superfamily Member 13B |
| Alternative Names | TACI, CD269 |
| Chromosomal Location | 17p11.2 |
| NCBI Gene ID | 9517 |
| OMIM | 604715 |
| Ensembl ID | ENSG00000165502 |
| UniProt ID | O14836 |
| Gene Family | TNF receptor superfamily |
| Associated Diseases | ALS, CVID, Autoimmune Encephalitis, MS |
| TACI (TNFRSF13B) | |
|---|---|
| Gene Symbol | TNFRSF13B (TACI) |
| Full Name | TNF Receptor Superfamily Member 13B |
| Chromosome | 17p11.2 |
| NCBI Gene ID | [9517](https://www.ncbi.nlm.nih.gov/gene/9517) |
| OMIM | 604715 |
| Ensembl ID | [ENSG00000165502](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000165502) |
| UniProt ID | [O14836](https://www.uniprot.org/uniprot/O14836) |
| Associated Diseases | ALS, CVID, Autoimmune Encephalitis, MS |
TACI (Transmembrane Activator and Calcium Modulator and Cyclophilin Ligand Interactor, or TNFRSF13B) is a critical member of the TNF receptor superfamily that plays essential roles in B cell function, immunoglobulin class switching, and immune system regulation. Encoded by the TNFRSF13B gene on chromosome 17p11.2, TACI is expressed primarily on B cells, particularly on marginal zone B cells, class-switched memory B cells, and plasma cells. The receptor binds two ligands, BAFF (B cell Activating Factor, also known as TNFSF13B) and APRIL (A Proliferation-Inducing Ligand, also known as TNFSF13), which are critical for B cell survival, maturation, and antibody production.
While TACI has been studied extensively in the context of humoral immunity and autoimmune diseases, emerging evidence suggests important roles in neuroinflammation and neurodegenerative diseases. The receptor is expressed in the central nervous system (CNS) by various cell types, including certain glial cells and possibly neurons, where it may contribute to B cell-mediated neuroinflammatory processes. This has implications for diseases such as multiple sclerosis (MS), autoimmune encephalitis, and potentially Alzheimer's disease (AD) and Parkinson's disease (PD).
The TNFRSF13B gene spans approximately 40 kilobases and consists of multiple exons encoding the TACI protein. Alternative splicing produces multiple isoforms, including a full-length membrane-bound form and soluble variants. The TACI protein is a type I transmembrane protein with the following structural features:
Extracellular Domain: The extracellular region contains multiple cysteine-rich domains (CRDs) characteristic of TNF receptor family members. These CRDs mediate ligand binding and interactions with BAFF and APRIL. The affinity for BAFF is moderate, while binding to APRIL requires higher concentrations. TACI can form both homomultimers and heteromultimers with other receptors.
Transmembrane Domain: A single pass transmembrane helix anchors the receptor in the cell membrane.
Cytoplasmic Domain: The intracellular region contains a "TRAF-binding motif" that allows recruitment of TRAF (TNF Receptor-Associated Factor) proteins, particularly TRAF2 and TRAF5, which mediate downstream signaling pathways.
The cytoplasmic domain also contains a "B cell linker" (BLINK) domain that interacts with proteins involved in B cell receptor signaling, linking TACI activation to calcium signaling and downstream transcriptional activation.
TACI activates multiple intracellular signaling cascades upon ligand binding:
TACI engagement leads to activation of both canonical and non-canonical NF-κB pathways:
TACI activates:
TACI name derives from its ability to modulate calcium signaling in B cells. The receptor interacts with calcium-modulating cyclophilin ligand (CAML), affecting calcium influx and downstream signaling events important for B cell activation and antibody production.
Recent evidence indicates TACI activates the mTOR (mammalian target of rapamycin) pathway, which is important for plasma cell differentiation and immunoglobulin production.
TACI is primarily expressed in the B cell compartment:
TACI expression is dynamic during B cell development and activation, regulated by factors including BAFF levels, B cell receptor signaling, and cytokines.
In the CNS, TACI expression is more limited but has been documented:
The CNS expression of TACI is upregulated in certain neuroinflammatory conditions, suggesting a role in B cell-mediated neuroinflammation.
BAFF and APRIL binding to TACI provides critical survival signals for B cells, particularly for transitional B cells and marginal zone B cells. TACI signaling upregulates anti-apoptotic proteins (Bcl-2, Bcl-xL) and promotes B cell maturation.
TACI is a key driver of immunoglobulin class switching from IgM to IgG, IgA, and IgE isotypes. This function is mediated through both BAFF and APRIL signaling and is critical for generating high-affinity antibodies with effector functions.
TACI signaling promotes plasma cell differentiation and supports antibody secretion. The receptor helps plasma cells survive in the bone marrow microenvironment, which is essential for long-term humoral immunity.
TACI is particularly important for T cell-independent (TI) antibody responses, which are critical for defense against encapsulated bacteria and blood-borne antigens. TI antigens can stimulate B cells without T cell help, and TACI signaling enhances IgM and IgG production in this context.
CVID is the most common symptomatic antibody deficiency in adults, characterized by low immunoglobulin levels and recurrent infections. Heterozygous TNFRSF13B mutations are found in approximately 5-10% of CVID patients, making this the most common genetic cause of CVID identified to date.
Pathogenic Mutations: The most common disease-causing mutations include:
Mechanism: TACI mutations cause disease through both haploinsufficiency (reduced protein levels) and dominant-negative effects (mutant proteins interfere with wild-type function). The result is impaired B cell survival, reduced class switching, and defective antibody production.
Phenotype: CVID patients with TNFRSF13B mutations often present with:
Autoimmune encephalitis (AE) is a group of disorders characterized by autoantibodies against neuronal antigens, leading to inflammation and dysfunction of the brain. TACI may play a role in the survival and function of autoantibody-producing B cells in the CNS.
Role: TACI expression on B cells within the CNS may support the survival of autoreactive B cell clones and the production of pathogenic autoantibodies. BAFF is elevated in the CSF of some AE patients, potentially creating a microenvironment that promotes autoantibody production.
Therapeutic Implications: B cell-depleting therapies (rituximab) and BAFF-targeting approaches may be beneficial in some forms of autoimmune encephalitis.
ALS is a fatal neurodegenerative disease characterized by progressive loss of upper and lower motor neurons. Neuroinflammation, involving both innate and adaptive immune responses, is a key pathological feature.
Evidence for TACI involvement:
Potential mechanisms: TACI may contribute to ALS through:
MS is an autoimmune demyelinating disease of the CNS with prominent B cell and antibody involvement.
Evidence for TACI involvement:
Potential mechanisms: TACI signaling may:
Emerging evidence suggests roles for B cells and BAFF/TACI signaling in AD and PD:
Several therapeutic strategies targeting the BAFF/TACI axis are in development or clinical use:
BAFF inhibitors:
TACI-Fc fusion proteins:
B cell depletion:
Clinical trials targeting BAFF/TACI pathways have shown: