B-cell activating factor
|Tumor necrosis factor (ligand) superfamily, member 13b|
PDB rendering based on 1jh5.
B-cell activating factor (BAFF) also known as tumor necrosis factor ligand superfamily member 13C is a protein that in humans is encoded by the TNFSF13C gene. BAFF is also known as B Lymphocyte Stimulator (BLyS) and TNF- and APOL-related leukocyte expressed ligand (TALL-1) and the Dendritic cell-derived TNF-like molecule (CD257 antigen; cluster of differentiation 257).
Structure and function
BAFF is a cytokine that belongs to the tumor necrosis factor (TNF) ligand family. This cytokine is a ligand for receptors TNFRSF13B/TACI, TNFRSF17/BCMA, and TNFRSF13C/BAFF-R. This cytokine is expressed in B cell lineage cells, and acts as a potent B cell activator. It has been also shown to play an important role in the proliferation and differentiation of B cells.
BAFF is a 285-amino acid long peptide glycoprotein which undergoes glycosylation at residue 124. It is expressed as a membrane-bound type II transmembrane protein  on various cell types including monocytes, dendritic cells and bone marrow stromal cells. The transmembrane form can be cleaved from the membrane, generating a soluble protein fragment. BAFF steady-state concentrations depend on B cells and also on the expression of BAFF-binding receptors. BAFF is the natural ligand of three unusual tumor necrosis factor receptors named BAFF-R (BR3), TACI (transmembrane activator and calcium modulator and cyclophilin ligand interactor), and BCMA (B-cell maturation antigen), all of which have differing binding affinities for it. These receptors are expressed mainly on mature B lymphocytes and their expression varies in dependence of B cell maturation (TACI is also found on a subset of T-cells and BCMA on plasma cells). BAFF-R is involved in the positive regulation during B cell development.  TACI binds worst since its affinity is higher for a protein similar to BAFF, called a proliferation-inducing ligand (APRIL). BCMA displays an intermediate binding phenotype and will work with either BAFF or APRIL to varying degrees. Signaling through BAFF-R and BCMA stimulates B lymphocytes to undergo proliferation and to counter apoptosis. All these ligands act as homotrimers (i.e. three of the same molecule) interacting with homotrimeric receptors, although BAFF has been known to be active as either a hetero- or homotrimer (can aggregate into 60-mer depending on the primary structure of the protein).
B-cell activating factor has been shown to interact with TNFRSF13B, TNFSF13 and TNFRSF17. Interaction between BAFF and BAFF-R activates classical and noncanonical NF-κB signaling pathways. This interaction triggers signals essential for the formation and maintenance of B cell, thus it is important for a B-cell survival. 
As an immunostimulant, BAFF (BLyS, TALL-1) is necessary for maintaining normal immunity. Inadequate level of BAFF will fail to activate B cells to produce enough immunoglobulin and will lead to immunodeficiency.
Human BLyS was expressed and purified in E. Coli. The BLyS protein in the engineered bacteria can be as rich as 50% to the bacteria’s total protein content and still retains activity after an easy purification procedure. Due to the ease of bacterial platform, the bacteria expressed BLyS will dramatically reduce the cost of BLyS as a potential pharmaceutical agent for immunodeficiency.
Excessive level of BAFF causes abnormally high antibody production, results in systemic lupus erythmatosis, rheumatoid arthritis, and many other autoimmune diseases.
Belimumab (Benlysta) is a monoclonal antibody developed by Human Genome Sciences and GlaxoSmithKline, with significant discovery input by Cambridge Antibody Technology, which specifically recognizes and inhibits the biological activity of B-Lymphocyte stimulator (BLyS) and is in clinical trials for treatment of Systemic lupus erythematosus and other auto-immune diseases.
BAFF has been found in renal transplant biopsies with acute rejection and correlate with appearance C4d. Increased levels of BAFF may initiate aloreactive B cell and T cell immunity, therefore may promote allograft rejection. Lower level of BAFF transcripts (or a higher level of soluble BAFF) show a higher risk of producing donor-specific antibodies in the investigated patients. Donor-specific antibodies bind with high affinity to the vascular endothelium of graft and activate complement. This process result in neutrophils infiltration, hemorrhage, fibrin deposition and platelet aggregation. Targeting BAFF-R interactions may provide new therapeutic possibilities in transplantation.
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