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BAFF (BLyS / TALL1)

BAFF /APRIL functions

BAFF (B cell activation factor of the TNF family, also known as BLyS or TALL1) is a cytokine expressed predominantly by cells of the immune system such as neutrophils, monocytes, macrophages, dendritic cells, follicular dendritic cells, activated T cells and some malignant B cells. Interferon-g stimulates the expression of BAFF by these cells. BAFF is a master regulator of peripheral B cell survival, and also acts in processes such as immunoglobulin isotype switch and B cell co-stimulation. Beside its major role in B cell biology, BAFF co-stimulates activated T cells. BAFF is found either at the cell membrane as a type II transmembrane protein or it is released in a soluble form after cleavage by furin-type convertases.[1]. Membrane-bound and soluble BAFF bind to CD19+ B cells, including both naive and memory B cells.

APRIL (A Proliferation-inducing ligand; TRDL-1) is a secreted protein related to BAFF and shares some of the BAFF receptors. APRIL is expressed in monocytes, dendritic cells and T cells. In contrast to BAFF, APRIL is also expressed in several tumor tissues or cell lines, such as colon carcinoma[2].

BAFF and APRIL both bind to TACI (transmembrane activator and calcium-modulator and cyclophilin ligand-interactor) and BCMA (B cell maturation antigen). BAFF, but not APRIL also signals through BAFF Receptor (BAFF-R, BR3). All three receptors are expressed during B cell differentiation, although activated T cells also express BAFF-R. A new third “receptor” for APRIL has been reported: the glycosaminoglycans side chains of proteoglycans are an APRIL-specific binding partner (Figure 1). April binding to the proteoglycans has been postulated to be a prerequisite for APRIL activation by inducing APRIL oligomerization[3, 4].

BAFF and diseases

Interactions of BAFF (B cell activating factor) with BAFF-R, one of three BAFF-binding receptors that are preferentially expressed on B cells, are essential for B cell development. Defects in either the ligand or the receptor arrest progression from immature type-1 B cells to type-2 B cells and mature B cells; B1 B cells are unaffected. Deregulated expression of BAFF leads to autoimmune disorders in mice. Mouse models of lupus-nephritis have been shown to exhibit increased serum BAFF levels correlating with disease severity. In the human, elevated levels of soluble BAFF have been detected in the serum of patients with various autoimmune diseases, such as rheumatoid arthritis (RA), Sjögren's syndrome (SS) and systemic lupus erythematosus (SLE) and in patients with tumors, such as multiple myeloma (MM) non-Hodgkin lymphoma (NHL), including B-cell chronic lymphocytic leukemia (B-CLL)[5]. Modulating the level and activity of BAFF in these patients may alleviate symptoms associated with their disease. Several potential therapeutic inhibitors targeting BAFF are under investigation, including an anti-BAFF antibody and receptor-Fc fusion proteins.

Mutations in the BAFF-R and TACI are associated with Common Variable Immunodeficiency (CVID) in humans[6, 7].

Figure 1: Functions of BAFF and APRIL and its receptors in the immune system. BAFF and APRIL interactions with their receptors and functional outcomes of BAFF and /or APRIL signaling are shown.

BAFF, Soluble (human) ELISA Kit (APO-54N-020)

Summary of features

	For the quantitative determination of soluble human BAFF from biological fluids (serum and cell culture supernatant)
	Monoclonal antibodies-based sandwich ELISA
	Detection limit: 0.95 ng / ml
	Range: 0-128 ng / ml
	Specificity : Detects only human BAFF
	Format: Microtiter plate, 96 wells (~80 tests)

Principle of the Kit

Standards, controls and samples containing human BAFF are added to wells of microplate coated with a high affinity monoclonal anti-human BAFF antibody. During the first incubation period, the antibody immobilized on the wall of the microtiter wells captures BAFF in the patient samples or in the cell culture supernatants. After washing away the unbound components from samples, a detection antibody (monoclonal anti-BAFF antibody) is added to each well. During the incubation step, the detection antibody is bound to the captured BAFF. A peroxidase-conjugated antibody is then added to each microtiter well and a “sandwich” of capture antibody - human BAFF – detection antibody - Peroxidase conjugate is formed. Tetramethylbenzidine (TMB) is used as a substrate for peroxidase. Finally, an acidic stop solution is added to terminate the reaction. The intensity of the yellow color is directly proportional to the BAFF concentration of sample. A dose response curve of absorbance unit (optical density, OD at 450 nm) vs. concentration is generated; using the values obtained from standard. BAFF present in the patient samples or cell culture supernatants, is determined directly from this curve.

Figure 2: Linearity of the STD curve.

Example: detection of endogenous soluble BAFF (human):

Figure 3: BAFF levels in Sjögren's syndrome (SS) patients. Numbers represent the different patients.

Literature Overview:

[1] Schneider, P. (2005). The role of APRIL and BAFF in lymphocyte activation. Curr Opin Immunol 17, 282-289.

[2] Rennert, P., Schneider, P., Cachero, T.G., Thompson, J., Trabach, L., Hertig, S., Holler, N., Qian, F., Mullen, C., Strauch, K., Browning, J.L., Ambrose, C., and Tschopp, J. (2000). A soluble form of B cell maturation antigen, a receptor for the tumor necrosis factor family member APRIL, inhibits tumor cell growth. J Exp Med 192, 1677-1684.

[3] Ingold, K., Zumsteg, A., Tardivel, A., Huard, B., Steiner, Q.G., Cachero, T.G., Qiang, F., Gorelik, L., Kalled, S.L., Acha-Orbea, H., Rennert, P.D., Tschopp, J., and Schneider, P. (2005). Identification of proteoglycans as the APRIL-specific binding partners. J Exp Med 201, 1375-1383.

[4] Hendriks, J., Planelles, L., de Jong-Odding, J., Hardenberg, G., Pals, S.T., Hahne, M., Spaargaren, M., and Medema, J.P. (2005). Heparan sulfate proteoglycan binding promotes APRIL-induced tumor cell proliferation. Cell Death Differ 12, 637-648.

[5] Mackay, F., and Tangye, S.G. (2004). The role of the BAFF/APRIL system in B cell homeostasis and lymphoid cancers. Curr Opin Pharmacol 4, 347-354.

[6] Salzer, U., Chapel, H.M., Webster, A.D., Pan-Hammarstrom, Q., Schmitt-Graeff, A., Schlesier, M., Peter, H.H., Rockstroh, J.K., Schneider, P., Schaffer, A.A., Hammarstrom, L., and Grimbacher, B. (2005). Mutations in TNFRSF13B encoding TACI are associated with common variable immunodeficiency in humans. Nat Genet 37, 820-828.

[7] Losi, C.G., Silini, A., Fiorini, C., Soresina, A., Meini, A., Ferrari, S., Notarangelo, L.D., Lougaris, V., and Plebani, A. (2005). Mutational Analysis of Human BAFF Receptor TNFRSF13C (BAFF-R) in Patients with Common Variable Immunodeficiency. J Clin Immunol 25, 496-502.