Davidson has received grant support from the National Institutes of Health. Glossary APRILa proliferation-inducing ligandBAFFB-cell-activating factorSLEsystemic lupus erythematosusTACItransmembrane activator and calcium modulator and cyclophilin ligand interactor Footnotes Disclosure Dr. Imm, immature B cell; LPC, long-lived plasma cell; Mem, memory B cell; MZ, marginal zone B cell; SPC, short-lived plasma cell; T1, transitional type 1; T2, transitional type 2 BAFF is cleaved from the cell surface to form a soluble homotrimer [3], whereas APRIL is cleaved intracellularly and secreted as a soluble protein. A small proportion Rabbit Polyclonal to TOP2A of circulating BAFF forms soluble 60-mer multimers, whereas APRIL multimerizes on cell surfaces by attaching to proteoglycans. Circulating BAFF homotrimers CHMFL-KIT-033 bind well to BAFF-R, but binding of both BAFF and APRIL to TACI or BCMA is markedly improved by multimerization [4]. Other forms of the cytokines and receptors can be generated by alternative splicing. Of these, the best studied is BAFF, an isoform that cannot be cleaved from the cell surface and appears to act as a dominant negative inhibitor of BAFF [5]. Mice deficient in BAFF or BAFF-R have a profound decrease in mature B2 cells. This is because the interaction of BAFF with BAFF-R is essential to the survival of B cells past the early transitional (T1) stage, with only a minor contribution from TACI and none from APRIL or BCMA [6C8]. T1 cells are subject to deletion or anergy induction when they receive a BCR signal because their immature rafts contain insufficient cholesterol to assemble signaling molecules. In the T2 stage, BCR signaling through the classical CHMFL-KIT-033 NF-kB pathway upregulates expression of BAFF-R and also generates p100, an essential substrate for the nonclassical NF-B signaling pathway used by BAFF-R [9]. Upon receiving both BCR- and BAFF-mediated signals, T2 cells differentiate and migrate to the marginal zone or to the B-cell follicles, where they require a source of BAFF for their continued survival. Autoreactive B cells that have downregulated their BCR as a consequence of antigen stimulation at the T1 stage produce less p100 and compete poorly for BAFF as they CHMFL-KIT-033 progress to the T2 stage. When B-cell numbers and BAFF levels are normal, stringent deletion of autoreactive B cells occurs. However, an increase in serum BAFF levels, such as occurs during B-cell lymphopenia or perhaps during inflammatory states, results in relaxation of B-cell selection, with survival of more autoreactive B cells [10, 11]. Importantly, however, BAFF excess has less effect on B-cell selection if physiologic competition is provided by non-autoreactive B cells [12?]. A few studies have addressed the fate of autoreactive B cells within a diverse repertoire under conditions of BAFF excess or BAFF inhibition. Findings in several different autoreactive B-cell transgenic models suggest that the effect of excess BAFF on na?ve B-cell selection can be quite variable, and that not all autoreactive B cells are equally susceptible to BAFF inhibition at the transitional B-cell checkpoint [13, 14]. It is therefore important to further dissect the factors that determine BAFF responsiveness of autoreactive B cells so as to find a means of determining which individuals are most likely to be responsive to BAFF inhibition. In SLE, class switching of autoreactive B cells from IgM to more pathogenic IgG is a critical checkpoint in the initiation of clinical CHMFL-KIT-033 disease. BAFF collaborates with cytokines and Toll-like receptor (TLR) signals to promote increased TLR expression, T-independent Ig class switching, and plasma cell differentiation [15, 16]. APRIL also, by binding to TACI, can mediate class switching but preferentially supports switching to IgA [2?]. In SLE, autoreactive B cells internalize nucleic acidCcontaining immune complexes or apoptotic material that can activate CHMFL-KIT-033 TLRs, thereby inducing increased expression of TACI [15, 17]. High serum levels of BAFF may therefore preferentially support the survival and induce class switching of autoreactive cells that recognize nucleic acids. In support of this notion, marginal zone B cells undergo T-independent class switching in BAFF transgenic mice and secrete antinuclear autoantibodies that cause SLE [17]. Some SLE patients have twofold to fivefold increases in serum BAFF levels [18]; this could be due to B-cell lymphopenia, BAFF production from inflammatory sites, or.