The Binding Site for TRAF2 and TRAF3 but Not for TRAF6 Is Essential for CD40-Mediated Immunoglobulin Class Switching

AbstractTo define the role of TRAF proteins in CD40-dependent isotype switching in B cells, we introduced wild-type (WT) and mutant CD40 transgenes that lacked the binding motifs for TRAF6 (CD40ΔTRAF6), TRAF2 and TRAF3 (CD40ΔTRAF2/3), or both (CD40ΔTRAFs) into B cells of CD40−/− mice. The in vivo isotype switch defect in CD40−/− mice was fully corrected by WT and CD40ΔTRAF6, partially by CD40ΔTRAF2/3, and not at all by CD40ΔTRAFs transgenes. CD40-mediated isotype switching, proliferation, and activation of p38, JNK, and NFκB in B cells were normal in WT and CD40ΔTRAF6 mice, severely impaired in CD40ΔTRAF2/3, and absent in CD40ΔTRAFs mice. These results suggest that binding to TRAF2 and/or TRAF3 but not TRAF6 is essential for CD40 isotype switching and activation in B cells

TACI and BAFF-R mediate isotype switching in B cells

The tumor necrosis factor family members BAFF and APRIL induce Ig isotype switching in human B cells. We analyzed the ability of BAFF and APRIL to induce isotype switching in murine B cells to IgG1, IgA, and IgE. APRIL and BAFF each engage two receptors, transmembrane activator and calcium-modulator and cytophilin ligand interactor (TACI) and B cell maturation antigen (BCMA), on B cells. In addition, BAFF engages a third receptor on B cells, BAFF-R. To determine the role of these receptors in isotype switching, we examined B cells from mice deficient in TACI, BCMA, and BAFF-R. The results obtained indicate that both TACI and BAFF-R are able to transduce signals that result in isotype switching

Impaired IgA class switching in APRIL-deficient mice

The tumor necrosis factor (TNF) family member APRIL binds to the receptors BCMA on B cells and TACI on B and T cells. To investigate the role of APRIL in immunity, we generated APRIL-deficient mice. APRIL(-/-) mice have normal T and B lymphocyte development, normal T and B cell proliferation in vitro, but increased numbers of CD44(hi)CD62L(lo) CD4(+) effector/memory T cells and increased IgG responses to T-dependent antigens. Serum IgA levels were significantly decreased, and serum IgA antibody responses to mucosal immunization with TD antigens and to type 1 T-independent antigens were impaired in APRIL(-/-) mice. APRIL by itself induced IgA as well as IgG1 isotype switching in CD40-deficient IgM(+)IgD(+) sorted B cells. These results suggest that APRIL down-regulates T cell-dependent antibody responses and promotes IgA class switching