Biotin
CD100/Sema4D belongs to the semaphorin family, factors known to act as repulsive cues for axons during neuronal development. Mouse CD100 plays a crucial role in both humoral and cellular immunity through ligation of the lymphocyte receptor, CD72. It remains controversial, however, whether human CD100 can function through human CD72 in a manner similar to mouse CD100. To determine the function of human CD100, we generated a recombinant soluble human CD100 protein comprised of the extracellular region of human CD100 fused to the human IgG1 Fc region (hCD100-Fc). hCD100-Fc specifically binds to cells expressing human CD72. As observed previously in the mouse, hCD100-Fc induces the tyrosine dephosphorylation of human CD72, leading to the dissociation of SHP-1 from the CD72 cytoplasmic tail. Consistent with findings for mouse CD100, hCD100-Fc exerts a co-stimulatory effect on B cells and dendritic cells that are stimulated with anti-CD40 mAb. Furthermore, both hCD100-Fc and anti-human CD72 agonistic mAb induce the production of the pro-inflammatory cytokines tumor necrosis factor-alpha, IL-6 and IL-8, even in the absence of anti-CD40 mAb. Collectively, our findings not only demonstrate that human CD100, interacting with human CD72, can function as a ligand in a manner similar to mouse CD100, but also suggest the involvement of human CD100 in inflammatory responses.
CD40 is essential for efficient humoral immune responses. CD40 has two cytoplasmic domains required for binding of tumor necrosis factor receptor-associated factors (TRAF). The TRAF6-binding site is within the membrane proximal cytoplasmic (Cmp) region, while a PXQXT motif in the membrane distal cytoplasmic (Cmd) region needs to engage TRAF2/3/5. To dissect CD40 signals necessary for B cell differentiation, we generated transgenic mice expressing wild-type and mutant human CD40 (hCD40) molecules in a mouse CD40-deficient (mCD40(-/-)) background. The B cell-specific expression of hCD40 in mCD40(-/-) mice resulted in T-dependent antibody responses including germinal center (GC) formation. Mutant hCD40 molecules that carry either a point mutation of the TRAF2/3/5-binding site or a deletion of the Cmd region rescued extrafollicular B cell differentiation but not GC formation. A mutant hCD40 that comprises of only the TRAF2/3/5-binding site in the cytoplasmic region also rescued low but significant titers of antigen-specific IgG1 without GC formation. These results demonstrated that two distinct signals either from the Cmp or from the Cmd region induced the extrafollicular B cell differentiation and Ig class switching; however, GC formation required both. We conclude that combinations of these two signals determine which of the extrafollicular or the follicular (GC) differentiation pathway B cells enter.
