CDB15:0001361 SEMA4D — CD72

ABSTRACT: 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.

ABSTRACT: Semaphorin 4D (sema4D; CD100) is an integral membrane protein and the ligand for two receptors, CD72 and plexin-B1. Soluble sema4D has been shown to evoke angiogenic responses from endothelial cells and impair monocyte migration, but the origin of soluble sema4D, particularly at sites of vascular injury, has been unclear. Here we show that platelets express sema4D and both of its receptors and provide evidence that these molecules promote thrombus formation. We also show that the surface expression of sema4D and CD72 increases during platelet activation, followed by the gradual shedding of the sema4D extracellular domain. Shedding is blocked by metalloprotease inhibitors and abolished in mouse platelets that lack the metalloprotease ADAM17 (TACE). Mice that lack sema4D exhibit delayed arterial occlusion after vascular injury in vivo, and their platelets show impaired collagen responses in vitro. In resting platelets, as in B lymphocytes, CD72 is associated with the protein tyrosine phosphatase SHP-1. Platelet activation causes dissociation of the complex, as does the addition of soluble sema4D. These findings suggest a dual role for sema4D in vascular responses to injury. As thrombus formation begins, platelet-associated sema4D can bind to its receptors on nearby platelets, promoting thrombus formation. As thrombus formation continues, sema4D is shed from the platelet surface and becomes available to interact with receptors on endothelial cells and monocytes, as well as continuing to interact with platelets.