No abstract

Herein we describe the molecular characterization of the human leukocyte activation antigen CD100 and identify it as the first semaphorin, to our knowledge, in the immune system. Semaphorins have recently been described as neuronal chemorepellants that direct pioneering neurons during nervous system development. In this study we demonstrate that CD100 induces B cells to aggregate and improves their viability in vitro. We show that CD100 modifies CD40-CD40L B-cell signaling by augmenting B-cell aggregation and survival and down-regulating CD23 expression. Thus, these results suggest that semaphorins as exemplified by CD100 also play a functional role in the immune system.

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.signaling ͉ thrombosis ͉ metalloprotease ͉ CD72 ͉ plexin-B1 P latelet activation typically begins with the exposure of collagen within a damaged vessel wall or the local generation of thrombin, but the establishment of a stable thrombus requires the recruitment of additional platelets and the development of stable contacts between platelets (1). Platelet activation also results in the release from platelets of molecules that can affect nearby cells, including endothelial cells and leukocytes as well as other platelets. In a continuing search for molecules that might contribute to contact-dependent events during thrombus formation, we screened human platelets for members of the semaphorin family. Although sempahorins are best known as regulators of neurite outgrowth and vascular development, individual family members have been shown to participate in a variety of events. Class IV semaphorin [semaphorin 4D (sema4D; CD100)] is a type I integral membrane protein first reported on T cells where it supports B cell development by binding to CD72 (2-4). However, sema4D receptors are not limited to B cells. Prior work has shown that a soluble sema4D extracellular domain fragment can activate endothelial cells by its other known receptor, plexin-B1. This causes endothelial migration, actin rearrangement, and the formation of tube-like structures in vitro, responses that are relevant for wound healing and angiogenesis (5-11). Soluble sema4D has also been shown to inhibit monocyte (12) and dendritic cell (13) migration. ...

Toxic epidermal necrolysis is an extremely severe drug reaction, manifesting itself by widespread apoptosis of keratinocytes, generally considered to result from Fas/CD95-FasLigand interaction, but of unknown primary mechanism. We looked at the role of cells present in the skin blisters as probable effectors of this immune reaction. In a patient suffering from cotrimoxazole-induced toxic epidermal necrolysis, blister fluid cells were phenotyped by FACS and tested without prior restimulation for cytotoxicity on autologous and allogeneic cells in the presence of the drug. Blister fluid lymphocytes were predominantly CD8+, DR+, CLA+, CD56+ T lymphocytes, perforin positive and expressing preferentially two Vbeta chains of the T cell receptor repertoire. These lymphocytes were cytotoxic only in the presence of the drug towards autologous EBV transformed lymphocytes and towards allogeneic cells sharing HLA-Cw4. Cytotoxicity occurred in the presence of either cotrimoxazole, sulfamethoxazole, or the nitroso metabolite of sulfamethoxazole, but not with the hydroxylamine metabolite of sulfamethoxazole. The lysis was blocked by an anti-MHC class I monoclonal antibody. It was abolished by EGTA and CMA, but neither by anti-fas, brefeldin A, nor by anti-TRAIL receptor monoclonal antibodies, strongly suggesting perforin/granzyme-mediated cytotoxicity, without implication of Fas or TRAIL at this stage. This is direct evidence that T lymphocytes present within the lesions of toxic epidermal necrolysis may exhibit, without any re-stimulation, a drug-specific cytotoxicity against autologous cells. Harboring the markers of classical CTL and MHC class I restriction these lymphocytes reacted against the parent drug and one of its reactive metabolites. These results challenge several current concepts and could support new therapeutic approaches.