Biological activity of soluble CD100. 1. The extracellular region of CD100is released from the surface of T lymphocytes by regulated proteolysis

CD100 is the first semaphorin described in lymphoid tissues, where it has b een shown to be associated with a serine kinase activity. Semaphorins are m olecules involved in axon pathfinding during nerve development and act as r epellent guidance cues. In the nervous system semaphorins exist as either m embrane-bound or secreted forms. We report here a spontaneous processing of membrane CD100, suggesting that it is also produced as a diffusable semaph orin from lymphoid cells. Monomeric and homodimeric forms of CD100 are expr essed by T lymphocytes and CD100-transfected fibroblasts. We demonstrate th at CD100 is released through a proteolytic process blocked by metalloprotea se inhibitors. In T cells, only soluble CD100 dimers are produced, suggesti ng that CD100 dimerization is required for proteolysis. In agreement, we ob serve that increasing membrane dimers strongly favors shedding of the molec ule. By expressing a CD100 molecule mutated at cysteine 674 into a COS cell system, we additionally demonstrate that this particular residue in the ex tracellular domain of the molecule is required for dimerization. Finally, w e show that staurosporine, a serine kinase inhibitor, enhances the membrane cleavage of CD100. Together these results demonstrate that membrane CD100 is cleaved by a metalloprotease-dependent process, which is probably regula ted by phosphorylation. Mainly, these findings shed light on a possible fun ction for the semaphorin region of CD100 as a long range guidance cue in th e immune system.