Soluble CD100 functions on human monocytes and immature dendritic cells require plexin C1 and plexin B1, respectively.

CD100 represents the first semaphorin described in the immune system. It is expressed as a 300-kDa homodimer at the surface of most hematopoietic cells, but is also found in a soluble form following a proteolytic cleavage upon cell activation. We herein established that soluble CD100 (sCD100) impaired the migration of human monocytes and immature dendritic cells (DCs), but not of mature DCs. Performing competition assays, we identified plexin C1 (VESPR/CD232) as being involved in sCD100-mediated effects on human monocytes. Interestingly, we observed a complete down-regulation of plexin C1 expression during the in vitro differentiation process of monocytes to immature DCs, while concomitantly the surface expression of plexin B1 was induced. The latter receptor then binds sCD100 on immature DCs, mediating its inhibitory effect on cell migration. Finally, we showed that sCD100 modulated the cytokine production from monocytes and immature DCs. Together these results suggest that sCD100 plays a critical role in the regulation of antigen-presenting cell migration and functions via a tightly regulated process of receptor expression.

Structure and function of the immune semaphorin CD100/SEMA4D.

Semaphorins are a large family of membrane-bound and secreted molecules involved in numerous functions, including axon guidance, morphogenesis, carcinogenesis, and immunomodulation. A growing number of semaphorins--namely, human CD100/SEMA4D, CD108/SEMA7A, and SEMA3A; viral semaphorins, SemaVA and SemaVB; and, very recently, mouse Sema4A--were reported to regulate immune cell responses. Among them, the role of CD100 has been well documented in both humans and mice. CD100, in particular, has been shown to influence monocyte migration, T-cell activation, B-cell survival as well as T/B and T/dendritic cell cooperation. In contrast to other semaphorins, CD100 is the only semaphorin for which membrane and soluble forms are endowed with functional properties, and for which bidirectional signaling has been suggested. The human membrane-bound CD100 engagement triggers costimulatory signals to T cells through its interaction with membrane protein tyrosine phosphatase CD45 and an intracellular serine kinase. Its soluble extracellular region acts most likely through its receptors, human PlexinB1 and mouse CD72, to promote T-cell priming, B-cell survival and antibody production in response to T-dependent antigens. Human soluble CD100 also induces monocyte paralysis and the arrest of its spontaneous and chemokine-induced migration by signaling through an as yet unknown receptor that is different from PlexinB1 and CD72. In this review, we discuss recent advances in research studies on human and murine CD100, and we describe the relationship of CD100 function to its expression and structure. The signaling events that support CD100 function are also discussed.