CD100 was originally described as an activation molecule on the surface of
human T lymphocytes. Its triggering through distinct epitopes leads to diff
erent signals of costimulation with phorbol myristate acetate (PMA) or with
CD3 and CD2. Interestingly, CD100 was shown to associate with different pa
rtner molecules in T cells. First, CD100 can associate with CD45, a key mol
ecule with protein tyrosine phosphatase activity involved in T-cell transdu
ction: this association is physical and has functional consequences for bot
h partners. Second, CD100 interacts in its cytoplasmic domain with a Ser/Th
r kinase for which it represents a preferential substrate. Recently, CD100
was identified as a member of the semaphorin gene family. This family compr
ises approximately 20 structurally related proteins. The first semaphorins
were identified in the developing nervous system. Function has been shown f
or only some of them and involves repulsion during growth cone guidance. Si
nce CD100 was the first semaphorin identified in the immune system, this ra
ises the possibility of the involvement of members of the semaphorin family
in other physiological phenomena outside the nervous system.