Human CD1a molecule expressed on monocytes plays an accessory role in the superantigen-induced activation of T lymphocytes.

The CD1 molecules exhibit characteristics of the MHC class I and class II molecules. They are expressed on cortical thymocytes and, similarly to MHC class II molecules, on antigen-presenting cells. In the present study, we investigated the role of the CD1 molecules in the T-cell response to bacterial superantigens. Indeed, we have observed that CD1 molecules could be detected on the CD14-positive population of some healthy donors (14% of donors tested). The CD1 expression on monocytes is correlated with an activation state of the donors as demonstrated by the increased expression of the CD25, CD38, CD45R0, and MHC class II molecules on their lymphocytes. On these donors, CD1a mAbs induced a clear inhibition (65%) of lymphocyte proliferation induced by either staphylococcal enterotoxin A or toxic shock syndrome toxin-1, whereas this proliferation was constantly unaffected by the addition of mAbs directed against CD1b or CD1c. Moreover, an intracellular calcium flux was induced in monocytes following CD1a engagement, and this calcium flux was partially inhibited by preincubation of these cells with the superantigen. These results attribute to the CD1a molecule expressed by monocytes a role in the transduction of signal(s) involved in superantigen-induced activation.