Expression of CD5, a lymphocyte surface antigen on the endothelium of blood vessels.

CD5 is a lymphocyte surface antigen considered to be a pan-T cell marker although it is also expressed on a small population of B cells. In this report we present evidence that four different monoclonal antibodies specific for the CD5 antigen also recognize a molecule present on the endothelium of blood vessels in certain regions (placentomes) of the pregnant sheep uterus. The identity of the CD5 antigen on the endothelium with that expressed on T lymphocytes was confirmed by molecular mass analysis of the CD5 antigen in membrane extracts of lymphocytes, thymocytes and placentomes which showed that all three antigens have an apparent molecular mass of 67 kDa. Possible implications of this finding in relation to the function of the CD5 molecule and lymphocyte migration are discussed.