Identification of a novel monocyte cell surface molecule involved in the generation of antigen-induced proliferative responses.

A monocyte molecule, identified by a monoclonal antibody (mAb) named 24, is involved in the antigen-specific proliferation of T cells. Several types of antigen-induced responses are blocked by mAb24, but mitogen responses, even at low doses, are not. The 24 molecule is found on circulating monocytes and no other cell type. Normally only a proportion of monocytes express the 24 molecule but it can be induced rapidly on monocytes in culture, possibly from intracellular sources. The epitope is present on a heterodimer of 175 and 95 kDa which is similar to the leukocyte cell adhesion molecule (Leu-CAM) family. These findings suggest that although the 24 molecule shows some functional and biochemical similarities with the Leu-CAM family of molecules, it represents a novel structure primarily associated with mononuclear phagocytes.