The B7/BB1 antigen provides one of several costimulatory signals for the activation of CD4+ T lymphocytes by human blood dendritic cells in vitro.

T cells respond to peptide antigen in association with MHC products on antigen-presenting cells (APCs). A number of accessory or costimulatory molecules have been identified that also contribute to T cell activation. Several of the known accessory molecules are expressed by freshly isolated dendritic cells, a distinctive leukocyte that is the most potent APC for the initiation of primary T cell responses. These include ICAM-1 (CD54), LFA-3 (CD58), and class I and II MHC products. Dendritic cells also constitutively express the accessory ligand for CD28, B7/BB1, which has not been previously identified on circulating leukocytes freshly isolated from peripheral blood. Dendritic cell expression of both B7/BB1 and ICAM-1 (CD54) increases after binding to allogeneic T cells. Individual mAbs against several of the respective accessory T cell receptors, e.g., anti-CD2, anti-CD4, anti-CD11a, and anti-CD28, inhibit T cell proliferation in the dendritic cell-stimulated allogeneic mixed leukocyte reaction (MLR) by 40-70%. Combinations of these mAbs are synergistic in achieving near total inhibition. Other T cell-reactive mAbs, e.g., anti-CD5 and anti-CD45, are not inhibitory. Lymphokine secretion and blast transformation are similarly reduced when active accessory ligand-receptor interactions are blocked in the dendritic cell-stimulated allogeneic MLR. Dendritic cells are unusual in their comparably higher expression of accessory ligands, among which B7/BB1 can now be included. These are pertinent to the efficiency with which dendritic cells in small numbers elicit strong primary T cell proliferative and effector responses.