Phorbol-12-Myristate-13-Acetate-Treated Human Keratinocytes Express B7-Like Molecules That Serve a Costimulatory Role in T-Cell Activation

In previous studies, Phorbol-12-myristate-13-acetate (PMA) — treated human keratinocytes (PMA-HNK) were shown to induce T-cell proliferation via a major histocompatibility complex (MHC)- and antigen (Ag)-independent mechanism, that was mediated in part by PMA-induced intercellular adhesion molecule (ICAM)-1 on HNK. Recently, the interaction of the B7 Ag on antigen-presenting cells with its ligand CD28 on T cells has been shown to deliver activation signals distinct from the interaction of MHC/Ag with the T-cell receptor. These findings led us to assess whether B7-dependent signals play a role in T-cell proliferation induced by PMA-HNK. We first examined B7 expression on HNK by staining with three different monoclonal antibodies (MoAbs). When analyzed by fluorescence-activated cell sorter, untreated HNK stained only faintly. By contrast, PMA induced a dose-dependent upregulation of B7 staining. This staining identifies a molecule closely related to B7 because it was blocked by purified recombinant B7 immunoglobulin. Upregulation of B7 staining was first observed 16 h after PMA treatment and persisted for at least 48 h; it was protein kinase C dependent and required de novo protein synthesis. Anti-B7 MoAbs reduced specifically the capacity of PMA-HNK to trigger proliferation of allogeneic peripheral blood mononuclear cells and T cells. The combination of anti-B7 and anti — ICAM-1 MoAbs further reduced this response. We conclude that PMA upregulates on HNK the expression of a B7-like molecule that contributes in concert with ICAM-1 to the capacity of PMA-HNK to induce proliferation of allogeneic T cells