Involvement of protein kinase C-δ in CD28-triggered cytotoxicity mediated by a human leukaemic cell line YT

Ligation of CD28 molecules expressed on the surface of human leukaemic natural killer-like YT cells triggers intracellular signals leading to cytolysis of target cells expressing CD80 or CD86 molecules. Known intracellular events include tyrosine phosphorylation, activation of phosphatidylinositol 3-kinase, and protein kinase C (PKC). In this study, we report that PKC-δ isoenzyme activity is required for CD28-triggered cytotoxicity mediated by YT cells and we also demonstrate that one of the primary targets of bryostatin 1, a modulator of PKC activity, is PKC-δ. Treatment of YT cells with bryostatin 1 caused degradation of PKC-δ, but not other PKC isoenzymes, and completely blocked the cytolytic activity of YT cells. In addition, PKC-δ-specific antibody introduced into YT cells by electroporation inhibited partially the YT cell-mediated cytotoxicity of B- lymphoblastoid cell line JY. This effect was specific, since addition of anti-PKC-δ antibody-blocking peptide in combination with anti-PKC-δ antibody to YT cells for electropotation, neutralized the effect of this antibody. These results demonstrate that YT cell cytolytic activity is dependent on PKC-δ, which is selectively down-regulated by bryostatin 1.