Early activation events render T cells susceptible to HIV-1-induced syncytia formation. Role of protein kinase C.

In human immunodeficiency virus-1 (HIV-1)-infected cell cultures, cell-to-cell fusion and the formation of multinucleated giant cells (syncytia) are induced as a consequence of interactions between the viral envelope glycoprotein on infected cells and cell surface CD4 molecules on uninfected cells. Although activated CD4+ T cells rapidly form syncytia when cultured with HIV-1 envelope glycoprotein expressing (env+) cells, freshly isolated, unstimulated CD4+ T cells do so more slowly. In these studies, we sought to explore the role of T cell activation in rendering CD4+ T cells susceptible to HIV-1-mediated syncytia formation. Our results indicate that within 2 h of exposure to immunologic stimuli, CD4+ T cells acquire the ability to form syncytia with HIV-1 env+ cells. Both cholera toxin, an inhibitor of protein kinase C (PKC) through its effects on inositol triphosphate and diacylglycerol production, and 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride, a noncompetitive inhibitor (with respect to ATP) of PKC, prevented unstimulated but not previously stimulated CD4+ T cells from forming syncytia with HIV-1 env+ cells. 1-Oleoyl-2-acetyl glycerol, an analog of the PKC activator, diacylglycerol, enhanced syncytia formation whereas ionomycin, a calcium ionophore, had no effect. These results suggest that activation of PKC is essential for previously unstimulated CD4+ T cells to become fusogenic.