Regulation of HLA class II antigen expression: intracellular signaling molecules responsible for the regulation by IFN-gamma and cross-linking of Fc receptors in HL-60 cells.

The cross-linking of Fc receptors (FcR) on HL-60 cells inhibited the ability of recombinant IFN-gamma to induce HLA class II antigens. This appeared to be correlated with intracellular mRNA level. HL-60 lacked detectable HLA class II mRNA. IFN-gamma led to appearance of these transcripts, which were canceled by the cross-linking of FcR. Therefore, experiments were designed to investigate the intracellular signaling molecules regulating the appearance of HLA class II molecules or transcripts. The expression of HLA class II antigen induced by IFN-gamma was blocked by a calmodulin antagonist, W-7, but not by a protein kinase C (PKC) inhibitor, H-7. Furthermore, a direct activator of PKC, phorbol myristate acetate, was not able to induce the HLA class II antigen expression. These results suggest that IFN-gamma induces HLA class II antigens on HL-60 cells via a calcium-calmodulin pathway and not via a PKC pathway. Calmodulin is activated by a transient rise in the cytosolic free calcium. In fact, the measurement of calcium influx into HL-60 cells showed that a remarkable and time-dependent calcium accumulation was caused by IFN-gamma, and that depletion of Ca2+ from culture medium resulted in failure of IFN-gamma to induce class II antigen expression. Furthermore, calcium ionophore, A23187, by itself induced HLA class II antigen expression. These results suggest that IFN-gamma stimulates calcium influx and activates the calmodulin branch of the calcium messenger system, resulting in the induction of class II antigen expression on HL-60 cells. On the other hand, cross-linking of FcR elicited the accumulation of intracellular cAMP, which appeared to suppress the IFN-gamma-induced calcium influx, resulting in annulling HLA class II antigen-inducing activity of IFN-gamma. These intracellular events of HL-60 regulate the expression of HLA class II transcripts and molecules.