Inhibition of recombinant interferon-gamma-induced Ia antigen expression by shed B16 F10 melanoma cell membrane vesicles.

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Abstract
The expression of immune region-associated (Ia) antigens by macrophages is a prerequisite for antigen presentation, which is necessary for the activation of T helper cell function. A decrease in macrophage Ia expression is associated with a decrease in immune function in vitro. However, the effect of diseases accompanied by immunosuppression, such as cancer, on macrophage Ia expression has not been studied. The expression of Ia antigen was induced by the culture of murine peritoneal macrophages with recombinant interferon-gamma (IFN). Maximal expression was achieved after 4 days of culture. Membrane vesicles shed from the murine B16 F10 melanoma cell line inhibited the in vitro induction of Ia expression by 40 to 90% in allogeneic and syngeneic systems. Inhibition was not due to toxicity, a reduction in IFN activity, phagocytosis or contamination of the vesicle preparation with endotoxin, which is an inhibitor of Ia expression. Inhibition exerted by vesicles was prostaglandin-dependent and was over-come by increasing concentrations of IFN. It is possible that the reduction of macrophage Ia antigen expression by tumor cell products, such as shed membrane vesicles, contributes to the immunosuppression of tumor-bearing hosts. Employing IFN to reverse the inhibition provides a strategy for improving the therapy of patients with cancer.