An intracellular action for IFN-gamma was detected by using microinjection technology. Human IFN-gamma (huIFN-gamma) does not ordinarily act on murine cells because it fails to bind to murine cell surface receptors. However, when huIFN-gamma was microinjected into murine macrophages, a time and dose-dependent induction of Ia was detected by autoradiography on the surface of injected and neighboring cells. These results imply a direct role for internalized IFN-gamma and show that huIFN-gamma, although it fails to be recognized by murine cell surface receptors, can act internally on murine cells. The effect on Ia gene expression induced by microinjected huIFN-gamma was in part indirect: granulocyte/macrophage-CSF (GM-CSF) was released by IFN-gamma-injected macrophages, and this secondary mediator appeared to induce Ia on neighboring cells, inasmuch as anti-GM-CSF blocked Ia induction. Anti-GM-CSF also partially blocked Ia induction by extracellular murine IFN-gamma on murine macrophages. Thus, at least some of the Ia induction attributed to IFN-gamma was mediated by GM-CSF.

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