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Interferon-gamma increases alveolar macrophage Ia antigen expression despite oral administration of dexamethasone to rats.

Corticosteroids have multiple effects on immune and inflammatory responses and decrease host resistance to a broad range of microorganisms. Resident tissue macrophages have been proposed as a target for the immunosuppressive effects of corticosteroids and are important in host defense against infections. During infection-induced immune responses, macrophages are activated after exposure to interferon-gamma (IFN-gamma), and class II major histocompatibility (Ia) antigens on their surface are increased. We investigated the effect of orally administered corticosteroids on alveolar macrophages, the resident macrophages of the lung parenchyma. We hypothesized that corticosteroids would inhibit the activation of alveolar macrophages and measured induction by IFN-gamma of Ia antigens as a marker of cell activation. Alveolar macrophages from normal and corticosteroid-treated rats were exposed to recombinant murine IFN-gamma (rMuIFN-gamma) in vitro and assayed for Ia transcription and surface Ia expression. Ia mRNA accumulation was induced in alveolar macrophages from normal and corticosteroid-treated rats after exposure in vitro to rMuIFN-gamma. Furthermore, rMuIFN-gamma increased surface expression of Ia proteins on alveolar macrophages from corticosteroid-treated rats, although to a lesser extent than on cells from control rats. Finally, surface Ia expression could also be increased in vivo by exposure of corticosteroid-treated rats to an aerosol containing rMuIFN-gamma. These results demonstrate that administration of oral corticosteroids, while establishing a state of immunosuppression in rats, does not abolish responsiveness of rat alveolar macrophages to rMuIFN-gamma. We speculate that IFN-gamma-induced augmentation of phagocytic cell function may constitute an important therapeutic modality to treat complications of immunodeficiency.