In this study, we examined the mechanisms by which avian reovirus infection of chickens depresses in vitro proliferative responses of spleen cells to T cell mitogens. We showed an enhanced production of nitric oxide (NO) by phytohemagglutinin (PHA)-stimulated spleen cells from reovirus-infected birds but not from virus-free birds. Since macrophages are a primary source of NO, we compared splenic adherent macrophages from virus-free and virus-exposed chickens. There was a fourfold increase in the number of adherent macrophages from the spleens of virus-exposed chickens. Production of NO by macrophages from virus-exposed chickens required T-cell-produced factors and was not due to direct stimulation of macrophages by PHA. Although T cell products were needed for NO production by macrophages, in an apparent paradox, we found significantly reduced levels of NO-inducing activity in the supernatants of PHA-stimulated spleen cells from virus-exposed chickens than in supernatants from PHA-stimulated normal spleen cells. Cocultures of adherent cells from infected chickens with normal spleen cells indicated that although macrophages secreted NO following PHA stimulation, macrophages ultimately suppressed the continued production of NO-inducing factors by normal spleen cells. We further showed in experiments utilizing NG-monomethyl-L-arginine, an NO synthesis inhibitor, that NO was not responsible for the mitogenic inhibition of spleen cells from virus-exposed chickens. In summary, our results indicated that following reovirus infection, macrophages are primed in vivo and activated in vitro by T-cell-produced factors. Despite the requirement of T cell cytokines for NO production, T cells did not proliferate to mitogenic stimuli, which indicated that the early events (i.e., cytokine secretion) but not the late events (i.e., proliferation) of the T cell activation cascade were functional. Macrophage priming following reovirus infection may have important implications for impaired T cell responsiveness.
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