Immunosuppression induced by nitric oxide and its inhibition by interleukin‐4

Abstract

Mice immunized with attenuated Salmonella typhimurium, strain SL3235, while protected against virulent challenge, are unable to mount in vivo and in vitro antibody responses to non-Salmonella antigens, such as tetanus toxoid and sheep red blood cells, and exhibit profoundly suppressed responses to B and T cell mitogens. Suppression of antibody responses is mediated by macrophage (Mϕ)-released soluble factors, and is completely reversed by treatment with interleukin (IL)-4. The present report identifies the suppressor factor as nitric oxide (NO), and provides evidence for a mechanism by which IL-4 abrogates suppression. Suppressed antibody responses correlated with high levels of NO secretion by splenocytes of SL3235-immunized mice. NO production was observed only in cultures consisting of the adherent cell fraction of immune splenocytes. Further, immunosuppression was reversed by N^G-monomethyl-L-arginine (NMLA), a competitive inhibitor of NO synthesis, and was completely blocked by the addition of excess L-arginine. Treatment with IL-4, or anti-interferon (IFN)-γ monoclonal antibody (mAb), also abrogated suppression. Optimal reversal of suppression was observed only when NMLA, IL-4, or anti-IFN-γ mAb, was added at day 0 of the 5-day plaque-forming cell assay. Treatment with either IL-4 or anti-IFN-γ mAb also lead to a sharp inhibition of NO production by immune spleen cells. Moreover, the addition of IL-4 to splenic adherent Mϕ inhibited their ability to generate NO. Our data characterize an immunoregulatory pathway, involving IFN-γ and NO, by which Mϕ mediate immunosuppression and identify IL-4 as a potent inhibitor of this pathway.