Immunosuppression induced by nitric oxide and its inhibition by interleukin-4*
Mice immunized with attenuated Salmonella typhirnurium, 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 Tcell mitogens. Suppression of antibody responses is mediated by macrophage (Ma)-released soluble factors, and is completely reversed by treatment with interleukin (1L)-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 NG-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 antiinterferon (1FN)-y monoclonal antibody (mAb), also abrogated suppression. Optimal reversal of suppression was observed only when NMLA, IL-4, or anti-IFN-y mAb, was added at day 0 of the 5-day plaque-forming cell assay. Treatment with either IL-4 or anti-IFN-y mAb also lead to a sharp inhibition of NO production by immune spleen cells. Moreover, the addition of IL-4 to splenic adherent M@ inhibited their ability to generate NO. Our data characterize an immunoregulatory pathway, involving IFNy and NO, by which M a mediate immunosuppression and identify IL-4 as a potent inhibitor of this pathway.