Interaction between intracellular pathogens of macrophages and the host: Microbe-induced, nitric oxide-mediated immunosuppression and morphine-mediated potentiation of oral infection to Salmonella typhimurium

This thesis addressed three projects, integrated by examination of modulation of host defenses to intracellular pathogens. In the first project, the role of nitric oxide (NO) in modulating immunosuppression was investigated after immunization of mice with an attenuated strain of Salmonella typhimurium , SL3235. Previous work had shown that this organism induced high levels of NO that mediated suppression of antibody formation in vivo and in vitro. In the present studies, aminoguanidine (AG), an inhibitor of NO, was used to block NO production in vivo. Chronic, oral AG treatment blocked the rise in NO normally induced by Salmonella infection, and also blocked the Salmonella-induced immunosuppression as assessed by the capacity to mount an antibody plaque-forming cell (PFC) response to sheep red blood cells, and to respond to the T-cell mitogen, Concanavalin A (Con A). AG-treatment also resulted in mortality of mice from this normally avirulent organism, which correlated with persistent bacteremia, showing that NO is a necessary molecule for antimicrobial host defense. In the second project, the role of NO in immunosuppression induced by virulent Listeria monocytogenes was investigated. Infection of mice resulted in suppression of the PFC response and mitogen responses to Con A. In vitro addition of NG-monomethyl-L-arginine (NMMA), an inhibitor of NO, to spleen cell cultures blocked suppression of the PFC responses. Co-culture of normal and immune splenocytes with NMMA and reduced levels of L-arginine completely restored responses to the mitogen. Collectively these results for Salmonella and Listeria underscore the dual biological consequences of NO production following infection with intracellular pathogens of macrophages, with NO being necessary for host defense, but also having the potentially adverse effect of immunosuppression.; Finally, this thesis examined the effect of morphine on oral infection with virulent S. typhimurium. Morphine markedly sensitized mice to Salmonella, as assessed by survival and colony culture, and the effects of the drug were significantly blocked by the opioid antagonist, naltrexone. These results have implications for potentiation of opportunistic infections in the gastrointestinal tract in intravenous drug abusers and in opioid-medicated post-surgical patients.