Nitric oxide mediates immunosuppression induced byListeria monocytogenesinfection: quantitative studies

Abstract

Our laboratory has shown that immunization of mice with an attenuated strain ofSalmonella typhimuriuminduces profound suppression in the capacity of splenocytes to mount anin vitroantibody plaque-forming cell (PFC) response to sheep red blood cells (SRBC) and to proliferate in response to mitogens.In vitroaddition of NG-monomethyl-L-arginine (NMMA), an inhibitor of nitric oxide (NO) synthase, to cell cultures fromSalmonella-immunized mice completely blocked suppression of the PFC responses, implicating that NO is the suppressor factor. The present study quantified the role of nitric oxide in immunosuppression induced byListeria monocytogenes, a gram positive intracellular pathogen of macrophages.Listeriainfection resulted in suppression of the PFC assay at inoculating doses of greater than 6.5×103colony forming units, with no suppression observed at lower doses. Suppression correlated with increased nitrite production. Addition of NMMA to spleen cell cultures taken fromListeria-infected mice completely blocked suppression of the PFC response, and returned nitrite production to baseline levels. In regard toListeria-induced suppression of responses to the mitogen, Concanavalin A (Con A), the parameters were different from those observed for the PFC response. There was a direct correlation between the log10of the inoculating dose ofListeriaand degree of immunosuppression, with suppression observed at doses as low as 1×103cells. Addition of NMMA to the Con A-stimulated cultures resulted in reduced nitrite levels, but only partial restoration of the proliferative responses. Co-culture of splenocytes fromListeriainoculated mice with normal splenocytes in media with NMMA and reduced levels of L-arginine resulted in complete reversal of suppressed responses to Con A. Similar differences in ease of reversing suppression of the PFC response, as compared with responses to Con A, were previously noted using cells taken fromSalmonella-infected mice. The present results show that a gram positive intracellular pathogen of macrophages,L. monocytogenes, induces immunosuppression in mouse spleen cells by a nitric oxide mediated mechanism that closely parallels that induced by the gram negative pathogen,S. typhimurium.