The role of the
nitric oxide synthase (NOS) pathway in inhibiting the ability of Rickettsia prowazekii to initially infect (invade) mouse
cytokine-treated, fibroblastic L929 cells and macrophagelike RAW264.7 cells and the ability of
nitric oxide (NO) to damage isolated rickettsiae were investigated. Substantial amounts of
nitrite (a degradation product of NO) were produced and the initial rickettsial
infection was suppressed in cultures of L929 cells treated with crude lymphokine preparations (LK) or with
gamma interferon (IFN-gamma) plus
tumor necrosis factor alpha (
TNF-alpha) but not in L929 cell cultures treated with IFN-gamma alone or
TNF-alpha alone. The NOS inhibitors N(G)-methyl-
L-arginine and
aminoguanidine both inhibited
nitrite production and prevented the suppression of the initial rickettsial
infection. Antibody-mediated neutralization of the IFN-gamma in the LK also inhibited both
nitrite production and suppression of the initial rickettsial
infection. Cultures of RAW264.7 cells treated with IFN-gamma plus
lipopolysaccharide exhibited suppression of the initial rickettsial
infection, and the suppression was relieved by
aminoguanidine. Addition of
oxyhemoglobin (a scavenger of extracellular NO) during the rickettsial
infection alleviated the suppression of the initial rickettsial
infection observed in appropriately treated L929 cells and RAW264.7 cells. In addition, the
oxyhemoglobin restored the rickettsia-mediated, rapid killing of the treated RAW264.7 cells. Incubation of isolated rickettsiae with NO inhibited their ability to infect L929 and IFN-gamma-treated RAW264.7 cells and to rapidly kill IFN-gamma-treated RAW264.7 cells. In contrast, incubation of L929 cells with a
solution that contained NO and/or degradation products of NO did not affect their ability to be infected by rickettsiae. The data are consistent with the hypothesis that NO released from appropriately stimulated potential host cells kills extracellular rickettsiae and thus prevents the rickettsiae from infecting the cells.