In a murine model of rickettsial disease in which, as in human rickettsioses, endothelial cells are the major target of infection, depletion of IFN-gamma or TNF-alpha converts a sublethal infection into a uniformly fatal disease with overwhelming rickettsial growth and decreased nitric oxide (NO) synthesis. The kinetics of NO production and rickettsial survival and growth were examined on Days 1, 2, and 3 after inoculation of endothelial cells with Rickettsia conorii under four different experimental conditions: (a) no cytokine treatment, (b) treatment with IFN-gamma and TNF-alpha, (c) treatment with cytokines and NG monomethyl-L-arginine, a competitive inhibitor of NO synthesis, and (d) treatment with sodium nitroprusside, a source of NO. Endothelial cells were examined for the presence of inducible nitric oxide synthase mRNA by specific reverse transcriptase-PCR after stimulation with IFN-gamma and TNF-alpha. Cytokine-stimulated and unstimulated rickettsiae-infected endothelial cells were examined by electron microscopy to observe the cellular and rickettsial events. Transformed and diploid mouse endothelial cells stimulated by the combination of recombinant murine IFN-gamma and TNF-alpha killed intracellular Rickettsia conorii by a mechanism that required the synthesis of NO. The antirickettsial effect and NO synthesis were inhibited by treatment of endothelial cells with NG monomethyl-L-arginine. Addition of nitroprusside, which released NO, also exerted a strong antirickettsial effect in the absence of IFN-gamma and TNF-alpha. Endothelial inducible nitric oxide synthase mRNA was detected 4 hours after cytokine stimulation, increased substantially at 8 hours, and decreased to low levels by 72 hours. Ultrastructural evaluation revealed that endothelial cells effected rickettsial killing in association with autophagy. Double membranes of endothelial cell granular endoplasmic reticulum surrounded rickettsiae, which were also observed being destroyed within phagolysosomes. This study demonstrated for the first time that endothelial cells are capable of killing rickettsiae. When stimulated by the combination of IFN-gamma and TNF-alpha, mouse endothelial cells kill Rickettsia conorii by an NO-dependent mechanism. Within the endothelium, NO exerts a rickettsicidal effect.
