Unique features of the primary site of rickettsial replication in typhus fevers, i.e., within the endothelial cells of small blood vessels in tissues, suggest that effector mechanisms, other than those dependent on phagocytosis by activated macrophages with enhanced microbicidal properties, most likely are necessary to explain the cell-mediated immune control of intracellular rickettsial replication in these sites. Theoretically, such mechanisms might involve contact between infected endothelial cells and activated T lymphocyte subpopulations or macrophages or immunologically induced soluble factors or lymphokines. Support for the existence of at least one of these alternative effector mechanisms is presented here for Rickettsia prowazekii. Cultures of human blood leukocytes, upon immunologically specific stimulation with R. prowazekii antigen or nonspecific stimulation with the mitogen phytohemagglutinin, produce soluble factor(s) in the supernatant fluid which, in culture, have (a) an intracellular antirickettsial action on R. prowazekii-infected human endothelial cells, fibroblasts, and macrophages, and (b) a specific cytolytic action on R. prowazekii-infected, but not uninfected bystander, human fibroblasts. Neither action is demonstrable in R. prowazekii-infected chicken embryo fibroblasts. The factor(s) has no direct antimicrobial action on extracellular rickettsiae and is inactivated by heating at 56 degree C for 1 h or by acid treatment at pH 2. Expression of the antirickettsial action requires new host cell messenger transcription and protein synthesis, whereas the cytolytic action does not. The circumstances of production and action and the properties of the factor(s) responsible for the intracellular antirickettsial, and perhaps also the cytolytic action are consistent with those of immune interferon (IFN-gamma).

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