Intracellular fate of phase I Coxiella burnetii in guniea pig peritoneal macrophages.

Cultivated guinea pig peritoneal macrophages were infected with radio-labeled phase I Coxiella burnetii in order to assess the intracellular distribution of ingested rickettsiae. Localization of organisms was determined by fractionation of macrophage homogenates by equilibrium density centrifugation on sucrose gradients. Macrophages isolated from either nonimmune or immune guinea pigs and infected with C burnetii opsonized with immune serum yielded equilibrium density distribution for rickettsiae similar to lysosomal enzymes, suggesting sequestration within macrophage lysosomes. To confirm these observations nonimmune or immune guinea pigs were injected with Triton WR-1339 prior to macrophage harvest to decrease the density of macrophage lysosomes. Triton-laden macrophages infected with opsonized rickettsiae resulted in equilibrium density distribution for lysosomal enzymes and organisms in less dense regions of the gradient. In contrast, when either nonimmune or immune macrophages were infected in the presence of normal guinea pig serum, the distribution of labeled rickettsiae in the gradient did not correspond with lysosomes. We conclude that in the absence of immune serum, ingested C burnetii are not sequestered within macrophage lysosomes. Phagolysomal fusion and subsequent degradation of rickettsiae within the lysosomes of the macrophages appear to occur only when C burnetii are opsonized with immune serum.