Mechanisms of immunity in typhus infections. 3. Influence of human immune serum and complement on the fate of Rickettsia mooseri within the human macrophages.

Preincubation of Rickettsia mooseri with human typhus convalescent serum, which is not rickettsiacidal but which confers passive protection to animals, opsonizes the rickettsiae for enhanced phagocytosis by monocyte-derived human macrophages in cell culture and renders them susceptible to destruction within the macrophages. Nonspecific opsonization by preincubation of the rickettsia with methylated bovine serum albumin enhances phagocytosis, but the rickettsiae are not prepared for intracellular destruction. Instead, they grow within the macrophages and eventually destroy these cells. Thus, immune serum and macrophages, neither of which is capable of killing these rickettsiae alone, act in concert to destroy the virulent organisms. In this system, immune serum appears to exert two distinct, possibly dissociable, actions on the rickettsiae: enhancement of phagocytosis and preparation for intracellular destruction. Complement is not required for this action but, when present with immune serum, markedly enhances phagocytosis of the rickettsiae, often leading to rapid destruction of the macrophage.