Host defenses in experimental scrub typhus: role of normal and activated macrophages.

Resident peritoneal macrophage from BALB/c mice were infected in vitro with Rickettsia tsutsugamushi strain Gilliam, and rickettsial growth was estimated by microscopic examination of Giemsa-stained cells. Both number of infected macrophage per culture and number of intracellular rickettsiae per cell increased with time during culture. Treatment of rickettsiae with immune serum before infection macrophage cultures reduced the number of infected macrophage by 50%. Macrophage treated in vitro with lymphokines were able to suppress rickettsial growth in the absence of detectable antibody and exhibited a 75% reduction in infection compared with normal macrophage. We also obtained activated macrophage from immune mice and found that they were refractory to in vitro rickettsial infection. Macrophage populations activated in vitro or in vivo contained a small percentage of cells which supported unrestrained growth of rickettsiae. These data suggest that an early immunological event in experimental scrub typhus infection may be the development of activated macrophage capable of suppressing rickettsial proliferation before the appearance of circulating antibody.