Production of gamma interferon in mice immune to Rickettsia tsutsugamushi.

C3H/He mice immunized by subcutaneous infection with Rickettsia tsutsugamushi Gilliam were examined for the production of immune interferon after intravenous administration of irradiated strain Gilliam antigen, in supernatants of immune lymphocytes stimulated with specific antigen, and after a secondary challenge with viable rickettsiae. Mice administered various doses of irradiated whole-organism antigen 28 days after immunization showed circulating levels of interferon which peaked 4 h after inoculation and were antigen dose dependent. The interferon produced was pH 2 sensitive and stable at 56 degrees C for 1 h and was neutralized by antiserum directed against immune, but not against alpha/beta, interferon. The production of another lymphokine, macrophage migration inhibition factor, paralleled that of interferon. The interferon produced by cultures of spleen cells obtained from immune animals was antigen specific and dose dependent. Peak levels were obtained 48 to 72 h after the addition of antigen. The interferon produced by spleen cell cultures after stimulation with Gilliam antigen was characterized as immune interferon by the same physical and antigenic criteria used for serum interferon. Interferon was produced in vitro by the Thy-1.2+ lymphocyte and required the presence of a spleen-adherent cell population. Immune mice produced high circulating levels of immune interferon after intraperitoneal challenge with viable rickettsiae, which suggested a possible role for interferon in the resistance of immune mice to rechallenge with R. tsutsugamushi.