Development of antigen-specific cell-mediated immune responses after infection of cynomolgus monkeys (Macaca fascicularis) with Rickettsia tsutsugamushi.

Cynomolgus monkeys were evaluated for cellular immune responses after infection with the Karp strain of Rickettsia tsutsugamushi. Antibody and clinical signs of localized and systemic infection were also evaluated. Animals challenged with homologous or heterologous strains at various times after a primary infection were also followed up. Naive monkeys developed eschars, lymphadenopathy, rickettsemia, and elevated body temperatures. Antibody in these animals was IgM followed by IgG. Lymphocyte proliferation and production of gamma-interferon by peripheral blood mononuclear leukocytes also were demonstrated. If challenged six years after the initial infection, clinical signs and cellular responses were indistinguishable from naive animals but an anamnestic IgG antibody response was noted. If challenged eight months after the initial infection, complete resistance was noted, but if challenged at one year, a localized cutaneous lesion developed. The majority of animals infected previously had preexisting lymphocyte activity, a characteristic suggesting long-term immunologic memory that was not protective against rechallenge.