Effect of immunosuppression on Rickettsia rickettsii infection in Guinea pigs

The role of the immune response in the pathogenesis of Rickettsia rickettsii infection in guinea pigs was investigated by immunosuppression, using antilymphocyte serum. Twenty guinea pigs were inoculated with R. rickettsii, Sheila Smith strain, on day 0. Fifteen animals received antilymphocyte serum on days -1, 0, 2, 4 and 6. Five animals received normal rabbit serum on the same schedule. At necropsy, specimens were collected for histological examination, rickettsial immunofluorescence, rickettsial titration, and antirickettsial antibody titration. All normal rabbit serum recipients and 12 of 15 antilymphocyte serum recipients developed typical disease. Comparison of animals in terminal stages of disease revealed the same clinical course and gross lesions, but differing rickettsial burden and cellular response. Immunosuppressed animals had higher titers of splenic rickettsiae and greater numbers of immunofluorescent rickettsiae. Thus, although antibody was undetectable in both groups, there appeared to be an inhibition of antirickettsial immunity. Microscopic vasculitis was similar quantitatively, but differed qualitatively, with immunocompetent animals having the typical mononuclear/lymphocytic inflammation and immunosuppressed animals having neutrophilic predominance. This study demonstrates that immunopathological mechanisms are not necessary for the pathogenesis of experimental Rocky Mountain spotted fever. The rickettsiae themselves seem capable of causing cellular and tissue damage.