Humoral immune response to Rocky Mountain spotted fever in experimentally infected guinea pigs: Immunoprecipitation of lactoperoxidase ¹²⁵I-labeled proteins and detection of soluble antigens of Rickettsia rickettsii

Rickettsia rickettsii, the etiologic agent of Rocky Mountain spotted fever, purified from infected L-929 cells by density gradient banding were extrinsically radioiodinated with lactoperoxidase. Immunodominant ¹²⁵I-labeled antigens were identified by radioimmunoprecipitation of detergent-solubilized antigens with protein A-Sepharose and anti-R. rickettsii sera collected 0, 3, 7, 11, 32, and 163 days after infection of guinea pigs. The average fever ≥40°C was detected by days 3 and 4 after infection with 6 x 10⁷ and 6 x 10⁶ PFU, respectively. By microagglutination and complement fixation assays, anti-R, rickettsii antibodies were detected as early as day 3 after infection, with titers increasing markedly between days 7 and 163. Convalescent sera, collected on day 163, from infected guinea pigs were used to identify seven ¹²⁵I-labeled antigens with apparent molecular sizes of 186,000 (I), 145,000 (II), 49,000 (III), 32,000 (IV), 27,500 (V), 17,500 (VI), and 16,500 (VII) daltons. Differences in antibody reactivity and specificity against the seven antigens were demonstrated with serially obtained sera. Sera from a guinea pig infected with 6 x 10⁷ PFU exhibited antibody-antigen interactions with all seven ¹²⁵I-labeled antigens by day 7, whereas the same antibody activity required 32 days for an animal infected with 6 x 10⁶ PFU. Prominent antibody activities toward proteins II and IV were demonstrated both early and late after infection. The fluids obtained from infected L-929 cells contained three soluble antigens which were detected with the 11-, 32-, and 163-day sera by an immunodiffusion assay. The soluble and ¹²⁵I labeled antigens of R. rickettsii identified in this study may be important candidates for vaccines against Rocky Mountain spotted fever.