Vibriolytic IgG immunocyte response of mice after primary and secondary immunization with cholera somatic antigens

Antibody plaque-forming cells (FC) to the somatic antigens of Vibrio cholerae were enumerated in the spleen of mice after primary and secondary immunization with a heat-killed vaccine prepared from the vibrios. Immunocytes releasing both high efficiency IgM and low efficiency IgG antibody were readily detected using a direct and facilitated plaque procedure in agar gel. Whereas the peak numbers of IgM-PFC after primary immunization occurred on days 12 to 14, the peak IgG-PFC response developed somewhat later (16-18 days). After a second injection of vaccine larger numbers of both IgM- and IgG-PFE appeared in the mouse spleens, with peak responses for both occurring between days 5 and 8. The largest number of IgG-PFC developed in spleens of mice given a second injection of vaccine 6-8 weeks after primary immunization. The dose of killed vibrios used for priming markedly affected both the magnitude and the class of antibody-forming cells appearing during the secondary response; 1--10 mug vaccine was more effective than higher or lower doses for priming the mice to a heightened secondary response. Furthermore, the antigenic specificity of both the IgM- and IgG-PFC appearing after secondary immunization was directly related to the strain of cholera bacilli used for priming. When mice were immunized with the Ogawa strains of cholera most of the secondary PFC after booster immunization with the serologically distinct Inaba strain was directed towards the common antigen shared by both strains and not to the type specific antigen of the Inaba vibrios. The specificity of the anti-vibrio PFC during both the primary and secondary responses was readily demonstrable by inhibition experiments using sonicated or soluble cholera antigens. Prior incubation of these antigens with test spleen cells in the agar gel effictively inhibited development of the vibriolytic plaques, regardless of antibody class. Similar antigen extracts from toher bacteria had no effect. The immunoglobulin nature of the plaques was also demonstrable by inhibition with low dilutions of rabbit anti-mouse globulin serum incorporated into the agar plates prior to testing; both IgM and IgG plaues were inhibited.