Detection and Enumeration of Immunocytes Producing Different Classes of Vibriolysins after Primary and Secondary Immunization with Cholera Vaccine

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Abstract
The vibriolytic plaque-forming cell (PFC) assay was used to study the kinetics of the primary and secondary immune responses of mice and rabbits immunized with heat-killed cholera vibrios. Immunocytes releasing IgG antibody could be detected as readily as those immunocytes secreting IgM antibody in spleens of BALB/c mice and New Zealand White rabbits after a single injection of vaccine. Peak numbers of indirect (IgG) PFC were detected 3 to 4 days after the peak direct (IgM) PFC response (12 to 14 days). In contrast, only direct vibriolytic PFC were detected in spleens of NIH Albino mice during the primary response to cholera antigens. After a second injection of vaccine, IgM, IgG, and IgA PFC were detected in both mouse strains with peak responses for each immunocyte class occurring within the first week after booster injection. Heat-killed vibrios or a lipopolysaccharide (LPS) extract, but not cholera exotoxin or E. coli LPS, inhibited the vibriolytic response. Furthermore, viable cholera vibrios were a more sensitive indicator cell in the immunoassay than sheep erythrocytes sensitized with a cellular extract from heated vibrios.

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