The early cellular events occurring during antibody formation to Vibrio cholerae somatic antigens were studied in the mouse spleen using a direct vibriolytic plaque assay in agar gel with viable bacteria as the indicator. Plaque-forming cells (PFC) with specificity for either the common A antigen or the type specific B antigen were distinguished by use of appropriate indicator strains of bacteria. No background of PFC to either of these antigens was detectable in spleens of normal non-immunized mice. Immunization with a relatively low dose of antigen (5 μg) resulted in appearance of the first PFC 42 to 46 hr later. There were consistently 9 to 10 PFC in the spleens of mice at this time, four of which were of anti-A specificity and five to six of anti-B specificity. Further accumulation of the cells was non-linear. There was a fivefold increase in the number of PFC at 48 hr, followed by a plateau between 48 and 60 hrt. A third increase, again about fivefold, occurred at 62 hr. Use of a higher dose of angien (500 μg) resulted in a longer latent period before appearance of the first PFC. This seemed due to a “masking” of PFC by excess antigen. Furthermore, no “staircase” accumulation was observed; in contrast there was a more rapid increase in the number of PFC, with periodic “waves.” The early differentiation of PFC could be inhibited by administration of various metabolic inhibitors of DNA between 24 and 36 hr after immunization. This suggests that DNA synthesis occurs during the latent period. The inhibitors had less, if any, effect when administered earlier after immunization.
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