Class-specific plaque-forming cell (PFC) (gammaM, gamma1, gamma2, and gammaA) responses to type III pneumococcal polysaccharide (SSS-III) were studied in BALB/c x C57BL/6F1 (CBF1) mice with and without induction of an allogeneic effect. Gamma1, gamma2, and gammaA PFC were detected in two ways: (a) With the sequential development of the assay slides, first for direct (gammaM)PFC followed by incubation with class-specific antiimmunoglobulin and complement for the development of additional gamma1, gamma2, and gammaA PFC (gammaM-independent gamma1, gamma2, and gammaA PFC); and (b) by blocking gammaM PFC with goat anti-gammaM and simultaneously developing gamma1, gamma2, and gammaA PFC (total gamma1-, gamma2-, and gammaA-secreting PFC). The results showed that whereas gammaM PFC arose on the 3rd d after immunization, gamma1-, gamma2-, and gammaA-secreting PFC arose on the 4th to 5th d after immunization. They appeared in association with gammaM-secreting PFC because they were detected with the gammaM blocking method but not with the sequential method. By the 7th d most gamma1, gamma2, and gammaA PFC were detected by the sequential method as well, indicating that those antibodies were secreted independently of cells secreting gammaM. When the numbers of double-class-secreting PFC were evaluated on the 5th d, the following results were obtained: 83% of gammaM PFC were secreting either gamma1 (25%), gamma2 (55%), or gammaA (2%). We interpret these data as evidence for an antigen-driven class differentiation from gammaM to gammaA and from gammaM to gammaG in the majority of anti-SSS-III-secreting clones without T-cell help. When an allogeneic effect was provided by inoculation of parental BALB/c spleen cells together with antigen, the numbers of all classes of PFC were increased. Furthermore, the frequency of gammaM-gammaG (108%) or gammaM-gammaA (9%) double-class secretors was increased, and gammaM-independent gammaG and gammaA secretors were detected earlier, indicating an overall maturation-promoting effect. In addition, prolonged appearance of gammaA PFC was dependent on the allogeneic effect.

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