Characterization of the murine immune response to type 6 pneumococcal polysaccharide.

The antibody response to type 6 (Danish type 6A) pneumococcal capsular polysaccharide (S6) was determined in various strains of mice. S6 elicited a very low plaque-forming cell (PFC) response in most strains of mice. BALB/c mice responded better than most other strains, and the response to S6 was further characterized in BALB/c mice. Immunization with 1 to 5 micrograms of S6 induced a plaque-forming cell response of short duration which consisted totally of immunoglobulin M-producing PFC. Athymic nude mice produced three- to fivefold more PFC in response to S6 than their euthymic littermates, suggesting that thymus-derived (T) cells negatively influence the S6 response. Regulation of the response by T cells was further suggested by the ability of antilymphocyte serum to enhance the S6 PFC response and by the suppressed response induced by priming with a low dose of S6 3 days before immunization with an optimal dose of S6 (low dose paralysis). When concanavalin A was given 2 days after immunization with S6, the response was enhanced two- to sevenfold, suggesting that the response is also positively influenced by T amplifier cells. When the avidity of the antibody produced by S6-specific PFC was measured by a plaque inhibition test, the avidity of the anti-S6 antibody was found to be very low. These results suggest that S6 is a poor immunogen because the affinity of the S6-specific antibody and B cell surface receptors is low.