The peak plaque-forming-cell (PFC) and serum antibody responses of diabetic mice to type III pneumococcal capsular polysaccharide (S3) were delayed compared with normals. Proliferation of PFC precursors was not inhibited in an insulin-deficient environment. The delay in the PFC response to S3 did not occur in diabetic nude mice but was demonstrable in their thymus-bearing heterozygote littermates. Therefore, T-cells appear to mediate the delay in the response of diabetic mice to S3 probably by delaying their differentiation into PFC. Diabetic mice responded normally to the induction of low-dose tolerance to S3, indicating the presence of active suppressor T-cells (Ts) in these mice. However, inactivation of Ts by anti-lymphocyte serum (ALS) required a higher dose in the diabetic mice. Furthermore, inactivation of Ts by ALS totally abolished the delay in peak PFC response. These findings suggest that the delayed PFC response to S3 in diabetic mice was the result of excessive splenic Ts activity. In peripheral blood, diabetic mice appeared to have more amplifier T-cell activity or less suppressor T-cell activity than normals. This response was normalized by insulin treatment.

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