Suppressor T cells and host resistance to tye 111 pneumococcus after treatment with antilymphocyte serum

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Abstract
The antibody response to type III pneumococcal polysaccharide (SSS-III) was significantly increased in mice treated with [rabbit] antilymphocyte serum (ALS). BALB/c mice given 0.25 ml of ALS on days -1, 0, and 1 relative to the days of immunization with 0.5 .mu.g of SSS-III had a 20-fold increment (11,383 increased to 199,917) in the number of splenic plaque-forming cells enumerated on day 5 compared with untreated, immunized controls. This effect was attributed to the elimination of a subpopulation of thymus-derived (T) lymphocytes that had suppressor function. The present series of experiments relate the augmented antibody response to SSS-III in mice treated with ALS to increased host resistance after infection with Streptococcus pneumoniae, type III (Pn-III). The 50% lethal dose of Pn-III in nonimmunized mice was 102 and the 100% lethal dose was 103 organisms. Mice immunized with 0.5 .mu.g of SSS-III and challenged 5 days later with Pn-III were completely protected against a dose of up to 108 organisms. Mice treated with 0.25 ml of ALS on days -1, 0, and 1, immunized with SSS-III on day 0, and challenged with 2.5 .times. 109 Pn-III on day 5 had a mean survival time of > 100 h compared with 16 h for immunized non-serum-treated controls. Animals given a single injection of ALS before immunization showed no increase in resistance; mice treated after immunization had significant prolongation of survival times. Untreated, immunized mice challenged with 5 .times. 109, 1 .times. 109 or 5 .times. 108 Pn-III survived 14-19 h; ALS-treated animals had mean survival times of 48, 174 and 222 h, respectively. Immunoregulatory T cells may have a biologically significant effect in a narrow zone in which the normal host immune response is insufficient but still potentially capable of providing some additional degree of protection if suppressor cells are eliminated.