Suppressor T cells and host resistance to type III pneumoccocus after treatment with antilymphocyte serum

Abstract

The antibody response to type III pneumococcal polysaccharide (SSS III) was significantly increased in mice treated with 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 μg 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 has been attributed to the elimination of a subpopulation of thymus derived lymphocytes (T cells) that has 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 μg 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 x 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, whereas mice treated after immunization had significant prolongation of survival times. Untreated, immunized mice challenged with 5 x 109, 1 x 109, or 5 x 108 Pn III survived 14 to 19 h, whereas ALS treated animals had mean survival times of 48, 174, and 222 h, respectively. These findings suggest that 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.