Evaluation of the immune response to type III pneumococcal polysaccharide as a means to evaluate T-independent immune function in the rat.

Since the rat is frequently the experimental animal of choice for toxicology testing, studies were undertaken to adapt assays routinely used to evaluate immune function in mice so that immune function could likewise be evaluated in collaborative projects employing toxicant-treated rats. Contrary to previous reports in the literature, Type III pneumococcal polysaccharide (S3) was immunogenic in rats. Specific antibody responses to S3 were demonstrated in two strains of rats following immunization by either the subcutaneous (sc) or intraperitoneal (ip) route with purified S3, with S3 contained in polyvalent pneumococcal polysaccharide vaccine (pneumovax), or with heat-killed Type III Streptococcus pneumoniae. Dose-response studies demonstrated that the optimal immunizing dose in Sprague-Dawley or Fischer rats was 25 micrograms S3. Reimmunization with S3 on d 21 did not produce an anamnestic response, and the kinetic data were consistent with S3 being a thymus-independent (T-independent) antigen in the rat. In contrast to our previous studies in the mouse, concurrent sc or ip injections of pertussis vaccine did not modify the response to S3 in rats. Sprague-Dawley rats acquired the capacity to respond immunologically to S3 between 24 and 31 d of age. In mature animals, sex had no effect on the ability to respond to S3. The utility of this model as a means of characterize toxicant-induced immune dysfunction was demonstrated using the prototype immunotoxicant cyclo-phosphamide.