Tissue cages for study of experimental streptococcal infection in rabbits III. Influence of immunization with erythrogenic toxin type A (ET A) and its toxoid on subsequent infection with an ET A producing strain

Purified erythrogenic toxin type A (ET A) and the corresponding toxoid, prepared by formalin treatment, were used for the immunization of rabbits (200 μg per rabbit). The impact of anti-erythrogenic toxin and toxoid immunity was investigated under the conditions of experimental infection with the ET A-producing Streptococcus pyogenes strain SF 130 (type 1). Whereas non of the immunized rabbits (n = 14) died after infection, 40% of nonimmunized animals did not survive (Table 1). The increase of the spleen weight after infection was significantly smaller in the immunized groups (Table 2). The immunized rabbits responded after infection with a significantly lower increase of fever which did not exceed 0.8 °C (2°C in infected non-immunized animals).

Humoral antibodies to ET A were detected after immunization by means of ELISA. The challenge infection acted as a booster leading to a further increase of antibodies.

The antibodies were found to be neutralizing the nonspecific mitogenicity of ET A in vitro in relation to the antibody titer. Cell-mediated immunity was tested in the lymphocyte transformation reaction with peripheral lymphocytes. The nonspecific mitogenicity of ET A, ET B, ET C and Con A was pronounced after immunization, whereas the nonimmunized rabbits responded to these antigens to a lower degree. The toxoid was found to be non-mitogenic. The altogether higher lymphocyte stimulation was also observed using spleen lymphocytes of immunized animals after infection.