Low T- and B-cell reactivity is an apparently paradoxical request for murine immunoprotection against Streptococcus mutans. Murine protection can be achieved by immunization against a B-cell mitogen produced by these bacteria.

C57BL/6 mice thymectomized as adults or depleted of CD4+ cells were much less susceptible than intact conventional mice to the B-cell mitogenic and specific immunosuppressive effects of a protein designated as F5'EP-Sm secreted by Streptococcus mutans. These mice were also considerably more resistant to infection by these bacteria than intact individuals. The immunosuppressor effect of F5'EP-Sm was also abrogated, however, in conventional intact mice when immunized intraperitoneally against heat-inactivated F5'EP-Sm. On the other hand, resistance to bacterial infection could be achieved by immunization of conventional intact C57BL/6 mice against heat-inactivated F5'EP-Sm by intraperitoneal or intradermal routes even when the animals were infected 3 months after immunization and even when the immunization procedure did not include Freund's adjuvant, which was the case with the intradermal route. Interestingly, the protection against the bacterial infection was accompanied by only a minor increase in specific serum antibodies against F5'EP-Sm. These results are discussed in the context of adequate strategies for immunoprotection against Streptococcus mutans and other micro-organisms which are secretors of substances that share both B-cell mitogenic and immunosuppressive properties and which are thus able to suppress the immune response by overstimulation of the immune system of the host.