Suppressive effects of the bacterial immunostimulant OK-432 on the incidence of spontaneous thymic lymphoma in AKR mice.

The mean survival age of female AKR/J mice was significantly prolonged, the enlargement of thymus was markedly suppressed, and the proliferation of ecotropic and recombinant murine leukemia viruses was inhibited when 2-month-old female AKR/J mice were injected intraperitoneally with attenuated Streptococcus pyogenes, strain Su (OK-432) twice weekly for 8 weeks. However, these effects of OK-432 in 2-month-old female AKR/J mice were not seen in 5-month-old female AKR/J mice. The difference in the effectiveness of OK-432 in these animals probably depends on the difference in the degree of the proliferation of ecotropic and recombinant murine leukemia viruses in the thymus which consequently lead to thymic lymphoma.