SUPPRESSIVE EFFECTS OF LACTOBACILLUS-CASEI CELLS, A BACTERIAL IMMUNOSTIMULANT, 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 prolifera tion of ecotropic and recombinant murine leukemia viruses (MuLV) was m arkedly inhibited when 8-week-old female AKR/J mice were injected intr aperitoneally (i. p.) with heat-killed Lactobacillus casei cells twice weekly for 8 weeks. In contrast, such actions of heat-killed L. casei cells were not seen in 20-week-old female AKR/J mice. The leukemogeni c activity of the cell-free extract of thymus from adult female AKR/J mice in newborn female AKR/J mice was drastically reduced by i. p. tre atment with heat-killed L. casei cells. The difference in adjuvant eff ectiveness of heat-killed L. casei cells on 8- and 20-week-old animals may be dependent on the difference in the enhancing activity of the c ell-mediated immune systems between the groups induced by heat-killed L. casei cells, and, as a result, on the difference in the degree of p roliferation of ecotropic and recombinant MuLV in thymus, which conseq uently causes thymic lymphoma.