[Involution of the thymus in tumor-bearing mice and its restoration by PSK (2)].

We examined the effect of PSK on involution of the thymus in tumor-bearing mice. The weight and cell number of the thymus decreased and the size distribution (scatter profile) of thymus cells was changed in X5563-bearing C3H/He mice. Also in these mice, 3H-thymidine incorporation into the thymus was reduced compared with that in control mice, as evaluated not only by per organ but also by per 1 mg of thymus tissue. Such an involution was also observed in tumor-free mice injected with immunosuppressive substance, which had been obtained from ascites of X5563-bearing mice and revealed suppressive activity against lymphocyte proliferation. PSK administration prevented such modulation in the thymus of tumor-bearing mice and immunosuppressive substance-injected tumor-free mice. In other words, it is considered that the various changes in the thymus demonstrated in tumor-bearing mice may be attributable to the suppression of cell proliferation in the thymus, that such suppression is caused at least partly by an immunosuppressive substance which possesses inhibitory activity against lymphocyte proliferation, and that PSK has an antagonistic activity against such a substance so as to restore the function of the thymus in tumor-bearing hosts.