Effect of a protein-bound polysaccharide preparation, PS-K, on dimethyl hydrazine induction of intestinal tumors in rats.

The effect of a protein-bound polysaccharide preparation, PS-K, on the induction of intestinal tumors by dimethyl hydrazine (DMH) was assessed in Wistar rats. One hundred and fifty-one rats were randomly divided into two groups. Seventy-two rats were treated with DMH alone and 79 rats were treated with DMH and PS-K. All animals were subjected to a sequential autopsy and all lesions within the gastro-intestinal tract were examined macroscopically and histologically. Tumor incidence in the DMH plus PS-K-treated group was significantly lower than that in the group treated with DMH alone. The most interesting histological finding was marked lymphoid infiltration in and around the tumors of the rats in the PS-K-treated group. The number of circulating lymphocytes dropped below the control range in the 25th and 35th weeks for the group treated with DMH alone, but the drop in the PS-K-treated group was smaller. Delayed-type hypersensitivity reaction to purified protein derivative was well maintained in the PS-K-treated rats. The most interesting findings in these experiments were differences in the serum blocking activities and serum immunosuppressive substance in these two groups; they were markedly reduced in the PS-K-treated rats. The present results may be explained in terms of competitive action of PS-K against the immunosuppressive factor produced by a tumor-bearing host.