Thymostimulin (TS), a partially purified thymic factor, has a significant impact on
tumor development in C57B1/6 mice inoculated with
Lewis lung carcinoma (3LL) cells, as judged by its effect on time of
tumor appearance after
tumor cell transplantation. In a previous study, we determined the conditions under which survival rate of the
tumor-bearing mice can be significantly increased by TS treatment. In the study communicated here we analyzed host defense mechanisms that are modified by TS treatment in the
tumor-bearing mice. In general, immune parameters that were increased or stimulated by the presence of the
tumor were further increased in the TS-treated animals (number of lymphoid spleen cells, their response in mixed lymphocyte
tumor cultures, their natural killer cell activity, and their ability to produce
colony-stimulating factor), or reached earlier maximum levels (spontaneous [3H]
thymidine incorporation, a reflection of in vivo spleen cell activation). Responses which reflect
tumor-induced immunosuppression (proliferative response induced by
phytohemagglutinin or
concanavalin A stimulation) were restored to normal level by TS. Specific
tumor-related reactions (specific cell-mediated cytotoxicity) were preserved in the TS-treated animals. The wide spectrum of TS effects had, nevertheless, certain elements of selectivity; e.g.
colony-stimulating factor, but no
interferon production is enhanced by TS in the
tumor-bearing mice in diametric contrast to TS effect in Mengo virus-infected mice. The spectrum of TS effects was also dependent on the type of
tumor cell used. The results indicate that the significant effect of TS on 3LL
tumor development in mice is associated with a strong, multifaceted effect of TS on the immune system.