Strain 13 guinea pigs with a transplanted, 3-methylcholanthrene induced fibrosarcoma produced a plasma factor that inhibited PHA induced blastogenesis of lymphocyte cultures from normal and tumor animals. The suppressive activity was not cytotoxic, was not adsorbed from plasma by lymph node cells, and was a noncompetitive inhibitor of PHA activation. The tumor plasma could not inhibit the uptake of [3H]thymidine if added during the period of maximal DNA synthesis, which suggested that the suppressive factor acts most effectively on metabolically resting cells. This nonspecific suppressive factor may represent an important mechanism for allowing rapidly progressive tumors to escape immunological destruction.
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