Tumor-induced changes in T cell mitogen responses in rats: suppression of spleen and blood lymphocyte responses and enhancement of thymocyte responses.

Concanavalin A (Con A) and phytohemagglutinin (PHA) responses of spleen and blood lymphocytes from tumor-bearing (TB) rats were found to be markedly depressed in 4 different models employing tumors of spontaneous origin. Removal of phagocytic cells from both spleen and blood lymphocyte suspensions led to a complete restoration of the responses, indicating that the decreased responses were not due to intrinsic defects in the lymphocytes. The reduction was shown to be due to the inhibitory effect of an increase in the percentage of phagocytic cells. In addition, TB induced an atrophy of the thymus and a decrease in the number of thymic lymphocytes, mainly due to severe lymphocyte depletion in the cortex. The cells that remained in the thymus exhibited increased responsiveness to PHA and Con A as compared to thymus cells from normal rats. Similar results were found in hydrocortisone acetate-treated rats, suggesting that TB leads to a decrease in nonresponsive, cortical corticosteroid-sensitive thymocytes.