[The role of apoptosis in the thymic involution during growth of the syngeneic transplanted tumor in mice].

Thymic involution accompanies the growth of many human and animals tumors but the precise mechanism of this phenomenon is unknown. We tried to elucidate the role of apoptosis as a possible mechanism of thymic involution during tumor growth. The mice were inoculated subcutaneously with syngeneic hepatoma 22a cells. Starting from the 3rd week after tumor inoculation, progressive thymic involution was observed. Histological studies revealed distinct delymphatization of the thymic cortex, reduced numbers of big lymphocytes in the subcapsular zone and of mitoses in the cortex, and an increased amount of mast cells. This was followed by decreased DNA synthesis in vitro as measured from 3H-thymidine incorporation. The number of pycnoses in the cortex was two to three times that in the thymus of control animals. DNA gel electrophoresis did not reveal any signs of apoptosis in the thymocytes without preincubation. Within 2 h of in vitro incubation of the thymocytes taken on the 7th day after the tumor inoculation, spontaneous apoptosis was more expressed than in the thymocytes from intact mice. This may reflect different induction mechanisms of apoptosis in the thymocytes from the tumor bearing and control mice. We propose that the mechanism of thymic involution during tumor growth is related to inhibition of thymocytes' proliferation, impaired differentiation and enhanced intrathymic death caused by cytokine release from the nonlymphoid thymic population.