Enhanced lymphocyte-mediated killing of tumour cells after tumour irradiation in vivo.

The effect of local X-irradiation of a syngeneic carcinoma of fibrosarcoma growing in the leg of W/Fu rats on the ability of host spleen lymphocytes to kill syngeneic and allogeneic tumour cells in vitro was examined. Lymphocytotoxicity was found to be enhanced one day after local X-irradiation (4000 R) when compared with unirradiated tumour-bearing rats (p = 0.05 to 0.01). This enhancing effect of local irradiation was not observed when the lymphocytes were tested 1 week or later after X-irradiation, nor when normal legs of non-tumour bearing rats were irradiated. Mechanisms which might possibly explain these results are a reduction in release of tumour-specific antigen which can act as an inhibitor of cell-mediated cytotoxicity, or depletion of suppressor cells in the lymphocyte population. These findings may be relavant to clinical studies of cellular immunity in patients undergoing radiotherapy of malignant tumours.