Radiation Can Inhibit Tumor Growth Indirectly while Depleting Circulating Leukocytes

The growth of certain UV-radiation-induced tumors is suppressed by T lymphocytes, but it has been proposed that non-T lymphocytes stimulate the growth of several of these tumors. In this study, the indirect effects of X irradiation on the growth of one such tumor in T-cell-deficient nude mice was tested. Even when the site of tumor cell injection was shielded, whole-body irradiation with 6 Gy before tumor challenge inhibited subsequent tumor growth significantly. The interval during which this indirect inhibition was observed correlated with the depletion of circulating leukocytes, which did not return to normal levels until 12-21 days after irradiation. These data are consistent with earlier results using an antibody to deplete ${\rm Gr}\text{-}1^{+}$ leukocytes and indicate that radiation can inhibit the growth of certain tumors indirectly without direct effects on the tumor cells or the tumor bed.