Synergistic antitumor activity of tumor-infiltrating lymphocytes, interleukin 2, and local tumor irradiation: Studies on the mechanism of action

The adoptive transfer of tumor-infiltrating lymphocytes (TIL) with the concomitant administration of IL-2 has been shown to mediate the regression of established 6- and 14-d murine hepatic and pulmonary metastases. For successful immunotherapy with TIL, however, pretreatment with either cyclophosphamide (CP) or whole body irradiation (WBX) was required. The exact mechanism of CP and WBX augmentation of TIL antitumor activity remains unknown, but the elimination of Ts cells has been frequently invoked as an explanation. To address this possibility and to determine if local tumor irradiation (LTX) could synergize with TIL as well as WBX, we investigated the effect of LTX on the therapeutic efficacy of TIL and IL-2 in the treatment of multiple 7-d murine hepatic metastases. Experiments studying the treatment of a weakly immunogenic murine adenocarcinoma, MC-38, showed prolonged survival of mice treated with the combination of IL-2, TIL, and either LTX or WBX, compared with treatment with radiation alone or radiation plus IL-2 controls (p<0.0001). In addition, therapy with LTX and IL-2 prolonged survival, compared with LTX administration alone, whereas therapy with WBX combined with IL-2 did not alter survival. This augmentation of TIL-mediated antitumor activity was dependent on the dose of radiation used. To assess the possibility that tumor-associated Ts cells inhibit the function of adoptively transferred TIL in animals with 7-d metastatic tumor and are eliminated by WBX and LTX, we repeated the above experiments leaving some tumor unirradiated. Mice underwent either LTX or limited LTX, which included only the right side of the liver (LTX( 1/2 ). The number of right- and left-sided metastases were then individually counted. These studies showed that the reduction in the number of right-sided metastases was identical between the two groups and that the presence of left-sided tumor in the LTX( 1/2 ) group did not suppress the observed antitumor activity of TIL against irradiated tumor. Additional evidence against the elimination of suppressor cells as an importnant mechanism in radiation-induced augmentation of TIL antitumor activity was provided by experiments studying the effectiveness of TIL in thymectomized, lethally irradiated, and reconstituted B mice. Unless CP was administered before the adoptive transfer of TIL, therapy with IL-2 and TIL in these B mice was ineffective in the absence of demonstrable T lymphocytes. Finally, clonagenic assays performed after treatment with CP, adriamycin, or WBX demonstrated that the degree of direct antitumor activity exhibited by these agents correlated well with their known ability to synergize with TIL and IL-2. Thus, these experiments not only provide evidence of a synergistic interaction between LTX and TIL in general, but they also provide indirect data contradicting a critical role for suppressor cells in the radiation- and CP-mediated therapeutic augmentation of TIL and IL-2. In addition, the marked synergy between LTX and TIL provides a rational basis for clinical trials designed to determine the extent to which LTX and TIL interact in the treatment of human malignancies.