Coinfusion of irradiated splenocytes with low titer tumor-infiltrating lymphocytes augments antitumor efficacy in adoptive immunotherapy

We hypothesized that adoptively increasing the density of antigen-presentin g cells (APCs) at a tumor site would improve tumor-infiltrating lymphocyte (TIL) in vivo antitumor efficacy. Irradiated splenocytes were used as crude APCs. Alone, they did nor have in vitro antitumor activity nor did they au gment TIL efficacy in vitro. Pulmonary metastases were established by intra venous (IV) injection of 5 x 10(5) MC-38 tumor into irradiated C57Bl/6 mice (500 cGy). After 3 days, MC-38 TIL (0.1, 0.5, and 1 x 10(6) cells) +/- irr adiated splenocytes (5,000 cGy) as APCs were administered intravenously (0. 25, 0.5, and I x 10(6) cells) to each group (n = 5/group). Interleukin-2 (6 0,000 TU) was injected intraperitoneally three times daily for 3 days. Mice were sacrificed 9 days later and metastases elaborated in blinded fashion. A titer of 1 x 106 TIL, completely eradicated pulmonary metastases. In two consecutive experiments, when increasing titers of irradiated splenocytes were coinfused with a constant titer of TIL that did not completely eradica te pulmonary metastases, a moderate reduction in pulmonary metastases was o bserved. The contribution of splenocytes to an improvement in TTL antitumor efficacy was not altered when irradiated splenocytes derived from mice bea ring 10-day subcutaneous MC-38 tumors were used. The coinfusion of nonirrad iated splenocytes did not Improve TIL antitumor in vivo activity. Activated B cells (expressing ICAM-1, B7.1 and B7.2) had no effect on in vitro tumor lysis and did not augment in vivo TIL efficacy. The results show a modest but statistically significant improvement in adoptive immunotherapy antitum or efficacy with fewer TIL by coinfusion of irradiated splenocytes. Further studies to characterize the active potential APC cell subpopulation and to clarify the mechanism(s) responsible for in vivo augmentation of TIL antit umor efficacy an in progress.