Systemic antitumor effect of intratumoral injection of dendritic cells in combination with local photodynamic therapy

Purpose: Photodynamic therapy (PDT), which is used clinically for the palliative treatment of cancer, induces local tumor cell death but has no effect on tumors in untreated sites. The purpose of this study was to determine if local PDT followed by intratumoral injection of naive dendritic cells (IT-DC) induces systemic antitumor immunity that can inhibit the growth of untreated as well as PDT + IT-DC - treated tumors. Experimental Design: BALB/c or C57Bl/6 mice were injected s.c. with CT26 colorectal carcinoma cells and 1316 melanoma cells, respectively, and following 10 to 12 days of tumor growth, the tumors were treated with PDT alone or PDT followed by IT-DC or IT-PBS. In other studies, tumors were established simultaneously in both lower flanks or in one flank and in the lungs, but only one flank was treated. Results: Whereas neither PDT nor IT-DC alone was effective, PDT + IT-DC eradicated both CT26 and B16 tumors in a significant proportion of animals and prolonged the survival of mice of which the tumors were not cured. The spleens of mice treated with PDT + IT-DC contained tumor-specific cytotoxic and IFN-gamma-secreting T cells whereas the spleens of control groups did not. Moreover, adoptive transfer of splenocytes from successfully treated CT26 tumor-free mice protected naive animals from a subsequent challenge with CT26, and this was mediated mainly by CD8 T cells. Most importantly, PDT plus IT-DC administered to one tumor site led to tumor regression at distant sites, including multiple lung metastases. Conclusions: PDT + IT-DC induces potent systemic antitumor immunity in mice and should be evaluated in the treatment of human cancer.