Role of the Immune System in Mediating the Antitumor Effect of Benzophenothiazine Photodynamic Therapy

The role of the host immune system in contributing to tumor regression following benzophenothiazine photodynamic therapy (PDT) was examined. Photodynamic therapy with 2-iodo-5-ethylamino-9-diethylaminobenzo[a]-phenothiazinium chloride (2I-EtNBS) eradicated EMT-6 mammary fibrosarcomas in 75-100% of treated mice. In contrast, PDT failed to inhibit tumor growth in T-cell-deficient nude mice. Furthermore, T-cell depletion studies with anti-CD8 antibody revealed that the CD8+ T-cell population was critical for an effective PDT response (tumor volume 14 days post-PDT: 262 mm3 vs 59 mm3 in controls; P < 0.01). Because anti-CD4 antibody inhibited tumor growth in the absence of PDT, the role of CD4+ T cells remains unclear. Depletion of natural killer (NK) cells in vivo with anti-asialo-GM1 antibody significantly reduced a suboptimal PDT effect relative to vehicle controls (tumor volume 13 days post-PDT: 513 mm3 vs 85 mm3, respectively; P < 0.001). However, splenic NK cells obtained from PDT-treated tumor-bearing mice were not cytotoxic in vitro against EMT-6 cells, suggesting that NK cells contribute to the PDT effect in vivo by an indirect mechanism. In addition, when mice with complete tumor regression following PDT were rechallenged 28 days later with 5 x 10(5) EMT-6 cells, tumor growth was significantly inhibited as compared to controls (tumor volume 40 days postrechallenge: 137 mm3 vs 833 mm3 in controls; P < 0.03; percent animals without tumor in five experiments: 67% vs 8% in controls). Collectively, these results demonstrate that CD8+ T cells are required to prevent tumor regrowth after 2I-EtNBS-PDT, NK cells contribute to this response and such PDT can elicit protective antitumor immunity.