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The purpose of this study was to investigate the induction of anti-tumor immunity by photodynamic therapy (PDT). We used EMT-6 mammary sarcoma, a moderately immunogenic tumor, with 10(6) cells injected s.c. in thighs of immunocompetent Balb/c mice. Mice were treated 10 days later when tumors were 6-mm diameter. Two PDT regimens were equally effective in curing tumors: 1-mg/kg of liposomal benzoporphyrin derivative (BPD) followed after 15 min by 150 J/cm2 690 nm light or 10-mg/kg chlorin(e6) (ce6) followed after 6 hours by 150 J/cm2 665 nm light. BPD-PDT produced a black eschar 24-48 hours after treatment with no visible tumor, followed by healing of the lesion. By contrast ce6-PDT showed no black eschar, but a slow disappearance of tumor over 5-7 days. When cured mice were rechallenged with 10(6) EMT-6 cells in the opposite thigh, all ce6-PDT cured mice rejected the challenge, but BPD-PDT cured mice grew tumors in a proportion of cases. When mice were cured by amputation of the tumor bearing leg, all mice subsequently grew tumors upon rechallenge. Mice were given two EMT6 tumors (1 in each leg) and the mouse was injected with ce6 or BPD but only one tumor was treated with light. Both tumors (PDT-treated and contralateral) regressed at an equal rate until they became undetectable, but in some mice the untreated tumor recurred. Those mice cured of both tumors rejected a subsequent EMT6 rechallenge. Amputation of the tumor bearing leg did not lead to regression of the contralateral tumor. Mice that rejected an EMT6 rechallenge failed to reject a subsequent cross-challenge with J774 reticulum cell sarcoma (an alternative Balb/c murine tumor). These data show that PDT generates a tumor-specific memory immune response, and in addition an active tumoricidal immune response capable of destroying distant established tumors. We hypothesize that ce6-PDT is more effective than BPD-PDT due to more necrotic rather than apoptotic cell death and/or generation of heat-shock proteins that are known for efficient presentation of tumor antigens via dendritic cells to cytotoxic T-cells. PDT could be used to treat a locally advanced tumor while at the same time destroying distant metastases via an induced immune response.
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