Potentiation of photodynamic therapy of cancer by complement: the effect of γ-inulin

Host response elicited by photodynamic therapy (PDT) of cancerous lesions is a critical contributor to the clinical outcome, and complement system has emerged as its important element. Amplification of complement action was shown to improve tumour PDT response. In search of a clinically relevant complement activator for use as a PDT adjuvant, this study focused on γ-inulin and examined its effects on PDT response of mouse tumours. Intralesional γ-inulin (0.1 mg mouse−1) delivered immediately after PDT rivaled zymosan (potent classical complement activator) in delaying the recurrence of B16BL6 melanomas. This effect of γ-inulin was further enhanced by IFN-γ pretreatment. Tumour C3 protein levels, already elevated after individual PDT or γ-inulin treatments, increased much higher after their combination. With fibrosarcomas MCA205 and FsaR, adjuvant γ-inulin proved highly effective in reducing recurrence rates following PDT using four different photosensitisers (BPD, ce6, Photofrin, and mTHPC). At 3 days after PDT plus γ-inulin treatment, over 50% of cells found at the tumour site were CTLs engaged in killing specific targets via perforin–granzyme pathway. This study demonstrates that γ-inulin is highly effective PDT adjuvant and suggests that by amplifying the activation of complement system, this agent potentiates the development of CTL-mediated immunity against PDT-treated tumours.