Can PDT be potentiated by immunotherapy?

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Abstract
Two principal aspects of interlinkage of the immune system with photodynamic therapy (PDT) treatment are discussed in detail: (1) participation of tumor associated macrophages (TAM) in tumor localization of photosensitizers, and (2) induction of an inflammatory immune response by phototoxic effects in endothelial cells, TAM and other cells in the tumor, leading to necrosis of the tumor tissue. It is illustrated how Photofrin levels in TAM, and consequently in the tumor as a whole, can be altered (increased or decreased) by specific agents that stimulate or impair physiological processes in TAM. It is suggested that the selectivity of tumor localization of some photosensitizers can be augmented by TAM targeted immunotherapy with agents that enhance the rate of tumor infiltration of these cells and stimulate their phagocytic activity. Preliminary results of ongoing studies suggest that PDT induces a massive infiltration of immune cells into the treated tumor. It is hypothesized that by combining PDT with an appropriate type of immunotherapy the immune reaction can be potentiated and directed against the surviving tumor cells. As as an example, it is shown that much better control of SCCVII tumor is achieved by combining PDT with the treatment by the immunoactivator SPG compared to PDT alone.

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