Stimulation of the host immune response by photodynamic therapy (PDT)

The tumor response to photodynamic therapy (PDT) involves a complex interplay between direct cytotoxicity to the tumor cells and secondary damage as a result of the effects of PDT on the vasculature and stimulation of the host inflammatory response. Pre-clinical and clinical studies have suggested that the combination of direct and indirect effects of PDT culminate in an activation of host anti-tumor immune responses. We have begun to examine the direct effects of PDT on tumor immunogenicity and have made the novel discovery that PDT treatment of tumor cells in vitro enhances tumor cell immunogenicity. We have further demonstrated that the increase in tumor cell immunogenicity by PDT can be correlated with the ability of PDT-generated tumor cell lysates to stimulate dendritic cell maturation and activation. The mechanisms by which PDT is able to enhance tumor cell immunogenicity and stimulate dendritic cell maturation and activation is unclear, however our finding suggest that alterations in tumor immunogenicity correlate with enhanced release of dendritic cell stimulating factors such as heat shock proteins.