Photodynamic therapy and immunity

Photodynamic therapy (PDT) is widely recognized as a technique with which to treat malignant tumors that are accessible to an activating light source. PDT utilizes light absorbing compounds that catalyse the formation of cytotoxic oxygen species to produce the antitumor effect subsequent to direct light irradiation. PDT also exhibits immunomodulatory attributes. The photodynamic treatment of solid tumors triggers a large influx of granulocytes and macrophages into the region, leading to T-cell mediated anti tumor immunity against residual cancer. In apparent contrast, the application of levels of light less than those required for tumor ablation and over a larger body surface lessened disease severity when applied in murine autoimmune models. Furthermore, PDT is inhibitory for immunologically-mediated reactions to topically applied chemical haptens. The capacity of PDT to influence immune responses appears related to its capacity to target activated T lymphocytes as well as influence the immunostimulatory attributes of antigen presenting cells.