The influence of photodynamic therapy on the immune response

Photodynamic therapy (PDT) is a clinically approved therapeutic modality used for the management of several types of tumors as well as non-malignant diseases. Most of the effects of this treatment regimen result from direct action of singlet oxygen and reactive oxygen species. However, accumulating evidence indicates that antitumor effects are also mediated by indirect stimulation of inflammatory and immune responses. These responses include rapid local infiltration of tumors by neutrophils and macrophages accompanied by systemic release of inflammatory mediators. This early response can initiate and translate into a more precise immune reaction that involves activation of specific T lymphocytes that seem to be necessary for the ultimate control of residual tumor cells. Although still incompletely understood, PDT can not only activate but also suppress the immune response depending on several variables. This review summarizes the influence of PDT on the immune response and discusses its importance in the management of human diseases.