Photodynamic therapy and anti-tumour immunity

Abstract

Photodynamic therapy (PDT) uses non-toxic photosensitizers and harmless visible light in combination with oxygen to produce cytotoxic reactive oxygen species that kill malignant cells by apoptosis and/or necrosis, shut down the tumour microvasculature and stimulate the host immune system. In contrast to surgery, radiotherapy and chemotherapy that are mostly immunosuppressive, PDT causes acute inflammation, expression of heat-shock proteins, invasion and infiltration of the tumour by leukocytes, and might increase the presentation of tumour-derived antigens to T cells.

Figure 1. The mechanism of action on tumours in photodynamic therapy

The photosensitizer (PS) absorbs light and an electron moves to the first short-lived excited singlet state. This is followed by intersystem crossing, in which the excited electron changes its spin and produces a longer-lived triplet state. The PS triplet transfers energy to ground-state triplet oxygen, which produces reactive singlet oxygen (¹O₂). ¹O₂ can directly kill tumour cells by the induction of necrosis and/or apoptosis, can cause destruction of tumour vasculature and produces an acute inflammatory response that attracts leukocytes such as dendritic cells and neutrophils.

Figure 2. Photodynamic therapy induces activation of antigen-specific T cells

When light (hν) is delivered to a photosensitizer (PS)-loaded tumour it induces both apoptotic and necrotic cell death. These cells are phagocytosed by dendritic cells (DCs) that have accumultated owing to the acute inflammatory response which is triggered by photodynamic therapy (PDT). DCs mature after stimulation by cytokines, which are released at the site of inflammation, and home to the regional lymph nodes where they present antigens to the T lymphocytes. Activated T lymphocytes become effector T cells and, attracted by chemokines, migrate to the tumour and kill the tumour cells.

Figure 3. Consequences of photodynamic therapy-induced inflammation

Damage to endothelial cells (ECs) activates a casade of events that lead to local inflammation, vessel dilatation and platelet aggregation. Much of this is caused by the release of thromboxane (TBX), cytokines such as interleukin 1β (IL1β), IL6 and IL8, the production of tumour-necrosis factor-α (TNFα), and infiltration of the treated tumour by cells of the immune system. Necrotic and apoptotic tumour cells express heat-shock proteins (HSPs) and provide antigens to dendritic cells (DCs) that migrate to lymph nodes. hν, light; PDT, photodynamic therapy.

Figure 4. Combination of photodynamic therapy with immunostimulants

The intratumoral injection of various Toll-like receptor (TLR) ligands: bacillus Calmette–Guerin (BCG), Mycobacterial cell-wall extract (MCWE), OK432, zymosan, schizophyllan (SPG) or Corynebacterium parvum (CP), effectively activates dendritic cells (DCs) and increases antigen presentation and local inflammation. The injection of various cytokines, such as granulocyte-macrophage colony-stimulating factor (GMCSF), granulocyte colony-stimulating factor (GCSF) and tumour-necrosis factor-α (TNFα), results in increased infiltration by macrophages, activation of neutrophils, and direct destruction of tumour vessels, respectively. hν, light; PDT, photodynamic therapy.

Figure 5. Mechanism of photodynamic therapy-induced immune suppression

Contact hypersensitivity (CHS) is induced by the application of a hapten, such as dinitrofluorobenzene (DNFB), to the skin, and is mediated by the expression of the major histocompatibility complex (MHC) class I on keratinocytes (KC). A subsequent rechallenge with DNFB elicits an inflammatory response caused by the cytotoxic T cells. Delayed type hypersensitivity (DTH) is induced by the injection of cellular antigens such as foreign proteins, and is mediated by MHC class II expressed by dendritic cells (DCs) that are recognized by T-helper (CD4⁺) lymphocytes. CHS is suppressed by photodynamic therapy (PDT) using blue light that does not penetrate the skin and red light that does penetrate. Only by using red light does the suppression of CHS depend on the secretion of interleukin 10 (IL10). DTH is not suppressed by PDT, whereas ultraviolet light (UVB) suppresses both CHS and DTH. These differences might explain the paradoxical observation that PDT can both simultaneoulsy stimulate and suppress parts of the immune system, whereas UVB is only found to be immunosuppressive. APC, antigen-presenting cells; CTL, cytotoxic T cells; Th, T-helper cells.