Photodynamic therapy and anti-tumour immunity

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Abstract
Photodynamic therapy (PDT) uses non-toxic dyes and harmless visible light in combination with oxygen to produce highly reactive oxygen species that kill cells. In addition to destroying tumour tissue by a process that can produce cellular necrosis and the expression of stress proteins, PDT produces an acute inflammation, and attracts leukocytes to treated tumours. PDT might increase the immunogenicity of dead tumour cells by exposing or creating new antigens, and by inducing heat-shock proteins that increase the efficiency of antigen cross-presentation to form more effective tumour-specific cytotoxic T cells. The pro-inflammatory effects of PDT might increase dendritic-cell migration, antigen uptake and maturation. PDT can produce tumour cures and long-lasting tumour-specific immunity (memory), as has been shown by the rejection of tumours on rechallenge in certain mouse and rat models. PDT has been combined with a range of immunostimulatory therapies, including microbial adjuvants, to increase the anti-tumour immunity produced by PDT alone. There are only a few reports of the immunostimulatory effects of PDT in humans, but increasing recognition of the effect should lead to further work and possibly to improved patient outcome.