Auto-Vaccination

One of the most interesting phenomena observed in the PDT/SPDT treatment of cancer is the potential to induce a strong and long-lasting anti-tumor immune response. It has been known for some years that under certain conditions, PDT can not only destroy the primary tumor that receives illumination but also sensitize the host immune system to recognize and potentially destroy any remaining tumor cells, whether left at the site of the primary tumor or present as distant metastases.

How the immune system is activated after PDT; When light (hν) is delivered to PS (photosensitizer) loaded tumors this induces both apoptotic and necrotic cell death. These dead and dying tumor cells are phagocytosed by dendritic cells (DC) that have accumulated owing to the acute inflammatory response triggered by PDT. Stimulated by cytokines released at the site of inflammation, DCs mature and home to the regional lymph nodes where they present antigens to the native T lymphocytes. Activated lymphocytes become tumor-specific effector T-cells and, attracted by chemokines, migrate to the tumor and kill the remaining tumor cells.

Successful PDT of tumors growing in immunocompetent syngeneic mice can in some cases cause a long-term memory anti-tumor immunity, as demonstrated by a resistance to rechallenge with the tumor from which they were cured, but not a different syngeneic tumor