Metronomic PDT induces innate and adaptive immune responses in murine models of skin cancer and pre-cancer

Aminolevulinate-based photodynamic therapy (ALA-PDT) is an effective treatment for cutaneous pre-cancers (Actinic Keratoses; AK) and Basal Cell Carcinoma (BCC), the most common skin malignancies. When administered in a conventional regimen with 1-4 h of ALA preincubation prior to light exposure, ALA-PDT elicits stinging pain during illumination that patients find objectionable. To avoid this pain, we have described a new regimen called metronomic PDT (mPDT) which is similar to daylight PDT but uses blue light (Kaw et al, J Am Acad Dermatol 2019). Metronomic PDT is not only painless but also nearly as effective as conventional PDT for AK lesion clearance. In this investigation, murine models of AK induced by repeated UVB exposure were treated with mPDT, followed by time-course analyses of immune responses in the lesions harvested. Our preliminary data showed that relative to conventional PDT, cell death (apoptosis) and generation of Reactive Oxygen Species (ROS) were compromised in mPDT samples. However, relative to untreated controls, enhanced recruitment/infiltration of immune cells that mediate innate immunity [neutrophils (Ly6G+) and macrophages (F4/80+)] was observed at early times after mPDT. Just as importantly, enhanced presence of cells regulating adaptive immune responses [T cells (CD3+, CD8+ and Foxp3+)] was observed at later times post mPDT. Activation of calreticulin and HMGB1 (markers of Damage Associated Molecular Patterns, DAMPs) were also observed in mPDT treated lesions. Our results suggest that mPDT can be just as effective as conventional PDT for treatment of skin cancer and pre-cancer, and that the therapeutic mechanisms may involve immune cell responses triggered by metronomic PDT.