Photoimmune suppression and photocarcinogenesis

The primary cause of non-melanoma skin cancer, the most prevalent form of human neoplasia, is the ultraviolet (UV) radiation found in sunlight. Exposing mice to UV radiation induces skin cancers that are highly antigenic. Upon transfer of an UV-induced skin cancer to a normal syngeneic mouse, the tumor cells are recognized and rapidly destroyed by the immune system of the recipient. This raises the question of how these cancers avoided immune destruction during their development in the UV-irradiated host. This question was answered when it was discovered that in addition to being carcinogenic, UV radiation was also immunosuppressive. Studies with immune suppressed transplantation recipients, and biopsy proven skin cancer patients have confirmed that UV-induced immune suppression is a risk factor for skin cancer development in humans. It is of great importance, therefore, to understand the mechanisms underlying UV-induced immune suppression. The focus of this manuscript will be to use some examples from the more recent scientific literature to review the mechanisms by which UV radiation suppresses the immune response and allows for the progressive outgrowth of antigenic skin tumors.