Sun and the immune system

Abstract

There is increasing epidemiological and laboratory evidence demonstrating the local and systemic immuno-modulatory (primarily suppressive) effects of solar ultraviolet radiation (UVR) exposure in humans. Solar UVR exposure, at levels consistent with day-to-day activities, can influence immune function directly via chromophores present in the skin or indirectly, via its central role in the synthesis of vitamin D pre-cursors. Using latitude gradient as a surrogate epidemiological exposure marker, high UVR exposure has been associated with lower rates ofautoimmune illnesses (e.g., multiple sclerosis), allergy states (e.g., asthma) and ANCA-associated vasculitis. Conversely, high UVR exposure has been associated with higher rates of skin cancer and infection (e.g., herpes virus, intracellular pathogens). Recent work has also focussed on the role of UVR exposure on vaccination response. Quantifying the relative contributions of direct or indirect (vitamin D) mechanisms towards UVR associated immune modulation has been challenging, as both involve activation of a similar milieu of local and systemic cytokines and influence the number, migration and function of antigen presenting cells and regulatory immune cells. Better understanding in this area will assist in fine-tuning the ‘sun smart' public health message and potentially better target populations who would benefit from vitamin D supplementation.