Skin and immune cells crosstalk via circadian regulations

Both innate and adaptive immune cells exist in the skin, predominantly in the dermis layer. Recent studies have focused on how and which circadian rhythms contribute to maintain good health. Over recent years, we have gained a better understanding of the molecular mechanisms that control biological clocks and circadian rhythms. Circadian rhythms maintain homeostasis by providing day and night information to various physiological functions of our body. However, excessively high immune system activity can lead to a risk of developing autoimmune or allergic diseases. Recently, increasing numbers of studies with human and mouse models have been conducted to investigate the mechanisms underlying circadian regulation of the skin homeostasis. In this review, circadian regulation in the skin will be discussed from different points of view. Skin is referred as the largest organ of the body and is directly exposed to the external environment, including large changes in diurnal temperature, light, and pathogens. Immune cells as well as skin cells are the ones protecting us from these stimulants. Associations of the circadian system and these cells have been revealed in many ways, however, the specific roles of the peripheral clocks in these cells remain unknown. Circadian regulation in the skin diseases is discussed specifically in atopic dermatitis and other skin allergic symptoms as well as psoriasis.