Acute exposure of human skin to ultraviolet or infrared radiation or heat stimuli increases mast cell numbers and tryptase expression in human skin in vivo

Summary Background Mast cells are key effector cells in diverse immunological and pathological processes. It is still unclear why there are more mast cells at peripheral and sun‐exposed skin sites than at sun‐protected sites. Objectives To investigate changes in mast cell numbers associated with natural ageing and photoageing, and to observe the effects of ultraviolet (UV) and infrared (IR) radiation and heat on the prevalence of mast cells and tryptase expression in human skin in vivo. Methods Sun‐exposed and sun‐protected skin samples were taken from individuals in four different age groups. UV, IR or heat‐treated buttock skin of young volunteers was also obtained. Mast cells were quantified by immunohistochemical staining of mast cell‐specific tryptase and chymase. The expression of tryptase was determined by Western blotting. Results Both sun‐exposed and sun‐protected skin showed a gradual decrease in total mast cells (MCTotal) number with ageing. The number of mast cells in sun‐exposed skin was significantly higher than that in sun‐protected skin. After UV irradiation (2 minimal erythema doses), MCTotal and mast cells expressing tryptase and chymase were significantly increased at 24 and 48 h postirradiation. After IR irradiation (3 minimal heating doses) and heat treatment (43 °C for 90 min), MCTotal reached peak induction at 8 and 48 h after stimulation, respectively. Tryptase expression was also clearly upregulated by UV, IR and heat. Conclusions Our data demonstrate that mast cell numbers decreased with ageing in human skin. Also, mast cells may be activated and recruited by UV, IR and heat. These findings should further our understanding of the reason for the high prevalence of mast cells at peripheral sun‐exposed skin sites.