Table 1.

Summary of the known anti-inflammatory mechanisms of UV light

Type of UV	Anti-inflammatory mechanisms
UVA	Induces expression of HO-1, which catalyzes the degradation of heme to biliverdin and bilirubin, themselves potent antioxidants, and to carbon monoxide, which suppresses proinflammatory cytokines. HO-1 activation may play a immunoprotective role in humans from increased IFN-γ as well. Langerhans cell counts are decreased in human epidermis after 4 weeks of UVA tanning bed exposure [60] Blood CD3 + and CD4 + counts are reduced in patients after exposure to UVA dominant tanning bed treatments [61]
UVB	UVB depletes of LC, the major antigen-presenting cell of the skin, through migration of damaged LCs to regional lymph nodes and through direct apoptosis. UVB exposed LCs preferential present antigens to Th2 and do not stimulate Th1. UVB irradiation induces T-suppressor and immunotolerant macrophages in the epidermis [62] Suppression of ICAM-1 expression by keratinocytes associated with a significant increase in intracellular thymine dimers in vivo with restoration of ICAM-1 expression via topical DNA repair enzyme [63]
Both	CGRP is released from cutaneous nerves after xposure to UVR, increasing cAMP levels in T cells, inhibiting T cell proliferation and inhibiting the production of IL-2 and expression of TNF-α, TNF-β and IFN-γ. CGRP also causes mast cells to degranulate and release TNF-α, which can interfere with APC’s ability to initiate the inflammatory cascade [64] The UV-induced mast cell degranulation releases the anti-inflammatory cytokine IL-10. UV irradiation damages keratinocyte DNA, activating p53 and subsequently increasing the transcription of POMC, which itself induces further production of IL-10 [65] Stimulates HDMEC to produce α-melanocyte-stimulating hormone, inhibiting expression of the adhesion molecules VCAM-1 and E-selectin, inhibiting the extravasation of leukocytes during inflammation [64]
PUVA	Induces cell death by inducing DNA damage, initiating a delayed apoptotic cascade [66]

APC antigen-presenting cell, CGRP calcitonin gene-related peptide, HDMEC human dermal microvascular endothelial cells, HO-1 heme oxygenase, ICAM-1 intracellular adhesion molecule 1, IFN Interferon, IL interleukin, LC langerhans cells, POMC proopiomelanocortin, PUVA psoralen ultraviolet A, Th1 type 1 T cells, Th2 Type 2 T cells, TNF tumor necrosis factor, UV ultraviolet, UVR ultraviolet radiation, VCAM-1 vascular cell adhesion molecule 1