Sunlight, mast cells, and photosensitivity: rethinking disease mechanisms in solar urticaria

PURPOSE OF REVIEW: Solar urticaria is a rare, life-altering photodermatosis caused by aberrant mast cell degranulation and cutaneous whealing following exposure to sunlight. Although mast cell degranulation is the key causal event in solar urticaria, upstream mechanisms linking sunlight exposure to mast cell activation remain unclear. This review will discuss existing evidence supporting our understanding of solar urticaria pathobiology. It will also incorporate evidence regarding interactions between sunlight, skin, and cutaneous mast cells to highlight a novel range of potential pathomechanisms. RECENT FINDINGS: Current understanding of solar urticaria is dominated by a photo-allergy model, whereby cutaneous chromophore(s) are modified by solar radiation and cause mast cell activation. Wavebands now known to frequently trigger solar urticaria, e.g., visible light, could help to highlight key disease chromophores. The occasional occurrence of solar urticaria-like reactions in cutaneous porphyrias could also suggest shared pathomechanisms mediated by reactive oxygen species. Evidence of suppressed immunoglobulin E-dependent mast cell activation following ultraviolet radiation and/or visible light exposure suggests a role for other factors, e.g., alterations in the cutaneous microenvironment, in priming mast cells towards activation in solar urticaria. SUMMARY: A diverse range of complex or complementary mechanisms may contribute to solar urticaria pathogenesis.