Photosensitivity is a sensitivity to UV radiation (UVR) commonly found in systemic lupus erythematosus (SLE) patients who have cutaneous disease. Upon even ambient UVR exposure, patients can develop inflammatory skin lesions that can reduce the quality of life. Additionally, UVR-exposed skin lesions can be associated with systemic disease flares marked by rising autoantibody titers and worsening kidney disease. Why SLE patients are photosensitive and how skin sensitivity leads to systemic disease flares are not well understood, and treatment options are limited. In recent years, the importance of immune cell–stromal interactions in tissue function and maintenance is being increasingly recognized. In this review, we discuss SLE as an anatomic circuit and review recent findings in the pathogenesis of photosensitivity with a focus on immune cell–stromal circuitry in tissue health and disease.

Type I IFN (IFN-I) mediates autoimmune and inflammatory conditions, and better understanding IFN-I-driven pathogenesis could expand therapeutic possibilities. Lupus is an autoimmune disease characterized by photosensitivity, inflammatory skin lesions, and systemic organ damage. Patients have an IFN-I signature in blood and tissues and anifrolumab (anti-IFNAR1), developed for lupus and recently FDA-approved, underscores the importance of IFN-I in pathogenesis. Anifrolumab is especially efficacious for cutaneous disease, but mechanisms are poorly understood. Langerhans cells (LCs) normally limit UVR-induced skin injury via ADAM17, a metalloprotease that clips from the cell membrane and activates skin-protective EGFR ligands. Downregulation of LC ADAM17 mRNA and activity in lupus models contributes to photosensitivity, and here we link IFN-I pathogenesis with LC dysfunction. We show that murine model and human lupus non-lesional skin have IFN-I signatures and that IFN-I reduces ADAM17 sheddase activity in LCs. Furthermore, anti-IFNAR1 in multiple murine lupus models restores LC ADAM17 function and reduces photosensitivity in an EGFR and LC ADAM17-dependent manner. These results suggest that IFN-I contributes to photosensitivity in lupus by downregulating LC ADAM17 function, providing mechanisms for IFN-I pathogenesis and anifrolumab efficacy and highlighting the importance of LCs as a potential therapeutic target.

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