Innate immune cell-produced IL-17 sustains inflammation in bullous pemphigoid

Bullous pemphigoid (BP) is an autoimmune skin disease characterized by the binding of autoantibodies to components of the hemidesmosome structure resulting in an inflammatory response and subepidermal blister formation. To investigate the role of immune orientation in the inflammatory processes associated to disease progression, blister fluid, serum and biopsy specimens were collected from thirty one consecutive BP patients. Blister fluids displayed high level of IL-6, IL-17, IL-22, IL-23, whereas TGF-β was increased in BP sera. However neither immunocytochemistry on a trans-differentiation model of IL-17-producing PBMCs nor immunohistochemistry on BP biopsy specimens could demonstrate the presence of T(h)17 lymphocytes. Instead innate immune cells, especially neutrophils, produced IL-17 at the skin lesional site. Of note, superpotent topical corticosteroid application quickly and dramatically reduced both IL-17 expression and clinical signs of BP. Consistently, IL-17 upregulated MMP-9 and neutrophil elastase expression, two proteases involved in blister formation, thereof further demonstrating its role in the progress of BP. Finally IL-17-induced matrix degradation originated from neutrophil activation, initiated the formation of an amplification loop of the inflammatory response that could represent the underlying phenomenon leading to the maintenance and even disease extent. Thus, our results could open new therapeutic strategies for BP patients.