The type I interferon (IFN) system has recently been suggested to play important and essential roles in the pathogenesis of myositis. However, a clarification of how type I IFNs could function as triggering factor(s) in the pathogenesis of myositis has yet failed. Through activation of the type I IFN system, the host defense peptide LL-37 carries numerous immunomodulatory properties and is implicated in the pathogenesis of several other autoimmune diseases, including systemic lupus erythematosus (SLE). The expression of LL-37 can be regulated by various endogenous factors including the active form of vitamin D (25(OH)D-3). The aim of this study was to explore a potential role of LL-37 in relation to the type I IFN system in patients with polymyositis (PM) and dermatomyositis (DM) and to compare these with SLE patients and healthy controls. We investigated muscle (3 PM, 5 DM) and symptomatic (5 DM) and non symptomatic (3 PM, 3 DM) skin biopsies from patients with short disease duration and muscle biopsies (3 PM, 1 DM) from patients with long disease duration. Six SLE patients with symptomatic and non symptomatic skin and five muscle and six skin biopsies from healthy individuals served as controls. Tissue specimens were immunohistochemically stained for LL-37, neutrophils (CD66b), plasmacytoid dendritic cells (BDCA-2), myxovirus resistance protein A (MxA), and macrophages (CDG8, CD163). In addition, LL-37 and CD66b double staining was also performed. Serum levels of 25(OH)D-3 were investigated in PM and DM patients with short disease duration (3 PM, 5 DM) and in 40 healthy controls. We found that the expression of LL-37, BDCA-2 (the major producer of type I IFNs), MxA (an interferon inducible protein), and macrophages were higher in muscle tissue of PM and DM patients compared to healthy controls. The LL-37 expression was mainly derived from neutrophils. Neutrophils were increased in both symptomatic and non-symptomatic skin of myositis and SLE patients and BDCA-2 was increased in symptomatic DM skin when compared to healthy controls. Moreover, the expression of MxA in symptomatic and non-symptomatic skin of SLE patients was higher when compared to both myositis patients and healthy controls. There was no difference in the expression of LL-37 in skin of myositis and SLE patients compared to healthy controls. All PM and DM patients with a short disease duration had low 25(OH)D-3 levels compared to healthy controls. In conclusion, the present study supports our hypothesis that LL-37 may activate type I IFNs, which could initiate and perpetuate an inflammatory process. The prolonged exposure of the immune system to type I IFNs may eventually break tolerance and lead to autoimmune myositis. (C) 2016 Elsevier Ltd. All rights reserved.