Monocytoid dendritic cells might be a specific marker for lung involvement in dermatomyositis

Dermatomyositis (DM) is a rare autoimmune disease leading to inflammatory and degenerative changes affecting skin and muscles. Interstitial lung disease is a major risk factor for morbidity and mortality in DM. The precise immune mechanism underlying DM, particularly in the context of lung involvement, remains poorly understood. This research focuses on the significance of dendritic cell subsets and Langerhans cells in the immune irregularities observed in DM.The study analyzed 12 cases of DM with lung involvement, and compared the findings to 12 cases with skin involvement. All subjects were untreated at the time of diagnosis, with ages ranging from the 20 to 80 years. Immunohistochemistry was utilized to characterize dendritic cell subsets and to assess cells expressing MDA5. Six cases of rheumatoid arthritis (RA) with fibrosing pneumonia were investigated for comparison.We identified dendritic cells and lymphocytes expressing the melanoma differentiation gene 5 (MDA5) protein. Monocytoid dendritic cells predominated in DM, and were absent in RA, therefore might serve as a diagnostic marker, separating DM from other autoimmune diseases. Our findings suggest that DCs and lymphocytes are constantly attacked by autoantibodies for MDA5, as these cells express the antigen. Furthermore, mature DC in skin and lung and Langerhans in the skin are replaced by immature DC. This might explain prolonged activation of the immune system resulting in fibrosis in both skin and lung tissues. The loss of Langerhans cells might be responsible for an increased penetrance of antigens through the epidermis.