The indirect immunofluorescence assay autoantibody profiles of myositis patients without known myositis-specific autoantibodies.

OBJECTIVES The indirect immunofluorescence assay (IIFA) is used to screen for the presence of autoantibodies. Our objective was to determine the prevalence and clinical features of IIFA positive myositis patients without known myositis-specific autoantibodies (MSA). METHODS Sera from healthy comparators (HC) and patients with dermatomyositis (DM), inclusion body myositis (IBM), and polymyositis (PM) with no detectable MSA were tested by IIFA on HEp-2 cells. The pattern of positivity was classified according to the International Consensus on Antinuclear Antibody Patterns. The prevalence and frequency of each IIFA pattern were compared between the different groups. RESULTS Sera from 100 HC, 71 DM, 53 IBM, and 69 PM subjects were included in the study. The IIFA was positive in 35% HC compared to 66% DM (p 69%). The most common pattern in myositis patients was fine speckled with no differences between groups. In general, IIFA positive and negative DM patients showed similar clinical features and disease activity. CONCLUSIONS Half of MSA-negative DM patients have moderate/strong IIFA positivity, predominantly with a fine speckled pattern. In contrast, MSA-negative PM, IBM, and healthy comparators are more often weakly positive for IIFA. These findings suggest that unidentified autoantibodies are more likely to exist in DM patients than in the other myositis groups.