In polymyositis and dermatomyositis (DM), identified autoantibodies occur in <50% of adult patients and in a smaller proportion of children. This study was undertaken as part of a larger effort to define novel autoantibodies that assist in the clinical evaluation of myositis.

Sera from children and adults satisfying criteria for idiopathic inflammatory myopathies and from patients with other connective tissue diseases (CTDs), patients with noninflammatory myopathies, and healthy individuals were tested for autoantibodies by immunoprecipitation (IP). A previously unrecognized autoantibody that immunoprecipitated a 155‐kd protein along with a weaker 140‐kd protein was seen. When the presence of this anti‐p155 autoantibody in test sera was suggested based on IP results, it was confirmed by immunoblotting of immunoprecipitates.

Sera from 51 of 244 myositis patients (21%), including 30 with juvenile DM (29%), 5 with juvenile CTD–associated myositis (33%), 8 with adult DM (21%), 6 with cancer‐associated DM (75%), and 2 with adult CTD–associated myositis (15%), were found to have anti‐p155 autoantibody. One of 49 patients with lupus, and none of 89 others without myositis, had anti‐p155. Caucasian patients with anti‐p155 had a unique HLA risk factor, DQA1*0301 (odds ratio 5.4, corrected P = 0.004). In adults with anti‐p155, of several clinical features assessed only the frequency of V‐sign rash was increased, but patients with this antibody were clinically distinct from those with autoantibodies to aminoacyl–transfer RNA synthetases.

A newly recognized autoantibody, anti‐p155, is associated with DM and cancer‐associated DM, and is one of the most common autoantibodies in this condition, occurring as frequently in children as in adults. The clinical features and immunogenetics associated with anti‐p155 differ from those associated with antisynthetases.