Abstract

Anti-Jo-1 antibodies (AJoA), which bind to and inhibit the activity of histidyl-transfer RNA synthetase (HRS), are found in a genetically and clinically distinct subset of myositis patients. This specificity suggests that understanding the antigenic epitopes and immunoregulation governing the production of AJoA may result in clues to disease pathogenesis. Limited digestion of human HRS by V8 protease resulted in four major antigenic polypeptides of 35, 34, 21, and 20 kD; digestion with subtilisin gave four fragments of the same sizes and two additional major antigenic polypeptides of 28 and 17 kD. Sera from 12 AJoA positive patients reacted indistinguishably with these proteolytic fragments by Western blotting, and AJoA elution studies suggested a common epitope(s) on all six. Isoelectric focusing showed a different polyclonal pattern of AJoA in each patient, although serial analyses in individual patients revealed stable AJoA spectrotypes over years of observation. Enzyme-linked immunosorbent assays showed that the AJoA response was mainly restricted to the IgG1 heavy chain isotype. The levels of IgG1 AJoA varied in proportion to disease activity over time but were independent of total IgG1 levels, and three patients became AJoA negative as their myositis remitted after treatment. These findings suggest that AJoA are induced by an antigen-driven mechanism, bind to a common epitope or epitopes on HRS, and are modulated by an immune response closely linked to that which is responsible for myositis in these patients.