Myositis specific autoantibodies; specificity and clinical applications.

Abstract

The sera of about half of the patients with myositis contain autoantibodies that are specific for this group of diseases compared to other inflammatory connective tissue disorders. In a recent study we showed that these myositis specific autoantibodies (MSAs) are also specific for myositis as compared to other neuromuscular disorders. The most prevalent MSA, the anti-Jo-1 autoantibody, is associated with the anti-synthetase syndrome consisting of myositis, interstitial lung disease, arthritis, and Raynaud's phenomenon. The anti-Jo-1 autoantibody is generally not encountered in inclusion body myositis (IBM), and the presence of this autoantibody in serum of a patient suspected of myositis virtually rules out the diagnosis IBM. Rarely, anti-Jo-1 autoantibodies are seen in patients with definite IBM. These patients have a remarkable clinical characteristic: a significant response to corticosteroids whereas IBM in general does not respond to treatment. The reason for anti-Jo-1 autoantibody formation is unknown. Recently, it has been suggested that anti-Jo-1 autoantibodies may be directed against fragments of tRNAHis synthetase, the Jo-1 antigen, with a chemokine function. Two other MSAs, anti-Mi-2 and anti-SRP autoantibodies, are less prevalent than anti-Jo-1 and their clinical associations were not well defined. In two recent studies we showed that anti-SRP autoantibodies are a marker of an aggressive immune-mediated necrotizing myopathy whereas anti-Mi-2 autoantibodies are not associated with a particular form of myositis. Autoantibodies against a particular fragment of the Mi-2 antigen may be associated with an increased risk of an underlying malignancy. MSAs have helped recognizing some specific clinical subtypes of myositis. The ability to recognize these different forms of myositis is of importance because of differences in associated disorders, complications, treatment responses, and prognosis. Even though we still do not know the cause(s) of the myositis syndromes or the reason(s) for MSA formation, the MSAs have facilitated our thinking on the pathophysiology of myositis.