The
myositides are systemic autoimmune conditions of which the most important are
polymyositis, dermatomyositis, and
inclusion body myositis. In addition to the classic clinical diagnostic criteria,
myositis-specific
autoantibodies were identified about 15 years ago. Among the dozen or so
myositis-specific
autoantibodies reported to date, the most characteristic are directed against cytoplasmic
antigens, such as
tRNA synthetase (Jo-1 or PL-1, PL-7, PL-12, EJ, OJ, JS, and KS),
signal-recognition particle (SRP), Mas, KJ, Fer (eEF1), and Wa.
Antibodies to
nuclear antigens include
anti-Mi-2, anti-PMS (PMS1, and PMS2), and related
antibodies (MLH1,
DNA protein kinase catalytic subunit (
DNA PKCS)...), and anti-56 kDa.
Myositis-associated
antibodies are not specific but may be found in patients with
myositis. They are directed to nuclear or nucleolar
antigens such as PM-Scl, Ku, RNP (U1-RNP and U2-RNP, U4/U6-RNP, and U5-RNP), Ro 52 kDa and, more rarely, Ro 60 kDa and La.
Myositis-specific
antibodies have proved useful on two fronts. They have improved the diagnosis of
myositis by leading to the identification of characteristic clinical patterns, such as anti-
synthetase syndrome. The place of
autoantibodies alongside classic clinical and laboratory criteria remains to be determined, however. First, standardized assays will have to be developed to replace current detection methods, which use widely variable techniques and
antigen preparations.
Myositis-specific
antibodies have also shed light on the pathogenesis of
myositis. For instance, the development of
antibodies to
tRNA synthetases constitutes an original autoimmunity model that shows how muscle damage, probably of a nonspecific nature, can lead to the production of
autoantibodies that perpetuate and aggravate the muscle lesions.