Pathomechanisms of anti–cytosolic 5′‐nucleotidase 1A autoantibodies in sporadic inclusion body myositis

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Sporadic inclusion body myositis (sIBM) is the most common acquired inflammatory myopathy in patients older than 50 years, especially in Western countries. The number of sIBM patients has been increasing in Japan.1 Long‐term observational studies showed that most patients could not ambulate independently at 7 years after symptom onset, the result being that patients were wheelchair‐bound after approximately 15 years.2 To date, no known effective treatments are available for the disease. Although both protein degradation dysfunction and immune abnormalities have been implicated in the etiology of sIBM, the precise pathogenic mechanisms remain unclear.
Recently, 2 different groups identified autoantibodies against cytosolic 5′‐nucleotidase 1A (cN1A) in plasma and serum samples from patients with sIBM.5 Larman et al,5 after utilizing a dot‐blot assay, reported that moderate reactivity of anti‐cN1A autoantibodies was 70% sensitive and 92% specific and that high reactivity was 34% sensitive and 98% specific for the diagnosis of sIBM. Pluk et al6 used immunoprecipitation to demonstrate high concentrations of anti‐cN1A autoantibodies in 33% of serum samples from sIBM patients. Measurement of anti‐cN1A autoantibodies is thus a potentially valuable method for clinical diagnosis of sIBM, as well as a clue to clarifying the role of autoimmunity in sIBM pathogenesis.
In contrast, another study found anti‐cN1A autoantibodies in a considerable proportion of sIBM patients but also in other patients with several different autoimmune diseases, including systemic lupus erythematosus and Sjögren syndrome, that were not associated with muscle diseases.7 Thus, whether the autoantibodies play a pathogenic role in sIBM is still unclear.
The purposes of this study were therefore to confirm the usefulness of a new cell‐based assay for detection of the anti‐cN1A autoantibodies in the serum from patients with various neuromuscular diseases including sIBM; to determine whether clinicopathological differences exist between sIBM patients with and without the autoantibodies; and to investigate a possible pathogenic role of the autoantibodies by using passive immunization models.

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