Anti–signal recognition particle (anti-SRP) autoantibodies are associated with severe acquired necrotizing myopathies. The role of these autoantibodies remains elusive, and the evolution of anti-SRP levels over time is unknown. In this study, we developed an addressable laser bead immunoassay (ALBIA) technique to investigate a correlation between anti-SRP levels, serum creatine kinase (CK) levels, and muscle strength in patients with necrotizing myopathy.
The diagnostic value of the ALBIA assay was determined by comparing serum levels of anti-SRP autoantibodies in 31 anti-SRP immunodot–positive patients to those in 190 healthy blood donors and 199 control patients with different inflammatory/autoimmune conditions or polyclonal hypergammaglobulinemia. Among the 31 anti-SRP–positive patients, serum samples from 8 patients were monitored over time for levels of anti-SRP autoantibodies and levels of CK (determined at least 3 times, consecutively, over a mean followup period of 783 days). The relationship between levels of anti-SRP autoantibodies and levels of CK was tested using a linear mixed model.
The assay yielded positive results for anti-SRP in all anti-SRP immunodot–positive serum samples tested, while all control sera tested negative. The 8 anti-SRP–positive patients who were followed up longitudinally were found to have normalized CK levels and improved muscle strength. There was a striking correlation between the degree of myolysis, as measured by CK levels, in patients receiving therapy and the anti-SRP54 autoantibody levels in these same patients (P = 0.002).
Anti-SRP–positive myositis appears to be one of the few autoimmune diseases in which specific autoantibody levels are correlated with surrogate disease activity markers. These results reveal the usefulness of monitoring anti-SRP autoantibody levels in patients receiving therapy, and may also suggest a possible pathogenic role for anti-SRP autoantibodies in the necrotizing myopathies.