Anti-signal recognition particle autoantibody ELISA validation and clinical associations

• Published in: Rheumatology (Oxford, England) Vol. 54; no. 7; pp. 1194 - 1199
• Main Authors: Aggarwal, Rohit, Oddis, Chester V., Goudeau, Danielle, Fertig, Noreen, Metes, Ilinca, Stephens, Chad, Qi, Zengbiao, Koontz, Diane, Levesque, Marc C.
• Format: Journal Article
• Language: English
• Published: England Oxford University Press 01.07.2015
• Subjects: Adult; Autoantibodies - blood; Biomarkers - blood; Case-Control Studies; Clinical Science; Enzyme-Linked Immunosorbent Assay - methods; Female; Humans; Male; Middle Aged; Muscle, Skeletal - enzymology; Myositis - blood; Myositis - diagnosis; Myositis - immunology; Predictive Value of Tests; Reproducibility of Results; Sensitivity and Specificity; Severity of Illness Index; Signal Recognition Particle - immunology; Time Factors; idiopathic inflammatory myopathy; anti-signal recognition particle (SRP) autoantibody; quantitative measure; immunoprecipitation; ELISA
• ISSN: 1462-0324; 1462-0332; 1462-0332
• DOI: 10.1093/rheumatology/keu436

The aim of this study was to develop and validate a quantitative anti-signal recognition particle (SRP) autoantibody serum ELISA in patients with myositis and longitudinal association with myositis disease activity. We developed a serum ELISA using recombinant purified full-length human SRP coated on ELISA plates and a secondary antibody that bound human IgG to detect anti-SRP binding. Protein immunoprecipitation was used as the gold standard for the presence of anti-SRP. Serum samples from three groups were analysed: SRP(+) myositis subjects by immunoprecipitation, SRP(-) myositis subjects by immunoprecipitation and non-myositis controls. The ELISA's sensitivity, specificity, positive predictive value and negative predictive value were evaluated. Percentage agreement and test-retest reliability were assessed. Serial samples from seven SRP immunoprecipitation-positive subjects were also tested, along with serum muscle enzymes and manual muscle testing. Using immunoprecipitation, we identified 26 SRP(+) myositis patients and 77 SRP(-) controls (including 38 patients with necrotizing myopathy). Non-myositis control patients included SLE (n = 4) and SSc (n = 7) patients. Anti-SRP positivity by ELISA showed strong agreement (97.1%) with immunoprecipitation (κ = 0.94). The sensitivity, specificity, positive predictive value, and negative predictive value of the anti-SRP ELISA were 88, 100, 100 and 96, respectively. The area under the curve was 0.94, and test-retest reliability was strong (r = 0.91, P < 0.001). Serial samples showed that anti-SRP levels paralleled changes in muscle enzymes and manual muscle testing. We developed a quantitative ELISA for detecting serum anti-SRP autoantibodies and validated the assay in myositis. Longitudinal assessment of SRP levels by ELISA may be a useful biomarker for disease activity.