Multi-center evaluation of autoantibodies to the major ribosomal P C22 epitope.

Anti-ribosomal P (Rib-P) autoantibodies have been demonstrated to be a specific diagnostic marker for systemic lupus erythematosus (SLE). The aim of this study was to evaluate the prevalence of anti-Rib-P (C22) antibodies in patients with SLE drawn from international, multi-center clinics. Sera collected from patients with SLE (n = 333) and various controls (n = 397) in four centers were tested for anti-C22 autoantibodies by ELISA (Dr. Fooke Laboratorien). SLE activity index 2000 (SLEDAI-2K) was assessed for each patient in two centers. Autoantibody profiles were generated for the SLE samples from Canada using two profile assays. Using the manufacturer`s cut-off value, the prevalence of anti-C22 autoantibodies in patients with SLE between the participating centers varied from 18.2 to 29.0%. In the control sera, the prevalence of anti-C22 autoantibodies was low and the titer in the individual control groups varied significantly. In patients with connective tissue disease other than SLE and in patients with infections disease, the anti-C22 reactivity was significantly higher than in healthy controls (P < 0.0001). Overall sensitivity/specificity was 23.1/99.0%, respectively. Anti-Rib-P reactivity was significantly higher in young (mean age 33.9 vs. 45.3 years) SLE patients (P < 0.0001) and was associated with decreased C3 (P = 0.0335) and C4 levels (P = 0.0129). Moderate association between anti-C22 reactivity and SLEDAI-2K was observed in one cohort (P = 0.02). Anti-C22 autoantibodies are frequently and specifically found in patients with SLE. Although an association between anti-C22 reactivity and SLEDAI score was observed in one center, measurement of anti-C22 autoantibodies is likely not appropriate for measuring global disease activity.