Anti-RNP autoantibodies occur either in mixed connective tissue disease (MCTD) (with a frequently favorable prognosis), or in systemic lupus erythematosus (SLE) cases with aggressive major organ disease. It is uncertain how to assess for the risk of severe disease in anti-RNP + patients.
Methods
Following institutional review board-approved protocols, clinical data and blood were collected from patients with known or suspected anti-RNP autoimmunity and normal controls in a cohort study. Samples were screened for parameters of immune activation. Groups were compared based on clinical diagnoses, disease classification criteria, disease activity and specific end-organ clinical manifestations.
Results
Ninety-seven per cent of patients satisfying Alarcon-Segovia MCTD criteria also met Systemic Lupus International Collaborating Clinic (SLICC) SLE criteria, while 47% of the anti-RNP + SLE patients also met MCTD criteria. Among SLICC SLE patients, MCTD criteria were associated with reduced rates of renal disease (odds ratio (OR) 4.3, 95% confidence interval (CI) 1.3–14.0), increased rates of Raynaud's phenomenon (OR 3.5, 95% CI 1.3–9.5) and increased serum B-cell maturation antigen, transmembrane activator and CAML interactor and TNFα levels. Circulating immune markers and markers of type I interferon activation were not effective at distinguishing clinical subgroups.
Conclusions
Among anti-RNP patients, the question of MCTD versus SLE is not either/or: most MCTD patients also have lupus. MCTD classification criteria (but not a broad set of immune markers) distinguish a subset of SLE patients at reduced risk for renal disease.
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