Complement activation occurs in patients with probable systemic lupus erythematosus and may predict progression to ACR classified SLE.
OBJECTIVE:To evaluate the frequency of cell-bound complement activation products (CB-CAPs) as a marker of complement activation in patients suspected of systemic lupus erythematosus (probable SLE [pSLE]) and the usefulness of this biomarker as predictor of the evolution of pSLE into classified SLE by the American College of Rheumatology (ACR) criteria. METHODS:Patients suspected of SLE by lupus experts and fulfilling 3 ACR classification criteria for SLE (pSLE) were enrolled, along with patients with established SLE by both ACR and Systemic Lupus International Collaborating Clinics (SLICC) criteria, primary Sjogren's syndrome (SjS), and other rheumatic diseases. Individual CB-CAPs were measured by flow cytometry and positivity rate was compared to that of commonly assessed biomarkers, including serum complement proteins (C3 and C4) and autoantibodies. The frequency of a positive multianalyte assay panel (MAP), which includes CB-CAPs, was also evaluated. pSLE were followed prospectively. RESULTS:The 92 pSLE were diagnosed more recently than the 53 established SLE and the use of antirheumatic medications was lower. More pSLE were positive for CB-CAPs (28%) or MAP (40%) than for low complement (9%) at the enrollment visit (p = 0.0001, for each). In pSLE, MAP score > 0.8 at enrollment predicted fulfillment of a fourth ACR criterion within 18 months (hazard ratio = 3.11, p<0.01). CONCLUSION:Complement activation occurs in some patients with pSLE and can be detected with higher frequency by CB-CAPs and MAP than traditional serum complement protein levels. MAP positivity above 0.8 predicts transition to classifiable SLE according to ACR criteria. This article is protected by copyright. All rights reserved.