Neutrophil FcγRIIA availability is associated with disease activity in systemic lupus erythematosus

Abstract

Background: Immune complexes (ICs) are detectable in a variety of inflammatory diseases, including systemic lupus erythematosus (SLE), reflecting autoantibody binding to antigens. Though ICs are the main contributors to disease pathogenesis through FcγR-mediated inflammation and organ damage, IC levels are not part of the clinical assessment of SLE. The aim of this study was to explore the clinical utility of analyzing levels of ICs in SLE patients using a novel technology, IC-FLOW.

Methods: Paired serum samples, at the time point of high and low disease activity (n = 92), were analyzed using two assays: an IC ELISA from a commercial company and a novel in-house flow cytometry-based method, IC-FLOW. IC-FLOW measures FcγRIIA availability on the neutrophil cell surface by flow cytometry, whereas the commercial ELISA measures IC binding to C1q.

Results: Using IC-FLOW, 90% of SLE patients with active disease had elevated levels of circulating ICs (p < 0.0001). Using the commercial assay, only 17% of SLE patients had elevated levels of circulating ICs. For both assays, levels of ICs reflected active disease as determined by SLEDAI (r = 0.45, p < 0.0001) and were associated with type I IFN activity (r = 0.37, p = 0.001), and complement consumption (p = 0.0002). Levels of ICs measured with IC-FLOW, but not with the commercial ELISA, were associated with active lupus nephritis (p = 0.004).

Conclusions: This novel FcγRIIA-IC assay can detect levels of circulating ICs in patients with SLE. Analyzing IC levels may facilitate monitoring of disease activity, as well as identify patients at risk of lupus nephritis, allowing for early preventive interventions.

Keywords: Biomarker; Immune complex; Interferon; Nephritis; Systemic lupus erythematosus.

Fig. 1

Development of a novel assay for the detection of circulating immune complexes. a Levels of FcγRIIA on neutrophils upon addition of heat-aggregated IgG (IC) as determined by binding of FcγRIIA antibody clones IV.3 (red) and FUN-2 (black). b Correlation plot for IV.3 and FUN-2. c, d Levels of ICs in healthy individuals (HC) and patients with SLE at the time point of low and high disease activity as measured by c IV.3 and d commercial assay. e Correlation plot for the two IC assays. Statistical analyses were performed using the Mann-Whitney U test and Spearman’s correlation with *p < 0.05,**p < 0.01, and ***p < 0.001

Fig. 2

Association between levels of circulating immune complexes and disease activity. a Levels of circulating immune complexes were correlated with disease activity, SLEDAI. b–f Levels of ICs were analyzed in patients with (+) or without (−) active b nephritis, c vasculitis, d rash, e oral ulcers, and f arthritis. Statistical analyses were performed using the Mann-Whitney U test and Spearman’s correlation with *p < 0.05,**p < 0.01, and ***p < 0.001

Fig. 3

Levels of circulating immune complexes are associated with autoantibodies and type I IFNs. a Levels of circulating ICs, analyzed by both IC-FLOW and commercial ELISA, were compared between time points of the absence or presence of anti-dsDNA antibodies. b Levels of circulating ICs were analyzed in patients with high disease activity and stratified based on the presence of anti-C1q antibodies (aC1q). c Levels of ICs were correlated with type I IFN activity score. Statistical analyses were performed using the Mann-Whitney U test and Spearman’s correlation with *p < 0.05,**p < 0.01, and ***p < 0.001

Fig. 4

Levels of immune complexes are associated with complement consumption. a Levels of circulating ICs, analyzed by both IC-FLOW and commercial ELISA, were compared between time points of low or high complement C3/C4 levels. b–d Levels of circulating ICs were correlated with levels of b complement C1q, c complement C3, and d complement C4 in patients with high disease activity. Statistical analyses were performed using the Mann-Whitney U test and Spearman’s correlation with *p < 0.05 and ***p < 0.001