Circulating angiopoietin-2 as a biomarker in ANCA-associated vasculitis

Abstract

The endothelial-specific Angiopoietin-Tie2 ligand-receptor system is an important regulator of endothelial activation. Binding of angiopoietin-2 (Ang-2) to Tie2 receptor renders the endothelial barrier responsive to pro-inflammatory cytokines. We previously showed that circulating Ang-2 correlated with disease severity in a small cohort of critically ill patients with anti-neutrophil cytoplasmic antibody (ANCA)-associated glomerulonephritis. The current study reassessed Ang-2 as a biomarker of disease activity and relapse in AAV. Circulating Ang-2 was measured in 162 patients with severe AAV (BVAS/WG≥3, with or without glomerulonephritis) in a clinical trial. Ang-2 levels during active AAV were compared to levels in the same patients during remission (BVAS/WG = 0). Levels in clinical subsets of AAV were compared, and association with future disease course was assessed. Ang-2 levels were elevated in severe disease (median 3.0 ng/ml, interquartile range 1.9-4.4) compared to healthy controls (1.2, 0.9-1.5). However, they did not reliably decline with successful treatment (median 2.6 ng/ml, interquartile range 1.9-3.8, median change -0.1). Ang-2 correlated weakly with BVAS/WG score (r = 0.17), moderately with markers of systemic inflammation (r = 0.25-0.41), and inversely with renal function (r = -0.36). Levels were higher in patients with glomerulonephritis, but levels adjusted for renal dysfunction were no different in patients with or without glomerulonephritis. Levels were higher in patients with newly diagnosed AAV and lower in patients in whom treatment had recently been started. Ang-2 levels during active disease did not predict response to treatment, and Ang-2 levels in remission did not predict time to flare. Thus, Ang-2 appears to have limited practical value in AAV as a biomarker of disease activity at time of measurement or for predicting future activity.

Figure 1. Angiopoietin-2 (Ang-2) levels at different levels of disease activity in ANCA-associated vasculitis.

A. Serum levels in individual patients in the RAVE trial before (Screening) and after (Month 6) treatment, stratified by whether patients were in remission (left panel) or had recurrent disease (right panel) at month 6. B. Plot of serum Ang-2 versus BVAS/WG score in individual subjects in RAVE at screening. Each patient had severe disease and therefore a BVAS/WG score of at least 3. C. Plot of plasma Ang-2 versus two measures of disease activity in the VCRC longitudinal study: Physician Global Assessment (PGA, left panel) and BVAS/WG score (right panel). Note that patients in remission (PGA = 0 and BVAS/WG = 0) are included, and that Ang-2 levels are lower in plasma than in serum.

Figure 2. Plots of angiopoietin-2 (Ang-2) levels versus markers of systemic inflammation.

A. Serum Ang-2 versus ESR (left panel) and CRP (right panel) in the RAVE trial. Values at screening, when each patient had severe disease (BVAS/WG≥3), are shown. B. Plasma Ang-2 versus ESR (left panel) and CRP (right panel) in the VCRC longitudinal study. Note that patients in remission (BVAS/WG = 0) are included, and that Ang-2 levels are lower in plasma than in serum.

Figure 3. Plots of serum angiopoietin-2 (Ang-2) levels versus GFR (top panels) or age (bottom panels).

Samples from subjects in RAVE were assayed at times of active AAV (screening, left panels) and remission 6 months after starting treatment (right panels).