Low-grade inflammation in symptomatic knee osteoarthritis: prognostic value of inflammatory plasma lipids and peripheral blood leukocyte biomarkers

Abstract

Objective: Inflammatory mediators, such as prostaglandin E2 (PGE2 ) and interleukin-1β (IL-1β), are produced by osteoarthritic (OA) joint tissue, where they may contribute to disease pathogenesis. We undertook the present study to examine whether inflammation, evidenced in plasma and peripheral blood leukocytes (PBLs), reflects the presence, progression, or specific symptoms of symptomatic knee OA.

Methods: Patients with symptomatic knee OA were enrolled in a 24-month prospective study of radiographic progression. Standardized knee radiographs were obtained at baseline and 24 months. At baseline, levels of the plasma lipids PGE2 and 15-hydroxyeicosatetraenoic acid (15-HETE) were measured, and transcriptome analysis of PBLs was performed by microarray and quantitative polymerase chain reaction.

Results: Baseline PGE2 synthase (PGES) levels determined by PBL microarray gene expression and plasma PGE2 levels distinguished patients with symptomatic knee OA from non-OA controls (area under the receiver operating characteristic curve [AUC] 0.87 and 0.89, respectively, P < 0.0001). Baseline plasma 15-HETE levels were significantly elevated in patients with symptomatic knee OA versus non-OA controls (P < 0.0195). In the 146 patients who completed the 24-month study, elevated baseline expression of IL-1β, tumor necrosis factor α, and cyclooxygenase 2 (COX-2) messenger RNA in PBLs predicted higher risk of radiographic progression as evidenced by joint space narrowing (JSN). In a multivariate model, AUC point estimates of models containing COX-2 in combination with demographic traits overlapped the confidence interval of the base model in 2 of the 3 JSN outcome measures (JSN >0.0 mm, JSN >0.2 mm, and JSN >0.5 mm; AUC 0.62-0.67).

Conclusion: The inflammatory plasma lipid biomarkers PGE2 and 15-HETE identify patients with symptomatic knee OA, and the PBL inflammatory transcriptome identifies a subset of patients with symptomatic knee OA who are at increased risk of radiographic progression. These findings may reflect low-grade inflammation in OA and may be useful as diagnostic and prognostic biomarkers in clinical development of disease-modifying OA drugs.

Figure 1

Baseline plasma PGE2 is elevated in NYUHJD learning, NYUHJD progression and Pfizer cohorts. Plasma levels of PGE2 were determined using ACE EIA kit as described in Methods. The NYUHJD learning cohort (A) consists of symptomatic knee osteoarthritis (SKOA) cases (n=42) and non-OA controls (n=20); the NYUHJD progression cohort (B, D) consists of SKOA cases (n=178) and non-OA controls (n=21). A) Mean PGE2 levels in the learning cohort were 91.8±44.9 pg/ml (range: 35.4–249.1) in SKOA and 63.2±14.22 (41.2–105.8) in controls. B) Mean PGE2 levels in the progression cohort were 159.0±83.8 (43.7–690.2) in SKOA and 72.4±33.5 (13.53–152.1) in controls. The solid horizontal bar in each group represents the mean. Non-parametric, Mann-Whitney test was performed to calculate statistical significance using GraphPad Prism 4.0. The p values for cases versus controls are shown for each biomarker. C) Baseline plasma PGE2 in Pfizer validation cohort consists of SKOA cases (n=66) and non-OA controls (n=12). D) ROC curve of NYUHJD progression cohort baseline plasma PGE2 for distinguishing NYUHJD progression cohort SKOA cases from non-OA controls with age, gender and BMI. The curve line depicts AUC in the 10-fold cross-validation repeated 100 times.

Figure 2

Baseline plasma 15-hydroxyeicosatetraenoic acid (15-HETE) is elevated in NYUHJD progression (A) and Pfizer validation cohorts (B). C) Baseline urinary type II collagen neoepitope (uTIINE) is elevated in Pfizer validation cohort [LC-MS/MS (Liquid Chromatography-Tandem Mass Spectrometry)]. The solid horizontal bar in each group represents the mean value. Non-parametric, Mann-Whitney test was performed to calculate statistical significance using GraphPad Prism 4.0. The p values for cases versus controls are shown for each biomarker. Mean (±SD) 15-HETE (ng/ml) levels in NYUHJD progression SKOA cases were 0.44±0.48 (0.0–2.3) and in controls and 0.22±0.26 (0.0–1.1), respectively. Mean (±SD) 15-HETE (ng/ml) levels in Pfizer SKOA cases were 1.3±0.45 (0.71–3.2), and in controls 0.72±0.16 (0.49–0.94), respectively.

Figure 3

Prostaglandin E synthase mRNA is elevated in SKOA patients. A) Hierarchical clustering of NYUHJD learning cohort SKOA cases and non-OA controls based on prostaglandin E synthase U133A probe sets. B) Receiver operating characteristic (ROC) curve of membrane PGE synthase (Affymetrix U133A array probe sets (207388_s_at, 200627_at, 210367_s_at, 218083_at) for distinguishing NYUHJD Learning cohort SKOA cases from non-OA controls with and without age, gender and BMI. The curve line depicts the AUC in the 10-fold cross validation repeated 100 times. The dotted line represents the ROC curve of null model AUC (0.5). C) ROC curve of plasma PGE2 levels from ex vivo blood PBL cultures distinguishing NYUHJD progression cohort SKOA cases from non-OA controls with and without age, gender and BMI. The curve line depicts AUC in the 10-fold cross validation repeated 100 times.