Interferon-regulated chemokines as biomarkers of systemic lupus erythematosus disease activity: a validation study

Abstract

Objective: Systemic lupus erythematosus (SLE) is a complex autoimmune disease characterized by unpredictable flares of disease activity and irreversible damage to multiple organ systems. An earlier study showed that SLE patients carrying an interferon (IFN) gene expression signature in blood have elevated serum levels of IFN-regulated chemokines. These chemokines were associated with more-severe and active disease and showed promise as SLE disease activity biomarkers. This study was designed to validate IFN-regulated chemokines as biomarkers of SLE disease activity in 267 SLE patients followed up longitudinally.

Methods: To validate the potential utility of serum chemokine levels as biomarkers of disease activity, we measured serum levels of CXCL10 (IFNgamma-inducible 10-kd protein), CCL2 (monocyte chemotactic protein 1), and CCL19 (macrophage inflammatory protein 3beta) in an independent cohort of 267 SLE patients followed up longitudinally over 1 year (1,166 total clinic visits).

Results: Serum chemokine levels correlated with lupus activity at the current visit (P = 2 x 10(-10)), rising at the time of SLE flare (P = 2 x 10(-3)) and decreasing as disease remitted (P = 1 x 10(-3)); they also performed better than the currently available laboratory tests. Chemokine levels measured at a single baseline visit in patients with a Systemic Lupus Erythematosus Disease Activity Index of < or =4 were predictive of lupus flare over the ensuing year (P = 1 x 10(-4)).

Conclusion: Monitoring serum chemokine levels in SLE may improve the assessment of current disease activity, the prediction of future disease flares, and the overall clinical decision-making.

Figure 1. Clinical profile of ABCoN SLE patient visits

The distribution of disease activity as measured by SLEDAI (panel A) and a physician’s global assessment (PGA, panel B) for 267 SLE patients followed longitudinally for one year (1152 total visits). The percentage of patients taking specific medications or exhibiting organ-specific manifestations at any time during the one year study are shown in panels C and D, respectively. HCQ - hydroxychloroquine, Pred - prednisone, mg/d - milligrams per day, Heme - hematologic, Cytotoxic − Imuran, Cytoxan, Chlorambucil, Methotrexate, and/or CellCept.

Figure 2. Active SLE patients have elevated IFN-regulated chemokine levels

A. Chemokine (CK) levels were compared between a group of single visits from active SLE patients (SLEDAI ≥6; n=76) and an independent group of inactive SLE patients (PGA=0 and SLEDAI ≤2; n=100). Chemokine scores and individual chemokine levels were compared using Mann-Whitney U-tests. Bars represent mean ± standard deviation. B. Individual chemokine levels were normalized to the average of the inactive patients and log₂-transformed. Chemokine scores were visualized on 100 point scale.

Figure 3. Patients with high chemokine levels are more likely to have elevated SLE disease activity

The percentage of patients with active SLE (SLEDAI ≥6) were compared between patients with high levels of chemokines (chemokine score ≥47, n=127) and an independent group of patients with low levels of chemokines (chemokine score ≤24, n=100). P-values were determined by Fisher’s exact test.

Figure 4. IFN-regulated chemokines as biomarkers for disease flares

Chemokine levels and standard laboratory tests were measured in 62 paired pre-flare and flare visits. Flares were defined as an increase in SLEDAI ≥3. Heat maps depict the magnitude and direction of the change in chemokine levels (A) or laboratory results (B) from the pre-flare to the flare visit. Nonparametric paired Wilcoxon tests were used to generate p-values for chemokine levels while paired Student’s t-tests were used for classic laboratory tests. *Changes in complement levels (C3 and C4) are reported inversely, so that a decrease in complement components appears yellow on the heat map. Chemokine levels generally increased with flare, although about 10% of flares were associated with ≥ 25% decrease in chemokine levels.

Figure 5. High baseline chemokine levels identify patients with elevated risk for future SLE flares

SLE patients with low baseline disease activity (SLEDAI <4) were followed for one year. A. Patients who had a flare in any organ system (increase in SLEDAI ≥3) were recorded. Kaplan-Meier plots show the percentage of patients who remained free of flare in any organ system. B. Renal flares were defined as an increase in the renal SLEDAI subscore from 0 to 4+. Plots show the percentage of patients in each group who remained free of renal flares. Vertical tick marks along each curve represent patients who remained flare-free but did not have a full year of clinical follow-up (censored data).