Circulating levels of IL-7 in rheumatoid arthritis

Abstract

We have previously demonstrated reduced circulating levels of IL-7 in active rheumatoid arthritis (RA) compared with health [1]. Controversy exist as to whether IL-7 correlates or not with C-reactive protein (CRP) in active disease or with other disease activity markers. Normal IL-7 levels were, however, found in 50% of patients in clinical remission and correlated with the recovery of thymic activity [1]. Patients in clinical remission may represent a heterogeneous group, and the aim of this work was to identify predictors of IL-7 recovery in demographic, clinical, imaging and functional data and to compare them with IL-7 in active disease. One hundred and six patients deemed to be in clinical remission were recruited: stable disease for the preceding 6 months, previous disease duration of at least 12 months, no clinically significant synovitis, CRP below 15 mg/l for the preceding 6 months. Blood and serum were collected from these patients (n = 106). Clinical data and imaging data were gathered at the time of sampling. High-sensitivity ELISA was used for cytokine analysis, proliferation assays for T-cell function (mitogen, T-cell receptor stimulation, IL-2, recall antigens); real-time PCR to quantify T-bet and GATA3 expression, and DNA sequencing was carried out to investigate two genetic polymorphisms in the IL-7 gene. Several studies in healthy controls indicate that normal levels of circulating IL-7 are between 10 and 25 pg/ml. In remission, circulating levels of IL-7 vary between 2.47 and 23.85 pg/ml. Recovery of T-cell function was directly related to levels of circulating IL-7 in vivo (P < 0.001, R = 0.873 for phytohaemagglutinin, R = 0.786 for T-cell receptor stimulation and R = 0.821 for recall antigen). Age and sex had no effect on IL-7. Combining active (n = 35) and remission patients (n = 106) showed no indication of a relationship between IL-7 and routine measures of disease activity (CRP, DAS28, erythrocyte sedimentation rate, plasma viscosity and joint counts), suggesting that recovery of IL-7 is not an indicator of remission. To confirm this observation, we used imaging data (MRI and US assessment of hand and wrist) to address whether subclinical disease could predict lack of IL-7 recovery. Evidence of subclinical synovitis was found in 96% of patients in remission [2] despite no evidence of clinically significant synovitis and there was no relationship between IL-7 and imaging scores. However, we found that levels of IL-7 in remission and the age of the patient at disease onset correlate (P < 0.001, R = 0.498, n = 86). A family history of RA and smoking at the time of onset (both self-reported) were also strongly associated with lower levels of IL-7 (both P < 0.001, n = 66). We analysed two polymorphisms in the promoter and enhancer regions of the IL-7 gene. We found no allele frequency difference between RA patients and healthy controls, remission patients with or without family history, or in relation to IL-7 levels. Finally we analysed circulating cytokines known to regulate IL-7 expression, and found a strong correlation between IL-7 and IFNγ (P < 0.03, R = 0.650, n = 10). Since IL-7 is a co-activator of Th1 polarisation, we analysed the expression of T-bet and found a direct relationship between the two (P < 0.01, R = 0.601, n = 15) but not with GATA3, which is a regulator of Th2 polarisation. Recent evidence suggests an important role for IL-7 in the pathogenesis of RA [3]. Our data demonstrate that circulating IL-7 is not an indicator of disease activity. Despite the relationship with age at onset (genetic anticipation) and the family history association, levels of IL-7 in the circulation are apparently not driven by these two polymorphisms. Our results suggest that Th1 polarisation and IL-7 are related and that the mechanism by which IL-7 is recovered in remission may be associated with the recovery of Th1 polarisation.