Interleukin 15 levels in serum may predict a severe disease course in patients with early arthritis.

BACKGROUND: Interleukin-15 (IL-15) is thought to be involved in the physiopathological mechanisms of RA and it can be detected in the serum and the synovial fluid of inflamed joints in patients with RA but not in patients with osteoarthritis or other inflammatory joint diseases. Therefore, the objective of this work is to analyse whether serum IL-15 (sIL-15) levels serve as a biomarker of disease severity in patients with early arthritis (EA). METHODOLOGY AND RESULTS: Data from 190 patients in an EA register were analysed (77.2% female; median age 53 years; 6-month median disease duration at entry). Clinical and treatment information was recorded systematically, especially the prescription of disease modifying anti-rheumatic drugs. Two multivariate longitudinal analyses were performed with different dependent variables: 1) DAS28 and 2) a variable reflecting intensive treatment. Both included sIL-15 as predictive variable and other variables associated with disease severity, including rheumatoid factor (RF) and anti-cyclic citrullinated peptide antibodies (ACPA). Of the 171 patients (638 visits analysed) completing the follow-up, 71% suffered rheumatoid arthritis and 29% were considered as undifferentiated arthritis. Elevated sIL-15 was detected in 29% of this population and this biomarker did not overlap extensively with RF or ACPA. High sIL-15 levels (ß Coefficient [95% confidence interval]: 0.12 [0.06-0.18]; p<0.001) or ACPA (0.34 [0.01-0.67]; p = 0.044) were significantly and independently associated with a higher DAS28 during follow-up, after adjusting for confounding variables such as gender, age and treatment. In addition, those patients with elevated sIL-15 had a significantly higher risk of receiving intensive treatment (RR 1.78, 95% confidence interval 1.18-2.7; p = 0.007). CONCLUSIONS: Patients with EA displaying high baseline sIL-15 suffered a more severe disease and received more intensive treatment. Thus, sIL-15 may be a biomarker for patients that are candidates for early and more intensive treatment.

Characterization of serum interleukin-15 in healthy volunteers and patients with early arthritis to assess its potential use as a biomarker.

As interleukin-15 (IL-15) has been implicated in the pathophysiology of rheumatoid arthritis, we analysed the serum IL-15 (sIL-15) levels in healthy subjects and patients with early arthritis to establish a cut-off point that might serve to define elevated sIL-15. This is an initial step to determine whether sIL-15 has the potential for use as a biomarker for patients with early arthritis. The IL-15 concentration was measured in serum obtained from 161 healthy controls and from 174 patients with early arthritis, and the relationship between the expression of the two IL-15 mRNA variants and the sIL-15 levels was also assessed. In healthy controls, the median sIL-15 value was 0.83 [interquartile range (IQR) 0-8.68] pg/mL; there was no significant difference in the sIL-15 values according to gender [median level in males was 1.99 (IQR: 0-8.68) pg/mL and in females 0.50 (0-8.25) pg/mL: p = 0.821]. Moreover, sIL-15 levels did not correlate with age (r = 0.033, p = 0.685), and they did not display a clear circadian rhythm in healthy donors, with the median values for IL-15 close to zero at each time tested. In the light of these findings, we considered that sIL-15 was elevated if its concentration was above 20 pg/mL, since this cut-off point corresponded to the 90th percentile for this healthy population. We found that 30% of the patients with early arthritis had sIL-15 values > 20 pg/mL. The levels of sIL-15 did not correlate with disease duration in early arthritis patients, nor did they fluctuate with changes in disease activity over the follow-up period. In addition, the high level of sIL15 in patients was not associated with alterations in the alternative splicing of the IL-15 mRNA, favouring the variant that produces the protein with a long signal peptide for secretion. Serum IL-15 levels were increased in a subpopulation of patients with early arthritis, indicating that this measure may serve as a biomarker for this condition. Further studies will be necessary to determine whether the clinical evolution or response to treatment of patients with high sIL-15 levels differs.