Differential expression of pro-inflammatory cytokines IL-15Ralpha, IL-15, IL-6 and TNFalpha in synovial fluid from Rheumatoid arthritis patients

Abstract

BackgroundPro-inflammatory cytokines are directly implicated in the pathogenesis of Rheumatoid arthritis (RA). Variable clinical response to cytokine targeted therapies as TNFalpha and IL-6, strongly highlights the heterogeneity of inflammatory process in RA. Another cytokine, IL-15 has also been related to the inflammatory process in RA. Recently we described for the first time, the presence of its specific receptor, IL-15Ralpha, in synovial fluid (SF). The aim of this work was to compare the expression profile of IL-15Ralpha, its ligand IL-15, TNFalpha and IL-6 and how these cytokines are correlated in SF from RA patients taking as a reference Osteoarthritis (OA), an articular but not autoinmmune disease.MethodsSynovial fluids were obtained from the knee joints of 60 patients, 30 with confirmed diagnosis of RA and 30 with OA diagnosis. The levels of TNFalpha, IL-6, IL-15 and IL-15Ralpha were measured by ELISA. A statistical analysis was performed with GraphPad Prism v5.0 using the Mann–Whitney U test and Spearman’s rank correlation. A cluster analysis was run in MeV software v4.9.0 and differences across clusters were evaluated by an ANOVA including post-test analysis.ResultsWe found higher and significant levels of TNFalpha, IL-6 and IL-15Ralpha but not of IL-15 in RA compared with the OA group. Additionally, a high inter-individual variability in the levels of these 4 cytokines was observed in RA, although we identified 4 patients’ subgroups by cluster analysis of cytokines concentration in SF. We also found a positive correlation between IL-15Ralpha-IL-6 and IL-15Ralpha-IL-15, but not for other pairs of cytokines in RA. In addition we found correlation between the value of IL-15Ralpha in SF and disease activity score, DAS28.ConclusionsIn our current work we found a high inter-individual variability in the levels of TNFalpha, IL-6, IL-15 and IL-15Ralpha in SF of RA patients and were identified four principal clusters of cytokines concentration in SF, suggesting the importance of identifying disease subset of patients for personalized treatment. Finally, we found a correlation between IL-15Ralpha-IL-6, IL-15Ralpha-IL-15, but we did not find any correlation between other pairs of studied cytokines in SF.

IL15RA is required for osteoblast function and bone mineralization

“…IL15 and IL15RA levels are elevated acutely during low-grade inflammation induced by exercise[8–12], but also chronically in pathological conditions such as rheumatoid arthritis, particularly in the synovial fluid[7]. IL15 SNPs correlate with bone mineral density and cortical bone volume in humans[17,45].…”

“…Bone resorption by osteoclasts releases factors such as Semaphorin-4D, TGFβl, and IGF1 from the bone matrix, which in turn regulate recruitment of mesenchymal progenitor cells and osteoblast differentiation[4]. In addition to RANKL, several other cytokines such as tumor necrosis factor α (TNFα) and interleukins (IL1, IL6, IL7, IL15, IL17) regulate bone formation and maintenance and have been implicated in bone and cartilage pathologies including osteoporosis and rheumatoid arthritis[5–7]. …”

“…Elevated IL15 receptor alpha (IL15RA) levels are found in the synovial fluid of patients affected by rheumatoid arthritis and other chronic inflammatory diseases that are associated with bone loss[7,13]. IL15 was shown to participate in both activation and turnover of osteoclasts mediated by NK cells[14,15].…”

Beyond Autoantibodies: Biologic Roles of Human Autoreactive B Cells in Rheumatoid Arthritis Revealed by RNA‐Sequencing

“…Since IL‐15Rα can also be converted to the soluble form by proteolytic cleavage via the TNFα‐converting enzyme (TACE) (), we next investigated the concentrations of soluble IL‐15Rα (sIL‐15Rα) in the RA patient cohort. Although elevated concentrations of sIL‐15Rα in the synovium are known to be associated with increased disease activity, there are no known reports that have documented elevated sIL‐15Rα concentrations in the serum of seropositive RA patients (). To systematically validate a large number of candidate proteins within the RA patients’ sera, including sIL‐15Rα, we took advantage of the aptamer‐based SOMAscan assay, which can detect analytes at picomolar concentrations by utilizing very small volumes of biologic samples () (as depicted in Supplementary Figure 6, http://onlinelibrary.wiley.com/doi/10.1002/art.40772/abstract).…”

Bone Control of Muscle Function

“…In humans, circulating IL-15 is elevated in response to a single session of resistance exercise in untrained and trained states [61]. Elevated IL-15 receptor alpha (IL15Rα) levels are found in the synovial fluid of patients affected by rheumatoid arthritis and other chronic inflammatory diseases that are associated with bone loss [62,63]. Indeed, IL-15 has a direct effect on Skeletal muscle, as an endocrine organ, as well as bone produce several secreted factors referred to as myokines.…”