Researchers investigating the role of the interleukin (IL)-17 producing T-helper (Th17) cell cytokine system have discovered that IL-23 is strongly associated with rheumatoid arthritis (RA) but not spondyloarthritis (SpA).
“Th17 cells are an effecter T-cell population that play a role in several chronic inflammatory conditions and are dependent on IL-23 for their survival and expansion,” explain Dirk Elewaut (Ghent University Hospital, Belgium) and colleagues.
“Recently, a genetic association was discovered between polymorphisms in the gene coding for the IL-23 receptor and SpA.”
To evaluate the role of the Th17 cytokine system in SpA pathogenesis, Elewaut and team obtained paired synovial fluid (SF), serum, and synovial biopsies from 22 psoriatic arthritis (PsA), 30 non-PsA SpA, and 22 RA patients.
They measured IL-17, IL-23, and chemokine ligand (CCL)20, a major Th17 attracting chemokine, levels in the SF and serum of the patients and correlated the results with systemic and local parameters of disease activity.
The researchers report that the degree of local as well as systemic inflammation did not differ between the cohorts.
“This indicates that eventual differences in the levels of inflammatory mediators between the three cohorts would be the result of genuine disease-specific traits rather than merely differences in the intensity of inflammation,” they say.
The results showed that levels of IL-17 in SF tended to be higher in non-PsA SpA compared with PsA and RA patients, whereas SF IL-23 levels were similar.
In serum, there was a tendency for higher IL-23 levels in RA, which correlated strongly with disease activity parameters.
Concentrations of CCL20 were higher in the joints of RA than in SpA patients. Interestingly, levels of CCL20 were markedly higher in synovial fluid compared with serum in all patients.
Of note, there was a significant association between the expression of the Th17 cytokine system and the presence of intimal lining layer hyperplasia in RA.
“These results point towards a differential regulation of the Th17 cytokine system in SpA compared to RA,” conclude Elewaut et al in the Annals of Rheumatic Diseases.
They add that further investigation into this cytokine system as a potential therapeutic target in chronic arthritis is warranted.