Background Rheumatoid arthritis (RA) is a chronic inflammatory disease. kynurenic acid has anti-inflammatory effects, because it is the most important agonist of the orphan G-protein-coupled receptor (GPR35) which expressed on various types of cells associated with the immune system. Stimulation of these receptors by kynurenic acid lead to reduction in the synthesis of proinflammatory cytokines, nitric oxide, and reactive oxygen species (1).
Objectives Detection and quantification of kynurenic acid in serum and synovial fluid obtained from the affected joints in patients with rheumatoid arthritis and its relation to different clinical aspects of disease activity and signs of synovitis and synovial hyperplasia detected by musculoskeletal diagnostic ultrasound.
Methods Thirty RA patients diagnosed according to ACR and EULAR revised criteria and thirty patients of idiopathic knee osteoarthritis as a control group were enrolled in the study. These patients were collected from outpatient clinic of rheumatology department Benha Teaching Hospital. Kynurenic acid was assessed in Serum samples from all patients and controls coupled synovial fluid samples aspirated from knee joint of all RA patients and fourteen OA patients after musculoskeletal ultrasonographic examination of these joints.
Results Serum and synovial level of kynurenic acid was assessed in the studied groups. Comparison between RA and OA patients as regard serum kynurenic acid showed no differences where it's level was 29.80±13.86 pg/ml in RA versus 30.98±11.03 pg/ml in OA patients), while synovial kynurenic acid was significantly lower in RA 16.38±6.45 pg/ml than in OA patients 26.22±2.99 pg/ml (p<0.001). kynurenic acid was significantly lower in synovial fluid (16.38±6.45 pg/ml) than in serum (29.80±13.86 pg/ml) in RA group of patients (p<0.001). Comparison among different grades of synovitis detected by grey scale U/S and by Doppler signals in RA patients as regard synovial kynurenic acid showed that it was significantly lower in higher grades of synovitis (P<0.001). Synovial kynurenic acid level was negatively correlated with grades of synovitis and Doppler signals (p<0.001).
Conclusions The negative correlation between Kynurenic acid concentration in the synovial fluid and both the synovial thickness detected by ultrasonography and the hyperaemia of synovial tissues as represented by the Doppler activity, may support its use as a local marker of the two faces of rheumatoid arthritis (chronicity and activity) at the joint level.
To the best of our knowledge, this is the first study that gives correlation between the serum and synovial levels of kynurenic acid concentrations and the grade of synovitis detected by grey scale and Doppler ultrasonography.
Tistzlavicz Z, Nemeth B, Fulop F et al (2011). Naunyn Schmiedebregs Arch Phrmacol; 383(5):447–55.
Disclosure of Interest None declared