Tryptophan and its metabolites have been suggested to play a role in inflammatory processes. However, studies in rheumatoid arthritis (RA) are scarce, which prompted us to investigate two cohorts of RA patients to better understand the importance of tryptophan metabolism in this disease.
Tryptophan and its metabolites were characterised by ELISA in a cross-sectional cohort 1 (81 RA, 55 OA) and a longitudinal cohort 2 (25 RA, 3 visits over 6 months) to investigate discriminatory power between diseases and predicitive value for radiologic outcome, respectively. Radiologic outcome was monitored by RA MRI Score (RAMRIS), including grading of synovitis, bone oedema and erosion, as well as delayed gadolinium-enhanced MRI of cartilage (dGEMRIC) index assessing cartilage quality of the MCP II joint.
RA patients showed higher levels of serum serotonin (RA: 206.8 ng/ml ± 156.7; OA: 81.2 ng/ml ± 63.6) and estimated indoleamine (2,3)-dioxygenase (IDO) activity (kynurenine / tryptophan ratio; RA: 0.065±0.067; OA: 0.021±0.010). IDO activity showed similar, or better discriminatory power between RA and OA (AUC 0.914) than anti-CCP antibody level (AUC 0.922) and rheumatoid factor (RF, AUC 0.783), respectively. In cohort 2, regression analysis revealed a predictive value of baseline serotonin levels and IDO activity for changes in RAMRIS score and erosions at month six, respectively.
This study supports the hypothesis that tryptophan and its metabolites can be used as biomarkers predicting radiologic outcome and discriminate between RA and OA patients. Overall, our results strengthen the notion that tryptophan metabolism is closely linked to RA disease mechanisms.