Mechanisms dependent on tryptophan catabolism regulate immune responses in primary Sjögren's syndrome.

To investigate the possible role of tryptophan metabolism in immune regulation of primary Sjögren's syndrome (pSS) the serum concentrations of tryptophan and its metabolite kynurenine were measured by reverse-phase high-performance liquid chromatography (HPLC) in 103 patients with pSS, 56 patients with sicca symptoms and 309 healthy blood donors. The kynurenine per tryptophan ratio (kyn/trp), which reflects the activity of the indoleamine-pyrrole 2,3-dioxygenase (IDO) enzyme involved in tryptophan catabolism, was calculated. Both female and male patients with pSS had significantly higher serum kynurenine concentrations and kyn/trp than subjects with sicca symptoms or healthy blood donors. The median (quartile range) concentration of kynurenine in female patients with pSS was 2.41 micromol/l (1.86-3.26) compared with 1.85 micromol/l (1.58-2.38, P < 0.0001) in subjects with sicca symptoms and 1.96 micromol/l (1.65-2.27, P < 0.0001) in healthy blood donors. Their kyn/trp x 1000 was 34.0 (25.1-44.3) compared with 25.3 (21.1-31.5, P < 0.0001) in subjects with sicca symptoms and 24.3 (21.0-28.9, P < 0.0001) in healthy blood donors. Female pSS patients with high IDO activity (kyn/trp x 1000 > or = 34.0) had significantly higher ESR, serum C-reactive protein, serum IgA and serum beta-2 microglobulin concentrations as well as higher serum creatinine levels, and they had positive antinuclear antibodies more frequently and presented with more American-European consensus group criteria than those with low IDO activity (kyn/trp x 1000 < 34.0). These data suggest that mechanisms dependent on tryptophan catabolism regulate immune responses in pSS. Tryptophan degradation is enhanced in patients with pSS, and high IDO activity is associated with severity of pSS.