Survival of lymphocytes is not restricted by IDO-expressing fibroblast from rheumatoid arthritis patients

Objective: Rheumatoid arthritis (RA) is characterized by expansion of fibroblast-like synoviocytes (FLS) in inflamed joints and activation of lymphocytes. Tryptophan (trp) is an essential amino acid indispensable for the biosynthesis of proteins and critical for survival of lymphocytes. Indoleamine 2,3-dioxygenase (IDO) that initiates the degradation of trp and tryptophanyl-tRNA synthetase (TTS) essential for tryptophan synthesis, regulate trp bioavailability. Here, we tested the hypothesis that triggered by cytokines, enhanced IDO activity modulate regulatory function of otherwise non-tolerogenic FLS isolated from RA patients. Materials and methods: IDO and TTS mRNA expression were evaluated by RT-PCR. IDO enzymatic activity was confirmed using HPLC. Resting or PHA-activated PBMC from healthy volunteers and RA patients were co-cultured with IDO expressing untreated (FLS_C) or IFNγ-treated (FLS_IFNγ) RA FLS. Lymphocyte survival and proliferation were evaluated by flow cytometry analysis and tritiated thymidine incorporation, respectively. Results: RA FLS_IFNγ produce functionally active IDO and constitutively express TTS. RA FLS_C and FLS_IFNγ increased survival of resting lymphocytes in both studied groups, and decreased proliferation of healthy, but not RA, PBMC. Only FLS_IFNγ diminished survival of activated CD3⁺CD4^-, but not CD3⁺CD4⁺, healthy T cells and similar tendency was observed in rheumatoid cells. Importantly, IDO inhibitor, 1-methyl-DL-tryptophan (1-MT), failed to reverse this effect. PBMC, irrespective of their state (resting versus activated) or origin (healthy or RA), expressed high level of TTS mRNA. Conclusions: We suggest that RA FLS express functionally active IDO but control survival and expansion of healthy cells in IDO-independent mechanism and exert weaker, if any, suppressive effect on rheumatoid cells.