Hypoxia decreases the T helper cell-suppressive capacity of synovial fibroblasts by downregulating IDO1-mediated tryptophan metabolism.

OBJECTIVE: The development of RA is linked to local infiltration of immune cells and to changes in the phenotype of synovial fibroblasts. Synovial fibroblasts possess the capacity to suppress T cell responses through indoleamine 2, 3-dioxygenase 1 (IDO1)-mediated tryptophan metabolism. However, synovial fibroblasts from RA patients are restricted in this immune-modulatory function. Moreover, hypoxic conditions are detected within synovial tissues of RA patients, with oxygen tensions of only 3.2% O2. This study aims at investigating the effects of hypoxia on the interaction between T cells and synovial fibroblasts, particularly on the T cell-suppressive capacities of synovial fibroblasts. METHODS: Synovial fibroblasts were cultured with Th cells under normoxic and hypoxic conditions (3% O2). Th cell proliferation was detected by flow cytometry. Tryptophan and kynurenine amounts were measured by HPLC. IDO1 expression and signal transducer and activator of transcription 1 (STAT1) phosphorylation were quantified by real-time PCR or western blot, and cytokine secretion by ELISA. RESULTS: Hypoxic conditions strongly diminished the Th cell-suppressive capacities of both OA synovial fibroblasts and RA synovial fibroblasts. Accordingly, IDO1 mRNA and protein expression, STAT1 phosphorylation and tryptophan metabolism were greatly reduced in OA synovial fibroblasts by hypoxia. MMP-3, IL-6, IL-10 and IFNγ secretion were significantly decreased under hypoxia in synovial fibroblast-Th cell co-cultures, while IL-17A levels were elevated. Supplementation with IFNγ, a well-known inducer of IDO1 expression, could rescue neither IDO1 expression nor Th cell suppression under hypoxic conditions. CONCLUSION: Hypoxia strongly affected the crosstalk between synovial fibroblasts and Th cells. By reducing the efficiency of synovial fibroblasts to restrict Th cell proliferation and by increasing the expression of IL-17A, hypoxia might have implications on the pathophysiology of RA.

Synovial Fibroblasts Selectively Suppress Th1 Cell Responses through IDO1-Mediated Tryptophan Catabolism.

The development of rheumatoid arthritis (RA) is linked to functional changes in synovial fibroblasts (SF) and local infiltration of T lymphocytes. Fibroblasts possess the capacity to suppress T cell responses, although the molecular mechanisms of this suppression remain incompletely understood. In this study, we aimed to define the mechanisms by which noninflammatory SF modulate Th cell responses and to determine the immunosuppressive efficacy of RASF. Hence, the influence of SF from osteoarthritis or RA patients on total Th cells or different Th cell subsets of healthy donors was analyzed in vitro. We show that SF strongly suppressed the proliferation of Th cells and the secretion of IFN-γ in a cell contact-independent manner. In cocultures of SF and Th cells, tryptophan was completely depleted within a few days, resulting in eukaryotic initiation factor 2α phosphorylation, TCRζ-chain downregulation, and proliferation arrest. Blocking IDO1 activity completely restored Th cell proliferation, but not IFN-γ production. Interestingly, only the proliferation of Th1 cells, but not of Th2 or Th17 cells, was affected. Finally, RASF had a significantly lower IDO1 expression and a weaker Th cell suppressive capacity compared with osteoarthritis SF. We postulate that the suppression of Th cell growth by SF through tryptophan catabolism may play an important role in preventing inappropriate Th cell responses under normal conditions. However, expansion of Th17 cells that do not induce IDO1-mediated suppression and the reduced capacity of RASF to restrict Th cell proliferation through tryptophan metabolism may support the initiation and propagation of synovitis in RA patients.