Impaired JAK/STAT1 Pathway Leads to Defective Interferon-gamma Induced IDO Expression in NOD Dermal Fibroblasts

Introduction: Indoleamine 2,3-dioxygenase (IDO) is the rate limiting enzyme in catabolism of the least abundant of all amino acids, tryptophan. IDO is known to be involved in the local suppression of T cell-mediated immune response.  It is shown that impaired IDO expression in dendritic cells of early prediabetic female nonobese diabetic (NOD) mice is involved in their defective immune tolerance. NOD strain of mouse is known as the prototype of autoimmune diseases and mainly type one diabetes. This strain develops a T cell-mediated destruction of the insulin producing β-cells with a higher incidence in female mice compared to male ones. Objectives: Knowing that IFN-γ-induced IDO expression in many different cell types including fibroblasts is important in controlling the number and viability of CD4⁺ and CD8⁺ cells, here, we asked the question of whether IFN-γ induces the expression of IDO in fibroblasts from NOD mice. Methods: We compared dermal fibroblasts from NOD and C57BL/6 mice in terms of: 1) IFN-γ-induced-IDO enzyme activity and expression, 2) Other IFN-γ-mediated pathways such as MHC-I expression, and 3) Modulation of collagen type I (Col-I) expression, 4) IDO protein translational capacity, 5) Phosphorylation of signal transducers and activators of transcription1 (STAT1), a key IFN-γ signal transducer in IFN-γ-mediated-IDO expression pathway and finally, 6) Ability of NOD fibroblasts to respond to an IFN-γ-independent stimulus such as LPS in terms of IDO expression. Results: Using Western blot and PCR analyses, we a showed that the fibroblasts from NOD mice failed to express IDO following IFN-γ treatment, regardless of the gender or status of diabetes. Evaluating the expression of other IFN-γ-modulated factors, we found an increase in MHC-I and suppression of Col-I expression in fibroblasts from both NOD and control mice following IFN-γ treatment; indicating that the impaired response to IFN-γ in NOD fibroblasts is specific to IDO gene and other IFN-γ-mediated gene expression pathways seem to be operative. Further assessing the mechanism underlying this defect, we found a defect of STAT1 phosphorylation in these cells relative to that of control.  Finally, we showed that an IFN-γ-independent IDO expression pathway i.e. LPS-mediated-c-Jun kinase is functional. Conclusions: The findings of this study for the first time indicate that IFN-γ fails to induce IDO expression in NOD dermal fibroblasts; this might partially be due to impaired STAT1 phosphorylation in IFN-γ-induced-IDO signaling pathway.