Objective: Indoleamine 2,3‐dioxygenase (IDO), the rate‐limiting enzyme that converts tryptophan to kynurenine, is driven in part by type I and type II interferons (IFNs). Naive T cells are polarized into FoxP3+ Treg cells upon exposure to either IDO+ cells or kynurenine. Recent studies have suggested that the kynurenine pathway reflects a crucial interface between the immune and nervous system. The aims of the present study were to evaluate whether Treg cell levels are elevated, in conjunction with increased IDO activity, in patients with primary Sjögren's syndrome (SS) who are positive for the IFN gene expression signature, and to investigate the downstream kynurenine pathway in these patients.Methods: Serum from 71 healthy controls, 58 IFN‐negative patients with primary SS, and 66 IFN‐positive patients with primary SS was analyzed using high‐performance liquid chromatography to measure the levels of tryptophan and kynurenine. Expression levels of messenger RNA (mRNA) for IDO and downstream enzymes in the kynurenine pathway were assessed in CD14+ monocytes using real‐time quantitative polymerase chain reaction. CD4+CD45RO+ T helper memory cell populations were analyzed by flow cytometry.Results: Significantly increased levels of IDO activity (assessed as the kynurenine:tryptophan ratio) (P = 0.0054) and percentages of CD25^highFoxP3+ Treg cells (P = 0.039) were observed in the serum from IFN‐positive patients with primary SS, and these parameters were significantly correlated with one another (r = 0.511, P = 0.002). In circulating monocytes from IFN‐positive patients with primary SS, the expression of IDO1 mRNA was up‐regulated (P < 0.0001), and this was correlated with the IFN gene expression score (r = 0.816, P < 0.0001). Interestingly, the proapoptotic and neurotoxic downstream enzyme kynurenine 3‐monooxygenase was up‐regulated (P = 0.0057), whereas kynurenine aminotransferase I (KATI) (P = 0.0003), KATIII (P = 0.016), and KATIV (P = 0.04) were down‐regulated in IFN‐positive patients with primary SS compared to healthy controls.Conclusion: These findings demonstrate enhanced IDO activity in conjunction with increased percentages of CD25^highFoxP3+ Treg cells in primary SS patients who carry the IFN signature. In addition, IFN‐positive patients with primary SS exhibit an imbalanced kynurenine pathway, with evidence of a shift toward potentially more proapoptotic and neurotoxic metabolites. Intervening in these IFN‐ and IDO‐induced immune system imbalances may offer a new array of possibilities for therapeutic interventions in patients with primary SS.

Childhood-onset systemic lupus erythematosus (cSLE) is an incurable multi-systemic autoimmune disease. Interferon type I (IFN-I) plays a pivotal role in the pathogenesis of SLE. The objective of this study was to assess the prevalence of the IFN-I signature and the contribution of cytosolic nucleic acid receptors to IFN-I activation in a cohort of primarily white cSLE patients. The IFN-I score (positive or negative), as a measure of IFN-I activation, was assessed using real-time quantitative PCR (RT-PCR) expression values of IFN-I signature genes (IFI44, IFI44L, IFIT1, Ly6e, MxA, IFITM1) in CD14+ monocytes of cSLE patients and healthy controls (HCs). Innate immune receptor expression was determined by RT-PCR and flow cytometry. To clarify the contribution of RNA-binding RIG-like receptors (RLRs) and DNA-binding receptors (DBRs) to IFN-I activation, peripheral blood mononuclear cells (PBMCs) from patients were treated with BX795, a TANK-binding kinase 1 (TBK1) inhibitor blocking RLR and DBR pathways. The IFN-I signature was positive in 57% of cSLE patients and 15% of the HCs. Upregulated gene expression of TLR7, RLRs (IFIH1, DDX58, DDX60, DHX58) and DBRs (ZBP-1, IFI16) was observed in CD14+ monocytes of the IFN-I-positive cSLE patients. Additionally, RIG-I and ZBP-1 protein expression was upregulated in these cells. Spontaneous IFN-I stimulated gene (ISG) expression in PBMCs from cSLE patients was inhibited by a TBK1-blocker. IFN-I activation, assessed as ISG expression, in cSLE is associated with increased expression of TLR7, and RNA and DNA binding receptors, and these receptors contribute to IFN-I activation via TBK1 signaling. TBK1-blockers may therefore be a promising treatment target for SLE.