Local cytotoxic T-lymphocyte-associated antigen-4 immunoglobulin inhibition of rejection response is dependent on indoleamine 2,3-dioxygenase activities in the allograft

The Kynurenine Pathway in Acute Kidney Injury and Chronic Kidney Disease

BACKGROUND

The kynurenine pathway (KP) is the major catabolic pathway for tryptophan degradation. The KP plays an important role as the sole de novo nicotinamide adenine dinucleotide (NAD+) biosynthetic pathway in normal human physiology and functions as a counter-regulatory mechanism to mitigate immune responses during inflammation. Although the KP has been implicated in a variety of disorders including Huntington's disease, seizures, cardiovascular disease, and osteoporosis, its role in renal diseases is seldom discussed.

SUMMARY

This review summarizes the roles of the KP and its metabolites in acute kidney injury (AKI) and chronic kidney disease (CKD) based on current literature evidence. Metabolomics studies demonstrated that the KP metabolites were significantly altered in patients and animal models with AKI or CKD. The diagnostic and prognostic values of the KP metabolites in AKI and CKD were highlighted in cross-sectional and longitudinal human observational studies. The biological impact of the KP on the pathophysiology of AKI and CKD has been studied in experimental models of different etiologies. In particular, the activation of the KP was found to confer protection in animal models of glomerulonephritis, and its immunomodulatory mechanism may involve the regulation of T cell subsets such as Th17 and regulatory T cells. Manipulation of the KP to increase NAD+ production or diversion toward specific KP metabolites was also found to be beneficial in animal models of AKI. Key Messages: KP metabolites are reported to be dysregulated in human observational and animal experimental studies of AKI and CKD. In AKI, the magnitude and direction of changes in the KP depend on the etiology of the damage. In CKD, KP metabolites are altered with the onset and progression of CKD all the way to advanced stages of the disease, including uremia and its related vascular complications. The activation of the KP and diversion to specific sub-branches are currently being explored as therapeutic strategies in these diseases, especially with regards to the immunomodulatory effects of certain KP metabolites. Further elucidation of the KP may hold promise for the development of biomarkers and targeted therapies for these kidney diseases.

IDO inhibitor synergized with radiotherapy to delay tumor growth by reversing T cell exhaustion

Previous studies suggest that radiotherapy (RT) can induce immune activation, which not only reduces the progression of tumors, but also causes the release of tumor antigens. The combination of RT and immune checkpoint blockade, such as the inhibition of programmed cell death 1 (PD‑1) and programmed cell death ligand 1 (PD‑L1), has been demonstrated to yield impressive response rates. However, a substantial proportion of patients develop resistance such therapies. Previous studies have shown that indoleamine 2,3‑dioxygenase (IDO) causes T cell exhaustion and increased formation of regulatory T cells (Tregs), upregulating the expression of inhibitory receptors and ligands. Therefore, the application of IDO inhibitors combined with RT may have a synergistic effect by relieving immunosuppression. Lewis lung cancer cell‑bearing mice were treated with the IDO inhibitor 1‑methyl‑tryptophan (1MT) and/or 10 Gy RT. Tumor size was measured every day, flow cytometry was performed to measure the expression of dendritic cell (DC) maturation markers, inhibitory receptors, ligands, cytotoxic T cells and Treg phenotypic markers. Reverse transcription‑quantitative PCR was used to evaluate the mRNA expression levels of IDO, PD‑L1, PD‑1, T cell immunoglobulin domain and mucin domain 3 (TIM‑3), B‑ and T‑lymphocyte attenuator (BTLA) and galectin‑9. Compared with the control group, treatment with 1MT or RT reduced tumor growth, however, the combination therapy was more effective than either treatment alone. Flow cytometry showed the upregulation of CD80, CD86 and the major histocompatibility complex II in spleen DCs and the concurrent downregulation of CD4, CD25 and forkhead box protein P3 in lymphocytes in the treatment groups. Compared with the control group, inhibitory receptors and ligands that affect DCs and T cells were observed, expression levels of PD‑L1, PD‑1, TIM‑3, BTLA and galectin‑9 are decreased in treatment group compared with control. IDO inhibition synergized with RT to downregulate Tregs, inhibitory receptors and ligands to prevent T cell exhaustion. By activating DCs and T cells, this combination therapy may strongly enhance antitumor immunity and inhibit tumor progression.

Indoleamine 2,3-Dioxygenase Activation by Interferon Gamma in Vascular Endothelial Rat Cells Requires Noncanonical NF-κB Signaling

Indoleamine 2,3-dioxygenase (IDO) is an important enzyme in immune response regulation; cells that express IDO can suppress T-cell responses. Endothelial cells (ECs) play an important role in graft rejection. Therefore, we investigated the role of IDO expression by vascular ECs in immunoregulation. We demonstrated that interferon gamma can upregulate IDO expression in cultured ECs. Moreover, IDO induction by interferon gamma required IKKα activation, a part of the noncanonical NF-κB signaling pathway. In addition, IκB kinase α (IKKα) silencing resulted in significantly reduced IPO expression, demonstrating an essential role of NF-κB signaling pathway in IDO activation in vitro. These results may have an implication for regulating the immune response to alloantigens. The results obtained are important not only in understanding the role of ECs in the regulation of the transplantation immune response, but also in determining a potential therapeutic target for inhibiting allograft rejection.

Targeting CTLA-4, PD-L1 and IDO to modulate immune responses in vitiligo

For decades, an extensive debate is continued on the pathophysiology of vitiligo. Numerous hypotheses have been put forward, and many supported by well-documented arguments. Regardless of the initiating steps, most experts agree that an immune-based melanocyte destruction is responsible for the final steps leading to epidermal depigmentation. It is remarkable that currently the only therapeutic approach to counter this phenomenon consists of non-specific local and systemic immunosuppressants. Immunotherapy for melanoma reveals that targeting factors involved in peripheral tolerance are sufficient to break the natural defense mechanisms to develop skin depigmentations. Therapeutically enhancing these immune checkpoints seems therefore a promising long-term therapy for vitiligo. In this viewpoint, we propose this strategy as a promising therapeutic option for vitiligo. Several approaches are proposed with a focus on cytotoxic T-lymphocyte-associated protein 4, programmed death ligand-1 and indoleamine 2,3-dioxygenase.

Local gene therapy with indoleamine 2,3-dioxygenase protects against development of transplant vasculopathy in chronic kidney transplant dysfunction

Chronic transplant dysfunction (CTD) is the primary cause of late allograft loss in kidney transplantation. Indoleamine 2,3-dioxygenase (IDO) is involved in fetomaternal tolerance and IDO gene therapy inhibits acute rejection following kidney transplantation. The aim of this study is to investigate whether gene therapy with IDO is able to attenuate CTD. Transplantation was performed in a rat Dark-Agouti to Wistar-Furth CTD model. Donor kidneys were incubated either with an adenovirus carrying IDO gene, a control adenovirus or saline. During the first 10 days recipients received low-dose cyclosporine. Body weight, blood pressure, serum creatinine and proteinuria were measured every 2 weeks. Rats were killed after 12 weeks. IDO had a striking beneficial effect on transplant vasculopathy at week 12. It also significantly improved body weight gain; it reduced blood pressure and decreased proteinuria during the follow-up. However, it did not affect the kidney function. In addition, IDO therapy significantly decreased the number of graft-infiltrating macrophages at week 12. The messenger RNA levels of forkhead box p3 and transforming grow factor-β were elevated in the IDO treated group at week 12. Here we show for first time a clear beneficial effect of local IDO gene therapy especially on transplant vasculopathy in a rat model of renal CTD.