br KYN Acts on AhR To Induce Tolerogenic Immunity IDO

KYN Acts on AhR To Induce Tolerogenic Immunity IDO1 and TDO2 are intracellular heme-containing metalloproteins that catalyze the committing and rate-limiting step of the KYN pathway (KP) that converts the essential amino Berberine mg tryptophan to a series of biologically active second metabolites (Figure 1). The physiological and pathological function of the homolog IDO2 is far less understood thus not discussed in this review article. Unlike cell-surface checkpoint receptor molecules that can be effectively targeted by antibody-based therapeutics, IDO1/TDO2 and their downstream effector molecules are intracellular targets that are still best addressed by small molecule drugs. The KYN pathway is responsible for the metabolism of a majority of the tryptophan intake. Although the sequence similarity between human IDO1 and TDO2 is poor (16%), high similarity of their catalytic domains is revealed by protein crystallography studies. While TDO2 is specific for metabolizing tryptophan to KYN, IDO1 recognizes a broad range of indole-containing substrates including the neurotransmitter melatonin. IDO1 is ubiquitously expressed in many tissues and cells including endothelial cells and cells that in part constitute the TME – fibroblasts, macrophages, myeloid derived suppressor cells (MDSCs), and DCs. By contrast, the expression of TDO2 is predominantly in the liver where it plays a key role in maintaining the systemic homeostasis of the tryptophan levels. Further, the induction of the two enzymes is differentially regulated. Thus, IDO1 is upregulated and sustained by inflammatory cytokines including interferon-γ (IFN-γ) and interleukin-6 (IL-6) while TDO2 is by tryptophan, cholesterol, as well as the lipid metabolite prostaglandin E2 (PGE2) 6, 7, 8, 9. Crystallographic studies have revealed the existence of an exo-binding site of tryptophan in TDO2. Tryptophan binding to this site stabilizes the TDO2 enzyme. The enzymatically active form of IDO1 is a monomer, and that of TDO2 is a homotetramer 10, 11. As a part of concerted mechanisms of evading immunosurveillance, numerous types of cancer cells upregulate preferentially IDO1 or TDO2, and in some cases both indiscriminately 12, 13, 14. In addition, DCs and MDSCs in the TME are coerced by cancer cells to express IDO1, which collectively support evasion of immunosurveillance. One prevalent hypothesis is that IDO1 induces immune tolerant effects via tryptophan starvation and the subsequent activation of the amino acid sensory molecule general control nonderepressible 2 (GCN2). Activated primary murine T cells neither proliferate nor differentiate to cytotoxic T cells when assay media were depleted of tryptophan; and deletion of tryptophan induces T cell anergy and apoptosis 15, 16. GCN2 is a serine/threonine (S/T) kinase that phosphorylates eukaryotic initiation factor 2α kinase (eIF2α) when the concentration of tryptophan is low, which results in diminished capacity in protein production [17]. IDO1-activated GCN2 also inhibits fatty acid synthesis in human primary T cells, which is important for T cell proliferation and effector function [18]. Conversely, GCN2-mediated tumor rejection was questioned in preclinical mouse melanoma models, which may reflect the capacity of cancer cells to escape immunosurveillance via multiple mechanisms. GCN2-deficient T cells showed equal effect to wild-type T cells against B16 melanoma [19]. Furthermore, GCN2 is a sensor of general amino acid starvation, not specific to tryptophan [20]. Therefore depletion of any amino acid or the combination of a few will activate GCN2, while IDO1 and TDO2 in particular are rather substrate-specific for the catabolism of tryptophan. Therefore, IDO1 mediated tryptophan depletion is likely a part of concerted immunological responses imparted by IDO. A different thesis [21] has emerged when KYN, as well as its downstream metabolic product kynurenic acid, was recognized as an endogenous aryl hydrocarbon receptor (AhR) agonist (Figure 2) 22, 23, 24, 25. AhR was first discovered as a high-affinity intracellular receptor for the industrial pollutant 2,3,7,8-tetrachlorodibenzodioxin (TCDD), and was thought to act as a detoxification mechanism against polyaromatic hydrocarbons. Further investigations have revealed a complex and central role of AhR in inducing tolerogenic immune responses. AhR regulates, and within a specific context controls, the functions of a plethora of cells of both the innate and adaptive immune system – DCs, macrophages, natural killer (NK) cells, innate lymphoid cells (ILCs), type 17 helper T (Th17) cells, Th22 cells, and regulatory T cells (Tregs). AhR is a ligand-activated transcription receptor that, upon ligation by organic small molecules, translocates from the cytosol to the nucleus, dissociates from the chaperone heat shock protein 90 (HSP-90), and forms a heterodimer with the AhR nuclear translocator (ARNT) protein. The AhR–ARNT heterodimeric complex binds to response elements in the promoter regions of the corresponding target genes. The AhR–ARNT heterodimer forms superstructures with several cotranscription factors to promote the transcription of diverse including IL10 in DCs and NK cells 26, 27, and IL6 in cancer cells and macrophages 6, 28. IL-6 and the immunosuppressive PGE2 augment the activity of IDO1 and TDO2, respectively, which amplifies signaling by KYN and AhR. AhR synergizes with the transcription factor c-Maf to promote the development of type 1 regulatory T (Tr1) cells, another major source of IL-10 [29]. IL-10 is a potent anti-inflammatory cytokine that promotes the differentiation, proliferation, and maintenance of FOXP+ Treg cells, which impairs the maturation and cytotoxicity of CD8+ T cells. The presence of Treg cells in tumor tissue from cancer patients often predicts resistance to immune checkpoint inhibitors and is inversely correlated to survival benefit. Furthermore, IL-10 is a signature cytokine that promotes the development and proliferation of tolerogenic DCs, tumor-associated macrophages (TAM), and MDSCs that populate the TME to support angiogenesis and immune escape of cancer cells 30, 31, 32.