The review, authored by Isabela Gontijo, Mariana Camurça and José Roberto Kfoury of the University of São Paulo, systematically dissects why IDO1-targeted drugs have underperformed in the clinic. The most famous failure was epacadostat, which, despite robustly lowering kynurenine when combined with pembrolizumab in a randomized phase 2 study in metastatic non-small-cell lung cancer, did not improve objective response rates over pembrolizumab alone. The authors argue that such disappointing results should not be read as evidence that the drug failed to engage its target. Instead, they propose, IDO1 operates within a distributed metabolic network in which substrate availability, alternative enzymes, non-enzymatic signaling, cell-specific metabolic programs, downstream receptors and even the gut microbiome can each sustain tryptophan-dependent immunoregulation after the primary enzyme is pharmacologically silenced.

To appreciate the scale of the problem, it helps to understand how IDO1 works. The enzyme is a cytosolic, heme-containing oxidoreductase expressed in dendritic cells, macrophages, stromal cells and cancer cells. Its transcription is induced when interferon-gamma binds its receptor, activating JAK1 and JAK2 kinases that phosphorylate STAT1. Phosphorylated STAT1 dimers, known as gamma-activated factor, translocate to the nucleus and bind gamma-activated sites in the IDO1 promoter, while a secondary wave of interferon regulatory factor 1 amplifies the signal through interferon-stimulated response elements. Once made, IDO1 is catalytically active only when its heme iron is in the reduced ferrous state, allowing it to cleave the indole ring of L-tryptophan and generate N-formyl-L-kynurenine, which arylformamidase rapidly converts to L-kynurenine. This step is the rate-limiting entry point of tryptophan into the kynurenine pathway, which ultimately feeds de novo NAD+ biosynthesis.

The downstream consequences of this reaction extend well beyond substrate consumption. Tryptophan depletion activates the GCN2 integrated stress response while dampening mTORC1 signaling, the nutrient-sensing hub that drives protein translation, cell-cycle progression and clonal expansion in activated T lymphocytes. Meanwhile, accumulated kynurenine acts as an endogenous ligand for the aryl hydrocarbon receptor, which sheds its cytoplasmic chaperone complex, enters the nucleus, pairs with ARNT and switches on a tolerogenic transcriptional program in antigen-presenting and myeloid cells. This favors regulatory T-cell differentiation and blunts effector immunity, although the review is careful to note that AHR biology is context dependent: in some settings, such as resident-memory CD8-positive T cells and natural killer cells, AHR activity actually enhances antitumor function. IDO1 also has a second, entirely non-enzymatic identity. In TGF-beta-conditioned dendritic cells, the enzyme is phosphorylated on immunoreceptor tyrosine-based inhibitory motifs by the Src-family kinase Fyn, recruiting the phosphatases SHP-1 and SHP-2 and triggering non-canonical NF-kappa-B signaling through p52/RelB complexes, a positive feedback loop that sustains long-term immune tolerance independently of catalysis.

Against this backdrop, the review catalogs a series of compensatory routes that survive IDO1 blockade. The first is enzymatic redundancy. Tryptophan 2,3-dioxygenase 2, or TDO2, is a hepatic, glucocorticoid-regulated heme enzyme that catalyzes exactly the same initial reaction as IDO1, oxidizing tryptophan to N-formyl-L-kynurenine. Because selective IDO1 inhibitors leave the substrate itself untouched, any cell expressing active TDO2 can simply absorb the surplus tryptophan and feed it back into the kynurenine pathway, preserving kynurenine-dependent AHR signaling without any restoration of IDO1 activity. Its paralog IDO2, encoded next to IDO1, has weak catalytic efficiency but may contribute through membrane-associated signaling functions of its own. This mechanistic logic underlies the development of dual IDO1/TDO2 inhibitors such as M4112 and SHR9146, which are designed to close both enzymatic entry points simultaneously.

A second form of compensation is metabolic rerouting. When IDO1-dependent consumption falls, more tryptophan remains available to competing enzymes, including tryptophan hydroxylase 1 and 2, which commit the amino acid to the serotonergic pathway instead. The resulting surge in 5-hydroxytryptophan and serotonin does not simply vanish into a metabolic dead end: serotonin is itself an extracellular signaling molecule, sensed by a family of G-protein-coupled receptors and the ion-channel receptor 5-HT3 on monocytes, macrophages, dendritic cells and lymphocytes, where it modulates cytokine production, migration and differentiation. Serotonin can be further converted to melatonin through AANAT and ASMT, adding another layer of immunomodulatory chemistry. Clinical evidence supports this rewiring: in ovarian cancer patients treated with an IDO1 inhibitor, tumor metabolic adaptation toward the serotonin pathway was documented and found to constrain antitumor immune responses.

A third route runs through IL4I1, a secreted FAD-dependent amino acid oxidase induced by interleukin-4 that oxidizes tryptophan to indole-3-pyruvic acid, releasing ammonia and hydrogen peroxide. Because this chemistry is fundamentally different from heme-dependent dioxygenation, IDO1 inhibitors have no direct effect on it. Yet the indole metabolites IL4I1 produces, including indole-3-aldehyde, are potent AHR ligands, meaning the pathway bypasses the kynurenine step entirely while converging on the same tolerogenic receptor. Recent studies have linked IL4I1/AHR signaling to macrophage polarization in allergic rhinitis and to anti-inflammatory programs in cytokine-primed muscle stem cells, underscoring how a single receptor can be fed by multiple, pharmacologically independent metabolic tributaries.

The review also elevates compensation from the single-cell to the tissue level. Tryptophan transporters such as LAT1 and SLC7A11 are distributed unevenly across cell types, so when IDO1-expressing cells stop consuming tryptophan, neighboring cells with high transporter activity act as alternative metabolic sinks, redirecting the shared substrate according to their own enzymatic repertoires. Single-cell RNA sequencing in inflammatory intestinal tissue has revealed sharply different tryptophan-metabolic profiles among macrophages, fibroblasts and epithelial cells, while work in macrophages shows that efferocytosis induces a coordinated program of tryptophan uptake, IDO1 expression and kynurenine production that aids tissue resolution. In this non-cell-autonomous model, substrate consumption, metabolite production and metabolite sensing can all occur in different compartments of the same tissue, which means that inhibiting one enzymatic source may merely reshuffle the cellular geography of tryptophan metabolism rather than extinguishing its outputs.

Perhaps the most surprising player is the gut microbiome. Commensal bacteria wielding tryptophanases, aminotransferases and reductases convert dietary tryptophan into a diverse library of indoles, including indole-3-aldehyde, indole-3-lactic acid, indole-3-acetic acid and indole-3-propionic acid, many of which activate AHR in epithelial and immune cells. Landmark work by Zelante and colleagues showed in IDO1-deficient mice that microbiota-derived catabolites engage AHR and balance mucosal immunity through interleukin-22, and more recent studies demonstrate that lactic acid bacteria are particularly prolific producers of these ligands. Microbial compensation is not uniformly immunosuppressive: cooperative metabolism between Lactobacillus johnsonii and Clostridium sporogenes boosts indole-3-propionic acid, which promotes H3K27 acetylation at the Tcf7 super-enhancer and maintains progenitor exhausted CD8-positive T cells, thereby enhancing the efficacy of anti-PD-1 therapy across multiple cancer models. The microbiome, in other words, can either restore immune suppression or sharpen antitumor immunity, depending on which community members dominate.

The translational message of the review is a shift from enzyme-specific inhibition to network-based regulation. The authors draw a sharp distinction between target engagement, which merely confirms that IDO1 catalysis has been suppressed, and network suppression, which asks whether the broader biological output has actually fallen. Clinical data reinforce the point: in a phase 1/2 study of the irreversible inhibitor linrodostat combined with nivolumab, kynurenine fell across patient groups regardless of response, while therapeutic benefit associated instead with an interferon-gamma transcriptional signature and, in non-melanoma cohorts, with the combination of low TDO2 expression and high interferon-gamma signaling. Patient stratification in head and neck cancer likewise showed benefit only in subgroups with high baseline IDO1 RNA and context-specific immune signatures. The authors argue that future trials should pair serial plasma metabolomics with tissue-resolved assessment of IDO1, TDO2 and IL4I1 expression, AHR-responsive transcriptional markers and stool metagenomics, using adaptive designs that identify which compensatory node dominates in each patient. Emerging tools such as IDO1-targeting PROTACs, which degrade the protein and thereby disrupt both its catalytic and non-enzymatic signaling functions, may widen the mechanistic reach of treatment, but even protein removal cannot touch TDO2, IL4I1, microbial metabolism or downstream AHR activity. The lesson is sobering and clarifying at once: IDO1 is not a switch but one node in a dynamic, redundantly wired metabolic network, and effective immunometabolic therapy will require mapping exactly where compensation lives in each patient before deciding where to strike.

Subject of Research: Compensatory tryptophan metabolism and immune regulation pathways activated following IDO1 enzyme inhibition

Article Title: Compensatory pathways in tryptophan metabolism and immune regulation following IDO inhibition

Article References: Gontijo, I., Camurça, M., & Kfoury, J. R. (2026). Compensatory pathways in tryptophan metabolism and immune regulation following IDO inhibition. Molecular Biology Reports, 53(1), Article 1567. https://doi.org/10.1007/s11033-026-12707-9

Image Credits: AI Generated

DOI: 10.1007/s11033-026-12707-9

Keywords: IDO1, tryptophan metabolism, kynurenine pathway, TDO2, IL4I1, aryl hydrocarbon receptor, immunotherapy resistance, epacadostat, gut microbiome, indole metabolites, serotonin pathway, immune regulation

Natural killer, or NK, cells are lymphocytes that can recognize and destroy abnormal cells without requiring the same antigen-specific priming used by conventional T cells. They release cytotoxic molecules, including perforin and granzymes, and produce signaling proteins that help coordinate broader antitumor immunity. Their activity is controlled by a balance of activating and inhibitory receptors, as well as signals from the surrounding tissue. In ovarian, breast, and prostate cancers, however, the tumor microenvironment can become metabolically hostile, nutrient-depleted, and rich in suppressive factors. Under these conditions, NK cells may accumulate inside tumors while progressively losing their ability to kill malignant cells. This state resembles exhaustion, but the researchers describe it more broadly as maladaptation because the cells are not simply inactive; they have been functionally reshaped by persistent environmental signals.

The study focused on AhR, a transcription factor that acts as a sensor for a wide range of chemical signals. AhR can be activated by compounds originating from pollutants, dietary components, drugs, and endogenous metabolism. Once activated, it moves into the cell nucleus and alters the expression of genes involved in immune development, metabolism, barrier function, and inflammatory responses. The researchers found that AhR activity has a context-dependent effect on NK cells. Brief or controlled activation can support NK-cell maturation and enhance antitumor functions. By contrast, prolonged stimulation appears to push the cells toward a dysfunctional state. The distinction is important because it shows that the same signaling system can be beneficial during an acute response but damaging when a tumor or chronic exposure keeps it switched on.

Using epithelial ovarian cancer as a model of an immunologically cold tumor, the investigators combined several experimental approaches to reconstruct this process. Their analyses included bulk gene-expression profiling, single-cell RNA sequencing, functional studies of isolated NK cells, mouse tumor models, clinical cohort data, and samples from patients with epithelial ovarian cancer. These methods allowed the team to examine both the molecular programs operating in individual immune cells and the clinical features associated with those programs. The single-cell analyses were particularly important because tumor-infiltrating NK cells are not a uniform population. Some retain cytotoxic activity, while others display gene-expression patterns associated with altered metabolism, persistent receptor signaling, and reduced effector function.

The proposed mechanism begins with GDF15, a stress-associated protein produced at elevated levels by chemoresistant tumor cells. GDF15 is known to participate in responses to cellular injury, metabolic imbalance, and inflammation, but in the tumor microenvironment it can also act as an immunomodulatory signal. According to the study, increased GDF15 is linked to enhanced IDO1 activity. IDO1 is an enzyme that converts tryptophan into kynurenine-pathway metabolites. This reaction can deprive local immune cells of tryptophan while generating molecules that influence immune-cell behavior. Kynurenine and related metabolites can enter NK cells and activate AhR, establishing a biochemical connection between tumor stress, altered amino-acid metabolism, and immune-cell reprogramming.

The resulting GDF15–IDO1–kynurenine–AhR axis appears to maintain NK cells in a state of chronic stimulation. At first, AhR signaling may help the cells adapt to the tumor environment and support their maturation. Over time, however, continual exposure to tumor-derived metabolites and stress signals changes the transcriptional and functional state of the cells. The NK cells become less effective at releasing cytotoxic molecules and killing cancer cells, even though they remain inside the tumor. This combination of retention and functional decline may be particularly advantageous to malignant tissue: immune cells are drawn into the tumor, but their capacity for surveillance and elimination is progressively weakened. The tumor can therefore appear immune-infiltrated without being genuinely immune-responsive.

The researchers also examined the possibility that environmental factors may reinforce this process. Because AhR responds to xenobiotics and other externally derived compounds, long-term exposure to AhR-activating chemicals could theoretically intensify signaling already initiated by tumor metabolism. The study places this possibility within a broader framework that links environmental toxicology to cancer immunology, including the potential influence of endocrine-disrupting compounds and dietary metabolites. The findings do not mean that any single environmental exposure directly causes ovarian cancer or NK-cell exhaustion. Rather, they raise the possibility that chronic exposure to chemical signals may interact with tumor-derived pathways and influence how immune cells behave after cancer has developed. This interaction could help explain why immune responses vary substantially among patients with apparently similar tumors.

A key experimental observation was that blocking AhR signaling restored aspects of NK-cell function. In the researchers’ models, inhibiting the pathway reduced the maladaptive program and improved the cells’ ability to respond against tumor targets. These results identify AhR as a possible therapeutic entry point, although the approach remains investigational. An AhR inhibitor would need to suppress harmful, persistent signaling without disrupting the receptor’s normal roles in immune development and tissue biology. The same caution applies to the upstream components of the pathway. GDF15 and IDO1 have functions in normal physiology, and broad inhibition could produce unintended effects. Future studies will need to determine which patients have pathway activity high enough to justify targeted treatment and whether blocking AhR can work safely alongside established immunotherapies.

The findings may also offer a strategy for predicting treatment response. The researchers propose that circulating GDF15 levels, combined with measurements of AhR activity in NK cells, could help identify patients whose immune systems are less likely to respond to checkpoint blockade. Such biomarkers would not replace clinical evaluation, but they could eventually help distinguish tumors that are merely infiltrated by dysfunctional immune cells from those with active antitumor immunity. The study further suggests that combining an AhR-targeting treatment with immune checkpoint inhibitors might help convert an immune-cold tumor into a more responsive one. For now, these implications require validation in larger patient groups and prospective clinical trials. Nevertheless, the work provides a mechanistic explanation for how chronic stress signals can transform a frontline immune defense into a source of persistent but ineffective tumor infiltration, and it places the GDF15–AhR pathway at the center of efforts to restore immune function in resistant cancers.

Subject of Research: Cells

Article Title: The Gdf15-xenobiotic receptor axis shapes NK cell maladaptation predicting cold tumors under environmental stress

News Publication Date: 5 June 2026

Web References: https://doi.org/10.1038/s41392-026-02690-9; https://www.pusan.ac.kr/eng/Main.do

References: Signal Transduction and Targeted Therapy, DOI: 10.1038/s41392-026-02690-9

Image Credits: Pusan National University

Keywords: Cancer immunotherapy, ovarian cancer, natural killer cells, cancer immunology, immune system, cold tumors, tumor microenvironment, immunotherapy, immune response, GDF15, IDO1, kynurenine, AhR, environmental stress, cancer research