Mechanisms underlying immunotherapy resistance in microsatellite-stable colorectal cancer
image: Created in BioRender.com. (a) Low neoantigen burden results in insufficient T-cell priming and activation. (b) An immunosuppressive tumor microenvironment underlies primary resistance in MSS CRC: ① Monocytes and Macrophages: Monocytes differentiate into distinct macrophage subsets; M2-polarized macrophages foster immune escape and tumor angiogenesis. Regulatory T cells (Tregs): Secrete immunosuppressive cytokines (IL-10, TGF-β), inhibit antitumor T-cell responses, and reprogram the microenvironment via fatty-acid accumulation and metabolic modulation. ② Myeloid-derived suppressor cells (MDSCs): Inhibit natural killer (NK) cell cytotoxicity and IFN-γ production, impairing NK-mediated activation of effector T cells; sustain immunosuppression through lipid-metabolism reprogramming. Neovasculature: Endothelial cells and tumor-associated vessels secrete VEGF, further dampening immune-cell infiltration and function. ③ Cancer-associated fibroblasts (CAFs): Resident fibroblasts, upon TGF-β stimulation, acquire a CAF phenotype that supports tumor growth and immune evasion. ④ Epithelial–mesenchymal transition (EMT): Tumor cells undergo phenotypic conversion, enhancing invasiveness and resistance to immune attack. CRC, colorectal cancer; IFN-γ, interferon-gamma; IL, interleukin; M-CSF, macrophage colony-stimulating factor; MHC1, major histocompatibility complex class I; MSS, microsatellite stable; PD-1, programmed cell death 1; PD-L1, programmed death ligand 1; TCR, T-cell receptor; TGF-β, transforming growth factor-beta; TMB, tumor mutational burden; VEGF, vascular endothelial growth factor.
Credit: Xiaochun Zhang
Microsatellite-stable (MSS) colorectal cancer (CRC) accounts for 80–85% of cases and remains largely refractory to immune checkpoint inhibitors (ICIs) compared with microsatellite instability-high (MSI-H) tumors. This review synthesizes current evidence on tumor-intrinsic and microenvironmental mechanisms driving ICI resistance in MSS CRC, including low neoantigen burden, impaired antigen presentation, activation of Wnt/β-catenin and MAPK signaling, an immunosuppressive cellular milieu (regulatory T cells, myeloid-derived suppressor cells, M2-like tumor-associated macrophages, cancer-associated fibroblasts), metabolic reprogramming, and gut microbiome dysbiosis. Translational strategies to overcome these barriers are evaluated. Preclinical and early-phase clinical data indicate that rational, mechanism-guided combinations (vascular normalization, myeloid reprogramming, metabolic inhibitors, antigen-priming approaches, and microbiome modulation) can enhance immune infiltration and produce benefits in biomarker-defined subgroups. Moving forward requires biomarker-driven, adaptive clinical trials with embedded translational endpoints.
Introduction
ICIs blocking PD-1/PD-L1 and CTLA-4 have revolutionized cancer therapy but show limited efficacy in MSS CRC, which lacks the high tumor mutational burden (TMB) and neoantigen load characteristic of MSI-H tumors. MSS CRC exhibits a “cold” tumor phenotype with poor T-cell infiltration and multiple resistance mechanisms. Understanding these barriers is essential to develop combination strategies that convert cold tumors into hot, immunotherapy-responsive lesions.
Low Neoantigen Expression
MSS CRC has a median TMB of ~4 mutations/Mb versus ~30 in MSI-H tumors, resulting in insufficient neoantigens for T-cell priming and activation. This leads to impaired CD8⁺ T-cell responses, progressive exhaustion, and ineffective cross-priming.
Immunosuppressive Microenvironment
The MSS CRC tumor microenvironment (TME) contains abundant immunosuppressive populations:
-
Regulatory T cells (Tregs): Secrete IL-10, TGF-β, and IL-35; express CTLA-4 and PD-1; suppress effector T cells.
-
Myeloid-derived suppressor cells (MDSCs): Produce reactive oxygen species, nitric oxide, and arginase-1; promote Treg expansion.
-
Tumor-associated macrophages (TAMs): Polarize toward M2 phenotype, promoting angiogenesis and immune evasion.
-
Cancer-associated fibroblasts (CAFs): Remodel extracellular matrix, block T-cell infiltration, and secrete immunosuppressive factors.
-
Tumor-associated neutrophils (TANs): Release NETs (neutrophil extracellular traps) that promote metastasis and shield tumor cells from immune attack.
Angiogenesis
VEGF/VEGFR signaling not only drives neovascularization but also impairs dendritic cell differentiation, induces T-cell exhaustion, and recruits Tregs and MDSCs, contributing to immune exclusion.
Wnt/β-Catenin Signaling
Activation of Wnt/β-catenin in MSS CRC suppresses CCL3 production by CD103⁺ dendritic cells, reducing CD8⁺ T-cell recruitment and promoting Treg survival, reinforcing the cold phenotype.
Tumor Metabolism
-
Lactate: Aerobic glycolysis produces lactate, acidifying the TME, inhibiting NK and CD8⁺ T cells, and skewing macrophages toward M2. Targeting LDH or MCT1/4 transporters may restore immunity.
-
Amino acid metabolism: IDO-mediated tryptophan depletion generates kynurenine, suppressing T cells and expanding Tregs. Methionine competition via SLC43A2 impairs T-cell function. Glutaminase (GLS) and MAT2A also contribute to immunosuppression. Branched-chain amino acid (BCAA) metabolism modulates mTOR signaling and T-cell activation.
Immunotherapy in Recurrent Disease
Recurrent MSI-H CRC responds well to ICIs, but recurrent MSS CRC remains resistant. Mechanism-based combinations (e.g., with radiotherapy, targeted agents, or local therapies) have shown occasional responses. ctDNA monitoring can guide adjuvant immunotherapy, and adoptive cellular therapies (TILs, CAR-T) are under investigation.
Gut Microbiota and Immunotherapy Response
Gut dysbiosis influences ICI efficacy. Fusobacterium nucleatum promotes immunosuppression and chemoresistance. Short-chain fatty acids (SCFAs) and tryptophan metabolites (via aryl hydrocarbon receptor) modulate Treg and myeloid function. Fecal microbiota transplantation (FMT) from ICI responders has shown promise in restoring anti-PD-1 activity. Antibiotic use around ICI initiation correlates with worse outcomes.
Clinical Prospects and Challenges
Multi-modal combination strategies are the most promising translational avenue (see Table 1 in original). Approaches include:
-
Anti-angiogenic agents + ICIs to normalize vasculature.
-
Targeting myeloid/stromal cells (CSF1R, CCR2, CXCR2, TGF-β).
-
Metabolic inhibitors (IDO1, GLS, LDH, MCT1/4) plus ICIs.
-
Antigen-priming (radiotherapy, oncolytic viruses, neoantigen vaccines).
-
Microbiome modulation (FMT, probiotics, dietary interventions).
Challenges include toxicity, trial design complexity, and need for robust biomarkers. The IDO1 inhibitor epacadostat failed in phase III, highlighting the importance of target engagement and patient selection.
Future Prospects
Emerging targets include IL-33/ST2 axis, NET formation (PAD4 inhibitors), and Wnt/β-catenin modulators (PORCN inhibitors). Metabolic reprogramming and microbiome-based interventions require standardization. Biomarker-driven, adaptive platform trials with serial biopsies and ctDNA are needed to identify which combinations benefit specific MSS CRC subgroups.
Conclusions
Immunotherapy resistance in MSS CRC arises from low neoantigen burden, an immunosuppressive TME (Tregs, MDSCs, TAMs, CAFs, NETs), angiogenic and Wnt/β-catenin signaling, and metabolic dysregulation (lactate, amino acid competition). Rational combination strategies—targeting myeloid cells, metabolism, vasculature, and microbiota—hold promise to convert cold MSS CRC into hot tumors. Rigorous biomarker-stratified trials are essential to translate these insights into durable clinical benefits.
Full text
https://www.xiahepublishing.com/2996-3427/OnA-2025-00021
The study was recently published in the Oncology Advances.
Oncology Advances is dedicated to improving the diagnosis and treatment of human malignancies, advancing the understanding of molecular mechanisms underlying oncogenesis, and promoting translation from bench to bedside of oncological sciences. The aim of Oncology Advances is to publish peer-reviewed, high-quality articles in all aspects of translational and clinical studies on human cancers, as well as cutting-edge preclinical and clinical research of novel cancer therapies.
Follow us on X: @xiahepublishing
Follow us on LinkedIn: Xia & He Publishing Inc.