One of the most frustrating realities in modern oncology is that treatments which succeed spectacularly in some patients fail completely in others. Neoadjuvant chemoimmunotherapy, the combination of chemotherapy with immune checkpoint inhibitors given before surgery, has transformed the outlook for many people with esophageal squamous cell carcinoma, an aggressive cancer that is particularly common in parts of East Asia. Yet a substantial fraction of patients derive little benefit, and clinicians have had limited ability to predict who will respond and who will not. A new study published in Cancer Immunology, Immunotherapy points to an unexpected culprit hiding within the tumor cells themselves: a cytochrome P450 enzyme called CYP2S1, which appears to orchestrate the recruitment of immunosuppressive myeloid cells and thereby undermine one of the most powerful weapons in the cancer treatment arsenal.
The research, led by Xiaoyan Li, Chunning Li and colleagues at the Cancer Hospital of China Medical University and Liaoning Cancer Hospital & Institute in Shenyang, began with an unbiased search for molecular features that distinguish tumors resistant to chemoimmunotherapy from those that shrink dramatically under treatment. Rather than examining genes in isolation, the team integrated two complementary high-resolution technologies. The first was proteomic profiling using liquid chromatography with tandem mass spectrometry, applied to clinical esophageal cancer specimens from eight patients, which allowed the researchers to catalogue thousands of proteins actually present inside the tumors. The second was single-cell RNA sequencing, performed on three esophageal tumors and one normal esophageal tissue sample, which provided a cell-by-cell map of gene expression across the entire tumor ecosystem, from malignant epithelial cells to stromal fibroblasts and infiltrating immune cells.
Converging evidence from these two approaches converged on a single molecule. CYP2S1, a member of the cytochrome P450 superfamily of enzymes best known for their roles in metabolizing fatty acids, drugs and other small molecules, stood out as significantly upregulated in tumors that had resisted chemoimmunotherapy. The single-cell data added a crucial layer of context: the enzyme was not produced uniformly throughout the tumor but was predominantly expressed by the malignant epithelial cells themselves. This tumor-intrinsic pattern of expression was important, because it suggested that the cancer cells were not merely passive victims of an unfavorable immune environment but were actively manufacturing a molecule associated with immune exclusion and treatment failure.
To move from correlation to biological plausibility, the investigators turned their attention to the tumor microenvironment, the complex ecosystem of immune and stromal cells that surrounds and infiltrates a tumor. Using immunohistochemistry, a technique that stains tissue sections to reveal the location and abundance of specific proteins, and multiplex immunofluorescence, which can visualize several protein markers simultaneously in the same tissue section, the team examined tumor samples at single-cell resolution. Their analyses revealed that tumors with high CYP2S1 expression contained markedly increased numbers of cells with the phenotypic characteristics of polymorphonuclear myeloid-derived suppressor cells, or PMN-MDSCs. These cells, which are related to neutrophils, are among the most potent immunosuppressive players in cancer biology. They accumulate in tumors under chronic inflammatory conditions and suppress the activity of cytotoxic T lymphocytes, the very cells that immune checkpoint inhibitors are designed to unleash against the cancer.
The clinical significance of this finding became apparent when the researchers examined the relationship between CYP2S1 protein levels and treatment response in a cohort of 46 evaluable patients who had undergone neoadjuvant chemoimmunotherapy. Response was assessed using the Mandard tumor regression grade, a pathological scoring system that pathologists use to quantify how much residual tumor remains after preoperative treatment, with lower grades indicating more complete tumor destruction. The results were striking: the CYP2S1 immunohistochemical score was positively associated with the tumor regression grade, meaning that patients whose tumors expressed more CYP2S1 tended to have less tumor regression and thus a poorer response to therapy. The statistical relationship was robust, with a linear regression analysis yielding an R-squared value of 0.271 and a P value of 0.0002, while a Spearman correlation coefficient of 0.528 with a P value below 0.001 confirmed the strength of the association.
With a firm clinical correlation established, the team turned to laboratory experiments to dissect the mechanism. Using short hairpin RNA and small interfering RNA techniques to reduce CYP2S1 expression in esophageal cancer cell lines, they observed that silencing the enzyme lowered intracellular levels of reactive oxygen species, the chemically reactive molecules containing oxygen that accumulate in stressed cells, and simultaneously reduced the expression of several chemokines known to attract myeloid cells. Conversely, when the researchers forced tumor cells to overproduce CYP2S1, reactive oxygen species accumulated and the cells secreted elevated amounts of two chemokines in particular: CXCL6 and CXCL8. Both of these signaling molecules belong to the CXC chemokine family and are well documented as chemoattractants for neutrophils and neutrophil-like suppressor cells, providing a plausible route by which tumor cell CYP2S1 could recruit PMN-MDSCs into the tumor bed.
To confirm that the chemokine induction was genuinely dependent on reactive oxygen species rather than some parallel effect of CYP2S1, the investigators performed a scavenging experiment using N-acetylcysteine, a well-established antioxidant that neutralizes intracellular reactive oxygen species. When cells overexpressing CYP2S1 were treated with N-acetylcysteine, the accumulation of reactive oxygen species was attenuated, the induction of CXCL6 and CXCL8 messenger RNA was reduced, and the secretion of CXCL8 protein declined. These results support a model in which CYP2S1 drives a reactive-oxygen-species-dependent signaling program that culminates in the production of myeloid-attracting chemokines. In this framework, the enzyme acts as a molecular switch inside malignant cells, converting metabolic and oxidative activity into an inflammatory signal that reshapes the immune composition of the surrounding tumor.
The broader implications of this work extend beyond esophageal squamous cell carcinoma. Cytochrome P450 enzymes have long interested cancer researchers because of their roles in drug metabolism and in the synthesis of signaling lipids, but CYP2S1 has remained comparatively understudied in the context of tumor immunology. By linking this enzyme to the reactive oxygen species, CXCL6 and CXCL8 axis and to the accumulation of immunosuppressive myeloid cells, the study adds a new node to the growing network of tumor-intrinsic mechanisms of immune evasion. It also offers a potential explanation for why some tumors with similar PD-L1 expression levels respond so differently to checkpoint inhibitors: the determinant of sensitivity may lie not in the checkpoint pathway itself but in whether the tumor has built a chemical wall of suppressive myeloid cells that antibodies cannot penetrate.
Translational possibilities follow naturally from these findings. If CYP2S1 expression in pretreatment biopsy samples proves predictive in larger cohorts, it could serve as a biomarker to identify patients unlikely to benefit from neoadjuvant chemoimmunotherapy, sparing them the toxicity of an ineffective regimen and directing them toward alternative strategies. More ambitiously, the CYP2S1–reactive oxygen species–CXCL6/CXCL8 pathway itself presents multiple candidate points of therapeutic intervention, from inhibiting the enzyme or its oxidative products to blocking the chemokine receptors that guide PMN-MDSCs into tumors. Such approaches would need careful validation, since reactive oxygen species play complex and sometimes protective roles in normal tissue biology, and myeloid cell biology differs substantially between mice and humans.
The study also carries methodological lessons for the field. By combining unbiased proteomics, single-cell transcriptomics, multiplex tissue imaging, patient-level pathological correlations and mechanistic cell biology within a single investigation, the researchers demonstrated how multi-omics integration can move a candidate biomarker from discovery to biological explanation in one continuous chain of evidence. The work adhered to the principles of the Declaration of Helsinki, received approval from the Ethics Committee of the Cancer Hospital of China Medical University, and all patients provided written informed consent. As chemoimmunotherapy becomes standard of care for more cancers and more patients, understanding why it fails will be as important as celebrating why it works. This study suggests that sometimes the answer lies in a quiet metabolic enzyme, humming away inside the tumor cell, quietly sending out the molecular invitations that summon the very cells that shield the cancer from destruction.
Subject of Research: CYP2S1-mediated recruitment of polymorphonuclear myeloid-derived suppressor cells and chemoimmunotherapy resistance in esophageal squamous cell carcinoma
Article Title: CYP2S1: a novel regulator of PMN-MDSC recruitment and immunotherapy resistance in esophageal squamous cell carcinoma
Article References: CYP2S1: a novel regulator of PMN-MDSC recruitment and immunotherapy resistance in esophageal squamous cell carcinoma. (n.d.). https://doi.org/10.1007/s00262-026-04556-z
Image Credits: AI Generated
DOI: 10.1007/s00262-026-04556-z
Keywords: CYP2S1, esophageal squamous cell carcinoma, immunotherapy resistance, PMN-MDSC, tumor microenvironment, reactive oxygen species, CXCL6, CXCL8, chemoimmunotherapy, tumor regression grade, single-cell RNA sequencing, cancer immunology
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Nathaniel Bowman. (September 22, 2026). CYP2S1 enzyme linked to immunotherapy resistance in esophageal cancer. Scienmag. https://scienmag.com/cyp2s1-enzyme-linked-to-immunotherapy-resistance-in-esophageal-cancer/
Nathaniel Bowman. “CYP2S1 enzyme linked to immunotherapy resistance in esophageal cancer.” Scienmag, 22 September 2026, https://scienmag.com/cyp2s1-enzyme-linked-to-immunotherapy-resistance-in-esophageal-cancer/. Accessed 22 September 2026.
Nathaniel Bowman. “CYP2S1 enzyme linked to immunotherapy resistance in esophageal cancer.” Scienmag. September 22, 2026. https://scienmag.com/cyp2s1-enzyme-linked-to-immunotherapy-resistance-in-esophageal-cancer/
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