Hidden Protein Signals Decide Why Immunotherapy Fails in Gut Cancers

Immune checkpoint inhibitors have rewritten the outlook for some patients with gastrointestinal cancers, turning once uniformly fatal diagnoses into manageable chronic conditions for a fortunate minority. Yet for every dramatic response there are many more patients whose tumours barely flinch, whose disease stalls briefly before resuming its advance, or whose initial remission gives way to acquired resistance. A comprehensive review published in the Journal of Translational Medicine argues that the explanation for this frustrating heterogeneity lies not primarily in the mutated genomes of the tumour cells themselves, but in a dense, constantly shifting web of secreted proteins that orchestrates the tumour microenvironment from the outside in.

The review, led by Kexun Li, Zilong Qian and Jie Mao with senior authors Yongtao Han and Xuefeng Leng, synthesises evidence across the major gastrointestinal malignancies: gastric and esophageal cancers, colorectal cancer, pancreatic ductal adenocarcinoma, hepatocellular carcinoma and biliary tract cancers. Its central claim is that the tumour secretome, the full complement of proteins released by tumour cells, stromal cells and immune cells into the extracellular space, functions as a dynamic signalling layer that determines whether immune cells are recruited to the tumour, whether they penetrate it, whether they function once they arrive, and whether they exhaust themselves in the struggle. When checkpoint blockade releases the brakes on T cells, the success of that manoeuvre depends on the state of the road ahead, and the secretome largely builds that road.

The authors catalogue an imposing roster of recurrent suppressive circuits. Transforming growth factor beta, long implicated in immune exclusion and fibroblast activation, appears across nearly every gastrointestinal tumour type as a driver of stromal barriers that physically wall off cytotoxic lymphocytes. Vascular endothelial growth factor, best known for promoting the chaotic, leaky vasculature of tumours, also actively repels T-cell infiltration and fosters immunosuppressive myeloid cells. The chemokine CXCL12, acting through its receptor CXCR4, excludes T cells from tumour nests in pancreatic and colorectal cancers, while CXCL8, also known as interleukin-8, signalling through CXCR1 and CXCR2, attracts neutrophils and suppresses T-cell function. The CCL2-CCR2 axis recruits inflammatory monocytes that can differentiate into tumour-promoting macrophages, and the CSF1-CSF1R pathway sustains those macrophages in a suppressive, pro-tumour state.

Beyond these canonical axes, the review highlights a second tier of secreted mediators whose roles have crystallised more recently. Interleukin-6 family cytokines drive chronic inflammatory programmes that blunt antitumour immunity and correlate with poor outcomes. SPP1, the gene encoding osteopontin, marks a distinctive population of tumour-associated macrophages and fibroblasts that sculpt an immunosuppressive niche. Periostin, secreted largely by cancer-associated fibroblasts, reinforces extracellular matrix barriers and promotes metastatic colonisation. Galectins, a family of beta-galactoside-binding lectins, can directly induce T-cell apoptosis and dysfunction. DKK1, a Wnt pathway antagonist, contributes to immune exclusion and stemness. Macrophage migration inhibitory factor, or MIF, sustains inflammatory suppression, while components of the complement cascade, traditionally viewed as blood-borne effectors of innate immunity, have been co-opted by tumours to remodel the microenvironment in their favour. Finally, soluble forms of PD-L1 and PD-L1 carried on extracellular vesicles circulate through the bloodstream, potentially mopping up therapeutic antibodies and dampening T-cell activity far from the tumour itself.

Against this suppressive chorus, the review sets out the secretome signatures of immune-permissive tumours. The chemokines CXCL9, CXCL10 and CXCL11, signalling through the receptor CXCR3, recruit effector T cells expressing that receptor, and their abundance consistently correlates with T-cell infiltration and responsiveness to checkpoint blockade. Even more striking is CXCL13, the chemokine that draws B cells and organises tertiary lymphoid structures, ectopic lymph-node-like aggregates that form within tumour tissue. Tumours rich in tertiary lymphoid structures, particularly in colorectal and gastric cancer, respond to immunotherapy at markedly higher rates, and CXCL13-associated signalling appears to be a key driver of their formation. The secretome, in other words, is not uniformly hostile; it can be reprogrammed toward a state that amplifies the effect of checkpoint inhibitors once stromal and myeloid barriers are relieved.

To bring analytical order to this complexity, the authors organise the suppressive and permissive circuits into four overlapping functional modules. The myeloid-enriched module encompasses the chemokines and colony-stimulating factors that flood tumours with suppressive macrophages, monocytes and granulocytes. The fibroblast-driven exclusion module centres on TGF-beta, periostin and matrix-remodelling signals that build physical and biochemical barriers to immune infiltration. The angiogenic-immunosuppressive module couples VEGF-driven vascular dysfunction to myeloid suppression and hypoxia. The immune-permissive module, by contrast, comprises the CXCR3 ligand axis and CXCL13-driven tertiary lymphoid structure programmes that characterise tumours primed for immunotherapy response. This modular framework allows clinicians and researchers to describe a tumour’s secretome state not as an undifferentiated list of molecules but as a pattern of dominant biological programmes with distinct therapeutic implications.

Perhaps the review’s most consequential methodological contribution is its insistence on a four-level evidence hierarchy that separates clinical validation from mechanistic inference. Many secreted proteins have been convincingly shown in cell culture and animal models to suppress or promote antitumour immunity, yet only a subset has been validated as predictive or prognostic biomarkers in large clinical cohorts, and fewer still have been targeted successfully in combination trials. TGF-beta, for example, has mechanistic support at every level, and signatures of TGF-beta-driven fibroblast activity have been shown to predict poor checkpoint response in multiple cancer types, yet TGF-beta inhibitors have delivered mixed results in the clinic, suggesting that timing, context and combination partners matter enormously. The hierarchy is designed to prevent over-interpretation of preclinical enthusiasm and to guide rational prioritisation of which secretome targets should advance toward biomarker-guided trials.

Equally important is the review’s argument that protein abundance alone is biologically meaningless without context. The same chemokine can recruit antitumour T cells or immunosuppressive myeloid cells depending on which receptor-bearing cells are present. The same cytokine can promote or restrain immunity depending on its spatial distribution within the tumour, whether it is produced by malignant epithelium, fibroblasts or infiltrating immune cells, and whether it is measured before treatment, during therapy or at the moment of acquired resistance. Metastatic sites differ from primary tumours in their secretome programmes, and host physiology, including liver function, microbiome composition and systemic inflammation, modulates the interpretation of circulating protein signals. A clinically useful secretome biomarker must therefore integrate cellular source, spatial localisation, receptor context, temporal dynamics and linkage to actionable immune-state transitions, a demand that far exceeds what a single blood test or immunohistochemical stain can deliver.

This contextual complexity helps explain why genomic biomarkers such as microsatellite instability and tumour mutational burden, while genuinely predictive in defined settings, leave most gastrointestinal cancer patients without a reliable answer. A colorectal tumour with high mutational burden may nonetheless be saturated with CXCL12-expressing fibroblasts and CSF1-dependent macrophages that render even reinvigorated T cells ineffective. A pancreatic cancer with modest genomic immunogenicity may be so thoroughly walled off by TGF-beta-driven stroma that no quantity of checkpoint blockade can achieve meaningful infiltration. Conversely, a gastric tumour with abundant tertiary lymphoid structures and a CXCL9-rich chemokine milieu may respond even with intermediate genomic predictors. The secretome is the layer at which these competing influences are integrated and expressed.

The therapeutic implications are substantial. Combination strategies already in clinical testing, including TGF-beta inhibition, CSF1R blockade, CXCR4 antagonism, VEGF pathway targeting and IL-6 pathway suppression, can be understood as attempts to dismantle specific suppressive modules and convert tumours from an excluded or suppressive secretome state into a permissive one. The review suggests that the rational design of such combinations should be guided by modular secretome profiling of individual tumours, with the goal of matching each patient to the barrier-removing strategy most likely to unmask checkpoint activity. Biomarker development, the authors argue, should focus on identifying actionable immune-state transitions, moments at which a tumour’s secretome programme is poised to flip from suppression to permissiveness, and on capturing that transition with spatially resolved, temporally informed measurements. As single-cell and spatial transcriptomic technologies mature and become clinically deployable, the prospect of reading a tumour’s secreted protein circuitry and intervening accordingly moves from aspiration toward practice. For the majority of gastrointestinal cancer patients who today derive little benefit from immunotherapy, that shift may ultimately determine whether the immunotherapy revolution reaches them at all.

Subject of Research: Secreted protein signalling circuits in the gastrointestinal tumour microenvironment that determine immunotherapy response and resistance

Article Title: Secreted protein circuits in the gastrointestinal tumour microenvironment: determinants of immunotherapy response and resistance

Article References: Li, K., Qian, Z., Mao, J., Han, Y., & Leng, X. (2026). Secreted protein circuits in the gastrointestinal tumour microenvironment: determinants of immunotherapy response and resistance. Journal of Translational Medicine. https://doi.org/10.1186/s12967-026-08945-x

Image Credits: AI Generated

DOI: 10.1186/s12967-026-08945-x

Keywords: gastrointestinal cancer, tumour microenvironment, secretome, immune checkpoint blockade, TGF-beta, VEGF, chemokines, tertiary lymphoid structures, cancer-associated fibroblasts, tumour-associated macrophages, immunotherapy resistance, biomarkers