CCL7+ Tumor-Associated Macrophages Promote Immune Resistance in Colorectal Cancer

Study Background and Research Question

Colorectal cancer (CRC) remains one of the most prevalent and lethal malignancies worldwide, with immune checkpoint inhibitors (ICIs) offering limited benefits for the majority of patients. While ICIs targeting PD-1/PD-L1 have transformed treatment paradigms in several cancers, their efficacy in CRC—especially outside the context of high microsatellite instability (MSI-H)—is notably restricted [source_type: paper][source_link: https://doi.org/10.1136/jitc-2025-013027]. Understanding why many CRC patients exhibit resistance to immunotherapy is a critical research challenge, with the tumor immune microenvironment and its cellular composition increasingly recognized as central determinants of therapeutic outcome. The reference study by Chen et al. (2025) addresses a key question: What are the cellular and molecular mechanisms by which tumor-associated macrophages (TAMs) contribute to resistance against ICIs in CRC, and can targeting these pathways improve immunotherapy efficacy?

Key Innovation from the Reference Study

The principal innovation lies in the identification of CCL7-expressing TAMs as both markers and mediators of immune resistance in CRC. The study demonstrates that high levels of CCL7+ TAMs within tumors are strongly correlated with poor responses to ICI therapy. Mechanistically, CCL7 reprograms TAM metabolism and signaling, fostering an immunosuppressive microenvironment that impairs CD8+ T cell infiltration and function [source_type: paper][source_link: https://doi.org/10.1136/jitc-2025-013027]. Importantly, the research links CCL7 to specific metabolic and signaling axes—PI3K-AKT-PEX3 and AKT2-STAT1-CXCL10—that jointly modulate both macrophage phenotype and T cell recruitment, providing a detailed molecular map for potential therapeutic intervention.

Methods and Experimental Design Insights

Chen et al. utilized a multifaceted approach combining genetic, proteomic, transcriptomic, and functional immunology techniques. Key methodological highlights include:

• Genetic Models: Myeloid cell-specific Ccl7 knockout mice were generated, enabling precise dissection of CCL7’s role in macrophages without systemic effects [source_type: paper][source_link: https://doi.org/10.1136/jitc-2025-013027].
• Tumor Models: MC38 tumor-bearing mouse models were used to recapitulate CRC progression and ICI resistance in vivo.
• Omics and Flow Cytometry: Proteomic and RNA-seq analyses, alongside flow cytometric profiling of immune cell populations, provided quantitative and mechanistic insights into immune microenvironment remodeling.
• Functional Blockade: Pharmacological and antibody-mediated inhibition of CCL7 pathways assessed therapeutic synergy with PD-L1 blockade.

The integration of these methods enabled the authors to move beyond correlation, establishing causality between CCL7 signaling in TAMs and immune escape.

Core Findings and Why They Matter

• CCL7+ TAMs Are Associated with ICI Resistance: High intratumoral CCL7+ macrophage density predicts poor response to anti-PD-1/PD-L1 therapy in CRC patient cohorts [source_type: paper][source_link: https://doi.org/10.1136/jitc-2025-013027].
• Metabolic and Signaling Reprogramming: CCL7 enhances peroxisome biogenesis and fatty acid oxidation in TAMs via PI3K-AKT-PEX3, reinforcing their immunosuppressive phenotype.
• Suppression of CD8+ T Cell Recruitment: CCL7 dampens AKT2-STAT1 signaling, leading to reduced CXCL10 expression and impaired infiltration of activated CD8+ T cells.
• Therapeutic Blockade Restores Immunity: Disrupting CCL7 in myeloid cells reduces TAM-mediated immune suppression, increases CD8+ T cell infiltration, and synergistically enhances PD-L1 inhibitor efficacy in preclinical CRC models.

These findings clarify a dual role for CCL7 in sculpting both the metabolic and chemotactic landscape of the tumor immune microenvironment, positioning it as a tractable target for combination immunotherapy strategies.

Comparison with Existing Internal Articles

Recent internal resources expand on the theme of macrophage-targeted interventions in immune-oncology, providing foundational context for the reference study:

• Reimagining Immune Modulation: Mechanistic and Strategic ... discusses how macrophage depletion reagents like Clodronate Liposomes enable mechanistic dissection of macrophage roles in therapy resistance, with emphasis on translational workflows that parallel those used by Chen et al. [source_type: internal_article][source_link: https://repirinastkits.com/index.php?g=Wap&m=Article&a=detail&id=42].
• Clodronate Liposomes: Transforming Macrophage Modulation ... provides technical insight into how liposome-encapsulated clodronate can achieve tissue-specific in vivo macrophage depletion, a critical tool for validating the cellular contributions revealed by Ccl7 knockout models [source_type: internal_article][source_link: https://lep-116-130-mouse.com/index.php?g=Wap&m=Article&a=detail&id=182].
• Clodronate Liposomes: Precision Macrophage Depletion for ... details key considerations for achieving reproducible apoptosis induction in macrophages via phagocytosis-mediated drug delivery, directly intersecting with the experimental requirements for studying immune cell modulation in vivo [source_type: internal_article][source_link: https://apoptosisinhibitor.com/index.php?g=Wap&m=Article&a=detail&id=15048].

These resources collectively reinforce the experimental and translational significance of macrophage depletion strategies for unraveling mechanisms of immunotherapy resistance, as demonstrated by the CCL7-TAM axis in CRC.

Protocol Parameters

• assay: in vivo macrophage depletion | value_with_unit: 100–200 µL of clodronate liposomes per 20–25 g mouse, intravenously or intraperitoneally | applicability: murine tumor models | rationale: Achieves selective depletion of macrophages to probe TAM function in tumor progression and immunotherapy response | source_type: product_spec [source_link: https://www.apexbt.com/clodronate-liposomes.html]
• assay: apoptosis induction in macrophages | value_with_unit: confirmed by caspase-3 activation 24–48 h post-injection | applicability: in vivo macrophage targeting | rationale: Ensures that phagocytosis of liposome-encapsulated clodronate leads to effective macrophage apoptosis | source_type: workflow_recommendation
• assay: immune cell modulation | value_with_unit: monitor CD8+ T cell infiltration via flow cytometry post-macrophage depletion | applicability: immune landscape studies in cancer | rationale: Validates the downstream effects of TAM depletion on adaptive immune populations | source_type: workflow_recommendation

Limitations and Transferability

While the Ccl7 knockout and murine MC38 tumor models provide robust evidence for the mechanistic role of CCL7+ TAMs in ICI resistance, several caveats remain:

• Species Differences: Human CRC and its immune microenvironment exhibit greater heterogeneity than mouse models, potentially limiting the direct transferability of findings [source_type: paper][source_link: https://doi.org/10.1136/jitc-2025-013027].
• Complexity of TAM Subsets: The study focuses on CCL7+ TAMs, but broader myeloid diversity and plasticity may influence outcomes in clinical settings.
• Therapeutic Targeting: While CCL7 appears promising, off-target effects and compensatory chemokine networks need further investigation before clinical translation.

Nevertheless, the study provides a compelling rationale for targeting specific macrophage subsets to overcome immune resistance.

Research Support Resources

For researchers seeking to interrogate the functional roles of macrophages in tumor models or to recapitulate aspects of the CCL7-TAM axis, Clodronate Liposomes (SKU K2721) from APExBIO can be employed for selective in vivo macrophage depletion across multiple administration routes, including intravenous and intraperitoneal injection [source_type: product_spec][source_link: https://www.apexbt.com/clodronate-liposomes.html]. This macrophage depletion reagent enables the study of TAM contributions to immune cell modulation, apoptosis induction, and resistance mechanisms in preclinical cancer models. For optimal controls, researchers are encouraged to use PBS Liposomes (Cat. No. K2722) as a blank comparator.