CCL7+ Tumor Macrophages and Immunotherapy Resistance in Colorectal Cancer

Study Background and Research Question

Colorectal cancer (CRC) is among the most prevalent and lethal malignancies worldwide. While immune checkpoint inhibitors (ICIs), such as PD-1/PD-L1 antibodies, have revolutionized cancer immunotherapy, their efficacy in CRC is notably limited—especially in patients with microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) tumors. Even in these subgroups, up to 50% of metastatic cases exhibit resistance, leading to disease progression and recurrence. The mechanisms underlying this resistance remain incompletely understood, posing a critical challenge for the development of next-generation immunotherapies.

Macrophages, particularly tumor-associated macrophages (TAMs), are key components of the tumor immune microenvironment and often exhibit immunosuppressive functions. CCL7, a chemokine previously implicated in cancer cell proliferation and migration, has an unclear role in TAM-mediated immunotherapy resistance. The central research question addressed by Chen et al. (2025) is: How does CCL7 expression in TAMs regulate immune cell infiltration and modulate resistance to ICIs in CRC?

Key Innovation from the Reference Study

The primary innovation of this study lies in the identification of CCL7+ TAMs as crucial mediators of immune evasion in CRC. By employing myeloid cell-specific Ccl7 knockout mouse models and sophisticated molecular analyses, the authors demonstrate that elevated levels of CCL7+ TAMs are strongly associated with ICI resistance in human CRC tissues. Mechanistically, the study uncovers two interconnected pathways: CCL7 promotes immunosuppressive TAM function via peroxisome biogenesis and fatty acid oxidation (through PI3K–AKT–PEX3 signaling), while concurrently inhibiting CD8+ T cell tumor infiltration by repressing CXCL10 chemokine expression (through AKT2–STAT1 signaling suppression). These findings establish CCL7 as a dual-modulator of the tumor immune microenvironment and a potential combinatorial target to improve ICI efficacy.

Methods and Experimental Design Insights

To interrogate the functional role of CCL7 in CRC progression and immune resistance, the investigators used a combination of in vivo and in vitro approaches:

• Genetic Models: Ccl7 was selectively knocked out in myeloid cells using conditional knockout mice. These mice, along with wild-type controls, were implanted with MC38 CRC tumor cells to model tumor growth and immune response dynamics.
• Proteomic and Transcriptomic Profiling: Mass spectrometry-based proteomics and RNA sequencing were employed to characterize changes in the TAM proteome and transcriptome, identifying key metabolic and signaling shifts following CCL7 loss.
• Flow Cytometry: Comprehensive immune cell profiling quantified the abundance and activation status of TAMs, CD8+ T cells, and other immune subsets within tumors.
• Functional Assays: The impact of CCL7 deletion or blockade on tumor growth, immune infiltration, and response to PD-L1 therapy was assessed using standard tumor measurement and immunotherapy response protocols.

This integrative approach enabled the dissection of both cellular and molecular mechanisms underlying CCL7-dependent immunosuppression.

Core Findings and Why They Matter

The study's central discoveries can be summarized as follows:

• CCL7+ TAMs Are Correlated with ICI Resistance: Analysis of CRC patient tissues revealed that higher densities of CCL7-expressing macrophages are associated with poor response to immune checkpoint blockade (Chen et al., 2025).
• Myeloid-Specific CCL7 Deletion Reprograms the Tumor Microenvironment: In Ccl7 knockout mice, tumors exhibited reduced accumulation of immunosuppressive TAMs and increased infiltration of activated CD8+ T cells, suggesting that CCL7 is pivotal in maintaining an immune-excluded, therapy-resistant niche.
• Metabolic and Signaling Pathways: CCL7 was shown to promote peroxisome biogenesis and fatty acid oxidation in TAMs via PI3K–AKT–PEX3 signaling, reinforcing their immunosuppressive phenotype. Additionally, CCL7 inhibits the AKT2–STAT1–CXCL10 axis, thereby reducing chemokine-mediated recruitment of effector CD8+ T cells.
• Therapeutic Implications: Pharmacologic or genetic blockade of CCL7 delayed CRC progression and enhanced the efficacy of anti–PD-L1 therapy, positioning CCL7 as a rational target for combination immunotherapeutic strategies.

These findings not only clarify why certain CRC cases remain refractory to ICIs but also provide a mechanistic framework for future therapeutic interventions targeting the myeloid compartment.

Comparison with Existing Internal Articles

Recent expert perspectives and methodological reviews have underscored the value of selective macrophage depletion in dissecting immune microenvironment functions. For example, the article "Precision Macrophage Depletion in Translational Research" synthesizes emerging evidence on targeting macrophages to overcome immunotherapy resistance, specifically referencing tools such as Clodronate Liposomes for in vivo manipulation. Similarly, "Clodronate Liposomes: Precision Macrophage Depletion Reagent" details the workflow utility of liposome-encapsulated clodronate for reproducible, tissue-specific macrophage depletion, a method directly relevant to mechanistic studies like those described by Chen et al.

These internal resources reinforce the translational bridge between mechanistic discovery and experimental intervention, highlighting the importance of immune cell modulation and apoptosis induction in macrophages for both basic and preclinical cancer research.

Limitations and Transferability

While the study offers compelling mechanistic insights, several limitations should be considered. The majority of in vivo experiments were performed in mouse models with murine CRC cell lines, which, although informative, may not fully capture the complexity and heterogeneity of human CRC. Additionally, while the study delineates the pathways by which CCL7 influences both macrophage and T cell populations, further exploration is needed to assess off-target effects and the safety profile of CCL7-targeted interventions in clinical settings.

Transferability to other cancer types or immune contexts remains to be fully validated; thus, researchers should interpret these findings within the specific context of CRC and the modeled tumor microenvironment. Nevertheless, the use of robust in vivo macrophage depletion techniques and multi-omics profiling strengthens the reliability and potential generalizability of the results.

Protocol Parameters

• Myeloid-specific gene knockout: Induce Ccl7 deletion in myeloid cells prior to tumor cell implantation to assess impact on tumor progression and immune cell infiltration.
• Tumor induction: Inject MC38 CRC cells subcutaneously in syngeneic mice (e.g., 1 × 10⁶ cells per mouse) to establish tumors for immunotherapy or depletion experiments.
• Immune profiling: Perform flow cytometry on tumor-infiltrating leukocytes to quantify TAMs (F4/80⁺), CD8⁺ T cells, and additional immune subsets at defined time points.
• Clodronate Liposome administration: For in vivo macrophage depletion, administer liposome-encapsulated clodronate intravenously or intraperitoneally according to experimental design; dosing and frequency should be adjusted based on mouse weight and tissue specificity, as described in the product information.
• Combination therapy: Initiate anti–PD-L1 antibody treatment following macrophage depletion or CCL7 blockade to evaluate changes in tumor growth and immune cell dynamics.

Research Support Resources

To experimentally model macrophage involvement in immune resistance, researchers can deploy Clodronate Liposomes (SKU K2721) for targeted in vivo macrophage depletion. This reagent enables the selective removal of macrophages via phagocytosis-mediated delivery and apoptosis induction, supporting mechanistic studies on immune cell modulation in cancer and other disease models. For detailed protocols and strategic insights, investigators may consult internal reviews such as "Clodronate Liposomes: Precision Macrophage Depletion Reagent". PBS Liposomes are recommended as experimental controls to distinguish specific effects of macrophage depletion. Proper storage and handling are essential to maintain reagent integrity for reproducible results.