1 Introduction

Colon cancer, a leading cause of cancer-related mortality globally, poses significant challenges despite the advancements in medical interventions such as surgery, chemotherapy, and targeted therapies. The survival rate for patients diagnosed with advanced-stage colon cancer remains dismally low, emphasizing the urgent need for more effective treatment strategies (Cao et al., 2022).

A growing body of evidence indicates that the tumor microenvironment (TME), particularly the immune microenvironment, plays a pivotal role in the progression and therapeutic response of colon cancer. The immune microenvironment comprises a complex network of immune cells, signaling molecules, and extracellular matrix components. These elements interact in ways that can either support or inhibit tumor growth. For instance, immune cells such as tumor-associated macrophages, dendritic cells, and T cells can influence the tumor’s behavior through various signaling pathways (Wang et al., 2020b).

The intricate interplay within the immune microenvironment has profound implications for the development of therapeutic strategies. Tumors often exploit immune checkpoints—regulatory pathways that normally maintain self-tolerance and modulate the duration and amplitude of physiological immune responses—to evade immune detection. Immune checkpoint inhibitors (ICIs), which block these regulatory pathways, have shown promise in unleashing anti-tumor immune responses. However, the success of ICIs in colon cancer has been limited to a subset of patients, particularly those with microsatellite instability-high (MSI-H) tumors, highlighting the variability in immune microenvironment characteristics among patients (Bao et al., 2020; Lazarus et al., 2018).

Recent research has underscored the potential of modulating the immune microenvironment to enhance the efficacy of existing therapies and develop new treatment modalities. Strategies such as combining ICIs with other forms of therapy, including oncolytic viruses and colony-stimulating factor 1 receptor (CSF-1R) inhibitors, have shown synergistic effects in preclinical models. These combination therapies work by reprogramming the immune microenvironment to favor anti-tumor immunity, increasing the infiltration and activity of cytotoxic T cells within the tumor (Shi et al., 2019).

Furthermore, understanding the molecular and cellular mechanisms underlying immune responses within the TME is critical for identifying biomarkers that predict response to immunotherapy. For example, higher tumor mutational burden (TMB) has been associated with better responses to ICIs, as it likely reflects a greater neoantigen load that can be recognized by the immune system (Wang et al., 2020b).

This study provides a comprehensive overview of the immune microenvironment of colorectal cancer and its impact on treatment, including the key components and dynamics of the colorectal cancer immune microenvironment. It discusses the roles of various immune cells, such as tumor-associated macrophages, dendritic cells, and T cells, in tumor progression and treatment response. The latest advances in therapeutic strategies aimed at modulating the immune microenvironment, including immune checkpoint inhibitors, vaccines, and combination therapies, are explored. This study highlights the critical importance of the immune microenvironment in colorectal cancer and its potential as a therapeutic target. Through systematic analysis, this paper offers scientific evidence and research directions for future colorectal cancer treatments.

2 Colon Cancer and the Immune Microenvironment

2.1 Overview of Colon Cancer Pathogenesis

Colon cancer is one of the most prevalent cancers worldwide, characterized by its complex pathogenesis involving genetic mutations, epigenetic alterations, and environmental factors. The disease often progresses from benign adenomas to malignant carcinomas through a series of well-defined stages known as the adenoma-carcinoma sequence. Key genetic mutations commonly associated with colon cancer include alterations in the APC, TP53, and KRAS genes, which contribute to unchecked cell proliferation and tumorigenesis (Schmitt and Greten, 2021). Additionally, microsatellite instability (MSI) and chromosomal instability (CIN) are significant pathways implicated in the pathogenesis of colon cancer, affecting the tumor's behavior and response to therapies (Bao et al., 2020).

2.2 The Role of the Immune System in Cancer Development

The immune system plays a dual role in cancer development by both suppressing and promoting tumor growth. On one hand, immune surveillance mechanisms can detect and eliminate nascent tumor cells. On the other hand, chronic inflammation and immune evasion strategies employed by tumor cells can facilitate cancer progression (Frigerio et al., 2021). In colon cancer, the immune microenvironment is critically involved in shaping tumor development and response to treatment. The presence of immune cells such as cytotoxic T lymphocytes (CTLs), regulatory T cells (Tregs), and myeloid-derived suppressor cells (MDSCs) within the tumor microenvironment influences the balance between anti-tumor immunity and immune suppression (Cao et al., 2022; Trimaglio et al., 2020).

2.3 Components of the Tumor Immune Microenvironment

The tumor immune microenvironment in colon cancer consists of various cellular and molecular components that interact to influence tumor growth and therapeutic response.

2.3.1 Immune Cells

Immune cells within the tumor microenvironment include both innate and adaptive immune cells. Tumor-associated macrophages (TAMs), dendritic cells (DCs), neutrophils, natural killer (NK) cells, and various subsets of T cells and B cells play distinct roles in modulating tumor progression. For instance, TAMs can exhibit either pro-tumor (M2) or anti-tumor (M1) phenotypes depending on their activation state and cytokine milieu. The infiltration and activation status of CTLs and Tregs are particularly crucial, as they can directly kill tumor cells or suppress immune responses, respectively (Cen et al., 2021; Wang et al., 2020b). 

2.3.2 Cytokines and Chemokines

Cytokines and chemokines are critical signaling molecules in the tumor microenvironment that regulate immune cell recruitment, differentiation, and function. Pro-inflammatory cytokines such as IL-6, TNF-α, and IL-1β can promote tumorigenesis by enhancing tumor cell proliferation, survival, and angiogenesis. Conversely, anti-inflammatory cytokines such as IL-10 and TGF-β can suppress effective anti-tumor immune responses. Chemokines such as CXCL12 play roles in recruiting immune cells to the tumor site and can contribute to either tumor suppression or promotion depending on the context (Lazarus et al., 2018; Zeng et al., 2021).

2.3.3 Extracellular Matrix

The extracellular matrix (ECM) provides structural support to the tumor and is composed of proteins such as collagen, fibronectin, and laminin. The ECM not only serves as a scaffold for tumor cells but also influences cellular behavior through biochemical and mechanical signals. ECM remodeling, driven by enzymes such as matrix metalloproteinases (MMPs), can facilitate tumor invasion and metastasis. Additionally, the ECM can modulate immune cell function and contribute to the establishment of an immunosuppressive microenvironment (Karlsson and Nyström, 2022; Schmitt and Greten, 2021).

3 Immune Evasion Mechanisms in Colon Cancer

Complex immune escape mechanisms are important cause of colon cancer progression (Figure 1). Clarification of these mechanisms is expected to propose new guidance or direction for tumor prevention and control.

3.1 Immune escape of tumor cells

3.1.1 Immune Checkpoints and Inhibitory Signals

Immune checkpoints are crucial modulators of the immune response, ensuring self-tolerance and preventing autoimmunity. However, tumors exploit these pathways to evade immune detection. In colon cancer, immune checkpoints such as PD-1, PD-L1, and CTLA-4 are often upregulated, leading to immune suppression and tumor progression. The engagement of PD-1 with its ligand PD-L1 inhibits T cell activity, thereby allowing tumor cells to evade immune surveillance. Studies have shown that blocking these checkpoints with antibodies can rejuvenate T cell responses and improve anti-tumor immunity (Gomez et al., 2020; Taghiloo and Asgarian-Omran, 2021).

3.1.2 Tumor Antigen Presentation and Recognition

Effective anti-tumor immunity relies on the presentation of tumor antigens by major histocompatibility complex (MHC) molecules to T cells. However, many tumors, including colon cancer, develop mechanisms to downregulate antigen presentation. This can occur through mutations in genes encoding components of the antigen presentation machinery, such as β2-microglobulin (B2M) and HLA class I molecules, or through epigenetic modifications that silence these genes. The reduced expression of MHC molecules on tumor cells leads to impaired recognition by cytotoxic T cells, thereby allowing the tumor to escape immune destruction (Grasso et al., 2018; Wang et al., 2020b). Immunocompromise, immune tolerance, or low antigen presentation capacity, contribute to tumor cells evading the immune system (Dersh et al., 2021).

3.1.3 Immune privilege

Tumor cells can also evade immune surveillance by creating a local immune privilege. FAS is a member of the TNF receptor superfamily and expressed virtually in all types of cells, whereas FASL, its ligand, is selectively expressed on the surface of activated T cells and NK cells (Golstein and Griffiths, 2018). CTLs use the FAS-FASL and perforin-granzyme pathways as major effector mechanisms of cytotoxicity(Yajima et al., 2019). However, a decreased FAS and highly FASL expression level help colon cancer cells capable to evade FAS-FASL cytotoxicity of tumor-reactive CTLs (Xiao et al., 2019).

3.2 Immunosuppressive cells in TME

3.2.1 Tumor-Associated Macrophages and Myeloid-Derived Suppressor Cells

Tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs) are prominent components of the tumor immune microenvironment that facilitate immune evasion. TAMs can polarize into a pro-tumor M2 phenotype, which secretes anti-inflammatory cytokines and promotes tissue remodeling, angiogenesis, and immune suppression. MDSCs, on the other hand, inhibit T cell activation and proliferation through the production of reactive oxygen species (ROS) and nitric oxide (NO), as well as the depletion of essential amino acids like arginine and tryptophan (Lee et al., 2020; Tang et al., 2021).

3.2.2 Regulatory T Cells and Immune Suppression

Regulatory T cells (Tregs) are essential for maintaining immune homeostasis, but in the context of cancer, they can suppress effective anti-tumor immune responses. Tregs accumulate in the tumor microenvironment and exert their immunosuppressive functions through the secretion of inhibitory cytokines such as TGF-β and IL-10, as well as through direct cell-cell contact-dependent mechanisms (Kang and Zappasodi, 2023; Shan et al., 2022). The presence of Tregs in tumors is often associated with poor prognosis due to their role in dampening cytotoxic T cell responses (Shibata, 2022; Tsuchiya and Shiota, 2021).

4 Therapeutic Strategies to Modulate the Immune Microenvironment

4.1 Immune Checkpoint Inhibitors

Immune checkpoint inhibitors have emerged as a transformative approach in cancer therapy, working by blocking inhibitory pathways that restrain T cell activation and function. This leads to the reactivation of cytotoxic T cells, which can then target and destroy cancer cells. Two major classes of immune checkpoint inhibitors are PD-1/PD-L1 inhibitors and CTLA-4 inhibitors.

4.1.1 PD-1/PD-L1 Inhibitors

The PD-1 receptor, expressed on T cells, interacts with its ligands PD-L1 and PD-L2, which are often overexpressed on tumor cells and within the tumor microenvironment. This interaction delivers an inhibitory signal to T cells, reducing their proliferation, cytokine production, and cytotoxic activity. PD-1/PD-L1 inhibitors, such as pembrolizumab and nivolumab, block this interaction, thereby restoring T cell function and enhancing the immune response against tumor cells (Gomez et al., 2020; Kroemer and Zitvogel, 2021) .

In colon cancer, particularly in cases with microsatellite instability-high (MSI-H) or mismatch repair-deficient (dMMR) tumors, PD-1 inhibitors have demonstrated significant efficacy. These tumors exhibit high mutational burdens, leading to the production of numerous neoantigens that make them more recognizable to the immune system (Lazarus et al., 2018). Clinical trials have shown that PD-1 inhibitors can achieve durable responses in a subset of patients with MSI-H colon cancer, providing a new therapeutic option for these individuals (André et al., 2020). Table 1 showed immunotherapy response in CRC patients based on clinical trials.

4.1.2 CTLA-4 Inhibitors

CTLA-4 is another critical immune checkpoint that downregulates immune responses. It is expressed on T cells and competes with the costimulatory receptor CD28 for binding to B7 molecules (CD80/CD86) on antigen-presenting cells (APCs). CTLA-4 has a higher affinity for B7 molecules than CD28, thereby outcompeting it and delivering an inhibitory signal that limits T cell activation. CTLA-4 inhibitors, such as ipilimumab, block this interaction, enhancing T cell activation and proliferation (Giannone et al., 2020).

Although CTLA-4 inhibitors have been primarily studied in melanoma, their application in colon cancer is being actively explored. The combination of CTLA-4 inhibitors with PD-1 inhibitors is particularly promising, as it can potentially overcome resistance mechanisms and provide synergistic effects (Lenz et al., 2022). This combination therapy aims to enhance both the priming and effector phases of the T cell response, leading to a more robust and sustained anti-tumor activity (Guerrouahen et al., 2019).

4.2 Adoptive Cell Transfer Therapies

Adoptive cell transfer (ACT) therapies represent a cutting-edge approach in cancer treatment, involving the ex vivo expansion and reinfusion of autologous or allogeneic immune cells with potent anti-tumor activity. This approach aims to overcome the immunosuppressive tumor microenvironment by introducing a large number of activated and tumor-specific immune cells directly into the patient (Galli et al., 2021).

4.2.1 CAR-T Cell Therapy

Chimeric Antigen Receptor T-cell (CAR-T) therapy involves genetically modifying a patient’s T cells to express CARs, which are engineered receptors that combine the antigen-binding domain of a monoclonal antibody with T cell-activating domains. This enables the T cells to specifically recognize and kill cancer cells expressing the target antigen. However, the shortage of TSAs, on-target off-tumour effects, low CAR-T cell infiltration and the immunosuppressive microenvironment are the current research dilemma (Aparicio et al., 2021).

In colon cancer, efforts to apply CAR-T therapy have faced challenges due to the immunosuppressive tumor microenvironment and the heterogeneous expression of tumor antigens. However, significant progress is being made in identifying suitable targets such as CEA (carcinoembryonic antigen) and HER2 (human epidermal growth factor receptor 2), which are overexpressed in some colon cancers. Preclinical studies have shown that CAR-T cells targeting these antigens can induce strong anti-tumor responses (Cook et al., 2018). To improve the efficacy and safety of CAR-T therapy in solid tumors, researchers are developing strategies to enhance CAR-T cell persistence, infiltration, and function within the tumor microenvironment (Jin et al., 2021).

4.2.2 TIL Therapy

Tumor-Infiltrating Lymphocyte (TIL) therapy involves the extraction of TILs from a patient’s tumor, followed by their expansion and activation in vitro before reinfusing them into the patient. TILs are a heterogeneous population of immune cells that have already encountered tumor antigens in vivo, making them highly specific to the patient’s cancer (Zhao et al., 2022).

TIL therapy has shown promise in treating metastatic melanoma and is now being explored for other solid tumors, including colon cancer. Clinical trials have demonstrated the potential of TIL therapy in inducing durable responses in patients with refractory solid tumors (Mullard, 2024). For colon cancer, research is focusing on optimizing the protocols for TIL expansion and activation, as well as combining TIL therapy with other treatments such as immune checkpoint inhibitors and targeted therapies to enhance efficacy (Trimaglio et al., 2020).

4.3 Cancer Vaccines

Cancer vaccines aim to elicit a robust immune response against tumor-specific antigens, leading to the eradication of cancer cells (Saxena et al., 2021; Schiller et al., 2022; Wang et al., 2020a). These vaccines can be categorized into peptide vaccines and dendritic cell (DC) vaccines, among other types. In colon cancer therapy, several promising vaccine strategies have emerged.

4.3.1 Peptide Vaccines

Peptide vaccines consist of short sequences of amino acids that correspond to tumor-specific antigens. These vaccines are designed to stimulate T cells to recognize and attack cancer cells. One significant advantage of peptide vaccines is their ability to be tailored to individual patients based on their tumor's antigenic profile. Recent advances have focused on identifying and validating novel peptide antigens that can effectively induce an anti-tumor immune response. For instance, studies have identified multiple peptides derived from colon cancer antigens that are capable of activating cytotoxic T lymphocytes (CTLs), leading to targeted killing of cancer cells (Liang et al., 2021; Naciute et al., 2020).

4.3.2 Dendritic Cell Vaccines

Dendritic cell (DC) vaccines leverage the natural ability of DCs to process and present antigens to T cells, thereby initiating a robust immune response. These vaccines involve loading patient-derived DCs with tumor antigens ex vivo and then re-infusing them into the patient. This process enhances the body's immune response to the cancer. For example, a study demonstrated that DCs loaded with colon cancer stem cell-derived antigens significantly suppressed tumor growth and extended survival in a mouse model of colorectal carcinoma (Fu et al., 2021). The complex crosstalk between CRC and DCs still need to be further unraveled (Subtil et al., 2021).

Another promising approach involves in situ DC vaccines, which are designed to generate tumor-associated antigens (TAAs) directly within the tumor microenvironment. These vaccines use polymersomal nanoformulations that combine immunogenic cell death inducers and photosensitizers to enhance antigen presentation and stimulate a strong immune response (Yang et al., 2019).

4.3.3 Additional Approaches and Combination Therapies

In addition to peptide and DC vaccines, other innovative strategies are being explored to improve the efficacy of cancer vaccines. For instance, personalized cancer vaccines, which are tailored to the unique mutational landscape of an individual’s tumor, have shown promise in clinical trials. These vaccines harness next-generation sequencing and bioinformatics to identify and target neoantigens, which are specific to the cancer cells.

Combination therapies that integrate cancer vaccines with other immunotherapies, such as immune checkpoint inhibitors, are also being investigated. This approach aims to overcome the immunosuppressive tumor microenvironment and enhance the overall anti-tumor response. Clinical trials have demonstrated that combining vaccines with agents like PD-1 inhibitors can significantly improve therapeutic outcomes (Azadi et al., 2021; Kim et al., 2021; Zhao et al., 2019).

4.4 Oncolytic Viruses

Oncolytic viruses are genetically engineered or naturally occurring viruses that selectively infect and kill cancer cells while sparing normal tissues. These viruses can replicate within tumor cells, causing cell lysis and the release of tumor antigens, which can subsequently stimulate an anti-tumor immune response. One significant advantage of oncolytic viruses is their ability to convert "cold" tumors, which are poorly infiltrated by immune cells, into "hot" tumors, which are more amenable to immune system attack (Omole et al., 2022).

Several oncolytic viruses have been studied for their potential in colon cancer therapy. For example, the oncolytic vaccinia virus has demonstrated the ability to rejuvenate the immune microenvironment in colon cancer by promoting the infiltration and activation of CD8+ T cells and dendritic cells within the tumor, enhancing the overall anti-tumor immunity (Lee et al., 2020). Additionally, combining oncolytic viruses with immune checkpoint inhibitors, such as anti-PD-1 antibodies, has shown synergistic effects, resulting in improved tumor control and prolonged survival in preclinical models (Shi et al., 2019).

Oncolytic viruses can also be engineered to express therapeutic genes, such as cytokines or other immune-stimulatory molecules, further enhancing their anti-tumor effects. For instance, viruses expressing granulocyte-macrophage colony-stimulating factor (GM-CSF) have been shown to boost local immune responses and attract immune cells to the tumor site. The versatility and multifaceted mechanisms of action of oncolytic viruses make them a promising therapeutic strategy for modulating the immune microenvironment in colon cancer, potentially overcoming resistance to conventional therapies and improving patient outcomes (Cook et al., 2018).

4.5 Modulating Tumor-Associated Macrophages

Tumor-associated macrophages (TAMs) can exhibit pro-tumor or anti-tumor phenotypes depending on the signals in the tumor microenvironment (Wang et al., 2021). Strategies to reprogram TAMs from a pro-tumor (M2) to an anti-tumor (M1) phenotype include the use of CSF-1R inhibitors, which have shown potential in enhancing the efficacy of other immunotherapies (Chamseddine et al., 2022).

4.6 Targeting Myeloid-Derived Suppressor Cells

Myeloid-derived suppressor cells (MDSCs) are key immunosuppressive cells that inhibit T cell responses and promote tumor growth. Targeting MDSCs with agents that block their recruitment, differentiation, or function is a promising approach to enhance the efficacy of cancer immunotherapies(Yin et al., 2020). Strategies include the use of small molecule inhibitors and monoclonal antibodies that disrupt MDSC-mediated suppression (Guerrouahen et al., 2019; Wu et al., 2022).

5 Combination Therapies

5.1 Rationale for Combining Immunotherapies

Combining immunotherapies aims to enhance the therapeutic efficacy by targeting multiple mechanisms of tumor immune evasion. While single-agent immunotherapies have shown promise, their effectiveness is often limited by the complex and adaptive nature of the tumor immune microenvironment. By employing combination therapies, it is possible to overcome resistance mechanisms, potentiate immune responses, and achieve more durable clinical benefits. For instance, combining immune checkpoint inhibitors with other immunomodulatory agents can synergistically enhance anti-tumor immunity (Shi et al., 2019).

5.2 Combination of Immune Checkpoint Inhibitors

The combination of different immune checkpoint inhibitors, such as PD-1/PD-L1 and CTLA-4 inhibitors, has shown to be effective in various cancers, including colon cancer. This approach leverages the distinct mechanisms of action of these inhibitors to enhance T cell activation and reduce immune suppression. Clinical studies have demonstrated that combining anti-PD-1 with anti-CTLA-4 can result in superior anti-tumor activity compared to monotherapy, detailed shown in Table 1 (Fan et al., 2019; Lenz et al., 2022).

Cuiffo et al. focused on investigating the antitumor effects of Phio Pharmaceuticals' self-delivering RNAi therapeutic technology, INTASYL™, in a mouse colorectal cancer model(Cuiffo et al., 2023). This technology targets two immune checkpoints, PD-1 and CTLA-4, demonstrating that intratumoral (IT) injections can significantly enhance antitumor effects while reducing systemic immune-related side effects. In the experiments, the treatment of the mouse colorectal cancer cell model CT26 was compared using single-target and dual-target INTASYL™. The study showed that dual-target INTASYL™ had a synergistic antitumor effect in vivo, and immune regulatory mechanisms within the tumor microenvironment were assessed using flow cytometry. Additionally, the study confirmed the dose-dependent antitumor effects of the therapy and highlighted the potential for further development of this strategy to maximize efficacy and minimize immune-related side effects.

5.3 Combining Immunotherapy with targeted therapy

To improve the effectiveness of immunotherapy, researchers have combined immunotherapy with targeted therapeutic agents, including epidermal growth factor receptor (EGFR) inhibitors, VEGF/VEGFR inhibitors, human epidermal growth factor receptor 2 (HER2) inhibitors and multikinase inhibitors for colorectal cancer, and several studies have achieved better results, listed in Table 2. The AVETUX study combined anti-PD-L1, cetuximab, and chemotherapy demonstrated a 75% objective response rate and a 95% disease control rate(Tintelnot et al., 2022). Researches showed that increased CXCL10 expression had a sensitizing effect on the combination therapy with cetuximab and anti-PD-1 antibody in CRC (Yan et al., 2023). Targeted therapy not only stops the proliferation of tumor cells, but also establishes a favorable immune microenvironment for the effectiveness of immunotherapy(Singh et al., 2024). Therefore, the combination of targeted therapy and immunotherapy may produce greater efficacy than either treatment alone, as shown in Table 2.

5.4 Combining Immunotherapy with Chemotherapy

Combining immunotherapy with chemotherapy can enhance anti-tumor immunity by inducing immunogenic cell death and modulating the tumor microenvironment. Chemotherapeutic agents can increase the release of tumor antigens, reduce immunosuppressive cells, and enhance the infiltration of effector T cells into the tumor. For example, the combination of doxorubicin with immune checkpoint inhibitors has shown significant improvements in tumor regression and immune response in preclinical models (Kuai et al., 2018; Taniura et al., 2020). Clinical trials combining immunotherapy with chemotherapy were shown in Table 3.

5.5 Combining Immunotherapy with Radiotherapy

Radiotherapy can potentiate the effects of immunotherapy by causing localized tumor cell death and releasing tumor-associated antigens, which can prime the immune system. Moreover, radiotherapy can modulate the tumor microenvironment to make it more conducive to immune infiltration and activation. Studies have shown that combining radiotherapy with immune checkpoint inhibitors can lead to enhanced anti-tumor responses and improved survival outcomes (Hanoteau et al., 2019; Tang et al., 2020). Table 4 showed clinical trials with ICIs and radiotherapy.

Abbreviations: Radiotheraty (RT), Objective response rate (ORR), Disease control rate (DCR), Relapse-Free Survival (RFS), Progression-Free-Survival (PFS), Overall survival (OS)

5.6 Emerging Combinatorial Approaches

Emerging combinatorial approaches aim to further enhance the efficacy of cancer immunotherapy by targeting various aspects of the tumor microenvironment and immune response. These include the use of oncolytic viruses, which selectively infect and kill tumor cells while stimulating an anti-tumor immune response, in combination with immune checkpoint inhibitors (Yoo et al., 2020). Additionally, combining metabolic modulators with immunotherapy to reprogram the immune cell metabolism within the tumor microenvironment has shown promise in preclinical studies (Bader et al., 2020).

By leveraging the synergistic effects of these various therapeutic strategies, combination therapies hold the potential to significantly improve the treatment outcomes for patients with colon cancer.

6 Biomarkers for Predicting Response to Immunotherapy

6.1 Immune-Related Biomarkers

6.1.1 Microsatellite Instability

Microsatellite Instability (MSI) is a key biomarker for predicting response to immunotherapy, particularly in colon cancer. MSI-high (MSI-H) tumors have defects in the DNA mismatch repair system, leading to a high mutation rate and increased neoantigen load. These tumors are more likely to respond to immune checkpoint inhibitors, and MSI status is an FDA-approved biomarker for selecting patients for pembrolizumab treatment in colon cancer (Hou et al., 2022). The predictive value of MSI status in predicting immunotherapy response in CRC is significant, and has been established in clinical practice. Regrettably, our previous research found that approximately 25% of patients with MSI-H CRC had intrinsic resistance to immunotherapy (Wang et al., 2023). Therefore, there is still a need to explore more appropriate biomarkers to predict the effect of immunotherapy in colorectal cancer.

6.1.2 Tumor Mutational Burden

Tumor Mutational Burden (TMB) is a measure of the number of mutations within a tumor genome. High TMB is associated with the presence of more neoantigens, which can make the tumor more recognizable to the immune system. As a result, patients with high TMB are often more responsive to immune checkpoint inhibitors. This biomarker has been validated in various cancers, including colon cancer, where a higher TMB has correlated with better responses to immunotherapy (Duffy and Crown, 2019; Wang et al., 2019). In fact, TMB as a marker for colorectal cancer is still fraught with problems. Although FDA approved pembrolizumab for advanced solid tumors with TMB-high according to KEYNOTE-158 clinical trial, there was none CRC patients in the clinical trial (Marabelle et al., 2020). Besides, the lack of adequate standards for optimal TMB cut-off still remains concern (Marques et al., 2024).

6.1.3 PD-L1 expression

PD-L1 expression in tumor cells and tumor-infiltrating immune cells is a common predictive biomarker for immunotherapy in various tumor types, but its role in CRC remains unclear. The substantial intra-tumor heterogeneity of PD-L1 expression complicates its detection. Besides, standardization and threshold setting for PD-L1 expression assays remain controversial.

6.2 Biomarkers of Immune Cell Infiltration

The presence and density of tumor-infiltrating lymphocytes (TILs), particularly CD8+ T cells, in the tumor microenvironment are critical indicators of the immune system's engagement with the tumor. High levels of TILs are associated with better responses to immunotherapy (Giatromanolaki et al., 2023). Quantification of TILs can be performed using techniques such as immunohistochemistry and digital pathology, which provide valuable prognostic and predictive information (Cao et al., 2022).

6.3 Predictive Biomarkers from Peripheral Blood

Non-invasive biomarkers from peripheral blood, such as circulating tumor DNA (ctDNA), immune cell profiles, and metabolic signatures, are being explored for their potential to predict responses to immunotherapy. For example, higher levels of specific immune cells, such as CD8+ T cells, and metabolic markers have been linked to better responses to treatment. Monitoring these biomarkers can provide real-time insights into the patient's immune status and treatment efficacy (Nixon et al., 2019; Triozzi et al., 2022).

6.4 Imaging Biomarkers

Imaging biomarkers, such as radiomics, utilize advanced imaging techniques combined with artificial intelligence to predict responses to immunotherapy. For instance, radiomic features derived from CT or PET scans can provide information on tumor heterogeneity, immune infiltration, and metabolic activity. These non-invasive biomarkers are valuable for assessing the tumor's immunogenic profile and predicting therapeutic outcomes, which still needs to be validated with larger samples in the future (Sun et al., 2018; Trebeschi et al., 2019) .

6.5 Specific gene mutation

6.5.1 POLE mutation

Polymerase ε (POLE), encoded by the POLE gene, plays an essential role in DNA replication and proofreading in cells, and has recently been proposed to be involved in the efficacy of immunotherapy (Garmezy et al., 2022). The association of POLE mutations with immunotherapy remission has been observed in clinical experience (Bikhchandani et al., 2023; Durando et al., 2022). Somatic POLE mutations are detected in approximately 1% of all CRC patients (Kawai et al., 2021; Ma et al., 2022). Further studies with large cohorts and long-term follow-up are critical to determine the role of POLE mutations as predictive markers of ICB efficacy in CRC treatment.

6.5.2 BRAF mutation

BRAF V600E mutations are associated with unfavorable clinical prognosis in CRC patients. The results of a clinical study combining the BRAF inhibitor darafenib, the MEK inhibitor trametinib, and the immunotherapeutic agent spartalizumab for BRAF V600E mutation CRC patients found a very good response rate (24.3% of all patients; 25% of microsatellite-stabilized patients) (Tian et al., 2023). Despite the small number of patients with BRAF mutations, this may still provide direction for the future search for immunotherapy biomarkers.

6.6 Gut microbiota

Recent studies have emphasized the potential of gut microbiota composition in predicting ICB responses in CRC patients. Interactions between the gut microbiota and colorectal cancer pathogenesis involve triggering pro-tumor inflammation and promoting immune evasion mechanisms, thereby influencing ICB administration therapeutic outcomes (Xu et al., 2020).

Although significant progress has been made in predictive biomarkers for CRC immunotherapy, many challenges remain, including standardized testing of biomarkers, integration and analysis of multi-omics data, and how to effectively translate basic research results into clinical applications. This requires more in-depth research in the future. By leveraging these biomarkers, clinicians can better stratify patients, personalize treatment plans, and improve outcomes in colon cancer immunotherapy.

7 Challenges and Future Directions

7.1 Overcoming Immune Resistance

One of the significant challenges in modulating the immune microenvironment for colon cancer therapy is overcoming immune resistance. Tumors employ various mechanisms to evade immune detection and destruction, including the upregulation of immune checkpoints, recruitment of immunosuppressive cells, and secretion of inhibitory cytokines. For instance, the immunosuppressive tumor microenvironment, characterized by high levels of regulatory T cells (Tregs) and myeloid-derived suppressor cells (MDSCs), significantly hampers the effectiveness of immunotherapies (Bao et al., 2020; Scolaro et al., 2024). Interestingly, recent developments in single-cell sequencing have advanced the study of reprogramming the immune microenvironment(Huang et al., 2024a). Strategies such as combination therapies using immune checkpoint inhibitors and agents that target these immunosuppressive cells are under investigation to overcome this resistance (Shi et al., 2019).

7.2 Enhancing Immune Memory

Another crucial direction for future research is enhancing immune memory to ensure long-term protection against cancer recurrence. Current immunotherapies often fail to induce robust and durable immune memory, which is essential for preventing tumor relapse. Enhancing the generation and persistence of memory T cells through vaccination strategies and adjuvants, as well as optimizing the timing and sequencing of immunotherapies, could significantly improve outcomes (Huang et al., 2024b; Jia et al., 2024; Song et al., 2024). Recent studies suggest that combining immunotherapies with agents that promote the survival and function of memory T cells may provide lasting anti-tumor immunity (Yu and Cui, 2018). Memory-like (ML) NK cells differentiated overcome many challenges to effective NK cell anti-tumor responses (Marin et al., 2024).

7.3 Personalized Immunotherapy Approaches

Personalized immunotherapy approaches tailored to the unique genetic and immunological landscape of each patient's tumor are crucial for improving therapeutic efficacy. Advances in genomic and transcriptomic technologies have enabled the identification of specific mutations, neoantigens, and immune signatures that can be targeted for personalized treatment. Integrating multi-omics data can help identify biomarkers for predicting response to therapy and guide the development of individualized treatment plans. For example, the use of tumor mutational burden (TMB) and microsatellite instability (MSI) as biomarkers has shown promise in selecting patients for immune checkpoint inhibitor therapy (Lazarus et al., 2018; Wang et al., 2020b). Currently, suitable biomarkers that can guide immunotherapy efficacy are still in continuous exploration (Li et al., 2024).

7.4 Integrating Multi-Omics Data for Better Understanding

The integration of multi-omics data, including genomics, transcriptomics, proteomics, and metabolomics, offers a comprehensive understanding of the tumor microenvironment and its interaction with the immune system. This holistic approach can uncover novel therapeutic targets and pathways involved in immune evasion and resistance. For instance, analyzing the molecular nature associated with microsatellite status in colon cancer can provide insights into the mechanisms of immune escape and identify potential targets for therapy (Bao et al., 2020). Microbiota-metabolite-immune crosstalk might also explain tumor resistance to immunotherapy. The gut microbiota, for example, F. nucleatum-derived succinic acid suppressed the cGAS-interferon-β pathway, limiting CD8 T cell and consequently inhibiting antitumor response (Jiang et al., 2023). However, current studies on microbiota are highly heterogeneous, which still needs more researches (Cheng et al., 2024; Derosa et al., 2024). Furthermore, the development of advanced computational models and bioinformatics tools is essential for integrating and interpreting these complex datasets, ultimately leading to the design of more effective and personalized immunotherapies (Visalakshan et al., 2023). 

In conclusion, addressing the challenges of immune resistance, enhancing immune memory, personalizing immunotherapy approaches, and integrating multi-omics data are critical for advancing the field of immunotherapy in colon cancer. These efforts hold the potential to significantly improve patient outcomes and pave the way for more effective and durable cancer treatments.

8 Concluding Remarks

In this study, we have comprehensively examined the critical role of the immune microenvironment in colon cancer therapy. Key findings highlight the importance of various immune cells, cytokines, and the extracellular matrix in modulating tumor progression and response to treatment. We discussed the mechanisms of immune evasion employed by colon cancer cells, including the upregulation of immune checkpoints, recruitment of immunosuppressive cells like tumor-associated macrophages (TAMs) and myeloid-derived suppressor cells (MDSCs), and impaired tumor antigen presentation. Therapeutic strategies to modulate the immune microenvironment, such as immune checkpoint inhibitors, adoptive cell transfer therapies, cancer vaccines, and oncolytic viruses, were explored for their potential to enhance anti-tumor immunity and improve clinical outcomes .

The insights gathered from recent research underscore the necessity of targeting the immune microenvironment in colon cancer therapy. The efficacy of immune checkpoint inhibitors, for instance, can be significantly enhanced when combined with therapies that modulate the immune microenvironment, such as TAM reprogramming and MDSC targeting agents. Additionally, personalized immunotherapy approaches that utilize biomarkers such as tumor mutational burden (TMB) and microsatellite instability (MSI) can better stratify patients and tailor treatments to achieve optimal outcomes.

Future research should continue to focus on overcoming the challenges associated with immune resistance and enhancing immune memory to ensure long-term protection against tumor recurrence. Integrating multi-omics data will be vital for a deeper understanding of the tumor microenvironment and for the development of novel therapeutic targets. Personalized immunotherapy approaches, guided by detailed molecular and immunological profiling, hold great promise for improving patient outcomes. By addressing these challenges and leveraging emerging technologies, we can pave the way for more effective and durable cancer treatments.

Acknowledgments

Thank you to the peer reviewers for their valuable feedback.

Funding

This work was supported by the National Natural Science Foundation of China (grant no. 62372141, 82303742).

Conflict of Interest Disclosure

The authors affirm that this research was conducted without any commercial or financial relationships that could be construed as a potential conflict of interest.