Phytochemicals remodel antitumor immunity via the “microbiota–metabolite–receptor” axis: focus on colorectal cancer and immunotherapy

Abstract

Colorectal cancer (CRC) is a malignancy with high mortality. Due to the suppressive state of the tumor immune microenvironment (TIME), approximately 95% of microsatellite-stable (MSS) or proficient mismatch repair (pMMR) cases exhibit poor responsiveness to immune checkpoint inhibitors (ICIs). According to research, immune cell function can be regulated by gut microbiota and their metabolites via receptor-mediated pathways. They act as critical extrinsic factors in modulating the TIME and enhancing the efficacy of ICIs. Therefore, we systematically summarize the immunological mechanisms mediated through gut-specific metabolites (short-chain fatty acids, secondary bile acids, indole derivatives) and their cognate receptors (GPR41/43, GPR109A, FXR, TGR5, AhR). We further discuss how phytochemicals, after microbial transformation, modulate immune cells via the “microbiota–metabolite–receptor” axis. Key examples of such compounds include polysaccharides, saponins (and triterpenes), polyphenols, and alkaloids, which influence cells like Tregs, Th17, and CD8⁺ T cells. Simultaneously, we analyze the different effects of CRC-specific microbiota and the interventional potential of phytochemicals, while evaluating synergistic treatment possibilities between CRC and ICIs, chemotherapy, anti-angiogenic therapy, and radiotherapy. We propose a verifiable framework for stratification mechanisms of “phytochemicals-microbiota-metabolites-receptors-immune system-TIME-ICIs”, and emphasize its potential application in MSS CRC immunotherapy to provide novel insights for precision treatment of CRC.

Introduction

Colorectal cancer (CRC) remains one of the most prevalent and deadly gastrointestinal tumors worldwide. In 2022, CRC ranked third globally in cancer incidence (9.6%) and second in mortality (9.3%) [1]. Despite continuous improvements in screening, early diagnosis, and comprehensive treatment strategies for CRC, the prognosis and quality of life for patients with advanced disease remain suboptimal. Current evidence demonstrates that the tumor immune microenvironment (TIME) plays a pivotal role in CRC development, with TIME-mediated immune escape being a key driver of tumor progression and treatment resistance [2]. As a critical component of the intestinal barrier and immune system, the gut microbiota exhibits strong associations with the risk and progression of gastrointestinal tumors. Research indicates that gut microbiota significantly influence the occurrence and progression of colorectal cancer [3]. Microbial metabolites can regulate the immune system through specific receptors and consequently influence the TIME. The gut microbiota can be influenced by phytochemicals, and the bacteria can further convert some of these substances into bioactive molecules. These metabolites impact TIME, indirectly control host immunological responses, and eventually have anticancer consequences [4]. Based on these considerations, we seek new breakthroughs in CRC treatment by exploring the interplay between metabolites, microbiota, and host immunity. We also examine the modulatory effects of phytochemicals on tumor growth, metastasis, and dissemination. Furthermore, we systematically review the mechanistic roles of the metabolite–microbiota–immunity axis in CRC and discuss the limitations and potential of current ICIs, aiming to provide new perspectives for individualized prevention and comprehensive precision therapy.

Background

Disease burden of colorectal cancer and the current imitations of its treatment

Colorectal cancer (CRC), as a malignant tumor with high incidence and mortality rates, ranks among the major diseases threatening human health. In most countries worldwide, the age-standardized incidence rate (ASIR) of CRC shows an upward trend [5]. Current clinical first-line treatment regimens for CRC include FOLFOX (calcium folinate + fluorouracil + oxaliplatin regimen), FOLFIRI (calcium folinate + fluorouracil + irinotecan combination chemotherapy) ± anti-VEGF/EGFR antibody combination regimens. While existing treatments improve survival rates, the median overall survival (OS) for advanced patients remains only 23–30 months [6], with most patients developing acquired resistance within 9–11 months [7]. Additionally, antitumor therapy with immune checkpoint inhibitors (ICIs) shows significant efficacy only in patients with dMMR/MSI-H tumors, which account for approximately 5% of CRC cases [8]. Nevertheless, most MSS/pMMR patients do not respond well to ICIs [9]. In order to investigate strategies for altering the TIME via the "microbiota-metabolite-receptor" axis, which is essential for improving ICI efficacy, we present phytochemicals as novel modulators.

Interactions between gut microbiota and phytochemicals

Phytochemicals are natural products widely distributed in plants. Most of them need to be metabolized by the gut bacteria in order to have therapeutic effects, and their real bioavailability is comparatively low. Consequently, the gut microbiota serves as a critical mediator for the action of phytochemicals. Research demonstrates that phytochemicals not only influence the internal composition of the gut microbiota [10] but also modulate microbial metabolic pathways, thereby affecting metabolite production and subsequently shaping host immune responses and the tumor immune microenvironment [11, 12].

The “microbiota–metabolite–receptor” axis in regulating the tumor immune microenvironment

The gut microbiota not only serves as the site for energy metabolism and nutrient absorption within the body, but also produces various small-molecule metabolites, such as short-chain fatty acids, bile acid derivatives, and indole compounds, through the breakdown of dietary components and substrates. These metabolites bind to specific receptors, activating distinct signaling pathways that modulate immune cell function and shape the gut-brain axis. The resulting “microbiota-metabolite-receptor” signaling axis serves as a crucial bridge linking exogenous bioactive compounds to internal immune regulation. (As shown in Fig. 1).

Fig. 1.

Phytochemicals Remodel Antitumor Immunity via the Microbiota–Metabolite–Receptor Axis in Colorectal Cancer. Schematic illustration of the mechanisms by which phytochemicals modulate antitumor immunity through gut microbiota. Phytochemicals (such as polysaccharides, saponins/triterpenoids, polyphenols, and alkaloids) can modulate the composition and metabolism of gut microbiota, thereby promoting the production of beneficial metabolites and further activating host-associated receptor pathways. In this process, the body’s immunological regulatory activities are greatly improved by gut microbiota-mediated communication, which also modifies the TIME. In addition to achieving direct anticancer benefits by preventing tumor development, these actions can increase the effectiveness of ICIs, enabling higher antitumor activity. Simultaneously, this mechanism synergizes with radiotherapy, chemotherapy, and anti-angiogenic therapies, thereby achieving comprehensive antitumor effects through multimodal treatment

Furthermore, it should be noted that the microbial biotransformation of plant bioactive compounds is highly individualized and does not result in the uniform production of beneficial metabolites across different hosts. Depending on variations in gut microbiota composition and related enzyme activities, the same phytochemical may be converted into metabolites with different biological activities, bioavailability, and even potential toxicity [13].

Characteristic metabolite profiles in the colorectal tumor immune microenvironment

Short-chain fatty acids (SCFAs), including acetate, propionate, and butyrate, are produced through the fermentation of polysaccharides, dietary fibers, and other phytochemicals [14]. SCFAs can promote CD8⁺ T cell memory formation and maintain Treg homeostasis [15, 16]. Among them, in vivo studies using germ-free mice and conventionally housed mice, along with in vitro colonic cell experiments, demonstrate that butyrate restores energy metabolism and suppresses excessive autophagy. In addition to acting as an HDAC inhibitor to control gene expression, butyrate is an important energy source for colorectal epithelial cells [17].

Secondary bile acids (sBAs) exert bidirectional regulatory effects within the immune microenvironment. For example, 3-oxolithocholic acid (3-oxoLCA) can selectively inhibit Th17 cell differentiation. It exerts this effect by directly binding to and suppressing the transcriptional activity of RORγt, thereby reducing the production of inflammatory cytokines and limiting the activity of effector T cells [18]. In contrast, by triggering tolerogenic reactions in dendritic cells (DCs), isodeoxycholic acid (isoDCA) increases Treg production [19].

Indole and tryptophan-derived metabolites can activate IL-22 via the aryl hydrocarbon receptor (AhR), promoting epithelial barrier repair while simultaneously regulating Th17/Treg balance [20, 21].

Other metabolites also have significant functions inside the TIME. Lactic acid accumulation in the tumor microenvironment suppresses effector T cell and NK cell function [22, 23]. Polyamines, such as spermidine, contribute to maintaining mucosal homeostasis, but excessive levels may promote immunosuppression. Trimethylamine N-oxide (TMAO) has been implicated in inflammation and an increased risk of CRC [24]. In summary, metabolites exert bidirectional effects in CRC: they can enhance antitumor activity while simultaneously facilitating immune evasion and elevating CRC risk.

Receptor-mediated immune signaling pathways and functional diversity

G protein-coupled receptors (GPCRs) constitute the largest family of cell surface receptors, transducing extracellular signals by activating intracellular G proteins [25]. Among them, GPR41/43 (FFAR3/FFAR2) mediates acetate and propionate signaling, thereby regulating inflammation and dendritic cell (DC) activation [26]. GPR109A, the primary receptor for butyrate, promotes Treg differentiation and establishes an anti-inflammatory environment [27]. In addition, GPR81 (the lactate receptor) [28] and GPR120 (the long-chain fatty acid receptor) [29] have also been shown to participate in immune regulation within the TIME.

Numerous types of nuclear receptors, including FXR, TGR5, and VDR, sense bile acids [30]. Among them, FXR serves as the primary receptor for bile acids, regulating bile acid metabolism and promoting immune homeostasis [31]. TGR5 is activated by secondary bile acids, inducing macrophages to shift from the pro-inflammatory M1 phenotype to the anti-inflammatory M2 phenotype upon activation, thereby exerting immunosuppressive effects. VDR is essential for controlling mucosal immunity and antimicrobial peptides [32].

The aryl hydrocarbon receptor (AhR), activated by indole derivatives and tryptophan metabolites, induces IL-22 secretion, promotes barrier repair, and enhances antimicrobial immunity. In summary, the complicated TIME is shaped by receptor-mediated signaling networks, which show variability in tumor immunity and play important functions in causing immunosuppression and boosting immune cell activation.

Immune pathway network of the “microbiota–metabolite–receptor” axis

The “microbiota-metabolite-receptor” axis constitutes a dynamic signaling network, where different metabolites can act on multiple immune cells via distinct receptors to exert diverse immune functions [33]. The following are the primary signaling routes in this network.

Within the SCFA-GPCR/HDAC pathway, SCFAs bind to G protein-coupled receptors to initiate downstream signaling, inducing anti-inflammatory factors and promoting Treg generation [16]. Simultaneously, upon entering the nucleus, SCFAs inhibit HDAC activity, enhancing the transcription of immune-related genes [34]. This dual mechanism helps maintain the Th17/Treg balance, mitigates excessive inflammation, and improves CD8⁺ T cell metabolism and function [15, 35], thereby providing theoretical support for the efficacy of ICIs.

The BA-FXR/TGR5 pathway exerts bidirectional effects depending on the context [36]. On one hand, FXR activation downregulates the NF-κB pathway, suppresses antigen presentation by dendritic cells (DCs), and promotes Treg differentiation, thereby exerting immunosuppression in the tumor microenvironment and promoting the “cold tumor” transformation of TIME [37]. Conversely, TGR5 promotes polarization of tumor-associated macrophages (TAMs) toward an M2-like immunosuppressive phenotype via cAMP-STAT3/STAT6 signaling, thereby suppressing CD8⁺ T cell activity and inhibiting antitumor immunity [38]. Collectively, this pathway both establishes an immune-tolerant microenvironment and suppresses tumor immunity under specific metabolic conditions.

Activation of the indole-AhR pathway plays a crucial role in maintaining the intestinal mucosal immune barrier and controlling inflammation. AhR signaling upregulates IL-22 expression, enhancing mucosal epithelial repair and antimicrobial peptide secretion [21], thereby reducing pathogen invasion and chronic inflammatory stimulation. Concurrently, AhR participates in regulating the dynamic equilibrium between Th17 and Treg cells. During excessive inflammatory responses, it suppresses Th17 expansion while promoting Treg functional stability [39], thereby maintaining immune homeostasis within TIME. Consequently, this pathway not only plays a pivotal role in the “inflammation-to-cancer” transition but also provides crucial evidence for phytochemicals inhibiting intestinal “inflammation-to-cancer” conversion via gut metabolites.

When taken as a whole, various route networks have different roles in TIME. Consequently, we propose phytochemicals as upstream modulators selectively regulating the internal composition and metabolic pathways of the gut microbiota. By maintaining equilibrium across different axes and pathways, these compounds drive TIME toward a direction conducive to antitumor immunity. The following table provides an overview of the metabolite-receptor-immune effector pathway. (As shown in Table 1).

Table 1.

Summary of metabolite–receptor–immune effector pathway

Metabolite type	Main receptor/sensor	Immune cell targets	Downstream (Typical) signaling pathways	Immune effects	Clinical significance (Related to ICIs)	Study model / evidence level	References
SCFAs (Butyrate)	GPR41, GPR43, GPR109A, Intracellular action: HDAC inhibition	Tregs, DC, CD8⁺T	GPCR signaling, Intracellular HDAC inhibition, Epigenetic regulation	↑Tregs, ↑IL-10, Context-dependent ↑ CD8⁺ memory formation and metabolic function, mainly shown in murine CRC models and infection models	Help restore barrier function. Reduce chronic inflammation. Have potential ICI-sensitizing effects. Animal models suggest it can enhance MSS CRC response to ICIs in a dose-dependent manner, while different doses may lead to excessive suppression or “cold” tumor effects	In vivo (mouse)	[15–17]
SCFAs (Propionate)	GPR41, GPR43	Tregs, Th17	GPCR signaling, relatively weaker HDAC inhibition compared with butyrate	Inhibit Th17, Promote Tregs, Maintain Th17/Treg balance	Improve immune-inflammatory status. May enhance ICI response rate	In vivo (mouse)	[26]
3-oxoLCA (Secondary BA)	Directly inhibit transcription factor RORγt	Th17	Block/Inhibit RORγt transcriptional activity	↓Th17 differentiation, ↓IL-17	Suppress inflammation-related pro-tumor pathways and oncogenic signals, but may also lead to an “anti-inflammatory” or “cold” immune tendency (immunosuppressive bias)	In vivo (mouse)	[18]
Iso-DCA (Isodeoxycholic Acid)	May act through FXR/TGR5 pathways. Influence DC phenotype and metabolic processes	DC, Tregs, Macrophages (TAM)	FXR → nuclear receptor–mediated transcriptional regulation, TGR5 → cAMP → STAT3/STAT6	Promote DC tolerogenic phenotype, ↑Tregs, Promote M2 polarization (immunosuppressive)	Animal models suggest immunosuppression and ICI resistance, associated with low ICI response/cold tumors, potentially promoting cold tumor formation; clinical validation is still needed	In vivo (mouse)	[38]
Indole Derivatives (IAA, IPA)	AhR	DC, Th17/Treg, epithelial cells	AhR → IL-22(and downstream STAT3) pathway	↑IL-22, ↑barrier repair, ↑antimicrobial peptide expression, in tumor-associated immune cells, AhR effects can be dual, with both barrier-protective and immunosuppressive outcomes depending on the context	Enhance mucosal defense, Reduce chronic inflammation, Support ICIs. AhR effects in the TIME are complex and require context-specific analysis	In vivo (mouse)	[20]
Compound K (Ginsenoside Metabolite)	Affect the Tryptophan → AhR pathway after microbial metabolism	Epithelial cells, Immune cells (DCs, T cells)	Activate AhR,Modulate the tryptophan metabolic pathway	↑IL-22, Repair barrier,Enhance antigen presentation	Can indirectly enhance ICI efficacy by improving barrier and immune function	In vivo (mouse)	[40]
DCA/LCA	FXR,TGR5	Macrophages, DC, Epithelial Cells	Nuclear receptor–mediated transcriptional regulation, Affecting NF-κB	In different contexts, can be pro-inflammatory (carcinogenic/damaging) or promote tolerance (immunosuppressive)	Involved in tumorigenesis and TIME modulation, with bidirectional effects on ICI response	In vivo (mouse), In vitro experiment	[19]

Explanation: The immune effects listed in the table represent only the experimental results described in the referenced literature. Depending on the species, dosage, and conditions of the tumor microenvironment, actual effects may differ. Unless otherwise specified, “clinical relevance” refers solely to potential associations with immune checkpoint inhibitor (ICI) responses or animal model data and does not imply a direct causal relationship.

Phytochemicals mediate the “microbiota–metabolite–receptor” axis to regulate the immune microenvironment in colorectal cancer

Types of phytochemicals

Polysaccharide

Polysaccharides can be categorized into soluble polysaccharides from plants [41] and natural polysaccharides derived from traditional Chinese medicine [42]. Representative components mainly include β-glucans and inulin. Most polysaccharides are metabolized by the gut microbiota in the colon to produce short-chain fatty acids (SCFAs), particularly butyrate, which can activate receptors such as GPR41/43, and act through GPR109A or HDAC inhibition to promote the generation of regulatory T cells (Tregs) [43]. In the end, this mechanism modulates the TIME by preserving the Th17/Treg balance [27].

Saponin

Saponins are renowned for their complex glycoside structures and are widely present in traditional Chinese medicines such as ginseng [44]. Among these, ginsenosides undergo deglycosylation by gut microbiota to generate secondary metabolites (compound K) with enhanced biological activity. These metabolites regulate the tryptophan-indole pathway to activate AhR signalling [40], promote IL-22 secretion, and strengthen the intestinal epithelial barrier, thereby maintaining relative TIME equilibrium.

Polyphenols

Flavonoids, phenolic acids, and monomeric polyphenols are examples of polyphenols. Most are broken down by gut microbes into small-molecule phenols or organic acids after reaching the colon. Among these, resveratrol increases the abundance of Akkermansia muciniphila, maintains intestinal barrier integrity [45], and promotes short-chain fatty acid (SCFA) production [46]. Epigallocatechin-3-gallate (EGCG) can enhance the growth of butyrate-producing bacteria and indirectly modulate Treg and CD8⁺ T cell activity via the SCFA–GPR43/HDAC pathway [15, 47], thereby contributing to its antitumor properties.

Alkaloid

Alkaloids are a class of nitrogen-containing natural compounds, among which berberine has been the most extensively studied. Berberine exhibits low oral bioavailability, with most of it interacting with gut microbiota within the intestinal tract. It significantly alters bile acid pool composition, such as reducing carcinogenic secondary bile acids (DCA), and inhibits M1 macrophage expansion by regulating FXR and TGR5 signaling [48, 49]. Additionally, it improves the microbiome dysbiosis linked to colorectal cancer by enriching beneficial bacteria including Bacteroides and Akkermansia [49].

Mechanisms by which different phytochemicals drive the microbiota–metabolite–receptor regulatory pathways

Phytochemicals exhibit remarkable diversity, and this variability results in distinct metabolic and immunomodulatory effects within the host gut [50, 51]. Furthermore, the majority of phytochemicals first undergo metabolic conversion by the gut bacteria in order for the body to use them [52]. Microbial action generates more bioactive secondary metabolites, which then bind to specific receptors on host cell membranes or within cells, activating downstream signaling pathways to ultimately regulate intestinal immune homeostasis [33] and reshape the colorectal cancer TIME.

Mechanisms by which polysaccharides modulate the colorectal tumor immune microenvironment

Polysaccharides primarily exert their regulatory effects on the “microbiota-metabolite-immune” axis through their prebiotic properties. Polysaccharides from diverse sources exhibit highly consistent mechanisms: metabolized by gut microbiota, they universally promote the proliferation of probiotic groups such as Bifidobacterium, Lactobacillus, and butyrate-producing bacteria, significantly increasing short-chain fatty acid (SCFA) production (particularly butyrate) [53].

Subsequently, SCFAs activate G protein-coupled receptors (GPR41 and GPR43) on the surface of intestinal immune cells, initiating signaling pathways that induce the expression of immunoregulatory genes. This upregulates anti-inflammatory factors (IL-10) [54] while suppressing pro-inflammatory factors (IL-6, TNF-α) [55]. Under inflammatory conditions, SCFAs simultaneously promote Th1/Th17 and IL-10⁺ T cell production while exerting dual regulatory effects on CD8⁺ T cell metabolism and memory formation.

Simultaneously, SCFAs can reach the cell nucleus and increase the acetylation of associated genomic proteins by inhibiting the activity of histone deacetylase (HDAC) [56]. Through both GPCR activation and HDAC inhibition, SCFAs exert anti-inflammatory effects, suppress oxidative stress and NF-Reduced inflammation aids in maintaining and repairing intestinal barrier function by enhancing tight junction protein expression, decreasing intestinal permeability, and preventing harmful substances and pathogens from penetrating the mucosa. This ultimately elevates intestinal barrier homeostasis improving the TIME in colorectal cancer and improves the TIME in CRC.

Collectively, the core mechanism of polysaccharides can be summarized as follows: “prebiotic effect → increased SCFAs → GPCR/HDAC pathways → Treg/CD8⁺ balance → TIME remodeling.”

Mechanisms by which saponins and triterpenoids drive regulatory pathways

Saponins and triterpenes enter the intestine and are metabolized by the gut microbiota, producing compound K or secondary bile acids through modulation of microbial populations such as Bacteroides and Clostridia and their 7α-dehydroxylation activity [57]. These metabolites influence short-chain fatty acids (SCFAs) and act on key host epithelial and immune cell receptors, including FXR, TGR5/GPBAR1, GPR41/43, and AhR, thereby modulating the FXR–FGF15/19 and TGR5–GLP-1 signaling axes [58–60].

The effects following activation of different receptors also vary. Activation of intestinal FXR alters the internal microbiota of the gut and influences systemic metabolism and immune plasticity via the TGR5/GLP-1 axis [61]. TGR5, activated by secondary bile acids and plant triterpenes, regulates cAMP-PKA/NF-κB pathways on macrophages and dendritic cells, thereby suppressing pro-inflammatory responses and NLRP3 inflammasome activation [62].

Regarding immune mechanisms, saponins (soybean-derived saponins) exert anti-inflammatory effects by downregulating TLR4-NF-κB and oxidative stress, inhibiting TLR4-mediated NF-κB and MAPK signaling, and significantly suppressing pro-inflammatory factor production (IL-1β, TNF-α), thereby preventing presentation defects caused by excessive inflammatory responses [63]. Furthermore, metabolites and FXR/TGR5 signaling jointly influence Treg cell homeostasis, Th1/Th17 polarization [18], and CD8⁺ T cell metabolism [64], creating a favorable environment for CD8⁺ T cell infiltration into tumors and enhancing their tumor infiltration and killing capacity.

Therefore, the core mechanism of saponins/triterpenes can be summarized as follows: “microbiota-mediated metabolite generation → receptor activation (AhR/FXR) → enhanced DC/Th1/CD8⁺ T cell function → augmented antitumor immunity.”

Mechanisms by which polyphenols drive regulatory pathways

Most polyphenolic compounds reshape the gut microbiota structure by promoting the growth of probiotic flora and suppressing pathogenic bacteria. Such alterations in microbial composition directly lead to changes in the metabolite profile, typically resulting in increased levels of short-chain fatty acids (butyrate, propionate) [65] and indole derivatives (indole-3-acetic acid, IAA) [66].

These metabolites (indole-3-acetic acid derivatives, phenolic acids) activate the AhR pathway, inducing IL-22 expression and promoting IL-22 production [67]. Intestinal inflammation is reduced by IL-22, which also improves tight junction protein and antimicrobial peptide expression and restores the epithelial barrier [68].

Short-chain fatty acids activate GPCRs through pathways such as GPR43/GPR109A, regulating Treg differentiation and upregulating IL-10. This suppresses excessive inflammatory responses, restores relative balance in the TIME, and supports CD8⁺ T-cell function and ICI efficacy [54, 69, 70].

Collectively, the core mechanism of polyphenols can be summarized as follows: “increased beneficial microbiota → elevated SCFAs/indoles → GPR/AhR activation → enhanced antitumor immunity.”

Mechanisms by which alkaloids drive regulatory pathways

Alkaloids mainly include quinoline and indole derivatives, with berberine exhibiting a unique regulatory pattern within the “microbiota-metabolite-immune” axis. Berberine primarily modulates bile acid metabolism via the gut microbiota, increasing the proportion of secondary bile acids [71].

Specifically, berberine promotes the expansion of beneficial bacteria such as Bacteroides and Akkermansia while reducing the production of pathogenic bacteria like Escherichia coli, thereby decreasing the generation of pro-carcinogenic secondary bile acids [72, 73]. Subsequently, secondary bile acids act on the FXR/TGR5 pathway, guiding macrophage polarization toward the M2 phenotype while inhibiting M1 polarization [74]. Concurrently, there is an increase in CD8⁺ T cell activity, which facilitates TIME’s shift to anti-tumor immunity [75].

Therefore, the mechanism of berberine can be summarized as follows: “microbiota remodeling → bile acid pool modulation → FXR/TGR5 activation → inflammation alleviation, Th1/CD8⁺ immunity enhancement → suppression of CRC progression.” The following table provides a detailed description of the mechanism. (As shown in Table 2)(As shown in Fig. 2).

Table 2.

Summary of mechanisms by which phytochemicals regulate colorectal cancer through the “microbiota–metabolite–receptor–immune” axis

Compound category	Representative compounds	Key microbiota modulation	Major metabolites	Target receptors	Immune effects	Impact on TIME and ICIs	Study model / Evidence level	References
Polysaccharides	β-Glucan	↑Lactobacillus, ↑Butyrate-producing beneficial bacteria	SCFAs (Butyrate)	GPR43/41GPR109A, Inhibit HDAC	Reported to enhance CD8⁺ T cell metabolic function and memory formation in preclinical models at specific doses, Improve antigen-presenting capacity	May alleviate TIME inflammation and potentially enhance PD-1 therapy efficacy	In vivo (mouse)	[27]
	inulin	↑ Bifidobacterium, ↑ Faecalibacterium	SCFAs (Acetate)	GPR43	Maintain Tregs, Homeostasis with mild anti-inflammatory effects	Protect barrier function, Improve TIME	In vitro experiment	[76]
			SCFAs (Propionate)	GPR41	Inhibit Th17, ↑Tregs	Anti-inflammatory, Promote metabolic balance, May improve TIME	In vivo (mouse)	[77, 78]
			SCFAs (Butyrate)	GPR109A, Inhibit HDAC	↑Tregs, ↑IL-10, Enhance barrier repair capacity	Immune tolerance, Anti-inflammatory function, Restore barrier,Increase sensitivity of MSS CRC to ICIs	In vivo (mouse)	[79]
Saponins/Triterpenes	Compound K	Alter the Bacteroides/Clostridium ratio, Modulate the gut microbiota	Secondary bile acids, Saponin metabolites	Indirect modulation of FXR/TGR5 and AhR signaling via microbiota and bile acid balance	Modulate DC phenotype,Influence Treg balance	Improve TIME and enhance the efficacy of PD-1/PD-L1 therapy. However, BA balance requires caution due to potential risks of immune suppression and metabolic toxicity	In vivo (mouse), In vitro experiment	[80]
	Astragaloside IV	↑Probiotics,Modulate bile acid metabolism	Secondary bile acids	FXR, GR5(indirect modulation)	Modulate DC antigen-presenting function	Anti-inflammatory, Have the potential to improve TIME	In vivo (mouse)	[21]
Polyphenols	Resveratrol	↑Akkermansia, ↑SCFA	Phenolic metabolites, May indirectly influence microbial indole production	AhR,GPCR(indirect)	↑IL-22	May improve PD-1 response and support synergy with ICIs in preclinical models, Improve resistance, Support synergy with ICIs or reversal of resistance	In vivo (mouse)	[43, 44]
	EGCG	↓Pathogenic bacteria	Microbial phenolic metabolites, May indirectly affect tryptophan/indole metabolism	AhR	↑IL-22	Anti-inflammatory function, Modulate TIME potentially assist ICIs	In vivo (mouse)	[45]
Alkaloids	Berberine	↑Bacteroides, ↑Akkermansia, Remodel the bile acid pool	Secondary bile acids	FXR,TGR5 (Affect BA signaling)	Modulate macrophage polarization and suppress pro-inflammatory pathways, Reported to enhance CD8⁺ T activity in some contexts	Alleviate immune evasion, Improve the TIME of CRC, Mitigate immune tolerance, and hold potential to reverse ICI resistance	In vivo (mouse)	[74]
Dietary-Nutrition/Feeding Patterns (As Intervention)	High-Fiber Diet, Fermented Foods, Plant-based formulations	Enhance microbial diversity, ↑SCFA production	Mixed metabolites (SCFAs, Indole derivatives)	Concurrent activation of multiple receptors (GPRs, AhR, FXR)	Collectively enhance mucosal and systemic immune homeostasis	Help convert “Cold Tumors” to a more immune-infiltrated state, High-Fiber/Prebiotics may increase ICI response rates	In vivo (mouse)	[17]

Explanation: “Key microbiome modulation” primarily originates from mouse models, while “major metabolites” and “receptors” are largely inferred mechanisms or laboratory test results; “effects on TIME and immune checkpoint s” are based on existing in vivo and in vitro evidence, with some conclusions still requiring clinical validation.

Fig. 2.

Phytochemicals Modulate TIME via Microbiota–Metabolite–Receptor Axis. This diagram illustrates the mechanism by which phytochemicals regulate the tumor immune microenvironment through the “gut microbiota-metabolite-receptor” axis. Phytochemicals such as polysaccharides, saponins/triterpenes, polyphenols, and alkaloids, after being acted upon by gut microbiota, can exert prebiotic effects, metabolize into short-chain fatty acids (SCFAs) or complex metabolites (compound K), and enrich specific bacterial populations (Akkermansia muciniphila) and indole metabolites. These metabolites regulate the functions of regulatory T cells (Tregs), CD8⁺ T cells, Th1/Th17 cells, and dendritic cells (DCs) by acting on receptors and signaling pathways including GPR, HDAC, FXR, TGR5/GLP-1, TLR4–NF-κB, and AhR. This balances immunological responses and encourages the polarization of immune effector cells. Simultaneously, phytochemicals can reshape the bile acid pool, repair intestinal barrier function, and synergistically promote the remodeling of the TIME, thereby enhancing antitumor immune responses

CRC-specific microbiota and the prospects of intervention with phytochemicals

Gut microbiota dysbiosis is a key factor in the initiation and progression of CRC. Research demonstrates that specific microbial populations form the TIME of CRC through their metabolites or direct interactions with host immune cells [81]. These specific microbial populations exert bidirectional effects: they may promote carcinogenesis and immune evasion, while also potentially enhancing the efficacy of immunotherapy [82]. In order to provide a theoretical foundation for phytochemical therapies, we examined representative microbiota to characterize their immune regulation processes.

Fusobacterium nucleatum,F.nucleatum

F. nucleatum is a “signature bacterium” of CRC. It typically binds to E-cadherin within the host and activates β-catenin signaling, accelerating tumor proliferation [83]. Additionally, it secretes the Fap2 protein, which binds to the inhibitory receptor TIGIT on NK cells and T cells, suppressing NK cell killing activity while also inhibiting the effector function of CD8⁺ T cells, thereby establishing an immune escape mechanism [84].

Subsequently, F. nucleatum selectively recruits myeloid-derived suppressor cells (MDSCs) into the tumor microenvironment, where they suppress T cell proliferation and promote tumor progression [85]. Concurrently, MDSCs suppress CD8⁺ T cell function and upregulate Arg-1, enhancing tumor cell immune escape [86]. This disrupts the relative equilibrium of the tumor immune microenvironment, leading to tumor progression, metastasis, and immune tolerance.

Numerous phytochemicals can alter the gut microbiome and enhance its internal structure, according to recent studies. Polyphenolic substances such as resveratrol and tea polyphenols inhibit the adhesion and biofilm formation of F. nucleatum by improving the internal structure of the gut microbiota [87, 88], thereby reducing tissue colonization in CRC.

Enterotoxigenic bacteroides fragilis, ETBF

Enterotoxigenic Bacteroides fragilis (ETBF) secretes Bacteroides fragilis toxin (BFT), which disrupts tight junctions by cleaving E-cadherin in epithelial cells. This compromises the epithelial barrier, allowing pathogens and metabolites to migrate into deeper tissues, thereby inducing local inflammation [89].

Concurrently, BFT activates the STAT3 signaling pathway, promoting the production of pro-inflammatory cytokines such as IL-6 and IL-23 by dendritic cells and intestinal epithelium. This further induces Th17 cell polarization and the expression of IL-17 and IL-22. Such abnormal overactivation not only exacerbates chronic inflammation but also suppresses anti-tumor T cell activity [90], accelerating the progression of CRC and disrupting the TIME balance.

Akkermansia muciniphila, A. muciniphila

The most well-known and thoroughly researched species in the genus Akkermansia is Akkermansia muciniphila [91]. Clinical studies have shown that it significantly enhances the efficacy of anti-PD-1 monoclonal antibody therapy and is markedly enriched in patients who benefit from ICIs [92, 93]. Its metabolites, particularly SCFAs, can promote the effector function of tumor-killing CD8⁺ T cells, enhancing their antitumor activity [94].

Additionally, A. muciniphila induces antigen-presenting cells to secrete IL-12, promoting Th1-type immune response polarization and increasing IFN-γ production [95], which further enhances T cells’ ability to recognize and eliminate tumor cells.

Notably, within a specific range, natural pharmaceutical substances can also increase A. muciniphila levels [96]. For instance, polysaccharides and saponins promote the upregulation of A. muciniphila abundance [97, 98] by improving the intestinal metabolic environment, providing favorable growth conditions, and establishing an environment conducive to immune activation. This indirectly enhances the efficacy of ICIs [93], improving their antitumor effects and treatment response rates.

In summary, CRC-specific microbial populations exhibit bidirectional effects. F. nucleatum and ETBF may promote tumor initiation, progression, and immune evasion, whereas A. muciniphila contributes to immune modulation and is recognized as a key responder species for ICIs.

Phytochemicals can change the microbiome specific to colorectal cancer (CRC), according to both basic and clinical research. By altering gut microbial composition, improving metabolic products, and activating corresponding receptors, they exert immunomodulatory effects [99]. Alternatively, they may achieve TIME regulation by suppressing or transforming harmful microbiota while synergizing with beneficial bacteria [100]. Consequently, phytochemicals offer novel approaches for CRC immunotherapy, particularly for MSS patients, potentially serving as key strategies for enhancing ICI sensitivity. (As shown in Fig. 3).

Fig. 3.

CRC-Specific Microbiota. The figure illustrates three typical interactions between bacterial communities and host immunity: Enterotoxin-producing Bacteroides fragilis (ETBF) disrupts the epithelial barrier and activates the E-cadherin/STAT3 pathway, inducing dendritic cells (DCs) to secrete IL-6 and IL-23. This further promotes Th17 cell differentiation, leading to IL-17 and IL-22 secretion that drives inflammation and tumor progression. Akkermansia muciniphila modulates antigen-presenting cells (APCs) and CD8⁺ T cells by producing short-chain fatty acids (SCFAs), promoting IL-12 secretion and Th1 differentiation, and enhancing IFN-γ production. This boosts antitumor immunity and enhances the immune checkpoint inhibitor (ICI) response

Synergistic effects of immune checkpoint inhibitors (ICIs) and combination therapy strategies

Current treatment landscape and limitations of ICIs

In the early stages of cancer, when tumor cells are confined to the primary site, surgery ± neoadjuvant/adjuvant chemotherapy (FOLFOX/CAPOX) constitutes the primary treatment regimen [101]. For advanced-stage tumors with metastasis, treatment strategies require case-by-case consideration. Patients with MSI-H/dMMR tumors demonstrate significant response to PD-1 monotherapy or dual-immunotherapy regimens, making these approaches viable treatment options [102]. In contrast, the majority of MSS or pMMR patients exhibit lower sensitivity to single-agent ICIs, presenting as “cold tumors” with limited immunotherapy efficacy. These cases require immune sensitization strategies [103–105] that remodel the TIME to enhance T-cell infiltration and overcome immune tolerance barriers in MSS tumors.

By influencing the composition of the gut microbiota and its metabolites, phytochemicals can activate downstream immune pathways by acting on receptors. As pretreatment for ICI therapy, this approach holds promise for enhancing immunotherapy responsiveness and expanding the patient population benefiting from ICI treatments.

Feasibility of the“compound → metabolites → receptors → immunity → → microbiota → consequence”

Feasibility of phytochemicals targeting gut microbiota: pharmacological strategies

Most phytochemicals are administered orally, enabling high exposure in the intestinal lumen and facilitating interaction with the gut microbiota. However, in clinical applications, the low bioavailability and significant interindividual metabolic variability of these compounds are key factors limiting their ability to exert health benefits through gut microbiota modulation [106, 107]. To address this challenge, pharmaceutical strategies such as microencapsulation and nanoparticle delivery enable selective, targeted release of active compounds within specific intestinal compartments. This approach increases effective exposure and prolongs interaction time with specific microbial populations, thereby enhancing bioavailability [108, 109]. Consequently, through appropriate medical techniques, it can influence metabolic substrates and amplify immune sensitization effects through the “microbiota-metabolite-receptor” axis.

Feasibility from gut microbiota to metabolites: multi-omics analysis

Unraveling the mechanisms by which gut microbiota exert effects through metabolites requires multi-omics technologies to serve as a bridge, systematically demonstrating causal links from microbial genes to host phenotypes. The integration of metagenomics and metabolomics [110] provides tools for the “microbiome-to-metabolite” translation pathway. By deciphering the internal composition and functional gene pool of individual microbiomes [111], it enables predictions of their potential activity in pathways such as bile acid metabolism. Fecal metabolomics can directly capture metabolites within the intestinal lumen, including short-chain fatty acids and indole derivatives, providing bidirectional validation between “gene function-metabolic phenotype” and offering direct evidence of microbial functional activity [112]. Plasma metabolomics reflects the types of gut-derived metabolites crossing the barrier into circulation, serving as a bridge between gut microbiota function and host immunity.

Multi-omics integration can validate the progression of drug-microbe interactions, providing real-time visualizable pharmacodynamic fingerprints for phytochemicals [113]. This makes it possible to measure the distribution and level changes of downstream metabolites, trace the transformation pathways of certain bioactive components, and eventually establish connections with ICI efficacy.

Feasibility from metabolites to receptor activation: detectability at the receptor level

The feasibility of the “metabolite-receptor” approach lies in the detectability of receptor activation and downstream effects. On one hand, multiple key receptors (FXR, TGR5) have been demonstrated to exhibit stable expression capabilities [114], and their activation can be monitored through transcriptional changes in target genes—for instance, AhR activation leads to IL-22 upregulation [115]. However, epigenetic markers can validate the effects of certain metabolites. For instance, SCFAs’ inhibition of HDAC can be corroborated by detecting decreased HDAC activity in tissues or cell nuclei [116] and elevated histone acetylation levels [117].

Clinical applications of receptor-level signaling are also possible. Peripheral blood RNA-seq can reveal dynamic expression changes of receptor target genes in patient immune cells [118], while immunohistochemistry and in situ hybridization on tumor or intestinal tissue sections can visualize the spatial distribution of receptor activation [119]. Existing studies have already observed differences in certain metabolite-receptor-gene expression patterns during ICI therapy, suggesting that this pathway not only provides crucial support for mechanism validation but also holds potential for clinical translation.

Feasibility from receptors to the immune system: dynamic changes in immune cells and cytokine levels

Metabolites activate receptors and translate into immune effects, which can be validated through dynamic changes in immune cell and cytokine levels. In tumor tissues and intestinal mucosal samples, flow cytometry enables quantitative analysis of the proportions and functional states of CD8⁺ T cells, Tregs, Th17 cells, M1/M2 macrophages, and others [120, 121], thereby assessing the capacity of receptor signaling to shape the TIME. In addition, peripheral blood immune profiling, as a minimally invasive sampling approach, can reflect systemic immune changes [122], providing a feasible method for monitoring ICI responses or immune-related adverse events in clinical studies.

These analytical approaches enable direct assessment of whether metabolite-receptor pathways successfully translate into specific immunological effects, while also providing a feasible pathway for incorporating the immunomodulatory effects of phytochemicals into clinical monitoring.

Feasibility from the immune system to TIME: cold tumor reversion and therapeutic synergy

The ultimate effect of the “metabolite-receptor-immune” pathway manifests in the remodeling of TIME, with its feasibility primarily stemming from the detectable process of transforming “cold tumors” into “hot tumors.” Extensive research demonstrates that phytochemicals significantly increase the density of tumor-infiltrating lymphocytes (TILs) by modulating the gut microbiota and its metabolites. This regulation upregulates effector signals such as IFN-γwhile reducing the proportion of immunosuppressive factors [123, 124]. This process helps improve the inflammatory state of TIME and creates conditions conducive to the action of ICIs.

In immunotherapy synergism, phytochemicals like polysaccharides, resveratrol, or berberine, when combined with anti-PD-1 therapy, enhance antigen presentation and promote CD8⁺ T cell infiltration into tumors [125], significantly improving response rates. Furthermore, some clinical data suggest the microbiota-metabolite axis may serve as a key mediator. For instance, high-fiber dietary patterns show a clear correlation with improved ICI response rates [126]. Thus, at the current stage of moving from mechanism validation to clinical exploration, the “plant factors + ICIs” combination demonstrates evidence-based synergistic effects in reversing TIME and enhancing immunotherapy, offering distinct clinical translation advantages.

In summary, integrating phytochemicals into the immunotherapy framework for CRC is not only theoretically feasible but has demonstrated preliminary efficacy across multiple models. Through larger-scale clinical trials combined with dynamic monitoring of the microbiota-metabolite-receptor signaling axis, this strategy holds promise for translation into a practical immune-sensitizing regimen, particularly for the MSS CRC patient population with limited current immunotherapy response rates. (As shown in Fig. 4).

Fig. 4.

Conceptual framework Therapeutic Concept in MSS CRC. This figure illustrates the potential mechanism of action for phytochemicals in MSS CRC. After being metabolized by gut microbiota, these compounds promote the production of multiple metabolites. These metabolites then activate receptor signaling pathways to upregulate CD8 + T cells and Th1 cells while suppressing Tregs, Th17 cells, and M2 tumor-associated macrophages, thereby improving the immunosuppressive microenvironment. This process not only enhances the efficacy of immune checkpoint inhibitors (ICIs) but also synergizes with treatments like chemotherapy and radiotherapy. It facilitates the transformation of “cold tumors” into “hot tumors,” ultimately achieving effective tumor suppression

Rather than proposing a fully validated causal loop, we aim to establish a stepwise and testable framework linking phytochemicals, gut microbiota, microbial metabolites, receptor signaling, and antitumor immunity. In this framework, each layer represents an independently verifiable biological process that may collectively contribute to the remodeling of the tumor immune microenvironment and influence ICI responsiveness in CRC.

Integration with existing therapeutic strategies

Parallel use with ICI monotherapy – focused on MSS CRC

In the current therapeutic landscape of CRC, the majority of patients are MSS and pMMR [127], and generally exhibit minimal response to ICI monotherapy [128]. Recent studies suggest that phytochemicals modulating the gut microbiota-metabolite-receptor axis may exert a synergistic sensitizing effect when used in parallel with ICI monotherapy [124, 129].

In mouse CRC models, pretreatment with plant factors for 2–4 weeks increased short-chain fatty acids and indole derivatives, thereby improving Treg/Th17 balance and promoting CD8⁺ T cell infiltration. This process enhances the initial response rate and sustained efficacy of ICIs in MSS CRC, positioning it as a potential first- or second-line adjuvant strategy for metastatic MSS CRC and offering novel adjuvant concepts for immune sensitization in locally advanced CRC.

Synergistic effects with chemotherapy and anti-angiogenic therapy

Chemotherapy and anti-angiogenic therapy are both critical treatment options for advanced colorectal cancer (CRC), but they commonly face dual constraints of acquired resistance and immunosuppressive microenvironments [130]. Against this backdrop, the intervention of phytochemicals may offer novel sensitization strategies for “chemotherapy/anti-angiogenesis + immune modulation.”

Regarding chemotherapy, polysaccharides and polyphenolic compounds can mitigate chemotherapy-related toxic side effects and enhance antigen recognition efficiency following chemotherapy-induced immunogenic cell death (ICD) [131, 132]. Furthermore, compounds like berberine and saponins can suppress the immunosuppressive effects of MDSCs and Tregs while promoting DC maturation and antigen presentation, thereby amplifying chemotherapy-triggered antitumor immune responses [133, 134].

Regarding anti-angiogenic therapy, natural plant-derived factors such as resveratrol and quercetin have been shown to maintain gut microbiota homeostasis and modulate local angiogenic signalling [135, 136], potentially facilitating the remodeling of tumor vasculature and creating more favorable conditions for immune cell infiltration.

Collectively, this multifaceted integrated strategy alleviates treatment-related toxicity while reshaping the TIME, positioning it as a promising complementary approach in the therapeutic pathway for MSS CRC patients.

Combination with radiation therapy and chemoradiation for immune enhancement

Radiation therapy (RT) and chemoradiation therapy (CRT) are applicable for locally advanced and metastatic CRC, exhibiting cytotoxic killing effects by inducing immune-mediated cell death (ICD) to activate innate immune responses. However, in clinical practice, radiotherapy is often accompanied by issues such as intestinal mucosal damage and dysbiosis, Furthermore, the TIME is readily reoccupied by immunosuppressive cells like myeloid-derived suppressor cells (MDSCs) and regulatory Tregs within a short timeframe, leading to limited therapeutic efficacy.

The intervention of plant-derived bioactive compounds in this pathway offers multiple potential advantages. First, polysaccharides and polyphenols can modulate gut microbiota composition, alleviating chemotherapy-induced intestinal inflammation and dysbiosis to maintain local TIME stability. Moreover, radiotherapy itself increases intestinal epithelial permeability, enhancing the local bioavailability of plant bioactives and their metabolites [137]. Moreover, phytochemicals can suppress immunosuppressive cells and promote antigen presentation, thereby creating a favorable immune environment for subsequent ICI therapy.

This “three-stage synergistic” combination strategy (RT/CRT releasing antigens → plant factors reshaping TIME → ICI treatment) has been preliminarily validated in some clinical explorations of neoadjuvant CRT combined with ICIs [138]. Therefore, the combination of phytochemicals with radiotherapy and chemoradiotherapy not only reduces toxicity and enhances sensitivity but also provides a favorable environment for subsequent ICI intervention, potentially becoming a direction for further in-depth research and clinical translation.

Clinical translation prospects and existing challenges

Clinical translation prospects

Strategies to sensitize immune checkpoint inhibitors

By altering the gut microbiota-immune axis via a variety of mechanisms, phytochemicals can improve the effectiveness of immune checkpoint inhibitors (ICIs). For instance, they increase the abundance of A. muciniphila [139] while suppressing the proliferation of F. nucleatum [140], thereby improving TIME and “warming” cold tumors. This sensitization effect has been validated in mouse CRC models, where phytochemicals demonstrated significantly enhanced tumor suppression when co-administered with anti-PD-1/PD-L1 therapy. Concurrently, high-fiber dietary patterns correlate positively with improved ICI response rates [141]. These findings provide both theoretical and experimental support for immunotherapy in MSS CRC.

Diet and nutrition-guided precision immunomodulation

Most phytochemicals (polyphenols, polysaccharides, saponins) can be obtained through daily dietary sources, offering natural accessibility. This characteristic enables them to serve not only as individual subjects for experimental research but also holds potential for development into functional dietary interventions or plant-based compound nutritional formulations, potentially representing a new direction for precision immune management in CRC patients. During long-term recovery phases, appropriate plant-based nutritional formulations aid in establishing and sustaining anti-tumor immune memory, thereby reducing recurrence risks [142]. Future integration of metagenomics and metabolomics approaches to achieve precise “nutrition-microbiome-immunity” matching may advance dietary interventions toward truly personalized immune nutritional management and preventive therapeutic strategies.

Biomarkers and stratified management

The multi-omics signature of “microbiota-metabolites-receptors-immune fingerprint” suggests the potential for progressively developing companion diagnostic tools to predict responses to immune checkpoint inhibitors (ICIs). Specifically, certain microbiota and metabolites have demonstrated potential biomarker value [143]. Potential biomarkers may include fecal SCFA levels, bile acid signatures, indole metabolites, CD8⁺ T-cell infiltration, Treg/Th17 balance, and receptor-associated transcriptional signatures. For instance, SCFAs have been demonstrated to enhance CD8⁺ T-cell effects or improve TIME by maintaining Treg/Th17 balance [16, 144], indicating that SCFAs possess not only immunoregulatory functions but also predictive value. Integrating these biomarkers into patient stratification may identify individuals more suitable for combined “phytochemicals + ICIs” therapy, thereby enhancing clinical trial efficiency and patient survival rates.

Despite encouraging mechanistic and preclinical findings, current evidence supporting phytochemical-mediated sensitization of ICIs in CRC remains largely derived from animal models and associative human studies. Direct interventional clinical evidence remains limited, particularly in MSS CRC populations.

Current key challenges

Heterogeneity analysis

The microbiota–metabolite–receptor axis is highly context-dependent and exhibits substantial interpatient heterogeneity in colorectal cancer (CRC). Increasing evidence suggests that right-sided and left-sided CRC differ significantly in microbial composition, metabolic signatures, immune infiltration patterns, and therapeutic responsiveness [145]. Right-sided tumors are more frequently associated with MSI-H status, increased immune-cell infiltration, and enrichment of specific bacterial taxa such as Fusobacterium and Ruminococcus, whereas left-sided tumors often display stronger metabolic dysregulation and lower baseline immunogenicity [146, 147].

In addition, host-related factors also play a significant role in this variability. Gender, age, obesity, and dietary patterns can all alter microbial diversity and metabolic capacity, thereby influencing the biotransformation of phytochemicals and downstream immune signalling [148]. In summary, heterogeneity itself may be one of the underlying causes of the inconsistencies among different study results.

Diversity of components and batch-to-batch variability

The primary challenge facing plant-derived active ingredients in clinical translation is the complexity of their composition and batch-to-batch variability [149]. The chemical structures of plant active ingredients are intricate, and their sources are influenced by multiple factors including the origin of medicinal materials, harvest season, processing methods, and extraction techniques. There is a lack of unified quality standards and standardized systems [150]. Furthermore, varying dosages and formulations may induce opposing shifts in microbial communities and metabolite profiles, introducing unpredictability and inherent randomness.

Individual variability of gut microbiota

The metabolic effects of phytochemicals are highly dependent on the composition and function of the host’s gut microbiota, which exhibits significant individual variation across populations. This diversity manifests not only in microbial types but also in the diversity of functional gene pools, directly influencing the metabolic pathways and final products of plant compounds [151]. Consequently, when certain microbial populations are absent or functionally imbalanced, phytochemicals may fail to deliver expected therapeutic effects or generate unintended metabolites.

Integration with existing clinical treatment regimens

During CRC treatment, patients typically undergo multimodal therapeutic regimens including chemotherapy, radiotherapy, targeted therapy, and immunotherapy. These interventions significantly alter the composition and metabolic environment of the gut microbiota, thereby influencing the mode of action of phytochemicals [152, 153]. Due to the complex interactions between phytochemicals and existing therapeutic regimens, clarifying their synergistic or antagonistic effects and determining the true benefit of these compounds within the context of multi-drug and complex treatment protocols remains a major challenge for clinical translation.

Safety and long-term monitoring

Although most plant-derived ingredients are administered orally and exhibit overall high safety profiles, long-term use or high-dose interventions still carry potential risks that warrant attention during clinical translation. For instance, berberine can reshape the bile acid pool by modulating bile acid metabolism and gut microbiota composition, thereby affecting receptor signaling pathways such as FXR and TGR5, potentially leading to lipid metabolism disorders or even hepatobiliary toxicity [154].

Furthermore, clinical patients often require long-term or cyclical treatment. Currently, there remains a lack of systematic quantification and assessment of the cumulative effects of chronic use of plant-derived active ingredients. Therefore, a comprehensive long-term safety monitoring system should be established, including but not limited to assessments of liver and kidney function, plasma metabolite profiling, gut microbiota homeostasis monitoring, and dynamic evaluation of potential drug interactions.

Conclusion

The development and progression of colorectal cancer (CRC) are closely associated with the gut microbiota and its metabolites, while the imbalance of the immune microenvironment plays a crucial role in its progression and resistance to immunotherapy. Recent studies have revealed that phytochemicals, as natural and modifiable exogenous intervention factors, can effectively remodel the immune microenvironment of CRC through the “microbiota-metabolite-receptor-immune” pathway: on one hand, they suppress carcinogenic microbiota and block their immune evasion signals; while simultaneously promoting the growth of probiotic microbiota and their metabolic products. This activates receptor pathways such as GPR, AhR, and FXR/TGR5, enhancing CD8⁺ T cell, NK cell, and Th1 responses to strengthen antitumor immunity.

Compared to existing conventional therapies, phytochemicals may offer certain advantages, such as a generally favorable safety profile, long-term intervention potential, and accessibility through dietary/traditional Chinese medicine formulations. This approach provides novel sensitization strategies for converting “cold tumors” into “hot tumors” and may synergize with radiotherapy, chemotherapy, or ICIs to enhance overall efficacy. This intervention model also enables patient stratification and precision immunotherapy based on microbial and metabolite fingerprints. However, the chemical complexity of phytochemicals, individual microbial variations, and integration challenges with existing therapeutic strategies remain significant hurdles for clinical translation. Future efforts may involve standardized formulations, mechanism-based causal validation, multi-omics integration analysis, and prospective clinical trials to better define their mechanisms of action and potential scope of clinical application.

Overall, phytochemicals demonstrate unique and broad potential in CRC prevention and treatment through the “microbiome-metabolite-receptor-immune” pathway. With deepening interdisciplinary research and the establishment of clinical evidence-based systems, this approach may emerge as a significant complementary and innovative direction in CRC immunotherapy. Finally, this study has several limitations that warrant clarification. Although mechanistic studies and preclinical findings are encouraging, there remains limited direct evidence from interventional clinical trials to support the use of phytochemicals to sensitize immune checkpoint inhibitors in colorectal cancer. Therefore, translation from the laboratory to the clinical setting still requires extensive clinical validation.

Author contributions

XQ, GH, and YH are co-first authors and contributed equally to this work. XQ, GH, and YH were responsible for literature search, data curation, and writing the original draft. MZ, MX, CX, YL, and XL contributed to investigation, data analysis, and validation. RC and FL are co-corresponding authors who provided supervision, funding acquisition, and reviewed and edited the manuscript. All authors contributed to the article and approved the submitted version.

Funding

This study was supported by the National Natural Science Foundation of China (NSFC) (Grant numbers: 82374585), Natural Science Foundation of Hubei Province (Grant numbers: 2024AFD272).

Data availability

No datasets were generated or analysed during the current study.

Declarations

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Ethics approval

Ethics declaration: not applicable.