The influence of gut microbiota on immunotherapy for colorectal cancer

ABSTRACT

Colorectal cancer (CRC) stands as the third most prevalent cancer worldwide and ranks second in mortality for both men and women. Novel therapeutic approaches have emerged to address this alarming situation, with immunotherapy showing great promise. This cutting-edge technique harnesses patients’ immune system to recognize and eliminate cancer cells. Despite its tremendous potential, the complex interplay between the intestinal microbiota and CRC development remains a critical area of investigation. The intestinal microbiota, comprised of diverse microorganisms such as bacteria, viruses, protozoa, and fungi, plays pivotal roles in digestion, vitamin production, defense against pathogens, and immune system modulation. Perturbations in this delicate balance have been implicated in carcinogenesis. Interestingly, the intestinal microbiota is now recognized to significantly impact immunotherapy’s effectiveness by influencing immune checkpoint regulation, particularly in the activation of cytotoxic T cells. Consequently, a better understanding of the intricate mechanisms underlying this interplay between microbiota and CRC could revolutionize cancer treatment outcomes. In this review, we aim to elucidate the mechanisms through which the intestinal microbiota directly influences CRC development and, specifically, explore its potential in generating improved prognoses for cancer patients. By delving into the link between the microbiome and CRC, this review highlights the significance of incorporating microbiota-related considerations in the design of novel therapeutic strategies, fostering a new era of precision medicine in colorectal cancer treatment.

Key words: colorectal cancer, immunotherapy, gut microbiota, immune checkpoints, cytotoxic activity of T cells, tumor microenvironment

INTRODUCTION

Colorectal cancer (CRC) ranks as the third most prevalent cancer worldwide and represents a significant health concern due to its high mortality rate, placing it as the second deadliest cancer for both men and women.^[1–3] The carcinogenesis of CRC is an intricate and multifactorial process, involving mucosal inflammation, interactions between the host and the microbiota, and genetic abnormalities. Accordingly, most CRC cases emerge from intestinal polyps that initiate inflammation, leading to genetic changes that culminate in tumor development.^[4–6] This process is influenced by a myriad of risk factors, including genetic predisposition, lifestyle habits (such as smoking, sedentary behaviors, and an unhealthy diet), as well as pre-existing conditions like diabetes and chronic inflammation of the gut.^[7–9]

The therapy of CRC typically relies on well-established methods such as surgery, radiotherapy, and chemotherapy.^[10–12] Nevertheless, novel therapeutic approaches are being explored and implemented in the treatment of cancer patients. Notably, immunotherapy has emerged as a promising strategy aimed at augmenting the efficacy of the individual’s immune system in combating tumors. This encompasses various techniques such as immune checkpoint inhibitors, adoptive cell transfer, and cancer vaccines, all of which are already employed strategies in CRC therapy.^[13–15]

The human microbiota, consisting of approximately 10 to 100 trillion microbes, including bacteria, viruses, fungi, and protozoa, is primarily concentrated in the human intestine. The gut microbiota functions as a distinct organ with vital roles in food digestion, vitamin production, protection against harmful/pathogenic microorganisms, and modulation of the immune system.^[16–19] Dysregulation of the microbiota has been linked to various health changes, including CRC, suggesting a potential role of microbiome dysregulation in the development or progression of CRC. Moreover, recent studies have demonstrated that the composition and diversity of the microbiome can impact the efficacy and toxicity of immunotherapeutic treatments.^[20,21]

Thus, this review provides an analysis of the direct association between the microbiota and colorectal cancer, examining its role in carcinogenesis and its impact on CRC immunotherapy. Additionally, the article discusses the potential of manipulating the microbiota in medicine to improve patient outcomes, enhance immunotherapy effectiveness, and improve the quality of life for individuals with cancer.

RELATIONSHIP BETWEEN GUT MICROBIOTA AND THE ONSET OF COLORECTAL CANCER

The gut microbiota, an intricate ecosystem, comprises a diverse assemblage of microorganisms that colonize the entire gastrointestinal tract. Predominantly, the bacterial phyla Bacteroidetes, Actinobacteria, Firmicutes, Proteobacteria, Verrucomicrobia, and Euryarchaeota constitute approximately 90% of its microbial composition.^[22,23] This highly complex environment serves as a critical determinant of human homeostasis, playing a pivotal role in various essential functions, including metabolic processes, synthesis of vitamins, and neuromodulation. The gut microbiota’s multifaceted regulatory functions further underscore its significance as both a metabolic and immunological regulator, impacting various aspects of human health and disease.^[24–26]

Dysbiosis, a condition marked by an altered balance of the gut microbiota, has garnered considerable attention as a potential contributory factor in the pathogenesis of autoimmune diseases, exemplified by irritable bowel syndrome.^[27–29] These investigations have unveiled previously undiscovered aspects of the complex gut microbiome. Additionally, a notable finding in the context of CRC is the discernible disparity in the gut microbiota composition between CRC patients and healthy individuals. Specifically, CRC patients frequently exhibit a diminished diversity of microorganisms constituting the gut microbiota, as evidenced by various studies.^[22,23,30]

The role of gut microbiota in the development of colorectal cancer can be comprehensively understood through an integrated theoretical model that combines insights from two prominent perspectives. This unified model highlights how both the intestinal microorganisms and the host’s genetic predisposition interact to shape the pathogenesis of CRC.^[31,32] The first aspect of the model focuses on the role of the host’s genetic predisposition in driving the establishment of a pro-inflammatory microenvironment in the gut. Individuals with specific genetic traits may exhibit recurrent and sustained inflammatory responses, leading to significant modifications in the normal cellular landscape of the gut microenvironment.^[33–37] These genetic factors influence the immune response, cellular signaling pathways, and inflammatory mediators, collectively contributing to the development of CRC.

In turn, the second aspect of the model emphasizes the active contribution of intestinal microorganisms in remodeling the gut environment, fostering a pro-carcinogenic milieu. According to this perspective, damage to the intestinal mucosa creates an opportunity for transient bacteria to infiltrate, thereby increasing susceptibility to carcinogenesis.^[38–40] Alterations in the composition and behavior of gut microorganisms play a critical role in initiating and promoting CRC. The interplay between these microorganisms and the host cells can lead to the generation of pro-inflammatory signals and other tumorigenic factors.^[41]

Recently, Chattopadhyay et al.^[42] comprehensively compiled a list of key bacteria associated with CRC, stemming from disturbances in the healthy gut microbiota. The bacteria implicated in CRC pathogenesis include Fusobacterium nucleatum, Bacteroides fragilis, Enterococcus faecalis, Escherichia coli (E. coli), Helicobacter pylori (H. pylori), and Streptococcus gallolyticus/Streptococcus bovis^[42]. Accordingly, Kharrat et al.^[43] conducted a metagenomic analysis, revealing a significant elevation of F. nucleatum in individuals with CRC compared to those with adenomas, implicating its potential role in adenoma-to-CRC progression.^[43] The principal pro-carcinogenic mechanism of F. nucleatum seems to involve the adhesion molecule FadA, which interacts with E-cadherin on intestinal epithelial cells, leading to the activation of β-catenin and subsequent stimulation of transcription factors and oncogenes.^[43,44] Furthermore, F. nucleatum appear to induce an increase in inflammatory cytokines (IL-1β, IL-6, IL-8, and TNF-α), fostering a pro-oncogenic microenvironment in the colorectal region.^[44]

Similarly, enterotoxigenic B. fragilis (ETBF) plays a significant role in CRC oncogenesis, particularly when prevailing in high proportions within a dysregulated gut microbiota.^[45] The toxin produced by ETBF stimulates β-catenin expression, eliciting inflammatory responses akin to those triggered by F. nucleatum. This process involves the induction of cyclooxygenase (COX)-2 production, resulting in the release of PGE2 and instigating the inflammatory cascade, leading to elevated pro-inflammatory cytokines, such as IL-17.^[46,47]

Enterococcus faecalis exhibits a dual role, serving as a protective agent under certain conditions, yet demonstrating pro-carcinogenic potential in cases of gut dysbiosis.^[48,49] While it modulates the immune system by inducing the release of anti-inflammatory cytokines and exerting a protective effect,^[48] E. faecalis can also activate β-catenin and transcription factors, potentially leading to cellular alterations.^[49] On the other hand, Pleguezuelos-Manzano et al.^[50] report the involvement of E. coli in CRC oncogenesis, particularly in strains harboring the pks genomic island, which synthesizes colibactin, causing direct DNA damage to cells.^[50,51] Furthermore, Iyadorai et al.^[52] further corroborate this association by observing a higher prevalence of pks+ E. coli in individuals with CRC compared to healthy individuals.

The involvement of H. pylori in CRC is partially elucidated, with underlying mechanisms still being investigated. Ralser et al.^[53] recently demonstrated that H. pylori triggers a Th17 pro-inflammatory response in the intestine, concurrently leading to a decrease in regulatory T cells, ultimately resulting in tumorigenesis. Moreover, the pro-inflammatory activity of H. pylori seems to activate transcription factors NF-κB and STAT3, thereby further augmenting its potential carcinogenic effects.^[53,54]

Finally, the relationship between S. bovis/gallolyticus and CRC also remains a subject of ongoing investigation, with mechanisms being actively explored. Abu-Ghazaleh et al.^[55] suggest that this bacterium elicits the release of inflammatory cytokines, creating an environment rich in free radicals and nitric oxide, which directly influence oncogenes in intestinal epithelial cells. Furthermore, it activates the COX-2 pathway, contributing to the altered environment.^[56] Table 1 presents an overview of the primary mechanisms associated with the carcinogenic impact of the aforementioned microorganisms.

In conclusion, the past few years have witnessed substantial progress in the field of research concerning the link between gut microbiota and CRC development. This advancement has primarily been centered around the exploration of potential bacterial profiles and the underlying mechanisms by which they might contribute to CRC initiation and progression. The emerging body of knowledge in this area offers promising insights into the complex interplay between the gut microbiome and colorectal carcinogenesis. However, it also highlights the need for further investigations to unravel the intricacies of this relationship fully.

IMMUNOTHERAPY FOR COLORECTAL CANCER

Immunotherapy has revolutionized the treatment outlook for several types of solid tumors, such as malignant melanoma, non-small-cell lung cancer, and renal cell carcinoma, and has emerged as the primary treatment approach for recurrent or metastatic solid tumors, surpassing traditional chemotherapy and targeted therapy.^[57] Immunotherapy is a treatment approach that utilizes peptides, cells, viruses, small molecules, or antibodies to activate or modulate the immune system to attack cancer cells. Research conducted in preclinical and clinical settings to evaluate the effectiveness of immunotherapy in treating colorectal cancer has yielded positive results.^[58] Here, we aim to synthesize the main concepts of immunotherapy in colorectal cancer and the main clinical results.

Due to their remarkable effectiveness, immune checkpoint inhibitors (ICIs) have quickly emerged as a prominent therapeutic approach for several solid tumors. ICI function by obstructing immunosuppressive tumor signaling pathways that target receptor or ligand checkpoint proteins, such as programmed cell death 1 (PD-1), PD-1 ligand 1 (PD-L1), and cytotoxic T lymphocyte antigen 4 (CTLA-4), thereby restoring the antitumor immune response.^[59,60] The FDA has authorized the use of treatments, including pembrolizumab, nivolumab, and ipilimumab, which target the PD-1, PD-L1, and CTLA-4 immune checkpoints, for managing dMMR/MSI-H CRC.^[57,60,62] Nevertheless, not all dMMR/MSI-H CRC cases respond to immunotherapy, and about 50% of patients with this subtype show primary resistance, which implies the existence of considerable molecular heterogeneity among dMMR/MSI-H CRCs. This heterogeneity could be attributed to high CIMP levels, oncogenic BRAF mutations, and CMS1 subtypes.^[63] Furthermore, recent research into combined ICI treatment has suggested that it may enhance the effectiveness of immunotherapy in this patient population. Several studies on combination therapy with ICIs have investigated their potential when administered alongside chemotherapy, targeted therapy, other ICIs, or radiotherapy to investigate methods of transforming cold tumors into hot ones, thereby enhancing their sensitivity to immunotherapy.^[64,65]

Despite existing for over a century, cancer vaccines have had limited success in treating CRC patients. However, with the promising efficacy of immunotherapy, cancer vaccines have garnered renewed attention. Several trials are currently underway to identify appropriate antigenic stimulants for cancer vaccines.^[66]

Adoptive cell therapy (ACT) is another eagerly anticipated novel treatment method aimed at stimulating tumor immunity. ACT involves selecting either host cells that demonstrate antitumor activity or host cells that have been genetically engineered with chimeric antigen receptors (CARs) or antitumor T cell receptors (TCRs) to enhance the host’s antitumor immune response. While both CAR T therapy and TIL (tumor-infiltrating lymphocyte) therapy have shown promising preliminary results, their wider applicability still needs to be demonstrated.^[67]

As further clinical trials produce valuable knowledge in the search for effective therapeutic approaches, the identification of novel combinations and biomarkers may assist clinicians in administering personalized treatments to CRC patients. These advancements in treatment are anticipated to transform the treatment landscape for CRC, with immunotherapy emerging as a potential game-changer.

INFLUENCE OF GUT MICROBIOTA COMPOSITION ON THE RESPONSE OF CRC TO IMMUNOTHERAPY

Recent research highlights the critical role of the gut microbiota in regulating immune responses within the tumor microenvironment and influencing immune-related cytokines.^[68] The composition of these microorganisms is crucial for maintaining gut symbiosis and producing metabolites like butyrate and tryptophan degrading metabolites, which stimulate lymphoid cells to produce IL-22.^[69,70] Moreover, they contribute to increased immune cell activity in the gut.^[71,72] Cytokines are essential for preserving gut homeostasis and supporting repair mechanisms during infectious events.^[72,73] Additionally, bacterial metabolites, such as short-chain fatty acids (SCFAs), play a role in immune responses and are closely associated with innate immunity and antibody production.^[74]

In the field of intestinal immune regulation, the adaptive response is of utmost importance, especially considering T cells’ high responsiveness to signals from the intestinal lumen. This responsiveness leads to the generation of both inflammatory and anti-inflammatory responses, as well as the facilitation of differentiation between CD8+ and CD4+ T cells.^[75] This phenomenon is intricately linked to tumors’ ability to induce cytokine production, bolstering Tregs and myeloid-derived suppressor cells (MDSCs), while inhibiting CD8+ cytotoxic T cell function.^[76] These events underscore the potential of certain microorganisms to modulate the function of specific intestinal immune cells through activation pathways, potentially contributing to anti-tumor defense by enhancing the activity of CD4+ and CD8+ effector T cells, thereby countering the suppressive effects induced by tumors.^[76]

Altered microbiota diversity significantly impacts disease development, with a decline in typical gut microbiome species, like Firmicutes and Bacteroidetes, playing a crucial role in triggering numerous diseases.^[70,77] In the context of CRCs, recent cohort studies have identified significant changes in the gut microbiome’s composition at the phylum level.^[78] Notably, the proliferation of the Fusobacterium nucleatum group, commonly observed in CRC patients, has been linked to its ability to recruit immune cells and create a pro-inflammatory environment that favors CRC development.^[78] Further investigation is necessary to elucidate its precise role in CRC pathogenesis and its impact on immune responses.

The relationship between immunotherapy and the gut microbiota has become an increasingly interesting topic of research, as it appears to exert both anti-tumor and pro-tumor influences.^[79] Some microorganisms have demonstrated beneficial effects in CRC immunotherapy and other cancers by exerting an anti-tumor role. For instance, Bacteroides fragilis has been associated with improved treatment outcomes, such as enhanced anti-CTLA4 therapy,^[80] leading to improved anti-tumor immune responses and increased activity of anti-tumor CD8+ T cells, similar to the effects observed with Lactobacillus acidophilus.^[81] Similarly, Akkermansia muciniphila has been found to increase IL-12 production, which restores anti-PD1 efficacy during immunotherapy.^[82] Bifidobacteria, when combined with PD-L1 inhibitors, demonstrated almost complete inhibition of cancer cell growth by enhancing the trafficking, penetration, and infiltration of effector CD8+ T cells into tumor tissue, addressing a critical challenge in cancer immunotherapy^[83] (Figure 1).

Figure 1. Microbiome-induced changes in the tumor microenvironment. DC: dendritic cell. CRC: colorectal cancer. PD-1: programmed cell death 1. PD-L1: programmed cell death ligand 1. IL-12: interleukin 12.

Pathways involving increased IL-12 production are associated with improved dendritic cell (DC) function, crucial for gut immune tolerance by promoting the differentiation of CD4+ T cells into regulatory T cells.^[84,85] These regulatory T cells induce an anti-inflammatory IL-10 response to defend against inflammatory lesions.^[85,86] Bacteroides fragilis in the gut microbiome also plays a regulatory role due to its production of polysaccharide A, which exerts anti-inflammatory effects by inhibiting IL-17 production and enhancing the activity of Tregs in the gut.^[86] Understanding these intricate interactions between microorganisms and immune responses holds promise for advancing cancer immunotherapy and gut immune regulation.

Another significant finding indicates that combining Lactobacillus acidophilus lysate with anti-CTLA4 can enhance antitumor immune responses by activating TME-infiltrated effector T cells.^[87] On the other hand, some microorganisms are associated with a pro-tumor environment, such as E. coli and F. nucleatum, which promote an increase in M2 macrophages and a decrease in FOXP3+.^[69–72] The effects of gut microbiota bacteria on CRC immunotherapy are summarized in Table 2.

Collectively, these studies suggest that the gut microbiota may play a key role in cancer immunotherapy, and certain bacterial species could be highly effective in combination therapies. However, due to the complexity and high diversity of microbiomes, further research is necessary to understand the specific functions of each microorganism. Additionally, the optimal composition of the gut microbiome for supporting anti-tumor immune responses remains unclear.

ROLE OF THE GUT MICROBIOTA IN THE MODULATION OF SPECIFIC IMMUNOTHERAPEUTIC APPROACHES

Influence of microbiota on anti-CTLA-4 efficacy

Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4) is a protein receptor with inhibitory features that plays pivotal role in T-cell homeostasis and immune regulation(88). This immune checkpoint can compete with CD28, a costimulatory molecule, by binding to CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells, thus inhibiting T cell activation and proliferation^[89] (Figure 2A). As a result, several studies have evaluated the effect of CTLA-4 blockade in enhancing the immune response against tumors, which led to the development of ipilimumab, a monoclonal antibody currently used in the treatment of certain types of cancer.^[90–92]

Figure 2. Immune checkpoint inhibition targets: PD-1/PDL-1, and CTLA-4/CD80 or CD86. A: CTLA-4/CD80 or CD86 inhibition. CTLA-4 is an additional immune checkpoint molecule predominantly found on the surface of regulatory T cells (Tregs) and certain cancer cells. When CTLA-4 interacts with its ligands CD80 or CD86 on antigen-presenting cells, it inhibits the activation of effector T cells, dampening the overall immune response. By blocking CTLA-4 with immune checkpoint inhibitors, this inhibition is relieved, enhancing the activation and proliferation of effector T cells, which can then more effectively target cancer cells or other threats. CTLA-4: cytotoxic T-lymphocyte-associated protein 4. B. PD-1/PDL-1 inhibition. PD-1, present on the surface of activated T cells, interacts with its ligand PDL-1, expressed on some tumor cells and other immune cells. This interaction acts as a 'brake' on the immune response, preventing T cells from attacking these cells, including tumor cells. Immune checkpoint inhibitors that target PD-1 or PDL-1 interrupt this interaction, releasing the brake and allowing T cells to launch a stronger attack against tumor cells. PD-1: programmed cell death 1. PD-L1: programmed cell death ligand 1. CRC: colorectal cancer.

Regarding CTLA-4-based immunotherapy in CRC, ipilimumab is the only approved for the treatment of colorectal cancer. Currently, its association with nivolumab for patients with microsatellite instability-high (MSI-H) or mismatch repair deficient (dMMR) metastatic colorectal cancer have been assessed and is apparently a promising alternative.^[65] On the other hand, tremelimumab has been the scope of several clinical trials, but is still pending approval.^[93]

Increasing evidence suggests that the microbiota plays a role in the effectiveness of immunotherapy. Certain types of bacteria may either enhance the immune system's response to cancer cells, reduce toxic effects, or even hinder immunotherapy.^[94,95] In this regard, Vétizou et al.^[80] demonstrated that CTLA-4 blockade efficacy is influenced by the presence of specific bacterial species in the gut, especially Bacteroides in mice and patients. The mechanism underlying this finding is probably related to the effect of the microbiota on IL-12 production. This cytokine stimulates Th1 cells and consequently enhances anti-tumor effects.^[80,96]

As for colorectal cancer, there are still few evidences assessing the role of microbiota in the efficacy of immunotherapy in humans. To date, Zhuo et al. ^[87] has pointed out a possible synergistic action between Lactobacillus acidophilus cell lysates and the anti-CTLA4 blocking antibody. The combined therapy was able to increase CD8+ T cells, IFN-γ and IL-2 production and reduce the percentage of regulatory T cells (Tregs) in mice, which suggests that combining L. acidophilus lysates with CTLA-4 blockade may enhance antitumor immunity and reduce immunosuppression.^[87]

Overall, even though there is increasing evidence on the benefits of anti-CTLA4 based immunotherapy, its use in colorectal cancer is still not well-established, mainly due to the high incidence of adverse effects. However, the modulation of the microbiota and association with CTLA-4 blockade may hold promising results. For this purpose, further studies are strongly required, especially in humans.

Influence of microbiota on anti-PD-1/PD-L1 efficacy

PD-1 is an essential immune checkpoint receptor expressed in activated T cells that regulates immunosuppression.^[97] On the other hand, membrane-bound PD-L1 engages PD-1, leading to T cell dysfunction and apoptosis. In turn, targeting PD-1 or its ligand PD-L1 with antibodies can rescue T cells from exhaustion and revive the immune response against cancer cells^[98–100] (Figure 2B). Increasing evidence suggest that the commensal microbiome might play a mechanistic role in the antitumor immune response of human cancer patients.^[82,101] Nevertheless, the influence of the microbiome in variable therapeutic outcomes of PD-1/PD-L1 therapy in CRC still needs to be addressed.^[102]

To investigate the effects of the gut microbiome on PD-1 antibody immunotherapy, Xu et al.^[68] compared its relative efficacy against established CT26 tumor-bearing mice treated with different antibiotics. These authors observed that after introducing the PD-1 antibody treatment, the administration of antibiotics resulted in an increase in tumor growth as compared to the group that consumed sterile drinking water without antibiotics. In addition, the combined use of broad-spectrum antibiotics (ampicillin, streptomycin, and colistin) hindered the antitumor effects of anti-PD-1.^[68] This was evident from the lack of response in the group that received the ampicillin, streptomycin, and colistin antibiotics. Alterations in the gut microbiome also resulted in changes of glycerophospholipid metabolism, which could impact the expression of immune-related cytokines IFN-γ and IL-2 within the tumor microenvironment. The observed changes and their impact suggest a potential association with the variable therapeutic outcomes of PD-1 antibody treatment.^[68] Overall, these findings indeed emphasize the critical role of the gut microbiota in the efficacy of PD-1 antibody-mediated anticancer effects.

In parallel, Goc et al.^[103] demonstrated that the establishment of microbiota-induced anti-tumoral type-1 immunity in the intestine and tumor microenvironment requires a conversation between T cells and group 3 innate lymphoid cells (ILC3s) through major histocompatibility complex class II (MHCII). Indeed, the lack of ILC3-specific MHC-II results in invasive CRC and resistance to anti-PD-1 immunotherapy in mice. Additionally, these authors showed that humans with dysfunctional intestinal ILC3s carry microbiota that do not induce type-1 immunity or promote immunotherapy responsiveness when transferred to mice.^[103] These results reinforce the importance of the ILC3s-microbiota binomial in modulating anti-tumor responses and demonstrate that disruption of their function in CRC leads to impaired adaptive immunity, tumor progression and resistance to immunotherapy.

Huang et al. ^[104], in turn, obtained exciting results by employing a multi-omics approach to evaluate the synergistic impact of fecal microbiota transplantation (FMT) and anti-PD-1 therapy in curing colorectal tumor-bearing mice. The combination therapy resulted in significantly higher tumor control and survival rate in mice compared to those treated with FMT or anti-PD-1 therapy alone. Metagenomic analysis showed that the gut microbiota composition of tumor-bearing mice treated with anti-PD-1 therapy was notably altered following FMT. Additionally, metabolomic analysis of mouse plasma identified several potential metabolites, such as punicic acid and aspirin, which increased after FMT and could promote the response to anti-PD-1 therapy via their immunomodulatory properties.^[104]

Similarly, Zhang et al.^[105] demonstrated that the gut microbiota of healthy individuals significantly increased the sensitivity of CRC tumor-bearing mice to anti-PD-1 therapy, whereas the gut microbiota of CRC patients did not have the same effect. Through 16S rRNA gene sequencing, these authors isolated a novel strain of Lacticaseibacillus, which was named L. paracasei sh2020. Mechanistically, the antitumor immune response induced by L. paracasei sh2020 was found to be CD8+ T cell-dependent. In vitro and in vivo experiments revealed that L. paracasei sh2020 also stimulated the upregulation of CXCL10 expression in tumors, which subsequently enhanced the recruitment of CD8+ T cells. Interestingly, the modulation of gut microbiota caused by L. paracasei sh2020 improved both its antitumor effect and gut barrier function.^[105]

Therefore, the differential composition of the patient microbiome has been shown to affect antitumor immunity and the efficacy of anti-PD-1/PD-L1 immunotherapy in preclinical mouse models. Nevertheless, further studies are needed to investigate the extension of these effects in humans.

BIOMARKERS FOR CRC DIAGNOSIS AND PROGNOSIS BASED ON GUT MICROBIOTA

Studies investigating the influence of gut microbiota on the development and treatment of CRC have led to a new stage, focused on exploring potential biomarkers for non-invasive diagnosis and prognosis evaluation. These biomarkers are identified through the detection of dysbiosis in microbial taxa and metabolites within the affected individual's microbiota.^[106] Genomic analysis of the microbiota has enabled the association of specific strains, such as F. nucleatum and B. fragilis, with CRC, making them promising biomarkers and prognostic targets.^[106,107]

Among the techniques adopted for identifying CRC-linked strains, fecal 16S rRNA sequencing has gained popularity due to its specificity and sensitivity, serving as a potential biomarker for prognosis. However, further research is required to address the limitations of this approach, such as defining its sensitivity and specificity more precisely.^[108] Additionally, exploring the relationship between intestinal metabolites and bacterial taxa associated with CRC holds promise for enhancing non-invasive diagnosis. In a specific study, the association of 11 metabolites and 6 bacterial species demonstrated substantial potential as colorectal cancer markers.^[109] This association includes SCFAs and their producing bacteria, which play a protective role on the intestinal mucosa.^[106] Investigating the interplay between microbial taxa and metabolites is crucial for identifying robust and reliable biomarkers for CRC diagnosis and prognosis. Future advancements in this area may lead to improved early detection and personalized treatment options for CRC patients.

MANIPULATION OF THE MICROBIOTA TO ENHANCE THERAPEUTIC RESPONSE

The therapeutic response of CRC patients is intricately linked to the composition of the microbiota, as previously mentioned above. To enhance treatment efficacy, researchers are investigating the effects of secondary interventions aimed at manipulating the microbiota.^[110]

Antibiotics

Recent studies have shown conflicting findings regarding the impact of antibiotics on immunotherapy efficiency in oncologic treatment. A meta-analysis of 2,740 patients revealed that antibiotic use negatively affected the efficacy of ICIs due to alterations in the individual's microbiota composition and potential elimination of beneficial bacterial strains linked to improved immunotherapy outcomes.^[111] However, some researchers advocate for the prophylactic use of antibiotics to prevent CRC development, particularly by eliminating aggressive pathogens like Fusobacterium nucleatum, which is associated with poor prognosis in CRC tissues.^[112] Physicians treating oncologic patients undergoing immunotherapy should carefully consider the use of antibiotics to preserve the resident microbiota while also exploring their potential role in preventing carcinogenesis.^[111–113]

Fecal microbiota transplant

Fecal microbiota transplant (FMT) involves the transfer of a healthy and complete microbiota ecosystem from a donor to a recipient to restore homeostasis in a dysbiotic region, potentially enhancing the response to immunotherapy.^[114,116] While FMT has shown promise in treating intestinal infections, such as Clostridium difficile infections, its application in CRC is still underexplored. Studies suggest that FMT may positively impact the treatment of cancer patients and even serve as a preventive measure against tumor genesis in high-risk individuals.^[117,118]

Nevertheless, research concerning the correlation between FMT and CRC remains scarce. While some knowledge exists regarding the advantageous effects on chronic inflammation, the association of this process with cancer remains unexplored. Consequently, further investigations in this domain are imperative, given the highly promising potential of FMT to enhance the efficacy of cancer treatment and even serve as a preventive measure against tumor development in high-risk individuals.^[117]

Prebiotics

Prebiotics are substrates that positively influence the development of the microbiota, promoting probiotic growth and strengthening the individual’s immune response.^[117] Studies have demonstrated that prebiotics may have a potential role in preventing CRC development by modulating the microbiome, restoring homeostasis, and producing SCFAs, which protect the intestinal mucosal barrier.^[118] Furthermore, SCFAs can reduce the release of pro-inflammatory cytokines, associated with cancer development, through their interaction with G protein-coupled receptors on colon cells and immune cells.^[118]

Probiotics

Probiotics consist of live microorganisms that, upon ingestion, induce a novel composition of an individual’s microbiota, resulting in positive health impacts and intestinal protection through modulation of the microbiome.^[117,119] In the field of medicine, probiotics are already employed for the prevention and treatment of various diseases, particularly those with inflammatory aspects. This efficacy stems from their ability to fortify the intestinal mucosa, thereby creating a natural barrier against harmful pathogens.^[119] As such, the potential of probiotics in the treatment and prevention of CRC is substantial.

Probiotics offer specific mechanisms that play a crucial role in both preventing and treating CRC. One such mechanism is the (1) reinforcement of the intestinal mucosal barrier. Within probiotics, there are selective bacteria that stimulate the production of mucus, leading to the strengthening of the intestinal barrier. Moreover, these beneficial bacteria outcompete aggressive pathogens, effectively preventing their colonization.^[117] Another significant mechanism is the (2) reduction of intestinal inflammation. Specific probiotic strains, including Bifidobacterium infantis and Lactobacillus acidophilus, possess the ability to modulate the immune system. Through the regulation of pro-inflammatory cytokine expression, they promote immune system homeostasis and effectively mitigate intestinal inflammation.^[117] Finally, probiotics also demonstrate the ability to (3) inhibit pathogenic toxic activity. In the gut, certain microorganisms release toxins that have been linked to carcinogenesis due to their aggressive nature. However, the introduction of probiotic bacteria creates a natural competition, regulating the production of these toxins and mitigating their harmful effects.^[119]

Furthermore, a randomized double-blind study involving 52 CRC patients, who received probiotics four weeks after the surgical procedure, demonstrated the safety and positive impact of this intervention on the treatment. Notably, inflammation was reduced, as evidenced by a clear decrease in pro-inflammatory cytokine levels among the subjects.^[120] This finding underscores the potential therapeutic value of probiotics in managing CRC.

Postbiotics

Postbiotics are by-products and metabolites excreted by microbiota microorganisms that play vital roles in host health. Specific postbiotics, such as SCFAs and the p40 protein produced by Lactobacillus rhamnosus, have been identified as crucial in preventing CRC development.^[121,122] SCFAs regulate gene expression, induce apoptosis in cancer cell lines, and contribute to mucosal protection, while the p40 protein aids in reducing intestinal epithelial inflammation and increasing IgA production.^[121,122] Although the use of postbiotics shows promise in CRC prevention, further research is also required to confirm their potential.

Diet

Research has indicated that diet, particularly non-digestible fermentable carbohydrates like resistant starches, plays a significant role in modulating the gut microbiota. Consuming whole grains, fruits, and vegetables has been associated with favorable changes in the microbiome and increased production of SCFAs, which possess anticancer effects.^[123,124] Studies also suggest that a Mediterranean diet may reduce inflammation, protect the intestinal barrier, and decrease aggressive microorganisms such as F. nucleatum.^[125] However, more extensive studies are needed to explore the impact of diet on microbiota modulation in CRC patient.

Manipulating the microbiota presents a promising avenue to enhance therapeutic responses in CRC patients. Secondary treatments such as antibiotics, prebiotics, probiotics, postbiotics, and FMT hold potential for optimizing treatment outcomes by influencing the composition of the microbiome. Additionally, diet plays a significant role in microbiota modulation and may have implications for CRC prognosis. However, further research is warranted to fully understand and leverage the therapeutic potential of microbiota manipulation in CRC treatment and prevention.

CONCLUSION

The relationship between the microbiota and colorectal cancer emerges as a complex process involving both positive and negative aspects. Dysregulation of the microbiota can impact carcinogenesis and lead to a poor prognosis, while a balanced microbiota can positively influence immunotherapy outcomes, thereby offering potential benefits for cancer patients. The microbiome’s direct involvement in intestinal homeostasis and its active role in organ protection through the production of mucus and stimulation of the immune system via cytokines are well-established.

Furthermore, the discovery of the microbiota’s influence on immune checkpoint inhibition has opened a new and promising avenue for immunotherapy. Nevertheless, while some advances have been made in manipulating the microbiota to enhance colorectal cancer treatment, many underlying processes remain incompletely understood, necessitating further and more targeted research.

Despite these uncertainties, the manipulation of the microbiota holds great promise, potentially revolutionizing the treatment of CRC and offering a renewed outlook for thousands of patients who face the challenges of this debilitating and lethal disease daily. By unlocking the full potential of these novel techniques, we may pave the way for improved quality of life and better prognoses for those affected by colorectal cancer. Therefore, comprehensive, and rigorous investigations in this field are warranted to advance our understanding and translate these discoveries into tangible clinical benefits.

DECLARATIONS

Author contributions

All authors equally contributed to this paper with conception and design of the study, literature review and analysis, drafting and critical revision and editing, and final approval of the final version. All authors agree to be accountable for all aspects of the work in ensuring that questions that are related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.

Source of funding

The coauthor Luz, MS and the coauthor Pinheiro, SLR are supported by the Scientific Initiation Scholarship Programme (PIBIC) of National Council for Scientific and Technological Development, CNPq, Brazil. The coauthor Lemos, FFB is supported by the Scientific Initiation Scholarship Programme (PIBIC) of Bahia State Research Support Foundation, FAPESB, Brazil. The last author – de Melo, FF – is a CNPq Research Productivity Fellow (PQ).

Conflict of interest

All authors declare no potential conflicts of interest.

REFERENCES

1. Sung H, Ferlay J, Siegel RL, Laversanne M, Soerjomataram I, Jemal A, et al. Global Cancer Statistics 2020: GLOBOCAN Estimates of Incidence and Mortality Worldwide for 36 Cancers in 185 Countries. CA Cancer J Clin. 2021;71:209–249.    DOI: 10.3322/caac.21660    PMID: 33538338
2. Siegel RL, Miller KD, Wagle NS, Jemal A. Cancer statistics, 2023. CA Cancer J Clin. 2023;73:17–48.    DOI: 10.3322/caac.21763    PMID: 36633525
3. Siegel RL, Wagle NS, Cercek A, Smith RA, Jemal A. Colorectal cancer statistics, 2023. CA Cancer J Clin. 2023;73:233–254.    DOI: 10.3322/caac.21772    PMID: 36856579
4. Schmitt M, Greten FR. The inflammatory pathogenesis of colorectal cancer. Nat Rev Immunol. 2021;21:653–667.    DOI: 10.1038/s41577-021-00534-x    PMID: 33911231
5. Khalyfa AA, Punatar S, Aslam R, Yarbrough A. Exploring the Inflammatory Pathogenesis of Colorectal Cancer. Diseases. 2021;9:79.    DOI: 10.3390/diseases9040079    PMID: 34842660
6. Al-Joufi FA, Setia A, Salem-Bekhit MM, Sahu RK, Alqahtani FY, Widyowati R, et al. Molecular Pathogenesis of Colorectal Cancer with an Emphasis on Recent Advances in Biomarkers, as Well as Nanotechnology-Based Diagnostic and Therapeutic Approaches. Nanomaterials (Basel). 2022;12:169.    DOI: 10.3390/nano12010169    PMID: 35010119
7. Lewandowska A, Rudzki G, Lewandowski T, Stryjkowska-Góra A, Rudzki S. Risk Factors for the Diagnosis of Colorectal Cancer. Cancer Control. 2022;29:10732748211056692.    DOI: 10.1177/10732748211056692    PMID: 35000418
8. Sawicki T, Ruszkowska M, Danielewicz A, Niedźwiedzka E, Arłukowicz T, Przybyłowicz KE. A Review of Colorectal Cancer in Terms of Epidemiology, Risk Factors, Development, Symptoms and Diagnosis. Cancers (Basel). 2021;13:2025.    DOI: 10.3390/cancers13092025    PMID: 33922197
9. Rawla P, Sunkara T, Barsouk A. Epidemiology of colorectal cancer: incidence, mortality, survival, and risk factors. Prz Gastroenterol. 2019;14:89–103.    DOI: 10.5114/pg.2018.81072    PMID: 31616522
10. Benson AB, Venook AP, Al-Hawary MM, Arain MA, Chen YJ, Ciombor KK, et al. Colon Cancer, Version 2.2021, NCCN Clinical Practice Guidelines in Oncology. J Natl Compr Canc Netw. 2021;19:329–359.    DOI: 10.6004/jnccn.2021.0012    PMID: 33724754
11. Benson AB, Venook AP, Al-Hawary MM, Azad N, Chen YJ, Ciombor KK, et al. Rectal Cancer, Version 2.2022, NCCN Clinical Practice Guidelines in Oncology. J Natl Compr Canc Netw. 2022;20:1139–1167.    DOI: 10.6004/jnccn.2022.0051    PMID: 36240850
12. Sakata S, Larson DW. Targeted therapy for colorectal cancer. Surg Oncol Clin N Am. 2022;31:255–264.    DOI: 10.1016/j.soc.2021.11.006    PMID: 35351276
13. Weng J, Li S, Zhu Z, Liu Q, Zhang R, Yang Y, et al. Exploring immunotherapy in colorectal cancer. J Hematol Oncol. 2022;15:95.    DOI: 10.1186/s13045-022-01294-4    PMID: 35842707
14. Ooki A, Shinozaki E, Yamaguchi K. Immunotherapy in Colorectal Cancer: Current and Future Strategies. J Anus Rectum Colon. 2021;5:11–24.    DOI: 10.23922/jarc.2020-064    PMID: 33537496
15. Carlsen L, Huntington KE, El-Deiry WS. Immunotherapy for colorectal cancer: mechanisms and predictive biomarkers. Cancers (Basel). 2022;14:1028.    DOI: 10.3390/cancers14041028    PMID: 35205776
16. Baquero F, Nombela C. The microbiome as a human organ. Clin Microbiol Infect. 2012;18 Suppl 4:2–4.    DOI: 10.1111/j.1469-0691.2012.03916.x    PMID: 22647038
17. Pham VT, Dold S, Rehman A, Bird JK, Steinert RE. Vitamins, the gut microbiome and gastrointestinal health in humans. Nutr Res. 2021;95:35–53.    DOI: 10.1016/j.nutres.2021.09.001    PMID: 34798467
18. Yoo JY, Groer M, Dutra SVO, Sarkar A, McSkimming DI. Gut microbiota and immune system interactions. microorganisms. 2020;8:1587.    DOI: 10.3390/microorganisms8101587    PMID: 33076307
19. Lo BC, Chen GY, Núñez G, Caruso R. Gut microbiota and systemic immunity in health and disease. Int Immunol. 2021;33:197–209.    DOI: 10.1093/intimm/dxaa079    PMID: 33367688
20. Temraz S, Nassar F, Nasr R, Charafeddine M, Mukherji D, Shamseddine A. Gut microbiome: a promising biomarker for immunotherapy in colorectal cancer. Int J Mol Sci. 2019;20:4155.    DOI: 10.3390/ijms20174155    PMID: 31450712
21. Koustas E, Trifylli EM, Sarantis P, Papadopoulos N, Aloizos G, Tsagarakis A, et al. Implication of gut microbiome in immunotherapy for colorectal cancer. World J Gastrointest Oncol. 2022;14:1665–1674.    DOI: 10.4251/wjgo.v14.i9.1665    PMID: 36187397
22. Flemer B, Lynch DB, Brown JMR, Jeffery IB, Ryan FJ, Claesson MJ, et al. Tumour-associated and non-tumour-associated microbiota in colorectal cancer. Gut. 2017;66:633–643.    DOI: 10.1136/gutjnl-2015-309595    PMID: 26992426
23. Gagnière J, Raisch J, Veziant J, Barnich N, Bonnet R, Buc E, et al. Gut microbiota imbalance and colorectal cancer. World J Gastroenterol. 2016;22:501–518.    DOI: 10.3748/wjg.v22.i2.501    PMID: 26811603
24. Feng Q, Chen W-D, Wang Y-D. Gut Microbiota: an integral moderator in health and disease. Front Microbiol. 2018;9:151.    DOI: 10.3389/fmicb.2018.00151    PMID: 29515527
25. Rowland I, Gibson G, Heinken A, Scott K, Swann J, Thiele I, et al. Gut microbiota functions: metabolism of nutrients and other food components. Eur J Nutr. 2018;57:1–24.    DOI: 10.1007/s00394-017-1445-8    PMID: 28393285
26. Levy M, Kolodziejczyk AA, Thaiss CA, Elinav E. Dysbiosis and the immune system. Nat Rev Immunol. 2017;17:219–232.    DOI: 10.1038/nri.2017.7    PMID: 28260787
27. Xu H, Liu M, Cao J, Li X, Fan D, Xia Y, et al. The dynamic interplay between the gut microbiota and autoimmune diseases. J Immunol Res. 2019;2019:7546047.    DOI: 10.1155/2019/7546047    PMID: 31772949
28. Shaheen WA, Quraishi MN, Iqbal TH. Gut microbiome and autoimmune disorders. Clin Exp Immunol. 2022;209:161–174.    DOI: 10.1093/cei/uxac057    PMID: 35652460
29. Christovich A, Luo XM. Gut microbiota, leaky gut, and autoimmune diseases. Front Immunol. 2022;13:946248.    DOI: 10.3389/fimmu.2022.946248    PMID: 35833129
30. Gao R, Gao Z, Huang L, Qin H. Gut microbiota and colorectal cancer. Eur J Clin Microbiol Infect Dis. 2017;36:757–769.    DOI: 10.1007/s10096-016-2881-8    PMID: 28063002
31. Hanus M, Parada-Venegas D, Landskron G, Wielandt AM, Hurtado C, Alvarez K, et al. Immune system, microbiota, and microbial metabolites: the unresolved triad in colorectal cancer microenvironment. Front Immunol. 2021;12:612826.    DOI: 10.3389/fimmu.2021.612826    PMID: 33841394
32. Xing C, Du Y, Duan T, Nim K, Chu J, Wang HY, et al. Interaction between microbiota and immunity and its implication in colorectal cancer. Front Immunol. 2022;13:963819.    DOI: 10.3389/fimmu.2022.963819    PMID: 35967333
33. Sun Y, Li H, Ma Z, Wang J, Yang H, Zhang X, et al. Identification of immune subtypes and their prognosis and molecular implications in colorectal cancer. PLoS One. 2022;17:e0278114.    DOI: 10.1371/journal.pone.0278114    PMID: 36417424
34. Gardner IH, Siddharthan R, Watson K, Dewey E, Ruhl R, Khou S, et al. A distinct innate immune signature of early onset colorectal cancer. Immunohorizons. 2021;5:489–499.    DOI: 10.4049/immunohorizons.2000092    PMID: 34162701
35. Chan RH, Chen PC, Yeh YM, Lin BW, Yang K, Shen M-R, et al. The expression quantitative trait loci in immune response genes impact the characteristics and survival of colorectal cancer. Diagnostics (Basel). 2022;12:315.    DOI: 10.3390/diagnostics12020315    PMID: 35204406
36. Pentheroudakis G, Raptou G, Kotoula V, Wirtz RM, Vrettou E, Karavasilis V, et al. Immune response gene expression in colorectal cancer carries distinct prognostic implications according to tissue, stage and site: a prospective retrospective translational study in the context of a hellenic cooperative oncology group randomised trial. PLoS One. 2015;10:e0124612.    DOI: 10.1371/journal.pone.0124612    PMID: 25970543
37. Colombo F, Illescas O, Noci S, Minnai F, Pintarelli G, Pettinicchio A, et al. Gut microbiota composition in colorectal cancer patients is genetically regulated. Sci Rep. 2022;12:11424.    DOI: 10.1038/s41598-022-15230-6    PMID: 35794137
38. Wong SH, Yu J. Gut microbiota in colorectal cancer: mechanisms of action and clinical applications. Nat Rev Gastroenterol Hepatol. 2019;16:690–704.    DOI: 10.1038/s41575-019-0209-8    PMID: 31554963
39. Clay SL, Fonseca-Pereira D, Garrett WS. Colorectal cancer: the facts in the case of the microbiota. J Clin Invest. 2022;132:e155101.    DOI: 10.1172/JCI155101    PMID: 35166235
40. Cheng Y, Ling Z, Li L. The Intestinal Microbiota and Colorectal Cancer. Front Immunol. 2020;11:615056.    DOI: 10.3389/fimmu.2020.615056    PMID: 33329610
41. Pandey H, Tang DWT, Wong SH, Lal D. Gut microbiota in colorectal cancer: biological role and therapeutic opportunities. Cancers (Basel). 2023;15:866.    DOI: 10.3390/cancers15030866    PMID: 36765824
42. Chattopadhyay I, Dhar R, Pethusamy K, Seethy A, Srivastava T, Sah R, et al. Exploring the role of gut microbiome in colon cancer. Appl Biochem Biotechnol. 2021;193:1780–1799.    DOI: 10.1007/s12010-021-03498-9    PMID: 33492552
43. Kharrat N, Assidi M, Abu-Elmagd M, Pushparaj PN, Alkhaldy A, Arfaoui L, et al. Data mining analysis of human gut microbiota links Fusobacterium spp. with colorectal cancer onset. Bioinformation. 2019;15:372–379.    DOI: 10.6026/97320630015372    PMID: 31312073
44. Ranjbar M, Salehi R, Haghjooy Javanmard S, Rafiee L, Faraji H, Jafarpor S, et al. The dysbiosis signature of Fusobacterium nucleatum in colorectal cancer-cause or consequences? A systematic review. Cancer Cell Int. 2021;21:194.    DOI: 10.1186/s12935-021-01886-z    PMID: 33823861
45. Li J, Zhu Y, Yang L, Wang Z. Effect of gut microbiota in the colorectal cancer and potential target therapy. Discov Oncol. 2022;13:51.    DOI: 10.1007/s12672-022-00517-x    PMID: 35749000
46. Allen J, Hao S, Sears CL, Timp W. Epigenetic Changes Induced by Bacteroides fragilis Toxin. Infect Immun. 2019;87:e00447-18.    DOI: 10.1128/IAI.00447-18    PMID: 30885929
47. Cheng WT, Kantilal HK, Davamani F. The Mechanism of Bacteroides fragilis Toxin Contributes to Colon Cancer Formation. Malays J Med Sci. 2020;27:9–21.    DOI: 10.21315/mjms2020.27.4.2    PMID: 32863742
48. De Almeida CV, Lulli M, di Pilato V, Schiavone N, Russo E, Nannini G, et al. Differential responses of colorectal cancer cell lines to Enterococcus faecalis’ strains isolated from healthy donors and colorectal cancer patients. J Clin Med. 2019;8:388.    DOI: 10.3390/jcm8030388    PMID: 30897751
49. de Almeida CV, Taddei A, Amedei A. The controversial role of Enterococcus faecalis in colorectal cancer. Therap Adv Gastroenterol. 2018;11:1756284818783606. .    DOI: 10.1177/1756284818783606    PMID: 30013618
50. Pleguezuelos-Manzano C, Puschhof J, Rosendahl Huber A, van Hoeck A, Wood HM, Nomburg J, et al. Mutational signature in colorectal cancer caused by genotoxic pks+ E. coli. Nature. 2020;580:269–273.    DOI: 10.1038/s41586-020-2080-8    PMID: 32106218
51. Wilson MR, Jiang Y, Villalta PW, Stornetta A, Boudreau PD, Carrá A, et al. The human gut bacterial genotoxin colibactin alkylates DNA. Science. 2019;363:eaar7785.    DOI: 10.1126/science.aar7785    PMID: 30765538
52. Iyadorai T, Mariappan V, Vellasamy KM, Wanyiri JW, Roslani AC, Lee GK, et al. Prevalence and association of pks+ Escherichia coli with colorectal cancer in patients at the University Malaya Medical Centre, Malaysia. PLoS One. 2020;15:e0228217.    DOI: 10.1371/journal.pone.0228217    PMID: 31990962
53. Ralser A, Dietl A, Jarosch S, Engelsberger V, Wanisch A, Janssen KP, et al. Helicobacter pylori promotes colorectal carcinogenesis by deregulating intestinal immunity and inducing a mucus-degrading microbiota signature. Gut. 2023;72:1258–1270.    DOI: 10.1136/gutjnl-2022-328075    PMID: 37015754
54. Zuo Y, Jing Z, Bie M, Xu C, Hao X, Wang B. Association between Helicobacter pylori infection and the risk of colorectal cancer. Medicine (Baltimore). 2020;99:e21832.    DOI: 10.1097/MD.0000000000021832    PMID: 32925719
55. Abu-Ghazaleh N, Chua WJ, Gopalan V. Intestinal microbiota and its association with colon cancer and red/processed meat consumption. J Gastroenterol Hepatol. 2021;36:75–88.    DOI: 10.1111/jgh.15042    PMID: 32198788
56. Vacante M, Ciuni R, Basile F, Biondi A. Gut microbiota and colorectal cancer development: a closer look to the adenoma-carcinoma sequence. Biomedicines. 2020;8:489.    DOI: 10.3390/biomedicines8110489    PMID: 33182693
57. Zhao W, Jin L, Chen P, Li D, Gao W, Dong G. Colorectal cancer immunotherapy-Recent progress and future directions. Cancer Lett. 2022;545:215816.    DOI: 10.1016/j.canlet.2022.215816    PMID: 35810989
58. Xie YH, Chen YX, Fang JY. Comprehensive review of targeted therapy for colorectal cancer. Signal Transduct Target Ther. 2020;5:22.    DOI: 10.1038/s41392-020-0116-z    PMID: 32296018
59. Wei SC, Duffy CR, Allison JP. Fundamental mechanisms of immune checkpoint blockade therapy. Cancer Discov. 2018;8:1069–1086.    DOI: 10.1158/2159-8290.CD-18-0367    PMID: 30115704
60. Weiner LM. Cancer immunology for the clinician. Clin Adv Hematol Oncol. 2015;13:299–306.    PMID: 26352774
61. Golshani G, Zhang Y. Advances in immunotherapy for colorectal cancer: a review. Therap Adv Gastroenterol. 2020;13:1756284820917527.    DOI: 10.1177/1756284820917527    PMID: 32536977
62. Overman MJ, McDermott R, Leach JL, Lonardi S, Lenz HJ, Morse MA, et al. Nivolumab in patients with metastatic DNA mismatch repair-deficient or microsatellite instability-high colorectal cancer (CheckMate 142): an open-label, multicentre, phase 2 study. Lancet Oncol. 2017;18:1182–1191.    DOI: 10.1016/S1470-2045(17)30422-9    PMID: 28734759
63. Endo E, Okayama H, Saito K, Nakajima S, Yamada L, Ujiie D, et al. A TGFβ-Dependent Stromal Subset Underlies Immune Checkpoint Inhibitor Efficacy in DNA Mismatch Repair-Deficient/Microsatellite Instability-High Colorectal Cancer. Molecular cancer research : MCR [Internet]. 2020 [cited 2023 Jul 21];18.    DOI: 10.1158/1541-7786.MCR-20-0308    PMID: 32493700
64. Overman MJ, Lonardi S, Wong KYM, Lenz HJ, Gelsomino F, et al. Durable clinical benefit with nivolumab plus ipilimumab in DNA mismatch repair-deficient/microsatellite instability-high metastatic colorectal cancer. J Clin Oncol. 2018;36:773–779.    DOI: 10.1200/JCO.2017.76.9901    PMID: 29355075
65. Lenz HJ, Van Cutsem E, Luisa Limon M, Wong KYM, Hendlisz A, Aglietta M, et al. First-line nivolumab plus low-dose ipilimumab for microsatellite instability-high/mismatch repair-deficient metastatic colorectal cancer: the phase II checkmate 142 study. J Clin Oncol. 2022;40:161–170.    DOI: 10.1200/JCO.21.01015    PMID: 34637336
66. Fan A, Wang B, Wang X, Nie Y, Fan D, Zhao X, et al. Immunotherapy in colorectal cancer: current achievements and future perspective. Int J Biol Sci. 2021;17:3837–3849.    DOI: 10.7150/ijbs.64077    PMID: 34671202
67. Rosenberg SA, Restifo NP. Adoptive cell transfer as personalized immunotherapy for human cancer. Science. 2015;348:62–68.    DOI: 10.1126/science.aaa4967    PMID: 25838374
68. Xu X, Lv J, Guo F, Li J, Jia Y, Jiang D, et al. Gut Microbiome influences the efficacy of PD-1 antibody immunotherapy on MSS-type colorectal cancer via metabolic pathway. Front Microbiol. 2020;11:814.    DOI: 10.3389/fmicb.2020.00814    PMID: 32425919
69. McDermott AJ, Huffnagle GB. The microbiome and regulation of mucosal immunity. Immunology. 2014;142:24–31.    DOI: 10.1111/imm.12231    PMID: 24329495
70. Shi N, Li N, Duan X, Niu H. Interaction between the gut microbiome and mucosal immune system. Mil Med Res. 2017;4:14.    DOI: 10.1186/s40779-017-0122-9    PMID: 28465831
71. Everts B. Metabolomics in immunology research. Methods Mol Biol. 2018;1730:29–42.    DOI: 10.1007/978-1-4939-7592-1_2    PMID: 29363063
72. Zenewicz LA, Yancopoulos GD, Valenzuela DM, Murphy AJ, Stevens S, Flavell RA. Innate and adaptive interleukin-22 protects mice from inflammatory bowel disease. Immunity. 2008;29:947–957.    DOI: 10.1016/j.immuni.2008.11.003    PMID: 19100701
73. Sonnenberg GF, Monticelli LA, Elloso MM, Fouser LA, Artis D. CD4(+) lymphoid tissue-inducer cells promote innate immunity in the gut. Immunity. 2011;34:122–134.    DOI: 10.1016/j.immuni.2010.12.009    PMID: 21194981
74. Pabst O. New concepts in the generation and functions of IgA. Nat Rev Immunol. 2012;12:821–832.    DOI: 10.1038/nri3322    PMID: 23103985
75. Lui JB, Devarajan P, Teplicki SA, Chen Z. Cross-differentiation from the CD8 lineage to CD4 T cells in the gut-associated microenvironment with a nonessential role of microbiota. Cell Rep. 2015;10:574–585.    DOI: 10.1016/j.celrep.2014.12.053    PMID: 25640181
76. Lin Y, Kong DX, Zhang YN. Does the microbiota composition influence the efficacy of colorectal cancer immunotherapy? Front Oncol. 2022;12:852194.    DOI: 10.3389/fonc.2022.852194    PMID: 35463305
77. Frank DN, St. Amand AL, Feldman RA, Boedeker EC, Harpaz N, Pace NR. Molecular-phylogenetic characterization of microbial community imbalances in human inflammatory bowel diseases. Proc Natl Acad Sci USA. 2007;104:13780–13785.    DOI: 10.1073/pnas.0706625104    PMID: 17699621
78. Purcell RV, Visnovska M, Biggs PJ, Schmeier S, Frizelle FA. Distinct gut microbiome patterns associate with consensus molecular subtypes of colorectal cancer. Sci Rep. 2017;7:11590.    DOI: 10.1038/s41598-017-11237-6    PMID: 28912574
79. Nejman D, Livyatan I, Fuks G, Gavert N, Zwang Y, Geller LT, et al. The human tumor microbiome is composed of tumor type-specific intracellular bacteria. Science. 2020;368:973–980.    DOI: 10.1126/science.aay9189    PMID: 32467386
80. Vétizou M, Pitt JM, Daillère R, Lepage P, Waldschmitt N, Flament C, et al. Anticancer immunotherapy by CTLA-4 blockade relies on the gut microbiota. Science. 2015;350:1079–1084.    DOI: 10.1126/science.aad1329    PMID: 26541610
81. Turnbaugh PJ, Ley RE, Mahowald MA, Magrini V, Mardis ER, Gordon JI. An obesity-associated gut microbiome with increased capacity for energy harvest. Nature. 2006;444:1027–1031.    DOI: 10.1038/nature05414    PMID: 17183312
82. Routy B, Le Chatelier E, Derosa L, Duong CPM, Alou MT, Daillère R, et al. Gut microbiome influences efficacy of PD-1-based immunotherapy against epithelial tumors. Science. 2018;359:91–97.    DOI: 10.1126/science.aan3706    PMID: 29097494
83. Sivan A, Corrales L, Hubert N, Williams JB, Aquino-Michaels K, Earley ZM, et al. Commensal Bifidobacterium promotes antitumor immunity and facilitates anti–PD-L1 efficacy. Science. 2015;350:1084–1089.    DOI: 10.1126/science.aac4255    PMID: 26541606
84. Colombo BM, Scalvenzi T, Benlamara S, Pollet N. Microbiota and mucosal immunity in amphibians. Front Immunol. 2015;6:111.    DOI: 10.3389/fimmu.2015.00111    PMID: 25821449
85. Round JL, Lee SM, Li J, Tran G, Jabri B, Chatila TA, Mazmanian SK. The Toll-like receptor 2 pathway establishes colonization by a commensal of the human microbiota. Science. 2011;332:974–977.    DOI: 10.1126/science.1206095    PMID: 21512004
86. Round JL, Mazmanian SK. Inducible Foxp3+ regulatory T-cell development by a commensal bacterium of the intestinal microbiota. Proc Natl Acad Sci USA. 2010;107:12204–12209.    DOI: 10.1073/pnas.0909122107    PMID: 20566854
87. Zhuo Q, Yu B, Zhou J, Zhang J, Zhang R, Xie J, et al. Lysates of Lactobacillus acidophilus combined with CTLA-4-blocking antibodies enhance antitumor immunity in a mouse colon cancer model. Sci Rep. 2019;9:20128.    DOI: 10.1038/s41598-019-56661-y    PMID: 31882868
88. Van Coillie S, Wiernicki B, Xu J. Molecular and Cellular Functions of CTLA-4. Adv Exp Med Biol. 2020;1248:7–32. .    DOI: 10.1007/978-981-15-3266-5_2    PMID: 32185705
89. Postow MA, Callahan MK, Wolchok JD. Immune Checkpoint Blockade in Cancer Therapy. J Clin Oncol. 2015;33:1974–1982.    DOI: 10.1200/JCO.2014.59.4358    PMID: 25605845
90. Leach DR, Krummel MF, Allison JP. Enhancement of antitumor immunity by CTLA-4 blockade. Science. 1996;271:1734–1736.    DOI: 10.1126/science.271.5256.1734    PMID: 8596936
91. Maker AV, Phan GQ, Attia P, Yang JC, Sherry RM, Topalian SL, et al. Tumor regression and autoimmunity in patients treated with cytotoxic T lymphocyte-associated antigen 4 blockade and interleukin 2: a phase I/II study. Ann Surg Oncol. 2005;12:1005–1016.    DOI: 10.1245/ASO.2005.03.536    PMID: 16283570
92. Hodi FS, O’Day SJ, McDermott DF, Weber RW, Sosman JA, Haanen JB, et al. Improved survival with ipilimumab in patients with metastatic melanoma. N Engl J Med. 2010;363:711–723.    DOI: 10.1056/NEJMoa1003466    PMID: 20525992
93. Boukouris AE, Theochari M, Stefanou D, Papalambros A, Felekouras E, Gogas H, et al. Latest evidence on immune checkpoint inhibitors in metastatic colorectal cancer: A 2022 update. Crit Rev Oncol Hematol. 2022;173:103663.    DOI: 10.1016/j.critrevonc.2022.103663    PMID: 35351582
94. Gopalakrishnan V, Spencer CN, Nezi L, Reuben A, Andrews MC, Karpinets TV, et al. Gut microbiome modulates response to anti-PD-1 immunotherapy in melanoma patients. Science. 2018;359:97–103.    DOI: 10.1126/science.aan4236    PMID: 29097493
95. Gori S, Inno A, Belluomini L, Bocus P, Bisoffi Z, Russo A, et al. Gut microbiota and cancer: How gut microbiota modulates activity, efficacy and toxicity of antitumoral therapy. Crit Rev Oncol Hematol. 2019;143:139–147.    DOI: 10.1016/j.critrevonc.2019.09.003    PMID: 31634731
96. Miller PL, Carson TL. Mechanisms and microbial influences on CTLA-4 and PD-1-based immunotherapy in the treatment of cancer: a narrative review. Gut Pathog. 2020;12:43.    DOI: 10.1186/s13099-020-00381-6    PMID: 32944086
97. Chen DS, Mellman I. Elements of cancer immunity and the cancer-immune set point. Nature. 2017;541:321–330.    DOI: 10.1038/nature21349    PMID: 28102259
98. Ganesh K, Stadler ZK, Cercek A, Mendelsohn RB, Shia J, Segal NH, Diaz LA. Immunotherapy in colorectal cancer: rationale, challenges and potential. Nat Rev Gastroenterol Hepatol. 2019;16:361–375.    DOI: 10.1038/s41575-019-0126-x    PMID: 30886395
99. Topalian SL, Hodi FS, Brahmer JR, Gettinger SN, Smith DC, McDermott DF, et al. Safety, activity, and immune correlates of anti-PD-1 antibody in cancer. N Engl J Med. 2012;366:2443–2454.    DOI: 10.1056/NEJMoa1200690    PMID: 22658127
100. Hudson K, Cross N, Jordan-Mahy N, Leyland R. The Extrinsic and Intrinsic Roles of PD-L1 and Its Receptor PD-1: Implications for Immunotherapy Treatment. Front Immunol. 2020;11:568931.    DOI: 10.3389/fimmu.2020.568931    PMID: 33193345
101. Matson V, Fessler J, Bao R, Chongsuwat T, Zha Y, Alegre ML, et al. The commensal microbiome is associated with anti-PD-1 efficacy in metastatic melanoma patients. Science. 2018;359:104–108.    DOI: 10.1126/science.aao3290    PMID: 29302014
102. Zhang X, Yang Z, An Y, Liu Y, Wei Q, Xu F, et al. Clinical benefits of PD-1/PD-L1 inhibitors in patients with metastatic colorectal cancer: a systematic review and meta-analysis. World J Surg Oncol. 2022;20:93.    DOI: 10.1186/s12957-022-02549-7    PMID: 35331250
103. Goc J, Lv M, Bessman NJ, Flamar AL, Sahota S, et al. Dysregulation of ILC3s unleashes progression and immunotherapy resistance in colon cancer. Cell. 2021;184:5015-5030.e16.    DOI: 10.1016/j.cell.2021.07.029    PMID: 34407392
104. Huang J, Zheng X, Kang W, Hao H, Mao Y, Zhang H, et al. Metagenomic and metabolomic analyses reveal synergistic effects of fecal microbiota transplantation and anti-PD-1 therapy on treating colorectal cancer. Front Immunol. 2022;13:874922.    DOI: 10.3389/fimmu.2022.874922    PMID: 35911731
105. Zhang SL, Han B, Mao YQ, Zhang ZY, Li ZM, Kong CY, et al. Lacticaseibacillus paracasei sh2020 induced antitumor immunity and synergized with anti-programmed cell death 1 to reduce tumor burden in mice. Gut Microbes. 2022;14:2046246.    DOI: 10.1080/19490976.2022.2046246    PMID: 35259052
106. Avuthu N, Guda C. Meta-analysis of altered gut microbiota reveals microbial and metabolic biomarkers for colorectal cancer. Microbiol Spectr. 2022;10:e0001322.    DOI: 10.1128/spectrum.00013-22    PMID: 35766483
107. Gethings-Behncke C, Coleman HG, Jordao HWT, Longley DB, Crawford N, Murray LJ, et al. Fusobacterium nucleatum in the colorectum and its association with cancer risk and survival: a systematic review and meta-analysis. Cancer Epidemiol Biomarkers Prev. 2020;29:539–548.    DOI: 10.1158/1055-9965.EPI-18-1295    PMID: 31915144
108. Wu Y, Jiao N, Zhu R, Zhang Y, Wu D, Wang AJ, et al. Identification of microbial markers across populations in early detection of colorectal cancer. Nat Commun. 2021;12:3063.    DOI: 10.1038/s41467-021-23265-y    PMID: 34031391
109. Coker OO, Liu C, Wu WKK, Wong SH, Jia W, Sung JJY, et al. Altered gut metabolites and microbiota interactions are implicated in colorectal carcinogenesis and can be non-invasive diagnostic biomarkers. Microbiome. 2022;10:35.    DOI: 10.1186/s40168-021-01208-5    PMID: 35189961
110. Schwartz S, Edden Y, Orkin B, Erlichman M. Perforated peptic ulcer in an adolescent girl. Pediatr Emerg Care. 2012;28:709–711.    DOI: 10.1097/PEC.0b013e31825d21c3    PMID: 22766592
111. Huang XZ, Gao P, Song YX, Xu Y, Sun JX, Chen XW, et al. Antibiotic use and the efficacy of immune checkpoint inhibitors in cancer patients: a pooled analysis of 2740 cancer patients. Oncoimmunology. 2019;8:e1665973.    DOI: 10.1080/2162402X.2019.1665973    PMID: 31741763
112. Bullman S, Pedamallu CS, Sicinska E, Clancy TE, Zhang X, Cai D, et al. Analysis of Fusobacterium persistence and antibiotic response in colorectal cancer. Science. 2017;358:1443–1448.    DOI: 10.1126/science.aal5240    PMID: 29170280
113. Khan MAW, Ologun G, Arora R, McQuade JL, Wargo JA. Gut microbiome modulates response to cancer immunotherapy. Dig Dis Sci. 2020;65:885–896.    DOI: 10.1007/s10620-020-06111-x    PMID: 32067144
114. Wang JW, Kuo CH, Kuo FC, Wang YK, Hsu WH, Yu FJ, et al. Fecal microbiota transplantation: Review and update. J Formos Med Assoc. 2019;118 Suppl 1:S23–S31.    DOI: 10.1016/j.jfma.2018.08.011    PMID: 30181015
115. Zhang J, Wu K, Shi C, Li G. Cancer Immunotherapy: Fecal Microbiota Transplantation Brings Light. Curr Treat Options Oncol. 2022;23:1777–1792.    DOI: 10.1007/s11864-022-01027-2    PMID: 36279081
116. Routy B, Lenehan JG, Miller WH, Jamal R, Messaoudene M, Daisley BA, et al. Fecal microbiota transplantation plus anti-PD-1 immunotherapy in advanced melanoma: a phase I trial. Nat Med. 2023;.    DOI: 10.1038/s41591-023-02453-x    PMID: 37414899
117. Fong W, Li Q, Yu J. Gut microbiota modulation: a novel strategy for prevention and treatment of colorectal cancer. Oncogene. 2020;39:4925–4943.    DOI: 10.1038/s41388-020-1341-1    PMID: 32514151
118. Mahdavi M, Laforest-Lapointe I, Massé E. Preventing Colorectal Cancer through Prebiotics. Microorganisms. 2021;9:1325.    DOI: 10.3390/microorganisms9061325    PMID: 34207094
119. Ding S, Hu C, Fang J, Liu G. The Protective Role of Probiotics against Colorectal Cancer. Oxid Med Cell Longev. 2020;2020:8884583.    DOI: 10.1155/2020/8884583    PMID: 33488940
120. Zaharuddin L, Mokhtar NM, Muhammad Nawawi KN, Raja Ali RA. A randomized double-blind placebo-controlled trial of probiotics in post-surgical colorectal cancer. BMC Gastroenterol. 2019;19:131.    DOI: 10.1186/s12876-019-1047-4    PMID: 31340751
121. Kvakova M, Kamlarova A, Stofilova J, Benetinova V, Bertkova I. Probiotics and postbiotics in colorectal cancer: Prevention and complementary therapy. World J Gastroenterol. 2022;28:3370–3382.    DOI: 10.3748/wjg.v28.i27.3370    PMID: 36158273
122. Pothuraju R, Chaudhary S, Rachagani S, Kaur S, Roy HK, Bouvet M, et al. Mucins, gut microbiota, and postbiotics role in colorectal cancer. Gut Microbes. 13:1974795.    DOI: 10.1080/19490976.2021.1974795    PMID: 34586012
123. Fratila TD, Ismaiel A, Dumitrascu DL. Microbiome modulation in the prevention and management of colorectal cancer: a systematic review of clinical interventions. Med Pharm Rep. 2023;96:131–145.    DOI: 10.15386/mpr-2526    PMID: 37197270
124. Rinninella E, Mele MC, Cintoni M, Raoul P, Ianiro G, Salerno L, et al. The facts about food after cancer diagnosis: a systematic review of prospective cohort studies. Nutrients. 2020;12:2345.    DOI: 10.3390/nu12082345    PMID: 32764484
125. Illescas O, Rodríguez-Sosa M, Gariboldi M. Mediterranean diet to prevent the development of colon diseases: a meta-analysis of gut microbiota studies. Nutrients. 2021;13:2234.    DOI: 10.3390/nu13072234    PMID: 34209683