This comprehensive review explores the multifaceted relationship between neoadjuvant immunotherapy, intestinal microbiota alterations, and anastomotic leakage risk in patients undergoing colectomy for colon neoplasia. With the advent of immunotherapy, remarkably immune checkpoint inhibitors, there has been a significant shift in the treatment paradigms for various cancers, including colon neoplasia. These treatments, while effective, have been associated with changes in the intestinal microbiome, which, in turn, may influence wound healing and the integrity of anastomotic sites. The review delves into the complex interactions between the immune system and the gut microbiota, examining how immunotherapy-induced dysbiosis could disrupt the delicate balance necessary for optimal anastomotic healing. It discusses the role of specific bacterial species in modulating the immune response to cancer and their impact on the efficacy of immunotherapeutic agents. Moreover, the review highlights the potential mechanisms through which the microbiota-immune system interaction could affect surgical outcomes, focusing on the development of anastomotic leaks. The implications of these findings for clinical practice are discussed, including the need for targeted strategies to modulate the gut microbiota in patients undergoing immunotherapy and colectomy. Through this discussion, the review aims to provide insights into improving patient outcomes by integrating microbiome management into colon neoplasia patients' treatment and perioperative care.

The integration of neoadjuvant therapies, particularly immunotherapy, into the treatment paradigm for colon neoplasia represents a pivotal advancement, offering significant improvements in patient outcomes [1-3]. 

Immunotherapy, by modulating the immune system's ability to recognize and eliminate cancer cells, introduces a novel approach for managing tumors characterized by specific molecular profiles, such as mismatch repair deficiency (dMMR) or high microsatellite instability (MSI-H) [4-6]. 

Despite the promising benefits of these therapies, their implications on postoperative complications, especially anastomotic leakage following colectomy, necessitate comprehensive investigation [7]. 

Anastomotic leakage is a severe complication that critically endangers patient recovery and oncological outcomes, influenced by an array of factors including patient health, tumor biology, surgical technique, and significantly, the intestinal microbiota [8-10].

Recent insights highlight the crucial role of the gut microbiome in modulating the host immune response, particularly within the context of cancer therapy. The intestinal microbiota maintains gut homeostasis, regulates immune function, and influences the efficacy and toxicity of immunotherapeutic agents [11,12]. 

Immunotherapy-induced alterations in the microbiota composition may disrupt this balance, potentially impairing wound healing, and anastomotic integrity. The underlying mechanisms involve intricate interactions among microbial-derived metabolites, immune cells, and intestinal epithelial cells [13-15].

The gut microbiota influences systemic and local immunity through various mechanisms. Short-chain fatty acids (SCFAs) produced Neoadjuvant immunotherapy and intestinal microbiota play important roles in anastomotic leakage and fistula formation after colorectal cancer surgery. Immunotherapy alters the host's immune response, particularly in the context of cancer therapy [16-18]. 

The intestinal microbiota maintains intestinal homeostasis, regulates immune function, and influences the efficacy and toxicity of immunotherapeutic agents. Changes induced by immunotherapy in the composition of the microbiota can disrupt this equilibrium, potentially impairing wound healing, and anastomotic integrity [19,20]. The underlying mechanisms involve intricate interactions between microbial-derived metabolites, immune system cells, and intestinal epithelial cells [21].

Intestinal microbiota influences systemic and local immunity through various mechanisms. Short-chain fatty acids (SCFAs), produced by the fermentation of dietary fibers by gut bacteria, enhance the regulatory functions of T cells (Treg), promote the production of anti-inflammatory cytokines, and strengthen the intestinal barrier. Immunotherapy may alter the abundance of SCFA-producing bacteria, potentially affecting these protective mechanisms and impacting wound healing [22-24]. 

Moreover, specific bacterial species can modulate the efficacy of immune checkpoint inhibitors, affecting antigen presentation and the activation of cytotoxic T lymphocytes against tumor cells. This interaction between the microbiota and the immune system is crucial for therapeutic response and potential side effects, including impacts on surgical outcomes. by the fermentation of dietary fibers by gut bacteria, enhance regulatory T cell (Treg) functions, promote anti-inflammatory cytokine production, and strengthen the gut barrier [25-28]. 

Immunotherapy may alter the abundance of SCFA-producing bacteria, potentially affecting these protective mechanisms and impacting wound healing [29]. Moreover, specific bacterial species can modulate the efficacy of immune checkpoint inhibitors by affecting antigen presentation and the activation of cytotoxic T lymphocytes against tumor cells. This interplay between the microbiota and the immune system is crucial for the therapeutic response and potential side effects, including impacts on surgical outcomes [30-32].

The interaction between the gut microbiota and the efficacy of immune checkpoint inhibitors (ICIs), as well as their influence on wound healing and anastomotic integrity, has become an area of intense research focus [33]. The modulation of gut microbiota not only affects the therapeutic efficacy of ICIs but also plays a significant role in various physiological processes, including the healing of wounds and maintenance of anastomotic integrity. Here's an overview based on current scientific understanding:

Specific Bacterial Species Modulating the Efficacy of Immune Checkpoint Inhibitors

Bacteroides Fragilis

Research has shown that Bacteroides fragilis can enhance the anticancer effects of CTLA-4 blockade, likely through the modulation of dendritic cell function and promotion of a favorable T cell response in the tumor microenvironment34,35.

Bifidobacterium

Studies have demonstrated that the presence of Bifidobacterium in the gut microbiota can improve the response to PD-L1 blockade by enhancing dendritic cell function and promoting T cell activation and proliferation36.

Akkermansia muciniphila

This bacterium has been associated with improved efficacy of PD-1 blockade in both preclinical and clinical settings, potentially by enhancing mucosal immunity and promoting the infiltration of effector T cells into the tumor microenvironment37.

Modulation of Gut Microbiota and Its Effects on Wound Healing and Anastomotic Integrity

Immune Modulation

The gut microbiota can influence systemic and local immune responses, affecting the balance between pro-inflammatory and anti-inflammatory signals crucial for wound healing. An imbalance could lead to either impaired healing or excessive inflammation, contributing to anastomotic leakage38-40.

Barrier Function

A healthy microbiota supports the integrity of the gut barrier, which can prevent the translocation of bacteria and reduce the risk of infection at surgical sites. Dysbiosis can weaken this barrier and compromise post-surgical recovery41.

Production of Metabolites

Short-chain fatty acids (SCFAs) produced by gut bacteria are essential for maintaining gut health and promoting healing. They can enhance the formation of collagen, promote angiogenesis, and regulate the immune response to foster a conducive healing environment42.

Clinical Implications of the Interaction Between the Gut Microbiota and the Immune System in Cancer Therapy

Personalized Medicine

Understanding the relationship between the gut microbiota and the efficacy of ICIs could lead to personalized medicine approaches, where microbiota modulation strategies (e.g., probiotics, prebiotics, fecal microbiota transplantation) are used to enhance therapeutic outcomes43,44.

Biomarker Development

The composition of the gut microbiota could serve as a biomarker to predict patient response to ICIs, allowing for more tailored treatment plans and potentially avoiding ineffective treatments45.

Management of Side Effects

Modulating the gut microbiota may also offer a strategy to manage or mitigate the immune-related adverse effects (irAEs) associated with ICIs, improving patient quality of life and treatment adherence46.

Enhancing Surgical Outcomes

For patients undergoing cancer surgery, strategies to optimize the gut microbiota before and after surgery could enhance wound healing, reduce the risk of anastomotic leakage, and potentially improve overall surgical outcomes47.

Immunotherapy, particularly with immune checkpoint inhibitors such as anti-PD-1/PD-L1 and anti-CTLA-4, primarily reactivates the immune system to recognize and attack tumor cells31-33. While these agents can lead to a hyperactive immune system in some cases, causing inflammation and a range of immune-mediated side effects, the concept of immunosuppression as a direct effect of immunotherapy is not accurate28. The common side effects of immunotherapy reflect an exacerbated immune response, including dermatitis, colitis, and pneumonitis12.

However, the healing process, especially concerning intestinal anastomoses, is complex and depends on a careful balance of pro-inflammatory and anti-inflammatory factors, cellular proliferation and migration, matrix deposition, and tissue remodeling48. 

A hyperactive immune system could potentially unbalance this process, increasing the risk of anastomotic dehiscence through mechanisms such as excessive inflammation and tissue damage19-21. 

Additionally, immunotherapy-induced alterations in the intestinal microbiota may influence anastomotic healing. A state of dysbiosis could lead to an unregulated inflammatory response at the anastomosis site, potentially compromising integrity and favoring the development of fistulas14-16.

Although immunotherapy does not cause immunosuppression in the traditional sense, its complex interactions with the immune system and the intestinal microbiota could theoretically influence the risk of postoperative complications, including intestinal fistulas39-42. Further research is required to elucidate these mechanisms and develop strategies to mitigate such risks in patients undergoing colectomy after neoadjuvant therapy for colon cancer24.

The primary objective of this review article is to explore the relationship between neoadjuvant immunotherapy, alterations in the intestinal microbiota, and the risk of anastomotic leakage in patients undergoing colectomy for colon neoplasia11. 

Through an in-depth analysis of the molecular mechanisms by which the gut microbiome influences immune responses in the context of immunotherapy and its subsequent effects on anastomotic healing, we aim to provide valuable insights into optimizing treatment protocols, surgical planning, and perioperative management to mitigate the risk of this severe complication in the era of cancer immunotherapy6-8.

The research methodology involved a comprehensive search of multiple reputable databases to ensure the inclusion of relevant studies while minimizing the risk of bias. PubMed, Scopus, Scielo, Embase, and Web of Science were chosen due to their comprehensive coverage of peer-reviewed literature in the medical field. Additionally, Google Scholar was utilized to access gray literature, which often includes valuable insights not found in traditional peer-reviewed articles. The study's selection criteria were centered on the focus: Neoadjuvant Immunotherapy, Intestinal Microbiota, Anastomotic Leakage, and Fistula after Colorectal Cancer Surgery. To refine the search and capture relevant studies, a combination of keywords was used, including "Colonic Neoplasms,” “Neoadjuvant Therapy," "Immunotherapy," "Postoperative Complications," " Intestinal Fistula," and " Surgical Oncology." This approach ensured that the selected studies were directly related to the topic of interest. The inclusion criteria encompassed various studies, such as systematic reviews, case-control studies, cross-sectional studies, case series, review articles, and editorial studies. This broad inclusion criteria aimed to gather a comprehensive range of evidence and perspectives on the subject matter. The process of analysis, review, and selection of materials was conducted rigorously to maintain the quality and relevance of the chosen studies. It involved a systematic and blinded approach, with pairs of reviewers independently assessing the title and abstract of each study. In cases of disagreement between the two reviewers, a third reviewer was involved to reach a consensus and ensure the final selection of studies was based on well-founded criteria. This meticulous research methodology guarantees that the findings and conclusions drawn in the article are rooted in a robust and diverse body of evidence, enhancing the credibility and reliability of the study's outcomes.

Delving deeper into the complexities of the interaction between neoadjuvant immunotherapy, the gut microbiota, and surgical outcomes in colorectal cancer surgery, we uncover a rich tapestry of biological interactions and potential clinical strategies48,49. This intricate interplay not only shapes the therapeutic efficacy of immunotherapy but also fundamentally influences the processes of wound healing and tissue regeneration critical to surgical success50.

The burgeoning field of immuno-oncology has illuminated the intricate interplay between the gut microbiota, immune checkpoint inhibitors (ICIs), and their collective impact on surgical outcomes, particularly in colorectal cancer surgery. This discussion unravels the multifaceted relationships and underlying mechanisms, drawing upon recent scientific advances and clinical observations51-53.

The emergence of ICIs targeting CTLA-4, PD-1, and PD-L1 has revolutionized cancer therapy, offering hope where conventional treatments have failed. However, the variability in patient responses underscores a complex interplay between the host's immune system and the gut microbiome36-38. 

At the heart of this interaction are the molecular and immunological mechanisms through which the gut microbiota influences the host's immune response to cancer and immunotherapy. The microbiota acts through various pathways to modulate systemic immunity, including the activation of pattern recognition receptors (PRRs) on immune cells, which detect microbial-associated molecular patterns (MAMPs)54-56. 

This interaction can lead to the maturation and activation of dendritic cells, which play a pivotal role in antigen presentation and the subsequent activation of T cells. The presence of specific microbial species can thus enhance the body’s immune response to tumors, potentially increasing the efficacy of ICIs57,58.

Conversely, the gut microbiota's role in regulating immune homeostasis and inflammation is paramount in the context of wound healing and anastomotic integrity. The balance between pro-inflammatory and anti-inflammatory signals, crucial for proper wound healing, can be tipped by alterations in the microbiome composition15,42-44. 

For example, an overabundance of pro-inflammatory bacterial species may exacerbate local and systemic inflammation, impeding the healing process and increasing the risk of anastomotic leakage. On the other hand, a healthy microbiome, rich in SCFA-producing bacteria, can promote an anti-inflammatory milieu, supporting tissue repair and integrity33-35.

Studies have identified specific bacterial species, such as Bacteroides fragilis, Bifidobacterium, and Akkermansia muciniphila, as pivotal in modulating the therapeutic efficacy of ICIs. These bacteria influence the tumor microenvironment by enhancing dendritic cell function, promoting T cell activation, and facilitating the infiltration of effector T cells, thereby increasing the anticancer immune response59-61.

Beyond the modulation of therapeutic efficacy, the gut microbiota plays a critical role in wound healing and the integrity of surgical anastomoses62. The physiological healing process is delicately balanced by pro-inflammatory and anti-inflammatory cues, cellular proliferation, and tissue remodeling—all significantly influenced by microbial metabolites and the immune system63. 

Short-chain fatty acids (SCFAs) by gut bacteria is essential for maintaining gut barrier function and regulating immune responses conducive to healing. Disruptions in the microbial composition, or dysbiosis, can lead to an imbalance in these healing processes, potentially culminating in anastomotic leakage—a dreaded complication with profound clinical implications64-66.

The clinical ramifications of the interaction between the gut microbiota and the immune system extend into personalized medicine. The potential of leveraging the microbiome to predict responses to ICIs opens new views for tailored cancer therapy, reducing the trial-and-error approach currently prevalent18-22.

The potential clinical implications of these findings are vast. Firstly, understanding the role of specific bacteria in modulating immune responses to ICIs could lead to the development of microbial-based biomarkers for predicting treatment outcomes3. Such biomarkers could significantly refine patient selection for immunotherapy, ensuring that only those likely to benefit are exposed to these potent drugs and their associated risks67.

Moreover, the prospect of microbiota modulation as an adjunct to cancer therapy offers exciting possibilities. Strategies such as dietary modifications, probiotics, prebiotics, and even fecal microbiota transplantation (FMT) could be employed to enhance the gut microbiome's beneficial effects on immune function and wound healing58-60.

However, these interventions must be approached with caution, as the gut microbiome's complexity and variability among individuals mean that effects can be unpredictable. Rigorous clinical trials are needed to ascertain the safety, efficacy, and optimal protocols for such interventions21-23. 

Furthermore, microbiota modulation strategies, such as probiotics, prebiotics, and fecal microbiota transplantation, present a promising adjunct to enhance ICI efficacy, manage immune-related adverse effects, and improve surgical outcomes5,6.

However, several challenges remain. The complexity of the gut microbiome and its variability among patients complicates the straightforward application of these insights2. Additionally, the mechanisms through which specific bacterial species influence immune responses and healing processes are not fully elucidated, requiring further research68.

In this sense, the nexus between neoadjuvant immunotherapy, gut microbiota, and surgical outcomes in colorectal cancer represents a fertile ground for future investigation. As we delve deeper into this interplay, the prospect of harnessing the gut microbiome to enhance cancer therapy and surgical care becomes increasingly tangible53,65.

As we venture further into this domain, several key areas require attention. Detailed mechanistic studies are needed to elucidate the exact pathways through which the gut microbiota influences immune responses to cancer and immunotherapy. Such knowledge could unlock novel therapeutic targets and strategies for modulating the immune system to improve cancer treatment outcomes27,69.

Additionally, the development of non-invasive methods for monitoring the gut microbiome in real-time could provide invaluable insights into the dynamic interplay between the microbiome, the immune system, and therapeutic agents58,70. This could lead to the development of real-time biomarkers for monitoring treatment efficacy and adjusting therapeutic strategies accordingly16.

Personalized medicine approaches, grounded in a profound understanding of the microbiome's impact on the immune system, could significantly alter the landscape of cancer treatment, offering more effective, tailored therapies with improved patient outcomes. While fraught with challenges, the journey from bench to bedside holds the promise of transforming the paradigm of cancer care in the era of immuno-oncology68-71.

In conclusion, the nexus of neoadjuvant immunotherapy, the gut microbiota, and surgical outcomes in colorectal cancer presents a complex but highly promising field of study. As we unravel the intricate web of interactions at play, the path toward more effective, personalized cancer therapy and improved surgical outcomes becomes increasingly clear. 

The integration of microbiome science into oncology and surgery heralds a new era of precision medicine, where treatments are tailored not only to the genetic makeup of the tumor but also to the unique microbiome of the individual, offering hope for more effective treatments and better patient outcomes in the future.

The authors thank the Federal University of Rio Grande do Norte, Potiguar University, and Liga Contra o Cancer for supporting this study.

The authors declare that there is no conflict of interest.