Targeting the Gut Microbiome to Improve Immunotherapy Outcomes: A Review

Abstract

The following narrative review embarks on a comprehensive exploration of the role played by the gut microbiome within the Diet-Microbiota-Immunity (DMI) tripartite, aiming to enhance anti-cancer immunotherapy efficacy. While revolutionizing cancer treatment, resistance to immunotherapy and immune-related adverse events (irAEs) remain challenges. The tumor microenvironment (TME), shaped by cancer cells, influences immunotherapy resistance. The gut microbiome, influenced by genetics, environment, diet, and interventions, emerges as a critical player in TME reshaping, thereby modulating immune responses and treatment outcomes. Dietary patterns like the Mediterranean diet, caloric restriction modifications, and specific nutritional components show promise in influencing the tumor microenvironment and gut microbiome for better treatment outcomes. Antibiotics, disrupting gut microbiota diversity, may compromise immunotherapy efficacy. This review emphasizes the need for tailored nutritional strategies to manipulate microbial communities, enhance immune regulation, and improve immunotherapy accessibility while minimizing side effects. Ongoing studies investigate the impact of dietary interventions on cancer immunotherapy, pointing toward promising developments in personalized cancer care. This narrative review synthesizes existing knowledge and charts a course for future investigations, presenting a holistic perspective on the dynamic interplay between dietary interventions, the gut microbiome, and cancer immunotherapy within the DMI tripartite.

Introduction

Immunotherapy has significantly advanced cancer treatment and is continually breaking new ground. Anticancer immunotherapy, which employs T lymphocyte-mediated antigen-directed cytotoxicity, is pivotal for engaging the immune system against cancer through approaches like adoptive cellular therapy, anti-cancer vaccines, and Immune Checkpoint Blockade (ICB).^1-3 However, initial resistance or eventually acquired resistance to immunotherapy is common. Moreover, the blocked natural immune checkpoint cascade can lead to powerful immune-related adverse events (irAEs) with varying severity (15%-90%), involving overactive memory T cells infiltrating organs and causing inflammation. Severe immune-related adverse events (irAEs), classified as grade 3 to 4 according to the Common Terminology Criteria for Adverse Events (CTCAE)—the widely accepted standard classification and severity grading scale for adverse events in cancer therapy—pose potential life-threatening risks, affecting 15% to 30% of patients undergoing anticancer immunotherapy. ² Currently, practical biomarkers serving as surrogates to predict treatment response are being examined in the context of irAEs, signaling the engagement of the immune system.

Resistance to immunotherapy is attributed to different pathways, including the presence of an immunosuppressive and immune-evasive tumor microenvironment (TME). In striving to support growth and proliferation, cancer cells’ high metabolic demands and their main dependency on glycolysis (Warburg effect) lead to nutrient deficiency and a hypoxic state. In response, immune and stromal cells release cytokines and chemokines and deposit an extracellular matrix that promotes cytotoxic T and NK cell exhaustion while supporting the proliferation of immunosuppressive immune cells, more suited for this environment. A key mechanism in cancer cells-induced immune evasion exploits the immune systems’ tight regulation by inhibitory checkpoints to avoid collateral damage and autoimmunity. For instance, An overexpression of PDL-1 on tumor cells and in the TME reduces the activation of PD-1 positive cells (ie, Activated T-cells, Natural-Killer [NK] cells, B cells, macrophages, Dendritic Cells [DCs]). In 2021, Verma et al demonstrated that post-ICB initiation, cancer cells’-induced absence or poor presentation of tumor antigenic proteins by antigen-presenting cells (APCs) may result in dysfunctional T cells (CD8 + PD-1 + CD38^hi phenotype).^4-7

Altogether, these findings justify further research into factors that could promote a balance between the immune system’s cytotoxic and regulatory elements, resulting in an effective treatment response while reducing irAEs.

The gut microbiome, consisting of bacteria, viruses, fungi, and protozoans, has recently gained recognition as a significant factor in immunotherapy. Increasingly recognized for its role in host health, the gut microbiome participates in host nutrition, metabolism, and physiology, protection against pathogens, and the development of the immune system, as well as producing vital vitamins (B and K) from otherwise undigestible molecules. Roughly 90% of all gut bacteria belong to the Firmicutes and Bacteroidetes phyla, while the remaining 10% are distributed among Verrucomicrobes, Proteobacteria, and Actinobacteria. ⁸ Gut dysbiosis, marked by changes in its composition and function, is linked to various diseases, including increased autoantibody production, leading to autoimmune disorders alongside other gastrointestinal issues, metabolic disorders, neurological conditions, and cancer development.^3,8

Emerging evidence indicates the manipulability of the gut microbiome. It becomes evident that distinct gut microbiota plays a crucial role in the immune response, tumor microenvironment (TME), and tumor characteristics that ultimately impact immunotherapy outcomes.^4,9-17 The complex interplay between the microbiome, host systems, and tumor immune surveillance is influenced by genetics, environment, diet, and various interventions like antibiotics, probiotics, and lifestyle changes. Although the precise nature of these interactions remains largely unexplored, these data emphasize the need to develop safe and viable strategies for modulating the microbiome in favor of immunotherapy.

Numerous studies have extensively investigated the impact of dietary patterns on gut microbiome composition and functionality, revealing rapid microbiome alterations with dietary changes. Short-term dietary interventions result in transient microbiome modifications, while long-term diets, especially those high in fermentable nutrients or animal proteins, demonstrate persistent changes in the gut microbiome, associating with distinct enterotypes, such as Bifidobacteria or Bacteroides and Clostridia, respectively.^3,8

Consequently, a deeper understanding of microbiome-host interactions and exploration of non-invasive microbiome modifications crucially underpin the development of strategies to enhance anticancer immunotherapy. In this review, we delve into the established mechanisms that explain how such influences on the gut microbiome—via factors like dietary regimens, specific dietary components, or antibiotics—may favorably affect anticancer immunotherapy.

The “Microbiome-Immunity” Axis

The bidirectional interplay between the gut microbiota and the host’s immune system maintains a delicate equilibrium for microbiota and guards against potential pathogens. Commensal gut bacteria shape the host immune response through two primary mechanisms. Firstly, they prompt cell differentiation in innate or adaptive immune cells, allowing them to function locally in the gut or at extraintestinal sites. This process involves the secretion of antimicrobial peptides and IgA into the gut lumen in response to dendritic cell (DC) activation by bacteria, microbial peptidoglycans, or other microbial metabolites (such as cyclic di-AMP, Inosine, Indole-3-Aldehide, and Tryptophan metabolites) ultimately restructuring the gut microbiota. DC migration to draining lymph nodes activates naive T cells, transforming them into effector T cells, including regulatory (Tregs), cytotoxic, and helper T cells (TH), which may return to the gut mucosa or enter the systemic circulation. Tregs foster an anti-inflammatory cytokine environment, while TH17 cells boost Paneth cell production of antimicrobial peptides and play a role in recruiting polymorphonuclear leukocytes (PMNs) from the bloodstream. Secondly, the concept of “molecular mimicry” comes into play, signifying a resemblance between the gut microbiome and tumor neoantigens.^13,14,18-24

While specific bacterial taxa linked to immunotherapy response are not yet well-established,^3,8,24 several clinical studies, including recent phase-1 trials involving fecal microbiota transplants (FMT) from the responder to non-responder patients to ICBs, have shown promising results. These studies correlate both microbiome taxa richness (α diversity) and specific taxa compositions (β diversity) with favorable changes in immune cell infiltrates and gene expression profiles, both in the gut lamina propria and the tumor microenvironment.^8,14,19,25 A 2021 systematic review found a correlation between an enriched abundance in the Firmicutes with both response to ICBs and incidence of AE. An abundance in the phylum Verrucomicrobia also demonstrated an almost universally better response from ICBs, whereas patients with enriched gut Proteobacteria presented unfavorable outcomes. The study found mixed correlations between Bacteroidetes-enriched microbiome and treatment response, while this phylum was correlated with reduced AE occurrence. ²⁶ Interestingly, a prospective study by Usyk et al ²⁵ found that while Bacteroides vulgatus was correlated with a lower risk of irAEs, Bacteroides dorei was correlated with a high risk of irAEs in ICB treatment for metastatic melanoma. These data align with pre-clinical data.

Given the variability observed in studies linking particular bacterial strains to positive responses in immunotherapy, metabolomics (the study of end products of microbial metabolism in feces and serum) combined with metagenomics (the study of structure and function of all genetic sequences isolated from microbial samples) are now widely used to further study the effect of microbiome on cancer immunotherapy, and promote the discovery of microbiota-linked biomarkers for response prediction of ICBs (eg, indole, aldehydes, and short-chain fatty acids), which could be a promising target for precision medicine.^27,28

The Diet-Microbiota-Immunity (DMI) Tripartite

A healthy diet not only influences cancer risk through associations with obesity, inflammation, and carcinogenesis but also shapes anti-cancer treatment outcomes. Plant-based diets demonstrate anti-carcinogenic effects by inhibiting carcinogens’ activation, angiogenesis, and inflammation, and promoting cell cycle and cell signaling regulation. This is in contrast to the detrimental impact of diets high in refined cereals, sugary beverages, alcohol, and red/processed meat. Cancer therapies frequently induce malnutrition and weight loss, jeopardizing treatment efficacy and quality of life. Thus, ensuring proper nutrition in cancer patients is imperative to prevent malnutrition and optimize therapy outcomes. ⁸ In this exploration, we delve into diverse dietary patterns showcasing a positive impact on the gut microbiota to bolster improved outcomes in anti-cancer immunotherapy.

Mediterranean Diet (MD)

MD is a highly recommended nutrition regimen characterized by increased plant fiber, vegetable-to-animal protein ratio, and a favorable polyunsaturated-to-saturated fat ratio. Studies show that saturated fatty acids, animal proteins, and certain vitamins affect inflammatory responses by activating TLR4, increasing trimethylamine N-oxide (TMAO) synthesis, resulting in gut epithelial barrier permeability, metabolic endotoxemia and inflammation. ⁸

Clinical research has demonstrated that adhering to the MD is associated with reduced systemic inflammation, heightened insulin sensitivity, and improved absorption of essential micronutrients (such as iron, vitamin D3, and folic acid). These positive effects align with the diverse functions of various gut bacteria, producing beneficial metabolites (such as Short-chain fatty acids—SCFAs) that can potentially enhance intestinal health, metabolism, and immunity.^8,29

SCFAs, stemming from dietary fiber fermentation, comprising acetate (C2), propionate (C3), and butyrate (C4), uphold gut epithelial balance, boost nutrient absorption, fortify antimicrobial defenses, and activate metabolic pathways, profoundly influencing mucosal and systemic immunity. Butyrate, in particular, promotes regulatory T-cell differentiation, generating anti-inflammatory interleukin-10 (IL-10), CTLA-4, and tumor growth factor-β (TGF-β). This immune regulation is mediated via interaction with G-protein coupled receptor 43 (GPR43) and inhibition of histone deacetylase production. Furthermore, dietary fibers influence the immune system through Bacteroidetes bacteria induced glycans. For example, polysaccharide A, produced by Bacteroides fragilis, acts as a toll-like receptor-2 (TLR2) ligand, fostering regulatory Tregs differentiation.^22,29-32 Indeed, the MD-induced adjustments in microbiota profiles exhibit elevated levels of fiber-degrading bacterial populations, including Bacteroidetes, Clostridium cluster XIVa, Faecalibacterium prausnitzii, Lactobacilli, and Bifidobacteria, or reduced levels of other Firmicutes and Proteobacteria.^21,29,33

Significantly, Bacteroides fragilis (mentioned before) and F. prausnitzii, both upregulated in some of the aforementioned studies, have been acknowledged for their ability to stimulate CD4+ T cells and the anti-inflammatory interleukin-10, thereby reducing inflammation.^29,34,35 These results were echoed in a recent study that evaluated the connections between microbiome clusters, response to ICB, and toxicity in 218 melanoma patients. In this study, responders’ taxa were enriched with F. prausnitzii, Butyricicoccus pullicaecorum, and Akkermansia muciniphila. ²¹ Previous studies have demonstrated that an increase in Akkermansia spp. is associated with metabolic improvements, including decreased liver triglyceride accumulation and alleviated intestinal inflammation. ³⁶

Recently, an observational study published in JAMA Oncology in 2023 revealed that among patients with advanced melanoma treated with ICB (n = 91), higher adherence to the MD was associated with an increased likelihood of treatment response (probability of .77 for ORR; P = .02; false discovery rate, .032; effective degrees of freedom, .83; probability of .74 for PFS-12; P = .01; false discovery rate, .021; effective degrees of freedom, 1.54). ³³

Dietary Interventions That Exploit the Metabolic Vulnerabilities of Cancer Cells

Reduced caloric intake, restricted meal timing, low-carbohydrate diets like the ketogenic diet, or even complete fasting share a disruptive metabolic effect on the organism’s nutrient availability and mediate a differential stress response. In stress conditions, normal cells prioritize repair and maintenance over growth and proliferation, while cancer cells lack this mechanism and thus become vulnerable to the combination of diet and cancer treatments.^37-40 Moreover, such unfavorable conditions dynamically shift the delicate balance between anti-cancer immunogenicity and immune evasion through a direct effect of starvation on the tumor microenvironment and post-starvation favorable gut microbiome changes.^6,37-47

Energy restriction (ER) diets

ER modalities, that is, short-term water fasting (STWF) diet, time-restricted eating (TRE, circadian-based), fasting mimicking diets (FMD), or the ketogenic diet (KD), have shown efficacy in reducing cancer development risk. They also exhibit positive effects on chemotherapy-induced AEs, including fatigue, weakness, gastrointestinal and hematological toxicity, ultimately contributing to an enhanced patients’ quality of life (QoL). ER’s impact on treatment outcomes has been substantiated in both preclinical and clinical studies of chemotherapy for cancer.^3,6,43,48,49 Despite the wealth of accumulated knowledge, ER modalities’ effects on immunotherapy treatments are at the beginning of exploration.

STWF consists of total food abstinence for 72 to 120 hours without limiting hydration; FMD consists of 5 to 6 days of caloric restriction that is low in protein and high in healthy fats, complex carbohydrates, and essential vitamins and minerals. KD is a high-fat, low-protein/carbohydrate diet. FMD mimics the STWF and KDs’ effects while avoiding potential side effects of lack of essential nutrients; TRE consists of 16 hours of fasting followed by 8 hours of Norma-caloric diet every 24 hours. Though TRE is the most feasible method to implement lack of nutrients stress during cancer treatments, STWF and FMD might be more effective.^3,6

Recent studies indicate that interventions involving STWF or FMD bring about the altered release of hormones and growth factors, decreased inflammation, and the restructuring of both tumor vasculature and extracellular matrix, influencing the differentiation and accumulation of immune cells, leading to the depletion of regulatory T cells (Tregs) and stimulation of cytotoxic cells. Post-ER positive metabolic changes—lower plasma glucose, insulin, and IGF-1, and increased ketones—correlate with a decline in peripheral monocytes and specific immunosuppressive cell types like Monocytic-derived suppressor cells (M-MDSCs), polymorphonuclear myeloid-derived suppressor cells (PMN-MDSCs), and Tregs.^43,45,46 Starvation-induced autophagy boosts the formation of tumoricidal M1 macrophages, enhancing antitumor immunity and promoting cytotoxic T cells, cytolytic NK cells, and activated/memory CD8+ cells . These systemic shifts align with beneficial metabolic and immunologic alterations in the TME. ⁶ ER-induced reductions in nutrient availability impede tumor survival, counteract tumor-driven nutrient diversion, and modify the typical TME conditions marked by high lactate and low pH. This shift prevents the exhaustion of cytotoxic T and NK cells while hindering the proliferation of immunosuppressive cells. ⁴³ Additionally, these TME changes increase the expression of MHC class II molecules in antigen-presenting cells (APCs). The decreased metabolic activity of tumor cells reduces the release of factors like adipokines (leptin, adiponectin), IL-6, CCL2, and CCL5, known contributors to a dense extracellular matrix that hinders T cell infiltration into the TME.^6,41-46,48 This positive impact extends beyond the cellular level, as alterations in the post-starvation gut microbiome further shape immune responses.

Both clinical trials and observational research on ER protocols, including Intermittent Fasting (IF), have uncovered striking similarities in microbiome and immune system characteristics compared to clinical trials involving Fecal Microbiota Transplantation (FMT). In the FMT trials, these shared features correlated with enhanced responses to ICBs in patients with advanced melanoma. Notable examples include an increase in gut microbiome α diversity and elevated levels of Firmicutes, particularly the butyric acid-producing Lachnospiraceae within the Clostridium cluster.^{3,14,16,19,48,50-52} Decreased levels of the potentially pathogenic Proteobacteria alongside increased levels of Akkermansia muciniphila were also demonstrated in some of these observational studies. ³

Currently, several ongoing studies are further investigating the effects of ER on cancer immunotherapy in melanoma, Non-Small Cell Lung Cancer (NSCC), and Triple-Negative Breast Cancer (TNBC). These studies are registered under the identifiers NCT04387084, NCT03700437, NCT05582538, and NCT05763992, respectively.

Ketogenic diets

The ketogenic diet involves restricting carbohydrates, which leads to lowered insulin levels and heightened cortisol levels. This, in turn, triggers the production of ketone bodies, enabling the body to shift its primary energy source from carbohydrates to fat, thus mimicking a state of fasting. ³ Ketone bodies can inhibit histone deacetylases (HDACs), and it is therefore hypothesized that ketosis may help slow tumor growth and promote differentiation via epigenetic chromatin modifications.^53,54 In a recent in vivo study, Weber et al ⁵⁵ demonstrated that ketogenic diets slow melanoma growth in vivo regardless of tumor genetics (mutations in BRAF, NRAS or wild type melanoma) and metabolic plasticity.

It is worth noting that the National Cancer Institute (NCI) recommends the ketogenic diet as a safe and feasible adjunct therapy for glioblastoma (GBM), and research has explored its therapeutic potential for other cancers. However, due to the challenges associated with adhering to a ketogenic diet, ongoing studies are investigating alternative ER-based diets, particularly the fasting mimicking diets (FMD), with the goal of achieving similar benefits. ⁵⁶

Specific Dietary Components

Specific food components, such as pro and pre-biotics, tryptophan and indole, urolithin A, vitamin A, vitamin D, and a balanced omega 6:3 ratio help maintain intestinal intra-epithelial homeostasis directly or through microbiota modulation. Vitamin D levels and the Omega 6:3 ratio, often disturbed in cancer patients, will be further discussed.^22,27

Pro and pre-biotics, high dietary fiber diet

Probiotics are defined as beneficial live microorganisms that can be consumed through commercially available supplements or fermented foods, such as kimchi, yogurt, kefir, pickled vegetables, and sauerkraut. ³ The most commonly used probiotics include Bifidobacterium, lactobacillus, and yeast. Probiotics exhibit immunomodulatory properties through direct actions such as boosting the activity of macrophages or NK cells, and influencing the secretion of immunoglobulins or cytokines, and indirect mechanisms like fortifying the gut epithelial barrier, modifying mucus secretion, and competitively excluding other (pathogenic) bacteria. ²³ In a study conducted by Dizman et al in 2021, it was demonstrated that supplementation with a Bifidobacterium-based probiotic yogurt could modulate the gut microbiome of patients with metastatic renal cell carcinoma who were receiving Vascular Endothelial Growth Factor Receptor Tyrosine Kinase Inhibitor (VEGF-TKI) therapy. The study found that Barnesiella intestinihominis, Akkermansia muciniphila, Bacteroides caccae, and F. prausnitzii were elevated in most samples from patients who experienced clinical benefits. Nevertheless, there was no statistical difference in clinical benefit between the probiotic and control groups (70%vs 80%, P = .606). ⁵⁷

On the other hand, antibiotic use, while controlling infections, disrupts gut microbiota diversity, possibly leading to gut dysbiosis, which may reduce immunotherapy efficacy. A 2020 meta-analysis by Wilson et al found a significant correlation between antibiotic use and decreased progression-free survival (PFS) and overall survival (OS) in 2889 cancer patients (most commonly lung (59%), renal cell carcinoma (RCC) or urothelial carcinoma (16.3%), and melanoma (18.7%)). Those without antibiotic exposure showed prolonged OS, especially when the exposure was 42 days prior to ICB initiation (HR 3.43, 95% CI 2.29-5.14, P < .0001). PFS was also longer in patients who did not receive antibiotics (pooled HR 1.65, 95% CI 1.3-2.1, P < .0001). Two observational studies later confirmed this. Xu and Ying’s ²³ study of 568 immunotherapy-receiving patients demonstrated a shorter median PFS in antibiotic-receiving patients (mPFS: 27.4 months vs 43.7 months). Nevertheless, as several studies found no apparent association or impact of antibiotic use on response to ICBs, it has been suggested that the timing and the duration of antibiotics intake alongside ICB treatment are important. Antibiotic exposure within 1 to 2 months before initiation of ICBs may negatively affect treatment response. ²³ This conflicting data warrants further investigation into antibiotics’ effect on immunotherapy treatments.

In a recent study published in Science in 2021, (n = 132) researchers investigated the correlations between higher dietary fiber intake and probiotic consumption and the response to ICBs in melanoma patients, hypothesizing that both dietary factors could influence the gut microbiome. The study revealed that a high dietary fiber intake and no probiotic use were associated with an improved response to ICB (with 82% responders compared to 59% in those with lower dietary fiber intake), as well as an extended PFS (median PFS not reached versus 13 months). The study suggests that these results may be primarily attributed to the microbiome within the tumor microenvironment (TME). The beneficial bacteria identified in this study align with recent findings, with microbial α diversity, the Ruminococcaceae family, and Faecalibacterium genus abundances being numerically higher in patients with sufficient dietary fiber intake and no probiotic use. ¹³ In their previously mentioned systemic review, Huang et al ²⁶ correlated plant-based diets, rich in dietary fibers, with a trend toward an “ICB favoring” gut microbiome (P = .0476). Furthermore, it is suggested that nutritional dietary fibers may assist in mitigating undesirable weight loss, which is common in cancer scenarios, partly through the increased abundance of cellulose-responsive bacteria. ⁵⁸ It is worth mentioning that a recent systematic review and meta-analysis of prospective studies and randomized controlled trials emphasized the importance of nondigestible fiber to human health, suggesting a notable decrease in all causes of cardiovascular-related mortality, type 2 diabetes, and colorectal cancer when comparing high- and low-fiber consumers. ⁵⁹

A current study being conducted at the M.D. Anderson Cancer Center (ClinicalTrials.gov Identifier: NCT04645680) also aims to evaluate gut microbiome characteristics following a high dietary fiber diet and the diet’s effect on ICB treatments in advanced melanoma patients.

These findings suggest that while dietary fibers should be recommended as a part of a healthy, primarily plant-based diet, there are still challenges to address regarding the safety of using commercially available probiotics in the context of Immunotherapy treatments.

Dietary Supplements

Omega-3 fatty acids, Omega 6:3 fatty acids ratio

Omega-3 polyunsaturated fatty acids (PUFAs), found in fish, seafood, nuts, and seeds, exhibit chemo and immune-protective effects. These involve suppressing nuclear factor-κB, activating AMPK/SIRT1, modulating cyclooxygenase (COX) activity, and up-regulating anti-inflammatory lipid mediators. Pre-clinical studies indicate that dietary fish oil rich in Omega-3 PUFAs enhances spleen B cell numbers and cytokine/IgM production and reduces autoantibodies in various conditions. In individuals at risk for rheumatoid arthritis, Omega-3 PUFA supplementation and higher levels in red blood cell membranes are associated with reduced anti-cyclic citrullinated peptide and rheumatoid factor positivity. ⁸ These beneficial effects are partially attributed to its influence on gut epithelial barrier integrity. Omega-3 PUFAs mitigate gut and systemic inflammation by promoting SCFA-producing bacteria and engaging GPR120, particularly in macrophages, thereby suppressing TNF-α and IL-6 and mitigating tissue inflammation. Maintaining a balanced tissue omega-6:omega-3 PUFA ratio in mice enhances intestinal alkaline phosphatase production, suppressing LPS-producing microbiome members. Simpson et al’s study links omega-3 fatty acids with higher α diversity.^8,21,22

The researched reported dosages of omega-3s vary from 1.5 to 3 g/day of EPA via oral administration, for periods of 8 to 12 weeks. Omega-3 fatty acids are well tolerated, with no serious adverse events reported. ⁶⁰

Vitamin D

Calcitriol, the active form of vitamin D (VD), plays a crucial role in the bidirectional relationship between gut microbiota and immunoregulation. VD deficiency is linked to gut dysbiosis and autoimmune diseases like inflammatory bowel disease, multiple sclerosis, type one diabetes, and systemic lupus erythematosus.^61,62 VD exhibits immunoregulatory characteristics by dampening T-cell-mediated immune responses. Given that irAEs likely stem from an abnormal T and B cell activation and an overall heightened inflammatory response, resembling hyper-immune reactions seen in autoimmune patients, the immunoregulatory attributes of VD, coupled with substantial evidence of its therapeutic benefits in autoimmunity, imply potential effectiveness in mitigating irAEs. VD enhances the integrity of the gut mucosa, influencing elements such as Paneth cells, tight junctions, and pro-inflammatory cytokines. In the gut microbiome, higher VD levels are associated with an increase in the plant glycans’ degrader Prevotella phylum and reduced Haemophilus and Veillonella. ³ Low VD levels correlate with fewer tolerogenic dendritic cells and more T cell receptor αβ cells in the lamina propria. VD intake in humans is tied to lower circulating levels of lipopolysaccharides (LPS).^3,22 In preclinical studies, VD supplementation induces the expression of PD-1 and CTLA-4 on gut epithelial cells and PD-1 on immune cells. These studies also reveal reduced IFNγ synthesis and increased IL-4 production when influenced by calcitriol. ³ The accumulative data suggest that the dual impact of VD, involving an adrenal-steroid-like suppression of excessive lymphocytic activity and a concurrent upregulation of PD-1 antibodies on immune cells, may confer potential benefits in the context of immunotherapy treatment. While the mentioned positive outcomes are promising, prospective clinical studies have not demonstrated a substantial impact on the progression of various cancer types or overall survival (OS). As a result, routine screening for vitamin D deficiency or vitamin D supplementation is not currently recommended in the context of cancer treatment. ⁶³

Figure 1: A flow chart showing the effects of dietary regimens and supplements on anti-cancer immunotherapy. TME—tumor microenvironment; irAEs—immune-related adverse events.

Figure 1.

Effects of dietary regimens and supplements on anti-cancer immunotherapy.

Conclusion and Possible Study Pathways

The intricate interplay between tumor cells, immunotherapy, and the gut microbiome underscores the challenges and opportunities in cancer treatment (Figure 1). Tumor cells orchestrate an immunosuppressive microenvironment, hampering immunotherapy effectiveness, while immunotherapy itself can lead to systemic immune overactivity and severe side effects. The pivotal role of the gut microbiome as a mediator of these dual effects has been elucidated through diverse studies, highlighting the impact of fecal microbiota transplantation (FMT), antibiotics, and probiotics. The evolving understanding of the complex connections involving nutrition, gut microbiome composition, diversity, and microbiome by-products presents yet another promising avenue for transformative research (Table 1).

Table 1.

Clinical Studies Focusing on the Effect of Dietary Manipulations on the Gut Microbiome and Anti-Cancer Immunotherapy Treatments.

Identifier	Title	Intervention	Immunotherapy treatment
NCT04645680	Effect of Diet on the Immune System in Patients With Stage III-IV Melanoma Receiving Immunotherapy, DIET Study	Isocaloric high-fiber diet	anti-PD1, anti-CTLA4, anti-LAG3
NCT04866810	The Effect of Diet and Exercise on Immunotherapy and the Microbiome (EDEN)	A plant-based, high-fiber diet + at least 150 min of moderate or 75 min of high-intensity exercise per week	Relatlimab, Pembrolizumab and Nivolumab
NCT06236360	Metastatic Melanoma Patients on Immunotherapy With Nutritive Intervention Based on Mediterranean Diet (MINI-MD)	Tele-recommendations based on the Mediterranean diet from nutritionists	Ipilimumab and Nivolumab
NCT05083416	Effect of Prolonged Nightly Fasting on Immunotherapy Outcomes in HNSCC - Role of Gut Microbiome	Prolonged nightly fasting combined with a regular eating pattern	ICBs (Nivolumab, Pembrolizumab, Atezolizumab, Avelumab, or Durvalumab) w/without chemotherapy
NCT04965129	Supplementation of n-3 PUFA in the Modulation of Lean Mass in Patients With Lung Cancer Receiving a High-protein Diet	PUFA n-3	undetailed chemotherapy, immunotherapy, or tyrosine kinase inhibitors
NCT03700437	Fasting-mimicking Diet With Chemo-immunotherapy in Non-small Cell Lung Cancer (NSCLC)	Fasting mimicking diet	Carboplatin/Pemetrexed and Pembrolizumab
NCT03709147	Metformin Plus/ Minus Fasting Mimicking Diet to Target the Metabolic Vulnerabilities of LKB1-inactive Lung Adenocarcinoma (FAME)	Fasting mimicking diet	Carboplatin/Cisplatin and Pemetrexed and Pembrolizumab w/without Metformin
NCT05318807	Personalized diet, exercise, and emotional support for lung cancer patients	A personalized plan of diet, exercise, and emotional support	Undetailed chemotherapy, radiotherapy, or immunotherapy
NCT05703997	FASTing-like Approach and Maintenance IMMunotherapy in ES-SCLC Patients Not Progressing on Chemoimmunotherapy Induction (FASTIMMUNE)	Cyclic, 5-D calorie restriction	Atezolizumab
NCT05356182	A Pilot and Feasibility Study of a Dietary Intervention With Low-protein Meals in Cancer Patients Receiving Immunotherapies	Low protein diet	PD1, PD-L1, CTLA-4 inhibitors
NCT04316520	Ketogenic Diet for Patients Receiving First-Line Treatment for Metastatic Renal Cell Carcinoma (CETOREIN)	Ketogenic diet	Nivolumab + Ipilimumab, Pembrolizumab + Axitinib, Sunitinib or Pazopanib
NCT05220124	A Study of Probiotics Administration in the Immunotherapy of Urothelial Bladder Carcinoma	Live Combined (Bifidobacterium,Lactobacillus and Enterococcus Capsules)	Not applicable
NCT04699721	Clinical Study of Neoadjuvant Chemotherapy and Immunotherapy Combined With Probiotics in Patients With Potential/ Resectable NSCLC	probiotics: Bifidobacterium trifidum live powder (BiFico, SINE)	Nivolumab and Paclitaxel and Carboplatin

The Mediterranean diet emerges as a potential ally, fostering anti-inflammatory states and supporting a gut microbiome conducive to balanced immunogenicity. Specific nutritional interventions, such as the ketogenic diet and fasting, exploit tumor cells’ high metabolic demands to diminish the unfavorable TME immunoediting. Additionally, dietary components like the omega 6:3 ratio and vitamin D, often deficient in cancer patients, may influence immune responses through their anti-inflammatory properties. Nonetheless, the current data is insufficient for a thorough assessment of how specific diets impact the effectiveness of immunotherapy. Moreover, given the prevalent imbalanced nutritional status among cancer patients, further investigations are essential to examine the optimal dietary regimens across the cancer trajectory.

As we navigate this intricate landscape, we encourage clinicians and researchers to consider tailored non-invasive strategies to manipulate microbial communities and metabolites, aiming to counter dysbiosis and enhance immune regulation. This holistic approach calls for a paradigm change that may potentially broaden the accessibility of immunotherapy for a broader patient population while mitigating the severity of treatment-related side effects.