Modulating Gut Microbiota to Boost Cancer Immunotherapy: Mechanisms, Targeting Strategies, and Clinical Insights

Role of Gut Microbiota in Cancer Immunotherapy

The gut microbiota exerts a profound influence on immunotherapeutic responses. The mechanisms by which the gut microbiota modulates anti-tumor immune responses are particularly significant, involving the direct or indirect (via metabolites) immunomodulation of the host immune system and tumor microenvironment (TME) (Fig. 1)^[6].

The gut microbiota functions within the intestines to maintain intestinal barrier function, defend against pathogens, and shape immune responses. Certain bacterial species, such as Bifidobacterium and Bacteroides fragilis, can interact with dendritic cells (DCs) and activate both CD8⁺ T cells and Th1 cells^[6].

The gut microbiota influences both systemic and tumor-specific immune responses through its metabolites-small molecules that diffuse from the intestinal lumen and modulate the immune system^[6].

These microbial metabolites can be broadly classified based on their origin and biosynthetic pathways: (1) metabolites derived from dietary components, such as short-chain fatty acids (SCFAs), tryptophan metabolites, TMAO, and Inosine; (2) host-derived metabolites modified by the gut microbiota, such as secondary bile acids; and (3) metabolites de novo synthesized by the gut microbiota, including branched-chain amino acids (BCAAs) and vitamins^[6].

SCFAs such as Acetate, Pentanoate, Butyrate, and Propionate regulate immune responses. SCFAs modulate T cell differentiation via G-protein-coupled receptor (GPCR) signaling and histone deacetylase (HDAC) inhibition. Butyrate and Pentanoate enhance IL-12 and IL-2 expression in CD8⁺ T cells, respectively. Acetate enhances CD8⁺ T cell function by downregulating PVR/CD155 through PI3K/AKT inactivation^[6-7]. The ability of SCFAs to induce IL-22 points to their wider impact on tumor immune regulation^[7] (Fig. 2).

Gut microbiota metabolism of tryptophan produces bioactive compounds including indoles, 5-hydroxytryptamine, and kynurenine, which modulate immunity. For instance, inosine, produced by Akkermansia muciniphila and Bifidobacterium longum, promotes anti-tumor immunity by inhibiting UBA6 in tumor cells and activating IL-12Rβ2/IFN-γ signaling in T cells^[6][12].

Bile acids, primarily converted by the gut microbiota in the cecum and colon, also influence immune function. Species within Firmicutes, particularly Clostridium, convert primary bile acids into secondary bile acids such as Lithocholic acid (LCA) and Deoxycholic acid (DCA). These metabolites regulate T cell differentiation and macrophage polarization, with secondary bile acids notably suppressing Th17 cell function. However, their effects on tumor immunity are complex. For instance, secondary bile acids derived from Clostridium scindens impair the tumor-suppressive function of natural killer T (NKT) cells. In contrast, LCA inhibits breast cancer cell growth by activating TGR5 and constitutive androstane receptors, leading to oxidative stress, suppression of epithelial-mesenchymal transition (EMT), and reduced angiogenesis and metastasis^[6].

The gut microbiota also produces BCAAs and Vitamins that influence anti-tumor immune responses. Bacteria degrade proteins and unabsorbed amino acids to synthesize BCAAs, such as Leucine, Isoleucine, and Valine, which promote the activation and proliferation of effector T cells through mTORC1 signaling. Meanwhile, Firmicutes (such as Lactobacillus and Enterococcus) and Bacteroides synthesize B vitamins. Bacteroides fragilis, Prevotella, and Ruminococcus lactis produce Vitamin B3, which binds to GPR109A receptors in myeloid cells, inhibiting NF-κB signaling, reducing the polarization of immunosuppressive myeloid cells, and enhancing CD8⁺ T cell cytotoxicity. These findings highlight the critical role of microbial metabolites in shaping anti-tumor immune responses^[6].

Targeting Gut Microbiota to Enhance Cancer Immunotherapy

Recent breakthroughs in the intricate interplay between host immunity and the gut microbiota have revealed novel approaches to improve immunotherapy efficacy. Here, we explore microbiota-targeted strategies with promising potential to augment antitumor immunity: (1) antimicrobial intervention, (2) microbial-derived metabolites, (3) probiotics, prebiotic supplementation and dietary modulation, (4) genetically modified bacterial strains, (5) oncolytic virus immunotherapy, (6) bacteriophage-targeted pathobiont therapy, (7) Fecal Microbiota Transplantation (FMT)^[3][6] (Fig. 4, 5).

Microbial metabolites are the focus of this section, as they play a vital part in shaping anti-tumor immune responses and do not show the low cell viability after oral administration observed in probiotics.

Gut microbial metabolites include short-chain fatty acids (SCFAs), Inosine, and TMAO. These metabolites enhance the efficacy of cancer immunotherapy by modulating host anti-tumor immunity^[3].

Gut microbiota and SCFAs (a mixture of Sodium acetate, Sodium propionate and Sodium butyrate; p.o.) enhanced the antitumor immune response in lung cancer-bearing mice. Butyrate treatment directly facilitated the antitumor responses of CD8⁺ T cell both in EG7 tumor cells and in EG7 tumor-bearing mice. Butyrate (150mg/kg) supplementation enhanced the antitumor immune responses of anti-PD-1 in B16F0-bearing mice. Combination therapy using anti-PD-1 and Butyrate exhibited greater tumor regression efficacy. However, in tumor-bearing mice, no significant difference in the degree of tumor reduction was observed between the combination therapy of anti-PD-1 with Acetate or Propionate and the single-administration of anti-PD-1. Butyrate (0.5 mM) treatment inhibited immunosuppressive factors (PD-L1 and IL-10) expression in the PBMCs from patients with gastric cancer (GC). Butyrate (200mg/kg; p.o.) suppresses the growth of GC cells in a humanized tumor mouse model with AGS cells. These findings indicate that exogenous Butyrate is an important SCFA to enhance the efficacy of cancer immunotherapy^[3][7-10].

Inosine supplementation exhibited a tendency to enhance the efficacy of immune checkpoint inhibitors against advanced solid tumors and reduced immunotherapy-related adverse events^[11].

TMAO (intratumoral injection of TMAO) promoted the antitumor immunity and augmented the efficacy of anti-PD-1 antibody in mice implanted with 4T1 TNBC cells^[12-13].
It is noteworthy that existing studies employ different administration routes for gut microbial metabolites. Whether these varying administration methods influence the efficacy of cancer immunotherapy warrants further characterization in future research. In summary, these findings suggest that exogenous supplementation of gut microbial metabolites represents a viable strategy for enhancing the efficacy of cancer immunotherapy.

Clinical Prospects of Gut Microbiota-Targeted Strategies

Currently, strategies such as antibiotics, fecal microbiota transplantation (FMT), probiotics, diet, and prebiotics have been clinically investigated to evaluate their effects on immunotherapeutic efficacy, further supporting the feasibility of gut microbiota-targeted strategies.

However, these strategies still have certain limitations^[6]. For FMT, the donor feces transplanted into the recipient contain numerous off-target microbiota components, and their impact remains unclear. Future research should focus on optimizing microbiota transplantation protocols and identifying optimal donor-recipient pairings to enhance anti-tumor efficacy^[6].
The key advances in the study of gut microbiota and immunotherapy are reviewed, as shown in Fig. 5, which shows that gut microbiota metabolites have become the latest research hotspot. SCFAs can regulate the differentiation and function of immune cells, as well as the production and release of cytokines, modulating tumor growth and metastasis through multiple signaling pathways. However, the evaluation of SCFAs in clinical trials is relatively insufficient. Future research should include more clinical trials to investigate the impact of SCFAs on cancer immunotherapy. Additionally, microbial populations in the oral cavity, skin, and TME may impact immune responses, thereby widening the scope for personalized therapeutic strategies^[6-7].

The gut microbiota plays a key role in regulating cancer immunotherapy. Strategies targeting the gut microbiota have opened up novel avenues for enhancing the efficacy of immunotherapy. Among these, microbial metabolites play a crucial role in shaping anti-tumor immune responses and hold great promise. However, the practical application of these strategies, including microbial metabolites, still faces significant challenges. Future research should include more clinical trials to investigate the impact of microbial metabolites on cancer immunotherapy.