Since cancer cells are under oxidative stress for a long time, they produce a large amount of ROS. ROS is a key driver of tumorigenesis, progression, metastasis and drug resistance. However, as the research progressed, the complex and diverse roles of ROS in TME have been gradually discovered (55).

Qu can regulate the recruitment and functional activation of effector T cells. Qu increased CXCL1-2 expression in MDA-MB-231 and MCF-7 cells in a dose-dependent manner (P < 0.05 or P < 0.01) (20μM-80μM) (65). PD-L1 in TME can inhibit the function of T cells, and Qu activates T cells by inhibiting the binding of PD-L1/PD-1. The anti-PD-L1 antibody (200ng, Santa) was used as a control, while BSA (0.5μg/ml) served as a negative control. Qu (5μM) can inhibit PD-1/PD-L1 binding in HEK293 cells, and the inhibition rate reaching as high as 90%. In MDA-MB-231 and PBMCs xenograft mouse models, 60 mg/kg of Qu significantly inhibits the proliferation of tumor cells compared with the control group (0 mg/kg of Qu) and increases the protein levels of CD8, GZMB, and IFN-γ in tumor tissues (66). Gamma delta T cells (γδT cells) are a type of innate immune cell that plays an immunomodulatory role in the immune response to many infections and immune diseases (67). Qu concentration at concentrations of 2.5μM to 5μM can significantly promote the proliferation of γδ T cells in vitro. The killing ability of γδ T cells against breast cancer (MCF-10A > MCF-10AT > MCF-7 > MDA-MB-231) was increased after treatment with 5μM of Qu. When the ratio of γδ T cells to target cells (E/T) was 10:1, the killing effect was most pronounced, and the mechanism of Qu action was related to the JAK-STAT1 signaling pathway (38).

In addition to effector T cells, Qu can influence the activity of NK cells. In a BALB/c mouse model constructed using 4T1 cells, the diet was supplemented with 1%, 2.5%, and 5% Qu, respectively, along with 50 mg/kg cyclophosphamide. The results showed that Qu combined with cyclophosphamide increased the activity of T cells and NK cells to a greater extent, and reduced the activity of Treg cells in the breast cancer microenvironment. Notably, 2.5% Qu had the most significant effect (68). This finding partially confirms the potential of Qu in regulating the chemotactic recruitment of immune cells. K562 and SNU1 cells were treated with 50 mM Qu for 30 hours, SNU-C4 cells with 20 mM Qu for 24 hours, and then the tumor cells were co-cultured with NK cells. The experimental results demonstrated that compared with tumor cells without Qu treatment, Qu increased the susceptibility of tumor cells to the toxic effects of NK cells. The cytotoxicity was reversed by the NKG2D receptor neutralizing antibody. Qu can effectively induce NKG2D ligands on the surface of tumor cells, thereby enhancing the tumor-killing effect of NK cells (69).

In the UK, obesity elevates the risk of breast cancer among the postmenopausal population and results in a diminished response rate to anti-tumor therapies in breast cancer patients, ultimately contributing to a poorer prognosis (73). Additionally, obesity-related chronic inflammation and metabolic syndrome have been identified as independent prognostic factors that influence treatment response rates and the risk of mortality from breast cancer (74). Obesity-related chronic inflammation is systemic, but can reach the local TME through the circulatory system to exert immunosuppressive effects and suppress the effects of breast cancer immunotherapy (75), by inducing T cell exhaustion and immunosuppressive cell recruitment. In addition, chronic inflammation is frequently linked to tumor cachexia, which significantly impacts patients’ quality of life (76).

Obesity-related chronic inflammation can promote the infiltration of TAMs and CAFs and inhibit anti-tumor immune response. The crownlike structure (CLS), which is widely present in adipose tissue, is a structure in which macrophages clear dead adipose cells, but causes macrophages to be exposed to high levels of saturated fatty acids (77). Saturated fatty acids can activate TLR4 on the surface of macrophages, induce NF-κB signaling, and inhibit anti-tumor immunity (78). Obesity also increases the activity of NLRP3 in adipose tissue (79), which promotes the infiltration of MDSCs and TAMs, forms an immunosuppressive microenvironment (80). ASCs, as the main component of adipose tissue, can differentiate into cancer-associated fibroblasts (CAFs) (81, 82). In breast cancer, CAFs are able to suppress anti-tumor immunity through their metabolism and secretion of cytokines (83).

Obesity can lead to a decrease in both the number and diversity of T cells, as well as promote T cell exhaustion (84). In obese breast cancer patients, leptin in breast adipose tissue inhibits CD8+ T cell effector function by activating STAT3-fatty acid oxidation (FAO) and inhibiting glycolysis (85). Obesity also increases the concentration of CXCL1, promotes CXCR2 mediated Fas ligand (FasL) activation, and recruitment of MDSCs, leading to apoptosis of CD8+ T cells and decreased immunotherapy efficiency (86).

Qu improves the prognosis of breast cancer patients by inhibiting obesity-related chronic inflammation, alleviating obesity-related metabolic syndrome, and increasing treatment sensitivity ( Figure 2 ). Treatment of adipose-derived macrophages with Qu (0, 3, 10, or 30 mM) for 5 hours inhibited the expression of inflammatory genes, such as tumor necrosis factor (TNF)-α, interleukin (IL)-6, IL-8, and IL-1β compared with untreated macrophages (87). Qu (10 mg/kg) can reverse the obesity caused by a high-calorie diet in vivo. Compared with the high-calorie diet groups, phentermine groups, and orlistat groups, Qu can significantly inhibit the expression of pro-inflammatory genes such as IL-6, IL-1β, IL-18, and TNF-α, thereby alleviating the inflammatory state caused by obesity (88). When fed 50 mg/kg, 100 mg/kg, and 200 mg/kg respectively, Qu inhibited serum IL-6 production in a dose-dependent manner compared with high-fat diet (HFD) alone, but there was no difference in the reduction of serum TNF-α between groups. Compared with the use of metformin, Qu (300 mg/kg), can assist metformin in reducing the levels of serum IL-6 and TNF-α, relieving the inflammatory response of obesity, and has good safety, with no obvious organ toxicity (89).

Obesity-related chronic inflammation can lead to the infiltration of inhibitory immune cells, promote T cell exhaustion, and diminish the effectiveness of breast cancer immunotherapy. Qu can directly influence the metabolism of adipose tissue and inhibit the development of obesity-related chronic inflammation by reducing the production of inflammatory factors and the recruitment of TAMs. This has positive implications for the clinical application of immunotherapy. Dietary Qu (17 mg/kg) for 9 weeks induced white fat remodeling and decreased IL-6 and adiponectin levels in adipose tissue in obese mice (90). Qu inhibits the infiltration of macrophages in adipose tissue to relieve chronic inflammation. After 12 weeks of supplementation with 0.1% Qu (Purity ≥98%) on the basis of HFD, Qu significantly reduced the infiltration of macrophages in adipose tissue compared with HFD alone. Qu increased the number of M2-type macrophages while decreasing the levels of pro-inflammatory cytokines TNF-α, IL-6, and MCP-1. Qu activates AMPKα1/SIRT1 signaling to inhibit the polarization and inflammation of bone marrow derived macrophages in vitro ( 91). In addition to feeding with a Western diet (high fat, high cholesterol, and high sucrose), 0.05% Qu was supplemented for 18 weeks. The results showed that Qu significantly inhibited the increase of serum TNF-α, leptin, and decreased the protein expression of NF-κB p65 in western diet mice. Immunohistochemistry showed that Qu inhibited Western diet-induced macrophage accumulation in adipose tissue (92).

In summary, the metabolism of adipose tissue frequently induces chronic inflammation, characterized by the accumulation of inflammatory factors and populations of white blood cells, which often inhibits the anti-tumor immune response. Tumor cells themselves also produce various chemokines to attract neutrophils, macrophages, myeloid-derived suppressor cells, and other immune cells. These cells, in turn, secrete a range of cytokines and cytotoxic mediators. Therefore, in cancer patients, regardless of obesity, chronic inflammation is prevalent and often adversely affects the response to tumor immunotherapy. Given the widespread nature of chronic inflammation in cancer patients, utilizing Qu as a routine clinical intervention to suppress chronic inflammation presents a promising treatment strategy. This approach may enhance the immunotherapy response rate and improve patient prognosis.

Many studies have demonstrated that the composition of intestinal flora significantly influences the effectiveness of breast cancer immunotherapy. Certain specific types of flora can even serve as biomarkers to predict the efficacy of this treatment. Qu has the ability to modulate the composition of intestinal flora, enhance its abundance, and increase the overall microbial diversity, thereby improving the effectiveness of immunotherapy. Additionally, the metabolites produced by these flora can impact the efficiency of immunotherapy by reshaping the immune microenvironment associated with breast cancer. Qu can also regulate the metabolism of intestinal flora, leading to an increased accumulation of beneficial metabolites, which in turn affects the efficacy of immunotherapy. Metabolites produced by gut microbiota get to local microenvironment of breast cancer through the circulatory system, and regulate key signaling pathways in cancer cells and various immune cells affecting the efficiency of anti-tumor immunotherapy (94). Butyrate, a metabolite of gut microbiota, improves the efficacy of anti-PD-1 immunotherapy by regulating TCR signaling in cytotoxic T cells (95). Intestinal B. pseudolongum enhances anti-tumor immunotherapy response by producing the inosine (96). Immunotherapy leads to a decrease in intestinal barrier function, and local inosine in the intestine is more likely to participate in the systemic circulation. Short chain fatty acids (SCFA) produced by Escherichia coli have cytotoxic effects on breast cancer cells and directly inhibit breast cancer cell proliferation (97). Gut microbiota and its metabolites can not only promote the immune response, but also inhibit the immune response of the body and promote the progression of tumors. In a C3-1-TAg mouse model of breast cancer, it has been observed that liver infection with Helicobacter hepaticus, a gut-resident bacterium, can induce breast cancer progression by promoting neutrophil recruitment and infiltration in TME, shaping an immunosuppressive microenvironment (98).

In addition to metabolites, extracellular polysaccharides or surface proteins in the structure of bacteria themselves can act as pathogen-associated molecular patterns (PAMPs), directly stimulating intestinal immune cells and inducing innate or adaptive immune responses to regulate anti-breast cancer immune responses. Exopolysaccharides produced by Lactobacillus delbrueckii subsp. bulgaricus OLL1073R-1 (EPS-R1) induce CCR6+CD8+T cells in mice and humans. In mice, ingestion of EPS-R1 enhanced the anti-tumor effects of anti-CTLA-4 or anti-PD-1 monoclonal antibodies against tumors expressing CCL20 (99). Immunostimulating exopolypolysaccharide (EPS) produced by Lactobacillus delbrueckii ssp. bulgaricus OLL1073R-1 enhanced activity of natural killer (NK) cell in mouse spleen cells and induced production of IFN-γ (100).

Qu can not only directly inhibit the proliferation of tumor cells by regulating intestinal flora ( Table 3 ) but also improve the efficiency of anti-tumor immunotherapy. Qu can regulate the proportion of intestinal flora and increase the number of beneficial flora ( Figure 3 ). In the TNBC mouse model, the addition of 2.5% Qu to the diet significantly induced Akkermansia enrichment and increased the activation of effector T cells and NK cells compared with mice without Qu supplementation (68). In a mouse model of hepatocellular carcinoma, the combination of Qu and immune checkpoint inhibitors increased the abundance of Firmicutes, Actinobacteria, and Vermilata microbiota, as well as Dubosiella and Acementia at the genus level, and increased the expression of CD8a, CD4, CD11b, IL-10, and IFN-γ in the TME compared with Qu or immune checkpoint inhibitors alone (106). In a mouse model of colitis, additional supplementation with Qu (30 mg/kg) increased the numbers of Bacteroides, Bifidobacterium, Lactobacillus, and Clostridium, and significantly decreased the numbers of Clostridium and Enterococcus (P < 0.05) (107). In the model of using antibiotics to disrupt the balance of intestinal flora, the experimental group added 0.2% Qu based on AIN-93G feeding, and the results showed that Qu increased the level of intestinal beneficial bacteria species. These included Faecalibaculum rodentium (103.13%), Enterorhabdus caecimuris (4.13%), Eggerthella lenta (4%), Roseburia hominis (1.33%), and Enterorhabdus mucosicola (1.79%). These bacteria can produce butyrate and reduce serum lipopolysaccharide and TNF-α levels (108). The mice fed with 0.05% Qu based on HFD lasted for 6 weeks. Compared with the mice in the HFD group, Qu significantly reduced liver fat and blood glucose levels. In feces, the relative abundance of Akkermansia was significantly increased (109).

Qu can directly regulate the metabolites of intestinal flora to exert anti-inflammatory effects, thereby shaping an effective anti-tumor immune microenvironment ( Figure 3 ). Through the antibiotic induced intestinal flora dysregulation mouse model, the experimental group was fed AIN-93G diet containing 0.2% Qu for 10 days. Through the analysis of mouse feces and serum metabolites, it was clear that isorhamnetin, a Qu methylated metabolite, was the main active component in serum. Compared with antibiotic intervention alone, Qu treatment was able to increase the diversity of intestinal flora (still lower than that of the group without antibiotics) and butyrate levels (110). Compared with HFD-fed mice, mice fed with 1% Qu for 16 weeks had lower body weight and total plasma cholesterol, reduced the ratio of Firmicutes, Bacteroidetes in the stool of HFD-fed mice and increased the production of short-chain fatty acids (SCFA) (111). Studies have shown that SCFA can improve the efficiency of breast cancer immunotherapy (112).

Based on the regulatory effects of Qu on intestinal flora, clinical treatment strategies can be influenced by the following three aspects. First, the sensitivity of intestinal flora richness can predict the efficacy of immunotherapy. A higher richness of intestinal flora often correlates with a better response to immunotherapy. Therefore, in the clinical application of immunotherapy, the bacterial flora of patients can be sequenced in advance to assess their microbial composition. Based on the results of microbiota sequencing, Qu can be administered beforehand to enhance the abundance of intestinal flora, thereby improving the response rate to immunotherapy. Second, the expression of beneficial bacteria often enhances the response to immunotherapy, including species such as Ackermannia and Ruminococcus.etc. Qu has been shown to improve the expression of these beneficial bacteria, thereby increasing the efficacy of immunotherapy. In addition to utilizing natural products, the incorporation of probiotics can also enhance the recruitment of beneficial bacteria in the gut. Third, some metabolites produced by intestinal flora can boost the effectiveness of immunotherapy. Qu can promote bacterial metabolism, making its application advantageous for increasing the accumulation of bacterial metabolites, which is beneficial for the clinical application of immunotherapy. Therefore, given Qu’s impact on intestinal flora, it has the potential to be developed as an adjuvant for tumor immunotherapy.