Nutritional modulation of anti-tumor immunity

Abstract

The composition and quantity of food we eat has a drastic impact on the development and function of immune responses. In this review, we highlight defined nutritional interventions shown to enhance anti-tumor immunity, including ketogenic, low protein, high fructose, and high fiber diets, as well as dietary restriction. We propose that incorporating such nutritional interventions into immunotherapy protocols has the potential to increase therapeutic responsiveness and long-term tumor control in cancer patients.

Introduction

Immunotherapy regimens include immune checkpoint blockade (e.g., anti-PD1/PDL1 and anti-CTLA4), which target receptors on T cells to remove suppressive signals and promote their anti-tumor function (1). Further, adoptive cell therapy involves the infusion of tumor-specific immune cells (e.g., expanded tumor-infiltrating lymphocytes, chimeric antigen receptor (CAR) T cells, and T cell receptor-engineered T cells) into patients to treat cancer (2). While immunotherapy has achieved great success in reducing the burden of cancer, not all patients respond, and not all responses are durable (3). Therefore, identifying novel strategies that improve the efficacy of immunotherapy is of the highest priority. Here, we propose that nutritional interventions are an accessible, untapped resource with the potential to enhance anti-tumor immunity and immunotherapy.

Nutritional interventions improve anti-tumor immunity and immunotherapy

Restricted food intake

Dietary restriction (DR) can be implemented in many forms, including calorie restriction (CR) (calories selectively reduced), fasting (complete elimination of food intake for short periods), and a fasting-mimicking diet (FMD) (cycling of a very low-calorie diet and ad libitum feeding). These different forms of DR have a similar impact on hormones (e.g., ghrelin, adiponectin, and insulin), systemic nutrient abundance (glucose, fatty acids, ketones), and the metabolic signaling components engaged (e.g., AMP-activated protein kinase (AMPK), phosphoinositide 3-kinases (PI3K), and the mammalian target of rapamycin (mTOR)) (4,5). It has been known for decades that restricting dietary intake without malnutrition improves metabolic profiles, and reduces cardiovascular disease, neurodegeneration, and the incidence of cancer in model organisms (6–8). Further, clinical studies showed that CR significantly improved biomarkers of human aging and longevity, and decreased energy expenditure and oxidative DNA damage (9,10). Restricting dietary intake has also recently been shown to enhance immune responses against pathogens and tumors (11–16). For example, CR improved the survival of mice bearing B16 melanoma tumors by enhancing tumor-specific memory CD8⁺ T cells (11). Similarly, CR inhibited the growth of MC38 colon adenocarcinoma tumors. This involved an enrichment in the intestinal commensal Bifidobacterium, which produced the short-chain fatty acid acetate that increased the frequency of IFN-γ-producing CD8⁺ T cells in tumors (15). These results suggest that CR-induced remodeling of microbiota composition and function enhances anti-tumor immunity. In the context of radiotherapy, CR was shown to limit the progression of triple-negative breast cancer by enhancing intra-tumoral CD8⁺ T cell infiltration and decreasing regulatory T (Treg) cells (17). Additionally, breast cancer patients undergoing radiotherapy and CR showed a lower concentration of anti-inflammatory cytokines and receptors, such as TGF-β and IL-10 receptor β, in serum compared to those received radiotherapy alone, suggesting that CR can skew anti-tumor immune responses toward a pro-inflammatory profile.

In a parallel manner, fasting and an FMD regimen were reported to favor anti-tumor immune responses and boost anti-cancer therapies. In the Lewis lung carcinoma model, which is poorly immunogenic, the application of PD-1 blockade alone was not effective in controlling tumor growth. However, together with fasting, significant tumor regression and improved survival were observed (18), which was associated with an increased frequency of intra-tumoral effector CD8⁺ T cells. In this setting, fasting-induced systemic reduction of insulin-like growth factor 1 (IGF-1) was a main driver in inhibiting tumor progression. CD8⁺ T cells were highly enriched in tumors of fasted mice, resulting in a lower Treg/CD8⁺ T cell ratio, and the expression of exhaustion markers such as PD-1 were downregulated on both intra-tumoral CD4⁺ T and CD8⁺ T cells. Comparably, an FMD regimen promoted immune checkpoint blockade (anti-PD-L1 and anti-OX-40 blockade) against breast cancer, which is known to be poorly immunogenic, by promoting the activation of T cells and reducing the infiltration of immunosuppressive myeloid cells (19). Moreover, an FMD regimen in combination with chemotherapy agents, such as doxorubicin or cyclophosphamide, was more effective in enhancing the control of breast cancer and melanoma cancers, compared to an FMD regimen or chemotherapy alone (20). Mechanistically, the FMD regimen reduced the expression of heme oxygenase-1 (HO-1), a stress-responsive enzyme, which promotes cancer cell proliferation and survival. The downregulation of HO-1 expression on breast cancer cells resulted in more frequent death of tumor cells, which was associated with increased infiltration of CD8⁺ T cells into the tumor (20). Further, an FMD with the chemotherapy agent vincristine systemically increased the frequency of CD8⁺ T cells in mice bearing acute B lymphoblastic leukemia, which promoted tumor-free survival (21). In the same setting, the beneficial effect of an FMD and chemotherapy was also observed in obese mice fed a high-fat diet, indicating the potential of dietary restriction to improve antitumor immunity in obese individuals with cancer. The combination of cyclic fasting or an FMD with chemotherapy was also shown to be beneficial in delaying chronic lymphocytic leukemia progression in mice (22). Additionally, periodic fasting or an FMD enhances the long-term therapeutic effect of hormone treatment such as tamoxifen and fulvestrant against breast tumors in animal models, which was mediated by reduced IGF-1 and leptin (23). Consistent with this, a clinical study showed that patients who underwent standard breast cancer surgeries and received an FMD regimen had changes in blood immune cell compartments (24). This included a decrease in immune-suppressive CD14⁺ monocytes and polymorphonuclear myeloid-derived suppressor cells (MDSCs), and increased cytotoxic lymphocytes, such as CD8⁺ T cells and natural killer (NK) cells. In tumors, the frequency of tumor-infiltrating cytotoxic CD8⁺ T cells that produced anti-tumor molecules, such as IFN-γ and granzyme B, was increased, while the frequency of CD3⁺CD25⁺ Treg cells was reduced (24). Altogether, restricting dietary intake reshapes immune cell distribution and function in multiple mouse tumor models, and in patients with breast cancer. This highlights the potential of harnessing dietary restriction to enhance anti-cancer immunity and the responsiveness to immunotherapy. However, several critical points remain to be addressed, including how CR impacts the intrinsic state of T cells to enhance their anti-tumor function, and an understanding of the impact of CR on the lethal wasting syndrome cachexia that is encountered by many cancer patients.

Calorie-restriction mimetics (CRMs)

While restricting dietary intake has been shown to promote anti-tumor immunity, it is unlikely to be feasible for all patients. Calorie-restriction mimetics (CRMs) are compounds that mimic the beneficial biochemical effects of CR or fasting without the need for a reduction in food intake (25). Hydroxycitrate is a CRM due to its ability to induce cellular autophagy (26). In mice implanted with fibrosarcoma tumors, the administration of hydroxycitrate and chemotherapy significantly delayed tumor growth compared to chemotherapy alone (27). This was due to increased autophagic flux in tumor cells, which enabled effective CD8⁺ T cell-mediated tumor control. Another study showed that hydroxycitrate administration facilitates the recruitment of myeloid cells, such as neutrophils and monocyte-derived dendritic cells to tumors, leading to improved tumor control in the context of chemotherapy (28). The synergistic effect of chemotherapy and CRM treatment was further enhanced by immunotherapy (anti-PD-1 and anti-CTLA-4 blockade) in the context of sarcoma tumors (28). Thus, CRMs have the potential to replace calorie restriction or fasting to enhance the efficacy of conventional cancer treatments, such as chemotherapy and immunotherapy.

Low protein diet

Multiple studies have demonstrated the enhanced anti-tumor effects of DR and fasting, but it is difficult for people to adhere to these regimens for sustained periods. An alternative is restricting the proportion of specific macronutrients in a diet, instead of reducing total caloric intake. Studies indicate that a low (4%) protein diet without caloric restriction promoted tumor control in mice compared to a diet with a standard (18%) protein content (29–31). Moreover, high protein intake was associated with an increased risk of cancer-related mortalities in individuals aged 50–65 (32). An underlying mechanism is that a low-protein diet promoted endoplasmic reticulum stress in colorectal carcinoma cells, which skewed the tumor microenvironment to favor the recruitment of CD8⁺ T cells and natural killer (NK) cells (30). In mice fed a low protein diet, the administration of either anti-CD8 or anti-CD86 antibodies, or liposome clodronate, abrogated tumor regression, indicating the importance of CD8⁺ T cells and phagocytic cells in this context. Further, a low-protein diet directly inhibited mTOR signaling in tumor cells by decreasing IGF-1 levels, which resulted in the reduced proliferation of tumor cells, including prostate and breast cancers (30). Altogether, these studies indicate that the composition and class of macronutrients play a critical role in modulating tumor growth and inducing anti-tumor immune responses.

Ketogenic diet

A ketogenic diet (KD) is defined by its relatively high content of fat (~70%) and a very low amount of carbohydrates (~1–3%). A KD induces the production of ketone bodies that act as a source of energy to fuel mitochondrial oxidative phosphorylation, and as a signaling molecule that regulates gene expression (33). As a result of this, the KD modulates immune cell migration and function, particularly T cells (34–36). For instance, ketone bodies were critical in promoting T cell effector responses against tumors, as well as bacterial (36) and viral infection (35), via epigenetic and metabolic reprogramming. Several studies using preclinical models have highlighted the immunomodulatory properties of the KD in anti-cancer therapy. For example, mice fed a KD showed improved glioma and colon tumor control, compared to mice fed with a standard diet (37,38). This nutritional intervention upregulated the expression of several chemokines (Ccl5, Cxcl9, Cxcl10, Cxcl11) in tumors, which promoted the recruitment of neutrophils, NK cells, and activated T cells (38). In these studies, the KD also increased the capacity of T cells to produce the cytokines IL-2 and IFN-γ, while the proportion of Treg cells was decreased (38). In CT26 tumor-implanted mice, KD-enhanced tumor control depended on CD8⁺ T cells (37). The administration of 3-hydroxybutyrate, the major ketone body generated by a KD, was sufficient to mimic the anti-tumor effect of a KD, which was abolished in the absence of T cells. Furthermore, 3-hydroxybutyrate administration synergizes with immune checkpoint blockade (anti-PD-1 plus anti-CTLA-4) to extend the survival of mice bearing either renal or lung cancers. Overall, the KD alters immune cell compartments to promote tumor control and strengthens the potency of immunotherapies.

High fiber diet

Dietary fiber is degraded by the intestinal microbiota into beneficial immunomodulatory metabolites such as short-chain fatty acids (SCFAs) (39). Inulin fiber has been shown to inhibit the development of melanoma tumors in mice by promoting the infiltration of CD4⁺ and CD8⁺ T cells (40). Specifically, inulin-dependent tumor control was mediated by gut microbiota-derived butyrate (40,41). Butyrate administration boosted anti-tumor responses of CD8⁺ T cells by regulating the expression of the transcriptional regulator ID2, which was critical for the anti-tumor function of CD8⁺ T cells following stimulation with the cytokine IL-12 (42). In the same study, the combined administration of butyrate and chemotherapy improved tumor control in mice inoculated with colon carcinoma tumors. In clinical studies, the consumption of fiber was associated with a lower risk of breast cancer (43) and gastric cancer (44), and a lower mortality of patients with colorectal cancer (45). Similarly, high fiber consumption was associated with significantly improved survival of melanoma patients, which correlated with findings that low consumption of fiber impairs the efficacy of anti-PD-1 immune checkpoint blockade in tumor-bearing mice (46). Of note, the concentration of butyrate in the blood correlated with the efficacy of chemotherapy in cancer patients treated with oxaliplatin (42). These data demonstrate the importance of abundant fiber intake, and its breakdown by the gut microbiota, for boosting anti-tumor immune responses and synergizing with traditional cancer therapies.

High fructose diet

High fructose consumption has a range of negative health consequences and has been shown to promote intestinal tumor growth (47,48). However, fructose is associated with a beneficial effect against lung cancer (49), and a recent study highlighted that a high fructose (60%) diet enhanced the anti-tumor function of CD8⁺ T cells against subcutaneous melanoma and LLC tumors (50). Mechanistically, a high fructose diet induced adipocytes to produce leptin, which prevented terminal exhaustion of tumor-specific CD8⁺ T cells, thereby facilitating tumor control. Thus, defined diets may have opposing effects depending on tumor type or site, highlighting the need for more granular studies on how each diet impacts immune responses in a variety of tumor contexts.

Summary and future directions

Emerging evidence indicates that specific dietary interventions enhance immune cells to promote tumor control. Diets consumed ad libitum with macronutrient alterations, such as a KD, low protein, high fiber, and high fructose diets improved anti-tumor immune responses by regulating the infiltration and function of T cells. Moreover, reduced caloric intake via DR, fasting, or an FMD, as well as administration of CRMs, enhance anti-tumor immunity and improve the success of immunotherapy. However, we are only beginning to scratch the surface, and several other diets with the potential to regulate anti-tumor immunity remain to be thoroughly investigated, including Mediterranean, fermented, vegetarian, and vegan diets, as well as Western diets that are detrimental to immune-mediated tumor control (51,52). A high priority moving forward will be to incorporate the effect of diet on anti-tumor immunity via the microbiota, both in terms of the specific commensals enriched and those that are reduced in abundance. This will provide the opportunity to design specific consortia of beneficial microbes for therapeutic use via fecal microbiota transfer (FMT), without the requirement of prolonged dietary changes. An important limitation is that most studies have been performed in preclinical models, and whether similar responses occur in people remains to be addressed. A major obstacle is that the response to different diets by different individuals is highly variable (53–55). Therefore, it will be essential to uncover the contribution of key drivers that promote inter-individual heterogeneity (e.g., genetics, age, microbiota status) regarding the impact of diet on immunity. Altogether, these findings demonstrate that nutrition has a major impact on cancer immunity and suggest that integrating these nutritional interventions into immunotherapy regimens has the potential to be of major benefit to cancer patients.

Figure 1. Nutritional modulation of anti-tumor immunity.

Dietary regimens such as restricting food intake without malnutrition, or altering dietary composition, can enhance tumor control by boosting immune responses. In the case of T lymphocytes, calorie restriction, fasting-mimicking diet (FMD), or calorie-restriction mimetics can support the effector function of CD8⁺ T cells and reduce the abundance of Treg cells in tumors. Mechanistically, acetate produced by the gut microbiota during calorie restriction boosted anti-tumor responses of CD8⁺ T cells. Further, cancer cell-intrinsic signaling via insulin-like growth factor 1 (IGF-1) receptor suppresses tumor progression in the context of an FMD, which is associated with increased intra-tumoral CD8⁺ T cells and reduced Treg cells. In the setting of a low protein diet, tumor-infiltrating CD8⁺ T and NK cells become enriched, while ketone bodies produced during a ketogenic diet enhance the production of anti-tumor cytokines by CD8⁺ and CD4⁺ T cells, thereby improving tumor control. Last, implementing a high-fiber diet induces an alteration in the composition and function of the intestinal microbiota, which enhances the infiltration of CD4⁺ and CD8⁺ T cells and their capacity for anti-tumor cytokine production. The bottom panel lists the cancer therapies shown to enhance immune-mediated tumor control by the indicated nutritional intervention.