Precision Probiotic Medicine to Improve ICB Immunotherapy

The recent development of immune-checkpoint blockade (ICB) has dramatically improved patients’ survival rates for several cancer types. This revolutionary therapy is based on monoclonal antibodies that block inhibitory receptors on immune cells, such as PD-1 and CTLA-4, unleashing the potent antitumor activity of the immune system. However, only a subset of patients and tumor types are fully responsive to ICB; the cellular mechanisms underlying these limitations are being actively investigated. Among various factors, the individual's microbiome composition is a critical player in the response to ICB. The gut microbiota directly shapes innate and adaptive immune responses, either by providing bacterial antigens that promote the expansion of specific immune cells or through production of immunomodulatory metabolites, such as indoles, exopolysaccharides, and short-chain fatty acids (1). Accordingly, the intestinal microbiota also affects the effectiveness of ICB therapy, which is strongly impaired in both germ-free mice and mice treated with broad-spectrum antibiotics. Several members of the commensal microbiota have been identified as putative probiotic strains that enhance ICB efficacy in preclinical mouse models, including Bacteroides thetaiotaomicron and Bacteroides fragilis (2), Akkermansia muciniphila (3), Enterococcus spp. (4), and Bifidobacterium spp. (5). Moreover, the relative abundance of these strains has been shown to correlate with ICB responsiveness in patients with cancer.

The significant correlation between gut microbiota composition and enhanced ICB responses (6) has prompted attempts to combine ICB treatments with fecal microbial transplantation as a way to transfer probiotic strains that mediate boosting effects from ICB responder to nonresponder patients (7, 8). However, this therapeutic strategy is still considered an experimental treatment and the transfer of bulk microorganism populations raises concerns for patients’ health. Thus, it is crucial to understand which commensal microbes and dietary probiotic bacteria can affect responses to ICB and to characterize the bioactive molecules they produce. Several studies have investigated muropeptides, c-di-AMP, inosine, and zwitterionic polysaccharides in general. These molecules, also known as postbiotics, collectively act as ICB adjuvants that drive type 1 antitumor immunity, although through different molecular mechanisms (9).

In this issue, Kawanabe-Matsuda and colleagues (10) show that the exopolysaccharide EPS-R1 produced by Lactobacillus delbrueckii subsp. bulgaricus enhances the efficacy of anti–CTLA-4 ICB therapy in mice through a novel cellular mechanism that may be effective against CCL20-producing tumors. Lactobacillus delbrueckii subsp. bulgaricus is one of the strains most frequently used in the industry of fermented diary food and is well accepted as a probiotic strain for human consumption with immunomodulatory effects. Kawanabe-Matsuda and colleagues first showed that oral administration of EPS-R1 in mice selectively increases the abundance of CCR6⁺ CD8⁺ T cells in Peyer's patches. More­over, they found that CCL20 is expressed at higher levels in several human cancer types, including colorectal adenocarcinoma, than in normal healthy tissues. Although the expression level of CCL20 did not correlate with clinical prognosis, the expression of the receptor for CCL20, CCR6, correlated with a favorable prognosis and longer overall survival in colorectal adenocarcinoma and breast invasive carcinoma. Thus, the authors hypothesized that the EPS-R1–induced mobilization of CD8⁺ T cells could be exploited to control growth of tumors that express the CCR6 ligand, CCL20. They examined CCL20 expression in experimental tumor models and found that Colon26 (colon adenocarcinoma) and 4T1 (mammary carcinoma) express higher levels of this chemokine compared with several other tumor cell lines. They next tested whether EPS-R1 administration could affect the growth of these tumor cells in vivo. They found that oral gavage of EPS-R1 to mice bearing Colon26 and 4T1 tumors specifically expanded the CCR6⁺ CD8⁺ T-cell population within tumor-infiltrating lymphocytes (TIL) and significantly enhanced the efficacy of anti–CTLA-4 blocking antibodies. These results suggest that CCR6⁺ CD8⁺ T cells migrate from Peyer's patches to the tumor site. Importantly, EPS-R1 combined with ICB therapy was selectively effective against CCL20-expressing tumors without altering the host commensal microbiota. These observations demarcate a new strategy by which novel molecules of microbial origin can be screened for their potential effects on cancer immunotherapy by leveraging tumor microenvironment immune signatures.

Furthermore, Kawanabe-Matsuda and colleagues showed that TILs isolated from the Colon26 tumor model produce high amounts of IFNγ and TNFα. In turn, IFNγ promotes lymphocyte recruitment to the tumor site by triggering the expression of several IFNγ-induced chemokines, such as CXCL9, CXCL10, and CXCL11, which engage CXCR3. Among TILs, CXCR3⁺ CCR6⁺ CD8⁺ T cells were identified as major producers of IFNγ and granzyme B, which promote inflammation in the tumor microenvironment and sustain T-cell effector functions. Interestingly, using the highly antigenic experimental tumor model 4T1-HA, the authors observed significant clonal expansion of HA-specific CD8⁺ T cells upon EPS-R1 treatment. The limited diversity of the TIL T-cell receptor repertoire a few days after EPS-R1 administration suggested that most of the effector CXCR3⁺ CD8⁺ T cells originating from the CCR6⁺ CD8⁺ T cells were boosted by the tumor antigen. Finally, Kawanabe-Matsuda and colleagues identify the receptor through which EPS-R1 induces CCR6 on T cells. They found that the phosphorylated Gro3P moiety of EPS-R1 binds the LPA receptor 2, a G protein–coupled receptor known to be expressed by lymphocytes. Further studies will be important to define the pathway by which LPA receptor 2 affects chemokine receptor expression.

Although future studies are required to determine whether EPS-R1 can be used to treat human cancers resistant to ICB therapy and ascertain its impact on patient health, the results reported by Kawanabe-Matsuda and colleagues identify a promising new adjuvant for immunotherapy that derives from a commonly consumed probiotic strain. Moreover, this study supports the idea that mining the gut microbiota and well-known dietary probiotics contained in fermented foods is a powerful approach to discover new, interesting ICB adjuvants. The use of postbiotics as drugs or potent adjuvants in combination with ICB is a propitious strategy to bypass the risks of fecal microbiota transplantation. Indeed, direct administration of active molecules would overcome obstacles affecting effectual bacterial transfer–based therapies, such as survival through the administration route, successful colonization, and efficient production of the relevant bioactive molecules on site. Studies like this describing the activity of probiotics exemplify a formidable approach to identify novel drugs and targets that may improve and extend the efficacy of cancer therapeutics.

M. Colonna reports grants and other support from NGM Bio­pharmaceutical and grants from Oncorus during the conduct of the study, as well as grants from Ono, Pfizer, and Aclaris, grants and other support from Vigil Neuro, and other support from Cell Signaling Technology outside the submitted work. No disclosures were reported by the other author.