A molecule released by gut bacteria could help determine whether cancer patients respond to one of modern oncology’s most powerful treatments, according to a new study in the journal Microbiome. Researchers in China report that a polysaccharide extracted from Wolfiporia cocos, a fungus used in traditional Chinese medicine, improved the performance of anti-PD-1 immunotherapy in several mouse models of cancer. Their experiments point to a specific biochemical link between diet-like complex carbohydrates, the gut microbiome and the immune cells that attack tumors: an enzyme called α-L-fucosidase breaks down part of the fungal polysaccharide and liberates the sugar L-fucose, which appears to help stimulate an immune response.
The findings address a central problem in cancer immunotherapy. Anti-PD-1 antibodies, often written as αPD1, work by blocking a molecular “brake” that tumors exploit to silence T cells. Under normal conditions, the PD-1 receptor helps prevent excessive immune activity; when it binds to PD-L1 or related ligands on tumor cells and immune cells, it reduces T-cell activation. Drugs that interrupt this interaction can restore the ability of cytotoxic T cells to recognize and destroy malignant cells. Yet most patients do not respond, and even among initial responders, resistance and relapse are common. Researchers have therefore been searching for safe adjuvants—treatments that can make checkpoint blockade more effective without adding substantial toxicity.
The team first screened polysaccharide-rich extracts prepared from six traditional medicines. Polysaccharides are long chains of sugar molecules whose biological effects depend on their precise chemical architecture, including the identity of their component sugars, the bonds connecting them and the branching patterns along the chain. The most promising candidate was Wolfiporia cocos polysaccharide, or WCP. In mice bearing tumors implanted beneath the skin, combining WCP with αPD1 produced stronger tumor control than either treatment alone. The improvement was observed across multiple subcutaneous tumor models, while the researchers found no obvious signs of systemic toxicity under the conditions tested. That distinction is important because many immune-stimulating compounds can cause inflammation throughout the body, potentially limiting their clinical usefulness.
The researchers then asked whether WCP acted directly on tumor cells or required the intestinal microbiome. When the animals’ gut bacteria were depleted, the polysaccharide largely lost its ability to enhance αPD1 therapy. This result suggested that WCP was not simply entering the bloodstream as an intact drug and acting directly on the tumor. Instead, microbes appeared to be processing it into one or more biologically active products. Microbiome analysis identified an increase in the abundance of Turicibacter in animals receiving the combined WCP and αPD1 treatment. The association did not by itself prove that the bacterium caused the therapeutic benefit, but it provided a lead for functional experiments aimed at tracing the chemical transformation.
Those experiments focused on Turicibacter sanguinis and an enzyme linked to a gene or protein designated FUC2. Fucosidases are glycosidases: enzymes that cut the chemical bonds joining fucose residues to larger carbohydrate structures. Fucose is a six-carbon sugar found in many biological glycans, including components of microbial cell surfaces, intestinal mucus and plant- or fungus-derived polysaccharides. The study’s biochemical evidence indicated that FUC2-associated α-L-fucosidase activity could release L-fucose from WCP. In practical terms, the gut bacterium appeared to act as a microscopic processing unit, converting a complex carbohydrate that the mammalian digestive system may not fully break down into a smaller metabolite capable of influencing host immunity.
Several lines of evidence supported this proposed pathway. The investigators used serum metabolomics to survey changes in small molecules circulating through the animals’ blood, then specifically measured L-fucose with targeted assays. They also carried out microbial add-back experiments, reintroducing selected bacteria into microbiota-depleted mice, and engineered Escherichia coli to express the FUC2 enzyme. Supplementing animals with T. sanguinis or with the engineered bacterium partially restored the ability of WCP to improve αPD1 treatment, even after the broader microbial community had been disrupted. Because the rescue was partial rather than complete, the enzyme is unlikely to be the only factor involved. Other microbes, metabolites, immune signals or interactions among bacterial species may also contribute, but the results identify fucosidase-mediated release of L-fucose as a mechanistically testable component.
The immune response inside the tumors offered a second critical piece of evidence. Both WCP and L-fucose increased the accumulation of CD8-positive T cells producing interferon-γ, or IFNγ. These cells are among the immune system’s most effective antitumor agents: after recognizing tumor-associated antigens, they can release cytotoxic molecules and inflammatory signals that damage malignant cells. IFNγ also reshapes the tumor microenvironment by increasing antigen presentation and influencing the behavior of neighboring immune and stromal cells. When the researchers depleted CD8-positive T cells, the antitumor effect disappeared, indicating that the therapeutic benefit was not merely a consequence of slowed tumor growth or a nonspecific metabolic change. It depended on the adaptive immune cells that checkpoint blockade is designed to reactivate.
The study also tested whether WCP could help in a setting meant to mimic treatment-resistant disease. The researchers colonized mice with fecal microbiota obtained from cancer patients who had not responded to immunotherapy. In that context, adding WCP still enhanced αPD1 efficacy. The result is potentially significant because previous research has linked the composition and function of the gut microbiome to checkpoint immunotherapy outcomes. However, a fecal microbiota transfer model is not the same as a human clinical trial. The bacterial communities established in mice may differ from those in their donors, and the animals’ immune systems, diets and tumor models do not reproduce the full complexity of human cancer. The experiment therefore suggests that the polysaccharide may overcome at least some features of a nonresponsive microbial environment, but it does not show that WCP will make anti-PD-1 drugs effective for patients who currently fail to benefit.
The work’s broader implication is that the microbiome may be more than a collection of bacterial species associated with treatment response. Its enzymes could be active determinants of how food-derived or medicinal carbohydrates are converted into immune-modulating molecules. This shifts attention from asking which bacteria are present to asking what biochemical reactions they can perform under particular dietary and therapeutic conditions. A structurally defined polysaccharide such as WCP might eventually be developed as a standardized companion to checkpoint blockade, provided its composition, dose, pharmacology and safety can be established. Before that possibility can be considered clinically, the pathway will need validation in additional models and human samples, along with studies of drug interactions, long-term immune effects and the variability of fucosidase genes across individuals. The authors’ findings nevertheless offer a striking example of how a gut microbial enzyme can unlock the hidden immunological potential of a complex carbohydrate and point toward more personalized combinations of cancer therapy and microbiome-directed treatment.
