Gut Microbes and Colon Cancer Prevention

Microsatellite-stable (MSS) colorectal cancer is the most common form of the disease, accounting for roughly 85% of cases. Unlike its microsatellite instability–high (MSI-H) counterpart, MSS CRC remains stubbornly resistant to immunotherapy drugs known as immune checkpoint blockade (ICB). New research argues this resistance is not simply due to a lack of immune cells, but is an active state maintained by the tumor’s entire ecosystem.

## The Immunometabolic Barrier: A Self-Reinforcing Resistance Circuit

The review by Yu Mi, Zerong Lin, and Qingyuan Zhang proposes a model where metabolic reprogramming, gut microbial imbalance, and immune suppression create a mutually reinforcing circuit of resistance. They call this the “immunometabolic barrier.”

In MSS tumors, cancer cells shift their metabolism to aerobic glycolysis, even when oxygen is present. This process, known as the Warburg effect, rapidly consumes glucose and produces a flood of lactate. The result is a tumor microenvironment that is both glucose-poor and lactate-rich. For cytotoxic T cells and natural killer (NK) cells, which need glucose to function, this is a hostile landscape. Their ability to attack cancer cells is directly impaired. Simultaneously, the metabolic waste products like lactate promote the activity and recruitment of regulatory T cells and suppressive myeloid cells, which actively shut down immune responses.

## How a Disrupted Gut Microbiome Fuels Tumor Resistance

The research connects this local tumor metabolism directly to the broader gut ecosystem. Colorectal cancer is frequently associated with significant microbial imbalance, or dysbiosis. This imbalance damages the integrity of the intestinal barrier, allowing bacterial products and other inflammatory signals to leak through. More critically, the altered microbial community changes the profile of metabolites it produces.

These microbial metabolites can feed directly into the inhibitory pathways that characterize the immunometabolic barrier. Two key pathways highlighted are adenosine signaling and the kynurenine–aryl hydrocarbon receptor (AhR) axis. Both are potent immunosuppressive signals that can be fueled by gut-derived molecules, further entrenching the tumor’s “cold,” non-inflamed state. This creates a direct link between gut health and the tumor’s ability to resist modern therapies, a concept that extends to broader health as discussed in the cross-site article on Gut Microbiome, Aging, and Longevity.

## Implications for Treatment: Beyond Single-Agent Immunotherapy

The core conclusion is that using ICB alone against MSS CRC is like trying to put out a fire while someone else pours gasoline. The treatment targets one component (immune checkpoints) while the ecosystem actively works to negate it. The authors argue that overcoming resistance requires strategies to “reprogram the entire ecosystem.”

This means developing combination therapies that pair immunotherapy with agents that target tumor metabolism—such as drugs that inhibit lactate production or improve glucose availability for T cells—and with microbiome-targeted interventions. The latter could include specific probiotic (probiotic 50 billion CFU)s, prebiotic (prebiotic fiber supplement)s, or even fecal microbiota transplantation (FMT) designed to correct the dysbiosis and restore a metabolite profile that supports immunity. The importance of diet in sustaining a healthy microbiome is a recurring theme, as seen in our article on how a Persistent Poor Diet Blocks Exercise, Inulin FMT Benefits.

Furthermore, the review stresses the need for better biomarkers to guide these complex combinations. Spatial omics, which maps where different cells and metabolites are located within the tumor, and multi-omic profiling will be essential to identify which patients have which dominant resistance circuits (metabolic, microbial, or both) and tailor treatment accordingly.

## A New Way to View MSS Colorectal Cancer

The most significant shift proposed is a redefinition of MSS CRC. It should not be seen merely as a genetic subtype defined by what it lacks (microsatellite instability). Instead, it is a dynamic, adaptive state of immune resistance, actively maintained by the integrated biology of the tumor cells, their local metabolism, and the host’s gut microbiome. Viewing it as an ecosystem problem opens the door to ecosystem-level solutions, moving the field beyond the search for a single magic bullet.

*Source: Mi, Y., Lin, Z., & Zhang, Q. (2024). The immunometabolic barrier: A new perspective on microenvironmental immunosuppression in MSS colorectal cancer. Cancer Biology & Therapy. DOI: 10.66505/cbtt.v1i2.39.*