Leveraging the Microbiome to Enhance Cancer Treatment

Alan P. Lyss, MD

April 19, 2021

Research suggests certain gut bacteria can reduce the efficacy of radiotherapy against cancers, but targeting those bacteria with vancomycin can reverse this effect.

Facciabene reviewed this research at the AACR Virtual Special Conference: Radiation Science and Medicine.

According to Facciabene, "gut microbiota" includes the more than 1,000 different strains of bacteria living in human intestines. He indicated that the average human has 10 times more bacteria than cells in the body and 150 times more genes in the gut microbiome than in the human genome.

In healthy individuals, the gut microbiota play a key role in intestinal function and digestive processes, modulation of hormones and vitamin secretion, energy extraction from food, and development and maintenance of a balanced immune system.

"Dysbiosis" is the term applied to a change in the composition, diversity, or metabolites of the microbiome from a healthy pattern to one associated with disease. Antibiotic therapy is a classic cause of dysbiosis, and dysbiosis has been implicated in a variety of inflammatory diseases.

The mechanisms by which the gut microbiome could influence systemic immunity is not known but is relevant to cancer therapy response. Augmenting the frequency and durability of response to immune-targeted treatments – potentially by manipulating the influence of gut microbiota on the immune system – could be highly impactful.

Gut Microbiota and Radiation-Induced Cell Death

Immunogenic cell death – a process by which tumors die and release their intracellular molecular contents – is one of the mechanisms by which radiotherapy kills cancer cells.

Tumor cells succumbing to immunogenic cell death stimulate antigen presenting cells, such as dendritic cells, that engulf tumor antigens and cross-present them to CD8+ cytotoxic T lymphocytes. This process culminates in the generation of a specific immune response capable of killing the malignant cells in the irradiated area, but it also impacts distant nonirradiated tumors – an abscopal effect.

Facciabene and colleagues hypothesized that alterations of the gut microbiota could have an impact on the effect of radiotherapy. To investigate this, they studied mouse models of melanoma.

The team allowed B16-OVA tumors to grow for 9-12 days, then delivered a single dose of radiotherapy (21 Gy) to one – but not all – tumors. Simultaneously with the delivery of radiotherapy, the investigators started some animals on oral vancomycin. The team chose vancomycin because its effects are localized and impact the gut microbiota directly, without any known systemic effects.

Results showed that vancomycin significantly augmented the impact of radiotherapy in the irradiated area and was associated with regression of remote tumors.

The effects of the combination treatment on tumor volume were significantly greater than the effects of either treatment alone. Since manipulation of the gut microbiome potentiated radiotherapy effects both locally and distantly, the investigators concluded that immunogenic cell death may be involved in both the local and abscopal effects of radiotherapy.

When the experiment was repeated with a lung tumor model, similar findings were observed.

Involvement of Cytotoxic T Cells and Interferon-Gamma

Facciabene and colleagues found that the irradiated and unirradiated B16 OVA melanoma tumors treated with the radiotherapy-vancomycin combination were infiltrated by CD3+ and CD8+ T cells.

The investigators selectively depleted CD8+ T cells by pretreating the mice with an anti-CD8 monoclonal antibody. Depletion of CD8+ cells prior to administering radiotherapy plus vancomycin abrogated the antitumor effects of the combination treatment, demonstrating that the CD8+ T cells were required.

To characterize the antigen specificity of the tumor-infiltrating CD8+ T cells, Facciabene and colleagues used OVA MHC class 1 tetramer. Tumors from mice treated with vancomycin alone, radiotherapy alone, or the combination were dissected. Individual dendritic cells were assayed for OVA tetramer by flow cytometry.

The investigators found that tumors from mice treated with radiotherapy plus vancomycin had a significantly higher number of OVA-specific CD8+ T cells, in comparison with untreated tumors or tumors treated with either vancomycin alone or radiotherapy alone. Since antibody that impaired recognition of MHC class I peptides by T cells ablated the effect, it was clear that antigen recognition was vital.

Interferon-gamma (IFN-gamma) is known to play a critical role in both differentiation and effector functions of CD8+ cytolytic T cells in the antitumor immune response. To determine whether IFN-gamma is involved in the antitumor effects of the radiotherapy-vancomycin combination, the investigators measured intratumoral expression of IFN-gamma in the tumors 5 days after radiotherapy.

IFN-gamma messenger RNA expression levels were significantly elevated in the combination treatment group when compared with either treatment alone. In B16-OVA melanoma–challenged knockout mice, the enhancement of the radiotherapy effects by vancomycin was ablated.

The investigators concluded that vancomycin remodels the tumor microenvironment and increases the functionality of tumor-infiltrating, tumor-specific, CD8+ T cells. Furthermore, IFN-gamma is required to augment the radiotherapy-induced immune effect against the tumor.

Potential Biochemical Mediators of Immune Effects

The gut microbiota aid host digestion and generate a large repertoire of metabolites after defermentation of fiber. Short-chain fatty acids (SCFAs) constitute the major products of bacterial fermentation.

Acetic acid, propionic acid, and butyric acid represent 95% of total SCFAs present in the intestine. SCFAs are known to directly modulate cytokine production and dendritic cell function.

In their study, Facciabene and colleagues focused on butyric acid. Using mass spectroscopy, they demonstrated that vancomycin treatment reduces butyrate concentrations in tumor and tumor-draining lymph nodes by eradicating the major families of SCFA-producing Clostridia species.

To test whether supplementing butyrate could influence the synergy of the radiotherapy-vancomycin combination in vivo, the investigators added sodium butyrate to the mice's drinking water when starting vancomycin treatment. The team then challenged the mice with B16-OVA tumors and treated them with radiotherapy.

In agreement with the group's prior findings, vancomycin enhanced the tumor-inhibitory effects of radiotherapy, but dietary butyrate inhibited the benefit. The investigators found a significant decrease in the population of B16-OVA–presenting dendritic cells in the lymph nodes of mice receiving the supplemental butyrate.

Facciabene said these findings were supported by a recent publication. The authors observed that butyrate inhibited type I IFN expression in dendritic cells and radiotherapy-induced, tumor-specific cytotoxic T-cell immune responses without directly protecting tumor cells from the cytotoxic effects of radiotherapy.

Wide-Ranging Implications

Overall, Facciabene's research has shown that:

  • Vancomycin significantly enhances the tumor inhibitory effect of targeted radiation, including abscopal effects.

  • The synergistic effects are dependent upon IFN-gamma and CD8+ cells.

  • Depletion of some gut microbiome species increases antigen presentation by dendritic cells. This is mediated by SCFAs produced by certain bacterial families.

  • There are promising new strategies to improve responses to radiotherapy, including targeting gut microbiota.

A clinical trial (NCT03546829) of vancomycin plus stereotactic body radiation in patients with locally advanced non–small cell lung cancer has been launched to investigate these findings further. Early data analysis has shown a significant impact of vancomycin on several species of gut microbiota, according to Facciabene.

Revolutionary results from immune-targeted therapy in the recent past have highlighted the important role the immune system can play in fighting cancer. Still, up to one-third of cancer patients fail to respond to overtly immune-targeted therapy.

The ability to inhibit cancer cells from evading immune surveillance by using new adjuvants – including those acting on non-traditional targets like gut microbiota – could herald the next major advances in cancer therapy. During his presentation, Facciabene gave participants an enticing hint of what could be coming for cancer patients in the years ahead.

Facciabene reported having no relevant disclosures.

Lyss was a community-based medical oncologist and clinical researcher for more than 35 years before his recent retirement. His clinical and research interests were focused on breast and lung cancers, as well as expanding clinical trial access to medically underserved populations. He is based in St. Louis. He has no conflicts of interest.

This article originally appeared on MDedge.com, part of the Medscape Professional Network.

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