Lung tumor bacteria plus vitamin B2 metabolite may boost immune response to cancer
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Bacteria in lung tumors may boost the activation of innate immune cells in the presence of a common vitamin metabolite, according to a preclinical study by researchers from the Johns Hopkins Kimmel Cancer Center and its Bloomberg~Kimmel Institute for Cancer Immunotherapy.
The study is published in the Proceedings of the National Academy of Sciences. It suggests a new way to stimulate the immune system to attack cancer. The study showed that adding a metabolite of vitamin B2 to bacteria found in lung tumors increased the cell-surface expression of a protein called MR1 on antigen-presenting immune cells, activating innate immune cells called mucosal-associated invariant T cells (MAIT).
"We are opening a new field of immunotherapy by using a metabolite that, in combination with these bacteria, increases the expression of MR1 and triggers the activation of MAIT cells toward the tumor," said Franck Housseau, Ph.D., PharmD, an associate professor of oncology at the Johns Hopkins University School of Medicine and now research director of the French National Institute of Health and Medical Research, who studies interactions between the microbiome and tumors in response to immunotherapy.
An overlooked immune first responder
Many emerging cancer therapies, such as immune checkpoint inhibitors, focus on reinvigorating the adaptive immune system's T cells to help them recognize and destroy tumor cells. Less attention has been paid to MAIT cells, Housseau said.
MAIT cells act as first responders to injury or infection, detecting and destroying threats in the mucosa that forms a barrier to the outside world. But emerging research suggests these cells might also be used to fight cancer.
MAIT cells recognize a protein called MR1 that antigen-presenting immune cells express in response to bacteria, injury or other dangers. But microbiome sequencing of human tumors and RNA sequencing of immune cells from patient samples, along with cell culture experiments, revealed an unexpected finding.
Housseau and his team expected bacteria that produce the B2 metabolite to activate MAIT cells, but one of the enterococcal bacteria that did not produce the B2 metabolite also greatly enhanced MAIT cell activation when a B2 metabolite was added to cell cultures.
"Tumor-associated bacteria may regulate the immune response to tumors by regulating the expression of MR1 on antigen-presenting cells and activating MAIT cells in the tumor microenvironment, which could have antitumor effects," said Pakhi Birla, Ph.D., a cancer immunologist who led the study while completing her doctorate in the laboratories of Housseau and Drew Pardoll, M.D., Ph.D.
A clue from patient response
In a previously published study of lung cancer patients treated with neoadjuvant PD-1 blockade immunotherapy by Pardoll and Kellie Smith, Ph.D., an associate professor of oncology at Johns Hopkins Medicine, Birla found that a patient who had a strong response to therapy had many MAIT cells, suggesting they may have contributed to the patient's therapeutic success.
This discovery was facilitated by an interdisciplinary collaboration among Housseau, an expert in mucosal immunology; Pardoll, director of the Bloomberg~Kimmel Institute for Cancer Immunotherapy and an expert in tumor immunology and immunotherapy; Cynthia L. Sears, M.D., an infectious disease expert and the Bloomberg~Kimmel Professor of Cancer Immunotherapy at Johns Hopkins; and Fyza Y. Shaikh, M.D., Ph.D., an assistant professor of cancer immunology at Johns Hopkins Medicine.
Testing a broader therapy concept
The team has already begun preclinical studies to determine whether injecting the B2 metabolite into lung tumors in mice can stimulate an immune response. They also plan to study whether they can engineer adaptive immune T cells to recognize MR1 and help them target tumor cells.
Unlike existing adaptive immune T cell therapies, which are customized to target proteins specific to an individual patient, T cell therapies targeting MR1 could work for almost any patient.
"It is very expensive and very complex to personalize immunotherapy, maybe not accessible to every patient," Housseau said. "But if we are successful, we could develop a new concept: tumor-agnostic, off-the-shelf therapy that works for every patient."
Publication details
Pakhi Birla et al, Select intratumoral riboflavin-auxotrophic Enterococcus species enhance cell surface MR1 expression and MAIT TCR activation in lung cancer, Proceedings of the National Academy of Sciences (2026). DOI: 10.1073/pnas.2617943123
Journal information: Proceedings of the National Academy of Sciences
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Sadie Harley
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