In lab, lung tumor bacteria and a B2 metabolite activate MAIT cells

A bacterium from a lung tumor sat in a cell culture beside a metabolite of vitamin B2. The combination increased the amount of MR1 on antigen-presenting immune cells and activated MAIT cells, a class of innate immune cells. Researchers at the Johns Hopkins Kimmel Cancer Center and the Bloomberg~Kimmel Institute for Cancer Immunotherapy reported the result in Proceedings of the National Academy of Sciences.

The mechanism gives the tumor microbiome a more active role in the experiment. MAIT cells recognize MR1 when antigen-presenting cells display it in response to bacteria, injury or other danger. The team expected bacteria that make the vitamin B2 metabolite to drive that response. Instead, one Enterococcus bacterium that did not make the metabolite also strongly enhanced MAIT-cell activation when the metabolite was added to the culture.

The clue came alongside human cancer research, but it is not yet proof of benefit. In an earlier study of lung cancer patients receiving neoadjuvant PD-1 blockade immunotherapy, which helps the immune system's T cells recognize tumors, Pakhi Birla found that a patient with a strong response had many MAIT cells. That observation suggested a possible contribution, not a clinical demonstration that MAIT cells caused the response.

So what changes, concretely? The team is testing whether injecting the B2 metabolite into lung tumors in mice can produce an immune response, and it plans to explore T cells engineered to recognize MR1. If those studies succeed, Franck Housseau says the approach could point toward a tumor-agnostic, off-the-shelf therapy rather than one customized to each patient's tumor. For now, the evidence remains in cell cultures and preclinical research; no human treatment has been established.