Gut Bacteria May Shape Immune Responses in Breast Cancer
- Sarah White
- 11 minutes ago
- 2 min read
At the 2025 San Antonio Breast Cancer Symposium, the UW Cancer Vaccine Institute shared new research revealing how a unique population of immune cells, shaped in part by the gut microbiome, may influence breast cancer growth and treatment response. Our team’s findings shed light on these immune cells called BAC‑TA T cells, their presence in newly diagnosed patients, and how precision probiotics could one day help shift the immune environment to better fight tumors.
The immune system can be a powerful ally against breast cancer. However, many breast tumors contain immune cells that actually suppress the body’s ability to fight cancer, allowing tumors to grow more aggressively. At the Cancer Vaccine Institute, we have discovered that some of these suppressive immune cells may be influenced by bacteria living in the gut.
We call these cells BAC-TA T cells. They recognize proteins found on certain gut bacteria as well as similar-looking proteins on tumor cells. In healthy settings, BAC-TA T cells may help protect beneficial gut bacteria, which play important roles in metabolism and overall health. But in our mouse studies, we found that these cells can leave the gut, travel to breast tumors, and accelerate tumor growth.
We wanted to know whether this same process might be happening in people with breast cancer.
To investigate, we collected blood and stool samples from 56 newly diagnosed breast cancer patients before they received any treatment, and from 14 age-matched individuals without cancer. We measured BAC-TA T cells in blood and analyzed stool samples to identify the types and amounts of bacteria present. For three patients, we also examined tumor biopsy tissue to look for BAC-TA T cells inside the tumor itself.
What we found:
Breast cancer patients had higher levels of BAC-TA T cells in their blood than individuals without cancer.
In all three patients with available tumor tissue, BAC-TA T cells were present in both the tumor and the blood.
Their stool samples showed increased levels of specific bacteria whose proteins closely resemble proteins found in tumors.
These findings support our hypothesis that BAC-TA T cells may contribute to an immune-suppressive environment in breast tumors, potentially promoting tumor growth and making treatments less effective.
Our work is ongoing. In separate mouse studies, we have shown that introducing a precision probiotic—containing bacteria that do not share similarities with tumor proteins—can reduce BAC-TA T cell levels. This leads to slower tumor growth and improves the effectiveness of cancer vaccines. We are now determining the optimal dosing, timing, and safety of this probiotic in preparation for a human clinical trial.
We are also studying whether certain groups of breast cancer patients are more likely to have high levels of BAC-TA T cells, and whether specific immune-suppressive proteins (cytokines) correlate with BAC-TA T cell levels.
Newly diagnosed patients could receive a blood test to measure BAC-TA T cells and a stool test to identify gut bacteria that stimulate these cells. If both are elevated, a precision probiotic could be used to reshape the gut microbiome, reduce BAC-TA T cells, and help create an immune environment more capable of killing tumor cells—ultimately improving patient outcomes.
