BCG Vaccine Cut Tau Tangles and Restored Memory in Dementia Mice

Mice vaccinated with BCG at three months of age were about five times more likely to show reduced tau deposits in the brain eight to nine months later, according to a Johns Hopkins University School of Medicine study published in Science Advances. The paper, titled "BCG vaccination mitigates tau pathology and restores cognitive functions in PS19 mice," marks what the researchers believe is the first study of BCG and dementia in a tau-pathology mouse model.

The work matters because it gives a mechanistic footing to a string of human observations. Prior studies cited by the team linked BCG immunotherapy for non-muscle invasive bladder cancer to a 12% to 78% reduced risk of dementia over three to 15 years. The shingles vaccine has been linked to a 20% drop in dementia incidence. None of those studies show cause and effect. This one tests the mechanism directly in animals.

What did the experiment measure?

The team, led by William Bishai, MD, PhD, co-director of the Johns Hopkins Center for Tuberculosis Research, used PS19 (P301S) mice, which overexpress P301S tau and develop neuronal loss, microgliosis, neurofibrillary tangle deposition, and cognitive impairment. Vaccinated cohorts received BCG at three months old. Controls received saline, alongside an age-matched group without dementia that also got saline.

Assessments came eight to nine months later, using brain imaging, a novel object recognition test, neuroinflammation markers, and expression of Alzheimer's-linked genes.

The results stack up. BCG-treated mice were four times less likely to show markers of sustained inflammation. Untreated mice activated 716 genes tied to Alzheimer's disease, cell death, and neuroinflammation; BCG normalized all but nine of them. Treated animals also showed less brain atrophy, better maintenance of body weight, longer survival, and stronger performance on object recognition tasks.

The authors wrote that BCG therapy "restored glutamate levels in vivo and improved recognition-memory performance, body weight, and survival of PS19 mice, suggesting key disease modifying effects."

How might a TB vaccine protect the brain?

Bishai points to trained immunity, the process by which innate immune cells such as monocytes and macrophages undergo epigenetic and metabolomic reprogramming after a first stimulus, then mount a stronger response to a second, unrelated antigen. That learned response is normally the business of adaptive cells like T cells.

"We're hard at work trying to understand the detailed mechanism for how it works, but this research points to early clues that aspects of trained immunity are likely driving this response," Bishai said. His working hypothesis: myeloid cells in the central nervous system use information from BCG to recognize plaques and tangles and respond to them. "BCG seems to be reducing the degree of hyperinflammation in the brain—which is a good thing—and it may be doing it through a trained immune cell mechanism," he added.

The team stated in the paper: "Mechanistically, our study identified BCG-induced trained immunity properties as playing key roles in the clearance of tau NFTs." Others have reported similar BCG-cognition findings in amyloid mouse models, which represent earlier, plaque-driven stages of disease.

What are the limits?

This is one preclinical study in a single mouse model. Doses, timing, and species don't translate directly to humans, and the observational human data that motivated the work can't establish causation. The authors themselves frame the finding as provisional: "Our study strongly suggests that dementia protection by BCG is a credible observation that merits future basic research and large-scale prospective clinical trials to provide definitive answers."

Co-author Barbara Slusher, PhD, professor of neurology and director of the Johns Hopkins Drug Discovery program, called it "a great example of translational science working in reverse—starting with an intriguing observation in people and then going back to the laboratory to understand why it might be happening." She cautions that researchers still need to understand more of the underlying biology before they can say who might benefit, when treatment would need to be given, and whether the effect can be safely reproduced in people at risk for dementia.

"If we can identify the specific immune changes that are protective, we may be able to develop even more targeted approaches for the prevention of Alzheimer's disease," Slusher said. The next steps are mechanistic studies in the lab and, if those hold, the large-scale prospective trials the authors say the finding now deserves.