Summary: A new study demonstrates that the BCG vaccine actively remodels the immune environment surrounding the human brain and spinal cord. By tracking older adults over 12 months, the research team discovered that BCG enhances the responsiveness of central nervous system immune cells without triggering destructive inflammation.
Crucially, in individuals completely free of pre-existing Alzheimer’s pathology, the vaccine systematically flipped the biological balance of amyloid-beta, facilitating the direct clearance of this neurotoxic protein out of the brain fluid and into the bloodstream.
Key Facts
- The Blood-Brain Immune Leap: While decades of trained immunity research have proved that BCG reprograms immune traits inside circulating blood cells, this study is the first to confirm that the vaccine’s therapeutic remodeling crosses into the cerebrospinal fluid protecting the brain and spinal cord.
- Enhanced Defense Without Inflammation: After receiving the skin vaccination, participants’ central immune cells showed an elevated, robust responsiveness to separate immune challenges. Remarkably, this hyper-vigilant state occurred without any increase in baseline inflammatory markers—a critical safety threshold, since chronic neuroinflammation is a known driver of brain cell death.
- The 12-Month Amyloid Cleansing Stream: In healthy older adults without pre-existing dementia pathology, BCG triggered a dramatic shift in amyloid-beta clearance pathways. Over 12 months, toxic amyloid levels declined significantly within the cerebrospinal fluid while concurrently rising inside blood samples, proving that the vaccine helps flush these dangerous plaques out of the central nervous system.
- The Timing Threshold: This protective amyloid-shifting mechanism vanished completely in participants who already possessed established biomarker evidence of Alzheimer’s disease pathology. This stark divergence suggests that the BCG vaccine acts strictly as an early-stage preventive intervention, capable of preserving brain health before structural neurodegenerative decay sets in.
- Deep History of Off-Target Success: Mass General Brigham has spent 20 years investigating BCG’s unique immunomodulatory properties. The vaccine is currently in Phase III clinical trials to treat type 1 diabetes by correcting blood sugars and reversing autoimmune destruction, following past trials evaluating its impact on COVID-19.
- Global Epidemiological Context: The study evaluates a targeted, late-life vaccination strategy in older American adults. It does not analyze the long-term neurological impacts of childhood BCG vaccinations, which remain standard at birth across sub-Saharan Africa, Southeast Asia, and Eastern Europe.
Source: Mass General
New research led by Mass General Brigham investigators suggests that the Bacillus Calmette-Guérin (BCG) vaccine — which is delivered through the skin to prevent tuberculosis — may remodel the human brain’s immune environment, offering a potential biological explanation for previously observed associations between BCG vaccination and lower Alzheimer’s disease risk.
The year-long study found that BCG promoted increased responsiveness in immune cells surrounding the brain and modified Alzheimer’s-related biomarkers in older adults without evidence of Alzheimer’s pathology, but not in those with biomarker evidence of the disease.
Findings are published in Communications Medicine.
“The immune system and the brain may be far more connected than we once thought,” said senior and co-corresponding author Steven Arnold, MD, managing director of the Interdisciplinary Brain Center at Mass General Brigham Neuroscience Institute.
“The next step is to test this rigorously in larger, controlled studies, particularly in prevention, where the hope would be to preserve brain health before significant Alzheimer’s disease develops.”
Over the last two decades, researchers at Mass General Brigham have been studying multiple “off-target” benefits of the BCG vaccine in autoimmunity and infection, including ongoing Phase III clinical trials in type 1 diabetes and past Phase II and Phase III trials in COVID-19.
Prior research involving preclinical models, retrospective studies, and randomized clinical trials has suggested that BCG can also reduce Alzheimer’s risk and bring about trained immunity, boosting defenses against unrelated infections and correcting blood sugars. Most prior research on trained immunity has focused on blood, leaving unclear whether BCG influences immune cells in the fluid surrounding the brain and spinal cord as well.
Seeking insight, the research team conducted two related one-year, open-label clinical trials of BCG immunotherapy in 23 adults aged 55 years and older. The cohort included 11 adults with Alzheimer’s pathology and 12 adults without. Cerebrospinal fluid (CSF) and peripheral blood samples were obtained from participants at regular intervals after vaccination.
The study, which was led by co-first authors Marc Weinberg, MD, PhD, Mahesh Chandra Kodali, PhD, and Zhaozhi Li, PhD, found that BCG promoted enhanced immune responses to other immune challenges, suggesting broader effects on immune function. Notably, the heightened immune responsiveness was not accompanied by an increase in inflammatory markers, which is a known risk factor for neurodegeneration.
BCG also shifted the levels of amyloid-beta (a key biomarker of Alzheimer’s disease) in the CSF and bloodstream. In healthier participants without Alzheimer’s disease pathology, amyloid levels declined significantly in brain and spinal fluid, while increasing in blood samples over 12 months.
This shift in balance was not observed in participants with Alzheimer’s pathology, indicating no measurable effect in this group, and suggesting that the timing of administering BCG might affect early disease dynamics and protein clearance from the central nervous system.
The authors note that further research, including placebo-controlled studies, are needed to investigate the relationship between BCG and Alzheimer’s disease. They also noted that the study tested a specific vaccination strategy in older adults and does not examine the effect of BCG vaccination in childhood, which is common in many countries in sub-Saharan Africa, Southeast Asia, and parts of Eastern Europe due to the prevalence of tuberculosis in these regions.
“Vaccines have traditionally been viewed through the lens of infectious disease prevention,” said Weinberg, who contributed to the study while working as a research scientist at Mass General Brigham. Weinberg is now an employee at AbbVie.
“Although more research is needed, these findings suggest they may also influence biological processes involved in brain aging and neurodegenerative disease.”
Authorship: In addition to Weinberg and Arnold, Mass General Brigham authors include Mahesh Chandra Kodali, Jake A. Galler, Arianna R. Tidball, William Cody Reynolds, Cathrine Young, Kelli Devitte-McKee, Hadia A. Fatima, Pia Kivisäkk, Willem M. Kühtreiber, Jessica Gerber, Alison J. McManus, Rudolph E. Tanzi, Sudeshna Das, and Denise L. Faustman. Additional authors include Zhaozhi Li, Mandovi Chatterjee, Arpita Kulkarni, James M. Billingsly, Alexandra L. Bartlett, and Shannan Ho Sui.
Disclosures: Weinberg is an employee of AbbVie Inc. The research reported here was conducted at Massachusetts General Hospital and Harvard Medical School, and AbbVie Inc. had no role in the design, conduct, or reporting of the study. Galler, Arnold, Kodali, Tidball, Reynolds, Gerber, Young, McManus, Devitte-McKee, Kivisäkk, Tanzi, Das, Faustman, and Kühtreiber are employees of The General Hospital Corporation (Massachusetts General Hospital). Faustman reports no consulting fees, funding, or involvement in trial design or manuscript preparation from the drug owner. All other authors declare no competing interests.
Funding: This project was funded by the National Institutes of Health (R25MH094612, T32-MH112485, U13AG067696, UL1 TR002541), National Institute on Aging (P30AG062421, UM1TR004408), the Alzheimer’s Association (AACSF-22-970716, PTC REG-20-653582), the Massachusetts Life Sciences Center, and the Cure Alzheimer’s Fund.
Key Questions Answered:
A: The answer lies in a fascinating biological phenomenon called “trained immunity.” When the live BCG vaccine is delivered to the skin, it doesn’t just teach the body to fight tuberculosis; it acts as a drill sergeant that completely retrains and upgrades the baseline habits of the body’s innate immune system. This study proved for the first time that this immune retraining travels all the way into the cerebrospinal fluid surrounding the brain. It boosts the responsiveness of the brain’s local immune cells, allowing them to clear out toxic waste more efficiently without triggering harmful inflammation.
A: In healthy older adults, the BCG vaccine acts like a metabolic sewage pump for the central nervous system. Over the course of the 12-month study, researchers watched amyloid-beta levels drop significantly inside the brain and spinal fluid, while simultaneously spiking inside the patients’ bloodstream. This mathematical flip proves that the vaccine helps open up biological clearance pathways, actively flushing loose, toxic amyloid proteins out of the brain tissue and draining them into the blood so the body can safely break them down and dispose of them.
A: Unfortunately, no. The study unmasked a very strict boundary: the vaccine’s protective amyloid-clearing effects were only observed in older adults who entered the trial without pre-existing Alzheimer’s pathology. For participants who already had established biomarker evidence of the disease, the vaccine had no measurable impact. Dr. Steven Arnold emphasizes that BCG is a tool for precision prevention, not a cure. The goal is to administer the vaccine to healthy, aging individuals to shield and preserve their brain health before any significant neurodegenerative damage can take root.
Editorial Notes:
- This article was edited by a Neuroscience News editor.
- Journal paper reviewed in full.
- Additional context added by our staff.
About this neurology research news
Author: Brandon Chase
Source: Mass General
Contact: Brandon Chase – Mass General
Image: The image is credited to Neuroscience News
Original Research: Open access.
“Bacillus Calmette–Guérin (BCG) immunotherapy reprograms CNS immunity and alters Alzheimer’s biomarkers: results from two open-label clinical trials” by Marc S. Weinberg, Mahesh Chandra Kodali, Zhaozhi Li, Jake A. Galler, Arianna R. Tidball, William Cody Reynolds, Cathrine Young, Kelli Devitte-McKee, Hadia A. Fatima, Pia Kivisäkk, Mandovi Chatterjee, Arpita Kulkarni, James M. Billingsly, Alexandra L. Bartlett, Shannan Ho Sui, Willem M. Kühtreiber, Jessica Gerber, Alison J. McManus, Rudolph E. Tanzi, Sudeshna Das, Denise L. Faustman & Steven E. Arnold. Communications Medicine
DOI:10.1038/s43856-026-01691-7
Abstract
Bacillus Calmette–Guérin (BCG) immunotherapy reprograms CNS immunity and alters Alzheimer’s biomarkers: results from two open-label clinical trials
Background
Immune aging may contribute to Alzheimer’s disease. Bacillus Calmette–Guérin (BCG), a vaccine known to induce trained immunity, has been linked to reduced Alzheimer’s risk in prior studies. However, whether trained immunity can be observed in the human central nervous system remains unclear. We assessed whether BCG induces trained immunity–like responses in adults with and without Alzheimer’s-related changes.
Methods
We conducted two related one-year, open-label clinical trials in adults aged 55 years or older (n = 12 without Alzheimer’s-related pathology; n = 11 with Alzheimer’s-related pathology) recruited at a single center. Participants received two intradermal BCG vaccinations one month apart. Protocol-defined objectives included safety, neurocognitive outcomes, and longitudinal immune and Alzheimer’s biomarker changes in blood and cerebrospinal fluid. Immune responses were assessed using cytokine assays and single-cell profiling. All enrolled participants were included where data were available; longitudinal changes were analyzed using mixed-effects models.
Results
Here we show that BCG induces persistent, trained immunity–like changes in immune cells in cerebrospinal fluid, including enhanced innate responsiveness and associated transcriptional programs. These responses differ from blood, suggesting compartment-specific immune imprinting. In participants without Alzheimer’s-related changes, these immune shifts are accompanied by decreased amyloid-β levels in cerebrospinal fluid and increased levels in blood. BCG was well tolerated, with no unexpected safety signals observed.
Conclusions
These findings suggest trained immunity–like responses in the central nervous system that may influence Alzheimer’s-relevant pathways. This approach may represent an early neurodegenerative intervention strategy, although larger controlled studies are needed to confirm these observations.
Trial registration
ClinicalTrials.gov NCT04507126 (June 23, 2020) and NCT05004688 (August 6, 2021).
