What starts Alzheimer's-like brain damage? Scientists find new clue

A surprising immune signal outside the brain may help drive damage—raising a key question about where Alzheimer's really begins. Scientists have discovered evidence that some of the immune activity linked to Alzheimer's-like brain damage may start outside the brain, opening a potential new avenue for future treatments. In a study in mice, published in Nature Neuroscience, researchers found that immune cells known as T cells, which have been associated with brain damage in Alzheimer's disease and related disorders, appear to be receiving instructions from lymph nodes rather than from inside the brain itself. The process involves another type of immune cell called dendritic cells. The finding could be significant because researchers have long known that T cells accumulate in the brains of people with Alzheimer's disease and related conditions. These cells are believed to contribute to damage in the brain, but scientists have not fully understood where they come from or what drives them to gather there. Immune Activity May Begin Outside the Brain To investigate, researchers studied mice that develop tau tangles, abnormal twisted clumps of tau protein that are a hallmark of Alzheimer's disease and other conditions known as primary tauopathies. The team found little evidence that a specific type of dendritic cell, known as classical dendritic cells type 1 , was directing T cell activity from within the brain. There were very few of these dendritic cells present in the brain, and those that were there did not appear to interact with the T cells that accumulated after tau tangles developed. The results led researchers to suspect that both dendritic cells and T cells were being activated elsewhere in the body. To test the idea, the scientists removed dendritic cells from lymph nodes and other locations in mice that would normally develop tau tangles and neurodegeneration. Blocking T Cells Reduced Brain Damage in Mice The effects were striking. The unusually high numbers of T cells found in the brain disappeared, and levels of CD8 T cells were significantly reduced. At the same time, brain damage associated with neurodegeneration was greatly reduced. Perhaps most notably, the amount of tau tangles in the brain remained unchanged. In other words, even though the protein clumps thought to drive disease were still present, the damage to the brain was reduced when the immune pathway involving dendritic cells and T cells was blocked. The mice also maintained their cognitive abilities, suggesting that limiting T cell activity could help slow or reduce cognitive decline associated with Alzheimer's disease. Researchers do not yet know exactly what activates the dendritic cells. However, they believe the most likely explanation begins with tau-related damage to brain cells. Material released from damaged cells may travel from the brain to lymph nodes in the neck, where dendritic cells identify it as something that should be targeted. The dendritic cells then activate T cells, which travel into the brain and contribute to neurodegeneration. The study did not prove that sequence of events, but researchers said it represents the most likely explanation based on their findings. Dr. David Holtzman, the Barbara Burton and Reuben M. Morriss III Distinguished Professor in WashU Medicine's Department of Neurology and the study's senior author, told Newsweek that there is already evidence linking T cells to human Alzheimer's disease and related disorders. "We know that in human Alzheimer's disease and in primary tauopathies such as FTD, PSP, CBD, and CTE that there is an increase in T cells including CD8 T cells in areas of the brain that contain tau pathology. Also, there are genetic changes in the HLA locus in Alzheimer's suggesting T cell involvement," Holtzman said. He added that while the mouse studies provide evidence that T cells can contribute to brain damage, confirming a meaningful role in people will require studies that directly test whether manipulating T cells improves outcomes in patients. Could the Immune System Offer a New Treatment Target? The discovery is particularly interesting because it points to a disease pathway that appears to begin outside the brain. One of the biggest challenges in treating neurological disorders is getting drugs across the blood-brain barrier, which protects the brain from many substances circulating in the body. If key immune signals are occurring in lymph nodes rather than inside the brain, they may be easier to target with treatments. Holtzman said a range of immune-targeting therapies could potentially be explored based on the findings, including drugs that suppress T-cell entry into the brain, JAK-STAT inhibitors, checkpoint inhibitors and T regulatory cell modulators. However, he said such approaches should first be evaluated in animal and cellular models of Alzheimer's disease to determine whether they are effective and safe before being tested in human trials. According to the researchers, many methods for manipulating T cells already exist, have been extensively studied and, in some cases, are already approved for other diseases. However, most have not yet been explored for neurodegenerative conditions such as Alzheimer's. Potential future approaches could include blocking dendritic cell activity, preventing dendritic cells from activating T cells, or identifying and interrupting the signal that guides T cells into the brain. The researchers are now investigating whether blocking dendritic cell function later in life, around the time tau tangles begin to form, could provide similar benefits. They are also working to identify the exact signal that directs T cells to the brain. One of the key unanswered questions, according to Holtzman, is what signals originating from the brain trigger immune activity in the lymph nodes. "I think understanding which antigen or more likely which groups of antigens from the brain are facilitating T cell priming in the lymph nodes is a very important unanswered question to address," he said. Reference Hao Hu et al. Priming of CD8+ T cells by peripheral dendritic cells exacerbates tau-mediated neurodegeneration. Nature Neuroscience. DOI: 10.1038/s41593-026-02427-5 Contact Newsweek editors on this story: Kara Dolman and Emma Lee-SangScientists have discovered evidence that some of the immune activity linked to Alzheimer's-like brain damage may start outside the brain, opening a potential new avenue for future treatments. , researchers found that immune cells known as T cells, which have been associated with brain damage in Alzheimer's disease and related disorders, appear to be receiving instructions from lymph nodes rather than from inside the brain itself. The process involves another type of immune cell called dendritic cells. The finding could be significant because researchers have long known that T cells accumulate in the brains of people withdisease and related conditions. These cells are believed to contribute to damage in the brain, but scientists have not fully understood where they come from or what drives them to gather there. , abnormal twisted clumps of tau protein that are a hallmark of Alzheimer's disease and other conditions known as primary tauopathies. The team found little evidence that a specific type of dendritic cell, known as classical dendritic cells type 1 , was directingactivity from within the brain. There were very few of these dendritic cells present in the brain, and those that were there did not appear to interact with the T cells that accumulated after tau tangles developed. The results led researchers to suspect that both dendritic cells and T cells were being activated elsewhere in the body. To test the idea, the scientists removed dendritic cells from lymph nodes and other locations in mice that would normally develop tau tangles and neurodegeneration. The effects were striking. The unusually high numbers of T cells found in the brain disappeared, and levels of CD8 T cells were significantly reduced. At the same time, brain damage associated with neurodegeneration was greatly reduced. In other words, even though the protein clumps thought to drive disease were still present, the damage to the brain was reduced when the immune pathway involving dendritic cells and T cells was blocked. The mice also maintained their cognitive abilities, suggesting that limiting T cell activity could help slow or reduce cognitive decline associated with Alzheimer's disease. Researchers do not yet know exactly what activates the dendritic cells. However, they believe the most likely explanation begins with tau-related damage to brain cells. Material released from damaged cells may travel from the brain to lymph nodes in the neck, where dendritic cells identify it as something that should be targeted. The dendritic cells then activate T cells, which travel into the brain and contribute to neurodegeneration. The study did not prove that sequence of events, but researchers said it represents the most likely explanation based on their findings. He added that while the mouse studies provide evidence that T cells can contribute to brain damage, confirming a meaningful role in people will require studies that directly test whether manipulating T cells improves outcomes in patients. The discovery is particularly interesting because it points to a disease pathway that appears to begin outside the brain. Holtzman said a range of immune-targeting therapies could potentially be explored based on the findings, including drugs that suppress T-cell entry into the brain, JAK-STAT inhibitors, checkpoint inhibitors and T regulatory cell modulators. However, he said such approaches should first be evaluated in animal and cellular models ofAccording to the researchers, many methods for manipulating T cells already exist, have been extensively studied and, in some cases, are already approved for other diseases. However, most have not yet been explored for neurodegenerative conditions such as Alzheimer's. Potential future approaches could include blocking dendritic cell activity, preventing dendritic cells from activating T cells, or identifying and interrupting the signal that guides T cells into the brain. The researchers are now investigating whether blocking dendritic cell function later in life, around the time tau tangles begin to form, could provide similar benefits. They are also working to identify the exact signal that directs T cells to the brain. One of the key unanswered questions, according to Holtzman, is what signals originating from the brain trigger immune activity in the lymph nodes. "I think understanding which antigen or more likely which groups of antigens from the brain are facilitating T cell priming in the lymph nodes is a very important unanswered question to address," he said. Hao Hu et al. Priming of CD8+ T cells by peripheral dendritic cells exacerbates tau-mediated neurodegeneration. Nature Neuroscience. DOI: 10.1038/s41593-026-02427-5