The immune system is a complex network involving two types of responses: innate and adaptive. The innate response acts as the body’s first line of defense against pathogens, while the adaptive system relies on specialized cells that learn to recognize and attack specific invaders. Microglia serve as the brain’s primary innate defenders. For many years, scientists believed that this microglial response was sufficient to protect the brain and that the blood-brain barrier (BBB) kept peripheral adaptive immune cells out. However, recent research has revealed significant communication between the brain and peripheral immune systems, even under normal conditions. More importantly, evidence suggests that during Alzheimer’s disease (AD), peripheral immune cells, particularly killer T cells, can contribute to brain inflammation and neurodegeneration. These cells, aided by microglia, infiltrate brain regions affected by tau protein buildup. Understanding these brain-peripheral immune interactions may provide crucial insights into new therapeutic avenues for treating AD that target the harmful effects of killer T cells.
Dr. Mathis recently discovered that in mice, the dura layer of the meninges—the membranous structure encasing the brain—is home to a small but potent population of a type of T cell called regulatory T cells (Tregs). Tregs normally act to end an immune response once a pathogen or other challenge is cleared, thereby preventing excessive killer T cell activity and protecting healthy cells from inadvertent damage. More recently, Tregs within tissues have been shown to regulate most types of innate immune cells and even non-immune cells like stem cells. As expected, the Tregs in the dura inhibit the activation of local immune cells, including killer T cells, and block their entry into the brain.
This discovery led Dr. Mathis to wonder: could these dural T cells help counter the harmful effects of killer T cells in AD? Her lab predicts that increasing Tregs in the brain could reduce neuroinflammation and other related AD pathologies. In their recent study in mice, the team found that experimentally removing dura Tregs allowed other immune cells (T and natural killer cells) to enter the brain and activate astrocytes and microglia, causing neuroinflammation and leading to the death of neuron precursor cells in the hippocampus. These findings show that dural Tregs play an indirect but crucial role in protecting the brain from harmful immune cell infiltration. Given these results in mice, the Mathis lab investigated whether similar populations of dural Tregs exist in human tissue—and they do. Dr. Mathis hypothesizes that these cells fail to function properly in AD patients, either as a driver or a consequence of the disease.
They are testing this hypothesis through two aims. In the first aim, they are cataloging the different types of immune cells present in human meninges. For these experiments, immune cells will be isolated and sorted using flow cytometry from dura layer tissue collected at autopsy from 30 individuals with varying conditions—some with AD, others with different neurodegenerative diseases, and some with no apparent brain disease. Once sorted, the immune cells will be identified based on their protein marker signatures. In addition to autopsy samples, the Mathis team is obtaining fresh meningeal tissues through a collaboration with a neurosurgeon at MGH. These tissues, removed during routine surgeries from approximately 10 individuals, are being used to validate findings from the autopsy brain tissue. In the second aim, they are determining if the Tregs in Alzheimer’s patients are different from those found in people without the disease. By measuring gene expression patterns (scRNA-seq) and T cell receptors (TCR-seq), the lab can assess how dural Tregs differ in AD brains. Gene expression patterns will reveal which genes are turned on or off in these cells, showing whether the Tregs are functioning normally or have become dysfunctional. T cell receptor analysis will help identify what specific targets these immune cells are programmed to recognize, indicating whether they’re responding to the right signals or are malfunctioning.
At the close of the first year of funding, the team has made substantial progress on both aims. For Aim 1, they have thus far obtained high-quality dural samples from 16 autopsy subjects and 30 surgical patients provided by MGH. In their early work characterizing immune cell populations, they discovered that the fresh samples obtained from surgical patients had more robust immune cell populations. Flow-cytometric analyses across multiple brain regions identified distinct populations of Tregs important for suppressing immune response in distinct regions of the brain, including the superior sagittal sinus, frontal, parietal, and temporal lobes. They also discovered that, consistent with their mouse studies, the Tregs found in human dura are different than those circulating in the bloodstream. For Aim 2, the team has analyzed four autopsy subjects with AD, four with frontotemporal dementia, and one with no neurodegeneration. While the sample size is very small, initial analyses suggest that there are fewer Tregs in specific areas of AD brains. To expand these analyses, the team has established an additional collaboration with Yale Medical School neuropathologists to increase access to post-mortem AD samples. Single-cell RNA sequencing studies are currently underway to further characterize dural Tregs and disease-specific alterations. In the next year of funding, they plan to complete single-cell RNA and TCR sequencing on additional autopsy and surgical samples, enabling a more comprehensive comparison of dural Treg populations across brain regions and disease states.
