One of the earliest clues that immune cells were involved in late-onset Alzheimer’s disease (LOAD) came out of large-scale human genetic studies. Many genes identified for their impact on LOAD risk are primarily expressed by microglia and astrocytes—the brain’s resident innate immune cells. This discovery launched a highly active area of research into understanding how these immune cells contribute to disease risk and Alzheimer’s pathologies, with the core idea that much of the neurodegenerative damage tied to cognitive decline comes from the immune response to amyloid beta and tau pathology. Recently, scientists have realized that a singular focus on the brain’s innate immunity is too narrow, and the aperture should be opened to include peripheral adaptive and innate immune responses. Since therapeutics that effectively alter peripheral immune responses exist for other diseases, understanding the role of this response in Alzheimer’s disease could spark the development of potential new interventions.
Innate immune cells have a set repertoire of responses to anything they recognize as harmful, whereas adaptive immune cells are trained through exposure to mount specific responses to specific stimuli. When a stimulus is present a second time, thanks to training from the first exposure, the adaptive immune system is ready to produce the specific cells specially structured to recognize and attack it (clonal expansion). The brain has only innate immune cells in permanent residence. In the past, scientists believed the brain needed only innate immune cells because, under healthy conditions, the blood-brain barrier kept the brain isolated from most materials from the body’s periphery, including immune cells resident there and pathogens.
However, recent discoveries have shown that signals from peripheral immune cells and the cells themselves—including innate immune cells like macrophages and adaptive immune cells like B and T cells—cross into and affect the brain environment. T cells originate in the body’s periphery and normally circulate in the blood but have now been found in the brain, the protective meninges around the brain, and cerebrospinal fluid. Levels of T cells in all these locations are higher in LOAD patients than in age-matched controls and are associated with tau pathology and neurodegeneration. Dr. Saligrama has been at the forefront of this research, collaborating with Dr. David Gate on a seminal publication during their postdoctoral training in the lab of Dr. Tony Wyss-Coray (both CureAlz-funded investigators; Dr. Wyss-Coray is also a member of the CureAlz Research Leadership Group). While this and other research support the overall hypothesis that T cells contribute to Alzheimer’s disease, additional factors could impact this interpretation—specifically age.
LOAD is defined as Alzheimer’s disease that becomes symptomatic only after an individual is 65 years old; approximately 95% of all Alzheimer’s cases are categorized as LOAD. Dr. Saligrama points out that since all prior work in this area was conducted in LOAD patients, the effects of aging itself on the immune system cannot be disentangled from the impact of Alzheimer’s pathology.
To address this issue, Dr. Saligrama proposed to study T cells in patients with early-onset genetic forms of Alzheimer’s, also called Autosomal Dominant Alzheimer’s Disease (ADAD). ADAD is caused by inheriting a mutant copy of the APP, PSEN1, or PSEN2 gene, which leads to an increased production of toxic forms of amyloid beta. The typical age of onset for ADAD is between 30–50 years old; therefore, this patient group provides the unique opportunity to examine whether and how amyloid or tau pathology causes T cell responses without the confounds of aging or age-related comorbidities. Dr. Saligrama is an investigator with the long-running federally funded Dominantly Inherited Alzheimer’s Network (DIAN) study at Washington University in St. Louis, which has followed 700 ADAD patients for the past 15 years. The study has collected cerebrospinal fluid, blood samples, and brain images along with clinical and demographic data, which provides Dr. Saligrama access to a rich resource of well-characterized patient samples. He has reported intriguing preliminary data that suggests T cell responses are different between ADAD and LOAD patients.
Dr. Saligrama’s team proposed two experimental aims to characterize the adaptive immune response of T cells in ADAD patients. In the first aim, they are measuring the levels and function of several key T cell types in blood samples collected from ADAD and control patients by utilizing single-cell genetic sequencing experiments (scRNAseq and scTCRseq). These data also enable them to identify which molecular signaling pathways are most likely impacted in T cells from ADAD patients. They are also measuring and comparing the amounts of different pro-inflammatory cytokines produced by cultured immune cells isolated from either ADAD patients or controls. In the second aim, they are assessing how T cells respond when exposed to pathological forms of amyloid or tau. They predict that a previous adaptive immune response earlier in the lifespan of ADAD patients leads to the production of specific T cells trained to recognize and respond to amyloid and tau pathology. This aim will help assess the impact these T cells have on Alzheimer’s disease pathogenesis. Finding and counting these T cells directly is technically challenging, so Dr. Saligrama devised a clever method for expanding these cells in culture. They are then collecting the T cells for genetic sequencing and analysis after the cells are exposed to amyloid and tau pathology.
In the first year of funding, Dr. Saligrama’s team made great strides across both aims. Upon completing much of Aim 1, the team reported a unique pattern of immune dysregulation in ADAD samples, characterized by a major clonal expansion of CD4 and CD8 T cells and significant changes in the inflammatory cytokines produced by these cells. The implications of clonal expansion typically depend on which cells are proliferating; in this case, the increased cell population seems to produce higher levels of the key pro-inflammatory cytokines TNFα and IFNγ, both of which have been implicated in neuroinflammation in Alzheimer’s disease cases. Dr. Saligrama did not observe the changes in his studies of LOAD cases, suggesting distinct patterns of T-cell changes in LOAD versus ADAD. The experiments for the second aim have been expanded to include samples from patients with Down syndrome (DS). Individuals living with DS have a third copy of chromosome 21, which contains the APP gene from which the amyloid beta protein derives, and they thus almost invariably develop AD pathology and do so by middle age. As such, they can provide additional critical insights into the pathogenesis of Alzheimer’s disease independent from advanced age. Sequencing data analysis for this aim is underway and expected to be completed in the second year of funding.
