Unbalances of the brain tissue resident memory CD8+ T cell compartment amplify Alzheimer’s disease like pathology

Alzheimer’s disease (AD) is a neurodegenerative disorder affecting several millions of people worldwide. Classical neuropathological hallmarks of AD are the presence of amyloid beta (Ab) deposits and hyperphosphorylated tau protein, leading to the formation of neurofibrillary tangles (NFTs). Neuroinflammation has been also shown to be a key contributor to AD pathology and chronic activation of microglial cells, which are brain resident immune cells, have a key role during the inflammatory responses in the central nervous system (CNS). Moreover, growing evidence demonstrated that peripheral innate and adaptive immune cells also contribute to AD development. However, the contribution of CD3+ T cells, and in particular of CD8 lymphocytes, in AD pathogenesis is still unclear. Recent studies demonstrated a detrimental role for CD8+ T cells in the AD course, reporting a clonal expansion of effector memory CD45RA+ (TEMRA) CD8+ T cells in the cerebrospinal fluid (CSF) of AD specimens and suggested a contribution of CD8+ T lymphocytes to neuronal dysfunction in mice with AD-like disease. However, no study investigated phenotypical and functional alterations of CD8+ T cells taking place in the AD brains during the early stages of disease. To understand the phenotype of brain CD8 T cells in AD, we obtained single-cell RNA sequencing data showing a profound dysregulation of CD8 T cell compartment in the CNS of 3xTg-AD mice during early disease. Notably, we found a significant increase of pro-inflammatory CD103- Trm (tissue resident memory) cells paralleled by a significant decrease of CD103+ Trm lymphocytes in the brain of 3xTg-AD mice compared to sex- and age-matched WT control animals. This suggests a loss of the immune protection exerted by memory-like CD103+ Trm CD8+ T cells and an increase of pro-inflammatory CD103- Trm CD8+ T lymphocytes, potentially promoting disease development. Moreover, we observed a significant decrease in the percentage of detrimental CD103- Trm CD8+ T lymphocytes, paralleled by an increase of CD103+ Trm CD8+ T cells in the brain of 3xTg-AD/Itgal- /- animals lacking LFA-1 integrin compared to conventional control 3xTg-AD animals, clearly indicating a role for LFA-1 integrin in the accumulation of CD103- Trm CD8+ T cells into the CNS during AD-like disease. These data were supported by single-cell RNAseq studies and flow cytometry experiments performed on peripheral blood lymphocytes, showing a drastic increase of LFA-1 expression on circulating activated T cells, further pointing to a role for LFA-1 integrin in the accumulation of pro-inflammatory CD8 T cells in the brain of AD mice. Notably, peripheral CD8+ T cells depletion ablated brain CD103- population and improved cognition and reduced neuropathology in 3xTg-AD mice, suggesting a peripheral origin and a detrimental role for CD8+ CD103- T cells in AD. CD103- Trm population upregulated Gzmk gene as well as granzyme K (GrK) protein in the brains of 3xTg-AD mice compared to controls, suggesting a potential key role for this molecule in AD. Imaging experiments using time-lapse wide-field microscopy revealed that neurons 5 in contact with pro-inflammatory CD103- Trm CD8+ T cells showed significantly higher cytoplasmic Ca2+ levels compared to neurons co-cultured with CD103+ Trm CD8+ T cells, clearly indicating that CD103- Trm CD8+ T cells invading the brain of 3xTg-AD mice induce intracellular Ca2+ dysregulation, an event previously associated with neuronal functional alterations. Importantly, GrK directly induced a significant increase of intracellular Ca2+ release in neurons from 3xTg-AD mice, suggesting that this molecule is responsible for neuronal dysfunction induced by CD103- cells. Moreover, we observed that GrK triggered neuronal dysfunctions in a dose-dependent manner, by binding to PAR-1 receptor expressed by neurons, uncovering a new neurotoxic mechanism mediated by CD8 T cells in AD-like disease. In conclusion, we found a novel CD8+ T cell-mediated GrK-dependent cytotoxic mechanism underlying AD, highlighting the role of CD8+CD103- T lymphocytes in neuronal dysfunction, accumulation of abnormal Ab and tau, and cognitive impairment and suggesting that targeting the neurotoxic mechanisms exerted by CD8+ T cells could interfere with early pathogenesis of AD.