Alzheimer's disease (AD) is a detrimental neurodegenerative disease with no effective treatments. Due to cellular heterogeneity, defining the roles of immune cell subsets in AD onset and progression has been challenging. Using transcriptional single-cell sorting, we comprehensively map all immune populations in wild-type and AD-transgenic (Tg-AD) mouse brains. We describe a novel microglia type associated with neurodegenerative diseases (DAM) and identify markers, spatial localization, and pathways associated with these cells. Immunohistochemical staining of mice and human brain slices shows DAM with intracellular/phagocytic Aβ particles. Single-cell analysis of DAM in Tg-AD and triggering receptor expressed on myeloid cells 2 (Trem2) Tg-AD reveals that the DAM program is activated in a two-step process. Activation is initiated in a Trem2-independent manner that involves downregulation of microglia checkpoints, followed by activation of a Trem2-dependent program. This unique microglia-type has the potential to restrict neurodegeneration, which may have important implications for future treatment of AD and other neurodegenerative diseases. VIDEO ABSTRACT.

The role of non-neuronal cells in Alzheimer’s disease (AD) progression has not been fully elucidated. Using single-nucleus RNA-seq, we identified a population of disease associated astrocytes (DAAs) in an AD mouse model. The DAA population appeared at early disease stages and increased in abundance with age. We discovered that similar astrocytes appeared in aged wild-type mice and in aging human brains, suggesting their linkage to genetic and age-related factors.

During ageing, microglia acquire a phenotype that may negatively affect brain function. Here we show that ageing microglial phenotype is largely imposed by interferon type I (IFN-I) chronically present in aged brain milieu. Overexpression of IFN-β in the CNS of adult wild-type mice, but not of mice lacking IFN-I receptor on their microglia, induces an ageing-like transcriptional microglial signature, and impairs cognitive performance. Furthermore, we demonstrate that age-related IFN-I milieu downregulates microglial myocyte-specific enhancer factor 2C (Mef2C). Immune challenge in mice lacking Mef2C in microglia results in an exaggerated microglial response and has an adverse effect on mice behaviour. Overall, our data indicate that the chronic presence of IFN-I in the brain microenvironment, which negatively affects cognitive function, is mediated via modulation of microglial activity. These findings may shed new light on other neurological conditions characterized by elevated IFN-I signalling in the brain.

Alzheimer’s disease (AD) is a heterogeneous disorder with multiple etiologies. Harnessing the immune system by blocking the programmed cell death receptor (PD)-1 pathway in an amyloid beta mouse model was shown to evoke a sequence of immune responses that lead to disease modification. Here, blocking PD-L1, a PD-1 ligand, was found to have similar efficacy to that of PD-1 blocking in disease modification, in both animal models of AD and of tauopathy. Targeting PD-L1 in a tau-driven disease model resulted in increased immunomodulatory monocyte-derived macrophages within the brain parenchyma. Single cell RNA-seq revealed that the homing macrophages expressed unique scavenger molecules including macrophage scavenger receptor 1 (MSR1), which was shown here to be required for the effect of PD-L1 blockade in disease modification. Overall, our results demonstrate that immune checkpoint blockade targeting the PD-1/PD-L1 pathway leads to modification of common factors that go awry in AD and dementia, and thus can potentially provide an immunotherapy to help combat these diseases.

The contributions of disease-associated microglia (DAM) and in ltrating monocyte-derived macrophages (MDM) to Alzheimer's disease (AD) are still controversial. Here, using Trem2 −/− 5xFAD DAM-de cient mice, we addressed this issue by targeting the programmed cell death ligand-1 (PD-L1) immune checkpoint, shown to modify AD via MDM recruitment. Treating Trem2 −/− 5xFAD mice resulted in cognitive improvement, rescue of synapses, and reduction of soluble-amyloid beta (Aβ) 1−42 , with no effect on insoluble Aβ 1−42 . In Trem2 +/+ 5xFAD mice, the treatment enhanced cognitive performance, led to elevation in DAM levels, and reduced insoluble Aβ 1−42 . Single-cell RNA-sequencing revealed that MDM, derived from both Trem2 −/− and Trem2 +/+ 5xFAD mouse brains, express a unique set of scavenger receptors and anti-in ammatory genes. Eliminating monocytes abrogated the bene cial effect of anti-PD-L1. The results highlight the need for MDM for neuroprotection even when microglia are fully activated, and demonstrate that their activity occurs through a TREM2-independent mechanism, with the potential to overcome TREM2 polymorphism in patients.

The contributions of disease-associated microglia (DAM) and infiltrating monocyte-derived macrophages (MDM) to Alzheimer’s disease (AD) are still controversial. Here, using Trem2−/−5xFAD DAM-deficient mice, we addressed this issue by targeting the programmed cell death ligand-1 (PD-L1) immune checkpoint, shown to modify AD via MDM recruitment. Treating Trem2−/−5xFAD mice resulted in cognitive improvement, rescue of synapses, and reduction of soluble-amyloid beta (Aβ)1−42, with no effect on insoluble Aβ1−42. In Trem2+/+5xFAD mice, the treatment enhanced cognitive performance, led to elevation in DAM levels, and reduced insoluble Aβ1−42. Single-cell RNA-sequencing revealed that MDM, derived from both Trem2−/− and Trem2+/+ 5xFAD mouse brains, express a unique set of scavenger receptors and anti-inflammatory genes. Eliminating monocytes abrogated the beneficial effect of anti-PD-L1. The results highlight the need for MDM for neuroprotection even when microglia are fully activated, and demonstrate that their activity occurs through a TREM2-independent mechanism, with the potential to overcome TREM2 polymorphism in patients.