Recent studies highlight the implication of innate and adaptive immunity in the pathophysiology of Alzheimer's disease, and foster immunotherapy as a promising strategy for its treatment. Vaccines targeting amyloid-β peptide provided encouraging results in mouse models, but severe side effects attributed to T cell responses in the first clinical trial AN1792 underlined the need for better understanding adaptive immunity in Alzheimer's disease. We previously showed that regulatory T cells critically control amyloid-β-specific CD4(+) T cell responses in both physiological and pathological settings. Here, we analysed the impact of regulatory T cells on spontaneous disease progression in a murine model of Alzheimer's disease. Early transient depletion of regulatory T cells accelerated the onset of cognitive deficits in APPPS1 mice, without altering amyloid-β deposition. Earlier cognitive impairment correlated with reduced recruitment of microglia towards amyloid deposits and altered disease-related gene expression profile. Conversely, amplification of regulatory T cells through peripheral low-dose IL-2 treatment increased numbers of plaque-associated microglia, and restored cognitive functions in APPPS1 mice. These data suggest that regulatory T cells play a beneficial role in the pathophysiology of Alzheimer's disease, by slowing disease progression and modulating microglial response to amyloid-β deposition. Our study highlights the therapeutic potential of repurposed IL-2 for innovative immunotherapy based on modulation of regulatory T cells in Alzheimer's disease.

Microglia are the CNS resident immune cells that react to misfolded proteins through pattern recognition receptor ligation and activation of inflammatory pathways. Here, we studied how microglia handle and cope with a-synuclein (a-syn) fibrils and their clearance. We found that microglia exposed to a-syn establish a cellular network through the formation of F-actin-dependent intercellular connections, which transfer a-syn from overloaded microglia to neighboring naive microglia where the a-syn cargo got rapidly and effectively degraded. Lowering the a-syn burden attenuated the inflammatory profile of microglia and improved their survival. This degradation strategy was compromised in cells carrying the LRRK2 G2019S mutation. We confirmed the intercellular transfer of a-syn assemblies in microglia using organotypic slice cultures, 2-photon microscopy, and neuropathology of patients. Together, these data identify a mechanism by which microglia create an ''on-demand'' functional network in order to improve pathogenic a-syn clearance. ll

Over the past few decades, research on Alzheimer’s disease (AD) has focused on pathomechanisms linked to two of the major pathological hallmarks of extracellular deposition of beta-amyloid peptides and intra-neuronal formation of neurofibrils. Recently, a third disease component, the neuroinflammatory reaction mediated by cerebral innate immune cells, has entered the spotlight, prompted by findings from genetic, pre-clinical, and clinical studies. Various proteins that arise during neurodegeneration, including beta-amyloid, tau, heat shock proteins, and chromogranin, among others, act as danger-associated molecular patterns, that—upon engagement of pattern recognition receptors—induce inflammatory signaling pathways and ultimately lead to the production and release of immune mediators. These may have beneficial effects but ultimately compromise neuronal function and cause cell death. The current review, assembled by participants of the Chiclana Summer School on Neuroinflammation 2016, provides an overview of our current understanding of AD-related immune processes. We describe the principal cellular and molecular players in inflammation as they pertain to AD, examine modifying factors, and discuss potential future therapeutic targets.

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder causing memory loss, language problems and behavioural disturbances. AD is associated with the accumulation of fibrillar amyloid-β (Aβ) and the formation of neurofibrillary tau tangles. Fibrillar Aβ itself represents a danger-associated molecular pattern, which is recognized by specific microglial receptors. One of the key players is formation of the NOD-, LRR-and pyrin domain-containing 3 (NLRP3) inflammasome, whose activation has been demonstrated in AD patient brains and transgenic animal models of AD. Here, we investigated whether Aβ oligomers or protofibrils that represent lower molecular aggregates prior to Aβ deposition are able to activate the NLRP3 inflammasome and subsequent interleukin-1 beta (IL-1β) release by microglia. In our study, we used Aβ preparations of different sizes: small oligomers and protofibrils of which the structure was confirmed by atomic force microscopy. Primary microglial cells from C57BL/6 mice were treated with the respective Aβ preparations and NLRP3 inflammasome activation, represented by caspase-1 cleavage, IL-1β production, and apoptosis-associated speck-like protein containing a CARD speck formation was analysed. Both protofibrils and low molecular weight Aβ aggregates induced a significant increase in IL-1β release. Inflammasome activation was confirmed by apoptosis-associated speck-like protein containing a CARD speck formation and detection of active caspase-1. The NLRP3 inflammasome inhibitor MCC950 completely inhibited the Aβ-induced immune response. Our results show that the NLRP3 inflammasome is activated not only by fibrillar Aβ aggregates as reported before, but also by lower molecular weight Aβ oligomers and protofibrils, highlighting the possibility that microglial activation by these Aβ species may initiate innate immune responses in the central nervous system prior to the onset of Aβ deposition. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

Neuroinflammatory responses in Alzheimer's disease (AD) are complex and not fully understood. They involve various cellular and molecular players and associate interaction between the central nervous system (CNS) and the periphery. Amyloid peptides within the senile plaques and abnormally phosphorylated tau in neurofibrillary tangles are able to initiate inflammatory responses, in brain of AD patients and in mouse models of this disease. The outcome of these responses on the pathophysiology of AD depends on several factors and can be either beneficial or detrimental. Thus, understanding the role of neuroinflammation in AD could help to develop safer and more efficient therapeutic strategies. This review discusses recent knowledge on microglia responses toward amyloid and tau pathology in AD, focusing on the role of Toll-like receptors and NOD-like receptor protein 3 (NLRP3) inflammasome activation in microglial cells.

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BackgroundActive immunization against Aβ was reported to have a therapeutic effect in murine models of Alzheimer’s disease. Clinical Aβ vaccination trial AN1792 was interrupted due to the development in 6 % of the patients of meningoencephalitis likely involving pro-inflammatory CD4+ T cells. However, the potential implication of auto-aggressive anti-Aβ CD8+ T cells has been poorly investigated.MethodsPotential MHC-I-restricted Aβ-derived epitopes were first analyzed for their capacity to recruit functional CD8+ T cell responses in mouse models. Their impact on migration of CD8+ T cells into the brain parenchyma and potential induction of meningoencephalitis and/or neuronal damage was investigated upon vaccination in the APPPS1 mouse model of AD.ResultsWe identified one nonamer peptide, Aβ33-41, which was naturally processed and presented in association with H-2-Db molecule on neurons and CD11b+ microglia. Upon optimization of anchor residues for enhanced binding to H-2-Db, immunization with the modified Aβ33-41NP peptide elicited Aβ-specific IFNγ-secreting CD8+ T cells, which are cytotoxic towards Aβ-expressing targets. Whereas T cell infiltration in the brain of APPPS1 mice is dominated by CD3+CD8− T cells and increases with disease evolution between 4 and 7 months of age, a predominance of CD3+CD8+ over CD3+CD8− cells was observed in 6- to 7-month-old APPPS1 but not in WT animals, only after vaccination with Aβ33-41NP. The number of CD11b+ mononuclear phagocytes, which significantly increases with age in the brain of APPPS1 mice, was reduced following immunization with Aβ33-41NP. Despite peripheral activation of Aβ-specific CD8+ cytotoxic effectors and enhanced infiltration of CD8+ T cells in the brain of Aβ33-41NP-immunized APPPS1 mice, no clinical signs of severe autoimmune neuroinflammation were observed.ConclusionsAltogether, these results suggest that Aβ-specific CD8+ T cells are not major contributors to meningoencephalitis in response to Aβ vaccination.