Neuroinflammation is now recognized as a central driver of Alzheimer's disease (AD) pathogenesis, functioning not merely as a secondary consequence of amyloid-beta (Aβ) and tau pathology but as an active, self-perpetuating contributor to disease progression. This pathway hub synthesizes current understanding of the neuroinflammatory cascade in AD, spanning from initial microglial activation through chronic cytokine storm, astrocyte reactivity, blood-brain barrier (BBB) dysfunction, and the bidirectional interplay between inflammation and protein aggregation. The page integrates molecular mechanisms, genetic evidence, therapeutic implications, and biomarkers, with cross-links to dedicated pages for deeper exploration.
Alzheimer's disease is characterized by two classical pathological hallmarks: extracellular amyloid plaques composed of Aβ peptides and intracellular neurofibrillary tangles (NFTs) composed of hyperphosphorylated [tau protein](/proteins/map-tau-protein). However, abundant evidence now demonstrates that chronic neuroinflammation — manifested through persistent activation of microglia and astrocytes, elevated pro-inflammatory cytokines, complement system engagement, and peripheral immune infiltration — is a third core pathological feature that interacts bidirectionally with both amyloid and tau pathology[@heneka2017].
Neuroinflammation is now recognized as a central driver of Alzheimer's disease (AD) pathogenesis, functioning not merely as a secondary consequence of amyloid-beta (Aβ) and tau pathology but as an active, self-perpetuating contributor to disease progression. This pathway hub synthesizes current understanding of the neuroinflammatory cascade in AD, spanning from initial microglial activation through chronic cytokine storm, astrocyte reactivity, blood-brain barrier (BBB) dysfunction, and the bidirectional interplay between inflammation and protein aggregation. The page integrates molecular mechanisms, genetic evidence, therapeutic implications, and biomarkers, with cross-links to dedicated pages for deeper exploration.
Alzheimer's disease is characterized by two classical pathological hallmarks: extracellular amyloid plaques composed of Aβ peptides and intracellular neurofibrillary tangles (NFTs) composed of hyperphosphorylated [tau protein](/proteins/map-tau-protein). However, abundant evidence now demonstrates that chronic neuroinflammation — manifested through persistent activation of microglia and astrocytes, elevated pro-inflammatory cytokines, complement system engagement, and peripheral immune infiltration — is a third core pathological feature that interacts bidirectionally with both amyloid and tau pathology[@heneka2017].
The innate immune system of the brain is primarily represented by [microglia](/cell-types/microglia), which constitute approximately 10–15% of all brain cells and serve as the resident macrophages of the central nervous system (CNS)[@ransohoff2016]. Under normal conditions, microglia exist in a surveillant state, continuously scanning their microenvironment through dynamic process motility and rapidly responding to pathological insults. In AD, this surveillant state transitions to a chronic activated state characterized by morphological changes, transcriptional reprogramming, and altered functional responses that initially may be protective but become progressively detrimental[@hepner2015].
The genetic architecture of AD provides compelling evidence for neuroinflammation as a disease driver. GWAS have identified multiple immune-related risk loci including [TREM2](/genes/trem2), [CD33](/genes/cd33), [PLCG2](/genes/plcg2), [ABI3](/genes/abi3), and [INPP5D](/entities/inpp5d)[@sims2017], collectively implicating microglial dysfunction as a key contributor to late-onset AD pathogenesis.
In the healthy brain, microglia exist in a surveillant (resting) state characterized by a small cell body with highly ramified processes that continuously sample the extracellular space[@patterson2018]. This surveillance function is mediated by numerous pattern recognition receptors (PRRs), ion channels, and signaling molecules that enable rapid detection of pathogen-associated molecular patterns (PAMPs) and damage-associated molecular patterns (DAMPs). The transformation from surveillant to activated microglia involves dramatic morphological changes (amoeboid shape, enlarged soma, retracted processes), transcriptional reprogramming, and functional shifts in phagocytosis, cytokine production, and antigen presentation.
Microglia detect Aβ through multiple PRR families:
[TLR2 and TLR4](/entities/toll-like-receptor-signaling) (Toll-like receptors) recognize Aβ and initiate MyD88-dependent signaling cascades leading to NF-κB activation and pro-inflammatory cytokine transcription[@isin2019]. TLR4 activation by Aβ induces production of IL-1β, TNF-α, and IL-6, driving the inflammatory cascade forward.
[TREM2](/genes/trem2) (Triggering Receptor Expressed on Myeloid Cells 2) is a critical microglial receptor for Aβ recognition, lipid binding, and phagocytosis[@colonna2016]. TREM2 is expressed exclusively on microglia in the CNS and signals through the adaptor protein DAP12 (TYROBP) to activate PI3K and CSF1R pathways. Rare loss-of-function variants in TREM2 significantly increase AD risk (3–5-fold for R47H variant), and TREM2 deficiency in mouse models results in reduced microglial clustering around plaques, increased plaque burden, and worsened cognitive outcomes[@jonsson2013].
[RAGE](/entities/rage-signaling-neurodegeneration) (Receptor for Advanced Glycation End-products) binds Aβ and contributes to microglial activation, ROS production, and NF-κB signaling[@yan2012]. RAGE-Aβ interactions at the BBB also facilitate Aβ entry into the brain, creating a vicious cycle.
Single-cell RNA sequencing has identified multiple microglial states in AD brains[@zhou2021]:
The DAM program represents a neuroprotective response that promotes Aβ clearance but becomes dysregulated over time. Single-nucleus transcriptomics reveals that TREM2-dependent DAM signatures are enriched in early-stage AD, while TREM2-independent interferon signatures predominate in advanced disease[@wolf2023].
[APOE](/proteins/apoe) (apolipoprotein E) is produced by astrocytes and microglia and plays central roles in lipid transport and Aβ binding[@yin2017]. The APOE ε4 allele, the strongest genetic risk factor for late-onset AD after TREM2, significantly impairs microglial Aβ clearance efficiency. APOE ε4 carriers demonstrate reduced microglial phagocytic capacity, enhanced inflammatory responses, and altered lipid metabolism compared to APOE ε3 carriers. TREM2 and APOE pathways intersect at the level of microglial lipid metabolism and inflammatory signaling, creating a genetically supported mechanism for immune dysfunction in AD.
One of the critical functions of microglia is Aβ clearance through phagocytosis[@wang2015]. This process involves receptor-mediated recognition (TREM2, TLRs, CD36), internalization into phagosomes, and lysosomal degradation. In AD, chronic exposure to Aβ and inflammatory cytokines leads to a "burnout" phenotype characterized by impaired phagocytic capacity, lysosomal dysfunction, and persistent inflammatory mediator production — paradoxically reducing clearance while maintaining neurotoxic inflammation.
TREM2 is a type I transmembrane receptor belonging to the immunoglobulin superfamily. Its extracellular domain binds diverse ligands including Aβ, lipids, apolipoproteins (APOE, APOJ), and phosphatidylserine暴露 on apoptotic cells[@colonna2016]. The cytoplasmic tail lacks signaling motifs and relies on the adaptor protein DAP12 (TYROBP) for signal transduction.
TREM2-DAP12 signaling activates multiple pathways:
Multiple therapeutic strategies targeting TREM2 are in development[@schneeberger2019]:
The complement system is heavily implicated in AD pathogenesis through both synaptic pruning and direct neurotoxicity[@stevens2007]. The classical complement cascade is activated by C1q binding to Aβ plaques and damaged synapses, leading to C3 convertase formation and downstream effector generation.
C1q localizes to synapses and tags them for complement-mediated elimination. In AD, excessive C1q tagging leads to abnormal synaptic pruning that correlates with cognitive decline[@hong2016].
C3 and its cleavage fragments (C3a, C3b) are upregulated in reactive astrocytes and microglia. C3a drives neuroinflammation through anaphylatoxin receptor signaling; C3b mediates opsonization and microglial phagocytosis of tagged synapses.
C5a (membrane attack complex pathway) promotes neuroinflammation and may directly damage neurons through the membrane attack complex (MAC).
The complement system mediates synaptic loss through a well-characterized pathway[@hong2016]: C1q tags synapses, C3 is deposited, microglia recognize C3-tagged synapses via CR3 (complement receptor 3), and complement-mediated phagocytosis eliminates the synapse. This mechanism normally operates during development for circuit refinement but becomes pathologically activated in AD, contributing to early synaptic loss even before neuronal death.
Activated microglia and astrocytes release a network of pro-inflammatory mediators that drive and amplify neuroinflammation:
| Cytokine | Primary Source | Key Effects in AD | CSF/Brain Levels |
|----------|---------------|-------------------|-----------------|
| IL-1β | Microglia, astrocytes | Promotes Aβ production, tau phosphorylation, synaptic dysfunction | Elevated |
| TNF-α | Microglia, neurons | Neurotoxicity, BBB disruption, enhances Aβ toxicity | Elevated |
| IL-6 | Microglia, astrocytes | Glial activation, impairs hippocampal plasticity | Elevated |
| IL-18 | Microglia | Inflammasome product, promotes IFN-γ responses | Elevated |
| CXCL1/KC | Astrocytes | Neutrophil recruitment, amplifies inflammation | Elevated |
| CCL2/MCP-1 | Astrocytes, microglia | Monocyte and microglial recruitment to plaques | Elevated |
IL-1β is among the most studied cytokines in AD pathogenesis[@lizio2018]. Elevated in both brain tissue and CSF, IL-1β levels correlate with disease severity. IL-1β creates feed-forward loops by:
The combination of elevated IL-1β, TNF-α, and IL-6 creates a "cytokine storm" effect in the AD brain that contributes to:
The [NLRP3 inflammasome](/mechanisms/nlrp3-inflammasome) is a cytosolic protein complex that catalyzes caspase-1 activation and maturation of IL-1β and IL-18[@heneka2013]. Aβ activates the NLRP3 inflammasome through two signals:
Signal 1 (priming): Aβ binding to TLRs and RAGE induces NF-κB-dependent transcription of NLRP3, pro-IL-1β, and pro-IL-18.
Signal 2 (activation): Aβ particles are phagocytosed, causing lysosomal damage and release of cathepsins; intracellular potassium efflux and ROS generation provide additional activation signals[@halle2008].
Once activated, NLRP3 recruits ASC (apoptosis-associated speck-like protein) and procaspase-1, forming the inflammasome complex that autocatalyzes caspase-1 activation.
Activated caspase-1 cleaves pro-IL-1β and pro-IL-18 to their active forms, which are then released from the cell. The IL-1β produced is orders of magnitude more potent than that produced via NF-κB alone, creating powerful autocrine and paracrine inflammatory signaling[@fan2019].
In AD mouse models, genetic deletion of NLRP3 components reduces IL-1β levels, improves synaptic plasticity, rescues cognitive deficits, and reduces amyloid pathology — demonstrating the therapeutic potential of inflammasome inhibition.
Recent research shows that ASC specks — large protein aggregates released from NLRP3-activated cells — can propagate inflammasome activation to neighboring microglia, spreading the inflammatory response throughout connected brain networks[@kumar2024]. This provides a mechanism for the spreading of neuroinflammation that parallels the prion-like spread of misfolded proteins.
[Astrocytes](/entities/astrocytes) undergo dramatic changes in AD, transitioning from a homeostatic to a reactive state[@liddelow2017]. Reactive astrocytes in AD adopt the A1 phenotype induced by activated microglia — characterized by upregulated complement component production (C3, C1q) and downregulated homeostatic functions.
Morphological changes: Hypertrophic cell bodies, extended processes, and upregulated GFAP (glial fibrillary acidic protein) expression. These changes are most prominent surrounding amyloid plaques and in regions of neuronal loss.
Functional changes: Reactive astrocytes release complement components that drive synaptic pruning, lose homeostatic functions (glutamate uptake, potassium buffering), and may acquire neurotoxic properties in the A1 state.
Bidirectional communication between astrocytes and microglia amplifies neuroinflammation:
Astrocytes contribute to Aβ clearance through phagocytosis and endocytic uptake, particularly in regions with limited microglial coverage. The MEGF10 and MERTK receptors mediate astrocytic phagocytosis of debris and Aβ[@iram2016]. Dysfunction of these pathways may contribute to accumulation of pathological aggregates. Astrocytes also produce matrix metalloproteinases (MMPs) that can degrade Aβ.
The [blood-brain barrier](/entities/blood-brain-barrier) is consistently disrupted in AD, facilitating peripheral immune cell infiltration and enabling toxic blood-borne substances to enter the brain parenchyma[@montagne2015]. Aβ directly damages endothelial cells and pericytes; inflammatory cytokines further compromise barrier integrity.
Tight junction proteins (claudin-5, occludin, ZO-1) show reduced expression and mislocalization in AD endothelial cells.
Pericytes — which regulate cerebral blood flow and BBB integrity — are particularly vulnerable in AD. Pericyte loss correlates with BBB breakdown and is observed early in disease progression. Pericyte deficiency in mouse models recapitulates key features of AD including neuroinflammation.
The glia limitans (perivascular and subpial glia limitans) forms a structural barrier between brain parenchyma and vascular/ventricular compartments. In AD:
BBB disruption allows peripheral immune cells to enter the brain parenchyma[@length2018]:
Neuroinflammation and tau pathology interact in a powerful bidirectional manner[@maphis2015]:
Inflammation drives tau pathology:
Microglia secrete exosomes containing hyperphosphorylated tau, inflammatory cytokines, complement components, and Aβ. These exosomes represent a mechanism for prion-like spreading of both pathological proteins and inflammatory signals throughout connected brain networks, potentially explaining the stereotyped progression of tau pathology through anatomically connected circuits.
[TSPO PET imaging](/mechanisms/tspo-pet-imaging-neuroinflammation) allows in vivo visualization of microglial activation in AD patients[@kreisl2013]. TSPO (18 kDa translocator protein), formerly called the peripheral benzodiazepine receptor, is expressed primarily on the outer mitochondrial membrane of activated microglia. Increased TSPO binding in AD brains correlates with disease severity and cognitive decline.
First-generation TSPO ligands (e.g., [¹¹C]-PK11195) showed increased binding in AD, but high non-specific binding limited signal quality.
Second-generation ligands (e.g., [¹⁸F]-DPA-714, [¹¹C]-ER176) provide improved signal-to-noise ratios, though off-target binding in brain vasculature remains a concern.
TSPO PET has demonstrated[@cagnin2001]:
Given the central role of neuroinflammation in AD pathogenesis, numerous anti-inflammatory strategies have been explored[@heppner2015]:
NLRP3 inhibitors: Direct inflammasome blockade (MCC950, dapansutrile) shows efficacy in preclinical models; human trials are underway.
TREM2 agonists: Enhance microglial clearance function while potentially modulating inflammatory responses; antibody and gene therapy approaches in development.
CSF1R antagonists: Modulate microglial survival and activation state; depletion followed by repletion with beneficial microglia shows promise.
Anti-cytokine therapies: IL-1β antibodies (anakinra), TNF-α inhibitors have shown mixed results in clinical trials.
Minocycline: Broad microglial inhibitor; failed in clinical trials due to timing and specificity issues.
Anti-inflammatory trials in AD have largely failed to date, with lessons including:
Rather than broadly suppressing microglial activation, emerging strategies aim to[@song2022]:
Related Hypotheses: