Neuroinflammation has emerged as a critical component of Alzheimer's disease (AD) pathogenesis, representing both a consequence of amyloid-beta (Aβ) and tau pathology and a driver of disease progression. This page synthesizes current understanding of the neuroinflammatory cascade in AD, from microglial activation to chronic neuroinflammation and its downstream effects on neurodegeneration.
Neuroinflammation in AD is characterized by persistent activation of brain immune cells, particularly microglia and astrocytes, leading to a chronic inflammatory state that contributes to neuronal dysfunction and death. While acute neuroinflammation serves protective functions, chronic dysregulation becomes deleterious, creating a feed-forward loop with pathological protein aggregates[@heneka2015].
The recognition that neuroinflammation is not merely a secondary phenomenon but an active driver of AD pathogenesis has led to significant research interest in inflammatory pathways as therapeutic targets. Genome-wide association studies (GWAS) have identified multiple immune-related genetic risk factors for AD, including TREM2, CD33, and PLCG2, underscoring the importance of immune dysfunction in disease etiology[@jansen2019].
Brain-resident microglia are the primary immune cells of the central nervous system. In AD, microglia undergo dramatic morphological and functional changes:
- Resting microglia ( surveillant state): Highly ramified morphology with small cell bodies and long processes
- Activated microglia: Amoeboid morphology with enlarged cell bodies and shortened processes
- Disease-associated microglia (DAM): A specialized subset that clusters around amyloid plaques
¶ TREM2 and Microglial Surveillance
TREM2 (Triggering Receptor Expressed on Myeloid Cells 2) is a critical receptor on microglia that recognizes amyloid plaques and other disease-associated signals. TREM2 variants, particularly the R47H mutation, significantly increase AD risk:
- TREM2 is expressed on microglia, not neurons
- R47H mutation reduces ligand binding (Aβ, lipids, apolipoproteins)
- Impaired microglial clustering around plaques
- Reduced plaque compaction and increased diffuse plaque burden
- Loss of protective microglial responses[@wang2015]
Single-cell RNA sequencing has identified multiple microglial states in AD brain:
- Homeostatic microglia: Express P2ry12, Tmem119, Cx3cr1 - typical surveillance
- DAM (Disease-Associated Microglia): Upregulate TREM2, APOE, CD74 - cluster at plaques
- IFN-responsive microglia: Express interferon-stimulated genes
- Proliferative microglia: Cell cycle genes - indicate expansion
Amyloid-beta directly activates microglia through multiple mechanisms:
-
Pattern Recognition Receptors (PRRs)
- TLRs (Toll-like receptors), especially TLR2 and TLR4
- RAGE (Receptor for Advanced Glycation Endproducts)
- CD14 co-receptor
-
NLRP3 Inflammasome Activation
- Aβ triggers NLRP3 inflammasome assembly
- Caspase-1 activation
- IL-1β and IL-18 maturation and release
- Chronic inflammation amplification[@heneka2013]
¶ Cytokine and Chemokine Release
Activated microglia release a array of pro-inflammatory mediators:
| Mediator |
Function |
AD Relevance |
| IL-1β |
Pro-inflammatory cytokine |
Elevated in AD brain, drives tau pathology |
| TNF-α |
Cytotoxicity, inflammation |
Neurotoxic, correlates with cognitive decline |
| IL-6 |
Acute phase response |
Associated with disease progression |
| CXCL8 (IL-8) |
Neutrophil chemoattractant |
Recruitment of peripheral immune cells |
| CCL2 (MCP-1) |
Monocyte recruitment |
Attracts microglia to plaques |
Astrocytes also undergo reactive changes in AD:
- Reactive astrocytes (A1 phenotype) release complement components
- Astrocytic plaques form around amyloid deposits
- Dysregulated glutamate uptake leads to excitotoxicity
- Impaired potassium buffering
- Reduced astrocyte-neuron metabolic coupling[@liddelow2017]
The complement system is heavily implicated in AD pathogenesis:
- C1q localizes to synapses and promotes complement-mediated elimination
- C3 is upregulated in reactive astrocytes and microglia
- C3a/C5a (anaphylatoxins) promote neuroinflammation
- MAC (Membrane Attack Complex) can directly damage neurons
The complement cascade may mediate synaptic loss in AD, with C1q and C3 participating in the tagging and elimination of synapses[@stevens2007].
¶ Glial Limiting Membranes and Barrier Dysfunction
The glia limitans (perivascular and subpial glia limitans) forms a barrier between the brain parenchyma and vascular/ventricular compartments. In AD:
- Dysfunction allows increased peripheral immune cell infiltration
- Breakdown enables toxic blood-borne substances to enter brain
- Contributes to cerebral amyloid angiopathy (CAA)
Blood-Brain Barrier dysfunction in AD includes:
- Reduced tight junction protein expression (claudin-5, occludin)
- Increased matrix metalloproteinase (MMP) activity
- Pericyte loss and capillary rarefaction
- Enhanced leukocyte trafficking
Neuroinflammation and tau pathology interact in a bidirectional manner:
-
Inflammation drives tau pathology
- IL-1β enhances tau phosphorylation via GSK3β and CDK5
- TNF-α promotes tau aggregation
- Microglial exosomes spread phosphorylated tau
-
Tau pathology promotes inflammation
- NFT-bearing neurons release inflammatory signals
- Tau oligomers activate microglia
- Neuronal death releases DAMPs[@maphis2015]
Microglia secrete exosomes containing:
- Hyperphosphorylated tau
- Inflammatory cytokines
- Complement components
- Aβ
These exosomes may represent a mechanism for prion-like spread of pathology throughout connected brain networks.
Multiple immune-related genes influence AD risk:
| Gene |
Variant |
Effect |
Mechanism |
| TREM2 |
R47H |
~3x risk |
Reduced microglial Aβ recognition |
| CD33 |
rs3865444 |
Protective |
Reduced inhibitory signaling |
| APOE |
ε4 |
~4x risk |
Impaired Aβ clearance, enhanced inflammation |
| PLCG2 |
P522R |
Protective |
Enhanced microglial signaling |
| ABI3 |
rs28348800 |
Risk |
Impaired microglial phagocytosis |
| INPP5D |
rs35349673 |
Risk |
Altered microglial signaling |
¶ HLA-DRB5 and Immune Regulation
The HLA-DRB5 region shows complex associations with AD, reflecting the role of MHC class II molecules in antigen presentation and immune regulation[@lambert2013].
Multiple therapeutic strategies target neuroinflammation:
- NLRP3 inhibitors: Direct inflammasome blockade
- TREM2 agonists: Enhance microglial clearance function
- CSF1R antagonists: Modulate microglial survival and activation
- Anti-cytokine therapies: IL-1β, TNF-α antibodies
- Minocycline: Broad microglial inhibitor (failed in trials)
Anti-inflammatory trials in AD have largely failed:
- Late intervention may miss therapeutic window
- Broad immunosuppression may be harmful
- Need for precise targeting of pathological inflammation
- Biomarker-guided patient selection required
flowchart TD
A["Amyloid Plaques"] --> B["Microglial Activation"]
A --> C["Astrocyte Reactivity"]
B --> D["PRR Recognition"]
B --> E["TREM2 Signaling"]
B --> F["NLRP 3 Inflammasome"]
D --> G["NF-κB Activation"]
E --> G
F --> H["IL-1β Release"]
F --> I["IL-18 Release"]
G --> J["Cytokine Storm"]
H --> J
I --> J
J --> K["Tau Hyperphosphorylation"]
J --> L["Synaptic Loss"]
J --> M["Neuronal Death"]
J --> N["BBB Dysfunction"]
C --> O["Complement Activation"]
O --> P["Synaptic Pruning"]
P --> L
N --> Q["Peripheral Immune Infiltration"]
Q --> B
K --> R["Neurofibrillary Tangles"]
style B fill:#FF6B6B
style J fill:#FF6B6B
style L fill:#FF6B6B
style M fill:#DC143C
- Microglial activation is a hallmark of AD, with TREM2 playing a critical role in plaque surveillance
- Chronic neuroinflammation creates a feed-forward loop with Aβ and tau pathology
- GWAS has identified multiple immune-related AD risk genes (TREM2, CD33, PLCG2)
- The complement system contributes to synaptic loss through excessive pruning
- Neuroinflammation-tau interaction is bidirectional, accelerating disease progression
- Anti-inflammatory therapies have failed in AD, highlighting need for targeted approaches