The innate immune system plays a dual role in Alzheimer's disease (AD): initially mounting protective responses to clear pathological protein aggregates, but ultimately driving chronic neuroinflammation that accelerates neurodegeneration. Pattern recognition receptors (PRRs) — including Toll-like receptors (TLRs), NOD-like receptors (NLRs), and cytosolic DNA sensors — detect amyloid-beta and other danger-associated molecular patterns (DAMPs), activating downstream signaling cascades involving NF-κB, the NLRP3 inflammasome, and the cGAS-STING pathway[heneka2015].
Genome-wide association studies (GWAS) have identified numerous AD risk loci in innate immune genes — including TREM2, CD33, INPP5D, PLCG2, ABI3, and ABCA7 — establishing neuroinflammation as a core pathogenic mechanism rather than a secondary consequence of neurodegeneration[el2017][sims2017].
TLR4 is the most extensively studied innate immune receptor in AD. It recognizes fibrillar and oligomeric amyloid-beta as a DAMP, initiating MyD88-dependent signaling that activates NF-κB and MAPK pathways[li2023].
Upon amyloid-beta binding, TLR4 forms a complex with MD-2 and CD14, triggering:
The role of TLR signaling in AD is profoundly context-dependent: acute TLR activation promotes microglial phagocytosis and amyloid clearance, while chronic activation drives sustained cytokine production, synaptic damage, and neuronal death.
TLR9, an endosomal receptor that recognizes unmethylated CpG DNA, is activated by mitochondrial DNA (mtDNA) released from damaged neurons. Mitochondrial dysfunction in AD leads to mtDNA release into the cytoplasm and extracellular space, where it acts as a potent DAMP.
The NLRP3 inflammasome is a key driver of chronic neuroinflammation in AD[heneka2015]. NLRP3 inflammasome activation requires two sequential signals:
The NLRP3 inflammasome contributes to tau pathology progression through IL-1β-mediated activation of kinases that phosphorylate tau, including GSK-3β and CDK5[ising2019].
A critical pathogenic mechanism is the discovery that ASC specks released from pyroptotic microglia cross-seed amyloid-beta in the brain[venegas2017]. ASC specks:
The cGAS-STING pathway is a cytosolic DNA-sensing mechanism activated by cGAS, which detects double-stranded DNA in the cytoplasm. In AD, mitochondrial and nuclear DNA released from damaged neurons activates cGAS-STING, driving type I interferon (IFN-I) production[xie2022].
STING activation contributes to tau pathology through interferon-mediated upregulation of kinases that phosphorylate tau, including GSK-3β and CDK5. In tauopathy mouse models, genetic deletion of STING reduces tau phosphorylation, neuroinflammation, and neurodegeneration.
STING activation in microglia promotes a neurotoxic inflammatory phenotype while impairing phagocytic function. The resulting type I interferon response upregulates complement components, chemokines (CXCL10, CCL2), and antigen presentation molecules (MHC-II), amplifying the broader neuroinflammatory cascade.
TREM2 (triggering receptor expressed on myeloid cells 2) is expressed primarily on microglia and serves as a critical regulator of microglial survival, proliferation, and function. TREM2 binds amyloid-beta and lipid ligands, triggering signaling through the adaptor protein DAP12 (TYROBP)[el2017].
Rare coding variants in TREM2 are associated with dramatically increased AD risk (heterozygous variants increase risk 2-4×)[el2017]. The R47H variant specifically impairs TREM2's ability to bind amyloid-beta and lipid ligands, reducing microglial activation around plaques.
Single-cell RNA sequencing has revealed multiple microglial states in AD[kerenshaul2017]:
The complement system provides innate immune defense through opsonization, inflammation, and membrane attack complex (MAC) formation. In AD, complement components C1q, C3, and C4 are significantly upregulated, particularly around amyloid plaques and degenerating synapses[hong2016].
C1q binds to synapses in an age- and amyloid-dependent manner, initiating the classical complement cascade that culminates in C3b/iC3b deposition on synaptic membranes. Complement receptor 3 (CR3/CD11b) on microglia then mediates phagocytic removal of tagged synapses[hong2016].
Key findings:
The dual protective/destructive nature of innate immunity in AD necessitates precision approaches rather than broad immunosuppression. Multiple clinical trials of NSAIDs failed in AD, likely because these agents suppress both harmful and protective immune functions.
Current therapeutic strategies:
| Target | Approach | Stage | Status |
|---|---|---|---|
| TREM2 | Agonist antibodies (AL002, JZH6) | Phase II | Active trials |
| NLRP3 | Small molecule inhibitors | Preclinical | Development |
| C1q | Anti-C1q antibody (ANX005) | Phase II | Recruiting |
| STING | STING antagonists (H-151) | Preclinical | Proof of concept |
Inflammatory biomarkers can identify patients with prominent neuroinflammation and guide therapeutic timing: