The CD33 (Siglec-3) gene encodes a sialic acid-binding immunoglobulin-like lectin (Siglec) that is predominantly expressed on microglia in the brain. CD33 is a significant Alzheimer's disease (AD) risk gene identified through genome-wide association studies (GWAS), with the protective C allele of rs3865444 reducing AD risk by approximately 5-10% per allele[@naj2011]. Unlike TREM2, which enhances microglial phagocytosis, CD33 contains an immunoreceptor tyrosine-based inhibition motif (ITIM) that suppresses microglial activity. This causal chain traces the molecular pathway from CD33 genetic risk through ITIM-mediated signaling to impaired amyloid clearance and cognitive decline.
Gene Summary
Genetic Architecture
| Feature | Details | |---------|---------| | Gene Symbol | CD33 | | Chromosome | 19q13.41 | | Protein | CD33 (Siglec-3), ITIM-bearing sialic acid receptor | | GWAS Locus | 19q13.41 (rs3865444) | | AD Risk | OR ~1.08-1.10 per risk allele (T→C protective) | | Brain Expression | Primarily microglia, increases with age and AD |
The primary AD-associated variant is rs3865444, where the C allele is protective. This variant is an expression quantitative trait locus (eQTL) that reduces CD33 expression in brain tissue. GWAS meta-analyses including over 350,000 individuals have confirmed the association with genome-wide significance[@tanzi2023].
The CD33 (Siglec-3) gene encodes a sialic acid-binding immunoglobulin-like lectin (Siglec) that is predominantly expressed on microglia in the brain. CD33 is a significant Alzheimer's disease (AD) risk gene identified through genome-wide association studies (GWAS), with the protective C allele of rs3865444 reducing AD risk by approximately 5-10% per allele[@naj2011]. Unlike TREM2, which enhances microglial phagocytosis, CD33 contains an immunoreceptor tyrosine-based inhibition motif (ITIM) that suppresses microglial activity. This causal chain traces the molecular pathway from CD33 genetic risk through ITIM-mediated signaling to impaired amyloid clearance and cognitive decline.
Gene Summary
Genetic Architecture
| Feature | Details | |---------|---------| | Gene Symbol | CD33 | | Chromosome | 19q13.41 | | Protein | CD33 (Siglec-3), ITIM-bearing sialic acid receptor | | GWAS Locus | 19q13.41 (rs3865444) | | AD Risk | OR ~1.08-1.10 per risk allele (T→C protective) | | Brain Expression | Primarily microglia, increases with age and AD |
The primary AD-associated variant is rs3865444, where the C allele is protective. This variant is an expression quantitative trait locus (eQTL) that reduces CD33 expression in brain tissue. GWAS meta-analyses including over 350,000 individuals have confirmed the association with genome-wide significance[@tanzi2023].
Allelic Series
| Variant Type | Effect | Mechanism | Disease Relevance | |-------------|--------|-----------|------------------| | rs3865444 C (protective) | Reduced risk (OR ~0.90-0.95) | Lower CD33 expression | Late-onset AD | | rs3865444 T (risk) | Increased risk (OR ~1.08-1.10) | Higher CD33 expression | Late-onset AD | | Rare LOF variants | Protective | Complete loss of function | AD protection |
Protein Function
Structure and Signaling
CD33 is a type I transmembrane protein of the Siglec family characterized by:
Intracellular signaling: Recruits phosphatases (SHP-1, SHP-2) that dephosphorylate signaling molecules
The ITIM structure is central to CD33's function. When CD33 engages with sialylated ligands on target cells (including Aβ plaques), the ITIM recruits Src homology 2 domain-containing phosphatases (SHP-1/SHP-2), which dephosphorylate key signaling molecules involved in phagocytosis and inflammatory responses[@matsumoto2022].
Expression Pattern
Microglial specificity: CD33 is expressed almost exclusively on microglia in the brain
Age-dependent increase: CD33 expression increases with normal aging
AD-specific elevation: Further upregulated in AD brains, particularly around amyloid plaques[@walker2023]
Specific subtypes: CD33+ microglia form a distinct population with reduced phagocytic capacity
Pathway Mechanisms
ITIM-Mediated Phagocytosis Suppression
Mermaid diagram (expand to render)
Molecular Mechanisms
Sialic acid recognition: CD33 binds to sialylated glycans on Aβ plaques, triggering inhibitory signaling
SHP-1/2 recruitment: ITIM phosphorylation recruits phosphatases that dampen activation signals
CD33 is unique among AD risk genes as a gain-of-function mechanism—the protective allele reduces expression, while the risk allele increases expression. This makes CD33 an attractive target for therapeutic inhibition rather than activation.
Clinical Biomarkers
CSF CD33: Elevated in AD, correlates with disease severity
Microglial CD33+ burden: Increases with disease progression
Amyloid PET: CD33 risk carriers show higher plaque burden
Tau PET: CD33 risk variants associated with increased tau signal independent of amyloid
Research Gaps
How does CD33 interact with the broader microglial ecosystem?
What are the downstream effectors of ITIM signaling in microglia?
Can selective CD33 inhibition enhance phagocytosis without disrupting beneficial immune functions?
What is the optimal timing for CD33-targeted interventions?
References
[Naj et al., Nat Genet 2011](https://pubmed.ncbi.nlm.nih.gov/21258336/) — GWAS identification of CD33 as AD risk gene
[Bradshaw et al., Nat Med 2013](https://pubmed.ncbi.nlm.nih.gov/24013771/) — CD33 as therapeutic target