CD33 Protein — Cluster of Differentiation 33 (Siglec-3)

CD33 Protein — Cluster of Differentiation 33 (Siglec-3)

<div class="infobox infobox-protein">
<table>
<tr><th colspan="2" style="background:#f0f0f0;">CD33 Protein</th></tr>
<tr><td><b>Full Name</b></td><td>Cluster of Differentiation 33</td></tr>
<tr><td><b>Alias</b></td><td>Siglec-3</td></tr>
<tr><td><b>Gene</b></td><td>[CD33](/genes/cd33)</td></tr>
<tr><td><b>UniProt ID</b></td><td>P07846</td></tr>
<tr><td><b>Protein Length</b></td><td>364 amino acids</td></tr>
<tr><td><b>Molecular Weight</b></td><td>~67 kDa</td></tr>
<tr><td><b>Structure</b></td><td>V-type Ig domain + 2 C2-type Ig domains + TM + ITIM tail</td></tr>
<tr><td><b>Expression</b></td><td>Microglia, monocytes, macrophages</td></tr>
<tr><td><b>Key Variants</b></td><td>rs3865444 (protective), rs12459419 (risk)</td></tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">ALS</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">ALZHEIMER</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">ALZHEIMER'S DISEASE</a>, <a href="/wiki/aging" style="color:#ef9a9a">Aging</a>, <a href="/wiki/als" style="color:#ef9a9a">Als</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">251 edges</a></td>
</tr>
</table>
</div>

Overview

CD33 Protein — Cluster of Differentiation 33 (Siglec-3)

<div class="infobox infobox-protein">
<table>
<tr><th colspan="2" style="background:#f0f0f0;">CD33 Protein</th></tr>
<tr><td><b>Full Name</b></td><td>Cluster of Differentiation 33</td></tr>
<tr><td><b>Alias</b></td><td>Siglec-3</td></tr>
<tr><td><b>Gene</b></td><td>[CD33](/genes/cd33)</td></tr>
<tr><td><b>UniProt ID</b></td><td>P07846</td></tr>
<tr><td><b>Protein Length</b></td><td>364 amino acids</td></tr>
<tr><td><b>Molecular Weight</b></td><td>~67 kDa</td></tr>
<tr><td><b>Structure</b></td><td>V-type Ig domain + 2 C2-type Ig domains + TM + ITIM tail</td></tr>
<tr><td><b>Expression</b></td><td>Microglia, monocytes, macrophages</td></tr>
<tr><td><b>Key Variants</b></td><td>rs3865444 (protective), rs12459419 (risk)</td></tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">ALS</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">ALZHEIMER</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">ALZHEIMER'S DISEASE</a>, <a href="/wiki/aging" style="color:#ef9a9a">Aging</a>, <a href="/wiki/als" style="color:#ef9a9a">Als</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">251 edges</a></td>
</tr>
</table>
</div>

Overview

CD33 (Cluster of Differentiation 33, also known as Siglec-3) is a type I transmembrane glycoprotein belonging to the sialic acid-binding immunoglobulin-type lectin (Siglec) family. It is expressed primarily on microglia—the brain's resident immune cells—where it functions as an inhibitory receptor regulating phagocytosis and inflammatory responses[@bradshaw2013]. Genetic variants in CD33 are among the most consistently replicated risk factors for late-onset Alzheimer's disease (LOAD), identified through genome-wide association studies (GWAS) and validated across multiple ethnic populations[@griciuc2013].

Unlike TREM2, which activates microglial phagocytosis, CD33 exerts an inhibitory effect on amyloid clearance. The GWAS-identified risk allele is associated with increased CD33 expression on microglia, leading to suppressed phagocytic capacity and greater amyloid accumulation[@malik2013]. This positions CD33 as a complementary therapeutic target to TREM2—where TREM2 agonists aim to enhance beneficial phagocytosis, CD33 antagonists seek to remove the inhibitory brake on microglial clearance[@song2022].

Protein Structure

Domain Architecture

CD33 is a 364-amino acid type I transmembrane protein with a characteristic multi-domain structure[@matsumoto2022]:

| Domain | Residues | Structure | Function |
|--------|----------|-----------|----------|
| V-type Ig domain | 1-120 | β-sandwich | Sialic acid binding, ligand recognition |
| C2-type Ig domain 1 | 121-220 | β-sandwich | Receptor stability, protein interactions |
| C2-type Ig domain 2 | 221-259 | β-sandwich | Dimerization support |
| Transmembrane helix | 260-290 | α-helix | Membrane anchoring, dimerization via cysteine |
| Cytoplasmic tail | 291-364 | Disordered | 3 ITIM motifs for signal inhibition |

V-Type Ig Domain (Sialic Acid Recognition)

The N-terminal V-type immunoglobulin domain is the functional heart of CD33:

• Sialic acid binding pocket: Formed by complementary determining regions (CDRs), with conserved arginine (R119) critical for sialic acid recognition
• Ligand specificity: Prefers α2-6-linked sialic acids over α2-3-linked, distinguishing host from pathogen
• Self-recognition function: Binds sialylated glycoproteins on host cells, generating an inhibitory "self" signal that prevents autoimmunity
• Dimerization interface: V-type domains can mediate CD33 homodimerization

ITIM Cytoplasmic Tail

The cytoplasmic domain contains three Immunoreceptor Tyrosine-based Inhibitory Motifs (ITIMs):

| ITIM | Position (YXXL/V) | Function |
|------|-------------------|----------|
| ITIM 1 | Y340 | Primary SHP-1/SHP-2 recruitment |
| ITIM 2 | Y358 | Secondary phosphatase binding |
| ITIM 3 | Y363 | Weak, regulatory role |

Upon ligand binding, ITIM tyrosine residues become phosphorylated by SRC family kinases, recruiting the phosphatases SHP-1 (PTPN6) and SHP-2 (PTPN11) to dephosphorylate downstream signaling molecules, thereby inhibiting cellular activation.

Post-Translational Modifications

• N-linked glycosylation: Multiple sites (N73, N130, N173, N209) bearing complex-type glycans
• Sialylation: Extensive sialylation of N-glycans creates a dense negative charge
• Disulfide bonds: Conserved cysteines in Ig domains form stabilizing disulfide bridges
• Phosphorylation: Tyrosine residues in ITIMs are phosphorylated upon activation

Isoform Diversity

CD33 generates multiple isoforms through alternative splicing, with distinct functional properties[@matsumoto2022]:

| Isoform | Features | Expression | Functional Effect |
|---------|----------|------------|------------------|
| CD33M (canonical) | Full-length, intact ITIM tail | Predominant in microglia | Full inhibitory signaling |
| CD33m (splice variant) | Exon 2 skipped, truncated | Lower expression | Reduced SHP recruitment |
| sCD33 (soluble) | Secreted, no TM domain | Detected in CSF | Decoy receptor? |

Protective Splice Variant (rs3865444)

The GWAS-identified protective variant (rs3865444) creates a splice form that skips exon 2:

• Exon 2 contains critical residues for proper Ig domain folding
• Skipping reduces surface expression of functional CD33M
• Lower CD33M levels means less inhibition and enhanced Aβ clearance
• Carriers show ~20-30% reduced AD risk per allele

Signaling Pathways

ITIM-Mediated Inhibition

CD33 signaling operates through a well-defined inhibitory cascade:

Molecular Mechanism

Step 1 — Ligand recognition:

• CD33 binds sialylated residues on target molecules
• Aβ oligomers are heavily sialylated, presenting CD33 ligands
• Receptor clustering upon ligand binding triggers signaling

• SRC family kinases (FYN, LYN) phosphorylate ITIM tyrosine residues
• Phosphorylated ITIMs recruit SHP-1 and SHP-2 via their SH2 domains
• SHP recruitment displaces activating kinases

• SHP-1/SHP-2 dephosphorylate key signaling molecules: SYK family kinases, PI3K lipid products, PLCγ
• Net effect: reduced microglial activation and phagocytosis

Downstream Effects on Microglial Function

| Function | CD33 Signaling Effect | Mechanism |
|----------|----------------------|-----------|
| Aβ phagocytosis | Inhibited | SHP-1 dephosphorylates phagocytosis machinery |
| Cytokine production | Reduced | Blocked NF-κB and MAPK activation |
| Cellular metabolism | Suppressed | Altered PI3K/AKT and mTOR signaling |
| Migration | Modulated | Changed chemokine receptor phosphorylation |
| Survival | Enhanced | SHP-2 can promote pro-survival signals |

Role in Alzheimer's Disease

Genetic Evidence

CD33 was identified as an AD risk gene through the International Genomics of Alzheimer's Project (IGAP)[@bradshaw2013]:

| Study | Cohort | Key Finding |
|-------|--------|-------------|
| Naj et al. (2011) | IGAP (74,046 cases) | First GWAS identification |
| Sims et al. (2017) | Meta-analysis | Replication across ancestries |
| Kunkle et al. (2019) | IGAP + GR | Fine-mapping of causal variants |
| Bellenguez et al. (2022) | IGAP (111,326 cases) | Confirmation in larger cohort |

Effect sizes:

• rs3865444 (C allele, protective): OR ~0.92 per allele
• rs12459419 (T allele, risk): OR ~1.08 per allele

Pathogenic Mechanisms

CD33 contributes to AD through multiple interconnected mechanisms[@deming2020]:

1. Impaired Amyloid Clearance

Sequence of events:

Evidence from models:

• Cd33 knockout mice: 50% reduction in amyloid plaque burden
• CD33 transgenic mice: Increased amyloid accumulation

2. Altered Neuroinflammation

CD33 shapes the microglial inflammatory response[@schwarting2022]:

| Cytokine/Mediator | Effect of High CD33 |
|-------------------|---------------------|
| TNF-α | Reduced production |
| IL-1β | Suppressed release |
| IL-10 | Enhanced (anti-inflammatory) |

The balance shifts toward a chronic, suppressed inflammatory state that fails to clear pathological aggregates.

3. Tau Pathology Interaction

CD33 influences tau pathology through microglial-mediated mechanisms[@schwarting2022]:

• CD33 genetic variants associated with increased tau tangle burden
• Microglial activation state affects extracellular tau uptake and degradation
• Combined anti-CD33 + anti-tau strategies may be synergistic

4. Metabolic Dysregulation

CD33 affects microglial metabolic reprogramming in AD[@yang2024]:

| Metabolic Parameter | Effect of High CD33 |
|---------------------|---------------------|
| Glycolysis rate | Suppressed |
| Mitochondrial function | Altered membrane potential |
| ATP production | Reduced |
| Lipid droplet formation | Impaired |

These metabolic changes compromise the energy-intensive process of Aβ phagocytosis and clearance.

CD33-TREM2 Axis in Neurodegeneration

CD33 and TREM2 represent opposing regulatory nodes in microglial function[@li2021]:

Key interactions:

• Opposing signals: TREM2 ITAM activates, CD33 ITIM inhibits
• Shared downstream: Both modulate SYK family kinase activity
• Genetic epistasis: Combined risk variants show additive effects
• Therapeutic synergy: Simultaneous TREM2 agonism + CD33 antagonism may maximize microglial clearance

Therapeutic Targeting

CD33 represents a compelling therapeutic target for AD-modifying therapy[@zhao2024]:

Strategy Overview

| Approach | Mechanism | Status |
|----------|-----------|--------|
| Anti-CD33 antibodies | Block ITIM signaling | Preclinical |
| Splice-modulating ASOs | Promote protective isoform | Preclinical |
| Small molecule ITIM blockers | Prevent SHP recruitment | Discovery |

Preclinical Efficacy

Anti-CD33 antibody therapy demonstrates[@yang2024]:

• Reduces amyloid plaque load by 40-60% in APP/PS1 mice
• Improves cognitive performance in Morris water maze
• Reduces microglial CD33 expression
• Enhances Aβ phagocytosis in primary microglial cultures

Animal Models

CD33 Knockout Mice

| Finding | Significance |
|---------|--------------|
| Viable and fertile | No baseline behavioral deficits |
| Enhanced microglial phagocytosis | 50% reduction in plaque burden |
| Improved spatial memory | Morris water maze improvement |
| Compensatory upregulation | Related Siglecs upregulated |

Summary

CD33 (Siglec-3) is a critical microglial inhibitory receptor encoded by one of the most consistently replicated AD risk genes. Through its ITIM-mediated signaling, CD33 suppresses microglial phagocytosis of amyloid-beta, and genetic variants that increase CD33 expression (rs3865444 risk allele) heighten AD susceptibility. The balance between CD33 (inhibitory) and TREM2 (activating) regulates the microglial response to neurodegeneration, and therapeutic strategies aimed at blocking CD33 have demonstrated proof-of-concept efficacy in preclinical models.

See Also

• [CD33 Gene](/genes/cd33) — Gene page with genetics and biology
• [TREM2 Protein](/proteins/trem2-protein) — Complementary microglial receptor
• [Microglia](/cell-types/microglia) — Cell type where CD33 is expressed
• [Alzheimer's Disease](/diseases/alzheimers-disease) — Primary disease association
• [Neuroinflammation](/mechanisms/neuroinflammation) — Broader inflammatory context
• [Microglial Phagocytosis](/mechanisms/microglial-phagocytosis) — The process CD33 inhibits