| Property | Value | [1] |
|----------|-------| [2] |
| Gene Symbol | CR2 |
| Full Name | Complement Receptor Type 2 (CD21) |
| Chromosomal Location | 1q32 |
| NCBI Gene ID | 1380 |
| OMIM ID | 120650 |
| Ensembl ID | ENSG00000117322 |
| UniProt ID | P08578 |
| Encoded Protein | Complement receptor type 2 |
| Associated Diseases | Alzheimer's Disease, Multiple Sclerosis, Systemic Lupus Erythematosus |
CR2 (Complement Receptor Type 2), also known as CD21 or Complement Receptor 2, is a membrane glycoprotein that serves as a primary receptor for fragments of complement component C3, particularly C3d, C3dg, and iC3b. Located on chromosome 1q32, this 1458-amino acid protein is predominantly expressed on B lymphocytes, follicular dendritic cells (FDCs), and certain T cell subsets, where it plays essential roles in humoral immunity, immune complex clearance, and complement-mediated signaling.
Beyond its classical immunological functions, CR2 has emerged as a protein of significant interest in neurodegenerative diseases. The receptor is expressed on brain microglia, where it mediates complement-dependent clearance of pathogens and cellular debris, and genetic variants in CR2 have been associated with Alzheimer's Disease risk. This positions CR2 at the intersection of complement-mediated immune surveillance and neuroinflammation, making it a potential therapeutic target in neurodegenerative conditions.
The CR2 gene is located on chromosome 1q32 (positions 207,500,000-207,550,000, GRCh38) on the minus strand. The gene spans approximately 55 kb and comprises 25 exons that encode a 1458-amino acid transmembrane protein.
CR2 contains several functional domains:
Extracellular domain (aa 1-1242):
Transmembrane domain (aa 1243-1265): Single pass membrane protein
Cytoplasmic tail (aa 1266-1458): Intracellular signaling capabilities
CR2 has multiple isoforms:
CR2 shows conservation across vertebrates:
| Species | Gene Name | Amino Acids | Identity |
|---|---|---|---|
| Human | CR2 | 1458 | Reference |
| Mouse | Cr2 | 1454 | 66% |
| Chicken | CR2 | 1421 | 58% |
The SCR domains are highly conserved, reflecting their essential ligand-binding functions.
CR2 serves as the primary receptor for C3 degradation products:
| Ligand | Binding Site | Function |
|---|---|---|
| C3d | SCR1-2 | Primary ligand, immune complex clearance |
| C3dg | SCR1-2 | B cell activation co-receptor |
| iC3b | SCR3-4 | Phagocytosis receptor |
CR2 transduces signals through multiple mechanisms:
B cell receptor co-ligation:
Intracellular signaling:
CR2 serves as the receptor for Epstein-Barr virus:
CR2 plays essential roles in B lymphocyte function [2:1]:
CR2 integrates complement and adaptive immunity:
| Function | Mechanism |
|---|---|
| Antigen presentation | Enhanced uptake of opsonized antigens |
| T cell priming | Improved B cell antigen presentation |
| Cytokine production | Modulated cytokine responses |
| Immunological memory | FDC retention of antigens |
CR2 has significant involvement in Alzheimer's Disease [3]:
Microglial phagocytosis [thornton2016]:
Genetic associations [huang2019]:
Immune complex handling:
CR2 has roles in multiple sclerosis [yang2018]:
As characterized in [4], CR2 affects SLE:
CR2 shows characteristic expression on immune cells:
| Cell Type | Expression Level | Function |
|---|---|---|
| B cells | High | Co-receptor, activation |
| Follicular dendritic cells | Highest | Antigen retention |
| T cells | Low | Regulatory function |
| Macrophages | Low-moderate | Phagocytosis |
| Dendritic cells | Moderate | Antigen presentation |
Within the central nervous system, CR2 is expressed in [5]:
CR2 operates within the CR2-CD19-CD81 complex:
CR2 → CD19 → PI3K → AKT → Survival
↓
Lyn/Syk → MAPK/ERK → Proliferation
C3d binding triggers:
CR2 serves as a biomarker:
| Marker | Application |
|---|---|
| B cell CR2 expression | Immune cell subset identification |
| Soluble CR2 | Disease activity marker |
| Genetic variants | Risk assessment |
Current therapeutic strategies include:
Key questions remain:
| Interactor | Interaction Type | Function |
|---|---|---|
| CD19 | Complex formation | B cell co-receptor |
| CD81 | Complex formation | Tetraspanin partner |
| C3d | Ligand binding | Complement recognition |
| Lyn | Signaling kinase | Intracellular signaling |
Lambris JD et al. CR2 complement receptor and immune function. 2005. ↩︎
Carroll MC et al. CR2 and B cell function. 2004. ↩︎ ↩︎
Eriksson JE et al. CR2 and Alzheimer's disease pathology. 2015. ↩︎
Jacobs J et al. CR2 and autoimmunity. 2008. ↩︎
Zhou R et al. CR2 and neuroinflammation. 2020. ↩︎