CD6 Human
Description
Functional Roles in Immunity
CD6 regulates immune responses through multiple mechanisms:
T Cell Activation and Synapse Formation
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CD6 colocalizes with the TCR/CD3 complex at the immunological synapse, stabilizing T cell-antigen-presenting cell (APC) interactions .
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CD6-deficient T cells exhibit augmented activation but reduced survival and proliferation, leading to diminished Th1/Th17 polarization .
Cell Adhesion and Migration
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CD6-CD166 interactions facilitate T cell migration across the blood-brain barrier (BBB), a key process in neuroinflammatory diseases like multiple sclerosis (MS) .
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Blocking CD166 reduces leukocyte infiltration into the central nervous system (CNS), attenuating experimental autoimmune encephalomyelitis (EAE) .
Dual Regulatory Effects
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CD6 enhances early T cell activation but restrains TCR-mediated signaling via associations with negative regulators like UBASH3A .
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It promotes cytokine production while preventing excessive inflammation, maintaining immune homeostasis .
Role in Autoimmune and Inflammatory Diseases
CD6 is implicated in several pathologies, supported by genetic and experimental evidence:
Therapeutic Targeting of CD6
CD6 is a promising target for immune modulation, with several clinical candidates in development:
Monoclonal Antibodies
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UMCD6: Reduces Th1/Th17 polarization and CNS infiltration in EAE models without T cell depletion .
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Itolizumab: An anti-CD6 IgG1 tested in lupus nephritis, graft-versus-host disease (GVHD), and COVID-19 complications .
Adoptive Cell Therapies
Table: Ongoing Clinical Trials Targeting CD6
| Trial ID | Treatment | Phase | Condition |
|---|---|---|---|
| NCT05993611 | CD6-CAR Tregs | Ib | Chronic GVHD post-transplant |
| NCT04007198 | Itolizumab | Ib | Moderate-to-severe uncontrolled asthma |
| NCT04128579 | Itolizumab | Ib | Systemic lupus erythematosus with nephritis |
Future Directions
While CD6-targeted therapies show potential, key challenges remain:
Product Specs
CD6 is a glycoprotein found on the surface of T cells, playing a crucial role in their function. It is also present on thymocytes (immature T cells), a specific subset of B cells (B-1 cells), and certain brain cells. As a member of the SRCR superfamily, CD6 participates in cell signaling and adhesion. It interacts with CD166, contributing to T cell activation and the development of T cells in the thymus.
This recombinant CD6 protein, produced in Sf9 insect cells using a baculovirus expression system, is a single, glycosylated polypeptide chain. It consists of 627 amino acids (specifically, amino acids 18-402a), resulting in a molecular weight of 68.3 kDa. The protein includes a 239 amino acid hIgG-His-Tag at its C-terminus and undergoes purification through proprietary chromatographic methods.
A clear, colorless solution that has been sterilized by filtration.
The CD6 protein is supplied in a solution at a concentration of 0.5 mg/ml. The solution also contains 10% glycerol and Phosphate-Buffered Saline (PBS) with a pH of 7.4.
For short-term storage (up to 2-4 weeks), keep the vial refrigerated at 4°C. For extended storage, freeze the protein at -20°C. Adding a carrier protein (either 0.1% HSA or BSA) is recommended for long-term storage. Repeated freezing and thawing of the protein should be avoided.
The purity of the CD6 protein is greater than 90% as determined by SDS-PAGE analysis.
The protein exhibits significant biological activity, as demonstrated by its ability to support the adhesion of Jurkat cells (a human acute T cell leukemia cell line). When Jurkat cells are added to plates coated with human CD6 at a concentration of 10 µg/ml, the adhesion is enhanced by more than 50%. This adhesion-promoting activity is a key characteristic of functional CD6 protein.
T-cell differentiation antigen CD6, T12, TP120, CD6, CD antigen, T-cell differentiation antigen CD6 isform1
Sf9, Baculovirus cells.
ADPHPSPAPP DQLNTSSAES ELWEPGERLP VRLTNGSSSC SGTVEVRLEA SWEPACGALW DSRAAEAVCR ALGCGGAEAA SQLAPPTPEL PPPPAAGNTS VAANATLAGA PALLCSGAEW RLCEVVEHAC RSDGRRARVT CAENRALRLV DGGGACAGRV EMLEHGEWGS VCDDTWDLED AHVVCRQLGC GWAVQALPGL HFTPGRGPIH RDQVNCSGAE AYLWDCPGLP GQHYCGHKED AGAVCSEHQS WRLTGGADRC EGQVEVHFRG VWNTVCDSEW YPSEAKVLCQ SLGCGTAVER PKGLPHSLSG RMYYSCNGEE LTLSNCSWRF NNSNLCSQSL AARVLCSASR SLHNLSTPEV PASVQTVTIE SSVTVKIENK ESRELMLLVE PKSCDKTHTC PPCPAPELLG GPSVFLFPPK PKDTLMISRT PEVTCVVVDV SHEDPEVKFN WYVDGVEVHN AKTKPREEQY NSTYRVVSVL TVLHQDWLNG KEYKCKVSNK ALPAPIEKTI SKAKGQPREP QVYTLPPSRD ELTKNQVSLT CLVKGFYPSD IAVEWESNGQ PENNYKTTPP VLDSDGSFFL YSKLTVDKSR WQQGNVFSCS VMHEALHNHY TQKSLSLSPG KHHHHHH
Q&A
What is the genomic organization of human CD6?
The human cd6 gene maps to chromosome 11 at position 11q13.1, spanning more than 25-kb. It is located adjacent to the cd5 gene within 200-kb telomeric to cd20 . The gene contains 13 exons, with exon 1 containing the start codon and exon 13 containing the stop codon. The coding sequence corresponds to a 668-amino acid protein. Exons 1-2 encode the 24-aa signal sequence, exons 3-5 each encode a 101/117-aa SRCR domain, exon 6 encodes a 22-aa spacer, exon 7 encodes the 26-aa transmembrane domain, and exons 8-13 encode the cytoplasmic domain .
How is CD6 expression regulated across immune cell populations?
CD6 is expressed by hematopoietic precursors, thymocytes, mature T cells, NK cells, and a subset of normal B lymphocytes (particularly those from lymphocytic chronic leukemia) . In B cells, CD6 expression predominates in mature B cells that produce IgM but have not undergone isotype switching, often co-expressing with CD5 . Expression can be modulated by various stimuli - exposure to phorbol myristic acetate (PKC activator) or phytohaemagglutinin increases CD6 mRNA levels . The regulation during B cell activation remains controversial, with conflicting reports about induction following BCR engagement or superantigen stimulation .
What functional domains characterize the CD6 protein structure?
CD6 contains three extracellular scavenger receptor cysteine-rich (SRCR) domains (D1, D2, and D3), a transmembrane region, and a cytoplasmic tail:
| Domain | Location | Function |
|---|---|---|
| SRCR-D1 | Extracellular (Exon 3) | Alternative ligand binding; pathogen recognition |
| SRCR-D2 | Extracellular (Exon 4) | Pathogen recognition |
| SRCR-D3 | Extracellular (Exon 5) | ALCAM binding domain |
| Spacer | Extracellular (Exon 6) | Separates SRCR-D3 from transmembrane domain |
| Transmembrane | Membrane-spanning (Exon 7) | Anchors protein in cell membrane |
| Cytoplasmic tail | Intracellular (Exons 8-13) | Contains 9 threonine and 32 serine residues; sites for protein interactions and signaling |
The cytoplasmic domain facilitates interactions with signaling molecules including SLP-76 and synthenin-1, which bind to phosphorylated Y662 .
What are the primary ligands for CD6 and how do they contribute to its function?
CD6 interacts with several ligands through different domains:
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ALCAM (CD166): The primary ligand, binds to SRCR-D3 domain of CD6. This interaction stabilizes the contact between T cells and antigen-presenting cells (APCs) at the peripheral supramolecular activation cluster (pSMAC) of the immunological synapse .
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3A11: A 130-kDa protein expressed on synovial fibroblasts, thymic fibroblasts, and keratinocytes following IFN-γ activation. This interaction involves the SRCR-D1 domain .
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Pathogen-associated molecular patterns (PAMPs): CD6 can bind directly to pathogen components in a calcium-dependent manner through all three SRCR domains, leading to MAPK/Erk activation and cytokine release .
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Additional uncharacterized ligands: Evidence suggests CD6 may interact with other proteins including a 90-kDa and a 45-kDa protein from human epithelial cells .
How does CD6 contribute to the immunological synapse formation?
CD6 plays dual roles at the immunological synapse:
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At the peripheral supramolecular activation cluster (pSMAC), CD6 binds to ALCAM, creating a stable, long-lasting interaction that initiates contact between T cells and APCs. Blocking this interaction with anti-CD6-SRCR-D3 antibodies or ALCAM-blocking antibodies significantly reduces T-cell/APC contact and impairs T-cell proliferation .
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At the central supramolecular activation cluster (cSMAC), CD6 physically associates with the TCR/CD3 complex, participating in T cell activation and proliferation. Anti-CD6-SRCR-D1 antibodies can inhibit T-cell proliferation without affecting T-cell/APC contact, suggesting this domain's involvement in activation signaling .
What is the functional significance of CD6-CD5 association in immune cells?
CD6 and CD5 show significant physical association, with approximately 12% of CD6 molecules associated with CD5 in resting T cells . Upon activation, this association strengthens, and the CD6-CD5 complexes migrate to the immunological synapse where they colocalize with TCR/CD3 within the cSMAC . This interaction:
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Proceeds through their extracellular domains (CD5 maintains CD6 binding capability even when its intracellular tail is truncated)
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Results in phosphorylation of CD6 on Y629 and CD5 on Y429, suggesting contribution to TCR or BCR-independent activation
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Can be demonstrated by co-immunoprecipitation, where CD5 associates with hyperphosphorylated 130-kDa CD6, and CD6 with hyperphosphorylated CD5
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Is tightly regulated during thymic maturation, contributing to thymocyte selection
How are CD6 genetic variants associated with autoimmune diseases?
Analysis of single-nucleotide polymorphisms (SNPs) in the CD6 locus has revealed important disease associations:
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Multiple Sclerosis: The SNP rs17824933 in exon 1 is significantly associated with multiple sclerosis (MS) . This variant is linked to lower CD6 expression.
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Other variants: Eight additional nonsynonymous SNPs have been identified in the CD6 coding region, though their functional effects remain largely unknown .
These genetic associations provide strong evidence for CD6's role in autoimmune pathogenesis and highlight its potential as a therapeutic target.
What are the experimental approaches for targeting CD6 in autoimmune diseases?
Anti-CD6 antibodies have been developed and tested in several autoimmune conditions:
| Antibody Type | Name | Disease Applications | Development Status |
|---|---|---|---|
| Murine anti-CD6 | Early versions | RA, psoriasis, MS | Discontinued due to immunogenicity |
| Humanized anti-CD6 mAb | T1h | RA, psoriasis | Evaluated in clinical trials |
The initial promising results with murine anti-CD6 antibodies in rheumatoid arthritis (RA), psoriasis, and multiple sclerosis had to be abandoned due to their murine origin causing immunogenicity issues . This led to the development of humanized anti-CD6 monoclonal antibody T1h, which has shown preliminary efficacy in active RA when combined with methotrexate, resulting in long-term remission with significant reduction of swollen and tender joints .
What mechanisms explain the therapeutic effects of anti-CD6 antibodies?
The therapeutic effects of anti-CD6 antibodies like T1h appear to work through several potential mechanisms:
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Not through T-cell depletion: Unlike some immune-targeting therapies, anti-CD6 antibodies do not appear to cause significant T-cell depletion .
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Blocking CD6-ALCAM interactions: While this may be one mechanism, in vivo experiments and in vitro competition assays have cast doubt on this being the primary mode of action .
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Immunomodulation: Anti-CD6-SRCR-D1 antibodies (like T1h) can inhibit T-cell proliferation without affecting T-cell/APC contact, suggesting they may modulate signaling rather than simply blocking adhesion .
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Interference with CD6's role at the immunological synapse: By targeting CD6, these antibodies may disrupt the proper formation or function of the immunological synapse, particularly at the cSMAC where CD6 associates with the TCR/CD3 complex .
What are the optimal approaches for studying CD6-mediated cell adhesion in experimental systems?
When investigating CD6-mediated cell adhesion, researchers should consider these methodological approaches:
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Blocking antibodies: Use domain-specific antibodies to determine which regions of CD6 are involved in particular interactions. Anti-CD6-SRCR-D3 antibodies block CD6-ALCAM interactions, while anti-CD6-SRCR-D1 antibodies target other functional aspects .
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Alternative isoform expression: Expressing CD6ΔD3 (an isoform lacking the ALCAM-binding domain) can help determine the specific contribution of CD6-ALCAM interactions to cellular adhesion .
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Confocal microscopy with FRET technology: This approach has been effective in demonstrating CD6-CD5 associations and their dynamic changes during immune cell activation .
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Co-immunoprecipitation studies: These have been valuable in identifying physical associations between CD6 and other proteins, such as CD5, and in studying the phosphorylation states of these complexes .
How can researchers effectively study CD6 involvement in pathogen recognition?
To investigate CD6's role in pathogen recognition:
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Binding assays: Evaluate direct binding between CD6 and pathogen components in dose-dependent, saturable, and calcium-facilitated conditions .
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Domain mapping: Determine which of the three SRCR domains are involved in particular pathogen interactions .
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Signaling analysis: Assess MAPK/Erk activation and cytokine release following CD6-pathogen interaction .
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Soluble CD6 studies: Measure levels of soluble CD6 in serum from healthy individuals and those with autoimmune diseases to explore potential feedback mechanisms that might reduce direct lymphocyte activation by pathogens .
What techniques are most effective for investigating CD6's role in the immunological synapse?
The most informative techniques include:
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Confocal microscopy with immunofluorescence: To visualize CD6 localization within the immunological synapse, particularly its distribution between the pSMAC and cSMAC .
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Förster resonance energy transfer (FRET): Useful for measuring protein-protein interactions, including CD6 association with CD5 and the TCR/CD3 complex .
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T-cell/APC contact measurements: Quantitative assays that can determine how CD6 blockade or genetic manipulation affects the stability and duration of T cell interactions with antigen-presenting cells .
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Proliferation assays: To assess functional outcomes of CD6 targeting at the immunological synapse, particularly using domain-specific antibodies that can differentiate between effects on adhesion versus signaling .
What are the unresolved questions regarding CD6 function in human immunity?
Despite significant advances, several aspects of CD6 biology remain unclear:
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Ligand diversity: The complete repertoire of CD6 ligands beyond ALCAM and 3A11 remains to be fully characterized .
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Domain-specific functions: While SRCR-D3 binds ALCAM and SRCR-D1 appears involved in signaling, the specific functions of SRCR-D2 need further investigation .
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CD6-CD5 interaction mechanism: The specific domains involved in CD6-CD5 binding and the functional consequences of this association require additional study .
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Isoform regulation: The factors controlling expression of alternative CD6 isoforms (like CD6ΔD3) and their functional significance need clarification .
How might novel technologies advance our understanding of CD6 biology?
Emerging technologies with potential to drive CD6 research forward include:
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CRISPR-Cas9 gene editing: Creation of CD6 knockout or domain-specific mutant human cell lines could provide valuable insights into CD6 function .
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Single-cell technologies: Single-cell RNA sequencing and proteomics could reveal cell-specific expression patterns and functional heterogeneity of CD6 .
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Advanced imaging techniques: Super-resolution microscopy and live-cell imaging would allow more detailed visualization of CD6 dynamics during immune cell interactions .
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Humanized mouse models: Development of better models expressing human CD6 would facilitate translational research into therapeutic applications .
Product Science Overview
CD6, also known as T-cell differentiation antigen CD6, is a type I transmembrane glycoprotein that plays a crucial role in the immune system. It is primarily expressed on the surface of T cells, a subset of B cells known as B1a cells, and natural killer (NK) cells. CD6 is also found on certain hematopoietic precursors and in specific regions of the brain .
The CD6 protein consists of three scavenger receptor cysteine-rich (SRCR) domains. These domains are essential for its function and interactions with other molecules. The membrane-proximal domain (D3) of CD6 binds to the amino-terminal domain (D1) of activated leukocyte cell adhesion molecule (ALCAM), also known as CD166 . This interaction is critical for the stabilization of cell-to-cell contacts and modulation of activation and differentiation events in the immune response .
CD6 is involved in the continuation of T cell activation and plays a role in the immune response. It acts as a co-stimulatory molecule, enhancing the activation and proliferation of T cells. CD6 also assists in the stabilization of the immunological synapse, which is the interface between T cells and antigen-presenting cells .
Recombinant human CD6 is a form of the CD6 protein that is produced using recombinant DNA technology. This involves inserting the gene encoding CD6 into a host cell, such as HEK293 cells, which then express the protein. The recombinant protein is typically purified and used for various research and therapeutic purposes .
Recombinant human CD6 has several applications in biomedical research and therapy. It is used to study the role of CD6 in immune responses and to develop potential therapeutic strategies for autoimmune diseases and cancer. For example, soluble forms of CD6 (sCD6) have been explored as decoy receptors to block CD6-mediated interactions, which can modulate antitumor lymphocyte effector function and tumorigenesis .
