CLEC4M (C-type lectin domain family 4 member M), also known as DC-SIGNR or CD299, is a type II transmembrane protein expressed in endothelial cells of liver, lung, and lymphoid tissues. It binds pathogens and glycoproteins, including SARS-CoV, HIV-1, and von Willebrand factor (VWF) . FITC (fluorescein isothiocyanate)-conjugated antibodies enable fluorescent detection of CLEC4M in flow cytometry and immunohistochemistry, enhancing visualization in research and diagnostic settings .
FITC-conjugated CLEC4M antibodies are used to detect surface expression on endothelial cells or immune cells. This method is critical for studying CLEC4M’s role in pathogen binding or clearance (e.g., VWF internalization) .
The antibody localizes CLEC4M in tissue sections, aiding in mapping its expression in liver, lymphoid, or tumor microenvironments. For example, CLEC4M is downregulated in hepatocellular carcinoma (HCC) but may inhibit tumor progression when overexpressed .
VWF Interaction: CLEC4M binds VWF via N-glycans, promoting its internalization and clearance from plasma. Polymorphisms in the CLEC4M gene (e.g., VNTR repeats) influence VWF levels and von Willebrand disease (VWD) susceptibility .
Viral Pathogens: CLEC4M homologs (e.g., DC-SIGN) enhance HIV-1 infection by binding gp120. FITC-conjugated antibodies could help study viral tropism or CLEC4M’s role in SARS-CoV entry .
HCC Prognosis: High CLEC4M expression correlates with improved survival in HCC patients. FITC-conjugated antibodies may aid in identifying CLEC4M-positive endothelial cells in tumor microenvironments .
JAK1/STAT3 Inhibition: CLEC4M overexpression suppresses the JAK1/STAT3 pathway, reducing proliferation and enhancing apoptosis in HCC cells .
Cross-Reactivity: While validated for human samples, potential cross-reactivity with DC-SIGN (77% identical to CLEC4M) requires confirmation .
Glycan Dependency: Binding to VWF and pathogens is calcium- and glycan-dependent, necessitating optimized experimental conditions .
Therapeutic Potential: Despite its role in tumor suppression, CLEC4M’s therapeutic targeting remains unexplored in clinical trials .
CLEC4M (C-type lectin domain family 4 member M) is a calcium-dependent mannose-specific receptor primarily expressed on endothelial cells in liver sinusoids and lymph nodes. It consists of an N-terminal cytoplasmic region, a polymorphic neck region containing variable numbers of tandem repeats (VNTR), and a carbohydrate recognition domain (CRD) . The protein functions include:
Binding and internalizing ligands with mannose-exposed N-linked glycans
Clearance of von Willebrand factor (VWF) and factor VIII (FVIII) from circulation
Recognition of various pathogens, including viruses such as SARS-CoV
Potential tumor suppression in hepatocellular carcinoma through inhibition of the JAK1/STAT3 pathway
The neck region stabilizes CLEC4M on the endothelial surface through tetramerization of monomers and influences the conformation of the carbohydrate recognition domain, affecting its binding capabilities .
FITC-conjugated CLEC4M antibodies provide specific advantages and limitations compared to other fluorescent conjugates:
FITC (Fluorescein isothiocyanate) has an excitation maximum at 488 nm and emission at approximately 520 nm, making it compatible with standard flow cytometers with 488 nm lasers
Unlike PE/Cy7 conjugates (excitation = 488 nm, emission = 778 nm), FITC has less compensation needs with PE channels but may show lower sensitivity
FITC-conjugated antibodies may be more susceptible to photobleaching compared to more photostable fluorophores like Alexa Fluor dyes
For multicolor panels, FITC can be paired effectively with PE, APC and far-red fluorochromes
Storage requirements are similar to other conjugates (4°C in the dark), but FITC may be more sensitive to pH changes than some alternatives
When designing multicolor flow cytometry panels, consider that FITC works best for abundantly expressed targets due to its moderate brightness compared to PE or APC conjugates.
A comprehensive experimental design for flow cytometry with FITC-conjugated CLEC4M antibodies should include:
Unstained control: Cells with no antibody to establish autofluorescence baseline
Isotype control: Matched isotype (IgG2 for clones like MM0241-2U25) conjugated to FITC to assess non-specific binding
Single-color controls: For compensation when performing multicolor analysis
Fluorescence Minus One (FMO) control: All antibodies in your panel except CLEC4M-FITC
Positive control: Cells known to express CLEC4M (e.g., liver sinusoidal endothelial cells or transfected HEK293 cells)
Negative control: Cells known not to express CLEC4M
Viability dye: Include a fixable viability dye (e.g., eFluor 780) in a non-overlapping channel to exclude dead cells
For optimal results, include blockers for Fc receptors if analyzing cells that may express them, and validate antibody performance using both positive and negative cell populations before conducting full experiments.
CLEC4M contains a polymorphic neck region with variable numbers of tandem repeats (VNTR), ranging from 3 to 9 repeats of a conserved 23-amino-acid sequence . This polymorphism presents several research challenges:
Impact on antibody binding:
Different VNTR lengths alter the spatial organization of the carbohydrate recognition domain (CRD)
Homozygous vs. heterozygous VNTR status affects oligomerization and binding capacity
The neck region stabilizes CLEC4M tetramers on the cell surface, affecting epitope accessibility
Strategies to address polymorphism challenges:
Epitope selection: Use antibodies targeting conserved regions outside the VNTR domain
Genotyping: Perform CLEC4M VNTR genotyping of samples before antibody studies to stratify results
Multiple clone approach: Compare binding patterns of different antibody clones (e.g., MM0241-2U25, 120604, OTI7D12)
Domain-specific controls: Include recombinant CLEC4M variants with defined VNTR lengths as controls
Cross-validation: Combine flow cytometry with immunoblotting to verify specificity across VNTR variants
When studying populations with known VNTR distribution differences, consider the potential impact of these polymorphisms on antibody recognition patterns and interpret data accordingly.
The optimal protocols for CLEC4M detection vary based on experimental goals and cell types:
For surface CLEC4M detection (recommended protocol):
Harvest cells using gentle methods (e.g., EDTA rather than trypsin) to preserve surface epitopes
Incubate with viability dye (e.g., eFluor 780) for 20 minutes at 4°C
Wash cells in calcium-containing buffer (e.g., TSM: 20 mM Tris HCl, 150 mM NaCl, 2 mM CaCl₂, 2 mM MgCl₂)
Block with 5% normal serum matched to secondary antibody species
Incubate with FITC-conjugated CLEC4M antibody at validated concentration for 30 minutes at 4°C
Wash twice with buffer and analyze immediately or fix in 1-2% paraformaldehyde
For intracellular trafficking studies:
Perform surface staining as above
Fix cells in 4% paraformaldehyde for 15 minutes at room temperature
Permeabilize with 0.1% saponin or commercial permeabilization buffer
Re-stain with antibodies against intracellular markers (e.g., early endosomal antigen-1)
Use confocal microscopy with z-stack analysis to distinguish surface from internalized CLEC4M
Special considerations:
For liver sinusoidal endothelial cells, gentle isolation techniques are critical as these cells are sensitive to mechanical stress
When studying CLEC4M-mediated internalization, compare 4°C (binding only) versus 37°C (binding and internalization) conditions
For dual staining with other markers (e.g., stabilin-2), optimize antibody concentrations to prevent cross-interference
CLEC4M-mediated internalization can be effectively studied using the following methodological approach:
Experimental design for internalization assays:
Establish CLEC4M-expressing cell system:
Dual fluorescence labeling strategy:
Time-course analysis protocol:
Incubate cells with ligand at 4°C (binding only)
Shift cells to 37°C for various time intervals (5-60 minutes)
Fix cells at each timepoint with 4% paraformaldehyde
Perform immunofluorescence with anti-CLEC4M-FITC and endosomal markers
Z-stack confocal microscopy analysis:
Flow cytometry-based internalization assay:
Label ligand with pH-sensitive fluorophore (quenched in acidic endosomes)
Track surface CLEC4M with FITC-conjugated antibody at different timepoints
Quantify receptor internalization by measuring surface CLEC4M reduction over time
Data analysis considerations:
Calculate percent colocalization between CLEC4M, ligand, and endosomal markers
Compare internalization rates between different CLEC4M VNTR variants
For inhibition studies, pretreat cells with mannan (1 mg/mL) to block CLEC4M binding
Research indicates that CLEC4M plays a tumor-suppressive role in hepatocellular carcinoma (HCC), making it a valuable target for prognostic and mechanistic studies:
Experimental approaches for HCC research:
Tissue expression analysis:
Prognostic marker assessment:
Mechanistic investigations:
Study JAK1/STAT3 pathway activity in relation to CLEC4M expression using phospho-specific antibodies
Perform flow cytometry on HCC cell lines (e.g., Huh7, PLC/PRF/5) with CLEC4M-FITC to track expression changes during experimental manipulation
Use CLEC4M overexpression models to study effects on cell proliferation, apoptosis, and signaling pathways
Data analysis framework:
Compare CLEC4M expression between different HCC stages and grades
Correlate CLEC4M levels with established HCC biomarkers
Analyze the relationship between CLEC4M polymorphisms and HCC risk/progression
Develop multiparameter flow cytometry panels combining CLEC4M-FITC with markers for cell cycle, apoptosis, and JAK/STAT signaling
The experimental data from Huang et al. demonstrated that CLEC4M overexpression inhibited proliferation and enhanced apoptosis in HCC cell lines while suppressing the JAK1/STAT3 pathway, suggesting its potential utility as both a prognostic marker and therapeutic target .
CLEC4M's role in viral binding and infection can be comprehensively studied using the following methodological approach:
Experimental design for viral studies:
Viral binding assays:
Express CLEC4M in suitable cell lines (HEK293 cells) with defined VNTR genotypes
Incubate cells with fluorescently labeled virus particles at 4°C (binding only)
Quantify binding by flow cytometry using CLEC4M-FITC antibodies to confirm receptor expression
Include mannan competition (1 mg/mL) to determine specificity of interactions
Viral internalization studies:
Compare cells expressing homozygous versus heterozygous CLEC4M VNTR variants
Track virus internalization using confocal microscopy with immunofluorescence for viral proteins
Monitor CLEC4M-virus colocalization in early endosomes and lysosomes over time
Use proteasome inhibitors to assess viral degradation pathways
Trans-infection assays:
Genetic association studies:
Key methodological considerations:
Maintain calcium in all buffers (2 mM CaCl₂) to preserve CLEC4M binding capacity
Include appropriate controls to distinguish specific from non-specific binding
For SARS-CoV or other BSL-3 pathogens, use pseudotyped viral particles or isolated spike proteins for initial studies
When comparing VNTR variants, ensure equivalent surface expression levels by titrating transfection conditions
Chan et al. demonstrated that homozygosity for CLEC4M VNTR was associated with protection against SARS-CoV infection, and CLEC4M-expressing cells showed increased viral binding but enhanced proteasome-dependent viral degradation, highlighting the importance of studying both binding and post-binding events .
CLEC4M binds both von Willebrand factor (VWF) and factor VIII (FVIII), making it an important regulator of coagulation factor clearance. The following methodological framework enables detailed investigation of these interactions:
Comprehensive experimental approach:
Solid-phase binding assays:
Flow cytometry analysis:
Glycan dependency studies:
Internalization and trafficking analysis:
Experimental data table from literature:
| Experimental Condition | Effect on CLEC4M-VWF Binding | Statistical Significance |
|---|---|---|
| De-O-glycosylation of VWF | 70% increase in binding | p = 0.041 |
| De-N-glycosylation of VWF | 75% decrease in binding | p = 0.046 |
| Mannan competition (1 mg/mL) | ~50% reduction in binding | p < 0.01 |
| Calcium chelation (EDTA) | Significant decrease in binding | p < 0.01 |
| High salt washing (500mM NaCl) | Reversal of binding | Demonstrated |
Advanced research considerations:
Study the impact of CLEC4M VNTR polymorphisms on coagulation factor clearance rates
Investigate how VWF glycoform variations affect CLEC4M binding affinity
Explore the potential competition between different CLEC4M ligands in physiological settings
Examine how CLEC4M-mediated clearance is affected by pathological states
These methodological approaches have revealed that CLEC4M binds to VWF through N-glycan-dependent mechanisms and can internalize both VWF alone and VWF-FVIII complexes, potentially regulating their plasma levels .
Primary liver sinusoidal endothelial cells (LSECs) present unique challenges for CLEC4M detection. A systematic troubleshooting approach includes:
Potential causes and solutions:
Cell isolation issues:
Antibody selection issues:
Test multiple antibody clones as epitope accessibility may vary
Titrate antibody concentration using positive control cells
Consider using signal amplification for low-abundance detection
Buffer composition problems:
Technical variables:
Validation strategy:
Use double-staining with established LSEC markers (stabilin-2, SE-1)
Include rat LSECs as biological controls with known CLEC4M expression
Perform parallel RNA analysis (RT-PCR) to confirm CLEC4M expression
Compare staining patterns between fresh isolates and cultured LSECs
Flow cytometry data from Elvevold et al. demonstrates that LSECs express both CLEC4M and other scavenger receptors, which can be used for co-staining validation approaches .
Multiple factors can significantly impact the performance of FITC-conjugated CLEC4M antibodies across different applications:
Critical performance factors:
Epitope accessibility factors:
CLEC4M's oligomeric state affects epitope exposure
Neck region VNTR polymorphisms alter protein conformation
Calcium-dependent conformational changes influence antibody binding
Ligand binding may induce epitope masking
Technical optimization parameters:
Flow cytometry: Optimal concentrations are typically 2-5 μg/mL
Immunohistochemistry: Antigen retrieval methods critically affect staining
Western blotting: Reducing vs. non-reducing conditions impact detection
Immunoprecipitation: Buffer composition affects complex stability
Sample-specific considerations:
Tissue fixation methods impact epitope preservation (fresh-frozen vs. FFPE)
Cell surface CLEC4M may be cleaved by proteases during sample processing
Expression levels vary significantly between liver, lymph nodes, and other tissues
Pathological conditions can alter glycosylation and expression patterns
Clone-specific characteristics:
Performance optimization matrix:
| Application | Optimal Fixation | Recommended Clone | Key Optimization Steps |
|---|---|---|---|
| Flow cytometry | 1-2% PFA, 10 min | MM0241-2U25, 120604 | Ca²⁺ in buffers, titrate antibody |
| Immunohistochemistry | 4% PFA, 15 min | MM0241-2U25 | Heat-mediated antigen retrieval |
| Western blotting | N/A | OTI7D12 | Non-reducing conditions preferred |
| Immunofluorescence | 4% PFA, 15 min | MM0241-2U25 | Confocal imaging for colocalization |
Validation approach:
Include both positive controls (CLEC4M-transfected cells) and negative controls
Perform parallel staining with multiple CLEC4M antibody clones
Validate results with complementary detection methods (flow + microscopy)
Consider genetic verification of CLEC4M VNTR status in heterogeneous samples
Distinguishing specific from non-specific binding is crucial for accurate CLEC4M analysis. A systematic verification approach includes:
Comprehensive validation strategy:
Essential control samples:
Positive control: CLEC4M-transfected HEK293 cells with confirmed expression
Negative control: Non-transfected parent cell line
Tissue controls: Liver sinusoidal endothelial cells (positive) vs. hepatocytes (negative)
Blocking controls: Pre-incubation with unconjugated antibody to demonstrate specificity
Molecular validation techniques:
siRNA knockdown: Demonstrate signal reduction following CLEC4M silencing
CRISPR knockout: Generate CLEC4M-null cells as definitive negative controls
Recombinant expression: Create dose-dependent expression models for calibration
Domain deletion mutants: Express CLEC4M lacking specific domains to map epitopes
Signal verification methods:
Dual-antibody approach: Use two antibodies targeting different CLEC4M epitopes
Fluorescence minus one (FMO): Include all antibodies except anti-CLEC4M-FITC
Isotype control: Match isotype (IgG2) and fluorophore concentration
Dilution series: Perform antibody titration to identify optimal signal-to-noise ratio
Technical optimization approaches:
Fc receptor blocking: Use commercial Fc blockers before antibody incubation
Filtering strategy: Apply hierarchical gating to exclude debris and aggregates
Live/dead discrimination: Include viability dye to exclude nonspecific binding to dead cells
Autofluorescence correction: Apply spectral unmixing for tissues with high autofluorescence
Data interpretation framework:
Calculate signal-to-noise ratio between positive and negative controls
Define positivity thresholds based on isotype control staining
Consider using fluorescence intensity (MFI) ratios rather than absolute values
For tissues, compare staining patterns with known CLEC4M expression in literature
When studying CLEC4M in heterogeneous cell populations like non-parenchymal liver cells, dual staining with established cell-type specific markers (stabilin-2 for LSECs, CD68 for Kupffer cells) can help distinguish specific CLEC4M expression from background .
CLEC4M detection can be effectively integrated into advanced single-cell analysis workflows:
Comprehensive single-cell strategy:
Single-cell suspension preparation protocol:
For liver tissue: Optimize enzymatic digestion to preserve CLEC4M epitopes
For cell lines: Use non-enzymatic dissociation methods when possible
Include calcium (2 mM) in all buffers to maintain CLEC4M conformation
Maintain cells at 4°C to prevent receptor internalization
Multi-parametric flow cytometry panel design:
Cell sorting parameters for single-cell applications:
Use 100 μm nozzle for gentle sorting of endothelial cells
Collect in media containing calcium and serum to maintain viability
Sort directly into lysis buffer for immediate RNA/protein analysis
Include index sorting to retain fluorescence intensity data
Integration with transcriptomic/proteomic platforms:
CITE-seq: Conjugate CLEC4M antibodies to DNA barcodes for simultaneous protein and RNA analysis
Single-cell RNA-seq: Use flow-sorted CLEC4M+ populations for transcriptomic profiling
Imaging mass cytometry: Combine CLEC4M-FITC with metal-tagged antibodies for spatial analysis
Spatial transcriptomics: Correlate CLEC4M protein expression with local gene expression patterns
Advanced analytical approaches:
Perform trajectory analysis to identify CLEC4M expression dynamics during cell differentiation
Create dimensional reduction visualizations (UMAP, t-SNE) incorporating CLEC4M as a key parameter
Develop automated classification algorithms to identify CLEC4M+ cell subtypes
Correlate CLEC4M expression levels with transcriptional signatures
These advanced workflows enable researchers to identify novel CLEC4M-expressing cell populations and characterize their functional states with unprecedented resolution.
CLEC4M's role in tumor microenvironments can be effectively investigated using advanced imaging strategies:
Comprehensive imaging methodology:
Multiplex immunofluorescence protocol:
Optimize CLEC4M-FITC staining on frozen or FFPE tumor sections
Combine with markers for: endothelial cells (CD31), tumor cells, immune cells
Include functional markers: JAK1/STAT3 pathway components, proliferation/apoptosis markers
Employ tyramide signal amplification for detecting low-abundance epitopes
Spatial analysis framework:
Quantify CLEC4M+ vessel density in tumor regions vs. adjacent normal tissue
Measure distances between CLEC4M+ vessels and tumor invasion fronts
Analyze colocalization of CLEC4M with markers of tumor progression
Create spatial maps of CLEC4M expression across tumor microenvironments
Live cell imaging approaches:
Generate tumor cell lines expressing fluorescent reporters for JAK/STAT activity
Co-culture with CLEC4M+ endothelial cells labeled with membrane dyes
Perform time-lapse imaging to track interactions and signaling dynamics
Use FRET-based biosensors to monitor CLEC4M-mediated signaling events
Intravital microscopy applications:
Develop fluorescently labeled anti-CLEC4M Fab fragments for in vivo imaging
Create window chamber models to visualize CLEC4M+ vessels in live tumors
Track tumor cell interactions with CLEC4M+ endothelium in real-time
Monitor effects of CLEC4M-targeting interventions on tumor vasculature
Data analysis considerations:
Employ machine learning algorithms for automated identification of CLEC4M+ structures
Develop spatial statistics to quantify CLEC4M distribution patterns
Correlate CLEC4M expression patterns with clinical outcome data
Integrate imaging data with molecular profiles from the same tumor regions
In hepatocellular carcinoma research, these imaging approaches can reveal how CLEC4M expression in tumor-associated endothelium influences cancer cell proliferation and apoptosis through JAK1/STAT3 signaling, potentially explaining the favorable prognosis associated with CLEC4M overexpression .
CLEC4M's unique expression pattern and functional properties make it a promising therapeutic target, which can be explored using the following research strategies:
Therapeutic research framework:
Target validation approaches:
Characterize CLEC4M expression across normal and diseased tissues using FITC-conjugated antibodies
Evaluate the safety profile based on restricted expression pattern (primarily liver and lymph node endothelium)
Determine the functional consequences of CLEC4M modulation in different pathological contexts
Assess the role of CLEC4M polymorphisms in predicting therapeutic responses
Antibody-based therapeutic development:
Generate and screen anti-CLEC4M antibodies with agonist vs. antagonist properties
Develop antibody-drug conjugates targeting CLEC4M+ cells in specific diseases
Create bispecific antibodies linking CLEC4M to immune effector cells
Design antibodies targeting specific CLEC4M epitopes to modulate ligand binding
Ligand-directed approaches:
Identify high-affinity mannose-containing glycan structures for CLEC4M targeting
Develop glycan-modified nanoparticles for liver-specific drug delivery
Create chimeric molecules combining CLEC4M ligands with therapeutic payloads
Design competitive inhibitors of pathogen binding to CLEC4M
Therapeutic applications by disease area:
Hepatocellular carcinoma: Enhance CLEC4M expression or signaling to suppress JAK1/STAT3 pathway
Viral infections: Block viral binding sites on CLEC4M to prevent pathogen entry
Coagulation disorders: Modulate CLEC4M-mediated clearance of VWF and FVIII
Liver-targeted drug delivery: Exploit CLEC4M for hepatic targeting of therapeutics
Target assessment considerations:
Evaluate the impact of CLEC4M VNTR polymorphisms on therapeutic targeting
Consider potential competition between therapeutic agents and endogenous ligands
Assess the risk of altering physiological clearance functions of CLEC4M
Develop biomarkers to identify patients likely to respond to CLEC4M-targeted therapies
These research approaches can guide the development of novel therapeutic strategies leveraging CLEC4M's unique properties, while FITC-conjugated antibodies provide essential tools for target validation and patient stratification studies.