FCGR2C Antibody

What is FCGR2C and what is its role in the immune system?

FCGR2C (Fc gamma receptor IIc) is a low-affinity receptor for the Fc region of immunoglobulin G (IgG). In humans, the canonical protein has 323 amino acid residues and a mass of 35.6 kDa, with subcellular localization in the cell membrane and cytoplasm . Up to four different isoforms have been reported for this protein. FCGR2C belongs to the family of Fc gamma receptors (FcγRs) that connect adaptive and innate immune systems by binding to the Fc region of complexed IgG and triggering various cellular immune responses .

Unlike the inhibitory FCGR2B, FCGR2C functions as an activating receptor. It is expressed on various immune cells including B cells, NK cells, macrophages, dendritic cells, neutrophils, and platelets, though expression patterns vary significantly based on genetic polymorphisms . The biological significance of FCGR2C lies in its ability to modulate antibody responses and potentially contribute to antibody-dependent cell-mediated cytotoxicity (ADCC), particularly in individuals with specific genetic variants.

How do genetic variants of FCGR2C affect its expression and function?

FCGR2C expression is highly dependent on genetic variants, particularly the open reading frame (ORF) versus stop codon (STP) alleles. The FCGR2C gene contains a polymorphic stop codon in exon 3 (Q57X), which determines whether the full receptor is expressed .

In individuals with the FCGR2C-ORF allele, functional FCGR2C protein is expressed on B cells, which can counterbalance the inhibitory effects of FCGR2B. RT-PCR analysis has confirmed the presence of FCGR2C mRNA in B cells from individuals homozygous for the ORF allele but not in those homozygous for the STP allele . This allelic variation has significant functional consequences:

• Individuals with the FCGR2C-ORF allele show enhanced humoral immune responses

• The FCGR2C-ORF allele is associated with increased risk of autoimmunity in both Caucasian and African American populations

• B cell-specific expression of FCGR2C appears to contravene FCGR2B-mediated negative feedback

These findings suggest that FCGR2C genetic testing may be valuable in personalized medicine approaches, particularly for therapies targeting B cells.

What are the standard applications for FCGR2C antibodies in research?

FCGR2C antibodies are utilized in various immunodetection applications, with Western Blot and Flow Cytometry being the most common methodologies . Specific applications include:

• Western Blot (WB): For detecting FCGR2C protein expression in cell or tissue lysates, particularly useful for determining protein size and relative abundance

• Flow Cytometry: For quantifying FCGR2C expression on the surface of specific cell populations

• Immunohistochemistry (IHC): For visualizing FCGR2C expression in tissue sections

• ELISA: For quantitative measurement of FCGR2C in solution

When selecting antibodies for these applications, researchers should consider:

• Specificity for FCGR2C versus other Fc gamma receptors

• Reactivity with human samples (most commercial antibodies are human-specific)

• Whether the antibody is conjugated or unconjugated

• Validation status for the intended application

Due to the high sequence homology between FCGR2A, FCGR2B, and FCGR2C, care must be taken to select antibodies that can distinguish between these closely related receptors, or to use additional experimental controls when using antibodies that recognize multiple FCGR2 family members.

How does FCGR2C expression on B cells affect humoral immune responses?

FCGR2C expression on B cells has been shown to significantly enhance humoral immune responses through several mechanisms:

• Enhanced BCR signaling: Co-crosslinking of FCGR2C and the B cell receptor (BCR) leads to FCGR2C tyrosine phosphorylation and enhanced BCR signaling .

• Counterbalancing inhibitory signals: FCGR2C expression counteracts the negative feedback provided by inhibitory FCGR2B receptors, resulting in a more robust B cell response to antigen .

• Increased antibody production: In both transgenic mouse models and human studies, FCGR2C expression is associated with enhanced antibody responses to immunization.

Experimental evidence supporting these effects includes:

• Transgenic mice expressing human FCGR2C showed a significant increase in IgM antibody titers comparable to changes observed in FCGR2B-deficient mice when immunized with TNP-Ficoll (T-independent antigen) .

• Most notably, a 2-fold increase in IgG1 production was observed in transgenic mice after immunization with TNP-CGG/alum (T-dependent antigen) compared to non-transgenic littermates .

• In a human Anthrax vaccine (AVA) trial, individuals with the FCGR2C-ORF allele demonstrated a 2.5-fold higher antibody production against protective antigen (AbPA) at 4 weeks post-vaccination compared to individuals lacking this allele .

These findings suggest that FCGR2C expression status could be an important consideration when evaluating vaccine responses or designing immunotherapeutic strategies targeting B cells.

What is the evidence linking FCGR2C to autoimmune diseases?

The FCGR2C-ORF allele has been associated with increased risk of autoimmunity in human populations. Research has demonstrated:

• Association with systemic lupus erythematosus (SLE) risk in both Caucasian and African American populations

• The potential mechanism involves enhanced B cell activation and antibody production in individuals expressing functional FCGR2C on B cells

• The balance between activating (FCGR2C) and inhibitory (FCGR2B) Fc receptors appears crucial for maintaining immune tolerance

This association makes biological sense considering FCGR2C's role in enhancing B cell responses. The presence of functional FCGR2C on B cells may lower the threshold for B cell activation, potentially allowing autoreactive B cells to escape normal tolerance mechanisms.

Understanding the role of FCGR2C in autoimmunity could help identify at-risk individuals and develop targeted therapies. Further research is needed to determine whether FCGR2C expression correlates with disease severity or response to B cell-targeted therapies in autoimmune conditions.

How can FCGR2C be utilized as a prognostic biomarker in sepsis?

Recent research has identified FCGR2C as a promising prognostic biomarker in sepsis with considerable clinical potential:

• Correlation with clinical scores: FCGR2C expression levels correlate with sepsis severity scores including SOFA (Sequential Organ Failure Assessment) and GCS (Glasgow Coma Scale) .

• Predictive ability: FCGR2C demonstrated strong prognostic value for discriminating between septic survivors and non-survivors across multiple validation datasets:

  • Discovery dataset: AUC = 0.73

  • First validation dataset (GSE95233): AUC = 0.67

  • Second validation cohort (Recruitment cohort): AUC = 0.84

• Comparison with established scores: The predictive assessment ability of FCGR2C was superior to that of the SOFA score (AUC = 0.80) and APACHE II score (AUC = 0.69) in the validation cohort .

• Immune cell correlations: FCGR2C levels showed significant correlations with immune cell populations including neutrophils, CD3+ T cells, and CD8+ T cells, suggesting its role in immune dysregulation during sepsis .

The data supporting FCGR2C as a sepsis biomarker comes from multiple cohorts with robust statistical validation. Researchers interested in this application should consider including FCGR2C expression analysis in sepsis studies, with potential for developing point-of-care tests that could improve risk stratification and guide personalized treatment approaches.

What methodologies are most effective for distinguishing FCGR2C from other Fc gamma receptors?

Distinguishing FCGR2C from other closely related Fc gamma receptors (particularly FCGR2A and FCGR2B) presents a significant challenge due to high sequence homology. Effective methodologies include:

• Genotyping approaches:

  • PCR-based methods targeting the Q57X polymorphism in exon 3

  • Sequence-specific primer PCR for distinguishing FCGR2C-ORF from FCGR2C-STP alleles

  • Next-generation sequencing for comprehensive variant analysis

• mRNA-based detection:

  • RT-PCR with primers specific to unique regions of FCGR2C

  • RNA-seq with computational approaches to distinguish between highly homologous transcripts

• Protein-level approaches:

  • Flow cytometry with carefully validated antibodies

  • Western blot with control samples of known FCGR2 expression patterns

  • Mass spectrometry to identify receptor-specific peptides

• Functional assays:

  • Comparing signaling patterns (FCGR2C delivers activating signals vs. FCGR2B's inhibitory signals)

  • Cell-type specific expression analysis (e.g., B cells express FCGR2B and potentially FCGR2C, but not FCGR2A)

Researchers should ideally combine genetic, transcript, and protein-level analyses to obtain the most accurate characterization of FCGR2C status in their experimental systems.

What are the optimal protocols for detecting FCGR2C expression in primary human cells?

Detection of FCGR2C expression in primary human cells requires careful consideration of both technical aspects and biological variability. Recommended protocols include:

Flow Cytometry Protocol:

• Isolate peripheral blood mononuclear cells (PBMCs) using density gradient centrifugation

• Block Fc receptors with human serum or commercial blocking reagents (critical to prevent non-specific binding)

• Stain with validated anti-FCGR2C antibodies alongside lineage markers for B cells (CD19, CD20), NK cells (CD56, CD16), or other relevant populations

• Include appropriate isotype controls and FMO (fluorescence minus one) controls

• Consider including genotype controls (cells from FCGR2C-ORF+ and FCGR2C-STP homozygous donors)

• Analyze using multiparameter flow cytometry with appropriate compensation

RT-PCR Protocol:

• Isolate RNA from purified cell populations (B cells, NK cells)

• Perform reverse transcription with oligo(dT) or random hexamer primers

• Design primers that specifically amplify FCGR2C but not FCGR2A or FCGR2B

• Include control amplifications of housekeeping genes and other FCGR2 family members

• Validate PCR products by sequencing to confirm specificity

Important considerations:

• Always determine the FCGR2C genotype of donors when possible

• Include positive and negative control samples with known FCGR2C expression

• Be aware that expression levels may vary with activation status of cells

The combination of genotyping, transcript analysis, and protein detection provides the most comprehensive assessment of FCGR2C status in primary cells.

How can researchers effectively use FCGR2C antibodies in immunohistochemistry applications?

Effective use of FCGR2C antibodies in immunohistochemistry (IHC) requires careful optimization and validation:

Recommended Protocol:

• Tissue preparation:

  • Fix tissues in 10% neutral buffered formalin (or appropriate fixative)

  • Process and embed in paraffin

  • Section at 4-5 μm thickness

• Antigen retrieval:

  • Heat-induced epitope retrieval (HIER) using citrate buffer (pH 6.0) or EDTA buffer (pH 9.0)

  • Optimize retrieval conditions for each specific antibody

• Blocking steps:

  • Block endogenous peroxidase with 3% hydrogen peroxide

  • Block endogenous biotin if using biotin-based detection systems

  • Use protein blocking solution containing BSA or serum

• Antibody incubation:

  • Titrate primary antibody to determine optimal concentration

  • Incubate at 4°C overnight or at room temperature for 1-2 hours

  • Include appropriate controls: isotype control, known positive tissue, known negative tissue

• Detection and visualization:

  • Use sensitive detection systems (polymer-based or tyramide signal amplification)

  • Counterstain with hematoxylin

  • Mount with permanent mounting medium

Validation approaches:

• Compare staining patterns with multiple antibodies targeting different epitopes

• Perform parallel staining for other FCGR2 family members to assess specificity

• Consider using tissue from genotyped donors (FCGR2C-ORF vs. FCGR2C-STP)

• Correlate IHC results with flow cytometry or Western blot data when possible

Troubleshooting common issues:

• High background: Increase blocking, reduce antibody concentration

• Weak signal: Optimize antigen retrieval, increase antibody concentration

• Non-specific staining: Validate antibody specificity, optimize blocking conditions

What experimental models are available for studying FCGR2C function?

Several experimental models are available for studying FCGR2C function, each with distinct advantages and limitations:

Human Cell Culture Systems:

• Primary B cells from genotyped donors (FCGR2C-ORF vs. FCGR2C-STP)

• EBV-transformed B cell lines with characterized FCGR2C expression

• NK cell lines or primary NK cells

• Advantages: Physiologically relevant, maintains natural genetic context

• Limitations: Donor variability, limited manipulation potential

Transgenic Mouse Models:

• B cell-specific human FCGR2C transgenic mice

• Advantages: In vivo analysis of FCGR2C function in B cell responses to immunization

• Findings: Enhanced antibody responses to both T-dependent and T-independent antigens

• Limitations: Species differences in Fc receptor biology

Cell Line Expression Systems:

• Transfection of FCGR2C into receptor-negative cells

• CRISPR/Cas9 editing of endogenous FCGR2 genes

• Inducible expression systems

• Advantages: Controlled expression, genetic manipulation

• Limitations: May lack physiological context

Ex Vivo Human Systems:

• Peripheral blood mononuclear cells from genotyped donors

• Humanized mouse models

• Advantages: More physiologically relevant than cell lines

• Limitations: Complex systems with multiple variables

When selecting a model system, researchers should consider:

• The specific aspect of FCGR2C biology being studied

• The need for genetic manipulation

• The importance of physiological context

• The downstream applications and readouts

The transgenic mouse model expressing human FCGR2C has provided particularly valuable insights into the function of this receptor in B cell responses to vaccination and its potential role in autoimmunity .

What are the key considerations for interpreting FCGR2C expression data in clinical samples?

Interpreting FCGR2C expression data in clinical samples requires careful consideration of several factors:

Genetic Variables:

• FCGR2C-ORF vs. STP allele status is critical for interpretation

• Copy number variations in the FCGR locus can affect expression levels

• Additional polymorphisms in regulatory regions may influence expression

Cell Type Heterogeneity:

• Expression patterns differ across cell types (B cells, NK cells, myeloid cells)

• Changes in cellular composition of samples can confound expression analysis

• Consider using cell-type specific markers or single-cell approaches

Disease Context:

• Expression may be altered in disease states

• In sepsis, FCGR2C correlates with disease severity and outcome

• In autoimmunity, functional FCGR2C may contribute to pathogenesis

Technical Considerations:

• Antibody cross-reactivity with other FCGR2 family members

• RNA expression vs. protein detection discrepancies

• Platform-specific normalization requirements

Reference Ranges and Controls:

• Establish appropriate healthy control reference ranges

• Include genotyped controls when possible

• Consider age, sex, and ethnicity-matched controls

Clinical Interpretation Framework:

• For sepsis prognosis, FCGR2C expression showed superior predictive value (AUC = 0.84) compared to established clinical scores

• The 2.5-fold increase in antibody responses seen in FCGR2C-ORF+ individuals after vaccination provides context for interpreting humoral response data

Researchers should ideally combine genetic information with expression data for the most accurate interpretation of FCGR2C's role in clinical samples, particularly when evaluating potential associations with disease outcomes or treatment responses.

How might FCGR2C-targeted therapies be developed for autoimmune diseases or sepsis?

The emerging understanding of FCGR2C biology suggests several potential therapeutic approaches:

For Autoimmune Diseases:

• FCGR2C-blocking antibodies: Developing antibodies that specifically block FCGR2C without affecting other Fc receptors could potentially reduce B cell hyperactivity in autoimmune conditions associated with the FCGR2C-ORF allele.

• Targeted B cell modulation: Since FCGR2C enhances BCR signaling, targeted approaches that selectively dampen this pathway in FCGR2C-expressing B cells could provide more precise immunomodulation than current B cell-depleting therapies.

• Personalized medicine approaches: Screening for FCGR2C-ORF status could help identify patients most likely to benefit from B cell-targeted therapies or those who might require more aggressive treatment due to enhanced B cell responses.

For Sepsis Management:

• Prognostic stratification: FCGR2C expression levels could be developed into a clinical test for risk stratification in sepsis, potentially guiding resource allocation and treatment intensity .

• Immunomodulatory approaches: Understanding FCGR2C's correlation with immune cell populations in sepsis could lead to targeted immunomodulatory interventions for high-risk patients.

• Combined biomarker panels: Integrating FCGR2C with other immune markers could improve the predictive power for sepsis outcomes beyond current clinical scoring systems.

Development Challenges:

• Ensuring specificity for FCGR2C over other Fc receptors

• Balancing immunomodulation without compromising protective immunity

• Developing rapid, cost-effective testing for clinical implementation

• Validating therapeutic approaches across diverse patient populations

As research progresses, FCGR2C-focused therapies could represent a new direction in precision immunomodulation for both autoimmune diseases and sepsis management.

What is the relationship between FCGR2C and other immune receptors in regulating B cell responses?

The relationship between FCGR2C and other immune receptors forms a complex regulatory network that fine-tunes B cell responses:

FCGR2C and FCGR2B Counterbalance:

• FCGR2C delivers activating signals that directly counteract the inhibitory signals from FCGR2B

• This balance appears crucial for appropriate antibody responses

• In individuals with FCGR2C-ORF, the presence of both receptors creates a more dynamic regulatory system

Interaction with B Cell Receptor (BCR):

• Co-crosslinking of FCGR2C and BCR enhances signaling and B cell activation

• This cooperative signaling may lower the threshold for B cell responses to antigen

• The mechanistic details of this synergy include FCGR2C tyrosine phosphorylation

Integration with TLR Signaling:

• Fc receptors can modulate Toll-like receptor responses in immune cells

• The integration of these pathways shapes both innate and adaptive immune responses

• Research exploring FCGR2C-TLR interactions is still emerging

Cytokine Receptor Crosstalk:

• Cytokine signals can influence FCGR expression and function

• This crosstalk creates context-dependent regulation of B cell responses

• Understanding these interactions may explain variability in immune responses

Research Gaps and Future Directions:

• Detailed signaling studies comparing FCGR2C vs. FCGR2B pathways

• Investigation of receptor clustering and membrane organization

• Exploration of differential effects across B cell subsets

• Systems biology approaches to model receptor interactions

This complex interplay between receptors highlights the importance of considering FCGR2C not in isolation, but as part of an integrated signaling network that collectively determines B cell fate and function.

What are the major knowledge gaps in FCGR2C research that require further investigation?

Despite significant advances in understanding FCGR2C biology, several important knowledge gaps remain:

• Tissue-specific expression patterns: More comprehensive characterization of FCGR2C expression across different tissues and cell types beyond peripheral blood cells is needed.

• Regulation of expression: The factors that control FCGR2C expression, including epigenetic regulation and influence of inflammatory stimuli, remain poorly defined.

• Signaling mechanisms: Detailed characterization of FCGR2C-specific signaling pathways and how they integrate with other receptor systems requires further investigation.

• Role in specific diseases: While associations with autoimmunity and sepsis have been established, the mechanistic contribution of FCGR2C to these and other conditions needs deeper exploration.

• Therapeutic targeting: Development of specific approaches to target FCGR2C for therapeutic benefit without affecting other Fc receptors remains challenging.

• Population variability: Understanding the distribution and functional consequences of FCGR2C variants across different human populations would enhance personalized medicine approaches.

• Developmental aspects: The ontogeny of FCGR2C expression and its role in early life immunity and tolerance development is largely unexplored.