FPR3 Antibody

What is FPR3 and why is it significant in immunological research?

FPR3 (Formyl peptide receptor 3, also known as FPRL2) is a G protein-coupled receptor that plays crucial roles in immune regulation. It functions as a low-affinity receptor for N-formyl-methionyl peptides, which are potent neutrophil chemotactic factors. FPR3 is particularly significant in immunological research because it contributes to neutrophil activation through a G-protein-mediated phosphatidylinositol-calcium second messenger system . Recent studies have demonstrated that FPR3 has a dual role - it is expressed in both vomeronasal sensory neurons and in immune cells, particularly neutrophils and bone marrow cells . Its expression in immune cells can be upregulated following stimulation with bacterial lipopolysaccharide (LPS), suggesting its importance in immune responses to bacterial infections . Understanding FPR3 function is essential for developing novel therapeutic approaches targeting inflammatory conditions, infectious diseases, and autoimmune disorders.

What types of FPR3 antibodies are available for research purposes?

Several types of FPR3 antibodies have been developed for research applications:

• Polyclonal antibodies: These include antibodies like PACO03723, raised in rabbits against synthesized peptides derived from human FPRL2 C-terminal regions . Polyclonal antibodies recognize multiple epitopes on the FPR3 protein.

• Monoclonal antibodies: These include specifically engineered antibodies like Fpr3-ECL2, which targets defined epitopes .

• Custom-developed antibodies: Researchers have generated specialized antibodies such as:

  • Fpr3-ECL1: A polyclonal rabbit antibody targeting the AMKEKWPFGWFLCKL epitope

  • Fpr3-ECL2: A monoclonal mouse antibody developed through epitope scoring

These antibodies vary in their specificity, sensitivity, and applications. For instance, commercial antibodies like M-20 (sc-18195) and N-20 (orb100776) showed insufficient detection of FPR3 in receptor-transfected cells, prompting the development of more specific alternatives with stock concentrations of 2 mg/ml .

How can I determine the specificity of an FPR3 antibody?

Determining antibody specificity is critical for reliable experimental results. For FPR3 antibodies, consider implementing the following methodological approach:

• Peptide-spot assay: Synthesize 15-amino acid peptides (with 10-residue overlaps) covering the entire length of FPR3 on cellulose membranes. Equilibrate membranes in 150 mM NaCl, 50 mM Tris/HCl (pH 7.5) for 30 minutes at room temperature. Incubate with the antibody (4 μg/ml in PBS with 5% milk powder) overnight at 4°C. After washing, add peroxidase-coupled secondary antibody, wash again, and visualize with enhanced chemiluminescence .

• Overexpression systems: Express FPR3 in HEK293T cells and test antibody recognition through immunocytochemistry. Compare staining patterns between transfected and non-transfected cells .

• Cross-reactivity testing: Evaluate antibody binding to related receptors like FPR1 and FPR2/ALX to confirm specificity. This is particularly important as these receptors share sequence homology .

• Knockout validation: If available, use natural Fpr3 knockout mouse strains to confirm antibody specificity. Several mouse strains carry the non-functional Fpr3Δ424–435 variant, which can serve as negative controls .

What are the recommended protocols for immunofluorescence detection of FPR3?

For optimal immunofluorescence detection of FPR3, follow this detailed protocol:

• Cell preparation:

  • Plate cells (e.g., HEK293) into poly-D-lysine-coated 8-chamber glass-bottomed slides

  • Transfect cells with FPR3 expression plasmids using Lipofectamine 2000 according to manufacturer's instructions

  • Allow 48 hours for expression

• Fixation and permeabilization:

  • Fix cells with 3% paraformaldehyde in PBS supplemented with 2% sucrose for 30 minutes at room temperature

  • Quench with 50 mM NH4Cl in RPMI 1640 for 15 minutes

  • Permeabilize with 0.1% Nonidet P-40 for 10 minutes

• Blocking and primary antibody:

  • Incubate with blocking buffer (2% BSA in RPMI 1640) for 30 minutes at room temperature

  • Apply primary antibody:

    • For untagged receptors: Use Fpr3-ECL1 (2 μg/ml) or Fpr3-ECL2 (0.2 μg/ml)

    • For HA-tagged receptors: Use anti-HA monoclonal antibody (1 μg/ml)

  • Incubate for 1 hour at room temperature

• Secondary antibody and visualization:

  • Wash three times

  • Incubate with appropriate secondary antibody (e.g., Alexa 488-conjugated goat anti-rabbit at 2 μg/ml) for 30 minutes

  • Wash again

  • Mount in anti-quenching medium (2.3% DABCO, 50% glycerol, 10% 0.2 M Tris-HCl, pH 8.0, 0.02% NaN3)

  • Visualize using fluorescence microscopy

For PACO03723 antibody specifically, a recommended dilution of 1:200-1:1000 is suggested for immunofluorescence applications .

How can I analyze FPR3 receptor internalization?

Analyzing FPR3 internalization requires special consideration as FPR3 exhibits constitutive internalization even without agonist stimulation. Here's a methodological approach:

• Antibody uptake assay for constitutive internalization:

  • Express N-terminally 3HA-tagged FPR3 in HEK293 cells

  • Incubate live cells with anti-HA antibody for 30 minutes at 37°C (allows tracking of receptors that reach the surface)

  • Wash, fix, and permeabilize cells

  • Visualize with fluorescently labeled secondary antibody

  • As a control, perform the same experiment at 4°C (inhibits internalization) to visualize surface-only receptors

• Comparative analysis:

  • Perform parallel experiments with FPR2/ALX, which predominantly localizes to the plasma membrane without constitutive internalization

  • Compare the distribution patterns - FPR3 typically appears as intracellular punctate structures, while FPR2/ALX shows even membrane distribution

• Chimeric receptor analysis:

  • Utilize chimeric receptors (e.g., FPR3-R2) to investigate the role of C-terminal phosphorylation in constitutive internalization

  • Compare localization patterns of wild-type and chimeric receptors

This unique constitutive internalization property of FPR3 is important to consider when designing experiments, as it results in limited cell surface expression even without stimulation.

What considerations should be made when choosing between different FPR3 antibody applications (ELISA, IF, etc.)?

When selecting FPR3 antibodies for specific applications, consider these methodological factors:

• For immunofluorescence (IF):

  • Select antibodies validated for IF applications, such as PACO03723

  • Consider dilution requirements (e.g., 1:200-1:1000 for PACO03723)

  • Ensure the antibody can detect the native conformation of the protein

  • Evaluate whether the antibody requires permeabilization (if targeting intracellular domains)

  • For studying receptor trafficking, N-terminally tagged constructs with anti-tag antibodies may provide cleaner results

• For ELISA:

  • Verify the antibody has been validated for ELISA applications

  • Consider whether the antibody recognizes linear or conformational epitopes

  • Determine if the antibody works with denatured proteins

• Antibody format considerations:

  • Storage buffer: PACO03723 comes in PBS containing 50% glycerol, 0.5% BSA, and 0.02% sodium azide

  • Purification method: Affinity purification increases specificity

  • Clonality: Polyclonal antibodies like PACO03723 recognize multiple epitopes, while monoclonal antibodies provide higher specificity for a single epitope

• Species reactivity:

  • Ensure the antibody recognizes your species of interest (e.g., PACO03723 is reactive with human samples)

  • Be aware of potential strain-specific differences in mice, as some strains carry non-functional Fpr3 variants

How do I address strain-specific variations in FPR3 expression when using mouse models?

Mouse strain-specific variations in FPR3 expression present significant challenges for researchers. To address these variations methodologically:

• Genotype analysis:

  • Screen for the presence of the 12-nucleotide in-frame deletion (Fpr3Δ424–435) that leads to an unstable, truncated, and non-functional receptor protein

  • At least 19 mouse strains encode this non-functional FPR3 variant, while at least 13 other strains express an intact receptor

• Functional validation:

  • Perform calcium imaging assays to confirm receptor functionality in your chosen strain

  • Compare results with known functional (intact receptor) and non-functional (truncated receptor) strains

• Immunofluorescence confirmation:

  • Use validated Fpr3 antibodies to verify protein expression in target tissues

  • Compare staining patterns between strains with functional and non-functional variants

• Experimental design considerations:

  • Include appropriate strain controls in all experiments

  • Document the strain background in publications

  • Consider using multiple strains to validate findings if FPR3 function is central to your hypothesis

This strain-specific variation provides natural knockout models that can be leveraged for experimental controls, but also requires careful consideration when interpreting results across different mouse strains.

How can I investigate the dual role of FPR3 in both sensory neurons and immune cells?

Investigating FPR3's dual functionality requires specialized methodological approaches:

• Tissue-specific expression analysis:

  • Perform immunohistochemistry on both vomeronasal organ (VNO) and immune tissues using validated FPR3 antibodies

  • Use appropriate positive controls, such as Gαo for VNO co-localization

  • Implement dual-labeling with cell-type specific markers:

    • For VSNs: Gαo co-expression

    • For immune cells: Neutrophil markers

• Functional studies in distinct cell populations:

  • For immune cells: Assess FPR3 upregulation following LPS stimulation to mimic bacterial infection

  • For VSNs: Evaluate responses to formylated bacterial signal peptides, which are natural FPR3 agonists

• Calcium imaging assays:

  • Compare ligand responsiveness in both cell types

  • Assess responses to FPR3-specific ligands versus broader formyl peptide ligands

• In vivo models:

  • Use tissue-specific knockout approaches to dissect the relative contribution of FPR3 in different cell types

  • Consider strain-specific variations, selecting strains with functional FPR3 expression

• Receptor trafficking studies:

  • Investigate whether the constitutive internalization observed in transfected cells occurs similarly in native VSNs and immune cells

  • Compare receptor dynamics between the two cell types

This dual-system approach will provide insights into whether FPR3 serves similar or distinct functions in these different biological contexts.

What are the best approaches to investigate FPR3 phosphorylation and its impact on receptor function?

Investigating FPR3 phosphorylation requires sophisticated biochemical and cellular approaches:

• Phosphorylation site mapping:

  • Perform site-directed mutagenesis of potential phosphorylation sites in the C-terminal domain

  • Create chimeric receptors, such as FPR3-R2, which exhibits constitutive phosphorylation

  • Use phospho-specific antibodies if available

• Functional correlation studies:

  • Compare wild-type and phosphorylation-deficient mutants in:

    • Constitutive internalization assays

    • Ligand-induced signaling

    • Receptor trafficking patterns

• Kinase involvement analysis:

  • Use specific kinase inhibitors to identify the kinases responsible for constitutive phosphorylation

  • Implement co-immunoprecipitation to detect kinase-receptor interactions

• β-arrestin recruitment assessment:

  • Co-transfect FPR3 with β-arrestin2-EGFP to visualize recruitment

  • Evaluate the impact of dominant-negative dynamin (dynamin K44A) and the β-arrestin1 fragment (318-419) on receptor trafficking

  • Compare arrestin recruitment between FPR3 and other FPR family members

• Constitutive vs. ligand-induced phosphorylation:

  • Design pulse-chase experiments to differentiate between these processes

  • Use phosphatase inhibitors to preserve phosphorylation status during biochemical analysis

The unique constitutive phosphorylation of FPR3, unlike its family member FPR2/ALX, may explain its distinctive intracellular distribution pattern and provides an important model for understanding GPCR regulation .

Why might I observe predominantly intracellular rather than membrane localization of FPR3 in transfected cells?

The predominantly intracellular localization of FPR3 is a well-documented phenomenon that differs from typical GPCR patterns. This is not necessarily an experimental artifact but reflects the receptor's unique biology:

• Constitutive internalization: FPR3 undergoes continuous internalization even in the absence of agonist stimulation, leading to accumulation in intracellular vesicles. This has been confirmed through antibody uptake experiments showing that FPR3 reaches the cell surface but is rapidly internalized .

• Methodological confirmation:

  • At 37°C, antibody uptake experiments show predominantly intracellular localization of 3HA-tagged FPR3

  • At 4°C (which inhibits internalization), only faint membrane labeling is visible, indicating limited surface expression at any given time

  • This contrasts with FPR2/ALX, which shows stable surface expression without agonist

• Possible biological significance:

  • This constitutive internalization may represent a regulatory mechanism to limit ligand accessibility

  • It could reflect continuous sampling of the extracellular environment in specific cell types

• Verification strategies:

  • Confirm antibody specificity using overexpression systems

  • Use N-terminally tagged constructs with antibody feeding assays to track surface-expressed receptors

  • Compare with related receptors like FPR2/ALX as positive controls for membrane localization

If you're specifically interested in studying surface-expressed FPR3, consider using internalization inhibitors or reduced temperature incubations to temporarily increase membrane localization.

How can I differentiate between technical challenges and biological properties when interpreting FPR3 antibody staining patterns?

Differentiating technical issues from true biological properties requires systematic controls and validation:

• Antibody validation controls:

  • Test antibodies on cells overexpressing FPR3 versus non-transfected cells

  • Include cells expressing related receptors (FPR1, FPR2) to assess cross-reactivity

  • Perform peptide competition assays with the immunizing peptide to confirm specificity

  • Use multiple antibodies targeting different epitopes (e.g., Fpr3-ECL1 and Fpr3-ECL2) and compare staining patterns

• Expression system considerations:

  • Compare staining patterns in transiently transfected cells versus stable cell lines

  • Assess native tissues known to express FPR3 (vomeronasal organ, neutrophils)

  • Use strains with non-functional FPR3 variants as negative controls

• Technical troubleshooting matrix:

• Biological validation:

  • Correlate staining with functional assays (e.g., calcium signaling)

  • Use stimulation conditions known to alter receptor distribution (e.g., LPS stimulation increases FPR3 expression in immune cells)

  • Compare with mRNA expression data to confirm expression patterns

By implementing these strategic controls, you can confidently distinguish between technical artifacts and genuine biological properties of FPR3.

What considerations should be made when comparing human FPR3 and mouse Fpr3 in research studies?

Comparing human FPR3 and mouse Fpr3 requires careful methodological considerations due to important species-specific differences:

• Sequence and structural variations:

  • Despite close sequence homology , there are distinct differences that may affect antibody recognition

  • Ensure antibodies are validated for the specific species being studied (e.g., PACO03723 is validated for human FPR3)

• Expression pattern differences:

  • Human FPR3 is predominantly found in immune cells

  • Mouse Fpr3 has a dual expression pattern in both vomeronasal sensory neurons and immune cells

  • Human lacks a functional vomeronasal organ, representing a fundamental species difference

• Experimental design considerations:

  • For immunological studies: Consider that quantitative PCR and in situ hybridization studies have yielded conflicting results regarding mouse Fpr3 expression in immune cells

  • For sensory studies: Remember that findings in mouse vomeronasal organ cannot be directly translated to humans

  • Always document the species origin of cells and tissues in publications

• Functional parallels:

  • Despite expression differences, human FPR3 and mouse Fpr3 respond to similar ligands, including formylated bacterial signal peptides

  • This suggests conserved functional roles in pathogen detection despite different tissue expression patterns

• Mouse strain considerations:

  • Account for the strain-specific variations in mouse Fpr3 functionality

  • At least 19 mouse strains encode a non-functional variant (Fpr3Δ424–435)

  • Ensure your model system uses a strain with functional Fpr3 expression if studying receptor function

These species differences provide an opportunity to understand the evolutionary divergence of FPR3 function while requiring careful experimental design to ensure valid cross-species comparisons.

How can I leverage FPR3 antibodies to study the role of this receptor in inflammatory disease models?

FPR3 antibodies offer powerful tools for investigating inflammatory conditions, with several methodological approaches:

• Expression profiling in disease tissues:

  • Use validated FPR3 antibodies for immunohistochemistry in tissue samples from inflammatory disease models

  • Quantify receptor expression levels in different cell populations using flow cytometry with FPR3 antibodies

  • Correlate expression with disease severity markers

• Functional blocking studies:

  • Determine if any available FPR3 antibodies have antagonistic properties

  • Use these antibodies to block receptor function in ex vivo cell preparations

  • Compare results with small-molecule FPR3 antagonists

• Receptor regulation in inflammatory conditions:

  • Leverage the finding that LPS stimulation upregulates FPR3 expression in immune cells

  • Design time-course experiments to track FPR3 expression during inflammatory response progression

  • Correlate with other inflammatory markers and cytokine production

• Dual-system investigation:

  • Explore whether neuroimmune communication involves FPR3 signaling

  • Investigate if vomeronasal expression in mice offers insights into chemosensory detection of inflammatory signals

• Therapeutic target validation:

  • Use antibody-based detection to validate FPR3 as a potential therapeutic target

  • Screen for compounds that modulate receptor expression or trafficking

  • Measure outcomes in disease models after intervention

These approaches can provide valuable insights into FPR3's role in conditions ranging from bacterial infections to autoimmune disorders, potentially identifying new therapeutic strategies.

What key publications should I consult for protocols and methodological approaches to FPR3 antibody applications?

Several key publications provide valuable methodological information for FPR3 antibody applications:

• Stempel et al. (2016) "Strain-specific Loss of Formyl Peptide Receptor 3 in the Murine Vomeronasal and Immune Systems" - This comprehensive study details the generation and validation of two specific FPR3 antibodies (Fpr3-ECL1 and Fpr3-ECL2), providing protocols for peptide-spot assays and immunofluorescence .

• Rabiet et al. (2011) "N-Formyl Peptide Receptor 3 (FPR3) Departs from the Homologous FPR2/ALX Receptor with Regard to the Major Processes Governing Chemoattractant Receptor Regulation, Expression at the Cell Surface, and Phosphorylation" - This paper offers detailed immunofluorescence protocols and antibody uptake assays to study FPR3 trafficking and constitutive internalization .

• Technical resources from antibody manufacturers, such as Assay Genie's datasheet for PACO03723, provide specific application recommendations including dilution factors for different techniques .