PIK3CD Antibody

What is PIK3CD and what cellular functions does it regulate?

PIK3CD encodes the p110δ catalytic subunit of phosphoinositide 3-kinase (PI3K), a heterodimeric enzyme predominantly expressed in leukocytes. While p110α and p110β subunits are ubiquitously expressed, p110δ is mainly restricted to leukocytes, making it a critical target for immunological research .

The PIK3CD-encoded protein generates phosphatidyl-inositol 3,4,5-trisphosphate (PIP3) and regulates multiple cellular processes including:

• Cell growth and survival

• Cellular proliferation

• Cell motility and morphology

• B and T cell development and function

• Humoral immune responses

PI3K signaling represents a key effector pathway downstream of B cell receptor (BCR), toll-like receptors (TLR), CD40, and various cytokine and chemokine receptors .

How do gain-of-function mutations in PIK3CD impact immune cell development and function?

Gain-of-function (GOF) mutations in PIK3CD cause immune dysregulation characterized by:

• T cell compartment: Increased proportions of effector memory cells (86%) and central memory cells (59%) among CD4+ T cells; increased proportions of effector memory cells (83%) and terminally differentiated effector memory T cells (38%) among CD8+ T cells

• B cell development: Severe defects in both bone marrow and peripheral B cell maturation

• Class-switch recombination (CSR): Intrinsic defects in CSR due to impaired induction of activation-induced cytidine deaminase (AID)

• Cell morphology: Increased B cell size compared to healthy controls

• B cell subpopulations: Higher proportions of IgM+ double-negative B (DNB) cells

• Cell death: Increased death of naïve B cells, potentially explaining decreased peripheral B cell numbers

These impacts manifest clinically as recurrent respiratory infections, susceptibility to herpesvirus infections, and impaired antibody responses following vaccination .

What are the optimal protocols for analyzing PIK3CD phosphorylation status using antibodies?

When analyzing PIK3CD phosphorylation:

• Sample preparation:

  • Fresh isolation of primary cells is critical as freeze-thaw cycles may affect phosphorylation status

  • Rapid processing (<30 minutes) minimizes changes in phosphorylation state

  • Use phosphatase inhibitors (sodium orthovanadate, sodium fluoride) in all buffers

• Antibody selection:

  • Use phospho-specific antibodies targeting key residues in the PI3K pathway

  • Primary antibodies against phospho-AKT (Ser473) serve as key downstream readouts of PI3K activity

  • Include total AKT antibodies for normalization

• Experimental approach:

  • Flow cytometry provides single-cell resolution of phospho-AKT levels

  • Western blotting allows quantification of phospho-AKT/total AKT ratios

  • Compare baseline and stimulated conditions to assess pathway hyperactivation

Research has shown that T cells from patients with PIK3CD E1021K mutations demonstrate elevated AKT phosphorylation and increased mTOR expression both before and after in vitro stimulation . When designing experiments, include both unstimulated and stimulated conditions to reveal differential activation patterns.

How can PIK3CD antibodies be used to characterize T cell dysfunction in research models?

A methodical approach to characterizing T cell dysfunction involves:

• Immunophenotyping panel design:

  • Use flow cytometry with antibodies targeting PIK3CD alongside:

    • Memory markers: CD45RA, CD27

    • Exhaustion markers: PD-1, CXCR3

    • Functional markers: CD38, HLA-DR

• Sorting strategy:

  • Isolate CD4+ T cell subpopulations:

    • Naïve (CD45RA+CD27+)

    • Central memory (CD45RA-CD27+)

    • Effector memory (CD45RA-CD27-)

    • Terminally differentiated (CD45RA+CD27-)

• Functional assays:

  • Proliferation assays (CFSE dilution)

  • Cytokine production (intracellular staining)

  • Phospho-flow to measure AKT/mTOR pathway activation

Research demonstrates that PIK3CD GOF mutations cause substantial increases in memory and follicular helper T (TFH) cells with dramatic changes in cytokine production . The dysregulated TFH phenotype is characterized by increased programmed cell death protein 1, CXCR3, and IFN-γ expression, representing a TFH cell subset with impaired B-helper function .

How do PIK3CD antibodies contribute to understanding B cell developmental abnormalities?

Advanced B cell developmental analysis requires:

• Comprehensive B cell phenotyping panel:

  • CD19+ (total B cells)

  • CD19+IgD+CD27- (naïve B cells)

  • CD19+IgD+CD27+ (marginal zone B cells)

  • CD19+IgD-CD27+ (memory B cells)

  • CD19+IgD-CD27- (double-negative B cells)

  • IgG+ vs. IgM+ subsets of memory B cells

  • CD19+CD20+CD38hiCD27hi (pre-plasmablasts)

  • CD20-CD38hi (plasmablasts)

• Developmental analysis strategy:

  • Compare frequencies of each population between experimental groups

  • Analyze surface marker expression intensity (MFI)

  • Assess class-switching by examining IgG+ vs. IgM+ frequencies

Research has revealed that PIK3CD GOF patients show altered B cell development patterns, including:

• Increased expression of CD5 and CD38 (approximately threefold higher) on transitional and naïve B cells

• Decreased expression of CD21, CD44, Bcl-2, CCR7, CXCR4, and CXCR5 (up to threefold lower) on various B cell subsets

• Expanded IgM+ double-negative B cell population

What methodologies should be employed when investigating the impact of PIK3CD inhibition on immune cell function?

When investigating PIK3CD inhibition effects:

• Inhibitor selection and validation:

  • Use specific p110δ inhibitors (e.g., leniolisib)

  • Verify target engagement via phospho-flow cytometry

  • Establish dose-response relationships

• Experimental design:

  • Include PIK3CD wild-type and mutant cells

  • Test both unstimulated and stimulated conditions

  • Analyze acute and chronic inhibition effects

• Functional readouts:

  • B cell differentiation into plasmablasts

  • Class-switch recombination efficiency

  • Antibody secretion (IgM vs. IgG)

  • Cell viability and proliferation

Research has demonstrated that defects in class-switch recombination, AID expression, and Ig secretion can be restored by leniolisib, a specific p110δ inhibitor . When designing inhibition studies, include appropriate time points to capture both immediate signaling effects and longer-term developmental consequences.

How can antibodies be used to establish mouse models that recapitulate human PIK3CD gain-of-function phenotypes?

Creating and validating PIK3CD mouse models requires:

• Model generation approach:

  • Use CRISPR/Cas9-mediated gene editing to introduce common pathogenic mutations (e.g., E1021K)

  • Verify mutation at genomic and protein levels using sequencing and PIK3CD antibodies

• Phenotypic validation:

  • Immunophenotyping of B and T cell compartments

  • Functional assessment of antibody responses

  • Verification of increased PI3K signaling

• Comparison to human disease:

  • Parallel analysis of mouse model and patient samples

  • Assessment of cellular and molecular similarities

  • Evaluation of therapeutic responses

Research has successfully established a mouse model using CRISPR/Cas9 to introduce the common E1021K mutation in Pik3cd . This model demonstrated that PIK3CD GOF CD4+ T cells acquire an aberrant follicular helper T cell phenotype and provide poor help to support germinal center reactions and humoral immune responses .

What is the relationship between PIK3CD mutations and autoimmunity, and how can this be investigated using antibodies?

Investigating PIK3CD-related autoimmunity requires:

• Cellular analysis:

  • Assess regulatory T cell (Treg) populations

  • Quantify T follicular helper (Tfh) and T follicular regulatory (Tfr) cells

  • Examine CD11c+ B and CD21lo B cell populations

• Autoantibody profiling:

  • Use antigen microarray analysis for comprehensive autoantibody detection

  • Quantify IgG and IgM autoantibodies

  • Correlate autoantibody levels with cellular abnormalities

• Correlation analysis:

  • Evaluate relationships between Tfr proportions and autoantibody levels

  • Compare immune phenotypes between patients with and without autoimmunity

Research findings show that APDS1 (activated PI3Kδ syndrome) patients with autoimmunity demonstrate:

• Decreased proportions of Tregs

• Increased proportions of Th9, Tfh, and Tfr cells

• Higher proportions of Th2 and Tfr cells compared to patients without autoimmunity

• Significantly increased proportions of CD11c+ B and CD21lo B cells

• A wide range of IgG/IgM autoantibodies

What controls and validation steps are essential when using PIK3CD antibodies in experimental systems?

Essential validation steps include:

• Antibody specificity validation:

  • Use PIK3CD-knockout or -knockdown cells as negative controls

  • Test antibody on cells with verified PIK3CD overexpression

  • Perform peptide competition assays

• Application-specific controls:

  • For Western blotting: Include molecular weight markers and loading controls

  • For flow cytometry: Use fluorescence-minus-one (FMO) and isotype controls

  • For immunoprecipitation: Include IgG control and input samples

• Cross-reactivity assessment:

  • Test antibody against related isoforms (p110α, p110β)

  • Verify specificity across species if using in multiple model systems

When analyzing phosphorylation status, include both positive controls (cells with known pathway activation) and negative controls (cells treated with PI3K inhibitors) to establish the dynamic range of detection .

What are the most effective systems for studying PIK3CD-dependent signal transduction using antibodies?

Optimal systems for PIK3CD signaling studies include:

• Cell models:

  • Primary human lymphocytes (most physiologically relevant)

  • Patient-derived cells carrying PIK3CD mutations

  • Genetically modified cell lines (CRISPR-engineered PIK3CD mutations)

• Stimulation conditions:

  • B cells: Anti-IgM, CD40L, IL-21, CpG

  • T cells: Anti-CD3/CD28, cytokines (IL-2, IL-4, IL-21)

• Readout approaches:

  • Phospho-flow cytometry for single-cell analysis of pathway activation

  • Western blotting for biochemical quantification

  • Imaging for spatial distribution of signaling components

Research has shown that when stimulating B cells to study PIK3CD function, class-switch recombination and plasmablast differentiation should be evaluated separately, as PIK3CD GOF mutations affect CSR but not plasmablast differentiation .

Table 1. Immune Cell Abnormalities in PIK3CD Gain-of-Function Mutations

Table 2. Comparison of PIK3CD Antibody Applications in Research