CD28 Antibody

What is CD28 and what is its role in T cell activation?

CD28 is a 44 kDa homodimeric cell surface glycoprotein expressed primarily on T lymphocytes, thymocytes, and plasma cells . It functions as a critical costimulatory receptor that provides the essential "second signal" required for complete T cell activation, working in concert with the "first signal" provided by T cell receptor (TCR) engagement .

When CD28 binds to its ligands CD80 (B7-1) and CD86 (B7-2) on antigen-presenting cells, it triggers signaling pathways that enhance TCR signaling, resulting in:

• Augmented interleukin-2 (IL-2) production and IL-2 receptor expression

• Enhanced T cell proliferation and survival

• Increased cytokine production

• Upregulation of anti-apoptotic molecules

Without this CD28-mediated costimulatory signal, T cells receiving only TCR stimulation typically become anergic or undergo apoptosis, highlighting the crucial role of CD28 in productive immune responses .

How do different types of anti-CD28 antibodies affect T cell function?

Anti-CD28 antibodies can be broadly classified based on their functional effects on T cell activation:

Costimulatory antibodies like clone CD28.2 enhance T cell proliferation and cytokine production when used in conjunction with T cell activators such as anti-CD3 antibodies or mitogens . In contrast, antagonistic antibodies like the domain antibody (dAb) described in the literature inhibit T cell activation by blocking the interaction between CD28 and its ligands . This diversity in functional effects makes CD28 antibodies versatile tools for both research and potential therapeutic applications .

What is the molecular basis for superagonistic versus conventional anti-CD28 antibody effects?

The distinct functional effects of conventional versus superagonistic anti-CD28 antibodies stem from their different epitope specificities:

Conventional agonistic antibodies:

• Bind near the ligand-binding site at the apex of CD28

• Require concurrent TCR engagement to activate T cells

• Provide true costimulation rather than independent activation

• Typically recognized as safer for research and potential therapeutic applications

Superagonistic antibodies:

• Bind to the lateral, C''D loop of the CD28 molecule, away from the ligand-binding site

• Can activate T cells without TCR engagement

• Induce robust cytokine release by crosslinking CD28 molecules

• Associated with severe cytokine release syndrome, as demonstrated by the TGN1412 clinical trial

The E1P2 antibody described in more recent research was specifically designed to bind at the apex of CD28 (similar to the natural ligands CD80/CD86), which allows it to avoid the superagonistic properties while maintaining binding capacity . This epitope mapping reveals how molecular recognition patterns directly influence antibody function and safety profiles.

How can researchers optimize CD28 antibody use in T cell activation assays?

When using CD28 antibodies for T cell activation assays, researchers should consider the following methodological approaches:

Optimal Protocol for T Cell Activation:

• Plate-bound versus soluble antibody: For maximal T cell activation, immobilize anti-CD28 antibodies (e.g., clone CD28.2 or CD28.6) on plates at 0.5-2 μg/mL along with anti-CD3 antibodies

• Concentration optimization: Titrate anti-CD28 antibodies (typically starting at 0.1-5 μg/mL) to determine optimal concentration for specific experimental systems

• Timing considerations: For proliferation assays, measure responses at 48-72 hours post-stimulation; for cytokine analysis, assess at earlier timepoints (6-24 hours)

• Experimental readouts: Measure proliferation (via 3H-thymidine incorporation or CFSE dilution), cytokine production (ELISA or intracellular cytokine staining), or activation marker upregulation (flow cytometry)

When designing controls, researchers should include:

• T cells with anti-CD3 alone (signal 1 only)

• T cells with anti-CD28 alone (to confirm absence of superagonistic effects)

• Unstimulated T cells as baseline

For antagonistic antibody studies, researchers should first pre-incubate T cells with the blocking antibody before adding stimulatory signals to ensure maximal inhibitory effects .

What are the considerations for using anti-CD28 antibodies in flow cytometry?

When using anti-CD28 antibodies for flow cytometric analysis, researchers should optimize their protocols based on these technical considerations:

Flow Cytometry Protocol Optimization:

• Antibody titration: For optimal staining, titrate antibodies to determine the concentration that provides maximal separation between positive and negative populations. For anti-CD28 clone CD28.2, starting concentrations of ≤1 μg per test are recommended

• Panel design: When designing multicolor panels, consider that CD28 is expressed at higher levels on CD4+ T cells compared to CD8+ T cells, which may affect fluorochrome selection

• Fluorochrome selection: Choose appropriate fluorochromes based on expression level (brighter fluorochromes for lower expressed markers)

• Controls: Include fluorescence-minus-one (FMO) controls and isotype controls (e.g., IgG1 for CD28.2) to establish proper gating strategies

• Buffer optimization: Use buffers containing sodium azide and protein (typically BSA or FBS) to minimize non-specific binding

For quantitative analysis of CD28 expression, researchers can use quantitative flow cytometry with antibody binding capacity (ABC) beads to determine the number of CD28 molecules per cell across different cell subsets or activation states .

How can researchers use CD28 antibodies to study costimulatory pathways?

CD28 antibodies serve as valuable tools for dissecting the complex roles of costimulatory pathways in T cell biology:

Methodological Approaches:

• Pathway analysis: Combine anti-CD28 antibodies with inhibitors of downstream signaling molecules (e.g., PI3K, AKT) to elucidate specific pathway contributions

• Comparative studies: Use both agonistic and antagonistic anti-CD28 antibodies alongside other costimulatory pathway modulators (e.g., CTLA-4-Ig, anti-CD80/CD86) to dissect redundancy and synergy between pathways

• Functional assays: Measure diverse T cell functions (proliferation, cytokine production, metabolic changes, survival) to comprehensively assess the impact of CD28 signaling

• Subpopulation analysis: Compare the effects of CD28 costimulation on naïve versus memory T cells or Th1 versus Th2 versus Treg populations

For more sophisticated analyses, researchers can combine CD28 antibodies with:

• Gene expression profiling to identify CD28-dependent transcriptional programs

• Phospho-flow cytometry to quantify signaling events downstream of CD28

• Live cell imaging to visualize immunological synapse formation in the presence or absence of CD28 costimulation

These approaches allow researchers to delineate the specific contributions of CD28 signaling to various aspects of T cell biology and immune responses.

What safety considerations should researchers be aware of when working with CD28 antibodies?

The TGN1412 clinical trial in 2006, which resulted in severe cytokine release syndrome in all six healthy volunteers, highlighted critical safety considerations for working with CD28 antibodies :

Key Safety Considerations:

• Antibody format: Monovalent formats (e.g., Fab fragments, domain antibodies) generally pose lower risks of unwanted T cell activation compared to bivalent IgG formats

• Epitope specificity: Antibodies binding to the C''D loop (lateral face) of CD28 are more likely to exhibit superagonistic properties than those binding at the CD80/CD86 binding site

• Species cross-reactivity: Human CD28 antibodies may exhibit different functional properties when tested on cells from different species due to subtle structural differences

• In vitro safety testing: Perform comprehensive cytokine release assays using human PBMCs from multiple donors before advancing to in vivo studies

• Fc receptor interactions: Consider using Fc-silent mutations or F(ab')2 fragments to prevent Fc-mediated effects that might contribute to unwanted activation

For researchers developing potentially therapeutic CD28 antibodies, additional preclinical testing should include:

• In vitro assessment across multiple donor samples

• Testing in humanized mouse models before clinical studies

• Careful dose escalation strategies beginning with very low doses

These considerations are essential not only for therapeutic development but also for basic research applications where unintended T cell activation could confound experimental results.

How are antagonistic anti-CD28 antibodies being developed for therapeutic applications?

The development of CD28 antagonists represents a promising approach for treating autoimmune diseases and preventing transplant rejection by selectively inhibiting T cell costimulation:

Current Development Approaches:

• Domain antibodies (dAbs): Single-domain antibodies that block CD28-CD80/86 interactions without activating T cells have shown promising results in preclinical models

• Monovalent formats: Fab fragments and other monovalent formats avoid the crosslinking that can lead to unwanted T cell activation

• Humanized antibodies: Fully human or humanized antibodies minimize immunogenicity concerns for therapeutic applications

• Bispecific constructs: Combining CD28 blockade with other immunomodulatory domains to enhance efficacy and specificity

The antagonistic anti-CD28 domain antibody described in the literature demonstrated potent inhibition of T cell proliferation with an EC50 of 35±14 ng/ml without any evidence of agonistic activity . Similarly, the E1P2 antibody showed promising safety profiles in humanized mouse models without inducing cytokine release syndrome .

These approaches represent potentially safer alternatives to existing therapies like CTLA-4-Ig (abatacept/belatacept), which block both CD28-CD80/86 and CTLA-4-CD80/86 interactions and may have broader immunosuppressive effects .

What lessons were learned from the TGN1412 clinical trial regarding CD28 antibody development?

The TGN1412 clinical trial in 2006 provided crucial insights that have fundamentally reshaped CD28-targeting therapeutic development:

Key Lessons and Resulting Methodological Changes:

• Epitope specificity is critical: The superagonistic properties of TGN1412 were linked to its binding to the lateral face (C''D loop) of CD28, leading to modern approaches that target the apical region instead

• Preclinical testing limitations: Standard in vitro assays and non-human primate models failed to predict the cytokine storm, leading to development of more predictive assay systems including:

  • Whole blood cytokine release assays

  • High-density culture systems

  • More comprehensive cytokine profiling panels

• Starting dose calculations: The "minimal anticipated biological effect level" (MABEL) approach replaced the "no observed adverse effect level" (NOAEL) approach for first-in-human studies

• Antibody engineering: Novel formats such as monovalent fragments and domain antibodies were developed to minimize crosslinking potential

• Species differences matter: Despite 100% homology in the extracellular domain between human and cynomolgus monkey CD28, functional responses differed significantly

These lessons have led to more cautious developmental approaches, including:

• Stepwise epitope mapping and functional characterization

• More comprehensive preclinical safety assessment

• Preference for antagonistic over agonistic mechanisms

• Development of novel antibody formats with improved safety profiles

The E1P2 antibody represents an example of applying these lessons, as it was specifically designed to avoid the superagonistic properties while maintaining CD28-binding capacity .

What are the optimal storage and handling conditions for CD28 antibodies?

Proper storage and handling of anti-CD28 antibodies are critical for maintaining their functional activity and specificity:

Recommended Storage and Handling Protocols:

• Storage temperature: Store antibodies at 2-8°C (short-term) or aliquot and store at -20°C to -80°C (long-term) to avoid freeze-thaw cycles

• Buffer conditions: Most purified antibodies are stable in PBS with sodium azide (0.09-0.1%) as a preservative

• Protein concentration: Higher concentration formulations (≥0.5 mg/mL) typically show better stability

• Avoid freeze-thaw cycles: Prepare single-use aliquots to prevent protein degradation from repeated freezing and thawing

• Avoid microbial contamination: Use sterile technique when handling antibodies for functional assays

• Protect from light: For fluorochrome-conjugated antibodies, minimize exposure to light to prevent photobleaching

For functional grade antibodies used in cell culture:

• Use low-endotoxin, azide-free, and serum-free formulations

• Store according to manufacturer recommendations (typically 2-8°C)

• Maintain sterility throughout handling procedures

Regular quality control testing (e.g., flow cytometry or ELISA) is recommended to verify antibody performance, particularly for critical experiments or after prolonged storage .

How should researchers select the appropriate anti-CD28 antibody clone for their experiments?

The selection of the appropriate anti-CD28 antibody clone should be guided by the experimental application and specific research questions:

Selection Criteria by Application:

When selecting between different clones, researchers should consider:

• Epitope specificity: Different epitopes may affect functional outcomes (e.g., agonistic vs. antagonistic effects)

• Format requirements: Purified, biotinylated, fluorochrome-conjugated, or low endotoxin preparations

• Species cross-reactivity: Some clones (e.g., E1P2) cross-react with mouse CD28, enabling translational studies

• Validation data: Review published literature and manufacturer data for validation in your specific application

• Isotype considerations: Match isotype controls appropriately (e.g., mouse IgG1 for CD28.2)

For novel applications, preliminary titration experiments and functional validation should be conducted to ensure optimal performance of the selected antibody clone .

What quality control parameters should researchers verify when using anti-CD28 antibodies?

To ensure experimental reliability and reproducibility, researchers should verify several quality control parameters when working with anti-CD28 antibodies:

Essential Quality Control Parameters:

• Purity: Antibody preparations should typically exceed 90-95% purity as determined by SDS-PAGE or HPLC analysis

• Endotoxin levels: For functional assays, endotoxin levels should be <0.001 ng/μg antibody to prevent LPS-mediated effects

• Aggregation: Protein aggregation should be <10% as determined by HPLC or other analytical methods

• Specificity validation: Confirm specific binding to CD28 using:

  • Flow cytometry with positive controls (T cells) and negative controls (CD28-negative cell lines)

  • Western blot against recombinant CD28 protein

  • Competitive binding assays

• Functional validation: Verify expected functional activity (e.g., costimulation or blocking) in relevant assay systems

• Lot-to-lot consistency: When changing antibody lots, perform side-by-side comparisons to ensure consistent performance

Documentation practices should include:

• Recording lot numbers, source, and concentration

• Maintaining detailed protocols for antibody use

• Documenting quality control testing results

• Noting any deviations from expected performance

These quality control measures are essential for ensuring experimental reproducibility and generating reliable scientific data when working with CD28 antibodies.

How are CD28 antibodies being integrated into bispecific and multispecific therapeutic approaches?

Recent advances in antibody engineering have led to innovative approaches combining CD28 targeting with other immunomodulatory mechanisms:

Emerging Multispecific Approaches:

• CD3×CD28 bispecific antibodies: These molecules simultaneously engage the TCR (via CD3) and provide costimulation (via CD28) to enhance T cell activation against cancer cells

• Tumor-targeted CD28 costimulators: These constructs combine a tumor-targeting domain with CD28 costimulatory domains to selectively activate T cells in the tumor microenvironment

• CD28×PD-1 bispecifics: These molecules combine CD28 costimulation with PD-1 checkpoint blockade to overcome T cell exhaustion

• CAR-T cell enhancement: CD28 costimulatory domains incorporated into CAR-T constructs have shown enhanced persistence and anti-tumor activity

The E1P2 antibody represents an example of this approach, designed to enhance T-cell activity when combined with CD3 bispecific antibodies while avoiding the safety concerns associated with superagonistic antibodies . This combinatorial approach demonstrated enhanced tumor cell killing and T-cell proliferation in preclinical models.

The key advantage of these approaches is the potential to provide targeted T cell activation while minimizing systemic inflammatory effects, potentially offering improved safety profiles compared to earlier CD28-targeting approaches .

What new methodological approaches are advancing our understanding of CD28 signaling?

Advanced methodological approaches are providing deeper insights into the molecular mechanisms and biological consequences of CD28 signaling:

Cutting-Edge Methodological Approaches:

• Single-cell technologies: Single-cell RNA-seq and CyTOF (mass cytometry) are revealing heterogeneity in CD28 expression and signaling responses across T cell subpopulations

• CRISPR-Cas9 gene editing: Precise manipulation of CD28 and downstream signaling components is enabling detailed dissection of signaling pathways

• Super-resolution microscopy: Advanced imaging techniques are revealing the spatial organization of CD28 within the immunological synapse at nanometer resolution

• Structural biology: Cryo-EM and X-ray crystallography of CD28-antibody complexes are providing atomic-level insights into binding mechanisms and epitope specificity

• Systems biology approaches: Integration of phosphoproteomics, transcriptomics, and metabolomics data is providing comprehensive views of CD28 signaling networks

These methodological advances are revealing:

• How CD28 signaling integrates with other costimulatory and inhibitory pathways

• The temporal dynamics of CD28-mediated signaling events

• How epitope-specific binding influences downstream signaling outcomes

• Potential novel therapeutic targets within the CD28 pathway

These approaches are not only enhancing our basic understanding of T cell biology but also informing more rational design of CD28-targeting therapeutics with improved efficacy and safety profiles.