VID28 Antibody

What is CD28 and why is it important in immunological research?

CD28 is a homodimeric receptor composed of disulfide-linked chains (44 kDa each) expressed primarily on T lymphocytes. It serves as a critical co-stimulatory molecule that interacts with CD80 (B7-1/BB-1) and CD86 (B7-2/B70) on antigen-presenting cells. This interaction provides essential second signals for T cell activation, proliferation, and cytokine production beyond the primary TCR stimulus. CD28 signaling is fundamental to proper T cell function, making it a significant target for immunomodulatory therapies and research . The CD28 family of receptors (including CD28, CTLA-4, ICOS, PD-1, and BTLA) plays a critical role in controlling adaptive immune responses through both TCR-dependent enhancement and independent signaling pathways .

How do CD28 antibodies differ in their functional properties?

CD28 antibodies can be categorized into three main functional classes based on their effects on T cell activation:

• Agonistic antibodies: These bind to CD28 on T cells and promote T cell responses by mimicking the natural CD80/CD86 interaction. They typically require TCR co-engagement for full T cell activation .

• Superagonistic antibodies: These can generate strong activating signals in mature T cells without additional TCR stimulation. They preferentially expand regulatory T cells (Tregs) and have been investigated for treating autoimmune conditions .

• Antagonistic antibodies: These block the interaction between CD28 and its ligands, inhibiting T cell activation. "Antagonist-only" versions have been developed with modifications to prevent Fc-mediated cross-linking, which could cause unwanted activation .

The binding epitope on CD28 significantly affects functional outcomes. For example, antibodies binding near the apex (like natural ligands) versus those binding lateral epitopes can produce dramatically different immunological effects .

What assays can effectively measure CD28 antibody functional activity?

Several methodologies can assess CD28 antibody functionality:

• T cell proliferation assays: Measure [³H]-thymidine incorporation or CFSE dilution in primary T cells or Jurkat cell lines treated with CD28 antibodies, with or without TCR stimulation.

• Cytokine production analysis: Quantify IL-2, IFN-γ, and other cytokines released following CD28 antibody treatment using ELISA or cytometric bead arrays.

• Luciferase reporter assays: Use cell lines like KIR3DL3/IL-2 Luciferase Reporter Jurkat cells to measure dose-dependent activation in response to anti-CD28 antibodies .

• Flow cytometry: Assess binding capacity to CD28-expressing cells and downstream activation markers such as CD25, CD69, and phosphorylated signaling molecules.

• In vitro co-culture systems: Evaluate T cell activation when combined with antigen-presenting cells or tumor cells to assess functional outcomes in more complex settings .

When designing these assays, it's essential to include appropriate controls, including isotype controls and comparative analysis with known agonistic or antagonistic antibodies.

How can researchers evaluate CD28 antibody binding characteristics and specificity?

To thoroughly characterize CD28 antibody binding:

• ELISA: Determine apparent affinity (EC50) values through direct binding to recombinant CD28. For example, the E1P2 antibody showed EC50 values of 2.7 nM for human CD28 and 18.5 nM for mouse CD28 .

• Flow cytometry on primary cells: Confirm binding to naturally expressed CD28 on human and mouse T cells. This approach validates antibody specificity in a physiological context .

• Epitope mapping: Use techniques like HDX-MS (hydrogen-deuterium exchange mass spectrometry), alanine scanning mutagenesis, or competitive binding assays to identify precise binding epitopes. This is critical as epitope location significantly affects functional properties - apical versus lateral binding can determine agonistic versus superagonistic activity .

• Species cross-reactivity analysis: Test binding across species (particularly human and mouse) to determine translational research potential. Not all anti-CD28 antibodies cross-react with murine CD28, as seen with TGN1412 which only bound human CD28 .

• Surface plasmon resonance (SPR): Measure association and dissociation kinetics to obtain kon, koff, and KD values for comprehensive binding characterization.

How do structural modifications in anti-CD28 antibodies affect their safety and efficacy profiles?

Structural modifications profoundly impact anti-CD28 antibody safety and functionality:

These structural considerations are critical when developing CD28-targeting therapeutics to balance efficacy and safety.

What are the current challenges in developing safe CD28-targeting therapeutic antibodies?

The development of CD28-targeting therapeutics faces several key challenges:

• Superagonism risk: Following the TGN1412 clinical trial in 2006 that caused severe cytokine release syndrome and multi-organ failure in healthy volunteers, preventing superagonistic activity has become paramount. This requires rigorous preclinical testing using human cells and appropriate animal models .

• Translational gaps: Significant differences exist between murine and human CD28 biology and downstream signaling. Pre-clinical models may not accurately predict human responses, as dramatically illustrated by the TGN1412 case, where no toxicity was observed in non-human primates despite catastrophic effects in humans .

• Balancing immunomodulation: CD28 signaling can promote both effector T cell and regulatory T cell responses. Creating antibodies with selective activity profiles that can either enhance anti-tumor immunity or promote tolerance in autoimmunity remains challenging .

• Heterogeneous T cell responses: CD28 expression varies across T cell subsets (higher on CD4+ than CD8+ T cells) and activation states, leading to differential responses to CD28-targeting antibodies . Understanding these variations is essential for predicting therapeutic outcomes.

• Combination therapy integration: Determining optimal combinations with other immunomodulatory agents (like CD3 bispecific antibodies) requires careful consideration of dosing, timing, and potential synergistic toxicities .

Addressing these challenges requires sophisticated preclinical testing platforms, including humanized mouse models, human PBMC assays, and detailed molecular and cellular characterization of antibody effects.

How are CD28 domains being utilized in CAR-T cell engineering?

CD28 domains play crucial roles in chimeric antigen receptor (CAR) T-cell design:

• Signaling domains: Second-generation CARs incorporate the CD28 intracellular domain as a co-stimulatory module, providing essential signals for T cell activation, proliferation, and cytokine production when the CAR binds its target antigen.

• Hinge regions: The CD28 hinge region (CD28H) is used in FDA-approved CAR constructs to connect the antigen-binding domain to the transmembrane domain. This region exhibits specific structural characteristics:

  • Contains 10-helix and polyproline II helix-like structural motifs

  • Forms an extended geometry that influences the flexibility and reach of the CAR

  • Contains proline residues that isomerize to promote structural plasticity and dynamics

• Structural considerations: The CD28 hinge contributes to CAR functionality by:

  • Constraining spacing between transmembrane and antigen recognition domains

  • Providing appropriate flexibility for antigen engagement

  • Contributing to recognition and signaling events through local structural elements

Understanding the structural properties of CD28 domains in CARs is enabling more rational design approaches for next-generation cellular therapeutics. Researchers are investigating how modifications to the CD28 hinge region might optimize CAR-T cell performance for specific target antigens and tumor types.

What are the mechanisms by which different anti-CD28 antibodies modulate regulatory T cell (Treg) function?

Anti-CD28 antibodies can differentially impact Treg function through several mechanisms:

• Superagonistic antibodies and Treg expansion: CD28 superagonists preferentially expand and strongly activate naturally occurring CD4+CD25+CTLA-4+FoxP3+ Tregs over conventional T cells. This occurs because:

  • Tregs constitutively express higher levels of CD25 (IL-2 receptor α-chain)

  • Tregs have a lower activation threshold for CD28 signaling

  • Superagonistic stimulation triggers robust IL-2 production which preferentially supports Treg proliferation and function

• Differential signaling pathways: Various anti-CD28 antibodies can activate distinct intracellular signaling pathways:

  • Conventional agonists typically activate PI3K-Akt-mTOR pathways

  • Superagonists may preferentially activate alternative pathways that favor Treg expansion

  • Antagonistic antibodies can block natural ligand binding without triggering intracellular signaling

• Therapeutic implications: In animal models of autoimmunity, CD28 superagonist administration (both prophylactic and therapeutic) significantly mitigated clinical symptoms and induced remission. Adoptive transfer experiments demonstrated that this protection was mediated through expansion and activation of CD4+CD25+ Tregs .

• Balance with effector functions: The ratio of conventional T cell to Treg activation determines the net immunological outcome. This balance depends on antibody concentration, epitope specificity, and the immunological context (presence of additional signals, cytokine milieu, etc.).

Understanding these mechanisms has significant implications for developing CD28-targeting therapies for autoimmune diseases, transplantation, and potentially cancer immunotherapy where modulating Treg/effector T cell balance is crucial.

How might bispecific antibodies incorporating CD28 targeting advance cancer immunotherapy?

Bispecific antibodies incorporating CD28 targeting represent a promising frontier in cancer immunotherapy:

• Addressing T cell exhaustion: CD3 bispecific T-cell engagers have shown remarkable clinical outcomes against several hematological malignancies, but the absence of costimulatory signals through CD28 often leads to insufficient T-cell activation and early exhaustion. Combining CD3 and CD28 targeting offers a strategy to boost T-cell activity and persistence .

• Enhanced tumor cell killing: In vitro studies demonstrate that the combination of non-superagonistic anti-CD28 antibodies (like E1P2) with CD3 bispecific antibodies significantly enhances tumor cell killing and T-cell proliferation compared to CD3 bispecifics alone .

• Potential designs include:

  • Trispecific antibodies targeting tumor antigen, CD3, and CD28

  • CD28×tumor antigen bispecifics used in combination with CD3×tumor antigen bispecifics

  • "Conditional" CD28 costimulators that activate only when bound to tumor-specific antigens

• Safety considerations: Learning from the TGN1412 disaster, next-generation CD28-targeting approaches must avoid superagonism. New designs like E1P2 that bind near the apex of CD28 (similar to natural ligands) rather than lateral epitopes show promising safety profiles in humanized mouse models .

• Target indications: These approaches may be particularly valuable for:

  • Solid tumors where T cell exhaustion limits efficacy of current therapies

  • "Cold" tumors lacking costimulatory ligands

  • Settings where enhanced memory T cell formation would provide long-term protection

Continued research in this area could overcome current limitations of CD3-only approaches while maintaining an acceptable safety profile.

What novel methodologies are being developed to predict and avoid cytokine release syndrome with CD28-targeting antibodies?

Following the TGN1412 incident, researchers have developed sophisticated methodologies to better predict cytokine release syndrome (CRS) risk:

• Humanized mouse models: NSG mice engrafted with human PBMCs provide a more predictive in vivo system for testing CRS potential. Comparative studies between known superagonists (TGN1412) and novel candidates (E1P2) in these models have successfully differentiated safe from hazardous antibodies .

• Advanced in vitro testing platforms:

  • Whole blood cytokine release assays

  • PBMC-based systems with aqueous versus solid-phase antibody presentation

  • Pre-culturing PBMCs at high density to restore sensitivity to superagonistic stimulation (addressing the issue that standard PBMC culture conditions failed to predict TGN1412 toxicity)

  • 3D organoid systems that better recapitulate tissue architecture

• Molecular design approaches:

  • Epitope mapping to avoid regions associated with superagonistic activity

  • Engineering antibodies with controlled valency to prevent excessive CD28 crosslinking

  • Developing conditional activation systems that require multiple signals

• Mechanistic biomarkers: Identifying early molecular signatures that predict CRS by:

  • Phosphoproteomic analysis of early signaling events

  • Single-cell RNA sequencing to identify transcriptional signatures

  • Monitoring specific cytokine patterns predictive of severe CRS (IL-6, IFN-γ, TNF-α cascades)

• Dose escalation strategies: Implementing ultra-low starting doses with careful pharmacokinetic/pharmacodynamic modeling to detect early signs of CRS at sub-clinical levels.

These methodologies collectively provide a more robust framework for developing safer CD28-targeting therapeutics and may eventually enable the clinical advancement of this promising but challenging target class.