CHARK Antibody

What distinguishes shark antibodies from conventional mammalian antibodies?

Shark antibodies, specifically the IgNAR (Immunoglobulin New Antigen Receptor) antibodies, are fundamentally different from conventional mammalian antibodies in several key aspects:

• Size and Domain Structure: Shark VNARs are significantly smaller (12-15 kDa) than conventional antibody fragments, measuring approximately one-tenth the size of human antibodies . Conventional antibodies have both heavy and light chains, while shark IgNAR antibodies contain only heavy chains .

• Binding Capabilities: The variable domains of shark antibodies can access "nooks and crannies" of proteins that human antibodies cannot reach due to their compact size and unique geometry . This allows them to recognize structures in proteins inaccessible to human antibodies.

• Stability: Shark antibodies demonstrate remarkable stability under extreme conditions. Some VNAR fragments remain functional even after boiling, and they maintain stability at room temperature without requiring freezing or cold storage .

• Evolutionary Origin: Sharks evolved some 350 million years before camels (which also produce heavy-chain antibodies), making shark antibodies among the most ancient antibody systems still in existence . They represent the earliest form of adaptive immunity known in vertebrates .

• Urea Resistance: Shark antibodies have evolved to function in high urea environments (which sharks use to prevent osmotic water loss in marine environments), a condition that would denature most mammalian proteins .

What are the different types of shark VNAR domains and how are they classified?

Shark VNAR domains are classified into several types based on their structural features, particularly the number and positioning of non-canonical cysteine residues:

• Type I: Characterized by two non-canonical cysteine residues in CDR3 that form disulfide bonds with cysteine residues in CDR1. This type represents approximately 11% of VNARs in nurse sharks .

• Type II: Contains two non-canonical cysteine residues within CDR1 and two within CDR3. These form intra-loop disulfide bonds that stabilize the structure. Type II represents the most abundant form, approximately 57% of VNARs in nurse sharks .

• Type III: Contains non-canonical cysteine residues in CDR1 but not in CDR3.

• Type IV: Has no non-canonical cysteine residues in either CDR1 or CDR3.

• Unclassified Types: Next-generation sequencing has revealed that approximately 30% of shark VNARs cannot be categorized into any of these classical types, suggesting greater diversity than previously recognized .

How are shark VNAR libraries constructed for antibody discovery?

The construction of VNAR libraries involves sophisticated molecular techniques to capture the diversity of these antibody domains:

• Sample Collection: Blood samples are collected from sharks, typically nurse sharks (Ginglymostoma cirratum) after immunization with target antigens. Researchers like Aaron LeBeau collect shark blood by carefully sedating the animals and drawing from a vein in the tail .

• Library Construction Methods: Advanced methods like PCR-Extension Assembly and Self-Ligation (EASeL) are used to construct large VNAR libraries. In one study, a library with a size of 1.2 × 10^10 individual clones was constructed from six naïve adult nurse sharks .

• Next-Generation Sequencing Analysis: The libraries are analyzed using NGS to characterize the diversity. In one study, analysis of 1.19 million full-length VNARs revealed coverage of all four classical VNAR types and many unclassified variants .

• Phage Display Technology: The VNAR sequences are typically displayed on phage particles for subsequent selection against target antigens. This allows for efficient screening of large antibody libraries to identify specific binders .

• Isolation of Specific Binders: Researchers perform rounds of selection (biopanning) to isolate VNAR phage binders against targets. Successful isolations have been reported against cancer therapy-related antigens (glypican-3, HER2, PD1) and viral antigens (MERS and SARS spike proteins) .

What methodologies are employed to analyze the structure-function relationships of shark antibodies?

Several advanced methodologies are used to understand the structural basis of shark antibody function:

• X-ray Crystallography: This technique provides atomic-resolution structures of shark antibody domains, revealing detailed information about their folding patterns and binding interfaces .

• Molecular Dynamics Simulations: These computational methods model the conformational flexibility of shark antibodies to understand their binding dynamics:

  • Well-tempered metadynamics simulations using GROMACS with PLUMED implementation

  • Use of collective variables based on ψ torsion angles of CDR loops

  • Simulations performed at 300K in NpT ensemble with parameters including Gaussian height of 10.0 kJ/mol and width of 0.3 radian

• Markov-State Models: Constructed based on backbone torsions of CDR regions to analyze conformational states:

  • Typically using k-means clustering algorithm with ~150 microstates

  • Application of lag times of ~10 ns for reliable state identification

• CDR Clustering Analysis: Computational methods to identify canonical structures of CDRs and classify them based on conformational similarity .

• Binding Kinetics Analysis: Techniques for measuring antibody-antigen interactions, including:

  • Surface plasmon resonance to determine binding affinities

  • Thermodynamic analysis of binding using techniques that calculate electrostatic and Van der Waals interactions

How can shark antibodies be applied to viral research, including coronavirus studies?

Shark antibodies show significant promise in viral research due to their unique binding properties:

• SARS-CoV-2 and Related Coronaviruses: Shark VNARs have been successfully used to neutralize SARS-CoV-2 (the virus causing COVID-19) and related coronaviruses. In a 2021 study, shark VNAR proteins prevented SARS-CoV-2 and its variants from infecting human cells by binding to the spike protein .

• Preparation for Future Outbreaks: Researchers are developing shark VNAR therapeutics against bat coronaviruses like WIV1-CoV that currently circulate only in animals but could potentially jump to humans. This represents a proactive approach to pandemic preparedness .

• Delivery Methods Development:

  • mRNA Technology: Researchers are exploring the delivery of mini antibodies via mRNA technology so that the antibodies assemble inside human cells .

  • Oral Delivery: VNARs could potentially be delivered in pill form to target pathogens in the digestive tract, which could be particularly useful against rotavirus and other gut-entering pathogens .

  • Engineered Microbes: Scientists have proposed genetically engineering spirulina (blue-green algae) or harmless bacteria like Lactobacilli to deliver therapeutic nanobodies via pill form .

• Blood-Brain Barrier Penetration: The small size of shark antibodies enables them to potentially cross the blood-brain barrier, which conventional antibodies typically cannot do, opening possibilities for targeting neurotropic viruses .

What approaches are used to humanize shark antibodies for therapeutic applications?

Humanization of shark antibodies involves several strategies to reduce immunogenicity while maintaining binding properties:

• CDR Grafting: This involves:

  • Identifying the CDRs and suitable human template frameworks

  • Transferring shark CDRs onto human antibody frameworks

  • Maintaining critical framework residues that influence CDR conformation

• Analysis of Antigen-Contacting Residues: Researchers define antigen-contacting residues by:

  • Using crystallography data from shark VNAR-antigen complexes

  • Applying computational tools like CCP4 with CONTACT/ACT program

  • Identifying residue pairs with closest atoms no more than 6Å apart

• Canonical Structure Assessment: This involves:

  • Evaluating CDR conformations in shark antibodies

  • Matching them with similar conformations in human antibodies

  • Selecting human frameworks with compatible canonical structures

• Stability Engineering: Additional modifications may be needed to maintain the stability of humanized constructs:

  • Transplanting specific stabilizing features from shark antibodies into human frameworks

  • Engineering disulfide bonds in strategic positions based on shark antibody patterns

• Assessment of Humanized Variants: The humanized variants are evaluated for:

  • Conformational flexibility using molecular dynamics simulations

  • Changes in binding affinity and specificity

  • Biophysical properties including thermal stability and solubility

How does affinity maturation affect shark antibody structures and what insights does this provide for antibody engineering?

Affinity maturation in shark antibodies leads to significant structural changes that provide valuable insights for antibody engineering:

• Structural Rigidification: Studies have shown that shark antibody variable domains undergo rigidification upon affinity maturation, which contributes to improved binding specificity .

• CDR Loop Changes: Specific changes observed include:

  • Decreased flexibility in CDR1 and CDR3 loops

  • Formation of more defined canonical structures

  • Restriction of conformational space that improves antigen complementarity

• Thermodynamic and Kinetic Effects:

  • Affinity-matured shark antibodies show more favorable binding energetics

  • Changed association and dissociation kinetics

  • Improved electrostatic complementarity with target antigens

• Applications to Antibody Engineering:

  • The understanding of rigidification patterns can guide the rational design of antibodies with improved affinities

  • Strategic introduction of stabilizing interactions in CDR loops can mimic natural affinity maturation

  • Engineering approaches can focus on residues that contribute to conformational restriction rather than just direct antigen contact

What are the optimal methods for expressing and purifying shark antibodies for research?

Shark antibodies can be expressed and purified using several systems, each with specific advantages:

Expression Systems:

• Bacterial Expression (E. coli):

  • Most commonly used due to simplicity and cost-effectiveness

  • Suitable for VNAR domains due to their single-domain nature and disulfide bond patterns

  • Typically uses pET or pComb3X expression vectors with T7 or lac promoters

  • Yields of 5-20 mg/L can be achieved in shake flask cultures

• Mammalian Expression:

  • Used for humanized variants or when post-translational modifications are required

  • HEK293 or CHO cells are commonly employed

  • Expression levels of humanized VNARs in Lab I studies showed comparable or better titers than clinical-stage antibodies

• Yeast Expression:

  • Pichia pastoris offers advantages for proper folding and disulfide bond formation

  • Secretion into the culture medium simplifies purification

Purification Strategies:

• Affinity Chromatography:

  • Immobilized metal affinity chromatography (IMAC) for His-tagged VNARs

  • Protein A affinity resin for Fc-fusion formats

• Size Exclusion Chromatography:

  • Critical for separating monomeric VNARs from aggregates

  • Typically using Superdex 75 or similar matrices

• Quality Control Assessments:

  • Size exclusion HPLC to assess monomer content

  • Differential scanning fluorimetry for thermal stability

  • Hydrophobic interaction chromatography for surface hydrophobicity assessment

Production Metrics Comparison:
Based on experimental data from Lab I comparing in-silico generated antibodies (GAN) to existing therapeutic antibodies (EXT) :

What screening techniques are most effective for identifying target-specific shark antibodies?

Multiple screening approaches have been developed to efficiently identify target-specific shark antibodies:

• Phage Display Biopanning:

  • Most widely used technique for shark VNAR selection

  • Typically involves 3-4 rounds of selection against immobilized target proteins

  • Includes stringent washing steps with detergents (typically Tween-20) to remove non-specific binders

  • Uses competitive elution with excess target or pH-based elution methods

• Next-Generation Sequencing (NGS) Analysis:

  • Enables deep profiling of selected VNAR libraries

  • Can identify enriched sequence families after selection

  • Provides insights into CDR diversity and potential cross-reactivity

  • Analysis of 1.19 million full-length VNARs revealed approximately 11% of classical Type I and 57% of classical Type II VNARs in nurse sharks

• ELISA-Based Screening:

  • Used to validate binding of selected VNARs to target antigens

  • Often employs detection of phage-displayed VNARs using anti-M13 antibodies

  • Direct ELISA using purified VNAR proteins for affinity assessment

• Functional Assays:

  • Cell-based assays to evaluate VNAR activity (e.g., virus neutralization)

  • Competition assays to determine epitope specificity

  • Surface plasmon resonance to determine binding kinetics and affinity

  • One Type II VNAR (PE38-B6) demonstrated high affinity (Kd = 10.1 nM) for its antigen

• CDR Cluster Analysis:

  • Computational method for assigning antigen specificity

  • Groups receptors with highly similar paratopes

  • Can help mitigate cell sorting errors in cases of potential cross-reactivity

  • Shows high sensitivity for identifying antigen-specific clusters (95% observed in COVID-19 data analysis)

What biophysical techniques are essential for characterizing shark antibody binding properties?

Several biophysical techniques are crucial for comprehensive characterization of shark antibody binding:

• Surface Plasmon Resonance (SPR):

  • Provides real-time binding kinetics (kon and koff rates)

  • Enables determination of equilibrium dissociation constants (KD)

  • Can be used to evaluate temperature dependence of binding

  • Allows epitope binning to classify antibodies binding to different regions

• Isothermal Titration Calorimetry (ITC):

  • Measures thermodynamic parameters of binding (ΔH, ΔS, ΔG)

  • Provides stoichiometry information

  • Useful for understanding the energetic basis of shark antibody-antigen interactions

  • Can reveal enthalpy-entropy compensation mechanisms

• Bio-Layer Interferometry (BLI):

  • Alternative to SPR for kinetic measurements

  • Allows high-throughput screening of multiple antibody candidates

  • Useful for crude sample analysis during initial screening

• Differential Scanning Fluorimetry (DSF) and Differential Scanning Calorimetry (DSC):

  • Assesses thermal stability of shark antibodies alone and in complex with antigens

  • Can reveal stabilization effects upon antigen binding

  • Provides melting temperatures (Tm) as a measure of conformational stability

• Molecular Dynamics Simulations:

  • Analyze conformational flexibility of CDR loops

  • Reconstruct binding thermodynamics and kinetics

  • Use inverse distances of native contacts between antibody and antigen

  • Apply tICA (time-lagged independent component analysis) and Markov-state modeling with lag times of approximately 50 ns

• X-ray Crystallography and Cryo-EM:

  • Reveal detailed binding interfaces at atomic resolution

  • Allow identification of key interacting residues

  • Provide structural basis for rational optimization

  • Complement and validate computational predictions

How are computational methods advancing shark antibody design and optimization?

Computational methods are revolutionizing shark antibody research through several advanced approaches:

• Deep Learning Models:

  • Generate novel antibody variable region sequences with desirable developability attributes

  • Train on datasets of antibodies that satisfy computational developability criteria

  • Can produce sequences that recapitulate intrinsic sequence, structural, and physicochemical properties of training antibodies

  • Generate antibodies with high expression, monomer content, and thermal stability

• Molecular Dynamics Simulations:

  • Characterize flexibility of CDR loops and paratope conformations

  • Simulate effects of mutations on stability and binding

  • Predict binding affinities and kinetics

  • Implement metadynamics approaches to overcome energy barriers and sample rare events

• Antibody Modeling Approaches:

  • Leverage knowledge of canonical structures for CDR modeling

  • Predict structures of antibody Fv regions with increasing accuracy

  • Apply specialized algorithms for CDR-H3/CDR3 prediction which remains challenging

  • Utilize software from groups like Accelrys, Chemical Computer Group, Schrödinger, and others

• Machine Learning for Affinity Optimization:

  • Develop models to predict binding affinity changes due to mutations (ΔΔG)

  • Random Forest Regressors show promise despite limited training data

  • Simulate selection of optimal mutations for affinity enhancement

  • Different models may select only ~50% of the same mutations despite similar performance metrics, suggesting the importance of diverse modeling approaches

• Next-Generation Sequencing Analysis:

  • Characterize diversity of shark VNAR libraries

  • Identify enriched sequences after selection

  • Analyze CDR length distribution and cysteine patterns

  • Discover novel VNAR types beyond the classical classifications

What are the challenges and potential solutions in developing shark antibodies as therapeutics?

Despite their promise, shark antibodies face several challenges in therapeutic development:

Challenges:

• Immunogenicity Concerns:

  • Non-human origin could trigger immune responses

  • Presence of non-canonical disulfide bonds may create novel epitopes

• Developability Issues:

  • Incidence of potential N-linked glycosylation sites in some in-silico generated sequences (~7.8%)

  • Presence of non-canonical unpaired cysteines in a small percentage (~0.5%) of sequences that could affect stability

  • Chemical liability motifs like Asn-deamidation that affect shelf-life

• Limited Access to Source Animals:

  • As noted by Ploegh, "mini antibodies are exceptionally useful... but they remain somewhat niche because of limited access to the animals that make them"

  • Few research facilities have the capability to house and maintain sharks

• Delivery Challenges:

  • Optimal route of administration for shark antibody therapeutics

  • Potential for rapid clearance due to small size

  • Blood-brain barrier penetration capabilities require further validation

Potential Solutions:

• Humanization Strategies:

  • CDR grafting onto human antibody frameworks

  • Maintaining only essential shark-specific residues

  • Strategic framework modifications to preserve binding properties

• Alternative Production Methods:

  • Development of "camelized" immune systems in mice

  • Creation of synthetic nanobodies that mimic shark antibody properties

  • Application of advanced computational design to overcome reliance on shark immunization

• Novel Delivery Approaches:

  • mRNA technology for in vivo antibody assembly

  • Engineered microorganisms (spirulina, Lactobacilli) for oral delivery

  • Combination with tissue-targeting moieties to enhance biodistribution

• Computational Screening for Liabilities:

  • Machine learning models to predict and eliminate chemical liability motifs

  • In-silico assessment of immunogenicity risk

  • Computational stability optimization

How might shark antibodies contribute to pandemic preparedness and emerging disease research?

Shark antibodies offer unique advantages for pandemic preparedness and emerging disease research:

• Pre-emptive Development Against Potential Threats:

  • Research has demonstrated shark VNARs can neutralize WIV1-CoV, a bat coronavirus with potential to jump to humans

  • This proactive approach creates "an arsenal of shark VNAR therapeutics that could be used down the road for future SARS outbreaks"

  • Represents "a kind of insurance against the future" for emerging pathogens

• Unique Binding Properties for Conserved Epitopes:

  • VNARs can access hidden epitopes that conventional antibodies cannot reach

  • May target conserved regions that are less susceptible to mutation

  • Potential to develop broadly neutralizing antibodies against viral families rather than specific strains

• Stability Advantages for Global Distribution:

  • Room temperature stability eliminates cold chain requirements

  • Resistance to harsh conditions (some can withstand boiling) enables deployment in resource-limited settings

  • Extended shelf-life potential compared to conventional antibodies

• Multiple Administration Routes:

  • Potential for oral administration targeting gut pathogens

  • Inhalation delivery for respiratory pathogens

  • Possibility of engineered microorganisms producing VNARs directly in the GI tract

• Rapid Discovery Pipeline:

  • Large naïve libraries can be screened against emerging pathogens

  • Phage display technology enables quick identification of binding candidates

  • Next-generation sequencing analysis accelerates characterization of potential therapeutic candidates