Single-domain antibodies (sdAbs), including VHHs derived from camelid heavy chain-only antibodies (HCAbs), are compact antigen-binding domains composed solely of a VH-like region without a paired VL domain. Their small size (~15 kDa), high solubility, and thermostability make them suitable for expression in microbial systems and for targeting otherwise inaccessible antigens.
VHHs are used extensively in imaging, oncology, and therapeutic delivery. Their modularity allows for construction of multivalent and bispecific formats with improved pharmacological profiles. Notably, VHHs penetrate tissues efficiently and can be engineered for intracellular targeting under reducing conditions, which are capabilities that conventional IgGs often lack.
However, due to their camelid origin, VHHs exhibit sequence divergence from human germline immunoglobulins. While some VHH frameworks share up to 95% identity with the human IGHV3 gene family, distinct features such as long CDR3 loops, exposed VH:VL interfaces, and non-canonical disulfide bonds increase the risk of immunogenicity and anti-drug antibody (ADA) development in humans.
Humanization of VHHs aims to reduce this immunogenic potential while preserving antigen affinity and structural integrity. This is particularly important for therapeutics requiring systemic or repeated dosing.
Related: Construction of a Synthetic Phage-Displayed VHH Library
The human immune system may recognize camelid-derived VHHs as foreign, particularly due to structural motifs not commonly present in human VH domains. These include:
Exposed VH:VL interfaces that are normally buried in conventional IgGs.
Long, diverse CDR3 regions that can introduce novel conformational epitopes.
Non-human Hallmark residues in Framework Region 2 (FR2) at positions 42, 49, 50, and 52 (IMGT numbering).
Studies have demonstrated that immunogenicity is not solely determined by species origin. Some fully human antibodies still elicit ADAs, while some non-human VHHs do not. Aggregation, epitope novelty, and T-cell epitope content all contribute.
Rossotti et al. (2021) highlighted clinical trial data where humanized and non-humanized VHHs exhibited minimal immunogenicity (<3% neutralizing ADAs in most trials). However, potent VHHs such as TAS266 (anti-DR5) and GSK1995057 (anti-TNFR1) encountered ADA-related setbacks, underscoring the importance of early risk mitigation.
In silico T-cell epitope prediction tools (e.g., NetMHCIIpan, IEDB) can identify potential immunodominant regions. However, these predictions must be validated experimentally, as they often overestimate immunogenic potential.
VHHs feature tight CDR-framework interactions. Unlike in conventional antibodies where CDRs protrude from the framework, VHH paratopes, especially CDR3, often fold over the VL interface, forming intricate networks supported by conserved framework residues.
Disruption of these networks during humanization can reduce binding affinity or destabilize the protein. For instance:
Hallmark mutations at positions 42 and 52 can displace CDR3 and reduce affinity.
Non-canonical disulfide bonds, common in ~25% of camelid VHHs, provide conformational stabilization.
These features necessitate a structure-informed humanization strategy that accounts for both the thermodynamic and functional impact of framework modifications.
Related: Camelid VHH Antibodies and scFvs: Structural Features, Applications, and Limitations
This approach involves grafting the antigen-specific CDRs from the parental VHH onto a human VH framework. However, because of the unique solubility-enhancing and stabilizing roles of VHH frameworks, full grafting is rarely sufficient.
Vincke et al. (2009) showed that:
Substitution of Hallmark residues E49G and H50L increased stability.
Humanizing positions F42V and G52W was detrimental to antigen binding due to CDR3 repositioning.
Hence, strategic back-mutations are often necessary to recover lost affinity or solubility.
Structural data from X-ray crystallography or AlphaFold2 modeling enables identification of residues critical for antigen interaction and conformational stability.
Fernández-Quintero et al. (2024) humanized two llama-derived anti-NKp30 VHHs by:
Mapping Vernier zone residues near the base of CDRs.
Evaluating the impact of non-canonical disulfide bonds on CDR3 conformation.
Performing stepwise humanization while monitoring affinity (BLI), expression yield, and SEC purity.
Key insights:
Hallmark residues in VHH1 directly interacted with antigen; those in VHH2 stabilized CDR3.
Some Vernier zone mutations (e.g., F78I) reduced binding affinity.
VHH1 tolerated humanization up to v1.9 with modest affinity loss (≤4.6×).
AI and molecular modeling tools increasingly guide humanization design:
SUMO pipeline (Merck): Evaluates multiple parameters for sequence optimization, including developability metrics.
T-cell epitope mapping: Identifies immunogenic hot spots.
Molecular dynamics: Predicts CDR flexibility, stability, and structural consequences of mutations.
Low aggregation and rapid clearance contribute to the low immunogenicity of sdAbs. However, improper humanization can induce aggregation, neutralizing the benefits of sequence humanness.
After sequence design, comprehensive in vitro and in vivo validation is essential.
Affinity Testing: Biolayer interferometry (BLI), surface plasmon resonance (SPR), and ELISA are used to confirm retention of binding.
Biophysical Profiling: SEC, DLS, and DSC to evaluate aggregation, thermostability, and purity.
Immunogenicity Assessment:
In silico TCE prediction (IEDB, NetMHCIIpan)
T-cell proliferation assays
Ex vivo dendritic cell (DC) activation tests
In Vivo Models: Rodent or primate studies to detect ADA formation.
Rossotti et al. reported that aggregation-prone VHHs are more immunogenic. Soluble monomeric formats tend to exhibit minimal ADA development even in multi-dose studies.
Humanization of camelid-derived VHHs is a critical step for minimizing immunogenicity while preserving therapeutic function. Strategies such as CDR grafting, structure-guided residue selection, and AI-supported design provide robust frameworks for balancing humanness, solubility, and affinity.
Structural motifs including Hallmark residues, Vernier zones, and non-canonical disulfide linkages must be considered during design. Evaluation must extend beyond sequence similarity to include stability, biophysical properties, and immunological response.
As computational models and structural data improve, more predictive and efficient humanization workflows are emerging. These efforts will enable safer, more effective single-domain antibody therapeutics.
What is the goal of humanizing VHH antibodies?
To reduce immunogenicity in humans by modifying camelid sequences to more closely resemble human VH domains while retaining function.
How do VHHs differ structurally from human VH domains?
VHHs lack a paired VL domain, often have longer CDR3s, distinct framework residues, and may contain non-canonical disulfide bonds.
Is humanization always required for camelid-derived antibodies?
Not always. Imaging agents with rapid clearance may not need humanization, but systemic therapeutics generally do.
What’s the difference between CDR grafting and framework resurfacing?
CDR grafting moves only the antigen-binding loops, while resurfacing replaces solvent-exposed framework residues to reduce immunogenicity without altering CDRs.
References:
Rossotti, M.A., Bélanger, K., Henry, K.A. and Tanha, J. (2022), Immunogenicity and humanization of single-domain antibodies. FEBS J, 289: 4304-4327. https://doi.org/10.1111/febs.15809
Fernández-Quintero, M. L., Guarnera, E., Musil, D., Pekar, L., Sellmann, C., Freire, F., Sousa, R. L., Santos, S. P., Freitas, M. C., Bandeiras, T. M., Silva, M. M. S., Loeffler, J. R., Ward, A. B., Harwardt, J., Zielonka, S., & Evers, A. (2024). On the humanization of VHHs: Prospective case studies, experimental and computational characterization of structural determinants for functionality. Protein science : a publication of the Protein Society, 33(11), e5176. https://doi.org/10.1002/pro.5176
Vincke, C., Loris, R., Saerens, D., Martinez-Rodriguez, S., Muyldermans, S., & Conrath, K. (2009). General Strategy to Humanize a Camelid Single-domain Antibody and Identification of a Universal Humanized Nanobody Scaffold. Journal of Biological Chemistry, 284(5), 3273–3284. https://doi.org/10.1074/jbc.m806889200
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