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Humanized antibodies usually lose affinity in the graft. How do you get it back?

By treating recovery as a selection problem rather than a modelling problem. Abwiz Bio grafts your mouse CDRs onto a human acceptor germline framework and restores binding-critical Vernier and interface residues by back-mutation — and where the graft still costs affinity, we rebuild it with STEM™, which can also humanize the CDRs themselves, lowering immunogenicity further than framework grafting alone can. Our humanization success rate is 15 of 15. You receive humanized IgG (approximately 1 mg), ELISA and kinetic data, and full VH/VL sequences.

Why the graft costs affinity

CDR grafting rests on a simplification: that specificity lives in the CDRs and the framework is scaffolding. It is close enough to be useful and wrong enough to cause the classic failure.

The framework positions the CDR loops. A set of framework residues underneath the loops — the Vernier zone — determines their conformation, and the VH/VL interface residues set the relative orientation of the two domains. Swap the mouse framework for a human one and those supporting residues change, the loops shift by fractions of an ångström, and affinity drops by one or two orders of magnitude even though every CDR residue was faithfully transferred.

Back-mutation is the standard remedy: return specific human framework positions to their mouse identity. But each back-mutation reintroduces non-human sequence, so the process is a negotiation between affinity and immunogenicity, and hand-guessing which positions matter can consume many rounds of build-and-measure.

How we humanize

• Acceptor framework selection. We choose a human germline framework based on similarity to the parent and on structural compatibility with the CDR canonical classes, so the graft starts from the closest available geometry rather than from a generic acceptor.
• Structure-guided back-mutation. Vernier zone and VH/VL interface positions are evaluated with a local AlphaFold instance plus germline and structural bioinformatics, so back-mutations are restricted to positions that actually support the loops instead of being applied broadly.
• Affinity recovery by selection. Where the graft still costs binding, STEM™ builds variant libraries around the humanized construct and selects for restored affinity on your antigen — an empirical search across a large sequence space rather than a handful of hand-designed variants.
• CDR humanization. STEM™ CDR libraries are designed from human antibody amino-acid usage, so the CDRs themselves can be made more human while function is held constant by the selection. This addresses the immunogenicity that framework grafting alone leaves behind, since CDRs are frequently the dominant T-cell epitope source in a humanized molecule.
• Developability filters throughout. Thermostability and polyreactivity filters run at every stage, so the humanized clone does not arrive with new aggregation or specificity problems.

Beyond humanization, in the same campaign

Because affinity recovery already runs a STEM™ selection, other properties can be selected for at the same time rather than in later projects:

• Cross-species reactivity — add cyno or mouse ortholog binding during humanization, avoiding the need to raise and defend a separate surrogate antibody for toxicology.
• Developability optimization — aggregation, high-concentration viscosity, sequence liabilities and PTM susceptibility.
• Expression rescue — humanized constructs sometimes titer worse than the parent, and the same library can fix it.
• pH-dependent binding — for recycling or tumour-microenvironment release.

We also humanize other source species: rabbit monoclonals, where well-defined canonical CDRs from a few germlines make the graft comparatively straightforward, and llama and alpaca VHH domains. If you need a fast function or isotype check before committing to full humanization, chimeric antibody generation is the shorter route.

You receive

• Humanized IgG (approximately 1 mg)
• ELISA and kinetic data, benchmarked against the parent mouse antibody
• Full VH/VL sequences
• Documentation of framework choice and back-mutations made
• No downstream royalties, stage-gated approval before each next step

Frequently asked questions

Why does a humanized antibody lose affinity?

Because the framework is not inert scaffolding. Vernier-zone residues beneath the CDR loops and the VH/VL interface residues determine loop conformation and domain orientation. Replacing the mouse framework with a human one changes those supporting residues, the loops shift slightly, and affinity can fall by one to two orders of magnitude even though every CDR residue was transferred correctly.

How do you recover affinity after CDR grafting?

First by structure-guided back-mutation of Vernier and interface positions, evaluated with a local AlphaFold instance plus germline and structural bioinformatics. Where that is not sufficient, STEM builds variant libraries around the humanized construct and selects on your antigen for restored binding, which searches a far larger sequence space than hand-designed variants can.

What is your humanization success rate?

15 of 15 across species. Success is defined as delivering a humanized molecule that retains the parent function, and it is confirmed by ELISA and kinetic data against the parent antibody rather than asserted.

Can you humanize the CDRs as well as the framework?

Yes. STEM CDR libraries are designed from human antibody amino-acid usage, so CDR positions can be made more human while the selection holds function constant. This matters because CDRs are often the dominant source of T-cell epitopes in a humanized molecule, which framework grafting alone does not address.

What do I need to send you?

The VH/VL amino-acid sequence of the mouse antibody and the antigen. No hybridoma material and no animal work are required.

Can humanization be combined with other engineering?

Yes, and it is usually more efficient to do so. Cross-species reactivity, developability optimization, expression rescue and pH-dependent binding can all be selected for in the same campaign that recovers affinity. Send your sequence to info@abwizbio.com for a scoped quote.