Synthetic Nanobody Library Yields New Resin for Purifying Clotting Factor VII

Researchers in Iran have shown that antibody fragments designed entirely in a computer and built in the laboratory, without ever immunizing an animal, can be turned into a working industrial purification column for a life-saving blood-clotting protein. The study, published in the open-access journal Heliyon, describes how two camelid nanobody binders isolated from a novel fully synthetic VHH library were used to construct immunoaffinity chromatography media for recombinant coagulation factor VII, a protein administered to hemophilia patients who have developed inhibitory antibodies against factor VIII and to people with hereditary factor VII deficiency. The work demonstrates, for the first time for this target, that a synthetic discovery route can substitute for the immune libraries that dominate commercial affinity resins.

Immunoaffinity chromatography occupies a special place in modern bioprocessing. By exploiting the exquisite specificity of antibodies for their antigens, it allows a desired protein to be pulled out of a complex mixture based on epitopes displayed on its surface, a capability that applies to nearly every antigen imaginable. Yet the technique is expensive and technically demanding at industrial scale, and its success hinges on the properties of the immobilized ligand. Conventional immunoglobulins are large, fragile molecules that can denature under the harsh acidic or alkaline conditions used to strip bound product from a column, shortening resin lifespan and raising costs. The variable domain of camelid heavy-chain antibodies, known as VHH or nanobody, has emerged as a compelling alternative. These roughly 15-kilodalton fragments are small, remarkably stable, and resistant to conditions that destroy ordinary antibodies, and they have already proven their worth commercially in resins such as the CaptureSelect family used across the biopharmaceutical industry.

Recombinant factor VII presents a particularly demanding purification challenge. The protein is expressed in baby hamster kidney cells and is both thermally sensitive and structurally fragile, requiring a rapid series of downstream steps to preserve its integrity. Because the drug is administered in its active form, activation at the end of the bioprocess is essential. At least one affinity chromatography step is indispensable, and the newest commercial solution, VIISelect from GE Healthcare, relies on a camelid VHH ligand discovered from an immune llama antibody library. Immune libraries, however, require animal immunization, a drawback that limits their use against toxic, poorly immunogenic, or otherwise unsuitable antigens. Synthetic libraries sidestep this limitation entirely, allowing binders to be discovered against an essentially unrestricted range of targets through rational design of molecular diversity.

The research team, led by Aliasghar Rahimian, Ali Nabati, Hooman Askari, and Mahdi Aminian, drew on two anti-factor VII nanobodies, designated VHH19 and VHH24, which they had previously isolated from a phage-displayed synthetic VHH library built on the cAbBCII-10 scaffold. This framework is prized for its exceptional stability, its plasticity for grafting diverse antigen-binding loops, and its acceptable expression yield in bacteria. The two binders carried affinity constants of 10 to the power of minus eight and 5.8 times 10 to the power of minus eight, respectively, indicating tight binding to the coagulation factor. The team expressed the nanobodies in the periplasm of TG1 bacterial cells on a five-liter scale, recovered them through osmotic shock, and purified them sequentially on nickel-affinity resin and Sephacryl S-100 gel filtration to remove aggregates and polymers. Yields reached approximately 0.8 milligrams per liter for VHH19 and 1 milligram per liter for VHH24, with reducing SDS-PAGE revealing the expected doublet of native and disulfide-reduced forms around 15 kilodaltons.

Before committing the binders to a column format, the researchers verified their specificity by enzyme-linked immunosorbent assay. Both purified VHHs produced strong, concentration-dependent signals on wells coated with recombinant factor VII, while signals on wells coated with diluted human blood plasma, used as a source of non-specific antigens, remained at baseline across all coating concentrations. This clean discrimination against the thousands of proteins present in plasma is a critical prerequisite for any ligand intended to purify a therapeutic protein from biological feedstocks. Encouraged by these results, the team coupled each nanobody to cyanogen bromide-activated cross-linked agarose beads, a classical chemistry that converts agarose hydroxyl groups into cyanate esters reactive with protein amino groups. By measuring protein remaining in the coupling supernatant, they determined that coupling efficiency exceeded 95 percent for both binders, yielding resins containing approximately 2 milligrams of immobilized VHH per milliliter of gel.

The decisive test came in column chromatography, where the chemistry of calcium binding to the Gla domain of factor VII shaped the optimization strategy. Calcium association is essential to the conformation and enzymatic activity of the protein, so the team screened four buffering systems that varied the presence of calcium chloride and EDTA during binding and elution. The outcome was unambiguous: only the VHH19-based resin, designated Seph4B-VHH19, captured factor VII effectively, and only when calcium chloride was present in the binding and washing buffers. Under those conditions the factor VII concentration in the flow-through was depleted almost completely, whereas procedures lacking calcium produced partial or negligible adsorption. EDTA proved unable to elute the bound protein, leaving acidic glycine-hydrochloric acid buffer at pH 2.2 as the effective desorption agent. The VHH24 column and an uncoupled control resin retained the protein poorly or not at all under every condition tested, and both were excluded from further study.

With the optimal conditions established, the researchers benchmarked their resin against the commercial VIISelect medium using breakthrough curve analysis on a partially purified, inactive factor VII feed. Seph4B-VHH19 processed 28 plus or minus 1 milliliter of feed before breakthrough, yielding 2.3 plus or minus 0.15 milligrams of eluted protein per milliliter of resin, compared with 42 plus or minus 1 milliliter and 4.08 plus or minus 0.06 milligrams per milliliter for VIISelect. The authors attribute roughly half of this capacity gap to ligand density: the commercial resin reportedly carries about 6 milligrams of ligand per milliliter, double the loading achieved with their research-scale periplasmic expression system. They argue the comparison should be read as a feasibility demonstration rather than a head-to-head performance contest, noting that industrial expression improvements and more sophisticated coupling chemistries could readily narrow the difference.

Two further results stand out for bioprocess engineers. First, the resin proved operationally robust: it performed consistently across four consecutive chromatography cycles, and although cleaning with 0.1 molar sodium hydroxide before a fifth cycle cost 15 percent of the dynamic binding capacity, the column retained that reduced capacity after a full month of storage at 4 degrees Celsius. The limited alkaline tolerance marks a clear target for future sequence optimization and cleaning-procedure refinement. Second, and perhaps more striking, the column accomplished on-column activation of the drug substance. When a mixture of partially purified inactive factor VII and human plasma was passed through Seph4B-VHH19, the resin excluded a substantial portion of the plasma contaminants, and the eluted factor VII appeared as two chains of 20 to 30 kilodaltons on reducing SDS-PAGE, the signature of autocleavage. Activity assays confirmed the transformation: eluate fractions reached 26.8 kilo-international units per milliliter, roughly two hundred times the 0.12 KIU per milliliter measured in the pre-column sample, while flow-through and wash fractions carried almost none of the activity.

This activation step matters because commercial factor VII production is not complete without it; in existing downstream processes, activation occurs as a byproduct of concentration during ion-exchange chromatography. The nanobody column reproduced that behavior naturally, concentrating the protein enough to trigger autocleavage as it bound. The authors place their work in a broader context of nanobody-based affinity media, which have already been engineered for demanding targets such as adeno-associated virus serotype 8 and for separating closely related product forms of low-abundance plasma proteins. Nanobodies resist aggregation because they lack the variable light-chain domain, their frameworks can be fully humanized, and the cAbBCII-10 scaffold offers exceptional plasticity for CDR grafting. The synthetic library approach eliminates animal immunization and permits rational targeting of difficult antigens, though the absence of in vivo affinity maturation means isolated clones may need additional in vitro optimization to match the affinity profiles of immune-derived counterparts.

The study also exposes a knowledge gap that the authors hope the field will fill. Few publications assess how different conjugation methods and linkers affect the performance of VHH affinity columns, and the cyanogen bromide chemistry used here, while simple and traditional, immobilizes ligands in random orientations. Site-specific immobilization strategies, including engineered tag systems, have been shown to improve both capacity and resin stability compared with random amine coupling, and the team has published the sequence of VHH19 to encourage such exploration. For an industry that already generates billions of dollars in value from VHH-based CaptureSelect resins, the demonstration that a fully synthetic, animal-free discovery pipeline can produce a functional purification ligand for a fragile, medically critical coagulation protein signals a practical new route to affinity media, one that could extend to toxic, poorly immunogenic, or otherwise inaccessible targets where immunized llamas simply cannot help.

Subject of Research: Synthetic camelid nanobody binders for immunoaffinity purification of recombinant coagulation factor VII

Article Title: Application of camelid nanobody binders derived from a novel full-synthetic VHH library in immunoaffinity purification of recombinant coagulation factor VII

Article References: Rahimian, A., Nabati, A., Askari, H., & Aminian, M. (2026). Application of camelid nanobody binders derived from a novel full-synthetic VHH library in immunoaffinity purification of recombinant coagulation factor VII. Heliyon, 12(15), Article e45524. https://doi.org/10.1016/j.heliyon.2026.e45524

Image Credits: AI Generated

DOI: Not provided

Keywords: nanobodies, VHH, synthetic antibody library, coagulation factor VII, immunoaffinity chromatography, bioprocessing, hemophilia, affinity resin, phage display, recombinant protein purification, cyanogen bromide coupling, downstream processing