This format consists of the variable heavy (V_H) and variable light (V_L) chains joined by a flexible peptide linker. The scfv format is popular due to its small size and efficient expression in E. coli systems.

A Fab consists of one constant and one variable domain of each of the heavy and light chains. The fab antibody is often preferred when the stability of a full immunoglobulin-like structure is required, though it can be more challenging to express than smaller fragments.

Derived from camelid heavy-chain-only antibodies, the VHH is the smallest functional antigen-binding unit. The VHH antibody is highly stable, can access hidden epitopes, and is relatively easy to sequence using HTS because it is encoded by a single gene.

HTS allows researchers to verify that the amino acid distribution at randomized positions matches the intended design. For a library with a theoretical diversity of 10⁹ variants, HTS can sample several million reads to calculate the "evenness" of the distribution and identify redundant sequences that might dominate the phage display library screening later on.

The core of phage display is the iterative selection process known as biopanning. In a typical experiment, a library is exposed to an immobilized target antigen. Non-binders are washed away, and binders are eluted and amplified in bacteria.

Fig 2 High-Throughput Sequencing in Phage Display

HTS is used during biopanning to track the enrichment of specific clones across multiple rounds. Instead of picking individual colonies after the third or fourth round, the entire pool of phagemid DNA is isolated and sequenced. This provides several technical advantages at the bench:

HTS can identify clones that show a significant increase in frequency as early as the second round. These clones might be lost in later rounds due to growth competition in the amplification step, even if they have high affinity for the target.

By comparing the frequency of a scfv antibody or a vhh antibody across rounds, researchers can observe how the library population converges. A successful selection usually shows a decrease in unique sequences and a sharp increase in the copy number of a few dominant families.

If HTS shows that the library is not converging after three rounds of biopanning, the scientist might decide to increase wash stringency. This could involve increasing the concentration of surfactants like Tween-20 to 0.1% or 0.5% or extending the duration of the incubation steps.

Some clones, such as a specific fab antibody variant, might enrich not because they bind the antigen, but because they have a growth advantage in E. coli. HTS helps identify these "false positives" by highlighting clones that enrich in "no-antigen" control selections.

Once the raw HTS data is obtained, bioinformatics pipelines are used to group sequences into families based on CDR (Complementarity-Determining Region) similarity. This is particularly important for a vhh antibody or an scfv where the CDR3 region of the heavy chain typically drives binding specificity.

In a standard phage display library screening workflow, the following parameters are often evaluated:

The total number of reads for a specific sequence.

The frequency of a sequence in Round N divided by its frequency in Round N-1. A high enrichment ratio is often a better predictor of affinity than simple abundance.

Grouping similar sequences to identify consensus motifs. For example, a fab antibody might belong to a cluster where several variations of the same framework exist, suggesting a robust binding solution.

Practically speaking, the transition from HTS data to a physical antibody for testing involves synthesizing the most promising sequences. This "next-generation" approach bypasses the need for large-scale colony picking and manual ELISA screening of thousands of clones. Researchers can instead focus their efforts on characterizing 50 to 100 highly diverse and enriched candidates.

There is a slight risk of over-relying on frequency data. Sometimes the most frequent scfv antibody in the final pool is not the one with the highest affinity. It might just be the one that was most soluble during the phage assembly process. Therefore, it is common practice to select candidates from different phylogenetic clusters identified in the HTS data to ensure a broad coverage of the epitope.

HTS has improved the efficiency of phage display. However, there are practical considerations to keep in mind. The length of the sequencing read is a major factor. For a VHH, a single Illumina MiSeq read (2x300 bp) can often cover the entire variable region. But for a Fab or a scfv, the distance between VH and VL sequences is often too large. It exceeds the capacity of standard short-read sequencing.

To solve this, researchers sometimes sequence the VH and VL pools separately. The downside is that the original pairing of the heavy and light chains is lost. Alternatively, long-read platforms like PacBio or Oxford Nanopore can be used. These platforms sequence the entire fab antibody or scfv antibody gene in one piece. You should note that these platforms generally have higher error rates than Illumina.

Another factor is the bias introduced during library preparation for sequencing. The number of PCR cycles used to add adapters for HTS should be kept to a minimum. Often, this means using less than 15-20 cycles. This is done to avoid distorting the relative abundance of the clones. Using Unique Molecular Identifiers (UMIs) is a helpful strategy. UMIs are used to count individual DNA molecules. This corrects for the PCR duplication.

The application of high-throughput sequencing has significantly refined the phage display workflow. By providing a deep, quantitative view of the selection process, HTS allows for a more informed transition from a large antibody library to a few high-quality leads. It enhances every stage of the process, from the initial characterization of a vhh, scfv, or Fab library to the final stages of phage display library screening.

Although technical challenges regarding read length and PCR bias remain, the ability to monitor biopanning in real-time has made the discovery of therapeutic proteins more predictable and efficient. Basically, the marriage of these two technologies ensures that the most promising vhh antibody or scfv antibody candidates are identified with greater speed and accuracy than ever before.

Alpha Lifetech has established a mature Antibody Discovery Platform to accelerate the high-affinity scFv, Fab, or VHH antibodies for your research projects. Utilizing advanced Phage Display Technology, we efficiently screen for both scFv, Fab and VHH antibodies.

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[3] Fahad AS, Gutiérrez-Gonzalez MF, Madan B, DeKosky BJ. Beyond Single Clones: High-Throughput Sequencing in Antibody Discovery. Cold Spring Harb Protoc. 2025 Jan 2;2025(1):pdb.top107772. doi: 10.1101/pdb.top107772. PMID: 39586681; PMCID: PMC12110794.
[4] Ngubane NA, Gresh L, Ioerger TR, et al. High-throughput sequencing enhanced phage display identifies peptides that bind mycobacteria. PLoS One. 2013 Nov 12;8(11):e77844. doi: 10.1371/journal.pone.0077844. PMID: 24265677; PMCID: PMC3827053.