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M13 Phage Display Library Construction for High-Affinity Monoclonal Antibody Discovery

2026-05-15

IntroductionDISPLAY

In the 1960s, Rodney Porter and Gerald Edelman successfully revealed the Y‑shaped chemical structure and amino acid sequence of antibodies through enzymatic digestion and sequencing, for which they were awarded the 1972 Nobel Prize in Physiology or Medicine.

A monoclonal antibody (mAb) is a highly homogeneous antibody produced by a single B‑cell clone, targeting a single specific epitope. Its production typically employs hybridoma technology. By clonal expansion of a single hybridoma cell, a large population of cells with identical genetic traits and secreting the same antibody can be obtained; the antibody produced in this way is a monoclonal antibody. Mouse‑derived hybridomas were the first reliable source of stable monoclonal antibodies, and the resulting mAbs have been widely used in various in vivo therapeutic applications.

Fig 1 Schematic Diagram of a Full‑Length Antibody

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Phage DisplayDISPLAY

Classification of Bacteriophages

The fundamental identity of a bacteriophage is determined by its genetic material (RNA or DNA); the two do not coexist in the same particle. Based on this criterion, bacteriophages can be classified into the following four types:

dsDNA

Double‑stranded DNA. This is the most common class, accounting for >95% of known phages. Representative examples include T2, T4, and λ.

ssDNA

Single‑stranded DNA. Representative: ΦX174.

dsRNA

Double‑stranded RNA. Representative: Φ6 (Pseudomonas phage).

ssRNA

Single‑stranded RNA. Representatives: MS2, Qβ.

M13 phage is a filamentous phage whose genetic material is circular ssDNA. The ssDNA encodes various proteins, including phage coat proteins and proteins involved in the phage packaging process. In display technologies applied in recent years, the foreign gene is typically inserted into the end of the coat protein G3P gene, thereby displaying the protein on the phage surface.

Fig 2 Schematic diagram of M13 phage display

Advantages of Phage Display

Compared to traditional hybridoma methods, phage display technology offers several advantages.

Table 1 Comparison between phage display and hybridoma technology

Technical Feature	Phage Display	Hybridoma Technology
Screening Format	In vitro	In vivo (requires animal immunization)
Time Efficiency	Fast, 10-12 weeks	4-6 months
Antibody Source	Species selectable	Mousederived, requires humanization
Challenging Targets	Can be screened	Often limited by immune tolerance
Library/Clone Diversity	High, 10⁹-10¹¹	Relatively low

Phage Display and Screening – Antibody Fragments

Immunogen Preparation

A target molecule with good immunogenicity needs to be prepared as an immunogen. The quality of the immunogen is key to obtaining high‑quality antibodies.

Animal Immunization

Select healthy animals with a well‑documented immunological background for immunization. Pre‑immune serum should be collected as a negative control. Immunization is performed by dispersed multi‑site injections on the back, once every two weeks.

PBMC Isolation

After immunization, serum is tested for antibody titer. Upon passing the titer test, whole blood is collected and peripheral blood mononuclear cells (PBMCs) are isolated using lymphocyte separation medium.

RNA Extraction and cDNA Library Preparation

Total RNA is extracted from PBMCs using an RNase‑free RNA extraction kit, reverse‑transcribed into cDNA, and the VH and VL regions of the single‑chain antibody are obtained by two rounds of nested PCR.

Phage Library Construction

The single‑chain antibody library is constructed based on phage display technology (especially M13 phage display). The amplified VH/VL fragments are ligated into a phage vector by homologous recombination, so that scFv antibodies are displayed on the phage surface. The immune library typically reaches a size of 10⁹-10¹⁰, with library diversity >90%, insertion rate of 95%, and positivity rate >95%.

Phage Library Screening

There are many screening methods, including solid‑phase, liquid‑phase, magnetic bead‑based, and cell‑based screening. One round consists of negative selection followed by positive selection. Typically, 3‑4 rounds are sufficient to obtain scFv antibody sequences with high affinity and specificity for the target molecule.

Fig 3 Schematic diagram of phage library screening

Expression of Monoclonal AntibodiesDISPLAY

After obtaining high‑affinity scFv sequences, the corresponding heavy chain constant region fragments of the appropriate species can be added to express full‑length monoclonal antibodies in vitro. This approach yields monoclonal antibodies with preserved structure and a guaranteed number of high‑affinity clones, while also reducing the time required.

Alpha Lifetech provides phage display and screening services. In addition, it offers downstream services such as antibody expression, antibody humanization, antibody affinity maturation, and other one‑stop solutions to meet diverse customer experimental needs, supporting customers on their research journey.

FAQsDISPLAY

1. When constructing an antibody library, how should one choose the display coat protein (pIII vs pVIII)?

(i) pIII Protein:
Located at the tip of the phage, present in only 3‑5 copies. It is commonly used to display antibody fragments (e.g., scFv, Fab). The monovalent display property is beneficial for isolating high‑affinity antibodies because it eliminates the “avidity effect” associated with multivalent display, thus avoiding enrichment of low‑affinity clones.

(ii) pVIII Potein:
The major coat protein, present in approximately 2700 copies on the phage surface. It is generally used for displaying short peptides. Although multivalent display can strongly amplify signals, the affinity of the selected peptides may be lower.
2. Should one choose an immune library or a naïve library for library construction? What are their respective characteristics?

(i) Immune Library:
Constructed after immunizing animals (e.g., mouse, rabbit, alpaca) with an antigen. The library is already enriched with specific antibody genes against that antigen, and the isolated antibodies usually exhibit high affinity (down to nM or even pM levels). It is suitable for targets with clear immunogenicity, but the construction cycle is relatively long.

(ii) Naïve Library:
Derived from unimmunized healthy donors (human or animal). In theory, it contains antibody genes against virtually all antigens in nature. Its greatest advantage is the ability to rapidly isolate fully human antibodies without the need for animal immunization. However, obtaining high‑affinity antibodies is more challenging, and affinity maturation steps are often required to improve affinity.
3. After screening, how can the affinity and specificity of the obtained antibody fragments be characterized?

(i) Binding Specificity Verification:
Use methods such as competitive ELISA or Western blot to confirm whether the antibody fragment specifically recognizes the target protein, including analysis of binding to conformational or linear epitopes on the target.

(ii) Binding Affinity Measurement:
Determine the dissociation constant (KD) to evaluate the tightness of binding between the antibody and the target. Common techniques include surface plasmon resonance (SPR), bio‑layer interferometry (BLI), or other quantitative methods.

(iii) Gene Sequencing Confirmation:
Sequence the antibody gene to verify the correctness and framework integrity of the antibody sequence.
4. How can the enrichment efficiency of high‑affinity clones be improved during the panning process?

(i) Use High‑Efficiency Helper Phage:
Ordinary helper phages (e.g., M13KO7) produce a large number of “empty” phage particles that do not display antibody fragments. Using helper phages such as Hyperphage ensures that nearly all phage particles display antibody fragments on their surface, increasing the effective display ratio from 30‑50% to over 90%.

(ii) Perform Monovalent Display:
Employ the pIII display system and low helper phage infection conditions to achieve monovalent display, eliminating the “avidity effect” and thereby screening for true, high‑affinity antibody clones.

(iii) Stringent Washing:
During panning, remove weakly bound phages by increasing the number of washes, raising the detergent concentration (e.g., 0.5% Tween‑20), or extending the washing time.
5. After obtaining antibody fragments from screening, how can their affinity be further improved through affinity maturation?

Introduce mutations, randomization, or chain shuffling into the initially isolated antibody genes to construct a secondary library, and then perform another round of more stringent panning. For example, introduce random mutations into the complementary determining regions (CDRs) of the antibody, or perform chain shuffling using naturally occurring antibody genes. After multiple rounds of selection, mutants with an affinity increase of 5‑fold or even higher can be obtained.

ReferenceDISPLAY

[1] Spadiut O, Capone S, Krainer F, Glieder A, Herwig C. Microbials for the production of monoclonal antibodies and antibody fragments. Trends Biotechnol. 2014 Jan;32(1):54-60.
[2] Ledsgaard L, Kilstrup M, Karatt-Vellatt A, McCafferty J, Laustsen AH. Basics of Antibody Phage Display Technology. Toxins. 2018; 10(6):236.
[3] Sawada T, Oyama R, Tanaka M, Serizawa T. Discovery of Surfactant-Like Peptides from a Phage-Displayed Peptide Library. Viruses. 2020 Dec 15;12(12):1442.