Phage display

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Phage display cycle. 1) fusion proteins for a viral coat protein + the gene to be evolved (typically an antibody fragment) are expressed in bacteriophage. 2) the library of phage are washed over an immobilised target. 3) the remaining high-affinity binders are used to infect bacteria. 4) the genes encoding the high-affinity binders are isolated. 5) those genes may have random mutations introduced and used to perform another round of evolution. The selection and amplification steps can be performed multiple times at greater stringency to isolate higher-affinity binders.

Phage display is a laboratory technique for the study of

amplified in a process called in vitro selection, which is analogous to natural selection
.

The most common bacteriophages used in phage display are

T4,[4] T7, and λ
phage have also been used.

History

Phage display was first described by

therapeutic protein engineering. Smith and Winter were awarded a half share of the 2018 Nobel Prize in chemistry for their contribution to developing phage display.[7] A patent by George Pieczenik claiming priority from 1985 also describes the generation of peptide libraries.[8]

Principle

Like the

DNA hybrid is then inserted (a process known as "transduction") into E. coli bacterial cells such as TG1, SS320, ER2738, or XL1-Blue E. coli. If a "phagemid" vector is used (a simplified display construct vector) phage particles will not be released from the E. coli cells until they are infected with helper phage, which enables packaging of the phage DNA and assembly of the mature virions
with the relevant protein fragment as part of their outer coat on either the minor (pIII) or major (pVIII) coat protein. By immobilizing a relevant DNA or protein target(s) to the surface of a
microtiter plate well, a phage that displays a protein that binds to one of those targets on its surface will remain while others are removed by washing. Those that remain can be eluted, used to produce more phage (by bacterial infection with helper phage) and to produce a phage mixture that is enriched with relevant (i.e. binding) phage. The repeated cycling of these steps is referred to as 'panning'
, in reference to the enrichment of a sample of gold by removing undesirable materials. Phage eluted in the final step can be used to infect a suitable bacterial host, from which the phagemids can be collected and the relevant DNA sequence excised and sequenced to identify the relevant, interacting proteins or protein fragments.[citation needed]

The use of a helper phage can be eliminated by using 'bacterial packaging cell line' technology.[9]

Elution can be done combining low-pH elution

buffer with sonification, which, in addition to loosening the peptide-target interaction, also serves to detach the target molecule from the immobilization surface. This ultrasound-based method enables single-step selection of a high-affinity peptide.[10]

Applications

Applications of phage display technology include determination of interaction partners of a protein (which would be used as the immobilised phage "bait" with a DNA library consisting of all

protein-DNA interactions[16] using specially-constructed DNA libraries with randomised segments. Recently, phage display has also been used in the context of cancer treatments - such as the adoptive cell transfer approach.[17] In these cases, phage display is used to create and select synthetic antibodies that target tumour surface proteins.[17] These are made into synthetic receptors for T-Cells collected from the patient that are used to combat the disease.[18]

Competing methods for in vitro protein evolution include yeast display, bacterial display, ribosome display, and mRNA display.[citation needed]

Antibody maturation in vitro

The invention of

tetanus toxin and the method was then extended to rapidly clone human anti-HIV-1 antibodies for vaccine design and therapy.[23][24][25][26][27]

Phage display of antibody libraries has become a powerful method for both studying the immune response as well as a method to rapidly select and evolve human antibodies for therapy. Antibody phage display was later used by Carlos F. Barbas at The Scripps Research Institute to create synthetic human antibody libraries, a principle first patented in 1990 by Breitling and coworkers (Patent CA 2035384), thereby allowing human antibodies to be created in vitro from synthetic diversity elements.[28][29][30][31]

Antibody libraries displaying millions of different antibodies on phage are often used in the pharmaceutical industry to isolate highly specific therapeutic antibody leads, for development into antibody drugs primarily as anti-cancer or anti-inflammatory therapeutics. One of the most successful was

TNF alpha, was the world's first fully human antibody[32] to achieve annual sales exceeding $1bn.[33]

General protocol

Below is the sequence of events that are followed in phage display screening to identify polypeptides that bind with high affinity to desired target protein or DNA sequence:[citation needed]

  1. Target proteins or DNA sequences are immobilized to the wells of a
    microtiter plate
    .
  2. Many genetic sequences are expressed in a bacteriophage library in the form of fusions with the bacteriophage coat protein, so that they are displayed on the surface of the viral particle. The protein displayed corresponds to the genetic sequence within the phage.
  3. This phage-display library is added to the dish and after allowing the phage time to bind, the dish is washed.
  4. Phage-displaying proteins that interact with the target molecules remain attached to the dish, while all others are washed away.
  5. Attached phage may be
    eluted
    and used to create more phage by infection of suitable bacterial hosts. The new phage constitutes an enriched mixture, containing considerably less irrelevant phage (i.e. non-binding) than were present in the initial mixture.
  6. Steps 3 to 5 are optionally repeated one or more times, further enriching the phage library in binding proteins.
  7. Following further bacterial-based amplification, the DNA within the interacting phage is sequenced to identify the interacting proteins or protein fragments.

Selection of the coat protein

Filamentous phages

pIII

pIII is the protein that determines the infectivity of the virion. pIII is composed of three domains (N1, N2 and CT) connected by glycine-rich linkers.[34] The N2 domain binds to the F pilus during virion infection freeing the N1 domain which then interacts with a TolA protein on the surface of the bacterium.[34] Insertions within this protein are usually added in position 249 (within a linker region between CT and N2), position 198 (within the N2 domain) and at the N-terminus (inserted between the N-terminal secretion sequence and the N-terminus of pIII).[34] However, when using the BamHI site located at position 198 one must be careful of the unpaired Cysteine residue (C201) that could cause problems during phage display if one is using a non-truncated version of pIII.[34]

An advantage of using pIII rather than pVIII is that pIII allows for monovalent display when using a phagemid (plasmid derived from Ff phages) combined with a helper phage. Moreover, pIII allows for the insertion of larger protein sequences (>100 amino acids)[35] and is more tolerant to it than pVIII. However, using pIII as the fusion partner can lead to a decrease in phage infectivity leading to problems such as selection bias caused by difference in phage growth rate[36] or even worse, the phage's inability to infect its host.[34] Loss of phage infectivity can be avoided by using a phagemid plasmid and a helper phage so that the resultant phage contains both wild type and fusion pIII.[34]

cDNA has also been analyzed using pIII via a two complementary leucine zippers system,[37] Direct Interaction Rescue[38] or by adding an 8-10 amino acid linker between the cDNA and pIII at the C-terminus.[39]

pVIII

pVIII is the main coat protein of Ff phages. Peptides are usually fused to the N-terminus of pVIII.[34] Usually peptides that can be fused to pVIII are 6-8 amino acids long.[34] The size restriction seems to have less to do with structural impediment caused by the added section[40] and more to do with the size exclusion caused by pIV during coat protein export.[40] Since there are around 2700 copies of the protein on a typical phages, it is more likely that the protein of interest will be expressed polyvalently even if a phagemid is used.[34] This makes the use of this protein unfavorable for the discovery of high affinity binding partners.[34]

To overcome the size problem of pVIII, artificial coat proteins have been designed.[41] An example is Weiss and Sidhu's inverted artificial coat protein (ACP) which allows the display of large proteins at the C-terminus.[41] The ACP's could display a protein of 20kDa, however, only at low levels (mostly only monovalently).[41]

pVI

pVI has been widely used for the display of cDNA libraries.[34] The display of cDNA libraries via phage display is an attractive alternative to the yeast-2-hybrid method for the discovery of interacting proteins and peptides due to its high throughput capability.[34] pVI has been used preferentially to pVIII and pIII for the expression of cDNA libraries because one can add the protein of interest to the C-terminus of pVI without greatly affecting pVI's role in phage assembly. This means that the stop codon in the cDNA is no longer an issue.[42] However, phage display of cDNA is always limited by the inability of most prokaryotes in producing post-translational modifications present in eukaryotic cells or by the misfolding of multi-domain proteins.

While pVI has been useful for the analysis of cDNA libraries, pIII and pVIII remain the most utilized coat proteins for phage display.[34]

pVII and pIX

In an experiment in 1995, display of Glutathione S-transferase was attempted on both pVII and pIX and failed.[43] However, phage display of this protein was completed successfully after the addition of a periplasmic signal sequence (pelB or ompA) on the N-terminus.[44] In a recent study, it has been shown that AviTag, FLAG and His could be displayed on pVII without the need of a signal sequence. Then the expression of single chain Fv's (scFv), and single chain T cell receptors (scTCR) were expressed both with and without the signal sequence.[45]

PelB (an amino acid signal sequence that targets the protein to the periplasm where a signal peptidase then cleaves off PelB) improved the phage display level when compared to pVII and pIX fusions without the signal sequence. However, this led to the incorporation of more helper phage genomes rather than phagemid genomes. In all cases, phage display levels were lower than using pIII fusion. However, lower display might be more favorable for the selection of binders due to lower display being closer to true monovalent display. In five out of six occasions, pVII and pIX fusions without pelB was more efficient than pIII fusions in affinity selection assays. The paper even goes on to state that pVII and pIX display platforms may outperform pIII in the long run.[45]

The use of pVII and pIX instead of pIII might also be an advantage because virion rescue may be undertaken without breaking the virion-antigen bond if the pIII used is wild type. Instead, one could cleave in a section between the bead and the antigen to elute. Since the pIII is intact it does not matter whether the antigen remains bound to the phage.[45]

T7 phages

The issue of using Ff phages for phage display is that they require the protein of interest to be translocated across the bacterial inner membrane before they are assembled into the phage.[46] Some proteins cannot undergo this process and therefore cannot be displayed on the surface of Ff phages. In these cases, T7 phage display is used instead.[46] In T7 phage display, the protein to be displayed is attached to the C-terminus of the gene 10 capsid protein of T7.[46]

The disadvantage of using T7 is that the size of the protein that can be expressed on the surface is limited to shorter peptides because large changes to the T7 genome cannot be accommodated like it is in M13 where the phage just makes its coat longer to fit the larger genome within it. However, it can be useful for the production of a large protein library for scFV selection where the scFV is expressed on an M13 phage and the antigens are expressed on the surface of the T7 phage.[47]

Bioinformatics resources and tools

Databases and computational tools for mimotopes have been an important part of phage display study.[48] Databases,[49] programs and web servers[50] have been widely used to exclude target-unrelated peptides,[51] characterize small molecules-protein interactions and map protein-protein interactions. Users can use three dimensional structure of a protein and the peptides selected from phage display experiment to map conformational epitopes. Some of the fast and efficient computational methods are available online.[50]

See also

Competing techniques:

References

Further reading

External links