In vivo selection of sFv from phage display libraries

High throughput methods to study protein-protein interactions during host-pathogen interactions

The ability of a pathogen to survive and cause an infection is often determined by specific interactions between the host and pathogen proteins. Such interactions can be both intra- and extracellular and may define the outcome of an infection. There are a range of innovative biochemical, biophysical and bioinformatic techniques currently available to identify protein-protein interactions (PPI) between the host and the pathogen. However, the complexity and the diversity of host-pathogen PPIs has led to the development of several high throughput (HT) techniques that enable the study of multiple interactions at once and/or screen multiple samples at the same time, in an unbiased manner. We review here the major HT laboratory-based technologies employed for host-bacterial interaction studies.

Progress on Phage Display Technology: Tailoring Antibodies for Cancer Immunotherapy

The search for innovative anti-cancer drugs remains a challenge. Over the past three decades, antibodies have emerged as an essential asset in successful cancer therapy. The major obstacle in developing anti-cancer antibodies is the need for non-immunogenic antibodies against human antigens. This unique requirement highlights a disadvantage to using traditional hybridoma technology and thus demands alternative approaches, such as humanizing murine monoclonal antibodies. To overcome these hurdles, human monoclonal antibodies can be obtained directly from Phage Display libraries, a groundbreaking tool for antibody selection. These libraries consist of genetically engineered viruses, or phages, which can exhibit antibody fragments, such as scFv or Fab on their capsid. This innovation allows the in vitro selection of novel molecules directed towards cancer antigens. As foreseen when Phage Display was first described, nowadays, several Phage Display-derived antibodies have entered clinical settings or are undergoing clinical evaluation. This comprehensive review unveils the remarkable progress in this field and the possibilities of using clever strategies for phage selection and tailoring the refinement of antibodies aimed at increasingly specific targets. Moreover, the use of selected antibodies in cutting-edge formats is discussed, such as CAR (chimeric antigen receptor) in CAR T-cell therapy or ADC (antibody drug conjugate), amplifying the spectrum of potential therapeutic avenues.

In vivo Phage Display: A promising selection strategy for the improvement of antibody targeting and drug delivery properties

The discovery of hybridoma technology, described by Kohler and Milstein in 1975, and the resulting ability to generate monoclonal antibodies (mAbs) initiated a new era in antibody research and clinical development. However, limitations of the hybridoma technology as a routine antibody generation method in conjunction with high immunogenicity responses have led to the development of alternative approaches for the streamlined identification of most effective antibodies. Within this context, display selection technologies such as phage display, ribosome display, yeast display, bacterial display, and mammalian cell surface display have been widely promoted over the past three decades as ideal alternatives to traditional hybridoma methods. The display of antibodies on phages is probably the most widespread and powerful of these methods and, since its invention in late 1980s, significant technological advancements in the design, construction, and selection of antibody libraries have been made, and several fully human antibodies generated by phage display are currently approved or in various clinical development stages. With evolving novel disease targets and the emerging of a new generation of therapeutic antibodies, such as bispecific antibodies, antibody drug conjugates (ADCs), and chimeric antigen receptor T (CAR-T) cell therapies, it is clear that phage display is expected to continue to play a central role in antibody development. Nevertheless, for non-standard and more demanding cases aiming to generate best-in-class therapeutic antibodies against challenging targets and unmet medical needs, in vivo phage display selections by which phage libraries are directly injected into animals or humans for isolating and identifying the phages bound to specific tissues offer an advantage over conventional in vitro phage display screening procedures. Thus, in the present review, we will first summarize a general overview of the antibody therapeutic market, the different types of antibody fragments, and novel engineered variants that have already been explored. Then, we will discuss the state-of-the-art of in vivo phage display methodologies as a promising emerging selection strategy for improvement antibody targeting and drug delivery properties.

High-Throughput Monoclonal Antibody Discovery from Phage Libraries: Challenging the Current Preclinical Pipeline to Keep the Pace with the Increasing mAb Demand

Simple Summary

Monoclonal antibodies are increasingly used for a broad range of diseases. Rising demand must face with time time-consuming and laborious processes to isolate novel monoclonal antibodies. Next-generation sequencing coupled to phage display provides timely and sustainable high throughput selection strategy to rapidly access novel target. Here, we describe the current NGS-guided strategies to identify potential binders from enriched sub-libraires by applying a user-friendly informatic pipeline to identify and discard false positive clones. Rescue step and strategies to boost mAb yield are also discussed to improve the limiting selection and screening steps.

Abstract

Monoclonal antibodies are among the most powerful therapeutics in modern medicine. Since the approval of the first therapeutic antibody in 1986, monoclonal antibodies keep holding great expectations for application in a range of clinical indications, highlighting the need to provide timely and sustainable access to powerful screening options. However, their application in the past has been limited by time-consuming and expensive steps of discovery and production. The screening of antibody repertoires is a laborious step; however, the implementation of next-generation sequencing-guided screening of single-chain antibody fragments has now largely overcome this issue. This review provides a detailed overview of the current strategies for the identification of monoclonal antibodies from phage display-based libraries. We also discuss the challenges and the possible solutions to improve the limiting selection and screening steps, in order to keep pace with the increasing demand for monoclonal antibodies.

T7 Phage as an Emerging Nanobiomaterial with Genetically Tunable Target Specificity

Bacteriophages, also known as phages, are specific antagonists against bacteria. T7 phage has drawn massive attention in precision medicine owing to its distinctive advantages, such as short replication cycle, ease in displaying peptides and proteins, high stability and cloning efficiency, facile manipulation, and convenient storage. By introducing foreign gene into phage DNA, T7 phage can present foreign peptides or proteins site-specifically on its capsid, enabling it to become a nanoparticle that can be genetically engineered to screen and display a peptide or protein capable of recognizing a specific target with high affinity. This review critically introduces the biomedical use of T7 phage, ranging from the detection of serological biomarkers and bacterial pathogens, recognition of cells or tissues with high affinity, design of gene vectors or vaccines, to targeted therapy of different challenging diseases (e.g., bacterial infection, cancer, neurodegenerative disease, inflammatory disease, and foot-mouth disease). It also discusses perspectives and challenges in exploring T7 phage, including the understanding of its interactions with human body, assembly into scaffolds for tissue regeneration, integration with genome editing, and theranostic use in clinics. As a genetically modifiable biological nanoparticle, T7 phage holds promise as biomedical imaging probes, therapeutic agents, drug and gene carriers, and detection tools.

Phage antibody library screening for the selection of novel high-affinity human single-chain variable fragment against gastrin receptor: an in silico and in vitro study

BACKGROUND

As a membrane G protein coupled receptors (GPCRs) family, gastrin/cholecystokinin-2 receptor (CCK2R) plays a key role in the initiation and development of gastric cancer.

OBJECTIVES

Targeting CCK2R by immunotherapeutics such as single-chain variable fragments (scFvs) may provide an effective treatment modality against gastric cancer. Thus, the main objective of this study was to isolate scFvs specific to CCK2R.

METHODS

To isolate scFvs specific to the CCK2R, we capitalized on a semi-synthetic diverse phage antibody library (PAL) and a solution-phase biopanning process. The library was panned against a biotinylated peptide of the second extracellular loop (ECL2) of CCK2R. After four rounds of biopanning, the selected soluble scFv clones were screened by enzyme-linked immunosorbent assay (ELISA) and examined for specific binding to the peptide. The selected scFvs were purified using immobilized metal affinity chromatography (IMAC). The binding affinity and specificity of the scFvs were examined by the surface plasmon resonance (SPR), immunoblotting and flow cytometry assays and molecular docking using ZDOCK v3.0.2.

RESULTS

Ten different scFvs were isolated, which displayed binding affinity ranging from 0.68 to 8.0 (nM). Immunoblotting and molecular docking analysis revealed that eight scFvs were able to detect the denatured form of CCK2R protein. Of the isolated scFvs, two scFvs showed high-binding affinity to the human gastric adenocarcinoma AGS cells.

CONCLUSIONS

Based on our findings, a couple of the selected scFvs showed markedly high-binding affinity to immobilized CCK2R peptide and CCK2R-overexpressing AGS cells. Therefore, these scFvs are proposed to serve as targeting and/or treatment agents in the diagnosis and immunotherapy of CCK2R-positive tumors. Graphical abstract ᅟ.

Current state of in vivo panning technologies: Designing specificity and affinity into the future of drug targeting

Targeting ligands are used in drug delivery to improve drug distribution to desired cells or tissues and to facilitate cellular entry. In vivo biopanning, whereby billions of potential ligand sequences are screened in biologically-relevant and complex conditions, is a powerful method for identification of novel target ligands. This tool has impacted drug delivery technologies and expanded our arsenal of therapeutics and diagnostics. Within this review we will discuss current in vivo panning technologies and ways that these technologies can be improved to advance next-generation drug delivery strategies.

Advances in the T7 phage display system (Review)

The present review describes the advantages and updated applications of the T7 phage display system in bioscience and medical science. Current phage display systems are based on various bacteriophage vectors, including M13, T7, T4 and f1. Of these, the M13 phage display is the most frequently used, however, the present review highlights the advantages of the T7 system. As a phage display platform, M13 contains single‑stranded DNA, while the T7 phage consists of double‑stranded DNA, which exhibits increased stability and is less prone to mutation during replication. Additional characteristics of the T7 phage include the following: The T7 phage does not depend on a protein secretion pathway in the lytic cycle; expressed peptides and proteins are usually located on the C‑terminal region of capsid protein gp10B, which avoids problems associated with steric hindrance; and T7 phage particles exhibit high stability under various extreme conditions, including high temperature and low pH, which facilitates effective high‑throughput affinity elutriation. Recent applications of the T7 phage display system have been instrumental in uncovering mechanisms of molecular interaction, particularly in the fields of antigen discovery, vaccine development, protein interaction, and cancer diagnosis and treatment.

Phage antibody display libraries: a powerful antibody discovery platform for immunotherapy

Phage display technology (PDT), a combinatorial screening approach, provides a molecular diversity tool for creating libraries of peptides/proteins and discovery of new recombinant therapeutics. Expression of proteins such as monoclonal antibodies (mAbs) on the surface of filamentous phage can permit the selection of high affinity and specificity therapeutic mAbs against virtually any target antigen. Using a number of diverse selection platforms (e.g. solid phase, solution phase, whole cell and in vivo biopannings), phage antibody libraries (PALs) from the start point provides great potential for the isolation of functional mAb fragments with diagnostic and/or therapeutic purposes. Given the pivotal role of PDT in the discovery of novel therapeutic/diagnostic mAbs, in the current review, we provide an overview on PALs and discuss their impact in the advancement of engineered mAbs.

T4 bacteriophage as a phage display platform

Analysis of molecular events in T4-infected Escherichia coli has revealed some of the most important principles of biology, including relationships between structures of genes and their products, virus-induced acquisition of metabolic function, and morphogenesis of complex structures through sequential gene product interaction rather than sequential gene activation. T4 bacteriophages and related strains were applied in the first formulations of many fundamental biological concepts. These include the unambiguous recognition of nucleic acids as the genetic material, the definition of the gene by fine-structure mutation, recombinational and functional analyses, the demonstration that the genetic code is triplet, the discovery of mRNA, the importance of recombination and DNA replications, light-dependent and light-independent DNA repair mechanisms, restriction and modification of DNA, self-splicing of intron/exon arrangement in prokaryotes, translation bypassing and others. Bacteriophage T4 possesses unique features that make it a good tool for a multicomponent vaccine platform. Hoc/Soc-fused antigens can be assembled on the T4 capsid in vitro and in vivo. T4-based phage display combined with affinity chromatography can be applied as a new method for bacteriophage purification. The T4 phage display system can also be used as an attractive approach for cancer therapy. The data show the efficient display of both single and multiple HIV antigens on the phage T4 capsid and offer insights for designing novel particulate HIV or other vaccines that have not been demonstrated by other vector systems.

Selection and characterization of single-chain recombinant antibodies against spring viraemia of carp virus from mouse phage display library

Antibody-displaying phage library was selected after three rounds of panning against spring viraemia of carp virus (SVCV) by phage display technology. Eight positive clones which could produce soluble single-chain fragment variable (scFv) antibody induced by isopropyl-beta-d-thiogalactopyranoside (IPTG) were obtained. Dot blot results showed that the eight scFv antibodies could recognize SVCV. The soluble scFv antibodies showed a molecular weight 29 kD by Western blot. All scFv antibodies could recognize SVCV proteins specifically without cross-reaction with other virus proteins by ELISA. Indirect immunofluorescence results showed that all of these scFv antibodies reacted positively with virus in the SVCV-infected cells. These scFv antibodies will be useful tools to establish immunological detection methods for SVCV.

Proteasome activator complex PA28 identified as an accessible target in prostate cancer by in vivo selection of human antibodies

Antibody cancer therapies rely on systemically accessible targets and suitable antibodies that exert a functional activity or deliver a payload to the tumor site. Here, we present proof-of-principle of in vivo selection of human antibodies in tumor-bearing mice that identified a tumor-specific antibody able to deliver a payload and unveils the target antigen. By using an ex vivo enrichment process against freshly disaggregated tumors to purge the repertoire, in combination with in vivo biopanning at optimized phage circulation time, we have identified a human domain antibody capable of mediating selective localization of phage to human prostate cancer xenografts. Affinity chromatography followed by mass spectrometry analysis showed that the antibody recognizes the proteasome activator complex PA28. The specificity of soluble antibody was confirmed by demonstrating its binding to the active human PA28αβ complex. Whereas systemically administered control phage was confined in the lumen of blood vessels of both normal tissues and tumors, the selected phage spread from tumor vessels into the perivascular tumor parenchyma. In these areas, the selected phage partially colocalized with PA28 complex. Furthermore, we found that the expression of the α subunit of PA28 [proteasome activator complex subunit 1 (PSME1)] is elevated in primary and metastatic human prostate cancer and used anti-PSME1 antibodies to show that PSME1 is an accessible marker in mouse xenograft tumors. These results support the use of PA28 as a tumor marker and a potential target for therapeutic intervention in prostate cancer.

In vivo phage display--a discovery tool in molecular biomedicine

In vivo phage display is a high-throughput method for identifying target ligands specific for different vascular beds. Targeting is possible due to the heterogeneous expression of receptors and other antigens in a particular vascular bed. Such expression is additionally influenced by the physiological or pathological status of the vasculature. In vivo phage display represents a technique that is usable in both, vascular mapping and targeted drug development. In this review, several important methodological aspects of in vivo phage display experiments are discussed. These include choosing an appropriate phage library, an appropriate animal model and the route of phage library administration. In addition, peptides or antibodies identified by in vivo phage display homing to specific types of vascular beds, including the altered vasculature present in several types of diseases are summarized. Still, confirmation in independent experiments and reproduction of identified sequences are needed for enhancing the clinical applicability of in vivo phage display research.

By-passing large screening experiments using sequencing as a tool to identify scFv fragments targeting atherosclerotic lesions in a novel in vivo phage display selection

Atherosclerosis is a chronic, progressive inflammatory disease that may develop into vulnerable lesions leading to thrombosis. To interrogate the molecular components involved in this process, single-chain variable fragments (scFvs) from a semi-synthetic human antibody library were selected on the lesions induced in a rabbit model of atherosclerosis after two rounds of in vivo phage display. Homing Phage-scFvs were isolated from (1) the injured endothelium, (2) the underlying lesional tissue and (3) the cells within the intima. Clones selected on the basis of their redundancy or the presence of key amino acids, as determined by comparing the distribution between the native and the selected libraries, were produced in soluble form, and seven scFvs were shown to specifically target the endothelial cell surface and inflamed intima-related regions of rabbit tissue sections by immunohistology approaches. The staining patterns differed depending on the scFv compartment of origin. This study demonstrates that large-scale scFv binding assays can be replaced by a sequence-based selection of best clones, paving the way for easier use of antibody libraries in in vivo biopanning experiments. Future investigations will be aimed at characterizing the scFv/target couples by mass spectrometry to set the stage for more accurate diagnostic of atherosclerosis and development of therapeutic strategies.

In vivo phage display to identify new human antibody fragments homing to atherosclerotic endothelial and subendothelial tissues [corrected]

In vivo phage display selection is a powerful strategy for directly identifying agents that target the vasculature of normal or diseased tissues in living animals. We describe here a new in vivo biopanning strategy in which a human phage single-chain antibody (scFv) library was injected into high-fat diet-fed ApoE(-/-) mice. Extracellular and internalized phage scFvs were selectively recovered from atherosclerotic vascular endothelium and subjacent tissues. After three successive biopanning rounds, a panel of six clones with distinct gene sequences was isolated. Four scFvs produced and purified in soluble form were shown to interact in vitro with a rabbit atheromatous protein extract by time-resolved fluorescence resonance energy transfer and to target the endothelial cell surface and inflamed intima-related regions of rabbit and human tissue sections ex vivo. These new scFvs selected in a mouse model recognized both rabbit and human tissue, underlying the interspecies similarities of the recognized epitopes. By combining immunoprecipitation and mass spectrometry, one of the selected scFvs was shown to recognize carbonic anhydrase II, an up-regulated enzyme involved in resorption of ectopic calcification. These results show that in vivo biopanning selection in hypercholesterolemic animals makes it possible to identify both scFvs homing to atherosclerotic endothelial and subendothelial tissues, and lesion-associated biomarkers. Such scFvs offer promising opportunities in the field of molecular targeting for the treatment of atherosclerosis.

On the synergistic effects of ligand-mediated and phage-intrinsic properties during in vivo selection

Phage display has been used as a powerful tool in the discovery and characterization of ligand-receptor complexes that can be utilized for therapeutic applications as well as to elucidate disease mechanisms. While the basic properties of phage itself have been well described, the behavior of phage in an in vivo setting is not as well understood due to the complexity of the system. Here, we take a dual approach in describing the biophysical mechanisms and properties that contribute to the efficacy of in vivo phage targeting. We begin by considering the interaction between phage and target by applying a kinetic model of ligand-receptor complexation and internalization. The multivalent display of peptides on the pIII capsid of phage is also discussed as an augmenting factor in the binding affinity of phage-displayed peptides to cellular targets accessible in a microenvironment of interest. Lastly, we examine the physical properties of the total phage particle that facilitate improved delivery and targeting in vivo compared to free peptides.

Use of superparamagnetic beads for the isolation of a peptide with specificity to cymbidium mosaic virus

A modified method for the rapid isolation of specific ligands to whole virus particles is described. Biopanning against cymbidium mosaic virus was carried out with a commercial 12-mer random peptide display library. A solution phase panning method was devised using streptavidin-coated superparamagnetic beads. The solution based panning method was more efficient than conventional immobilized target panning when using whole viral particles of cymbidium mosaic virus as a target. Enzyme-linked immunosorbent assay of cymbidium mosaic virus-binding peptides isolated from the library identified seven peptides with affinity for cymbidium mosaic virus and one peptide which was specific to cymbidium mosaic virus and had no significant binding to odontoglossum ringspot virus. This method should have broad application for the screening of whole viral particles towards the rapid development of diagnostic reagents without the requirement for cloning and expression of single antigens.

Isolation of human antibodies to tumor-associated endothelial cell markers by in vitro human endothelial cell selection with phage display libraries

Human antibodies selectively targeting angiogenic vessels hold great promise for the immunotherapy of human malignancies and can help to elucidate the molecular mechanisms regulating angiogenesis. By selecting a large antibody phage display library on proliferating stimulated HUVEC, we have isolated 15 human antibodies that bind to HUVEC in flow cytometric analysis, 11 of which target the vasculature of colorectal carcinomas as demonstrated by immunohistochemical analysis. The four most specific antibodies, TEM-A, TEM-C, TEM-M and TEM-Q, also stain the vasculature of a series of carcinomas derived from liver, ovary, kidney, bladder, lung and breast, and either react weakly or not all with the vasculature of corresponding normal tissues. All four antibodies react with the vasculature of endometrium, but only TEM-M and TEM-Q react with the vasculature of placenta. As shown by non-reducing western blot analysis, 9/15 of the antibodies recognize either one or two distinct bands on HUVEC cell lysates, with molecular weights of 175 and/or 110-125 kDa. Antibodies identified by this approach may be used for the identification of new markers of angiogenesis and/or tumor vasculature. The selected antibodies may prove useful as new prognostic markers, for in vivo tumor imaging purposes and for further development of targeted therapies.

Epitope selection from an uncensored peptide library displayed on avian leukosis virus

Phage display libraries have provided an extraordinarily versatile technology to facilitate the isolation of peptides, growth factors, single chain antibodies, and enzymes with desired binding specificities or enzymatic activities. The overall diversity of peptides in phage display libraries can be significantly limited by Escherichia coli protein folding and processing machinery, which result in sequence censorship. To achieve an optimal diversity of displayed eukaryotic peptides, the library should be produced in the endoplasmic reticulum of eukaryotic cells using a eukaryotic display platform. In the accompanying article, we presented experiments that demonstrate that polypeptides of various sizes could be efficiently displayed on the envelope glycoproteins of a eukaryotic virus, avian leukosis virus (ALV), and the displayed polypeptides could efficiently attach to cognate receptors without interfering with viral attachment and entry into susceptible cells. In this study, methods were developed to construct a model library of randomized eight amino acid peptides using the ALV eukaryotic display platform and screen the library for specific epitopes using immobilized antibodies. A virus library with approximately 2 x 10(6) different members was generated from a plasmid library of approximately 5 x 10(6) diversity. The sequences of the randomized 24 nucleotide/eight amino acid regions of representatives of the plasmid and virus libraries were analyzed. No significant sequence censorship was observed in producing the virus display library from the plasmid library. Different populations of peptide epitopes were selected from the virus library when different monoclonal antibodies were used as the target. The results of these two studies clearly demonstrate the potential of ALV as a eukaryotic platform for the display and selection of eukaryotic polypeptides libraries.

Isolating ligands specific for human vasculature using in vivo phage selection

The endothelium lining blood vessels expresses molecules that are restricted in their expression to a particular tissue or organ. These molecules are attractive targets for therapy and diagnosis because they allow agents to be delivered specifically to the blood vessels supplying the desired tissue. However, it is difficult to identify these tissue-specific molecules because endothelium loses much of its tissue-specific nature when it is removed from the organ. This can be overcome by using in vivo phage selection - injecting libraries of phage bearing antibodies or peptides into an animal and isolating phage that bind to the relevant tissue. A variation on this approach, in which in vivo phage selection is performed in animals bearing human tissue xenografts, allows the isolation of peptides (and presumably other molecules) specific for human vasculature.

Therapeutic cancer targeting peptides

Antitumor monoclonal antibodies have shown clinical promise as cancer cell surface targeting agents. More tumor targeting antibodies are likely to be approved by the FDA in the next few years. However, there are two major limitations in antibody-targeted therapy: large size and nonspecific uptake of the antibody molecules by the liver and the reticuloendothelial system. These result in poor tumor penetration of antibody pharmaceuticals and dose-limiting toxicity to the liver and bone marrow. Peptides are excellent alternative targeting agents for human cancers, and they may alleviate some of the problems with antibody targeting. In the last decade, several investigators have successfully used combinatorial library methods to discover cell surface binding peptides that may be useful for cancer targeting. The phage-display library technique and the "one-bead one-compound" combinatorial library method are the two approaches that have been used. Cancer cell surface receptors or endothelial cell surface receptors of the neovasculature are the two popular therapeutic targets for cancer. Results from preclinical studies with some peptides are encouraging in their targeting potential.

Antibody discovery: phage display

Phage display has proven to be a robust and convenient technology for the selection of high-quality human antibodies from diverse libraries. Besides enabling the identification of antibodies in a fast, high-throughput mode, which allows comprehensive protein expression analyses, phage display has been used to identify a fully human therapeutic antibody presently undergoing the regulatory process for market approval.

Identification of synovium-specific homing peptides by in vivo phage display selection

OBJECTIVE

To identify homing peptides specific for human synovium that could be used as targeting devices for delivering therapeutic/diagnostic agents to human joints.

METHODS

Human synovium and skin were transplanted into SCID mice. A disulfide-constrained 7-amino acid peptide phage display library was injected intravenously into the animals and synovial homing phage recovered from synovial grafts. Following 3-4 cycles of enrichment, DNA sequencing of homing phage clones allowed the identification of specific peptides that were synthesized by a-fluorenylmethyloxycarbonyl chemistry and used in competitive in vivo assays and immunohistochemistry analyses.

RESULTS

We isolated synovial homing phages displaying specific peptides that distinctively bound to synovial but not skin or mouse microvascular endothelium (MVE). They retained their tissue homing specificity in vivo, independently from the phage component, the original pathology of the transplanted tissue, and the degree of human/murine graft vascularization. One such peptide (CKSTHDRLC) maintained synovial homing specificity both when presented by the phage and as a free synthetic peptide. The synthetic peptide also competed with and inhibited in vivo the binding of the parent phage to the cognate synovial MVE ligand.

CONCLUSION

This is the first report describing peptides with homing properties specific for human synovial MVE. This was demonstrated using a novel approach targeting human tissues, transplanted into SCID mice, directly by in vivo phage display selection. The identification of such peptides opens the possibility of using these sequences to construct joint-specific drug delivery systems that may have considerable impact in the treatment of arthritic conditions.

Tumor cell-targeting by phage-displayed peptides

We isolated cancer cell-specific phages by subtracting and selecting complex peptide display phage libraries on cultured human cancer cells. The best candidate was selected by performing three rounds of subtraction before each of five selections on the human colorectal WiDr cell line. The phage showed more than 1000-fold higher binding efficiency for WiDr cells when compared to five other human cancer cell lines, including two of colorectal origin, and when compared to wild-type M13 phage. Fifty-fold higher binding efficiency was also seen for a human breast cancer cell line. We show that the WiDr cell binding of the selected phage was efficiently competed by the synthetic peptide HEWSYLAPYPWF, predicted from the phage sequence. This confirms that the specificity of the peptide is independent of the display by the phage coat proteins. The identified peptide may target biomarkers linked to colorectal cancer, and thus be useful for designing gene transfer vectors as well as diagnostic and prognostic tools for this disease.

In vivo phage display and vascular heterogeneity: implications for targeted medicine

The vascular endothelium expresses differential receptors depending on the functional state and tissue localization of its cells. A method to characterize this receptor heterogeneity with phage display random peptide libraries has been developed. Using this technology, several peptide ligands have been isolated that home to tissue-specific endothelial cell receptors following intravenous administration. Such peptide ligands, or antibodies directed against specific vascular receptors, can be used to target therapeutic compounds or imaging agents to endothelial cells in vitro and in vivo. Recent advances in the field include identification of novel endothelial receptors expressed differentially in normal and pathological conditions and the isolation of peptides or antibody ligands to such receptors in in vitro assays, in animal models and in a human patient. These milestones, which extend the 'functional map' of the vasculature, should lead to clinical applications in diseases such as cancer and other conditions that exhibit distinct vascular characteristics.

Novel strategy for the selection of human recombinant Fab fragments to membrane proteins from a phage-display library

Traditionally, the selection of phage-display libraries is performed on purified antigens (Ags), immobilized to a solid substrate. However, this approach may not be applicable for some Ags, such as membrane proteins, which for structural integrity strongly rely on their native environment. Here we describe an approach for the selection of phage-libraries against membrane proteins. The envelope glycoproteins (Env) of the Human Immunodeficiency Virus type-1 (HIV-1) were used as a model for a type-1 integral membrane protein. HIV-1IHI Env, expressed on the surface of Rabbit Kidney cells (RK13) with a recombinant vaccinia virus (rVV), was solubilized using the non-ionic detergent n-Octyl beta-D-glucopyranoside (OG). Membrane associated Env was reconstituted into vesicles by the simultaneous removal of detergent and free monomeric Env subunits by gel-filtration. The resulting antigen preparation, termed OG-P1IHI, was captured on microtiter plates coated with Galanthus nivalis agglutinin (GNA) and used for rounds of selection (panning) of a well-characterized phage-display library derived from an HIV-1 seropositive donor. Simultaneously, an identical experiment was performed with OG-P1IHI vesicles disrupted by Nonidet P-40 (NP-P1IHI). Both membrane-associated and soluble Ags were selected for vaccinia-specific clones (OG-P1IHI: 59/75 and NP-P1IHI: 1/75) and HIV-1-specific clones (OG-P1IHI: 11/75 and NP-P1IHI: 65/75) using our approach. Hence, the novel panning strategy described here may be applicable for selection of phage-libraries against membrane proteins.

Uptake and processing of modified bacteriophage M13 in mice: implications for phage display

Internalization and degradation of filamentous bacteriophage M13 by a specific target cell may have major consequences for the recovery of phage in in vivo biopanning of phage libraries. Therefore, we investigated the pharmacokinetics and processing of native and receptor-targeted phage in mice. (35)S-radiolabeled M13 was chemically modified by conjugation of either galactose (lacM13) or succinic acid groups (sucM13) to the coat protein of the phage to stimulate uptake by galactose recognizing hepatic receptors and scavenger receptors, respectively. Receptor-mediated endocytosis of modified phage reduced the plasma half-life of native M13 (t(1/2) = 4.5 h) to 18 min for lactosylated and 1.5 min for succinylated bacterophage. Internalization of sucM13 was complete within 30 min after injection and resulted in up to 5000-fold reduction of bioactive phage within 90 min. In conclusion, these data provide information on the in vivo behavior of wild-type and receptor-targeted M13, which has important implications for future in vivo phage display experiments and for the potential use of M13 as a viral gene delivery vehicle.

Expression and bioactivity identification of soluble MG7 scFv

AIM

To examine the molecular mass and identify the bioactivity of MG7 scFv for its application as a targeting mediator in gene therapy of gastric cancer.

METHODS

Two strongly positive recombinant phage clones screened from MG7 recombinant phage antibody library were separately transfected into E.coli TG1. Plasmid was isolated from the transfected E.coli TG1 and digested by EcoR I and Hind III to examine the length of exogenous scFv gene. Then, the positive recombinant phage clones were individually transfected into E.coli HB2151. The transfectant was cultured and induced by IPTG. Perplasmic extracts was prepared from the induced transfectant by osmotic shock. ELISA was used to examine the antigen-binding affinity of the soluble MG7 scFv. Immunodotting assay was adopted to evaluate the yield of soluble MG7 scFv produced by transfected E.coli HB2151. Western blot was used to examine the molecular mass of MG7 scFv. Finally, the nucleotide sequence of MG7 scFv was examined by DNA sequencing.

RESULTS

Two positive recombinant phage clones were found to contain the exogenous scFv gene. ELISA showed that MG7 scFv had strong antigen-binding affinity. Immuodotting assay showed that transfected E.coli HB2151 could successfully produce the soluble MG7 scFv with high yield via induction by IPTG. The molecular mass of MG7 scFv was 30 kDa by western blot. DNA sequencing demonstrated that the VH and VL genes of MG7 scFv were 363 bp and 321 bp,respectively.

CONCLUSION

We have successfully developed the soluble MG7 scFv which possessed strong antigen-binding affinity.

High-throughput antibody isolation

To define the proteome of an organism, there is a need for robust and reproducible methods for the quantitative detection of all the polypeptides in a cell. High-density arrays of receptors specific for each of the polypeptides in a complex sample hold great promise for the analysis of complex protein mixtures. Because of their high affinity, specificity and their ability to bind to virtually any protein, antibodies appear particularly promising as the receptor element in protein-detection arrays. For proteomic-scale analyses, the ability to isolate and produce antibodies en masse to large numbers of target molecules is critical. A variety of systems for the high-throughput isolation of antibodies from combinatorial libraries are being developed and are outlined in this review. However, there are several other important considerations to be borne in mind before such systems can realistically be applied on a large scale.

Insertional mutagenesis of the adeno-associated virus type 2 (AAV2) capsid gene and generation of AAV2 vectors targeted to alternative cell-surface receptors

Recombinant adeno-associated virus (AAV) vectors are of interest in the context of gene therapy because of their ability to mediate efficient transfer and stable expression of therapeutic genes in a wide variety of tissues. However, AAV-mediated gene delivery to specific cell populations is often precluded by the widespread distribution of heparan sulfate proteoglycan (HSPG), the primary cellular receptor for the virus. Conversely, an increasing number of cell types are being identified that do not express HSPG and are therefore poor targets for AAV-mediated gene transfer. To address these issues, we have developed strategies to physically modify AAV vectors and allow efficient, HSPG-independent, receptor-targeted infection. We began by generating a series of 38 virus capsid mutants containing peptide insertions at 25 unique sites within the AAV capsid protein. The mutant viruses were characterized on the basis of their phenotypes and grouped into three classes: class I mutants (4 of 38) did not assemble particles; class II mutants (14 of 38) assembled noninfectious particles; and class III mutants (20 of 38) assembled fully infectious particles. We examined the HSPG-binding characteristics of the class II mutants and showed that a defect in receptor binding was a common reason for their lack of infectivity. The display of foreign peptide epitopes on the surface of the mutant AAV particles was found to be highly dependent on the inclusion of appropriate scaffolding sequences. Optimal scaffolding sequences and five preferred sites for the insertion of targeting peptide epitopes were identified. These sites are located within each of the three AAV capsid proteins, and thus display inserted epitopes 3, 6, or 60 times per vector particle. Modified AAV vectors displaying a 15-amino acid peptide, which binds to the human luteinizing hormone receptor (LH-R), were generated and assessed for their ability to target gene delivery to receptor-bearing cell lines. Titers of these mutant vectors were essentially the same as wild-type vector. The LH-R-targeted vector was able to transduce ovarian cancer cells (OVCAR-3) in an HSPG-independent manner. Furthermore, transduction was shown to proceed via the LH-R and therefore treatment of OVCAR-3 cells with progesterone, to increase LH-R expression, accordingly increased LH mutant-mediated gene transfer. This technology may have a significant impact on the use of AAV vectors for human gene therapy.

Preparation of single chain variable fragment of MG7 mAb by phage display technology

AIM: To develop the single chain variable fragment of MG₇ murine anti-human gastric cancer monoclonal antibody using the phage display technology for obtaining a tumor-targeting mediator.

METHODS: mRNA was isolated from MG₇ producing murine hybridoma cell line and converted into cDNA. The variable fragments of heavy and light chain were amplified separately and assembled into ScFv with a specially constructed DNA linker by PCR. The ScFvs DNA was ligated into the phagmid vector pCANTAB5E and the ligated sample was transformed into competent E. Coli TG1. The transformed cells were infected with M13K07 helper phage to form MG₇ recombinant phage antibody library. The volume and recombinant rate of the library were evaluated by means of bacterial colony count and restriction analysis. After two rounds of panning with gastric cancer cell line KATOIII of highly expressing MG₇-binding antigen, the phage clones displaying ScFv of the antibody were selected by ELISA from the enriched phage clones. The antigen binding affinity of the positive clone was detected by competition ELISA. HB2151 E.coli was transfected with the positive phage clone demonstrated by competition ELISA for production of a soluble form of the MG₇ ScFv. ELISA assay was used to detect the antigen-binding affinity of the soluble MG₇ ScFv. Finally, the relative molecular mass of soluble MG₇ ScFv was measured by SDS-PAGE.

RESULTS: The V-H, V-L and ScFv DNAs were about 340 bp, 320 bp and 750 bp, respectively. The volume of the library was up to 2 × 10⁶ and 8 of 11 random clones were recombinants. Two phage clones could strongly compete with the original MG₇ antibody for binding to the antigen expressed on KATOIII cells. Within 2 strong positive phage clones, the soluble MG₇ ScFv from one clone was found to have the binding activity with KATOIII cells. SDS-PAGE showed that the relative molecular weight of soluble MG₇ ScFv was 32.

CONCLUSION: The MG₇ ScFv was successfully produced by phage antibody technology, which may be useful for broadening the scope of application of the antibody.

Antibodies in haystacks: how selection strategy influences the outcome of selection from molecular diversity libraries

Antibodies against most antigens can be isolated from high quality phage antibody libraries. However, not all antibodies binding a particular antigen are necessarily found when standard selections are performed. Here we investigate the effect of two different selection strategies on the isolation of antibodies against a number of different antigens, and find that these different strategies tend to select different antibodies, with little overlap between them. This indicates that the full diversity of these libraries is not tapped by a single selection strategy and that each selection strategy imposes different selective criteria in addition to that of antigen binding. To fully exploit such libraries, therefore, many different selection strategies should probably be employed for each antigen. The use of alternative strategies should be considered when selection apparently fails, or when the number of different antibodies recognizing an antigen needs to be maximised. Furthermore, the microtitre selection strategy developed is likely to prove useful in the application of phage antibody libraries to the human genome project, allowing the high throughput selection of antibodies against multiple antigens simultaneously.

Molecular addresses in blood vessels as targets for therapy

We have isolated several organ- and tumor-homing peptides by using in vivo phage display. This technology involves the screening of peptide libraries in a living animal. The peptides that result from such a selection home to specific organs or tissues because they recognize molecular 'addresses', receptors that are differentially expressed in vascular beds. Targeted delivery of chemotherapeutics, pro-apoptotic peptides and cytokines to tumors using these peptides improved therapeutic efficacy in animal models. Translation of this technology into clinical applications will form the basis for targeting therapeutic and imaging agents in the context of cancer and other diseases.

Antibody libraries in drug and target discovery

Antibody libraries have come of age in the generation and evolution of monoclonal antibodies for therapeutic applications. Here, with an emphasis on cancer therapy, several examples are presented that illustrate the ability to design, engineer and select antibody libraries for different rationales in drug and target discovery.