Human anti-CD30 recombinant antibodies by guided phage antibody selection using cell panning

Phage display for discovery of anticancer antibodies

Antibodies and antibody-based immunotherapeutics are the mainstays of cancer immunotherapy. Expanding the repertoire of cancer-specific and cancer-associated epitopes targetable with antibodies represents an important area of research. Phage display is a powerful approach allowing the use of diverse antibody libraries to be screened for binding to a wide range of targets. In this review, we summarize the basics of phage display technology and highlight the advances in anticancer antibody identification and modification via phage display platform. Finally, we describe phage display-derived anticancer monoclonal antibodies that have been approved to date or are in clinical development.

Role of antibody engineering in generation of derivatives starting from MOv19 MAb: 40 years of biological/therapeutic tools against folate receptor alfa

In the 1980s, we developed and characterized numerous murine monoclonal antibodies (MAbs) directed against human tumor-associated antigens. This mini review is focused on the generation of derivatives of an anti-folate receptor α (FRα) MAbs, named MOv19, exploiting the antibody-engineering progresses in the last 40 years. The FRα location on the luminal surface of proliferating epithelial cells, inaccessible to circulation, versus its over-expression in the entire surface of numerous carcinomas suggested a role for anti-FRα MAbs in the diagnosis and/or treatment of solid tumors. Presently, two MOv19 derivatives are in clinical trials: a chimeric resurfaced version in an antibody-drug conjugate format (SORAYA trial, 2022) and the murine scFv in a second generation chimeric antigen receptor, CAR-T (Phase Ia, 2021). MOv19 and its derivatives could be considered a relevant example that well-characterized anti-tumor murine Mabs and antibody engineering could be combined to generate useful therapeutic tools.

A single-chain antibody generation system yielding CAR-T cells with superior antitumor function

Cancer immunotherapy using T cells redirected with chimeric antigen receptor (CAR) has shown a lot of promise. We have established a single-chain antibody (scFv) generation system in which scFv library-expressing CAR-T cells can be screened appropriately based on their antitumor functions. A variable region library containing the variable and J regions of the human immunoglobulin light or heavy chain was fused with the variable region of a heavy or light chain encoded by an existing tumor-specific antibody to generate a new scFv library. Then, scFv library-expressing CAR-T cells were generated and stimulated with target cells to concentrate the antigen-specific population. Using this system, target-specific recognition of CAR-T cells appeared to be finely tuned by selecting a new variable region. Importantly, we have demonstrated that the newly optimized scFv-expressing CAR-T cells had better proliferation capacity and durable phenotypes, enabling superior reactivity against advanced tumors in vivo in comparison with the original CAR-T cells. Therefore, the optimization of an scFv is needed to maximize the in vivo antitumor functions of CAR-T cells. This system may allow us to adjust an immunological synapse formed by an scFv expressed by CAR-T cells and a target antigen, representing an ideal form of CAR-T-cell immunotherapy.

Selection and characterization of scFv antibody against nucleocapsid protein of Porcine reproductive and respiratory syndrome virus

Porcine reproductive and respiratory syndrome virus (PRRSV) is a widespread infectious agent in pigs. Nucleocapsid (N) protein of PRRSV has been identified as the most immunodominant viral protein. The main goal of the work was the selection and characterization of a single-chain antibody fragments (scFv) antibody specific to the N protein. Specific scFv antibody clone D5 was selected from the Tomlinson phagemid library and purified by immobilized metal affinity chromatography from the periplasmatic space of E. coli cells. The antibody was then characterized by sequencing and the ability to recognize the native virus N protein by Western blot and competitive ELISA. Pepscan analysis identified the position of the binding epitope between amino acids 62–84 of the N protein. Our study could help to improve the diagnostics and prevention of PRRSV in Central Europe.

Isolation of Neutralizing Human Single Chain Antibodies Against Conserved Hemagglutinin Epitopes of Influenza a Virus H3N2 Strain

BACKGROUND

Immunotherapies using monoclonal antibodies against influenza A hemagglutinin (HA) has been an effective means for controlling Influenza spread. An alternative method for viral prophylaxis and treatment is the development of human single-chain variable fragment (scFv) antibodies with no human anti-mouse antibody (HAMA) response and high specificity. In the present study, two highly conserved sequences of HA were used to select specific neutralizing scFvs against H3N2 strain of influenza A virus.

METHODS

Biopanning process was performed to isolate specific scFv antibodies against highly conserved HA sequences, aa173-181 and 227-239, of the influenza A H3N2 strain from a scFv library. The peptide-binding specificity of the selected clones was examined via phage ELISA. The soluble forms of the clones were prepared and assessed using western blot analysis and neutralization efficiency of the selected clones were examined by TCID50 neutralizing assay and real-time PCR.

RESULTS

scFv 1 and scFv 2 were selected against HA of H3N2 influenza A virus with frequencies of 95% and 30% in the panning process, respectively. Western blot analysis confirmed the scFv band size. Significant neutralization in the presence of scFv 1 and scFv 2 were obtained. Real time PCR revealed significant decrease in viral copy number.

CONCLUSION

Two specific neutralizing scFvs against two highly conserved neutralizing epitopes of the influenza A virus HA glycoprotein were selected. A strong neutralization effect of scFv1, showed the potential of this antibody for H3N2 influenza A controlling in the viral spread.

Escherichia coli surface display for the selection of nanobodies

Nanobodies (Nbs) are the smallest functional antibody fragments known in nature and have multiple applications in biomedicine or environmental monitoring. Nbs are derived from the variable segment of camelid heavy chain-only antibodies, known as VHH. For selection, libraries of VHH gene segments from naïve, immunized animals or of synthetic origin have been traditionally cloned in E. coli phage display or yeast display systems, and clones binding the target antigen recovered, usually from plastic surfaces with the immobilized antigen (phage display) or using fluorescence-activated cell sorting (FACS; yeast display). This review briefly describes these conventional approaches and focuses on the distinct properties of an E. coli display system developed in our laboratory, which combines the benefits of both phage display and yeast display systems. We demonstrate that E. coli display using an N-terminal domain of intimin is an effective platform for the surface display of VHH libraries enabling selection of high-affinity Nbs by magnetic cell sorting and direct selection on live mammalian cells displaying the target antigen on their surface. Flow cytometry analysis of E. coli bacteria displaying the Nbs on their surface allows monitoring of the selection process, facilitates screening, characterization of antigen-binding clones, specificity, ligand competition and estimation of the equilibrium dissociation constant (KD ).

Antibody Engineering for Pursuing a Healthier Future

Since the development of antibody-production techniques, a number of immunoglobulins have been developed on a large scale using conventional methods. Hybridoma technology opened a new horizon in the production of antibodies against target antigens of infectious pathogens, malignant diseases including autoimmune disorders, and numerous potent toxins. However, these clinical humanized or chimeric murine antibodies have several limitations and complexities. Therefore, to overcome these difficulties, recent advances in genetic engineering techniques and phage display technique have allowed the production of highly specific recombinant antibodies. These engineered antibodies have been constructed in the hunt for novel therapeutic drugs equipped with enhanced immunoprotective abilities, such as engaging immune effector functions, effective development of fusion proteins, efficient tumor and tissue penetration, and high-affinity antibodies directed against conserved targets. Advanced antibody engineering techniques have extensive applications in the fields of immunology, biotechnology, diagnostics, and therapeutic medicines. However, there is limited knowledge regarding dynamic antibody development approaches. Therefore, this review extends beyond our understanding of conventional polyclonal and monoclonal antibodies. Furthermore, recent advances in antibody engineering techniques together with antibody fragments, display technologies, immunomodulation, and broad applications of antibodies are discussed to enhance innovative antibody production in pursuit of a healthier future for humans.

High affinity nanobodies against human epidermal growth factor receptor selected on cells by E. coli display

Most therapeutic antibodies (Abs) target cell surface proteins on tumor and immune cells. Cloning of Ab gene libraries in E. coli and their display on bacteriophages is commonly used to select novel therapeutic Abs binding target antigens, either purified or expressed on cells. However, the sticky nature of bacteriophages renders phage display selections on cells challenging. We previously reported an E. coli display system for expression of VHHs (i.e., nanobodies, Nbs) on the surface of bacteria and selection of high-affinity clones by magnetic cell sorting (MACS). Here, we demonstrate that E. coli display is also an attractive method for isolation of Nbs against cell surface antigens, such as the epidermal growth factor receptor (EGFR), upon direct selection and screening of Ab libraries on live cells. We employ a whole cell-based strategy using a VHH library obtained by immunization with human tumor cells over-expressing EGFR (i.e., A431), and selection of bacterial clones bound to murine fibroblast NIH-3T3 cells transfected with human EGFR, after depletion of non-specific clones on untransfected cells. This strategy resulted in the isolation of high-affinity Nbs binding distinct epitopes of EGFR, including Nbs competing with the ligand, EGF, as characterized by flow cytometry of bacteria displaying the Nbs and binding assays with purified Nbs using surface plasmon resonance. Hence, our study demonstrates that E. coli display of VHH libraries and selection on cells enables efficient isolation and characterization of high-affinity Nbs against cell surface antigens.

Efficacy of an adapted granzyme B-based anti-CD30 cytolytic fusion protein against PI-9-positive classical Hodgkin lymphoma cells in a murine model

Tumors develop when infiltrating immune cells contribute growth stimuli, and cancer cells are selected to survive within such a cytotoxic microenvironment. One possible immune-escape mechanism is the upregulation of PI-9 (Serpin B9) within cancer cells. This serine proteinase inhibitor selectively inactivates apoptosis-inducing granzyme B (GrB) from cytotoxic granules of innate immune cells. We demonstrate that most classical Hodgkin lymphoma (cHL)-derived cell lines express PI-9, which protects them against the GrB attack and thereby renders them resistant against GrB-based immunotherapeutics. To circumvent this disadvantage, we developed PI-9-insensitive human GrB mutants as fusion proteins to target the Hodgkin-selective receptor CD30. In contrast to the wild-type GrB, a R201K point-mutated GrB construct most efficiently killed PI-9-positive and -negative cHL cells. This was tested in vitro and also in vivo whereby a novel optical imaging-based tumor model with HL cell line L428 was applied. Therefore, this variant, as part of the next generation immunotherapeutics, also named cytolytic fusion proteins showing reduced immunogenicity, is a promising molecule for (targeted) therapy of patients with relapsing malignancies, such as cHL, and possibly other PI-9-positive malignancies, such as breast or lung carcinoma.

An efficient strategy for cell-based antibody library selection using an integrated vector system

Background

Cell panning of phage-displayed antibody library is a powerful tool for the development of therapeutic and imaging agents since disease-related cell surface proteins in native complex conformation can be directly targeted. Here, we employed a strategy taking advantage of an integrated vector system which allows rapid conversion of scFv-displaying phage into scFv-Fc format for efficient cell-based scFv library selection on a tetraspanin protein, CD9.

Results

A mouse scFv library constructed by using a phagemid vector, pDR-D1 was subjected to cell panning against stable CD9 transfectant, and the scFv repertoire from the enriched phage pool was directly transferred to a mammalian cassette vector, pDR-OriP-Fc1. The resulting constructs enabled transient expression of enough amounts of scFv-Fcs in HEK293E cells, and flow cytometric screening of binders for CD9 transfectant could be performed simply by using the culture supernatants. All three clones selected from the screening showed correct CD9-specificity. They could immunoprecipitate CD9 molecules out of the transfectant cell lysate and correctly stain endogenous CD9 expression on cancer cell membrane. Furthermore, competition assay with a known anti-CD9 monoclonal antibody (mAb) suggested that the binding epitopes of some of them overlap with that of the mAb which resides within the large extracellular loop of CD9.

Conclusions

This study demonstrates that scFv-Fc from mammalian transient expression can be chosen as a reliable format for rapid screening and validation in cell-based scFv library selection, and the strategy described here will be applicable to efficient discovery of antibodies to diverse cell-surface targets.

Conversion of a murine monoclonal antibody A13 targeting epidermal growth factor receptor to a human monoclonal antibody by guided selection

Epidermal growth factor receptor (EGFR) is an attractive target for tumor therapy because it is overexpressed in the majority of solid tumors and the increase in receptor expression levels has been linked with a poor clinical prognosis. Also it is well established that blocking the interaction of EGFR and the growth factors could lead to the arrest of tumor growth and possibly result in tumor cell death. A13 is a murine monoclonal antibody (mAb) that specifically binds to various sets of EGFR-expressing tumor cells and inhibits EGF-induced EGFR phosphorylation. We isolated human immunoglobulin genes by guided selection based on the mAb A13. Four different human single chain Fvs (scFvs) were isolated from from hybrid scFv libraries containing a human VH repertoire with the VL of mAb A13 and a human VL repertoire with the VH of mAb A13. All the 4 scFvs bound to EGFR-expressing A431 cells. One scFv (SC414) with the highest affinity was converted to IgG1 (ER414). The ER414 exhibited ∼17 fold lower affinity compared to the A13 mAb. In addition the ER414 inhibited an EGF-induced tyrosine phosphorylation of EGFR with much lower efficacy compared to the A13 mAb and Cetuximab (Merck KgaA, Germany). We identified that the epitope of A13 mAb is retained in ER414. This approach will provide an efficient way of converting a murine mAb to a human mAb.

scFv antibody: principles and clinical application

To date, generation of single-chain fragment variable (scFv) has become an established technique used to produce a completely functional antigen-binding fragment in bacterial systems. The advances in antibody engineering have now facilitated a more efficient and generally applicable method to produce Fv fragments. Basically, scFv antibodies produced from phage display can be genetically fused to the marker proteins, such as fluorescent proteins or alkaline phosphatase. These bifunctional proteins having both antigen-binding capacity and marker activity can be obtained from transformed bacteria and used for one-step immunodetection of biological agents. Alternatively, antibody fragments could also be applied in the construction of immunotoxins, therapeutic gene delivery, and anticancer intrabodies for therapeutic purposes. This paper provides an overview of the current studies on the principle, generation, and application of scFv. The potential of scFv in breast cancer research is also discussed in this paper.

The use of scFv-displaying yeast in mammalian cell surface selections

Yeast surface display has proven to be a powerful tool for the directed evolution of immunological proteins when soluble ligands are available (Cho, B.K., Kieke, M.C., Boder, E.T., Wittrup, K.D., Kranz, D.M., 1998. A yeast surface display system for the discovery of ligands that trigger cell activation. J. Immunol. Methods 220, 179; Boder, E.T., Midelfort, K.S., Wittrup, K.D., 2000. Directed evolution of antibody fragments with monovalent femtomolar antigen-binding affinity. Proc. Natl. Acad. Sci. U. S. A. 97, 10701; Shusta, E.V., Holler, P.D., Kieke, M.C., Kranz, D.M., Wittrup, K.D., 2000. Directed evolution of a stable scaffold for T-cell receptor engineering. Nat. Biotechnol. 18, 754; Esteban, O., Zhao, H., 2004. Directed evolution of soluble single-chain human class II MHC molecules. J. Mol. Biol. 340, 81). This investigation extends the utility of this display platform by demonstrating its capacity for use in cell panning selections. This was accomplished by employing a model single-chain antibody (scFv)-hapten system that allowed for detailed investigation of the factors governing panning success. Yeast displaying anti-fluorescein scFv (4-4-20) exhibited specific interactions with the fluoresceinated endothelial cells and could be recovered from large backgrounds of irrelevant yeast in just three rounds. Successful selections required as few as 1700 fluorescein ligands per cell, and a three-round enrichment ratio of 10(6) was possible. These results indicate that yeast surface display is a viable option for use in cell or tissue-based selections.

Light-chain shuffling results in successful phage display selection of functional prokaryotic-expressed antibody fragments to N-glycolyl GM3 ganglioside

Phage display technology makes it possible to introduce and rapidly screen diversity in antibody binding sites. Chain shuffling has been successfully used to humanize murine antibody fragments and also to obtain affinity matured variants. Here we report a different application of this method: the use of chain shuffling to overcome improper prokaryotic expression behavior of a hybridoma-derived single-chain antibody fragment. Construction and expression of such recombinant antibody fragments remain as empirical entities, hampered by the inability to express some antibody genes coming from eukaryotic cells in bacterial expression systems. Such problems are different for each combination of variable regions and can be serious enough to preclude the use of some hybridomas as sources of V regions to obtain recombinant antibody fragments. The particular binding properties and potential usefulness of some monoclonal antibodies make it highly desirable to bypass these technical limitations in order to develop smaller size therapeutic agents in the form of antibody fragments. The 14F7 mouse monoclonal antibody is one such attractive candidate due to its high specificity for the N-glycolyl GM3 ganglioside overexpressed in tumor cells and its ability to distinguish this antigen from closely related gangliosides like N-acetyl GM3. Our goal was to construct a phage-displayed single-chain Fv antibody fragment derived from 14F7. After cloning the original variable regions from the 14F7 hybridoma in a phagemid vector, we were unable to detect either binding activity or even expression of antibody fragments in bacteria, despite repetitive efforts. We constructed light-chain shuffling libraries, from which functional antibody fragments were readily selected. These combined the original 14F7 heavy chain variable region with a wide variety of unrelated murine and human light-chain variable regions. New antibody fragments retained the valuable properties of the monoclonal antibody in terms of fine specificity, affinity and tumor recognition. They were readily produced by bacteria, either in phage-displayed form or as soluble molecules, and provided a panel of potentially useful variants for cancer diagnosis and immunotherapy. Chain shuffling and phage display were found to be useful strategies for selecting antibody fragments on the basis of both prokaryotic expression and antigen binding criteria.

A Highly Conserved Interspecies V H in the Human Genome

Idiotype conservation between human and mouse antibodies has been observed in association with various infectious and autoimmune diseases. We have isolated a human anti-idiotypic antibody to a mouse monoclonal anti-IgE antibody (BSW17) suggesting a conserved interspecies idiotype associated with an anti-IgE response. To find the homologue of BSW17 in the human genome we applied the guided selection strategy. Combining V(H) of BSW17 with a human V(L) repertoire resulted in three light chains. The three V(L) chains were then combined with a human V(H) repertoire resulting in three clones specific for human IgE. Surprisingly, one clone, Hu41, had the same epitope specificity and functional in vitro activity as BSW17 and V(H) complementarity-determining regions identical with that of BSW17. Real-time PCR analysis confirmed the presence of the Hu41 V(H) sequence in the human genome. These data document the first example of the isolation of a human antibody where high sequence similarity to the original murine V(H) sequence is associated with common antigen and epitope specificity.

Identification of scFv antibody fragments that specifically recognise the heroin metabolite 6-monoacetylmorphine but not morphine

Use of phage display of recombinant antibodies and large repertoire naïve antibody libraries for identifying antibodies of high specificity has been extensively reported. Nevertheless, there have been few reported antibodies to haptens that have originated from naïve antibody libraries with potential use in diagnostics. We have used chain shuffling of lead single-chain fragment variable (scFv) antibodies, isolated from a naïve antibody library, to screen for antibodies that specifically recognise the major metabolite of heroin, 6-monoacetylmorphine (6MAM). The antibodies were identified by screening high-density colonies of Escherichia coli expressing soluble scFv antibody fragments without prior expression on bacteriophage (phage display). The antibodies recognise 6MAM with affinities of 1-3x10(-7) M with no crossreactivity to morphine. These antibodies can potentially be used for developing a rapid immunoassay in drug-testing programs. To our knowledge, this is the first report of an antibody that distinguishes 6MAM from its de-acetylated form, morphine.

Clinical evaluation of ricin A-chain immunotoxins in patients with Hodgkin's lymphoma

BACKGROUND

Immunotoxins (ITs) consist of cell binding ligands coupled to toxins or their subunits. Hodgkin's lymphoma (HL) is an excellent target for ITs since lymphocyte activation markers such as CD25 and CD30 are expressed in large numbers. The ITs RFT5.dgA (anti CD25) and Ki-4.dgA (anti CD30) were constructed by linking the monoclonal antibodies RFT5 and Ki-4 to deglycosylated ricin A-chain (dgA). Both ITs showed potent specific activity against HL cells in vitro and in vivo in animal models, and were subsequently evaluated in phase I/II clinical trials in humans.

PATIENTS AND METHODS

In two separate trials, the ITs were administered i.v. four times every other day over 4 h. The objectives of the phase I trials included the determination of the maximum tolerated dose (MTD), dose-limiting toxicities (DLTs), pharmacokinetics, antitumor activity and immune response against the IT.

RESULTS

Twenty-seven patients with refractory HL were included in the phase I/II study of RFT5.dgA and 17 patients were included in the phase I study of Ki-4.dgA. The MTD of RFT5.dgA was 15 mg/m(2), whereas that of Ki-4.dgA was 5 mg/m(2). DLTs were related to vascular leak syndrome, consisting of edema, tachycardia, dyspnea, weakness and myalgia. Measurement of serum levels of RFT5.dgA demonstrated a C(max) of 0.2-9.7 micro g/ml with a half-life (t()) varying from 4 to 10.5 h. Peak serum concentration of Ki-4.dgA ranged from 0.23 to 1.7 micro g/ml. In both trials approximately 60% of patients developed human anti-mouse and/or anti-dgA antibodies. Seventeen of 18 patients treated at the MTD of RFT5.dgA were evaluable for clinical response. Responses included two partial remissions (PR), one minor response (MR) and five stable diseases (SD). Fifteen of 17 patients treated with Ki-4.dgA were evaluable for clinical response. Responses included one PR, one MR and two SD.

CONCLUSIONS

RFT5.dgA and Ki-4.dgA showed moderate efficacy in heavily pretreated refractory patients with HL. Ki-4.dgA was less well tolerated than RFT5.dgA. This might be due, at least in part, to the formation of Ki-4.dgA/sCD30 complexes.

Current strategies of antibody-based treatment in Hodgkin's disease

Many new approaches involving antibody-based agents have given promising results in experimental Hodgkin's disease (HD) models. Clinical trials with monoclonal antibodies, immunotoxins, bispecific constructs and radioimmunoconjugates have demonstrated some clinical efficacy in patients with advanced refractory HD. Although it seems unlikely that resistant patients with larger tumor masses will be cured by either of these approaches, it might be feasible to treat bulky disease by conventional therapy and then administer biological agents to kill residual Hodgkin and Reed-Sternberg cells. Future phase III trials will have to prove a possible superior effect of this combined immunochemotherapy. Currently, the evaluation of the most promising approaches continues.

Directed evolution of high-affinity antibody mimics using mRNA display

We constructed a library of >10(12) unique, covalently coupled mRNA-protein molecules by randomizing three exposed loops of an immunoglobulin-like protein, the tenth fibronectin type III domain (10Fn3). The antibody mimics that bound TNF-alpha were isolated from the library using mRNA display. Ten rounds of selection produced 10Fn3 variants that bound TNF-alpha with dissociation constants (K(d)) between 1 and 24 nM. After affinity maturation, the lowest K(d) measured was 20 pM. Selected antibody mimics were shown to capture TNF-alpha when immobilized in a protein microarray. 10Fn3-based scaffold libraries and mRNA-display allow the isolation of high-affinity, specific antigen binding proteins; potential applications of such binding proteins include diagnostic protein microarrays and protein therapeutics.

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.

Protein-protein interactions in hematology and phage display

Phage display, which exploits fundamental tools and principles of immune repertoire diversity, antigen-antibody interactions, and clonal and immunologic selection, is used increasingly to advance experimental and clinical hematology. Phage display is based on the ability of bacteriophage to present engineered proteins on their surface coat. Diverse libraries of proteins such as peptides, antibody fragments, and protein domains corresponding to gene fragments or cDNAs may be displayed. Interactions between phage-displayed proteins and target antigens can be identified rapidly and characterized using high throughput methodologies. Peptide and gene fragment libraries are particularly useful to characterize binding interactions between proteins, such as ligand-receptor interactions. This approach allows rapid generation of human antibodies, often against nonimmunogenic, conserved proteins. Phage antibodies against surface and intracellular antigens are used as reagents for flow cytometry, in vivo imaging, and therapeutic targeting. Phage-derived antibodies also facilitate analyses of the humoral antibody response. Finally, cellular delivery of phage-displayed peptides and gene fragments can be used to modulate functional pathways and molecules in vitro and in vivo. The combinatorial power of phage display enables identification of candidate epitopes without knowledge of the protein interaction, a priori. Overall, these capabilities provide a versatile, high-throughput approach to develop tools and reagents useful for a plethora of experimental hematology applications. This paper focuses on current and future applications of antibody and epitope phage display technology in hematology.

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.