Cell-free synthesis of functional antibodies using a coupled in vitro transcription-translation system based on CHO cell lysates

Antibodies are indispensable tools for basic research as well as diagnostic and therapeutic applications. Consequently, the development of alternative manufacturing strategies which circumvent the hurdles connected to conventional antibody production technologies is of enormous interest. To address this issue, we demonstrate the synthesis of complex antibody formats, in particular immunoglobulin G (IgG) and single-chain variable fragment Fc fusion (scFv-Fc), in a microsome-containing cell-free system based on translationally active chinese hamster ovary (CHO) cell lysates. To mimic the environment for antibody folding and assembly present in living cells, antibody genes were fused to an endoplasmic reticulum (ER)-specific signal sequence. Signal-peptide induced translocation of antibody polypeptide chains into the lumen of ER microsomes was found to be the prerequisite for antibody chain assembly and functionality. In this context, we show the rapid synthesis of antibody molecules in different reaction formats, including batch and continuous-exchange cell-free (CECF) reactions, depending on the amount of protein needed for further analysis. In addition, we demonstrate site-specific and residue-specific labeling of antibodies with fluorescent non-canonical amino acids. In summary, our study describes a novel antibody production platform which combines the highly efficient mammalian protein folding machinery of CHO cells with the benefits of cell-free protein synthesis.

Multimerization domains for antibody phage display and antibody production

High-throughput generation of antibodies for proteome research has become feasible by using antibody gene libraries and in vitro selection methods like phage display. Typically monovalent antibody fragments like scFv, Fab or scFab are obtained by this technology. To mimic the IgG molecule and gain avidity, resulting in stronger binding, multimerization domains can be fused to antibody fragments. Here we systematically analyzed different multimerization domains in respect to three key parameters, crucial for the high-throughput generation of binders. (i) The compatibility to be displayed on phage (assessed for at least three different antibody formats, scFv, Fab and scFab) in combination with five different multimerization domains; (ii) production yields and (iii) oligomerization properties were analyzed for three different scFv fragments. We found that the use of a biotin acceptor domain in combination with an in vivo biotinylation system performed best concerning the key parameters and thus would be a useful tool to generate multimeric antibody complexes on demand from phage display selected antibody fragments with the least effort.

Functional knockdown of VCAM-1 at the posttranslational level with ER retained antibodies

Vascular cell adhesion molecule 1 (VCAM-1) is involved in the recruitment of leukocytes to inflammatory sites. In this study we present the first functional knockdown of VCAM-1 using an ER retained antibody construct. We generated a knockdown construct encoding the VCAM-1 specific single chain variable fragment scFv6C7.1 fused to the C-terminal ER retention sequence KDEL. HEK-293:VCAM-YFP cells stably expressing a VCAM-YFP fusion protein were transiently transfected with the knockdown construct and showed down-regulation of surface VCAM-1. Knockdown efficiency of the system is time-dependent due to used transient transfection of the intrabody construct. Furthermore, intrabody mediated knockdown of HEK-293:VCAM-YFP cells also impaired cell-cell interaction with Jurkat cells that are endogenously expressing VLA-4, the physiological partner of VCAM-1. Posttranslational knockdown with ER retained antibodies seems to be a promising technique, as shown here for VCAM-1.

Recombinant variants of antibody 138H11 against human gamma-glutamyltransferase for targeting renal cell carcinoma

The monoconal antibody 138H11 recognizes human renal gamma-glutamyltransferase (GGT), an antigen shown to allow targeting primary and metastatic renal cell carcinoma (RCC). We determined the primary structure of the antigen binding region of mAb 138H11 and generated several different recombinant antibody variants. First, monomeric single-chain Fv antibody fragments, diabodies and triabodies were obtained by constructing linker variants consisting of 18, 10, 8, 5, 3, 2, 1 and zero amino acid residues, resulting in significant differences in yield and molecular architecture. Second, two variants of disulphide bond-stabilised Fv fragments (dsFvs) were generated using two different pairs of complementary framework amino acid positions of VH and VL for disulphide stabilisation. The binding activities of diabodies to human GGT located on its tissue decreased when using shorter linker lengths. The scFv dimer containing a 3 amino acid residue linker peptide and one of the dsFv variants were not functional. Further, the work paves the way for generating new effector constructs and a systematic optimisation of the pharmacokinetics of this tumor targeting antibody by offering variants with a broad range of valencies and molecular masses.

Parameters affecting the display of antibodies on phage

Despite the fact that a multitude of antibody phage display libraries has been built, systematic comparisons of critical design parameters are rare. Here we analysed the impact of various factors on the performance of the phage display system. First, we compared several vector designs for the display of Fab fragments of antibodies. Bicistronic as well as monocistronic expression of the antibody/pIII operon and vectors using fd-pIII as well as LC-pIII fusions were tested. Further, we evaluated the influence of glucose on the promoter induction. We compared monovalent versus oligovalent display of the antibody fragments and we used antibody fragments with different folding efficiency to assess the influence of the individual antibody sequences on the performance of the system. Finally, both phage display efficiency and yield of soluble Fab fragments were analysed. The significant differences found for phage yield, display of Fabs on the phage and expression of soluble Fabs suggest to use a bicistronic vector with an fd-fragment-pIII fusion for the construction of future Fab phage display libraries.

Functional Characterization of Recombinant Chloroplast Signal Recognition Particle

The signal recognition particle (SRP) is a ubiquitous system for the targeting of membrane and secreted proteins. The chloroplast SRP (cpSRP) is unique among SRPs in that it possesses no RNA and is functional in post-translational as well as co-translational targeting. We have expressed and purified the two components of the Arabidopsis thaliana chloroplast signal recognition particle (cpSRP) involved in post-translational transport: cpSRP54 and the chloroplast-specific protein, cpSRP43. Recombinant cpSRP supports the efficient in vitro insertion of pea preLhcb1 into isolated thylakoid membranes. Recombinant cpSRP is a stable heterodimer with a molecular mass of approximately 100 kDa as determined by analytical ultracentrifugation, gel filtration analysis, and dynamic light scattering. The interactions of the components of the recombinant heterodimer and pea preLhcb1 were probed using an immobilized peptide library (pepscan) approach. These data confirm two previously reported interactions with the L18 region and the third transmembrane helix of Lhcb1 and suggest that the interface of the cpSRP43 and cpSRP54 proteins is involved in substrate binding. Additionally, cpSRP components are shown to recognize peptides from the cleavable, N-terminal chloroplast transit peptide of preLhcb1. The interaction of cpSRP43 with cpSRP54 was probed in a similar experiment with a peptide library representing cpSPR54. The C terminus of cpSRP54 is essential for the formation of the stable cpSRP complex and cpSPR43 interacts with distinct regions of the M domain of cpSRP54.

Identification of epitope regions recognized by tumor inhibitory and stimulatory anti-ErbB-2 monoclonal antibodies: implications for vaccine design

The self-oncoprotein ErbB-2 is overexpressed in a number of malignancies. The presence of endogenous anti-ErbB-2 Ab and T cell immune responses to this protein in cancer patients has made ErbB-2 an attractive target for active immunization. However, the finding that murine anti-ErbB-2 Abs can have stimulatory, inhibitory, or no effects on cancer cell growth suggests that an inappropriately induced immune response may have an adverse effect. To ensure the induction of a beneficial Ab response, it is important to identify the epitopes recognized by these Abs. In this study we have used phage-displayed ErbB-2 gene fragment libraries and synthetic peptides to epitope-map a panel of anti-ErbB-2 mAbs. The epitopes of three mAbs, N12, N28, and L87, were successfully located to C531-A586, T216-C235, and C220-C235 of ErbB-2, respectively. It was found that while N12 inhibited tumor cell proliferation, N28 stimulated the proliferation of a subset of breast cancer cell lines overexpressing ErbB-2. The peptide region recognized by N12, (C531-A586; EP531), was used as an immunogen to selectively induce an inhibitory immune response in mice. Mice immunized with the GST fusion peptide (GST-EP531) recognized the peptide region EP531 as well as native ErbB-2. More importantly, Igs purified from mouse sera were able to inhibit up to 85% of tumor cell proliferation. In conclusion, our study provides direct evidence of the function-epitope relationship of anti-ErbB-2 Abs and also emphasizes the value of inducing a potent tumor inhibitory polyclonal Ab response by rationally selecting regions of ErbB-2 used for immunization.

A helper phage to improve single-chain antibody presentation in phage display

We show here that the number of single-chain antibody fragments (scFv) presented on filamentous phage particles generated with antibody display phagemids can be increased by more than two orders of magnitude by using a newly developed helper phage (hyperphage). Hyperphage have a wild-type pIII phenotype and are therefore able to infect F(+) Escherichia coli cells with high efficiency; however, their lack of a functional pIII gene means that the phagemid-encoded pIII-antibody fusion is the sole source of pIII in phage assembly. This results in an considerable increase in the fraction of phage particles carrying an antibody fragment on their surface. Antigen-binding activity was increased about 400-fold by enforced oligovalent antibody display on every phage particle. When used for packaging a universal human scFv library, hyperphage improved the specific enrichment factor obtained when panning on tetanus toxin. After two panning rounds, more than 50% of the phage were found to bind to the antigen, compared to 3% when conventional M13KO7 helper phage was used. Thus, hyperphage is particularly useful in stoichiometric situations, when there is little chance that a single phage will locate the desired antigen.

Baculovirus expression cassette vectors for rapid production of complete human IgG from phage display selected antibody fragments

For the expression of human intact IgG antibodies, we have constructed a set of baculovirus expression vectors designed to facilitate rapid insertion of heavy and light chain genes of Fab or scFv antibodies derived from phage display antibody libraries. By linking them to human constant or Fc regions, expression of complete human immunoglobulin molecules was achieved in insect cells by infection with recombinant baculovirus. The IgG expression cassette vectors are based on the backbone vector which contains two back to back polyhedron and p10 promoters. The IgG expression cassette elements, including the authentic IgG lambda or kappa and heavy chain signal sequences, as well as light chain (lambda or kappa) and heavy chain constant region genes are combined in a single vector and are controlled by the p10 and polyhedron promoter respectively. Either of VL or Fab-L and VH or Fab-Fd genes from common phage display systems can be directly inserted into one of the cassette vectors through in-frame cloning sites. This design of a single cassette vector combining heavy and light chain expression elements allowed rapid production and secretion of correctly processed and assembled intact immunoglobulins from recombinant baculovirus infected insect cells. The recombinant antibodies showed the expected molecular size of the H2L2 heterodimer in non reducing SDS-PAGE. No apparent differences were found between the expression level of heavy and light chains, and antigen binding function was preserved. For various antibodies, yields between 6 and 18 mg/l IgG were obtained.

Expression of a bispecific dsFv-dsFv' antibody fragment in Escherichia coli

A bispecific disulfide-stabilized Fv antibody fragment (dsFv-dsFv') consisting of two different disulfide-stabilized Fv antibody fragments connected by flexible linker peptides was produced by secretion of three polypeptide chains into the periplasm of Escherichia coli. The dsFv-dsFv' molecules were enriched by immobilized metal affinity chromatography and further purified by anion-exchange chromatography. The recombinant antibody constructs retained the two parental antigen binding specificities and were able to cross-link the two different antigens. The described dsFv-dsFv' design might be of particular value for therapeutic in vivo applications since improved stability is expected to be combined with minimal immunogenicity.

Effects of unpaired cysteines on yield, solubility and activity of different recombinant antibody constructs expressed in E. coli

New E. coli vectors based on the pOPE/pSTE vector system [Gene 128 (1993) 97] were constructed to express a single-chain Fv antibody fragment (scFv), a scFv-streptavidin fusion protein and two disulfide bond-stabilized Fv antibody fragments (dsFvs) utilizing different side chain positions for disulfide stabilization. All of these constructs encoded fusion proteins carrying five C-terminal histidine residues preceded by an unpaired cysteine. The influence of this cysteine, which was originally introduced to allow the chemical modification of the fusion proteins, was assessed by exchanging the two amino acids CysIle in front of the carboxy terminal His-tag to SerHis in all constructs. Yield and antigen-binding activity of the antibody constructs were compared after standard lab-scale periplasmic expression in Escherichia coli. The removal of the unpaired cysteine resulted in a significant increase in antigen-binding activity of the crude periplasmic extracts. Further, a three-five fold increase of yield and a significantly improved purity were observed after immobilized metal affinity chromatography (IMAC) with all four constructs.

Protein stabilization through phage display

RNase S consists of two proteolytic fragments of RNase A, residues 1-20 (S20) and residues 21-124 (S pro). A 15-mer peptide (S15p) with high affinity for S pro was selected from a phage display library. Peptide residues that are buried in the structure of the wild type complex are conserved in S15p though there are several changes at other positions. Isothermal titration calorimetry studies show that the affinity of S15p is comparable to that of the wild type peptide at 25 degrees C. However, the magnitudes of DeltaH(o) and DeltaC(p) are lower for S15p, suggesting that the thermal stability of the complex is enhanced. In agreement with this prediction, at pH 6, the T(m) of the S15p complex was found to be 10 degrees C higher than that of the wild type complex. This suggests that for proteins where fragment complementation systems exist, phage display can be used to find mutations that increase protein thermal stability.

Primary structure of the antigen-binding domains of a human oligodendrocyte-reactive IgM monoclonal antibody derived from a patient with multiple sclerosis

Several murine IgM monoclonal antibodies (mAbs) promoting remyelination in mice were shown to be germline gene-encoded natural autoantibodies that react with oligodendrocytes and intracellular antigens. Here, we show that human oligodendrocyte-reactive IgM mAb DS1F8 derived from a patient with multiple sclerosis targets microtubule-like structures similar to the murine mAbs. Sequencing of the cDNAs of the variable regions revealed that the antigen-binding domains are also encoded by germline genes. These similarities of mAb DS1F8 to the murine mAbs promoting remyelination suggest that this human mAb is a natural autoantibody. This may imply that the engineering of human autoantibodies for therapy of demyelinating diseases is feasible.

Determination of phage antibody affinities to antigen by a microbalance sensor system

Over the past decade, phage display has maturated to be a frequently used method for the generation of monoclonal antibodies of human origin. The essential step of this method is the "biopanning" of phage carrying functional antibody fragments on their surface on an immobilized antigen. The screening of large combinatorial gene libraries with this method usually leads to a set of diverse clones specifically binding to the antigen that need to be characterized further. Beside its specificity, the key parameter to be determined is the affinity of the recombinant antibody fragment to its antigen. Here, we present a mass sensitive microsensor method that allows the estimation of antibody affinity directly from the phage supernatant. Binding of phage antibodies to the antigen immobilized on a quartz crystal microbalance (QCM) induced a mass dependent decrease in frequency. This principle was used to determine the apparent affinity of a single-chain (sc)Fv antibody against the RNA polymerase of Drosophila melanogaster presented on the surface of a filamentous phage (M13) from its association and dissociation rates. The apparent affinity obtained is in accordance with the affinity of the scFv fragment as determined by conventional equilibrium enzyme-linked immunosorbent assay (ELISA) and plasmon resonance methods.

Epitope structures recognised by antibodies against the major coat protein (g8p) of filamentous bacteriophage fd (Inoviridae)

To map the accessible surface of filamentous bacteriophage fd particles, the epitope structures of polyclonal rabbit serum and three mouse monoclonal antibodies raised against complete phage were analysed. Western blot analysis confirmed the major coat protein, gene VIII product (g8p or pVIII), to be the antigen. Overlapping peptides were synthesised by spot synthesis on cellulose membranes, covering the whole sequence of g8p. Each of the three tested monoclonal antibodies, B62-FE2, B62-GF3/G12 and B62-EA11, reacted with a core epitope covering ten amino acid residues at or near the amino terminus of g8p. The epitope recognised by B62-FE2 consists of the ten N-terminal amino acid residues of g8p. Extension of the amino terminus by various sequences did not inhibit binding, indicating that a terminal amino group is not essential for the interaction. Both B62-GF3/G12 and B62-EA11 recognise internal epitopes covering amino acid residues 3 to 12 of g8p. The epitopes of the polyclonal rabbit serum were also confined to the 12 N-terminal amino acid residues. The contribution of individual amino acid residues to the binding was analysed by a set of peptides containing individual amino acids exchanged by glycine. Accessible residues were Glu2, Asp4, Asp5, Pro6, Lys8, Phe11 and Asp12. The positions of the essential amino acid residues within the epitope are in accordance with a helical conformation of the amino-terminal region of g8p. Further, the results suggest new designs of phage display screening vectors to improve their performance in analysing non-linear epitopes.

Fine mapping of the antigen-antibody interaction of scFv215, a recombinant antibody inhibiting RNA polymerase II from Drosophila melanogaster

A bacterially expressed single chain antibody (scFv215) directed against the largest subunit of drosophila RNA polymerase II was analysed. Structure and function of the antigen binding site in scFv215 were probed by chain shuffling and by site-specific mutagenesis. The entire variable region of either the heavy or light chain was replaced by an unrelated heavy or light chain. Both replacements resulted in a total loss of binding activity suggesting that the antigen binding site is contributed by both chains. The functional contributions of each complementarity determining region (CDR) were investigated by site specific mutagenesis of each CDR separately. Mutations in two of the CDRs, CDR1 of light chain and CDR2 of heavy chain, reduced the binding activity significantly. Each of the amino acids in these two CDRs was replaced individually by alanine (alanine walking). Seven amino acid substitutions in the two CDRs were found to reduce the binding activity by more than 50%. The data support a computer model of scFv215 which fits an epitope model based on a mutational analysis of the epitope suggesting an alpha-helical structure for the main contact area.

Epitopes fused to F-pilin are incorporated into functional recombinant pili

In order to develop a system which allows infection by an epitope-specific phage-antibody via an F-pilus expressing that epitope, a study on the expression of foreign sequences on F-pilin was undertaken. Initially, a plasmid library was constructed with random sequences encoding one to five amino acid residues fused to the C terminus of F-pilin (traA) which was used to complement an F-plasmid with an amber mutation in traA. Functional F-pilin fusions were detected using the filamentous phage, fUSE2, which transduces tetracycline resistance, as well as immunoblots using a monoclonal antiserum specific for the acetylated N terminus of pilin. All the clones selected expressed the pilin-fusions and displayed full sensitivity towards fUSE2 infection, which was indistinguishable from the wild-type F-pilin. The sequences of fUSE2-sensitive clones when compared to randomly selected clones which were not fUSE2-sensitive, revealed no obvious pattern in the amino acid residues fused to the C terminus, except for a preference for a hydrophilic amino acid at position +1. Mutating the C-terminal Leu in wt (wild-type) pilin to Ser blocked pilus assembly and fUSE2 infection; the pilin was correctly processed but the level of acetylation at the N terminus appeared to decrease. Fusing a known epitope (myc) directly to the C terminus blocked processing of F-pilin leading to loss of F-pilus assembly and function. The introduction of random sequences between traA and this epitope yielded fully recombinant, functional F-pili but this appeared to be due to processing of the extension by an unidentified protease leading to loss of the epitope. Surface expression of another epitope (G2-10) was clearly demonstrated by immuno-electron microscopy of pili with a G2-10 monoclonal antibody. A different five amino acid residue spacer between the F-pilin C terminus and the G2-10 epitope produced a system that was transfer-proficient and fUSE2-sensitive, but the pili were barely detectable by immunoblots or by electron microscopy. While the underlying rules that govern successful epitope expression at the C terminus of F-pilin remain elusive, many types of foreign sequences can be displayed with varying degrees of success. Our results also suggest that pilin sequence determines a number of steps in the complex pathway for pilus assembly.

Cloning and Expression of Single-Chain Fragments (scFv) from Mouse and Rat Hybridomas

For cloning and expressing the antigen-binding variable (Fv) portion of an antibody in Escherzchia coli, vectors have been constructed that combine the two variable regions (V(H) and V(L)) with a peptide linker (1-3). The genetic information for V(H) and V(L) is generally amplified from hybridoma cells using the polymerase chain reaction (PCR) with antibody-specific primers A variety of primer sets for the amplification of mouse-variable domams has been developed that are particulary suitable for the generation of complex mouse libraries consisting of more than 10(5) different antibody sequences (4,5). For this purpose, an equimolar amplification of all different antibody genes present in the cDNA mixture should be sought. Therefore, complex sets of primers have been designed Everyone of the target cDNAs should be prrmed with an oltgonucle-otide hybridizing with a standardized affinity to prevent stronger amplification of sequences with a better match to a primer. However, for the amplification of the variable region genes of a particular single hybridoma clone, a much simpler set can be employed. Long primers allowing a high number of mismatches have been successfully used to specifically amplify antibody DNA from a variety of cell lines, including rat hybridomas (6-8). However, some of the restriction sites (in particular P(st)I and BarnHI) introduced for subsequent cloning were found to be frequently present as internal sites in the DNA coding for mouse-antibody-variable domains. Therefore, additional restrtction sites were introduced that occur only rarely. The resulting primer list IS described in Table 1, and a protocol for the amplification of V(H) and V(L) DNA is given in Section 3.1.. Table 1 Oligonucleotides for the Amplification of Mouse and Rat Immunoglobulin-Variable Region DNA γ-Chain CHl domain: Bi4 5 ' -CCAGGGGCCAGTGGATAGACAAGCTTGGGTGTCGTTTT Hind III Heavy-cham variable domain: Bi3f 5 ' -CAGCCGGCCATGGCGCAGGT (C/G) CAGCTGCAG (C/G) AG NcoI PvuII, Pst I κ-Chain constant domain: Bi5c 5 ' -GAAGATGGATCCAGCGGCCGCAGCATCAGC BamHI NotI κ-Chain variable domain Bi8b 5 ' -ATTTTCAGAAGCACGCGTAGATATC (G/T) TG (A/C) T (G/C) ACCCAA (T/A) CTCCA MluI EcoRV.

Recent developments in antibody engineering

Rapid growth in the field of antibody engineering occurred after it was shown that functional antibody fragments could be secreted into the periplasmic space and even into the medium of Escherichia coli by fusing a bacterial signal peptide to the antibody's N-terminus (1,2). These findings allowed scientists to transfer the principles of the immune system for producing specific antibodies to a given antigen into a bacterial system (3). It was now possible to establish antibody libraries in E. coli that could be directly screened for binding to antigen. This was accomplished at first by transforming E coli with plasmids containing polymerase chain reaction (PCR)-amplified immunoglobulin families from the lymphocytes of immunized mice. Immunogen-reactive recombinant antibodies were then selected by an enzyme-linked immunosorbent assay (ELISA) of the bacterial supernatant from isolated bacterial colonies (4). This procedure was subsequently improved upon by inserting the antibody operon into bacteriophage λ. Antibody libraries were then able to be efficiently transfected into E. coli and plaque lift-offs of lysed bacterial colonies on nitrocellulose could be screened for reactivity to a radioactive labeled immunogen (5-7).

A TNF receptor antagonistic scFv, which is not secreted in mammalian cells, is expressed as a soluble mono- and bivalent scFv derivative in insect cells

Single chain antibodies (scFv) are usually produced in E. coli, but generation of certain scFv derivatives, such as complex fusion proteins or glycosylated forms of scFv is restricted to eukaryotic expression systems. We investigated the production of soluble mono- and bivalent single chain antibodies (scFv) in eukaryotic cells and describe a cassette vector system for mammalian and baculovirus expression which is compatible with an established vector system for bacterial expression and phage display selection of scFvs. The applied model scFv was derived from a murine antibody (H398) against human tumor necrosis factor receptor 1 (TNFR60), known to be a potent antagonist of TNF action in its monomeric form and a potential therapeutic agent for treatment of TNF-mediated diseases. Surprisingly, the monomeric scFv form of H398 (scFv H398) is expressed but not secreted in different mammalian cells. In contrast, in insect cells using recombinant baculovirus, a monovalent scFv H398 and a bivalent scFv fusion protein with an human IgG1 Fc region were expressed and secreted with correctly processed signal sequence. Concerning the influence of valency of the model Ab and its derivatives on antigen binding affinity and neutralisation of TNF activity, we found that the mono- and bivalent form of scFv H398 possesses the same characteristics as proteolytically produced Fab H398 and original mAb H398, respectively. Furthermore, fusion of the Ig Fc protein to scFv H398 increase the in vitro half-life at 37 degrees C. We conclude that the described cassette vectors readily allow the eukaryotic expression of mono- and bivalent scFv derivatives to analyse the influence of valency of scFv molecules on antigen binding and biological activity.

Primary structure and functional expression of heavy- and light-chain variable region genes of a monoclonal antibody specific for human fibrin

The immunoglobulin heavy- and light-chain variable region (VH and VK) genes were isolated from 8E5 hybridoma cells, which secreted monoclonal antibody against human fibrin by RT-PCR. An expression vector pOPE51-8E5 was constructed for the recombinant VH-VK scFv expression. The primary sequence of the variable regions was determined. Expression product was found in the periplasmic space and inclusion bodies by SDS-PAGE and immunoblotting. It was a 30 KDa single chain fragment (scFv) with the antigen-binding specificity of the parental monoclonal antibody. A light chain shuffling with an unspecific VL did not result in a loss of fibrin binding specificity.

Primary structure and functional scFv antibody expression of an antibody against the human protooncogen c-myc

The immunoglobulin heavy- and light-chain variable region (Vh and Vl) genes were isolated from Myc1-9E10 hybridoma cells, which secreted monoclonal antibody against human oncogen c-myc. The expression vector pOPE52-c-myc was constructed for the recombinant production in E. coli. A 30 kDa single chain fragment (scFv) expression product was found in the periplasmic space by SDS-PAGE and immunoblotting. A significant fraction was processed correctly as demonstrated with an antiserum recognizing the processed aminoterminus only. The specific binding of the scFv fragment to the peptide epitope of the maternal monoclonal antibody was demonstrated and the primary sequence of the variable regions was determined. Sequence comparison with previously published partial Vh and Vl sequences from this hybridoma cell line revealed a genetic heterogeneity for the light chain variable region. The potential use of this scFv as a new tool for detection and purification of tagged proteins, for adding costimulatory signals to the surface of cancer cells as well as for analyzing c-myc function in the living cell by cytoplasmic expression is discussed.

Cloning and cytotoxicity of a human pancreatic RNase immunofusion

BACKGROUND

Immunotoxins based on plant and bacterial proteins are usually very immunogenic. Human ribonucleases could provide an alternative basis for the construction of less immunogenic reagents. Two members of the human RNase family, angiogenin and eosinophil-derived neurotoxin (EDN), have been fused to a single chain antibody against the transferrin receptor, which is known to be internalised by endocytosis. The fusion proteins proved to be very efficient inhibitors of protein synthesis using various cell lines. It is not yet known whether the side effects of angiogenin and EDN will compromise their potential use as immunotoxins.

OBJECTIVES

The goal of this work was to construct a human immunotoxin with no harmful side effects. Bovine pancreatic ribonuclease has been shown to be as potent as ricin at abolishing protein synthesis on injection into oocytes. We therefore decided to clone its human analogue, which is fairly ubiquitous and per se non-toxic. An immunofusion of human pancreatic RNase with a single chain antibody against the transferrin receptor was tested for its ability to inhibit protein synthesis in three different human tumor cell lines.

STUDY DESIGN

DNA coding for the human pancreatic RNase was cloned partially from a human fetal brain cDNA library and then completed by PCR using a human placental cDNA library as a template. The RNase gene was then fused with a DNA coding for an single chain antibody against the transferrin receptor (CD71). After expressing the fusion protein in E. coli, the gene product was isolated from inclusion bodies and tested for cytotoxicity.

RESULTS

This fusion protein inhibited the protein synthesis of three human tumor cell lines derived from a melanoma, a renal carcinoma and a breast carcinoma, with IC50s of 8, 5 and 10 nM, respectively. These values were comparable with those using a similar fusion protein constructed with eosinophil derived neurotoxin (EDN) as the toxic moiety (IC50s of 8, 1.2 and 3 nM, respectively). The slightly lower activities of the human pancreatic RNase-scFv (pancRNase-scFv) with two of the cell lines suggests that fewer molecules are reaching the cytoplasmic compartment, since it was twice as active as EDN-scFv in inhibiting the protein synthesis of a rabbit reticulocyte lysate.

CONCLUSION

These results demonstrate that the human pancreatic RNase, which is expected to have a very low immunogenic potential in humans with no inherent toxicity, may be a potent cytotoxin for tumor cells after antibody targeting.

The antigen-binding domain of a human IgG-anti-F(ab')2 autoantibody

Recent studies revealed an immunoregulatory role of natural IgG-anti-F(ab')2 antibodies in both healthy individuals and patients with certain diseases. The implication of anti-F(ab')2 antibodies in the pathogenesis of diseases prompted us to study the gene segment structure of their antigen-binding domains and their binding characteristics. cDNA was prepared from the lymphocytes of a patient with a high IgG-anti-F(ab')2 serum titer. Variable heavy and light gene segments were amplified by PCR and inserted into a phagemid surface expression vector. Single-chain antibodies displayed on the phage surface were screened for binding to F(ab')2 fragments. The subsequent analysis of 95 single clones demonstrated that they all bound specifically to F(ab')2. Sequence analyses of 12 clones showed that 11 were identical and 1 contained a silent point mutation in the heavy chain and three amino acid exchanges in the light chain. The heavy chains belonged to the V(H)3 and the light chains to the V(kappa)2 gene family. The 11 identical light-chain genes were completely homologous to a germ-line sequence (DPK-15). Binding assays showed that the single-chain antibodies bind to F(ab')2, but not to Fab, Fc, or intact IgG. This binding pattern was confirmed by surface plasmon resonance studies, which revealed a relatively high affinity (Ka = 2.8 x 10(7) M(-1)). The strong binding capacity was further demonstrated by competitive inhibition of the serum anti-IgG antibody's interaction with antigen. The present study defines for the first time to our knowledge the gene segment structure of the antigen-binding domain of two human IgG-anti-F(ab')2 autoantibody clones and describes the binding kinetics of the purified monomeric fragments.

Cell surface display of a single-chain antibody for attaching polypeptides

To provide an efficient means of coupling proteins, peptides and other suitable moieties to cells, we have constructed a retroviral expression vector for cell surface display of a single-chain antibody (scFv) against the hapten 4-ethoxymethylene-2-phenyl-oxazo-line-5-one (phOx). The hapten phOx can be easily conjugated to primary amino and sulfhydryl groups, thus providing points of attachment for the cell surface-bound anti-phOx scFv. This universal cell coupling system could prove to be particularly useful for anchoring monoclonal antibodies for tumor targeting and to present co-stimulatory molecules and other ligands (even mixtures) at the cell surface for gene therapy.

Separation of E. coli expressing functional cell-wall bound antibody fragments by FACS

BACKGROUND

The rapid development of recombinant antibody technology in the last few years has facilitated the generation of antibody libraries in bacteria. Recombinant antibodies against various antigens have been selected from these libraries by presenting each antibody on the surface of a phagemid particle that contains the antibody's gene. An alternative screening system is the display of antibody fragments on bacteria. A major advantage is the possibility to select single cells directly from a large number of bacteria by using fluorescently labeled antigens and fluorescence assisted cell sorting (FACS).

OBJECTIVES

pAP is an expression vector for the bacterial display of antibody fragments. E. coli transformed with pAP express a single chain antibody (scFv) fused to the peptidoglycan-associated-lipoprotein (PAL). This fusion protein binds strongly to the cell wall. To employ this system for screening, we have investigated the possibility of selecting antigen-specific clones by FACS.

STUDY DESIGN AND RESULTS

Several DNA fragments coding for various scFvs were inserted into the pAP expression vector. E. coli were transformed with these plasmids and immunostained with fluorescent antigens under given conditions. We were able to select stained E. coli expressing a specific scFv from unstained E. coli expressing a non-binding scFv by FACS. The specific selection of the bacteria was demonstrated by amplifying their genes by PCR.

CONCLUSIONS

Conditions are described for separating E. coli containing scFv bound to their cell wall by FACS using fluorescently labeled antigens. These studies provide a basis for screening libraries of scFv antibodies.

Molecular cloning of tissue-specific transcripts of a transketolase-related gene: implications for the evolution of new vertebrate genes

As part of a systematic search for differentially expressed genes, we have isolated a novel transketolase-related gene (TKR) (HGMW-approved symbol TKT), located between the green color vision pigment gene (GCP) and the ABP-280 filamin gene (FLN1) in Xq28. Transcripts encoding tissue-specific protein isoforms could be isolated. Comparison with known transketolases (TK) demonstrated a TKR-specific deletion mutating one thiamine binding site. Genomic sequencing of the TKR gene revealed the presence of a pseudoexon as well as the acquisition of a tissue-specific spliced exon compared to TK. Since it has been postulated that the vertebrate genome arose by two cycles of tetraploidization from a cephalochordate genome, this could represent an example of the modulation of the function of a preexisting transketolase gene by gene duplication. Thiamine defiency is closely involved with two neurological disorders, Beriberi and Wernicke-Korsakoff syndromes, and in both of these conditions TK with altered activity are found. We discuss the possible involvement of TKR in explaining the observed variant transketolase forms.

Affinity enhancement of a recombinant antibody: formation of complexes with multiple valency by a single-chain Fv fragment-core streptavidin fusion

In antigen-antibody interactions, the high avidity of antibodies depends on the affinity and number of the individual binding sites. To develop artificial antibodies with multiple valency, we have fused the single-chain antibody Fv fragments to core streptavidin. The resulting fusion protein, termed scFv::strep, was found after expression in Escherichia coli in periplasmic inclusion bodies. After purification of the recombinant product by immobilized metal affinity chromatography, refolding and size-exclusion FPLC, tetrameric complexes resembling those of mature streptavidin were formed. The purified tetrameric scFv::strep complexes demonstrated both antigen- and biotin-binding activity, were stable over a wide range of pH and did not dissociate at high temperatures (up to 70 degrees C). Surface plasmon resonance measurements in a BIAlite system showed that the pure scFv::strep tetramers bound immobilized antigen very tightly and no dissociation was measurable. The association rate constant for scFv::strep tetramers was higher than those for scFv monomers and dimers. This was also reflected in the apparent constants, which was found to be 35 times higher for pure scFv::strep tetramers than monomeric single-chain antibodies. We could also show that most of biotin binding sites were accessible and not blocked by biotinylated E.coli proteins or free biotin from the medium. These sites should therefore facilitate the construction of bispecific multivalent antibodies by the addition of biotinylated ligands.

Characterization of the epitope recognized by a monoclonal antibody directed against the largest subunit of Drosophila RNA polymerase II

The epitope recognized by monoclonal antibody MAb215 generated previously against Drosophila melanogaster RNA polymerase II was mapped to amino acid residues 806-820 of the largest, 215 kDa, subunit located in a region conserved within the largest subunits of pro- and eukaryotic RNA polymerases. The affinities of MAb215 and of a recombinant single-chain Fv fragment (scFv215) were determined for binding to the enzyme as well as the fusion protein and synthetic peptides used for epitope mapping. In addition, amino acid residues of the epitope important for binding to MAb215 were identified using peptides carrying single amino acid substitutions. The epitope is not involved in the polymerization reaction or the DNA unwinding process since no inhibitory effects of the monoclonal antibody were observed in nonspecific in vitro transcription using denatured calf thymus DNA or double stranded oligo dC-tailed T7 DNA as template. In contrast, MAb215 inhibits accurate in vitro transcription from the Krüppel gene promoter and from the adenovirus-2 major late promoter. Preincubation of template DNA with the nuclear extract had no effects on inhibition supporting the notion that the epitope does not participate directly in the formation of preinitiation complexes. The same inhibitory effects were observed using scFv215. The results provide further evidence that recombinant antibody fragments produced in Escherichia coli possess the same specificity and similar affinity as their parental antibodies and demonstrate that scFv fragments are useful tools for analysis of transcriptional processes.

Human antibody libraries in Escherichia coli

A potentially vast pool of human antibodies with novel specificities for diagnostic and therapeutic purposes can be generated in Escherichia coli. Antibodies to infectious agents have already been isolated by amplifying the heavy and light chain repertoires of donor lymphocytes and they have even been rescued many years after the initial infection from memory cells cultivated in SCID mice. Eventually, however, the creation of extremely large and diverse libraries from the naive antibody repertoire of unactivated B lymphocytes or by gene synthesis using random oligonucleotides for the hypervariable regions could provide a rapid means of obtaining human antibodies to any particular antigen. An important breakthrough for exploiting the potential size and diversity of these libraries has been the development of systems for the surface display of antibodies that are physically linked to their own genes. This allows large numbers of clones to be screened simultaneously and antibodies with affinities of up to 10(8) M-1 have already been obtained using these vectors. It seems quite feasible, therefore, that antibodies with affinities approaching those obtained in the secondary immune response can be obtained by systematically optimizing the strategies for making antibody libraries. Furthermore, it might be possible to establish extremely large antibody repertoires in E. coli by the in vivo recombination of phage and plasmid antibody libraries. The affinity of the selected antibodies could be increased by chain shuffling or random mutagenesis followed by several rounds of selection under increasingly stringent conditions.

Bacterial expression and refolding of single-chain Fv fragments with C-terminal cysteines

Two antibody single-chain Fv (scFv) fragments carrying five C-terminal histidine residues were expressed in Escherichia coli as periplasmic inclusion bodies. Their variable heavy (VH) and light (VL) domains are derived from the mouse monoclonal antibody 215 (MAb215), specific for the largest subunit of RNA polymerase II of Drosophila melanogaster and rat MAb Yol1/34, specific for pig brain alpha-tubulin. ScFv-215 contains an additional cysteine residue near to its C-terminus. After solubilization of inclusion bodies followed by immobilized metal affinity chromatography (IMAC) in 6M urea and a renaturation procedure, scFv monomers, noncovalent dimers, and aggregated antibody fragments were separated by size exclusion chromatography. In addition, a fraction of disulfide-bonded scFv-215 homodimers (scFv')2 was also isolated. The various antibody forms appear to be in equilibrium after renaturation since first peak composed mainly of aggregates could be resolved into a similar pattern of aggregates, dimers, and monomers after repeating the denaturation/renaturation procedure. All fractions of the recombinant scFv-215 demonstrated high antigen-binding activity and specificity as shown by enzyme-linked immunosorbent assay (ELISA) and Western blot analysis. Affinity measurements carried out by competitive immunoassays showed that covalently linked (scFv')2 have binding constants quite close to those of the parental MAbs and fourfold higher than scFv' monomers. ScFv derivatives, specifically biotinylated through the free sulfhydryl group, recognize the corresponding antigen in ELISA and Western blot analysis, thus demonstrating the possibility of using chemically modified scFv antibodies for immunodetection.

Amino acid sequence based PCR primers for amplification of rearranged human heavy and light chain immunoglobulin variable region genes

Previously described primers for PCR amplification of variable immunoglobulin (Ig) genes were based on gene sequences. To include the large number of amino acid sequences of antibodies whose DNA has not been sequenced and to ensure a maximal fit to rearranged human Ig variable region genes, we have made a comprehensive comparison of both protein and nucleotide sequences. The resulting set of 15 primers was able to amplify a wide range of rearranged antibody variable region genes. Restriction sites included in the primers facilitate cloning of the PCR products into various expression vectors. Sequence analyses of PCR-amplified cDNA derived from a polyclonal B cell population showed that maximal enrichment is obtained for highly represented variable Ig gene subgroups. Rarely occurring V kappa 4 and V lambda 5 subgroups were not detected. Rearranged Ig variable region genes from each of 19 human B cell lines were also amplified. Comparisons to germline sequences allowed the allocation of rearranged genes to the original Ig genes. This primer set should be very useful for generating large repertoires of rearranged V genes and for amplifying genes of individual B cell clones.

A single-chain TNF receptor antagonist is an effective inhibitor of TNF mediated cytotoxicity

Tumour necrosis factor (TNF) is an important mediator of immune and inflammatory responses and has been recognized as a major pathogenic factor in several autoimmune and inflammatory diseases. TNF receptor TR60 plays a critical role in signalling the pathogenic activities of TNF. We here describe molecular cloning and bacterial production of a single-chain antibody (scFv H398) directed against TR60 which possesses antagonistic activity. VH and VL encoding sequences were isolated by PCR from the murine hybridoma cell line H398, cloned into a scFv expression vector and expressed in Escherichia coli. The recombinant antibody (Ab) fragment was found as an active soluble protein in the periplasm but also formed inclusion bodies. Re-folded scFv H398 purified from inclusion bodies was shown to be functional and stable at 37 degrees C with a half-life of 50 h. Comparison of the antigen binding characteristics of scFv with the parental enzymatically produced Fab H398 revealed that both Ab fragments have the same epitope specificity and an identical antigen binding affinity of 1.5 nM. In an in vitro assay it was demonstrated that scFv H398 is an efficient inhibitor of TNF mediated cytotoxicity with an IC50 of 22 nM, which is comparable to the antagonistic activity of natural Fab H398 with an IC50 of 12 nM. As scFv H398 possesses the high affinity TR60 binding and receptor antagonistic activity of the parental Ab H398 but is expected to be less antigenic in man, it provides a valuable tool for the development of novel therapeutic reagents against TNF mediated diseases.

Bifunctional and multimeric complexes of streptavidin fused to single chain antibodies (scFv)

Multivalent and multispecific antibodies with defined stoichiometry could provide valuable tools for biological and medical research and for the diagnosis and therapy of cancer. We have therefore fused single chain antibodies (scFv) with core-streptavidin. This chimeric protein, expressed by the vector pSTE-215 (plasmid for streptavidin-tagged expression), can form tetrameric complexes, binds antigen and contains the biotin binding site which may be used for further complex formation. An additional cysteine was inserted near the carboxy terminus to facilitate the construction of covalently linked bifunctional molecules. The scFv fusion protein could be purified by affinity chromatography using biotin analogues. We have also shown that the scFv fusion protein could be used for direct detection of its antigen in ELISA and Western blots when stained with biotinylated horseradish peroxidase.

Single-chain antibody streptavidin fusions: tetrameric bifunctional scFv-complexes with biotin binding activity and enhanced affinity to antigen

To increase the avidity of single-chain antibodies (scFv) for their antigen, we have fused them to core-streptavidin. The chimeric protein, expressed by the vector pSTE (plasmid for streptavidin-tagged expression) from Escherichia coli, can form tetrameric complexes, binds its antigen and contains four biotin binding sites per tetrameric complex. An additional cysteine inserted near the carboxy terminus further stabilised the complex. The scFv fusion protein tetramers could be enriched by affinity chromatography using the biotin analog 2-iminobiotin from periplasmic inclusion bodies after refolding. We have also shown that the scFv fusion protein could be used for direct detection of its antigen in ELISA when stained with biotinylated horseradish peroxidase. The affinity of the scFv-antibody complex was substantially increased by avidity effects due to the tetrameric structure. The biotin binding sites may be used for coupling other antibodies and molecules to form bispecific and bifunctional reagents.

Recombinant single-chain Fv fragments carrying C-terminal cysteine residues: production of bivalent and biotinylated miniantibodies

A murine antibody single-chain Fv (scFv) fragment carrying five C-terminal histidine residues preceded by a cysteine residue and a marker peptide was expressed in Escherichia coli. Its variable heavy (VH) and light (VL) domains are derived from the mouse monoclonal antibody mAb215, which is specific for the largest subunit of RNA polymerase II of Drosophila melanogaster. ScFv' monomers, covalently linked (scFv')2 and non-covalent dimers, as well as aggregated antibody fragments, were isolated from an E. coli cell paste by immobilized metal affinity chromatography in 6 M urea followed by a renaturation procedure that does not use any sulfhydryl agents. In a final step, the components were separated by size exclusion chromatography. All the recombinant antibody fractions demonstrated high antigen-binding activity and specificity as shown by ELISA and Western blot analysis. Affinity measurements carried out by competitive immunoassays showed that covalently linked (scFv')2 have binding constants quite close to those of the parental monoclonal antibodies and four-fold higher than scFv' monomers. ScFv derivatives, specifically biotinylated through the free sulfhydryl group, recognize the corresponding antigen in ELISA and Western blot analysis, thus demonstrating the possibility of using chemically modified scFv antibodies for immunodetection.

Isolation of IgG antibody Fv-DNA from various mouse and rat hybridoma cell lines using the polymerase chain reaction with a simple set of primers

To facilitate the isolation of IgG antibody Fv-DNA sequences from hybridoma cell lines, we have established a polymerase chain reaction (PCR) procedure requiring only a small number of primers. The sense primers homologous to DNA coding for the first framework sequences were designed to hybridize to all the known antibody sequences under conditions that permit a high number of mismatches. The antisense primers were homologous to DNA coding for the beginning of the constant regions of the gamma and kappa chains. Restriction sites introduced by the primers enable the DNA to be cloned into bacterial expression vectors. Only three sense VH primers and two sense VL primers paired with one backward primer for the heavy and light chains, respectively, were necessary for the amplification of Fv-DNA from a total of 17 rodent cell lines that we have so far worked with. These consisted of 12 mouse cell lines and five rat cell lines. This procedure will therefore probably be sufficient to isolate the Fv-DNA from most mouse cell lines and possibly also from most rat cell lines.

Simultaneous mutagenesis of antibody CDR regions by overlap extension and PCR

A method for the facile simultaneous mutagenesis of complementary-determining regions (CDRs) in a single chain antibody (scFv) is described. Overlapping sets of oligonucleotides containing random sequences within the CDRs corresponding to the heavy chain variable region (VH) jointed to a linker peptide (J) and the light chain variable region (VL) were extended under PCR conditions to full-length genes. These gene products were then further amplified using short PCR primers containing complementary overlaps between the 3' and 5' ends of the VH-J and VL genes respectively. In a final step, the VH-J and VL gene products were mixed and assembled into scFv DNA products by overlap extension under standard PCR conditions. Sequence analyses indicated that the method is basically successful. However, some deletions were observed, which probably reflects difficulties in the automatic synthesis of long degenerate oligonucleotides.

Production of monoclonal antibodies against epitopes of the main coat protein of filamentous fd phages

Three monoclonal antibodies (MAbs) were produced which react with epitopes of the main structural coat protein (pVIII) of filamentous fd phages as demonstrated by solid-phase fluorometric enzyme immunoassays and by immunoelectron microscopy. The antibodies are of the IgG1, IgG2a and IgG2b immunoglobulin subclasses. Since they also react with recombinant phages expressing antigen fragments in their pIII region they may be suitable reagents for the demonstration and isolation of filamentous phages used in recombinant protein technology.

Surface display of antibodies

To screen antibody libraries that contain many millions of different clones, a selection system is required with an efficiency comparable to that of the immune system. This can be achieved by displaying antibodies on the surface of microorganisms containing the antibody's gene, analogous to the expression of the IgM antigen receptor on the surface of unactivated B-lymphocytes. Specific clones can then be selected using immobilized antigens. The minor coat protein of filamentous phages, pIII, which initiates the infection of E.coli by binding to their F-pili, and the major coat protein, pVIII, have been used as carriers for displaying antibodies on the phage surface. Recombinant antibodies have also been targeted to the cell surface of bacteria by fusing them with outer membrane components derived from lipoproteins, OmpA and an IgA protease. However, only the pIII system has been routinely used for screening antibody libraries. Here we describe the various antibody surface display systems and the screening of antibody libraries generated from the gene repertoire of lymphocytes and by gene synthesis. Finally, we have made a short comparison of the bacterial production of Fabs versus single chain antibodies (scFv).

A family of vectors for surface display and production of antibodies

Expression vectors for surface display and production of single-chain (Fv) antibodies (scAb) have been constructed based on the phagemid pSEX, which expresses DNA encoding a scAb fused to the gene III product of filamentous phage [Breitling et al., Gene 104 (1991) 147-153]. A smaller version of this phagemid, pSEX20, was made by removing an unnecessary cat. To produce a vector for the surface display of other proteins and peptides, the scAb of pSEX20 was substituted by a polycloning site (MCS) to give pSEX40. For the presentation of Ab on the surface of Escherichia coli, phagemid pAP10 was derived from pSEX20 by substituting gene III with a gene encoding the peptidoglycan-associated lipoprotein (PAL). Vectors for producing scAb that can be purified by antibody and metal affinity chromatography were constructed by substituting gene III in the vector pSEX20 with DNA encoding a peptide with a C-terminal epitope recognised by a monoclonal antibody (phagemid pOPE40) or with five C-terminal histidines (pOPE 90).

Recombinant human monoclonal antibodies. Basic principles of the immune system transferred to E. coli

To produce human monoclonal antibodies in bacteria, a gene repertoire of IgM variable regions was isolated from human peripheral B lymphocytes by the polymerase chain reaction. Alternatively, synthetic antibody genes with random hypervariable regions are being generated that may provide libraries of even higher complexity. For the selection of specific monoclonal antibodies from these libraries, we have developed two E. coli vector systems that facilitate the surface display of an antibody physically linked to its own gene. The phagemid pSEX encodes a fusion protein of an antigen binding domain (Fv-antibody) with the docking protein (pIII) of filamentous phages. Specific antibody genes can therefore be enriched by antigen affinity chromatography. The plasmid pAP1 encodes a fusion protein of an Fv-antibody with a bacterial cell-wall protein. Bacteria carrying this plasmid express functional Fv-antibodies tightly bound to their surface. This should enable the selection of single cells with a fluorescence-assisted cell sorter (FACS) using labeled antigen or by adsorption to immobilized antigen. These vectors permit three major principles of the antibody response to be mimicked in E. coli: 1. Generation of a highly complex antibody repertoire; 2. Clonal selection procedures for library screening; and 3. The possibility of increasing a given affinity by repeated rounds of mutation and selection.

Regulated secretion and purification of recombinant antibodies in E. coli

A plasmid for optimized protein expression of recombinant Fv antibodies (pOPE) in E. coli was used to express the variable domains of the murine monoclonal antibody HD39 specific for the human B-cell surface antigen CD22. The production of Fv antibodies by pOPE can be regulated over a wide range by varying the IPTG concentration. Antibodies that can discriminate between secreted and nonsecreted Fv antibody fragments were used to show that secretion is the limiting step for the production of functional Fv antibodies. IPTG concentrations above 20 microM increased the total antibody production, but did not yield larger amounts of secreted Fv antibodies. The addition of five histidines to the C terminus facilitates an easy single-step enrichment procedure based on immobilized metal affinity chromatography.

A vector for the removal of deletion mutants from antibody libraries

To reduce the number of deletion mutants from antibody (Ab) libraries that had been amplified by PCR from peripheral blood lymphocytes, we constructed the Ab expression vector, pLAB, in which DNA coding for a single-chain Ab was inserted into the gene encoding beta-lactamase (Bla) at the 3'-terminus of its signal sequence. After transforming Escherichia coli with this vector, a fusion protein with a functional Bla domain was produced that was able to protect the bacteria from the action of ampicillin (Ap). Libraries can therefore be usefully propagated with this vector, since only those clones carrying inserts that are in frame with Bla will survive Ap selection, while others that carry out-of-frame deletions or internal stop codons are eliminated.

Targeting Recombinant Antibodies to the Surface of Escherichia coli: Fusion to a Peptidoglycan Associated Lipoprotein

To target recombinant antibodies to the surface of Escherichia coli, we have fused single-chain variable domains to its peptidoglycan associated lipoprotein (PAL). The fusion protein was able to bind antigen and was tightly bound to the murein layer of the cell envelope. Antibody-PAL had little effect on cell growth and viability. In contrast, the expression of single chain antibody alone eventually resulted in cell lysis. Immunofluorescence studies on unfixed cells showed that functional antibodies were accessible at the surface of intact bacteria. This could provide a means of isolating single cells producing specific antibodies from libraries in E. coli by fluorescence assisted cell sorting (FACS). Pal fusions may also be of general interest for the presentation of proteins at the surface of E. coli as, for example, in the production of live vaccines.

A surface expression vector for antibody screening

To select specific antibodies (Ab) from large recombinant libraries using small amounts of antigen, we have constructed a phagemid that expresses a single-chain Ab fused to pIII, a coliphage protein product of gene III that initiates infection by binding to F pili. Surprisingly, the production of the fusion protein (Ab::pIII) was induced by wild-type (wt) phage fd in the absence of IPTG. Ab::pIII was identified by a monoclonal Ab to an epitope in the linker sequence between the heavy and light chains, and by antisera to their N-terminal sequences. It is able to bind antigen and be assembled into infectious phagemid particles that can be enriched on columns of immobilised antigen. The phagemid DNA is even smaller than that of wt fd phages and can easily be propagated in plasmid form. Most importantly, its Ab::pIII-encoding gene can be tightly repressed so that Ab libraries can be amplified without risk of being dominated by deletion mutants. After induction, however, large quantities of the fusion protein can be produced, thus greatly facilitating its analysis.