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.