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

Inhibition of SARS-CoV-2 Spike Protein Pseudotyped Virus Infection Using ACE2-Tethered Micro/Nanoparticles

Coronavirus disease 2019 (COVID-19) has caused a global pandemic of severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The viral infection is reliant upon the binding between angiotensin-converting enzyme 2 receptor (ACE2) and spike protein (S). Therefore, ACE2 is a key receptor for SARS-CoV-2 to infect the host. Nonetheless, as SARS-CoV-2 is constantly mutating into new variants that cause high infection rates, the development of prophylactic and therapeutic approaches remains a necessity to continue fighting mutated SARS-CoV-2 variants. In this study, ACE2-streptavidin fusion proteins expressed by recombinant DNA technology were anchored on biotinylated fluorescent polystyrene particles of various sizes ranging from 0.15 to 5 µm. The ACE2-tethered micro/nanoparticles were shown to prevent spike protein pseudotyped lentivirus entry into ACE2-expressing HEK293T cells. Compared to ACE2 in soluble form, micro-sized particles (2 and 5 µm) immobilized with ACE2 interfered more efficiently with viral attachment, entry, and the ensuing infection. Our results showed that particles functionalized with ACE2 could be used as efficient decoys to block the infection of SARS-CoV-2 strains.

Eradication of Myrosinase-Tethered Cancer Cells by Allyl Isothiocyanate Derived from Enzymatic Hydrolysis of Sinigrin

Sinigrin is present in significant amounts in cruciferous vegetables. Epidemiological studies suggest that the consumption of such vegetables decreases the risk of cancer, and the effect is attributed mainly to allyl isothiocyanate (AITC), a hydrolysis product of sinigrin catalyzed by myrosinase. Anticancer activity of AITC has been previously investigated for several cancer models, but less attention was paid to delivering AITC on the target site. In this study, the gene sequences of core streptavidin (coreSA) and myrosinase (MYR) were cloned in a pET-30a(+) plasmid and transformed into BL21(DE3) E. coli competent cells. The MYR-coreSA chimeric protein was expressed and purified using immobilized metal affinity chromatography and further characterized by gel electrophoresis, Western blot, and enzyme activity assay. The purified MYR-coreSA chimeric protein was tethered on the outer membrane of biotinylated adenocarcinoma A549 cells and then treated with various concentrations of sinigrin. Our results showed that 20 µM of sinigrin inhibited the growth of A549 cells tethered with myrosinase by ~60% in 48 h. Furthermore, the levels of treated cells undertaken apoptosis were determined by Caspase-3/7 activation and Annexin-V. In summary, sinigrin harnessed like a prodrug catalyzed by myrosinase to the production of AITC, which induced cell apoptosis and arrested the growth of lung cancer cells.

Less phagocytosis of viral vectors by tethering with CD47 ectodomain

Many viral vectors, which are effective when administrated in situ, lack efficacy when delivered intravenously. The key reason for this is the rapid clearance of the viruses from the blood circulation via the immune system before they reach target sites. Therefore, avoiding their clearance by the immune system is essential. In this study, lentiviral vectors were tethered with the ectodomain of self-marker protein CD47 to suppress phagocytosis via interacting with SIRPα on the outer membrane of macrophage cells. CD47 ectodomain and core-streptavidin fusion gene (CD47ED-coreSA) was constructed into pET-30a(+) plasmid and transformed into Lemo21 (DE3) competent E. coli cells. The expressed CD47ED-coreSA chimeric protein was purified by cobalt-nitrilotriacetate affinity column and characterized by SDS-PAGE and western blot. The purified chimeric protein was anchored on biotinylated lentivirus via biotin-streptavidin binding. The CD47ED-capped lentiviruses encoding GFP were used to infect J774A.1 macrophage cells to assess the impact on phagocytosis. Our results showed that the overexpressed CD47ED-coreSA chimeric protein was purified and bound on the surface of biotinylated lentivirus which was confirmed via immunoblotting assay. The process to produce biotinylated lentivirus did not affect native viral infectivity. It was shown that the level of GFP expression in J774A.1 macrophages transduced with CD47ED-lentiviruses was threefold lower in comparison to control lentiviruses, indicating an antiphagocytic effect triggered by the interaction of CD47ED and SIRPα. Through the test of blocking antibodies against CD47ED and/or SIRPα, it was confirmed that the phagocytosis inhibition was mediated through the CD47ED-SIRPα axis signaling. In conclusion, surface immobilization of CD47ED on lentiviral vectors inhibits their phagocytosis by macrophages. The chimeric protein of CD47 ectodomain and core-streptavidin is effective in mediating the surface binding and endowing the lentiviral nanoparticles with the antiphagocytic property.

Construction, characterization and crystal structure of a fluorescent single-chain Fv chimera

In vitro display technologies based on phage and yeast have a successful history of selecting single-chain variable fragment (scFv) antibodies against various targets. However, single-chain antibodies are often unstable and poorly expressed in Escherichia coli. Here, we explore the feasibility of converting scFv antibodies to an intrinsically fluorescent format by inserting the monomeric, stable fluorescent protein named thermal green, between the light- and heavy-chain variable regions. Our results show that the scTGP format maintains the affinity and specificity of the antibodies, improves expression levels, allows one-step fluorescent assay for detection of binding and is a suitable reagent for epitope binning. We also report the crystal structure of an scTGP construct that recognizes phosphorylated tyrosine on FcεR1 receptor of the allergy pathway.

Mesenchymal stem cells anchored with thymidine phosphorylase for doxifluridine-mediated cancer therapy

Many tumors express thymidine phosphorylase (TYMP) with various levels, however due to tumor heterogeneity, the amount of TYMP is usually not enough to convert prodrug doxifluridine (5'-DFUR) to toxic drug 5-fluorouracil (5-FU). Since human mesenchymal stem cells (hMSCs) have unique features of tumor-tropism and low immunogenicity, the purpose of this study is to use mesenchymal stem cells as carriers to deliver TYMP to cancer cells and then trigger their death by administrating doxifluridine. First, the TYMP gene sequence and core streptavidin (core SA) were constructed into pET-30a(+) plasmid. After bacterial transformation and colony screening, TYMP-SA fusion protein was expressed by IPTG induction and purified by immobilized metal affinity chromatography and characterized by SDS-PAGE and western blot with a clear band at 75 kDa. The characterized TYMP-SA was further anchored on the cell membrane of biotinylated hMSCs via biotin-streptavidin binding. hMSCs anchored with TYMP-SA were then co-cultured with adenocarcinoma A549 cells (with different ratios) and treated with 100 μM prodrug doxifluridine over the course of four days. Our results showed that a 2 : 1 ratio led to the eradication of A549 cells at the end of the experiment with less than 5% confluency, in comparison with the 1 : 1 and 1 : 2 ratios which still had about 13% and 20% confluency respectively. In conclusion, harnessing hMSCs as cell carriers for the delivery of TYMP enzyme to cancer cells could lead to significant cell death post-treatment of the prodrug doxifluridine.

Efficient Expression of Soluble Recombinant Protein Fused with Core-Streptavidin in Bacterial Strain with T7 Expression System

The limited amount of fusion protein transported into cytosol milieu has made it challenging to obtain a sufficient amount for further applications. To avoid the laborious and expensive task, T7 promoter-driving pET-30a(+) coding for chimeric gene of thymidine phosphorylase and core streptavidin as a model system was constructed and transformed into a variety of E. coli strains with T7 expression system. Our results demonstrated that the pET-30a(+)-TP-coreSA/Lemo21(DE3) system is able to provide efficient expression of soluble TP-coreSA fusion protein for purification. Moreover, the eluted TP-coreSA fusion protein tethered on biotinylated A549 carcinoma cells could effectively eliminate these malignant cells after administrating prodrug 5′-DFUR.

Blockade of macrophage adhesion to CD200-treated polystyrene culture surface

CD200 is an anti-inflammatory transmembrane glycoprotein in the immunoglobulin superfamily. The interaction of CD200 and its receptor CD200R has shown to inhibit inflammatory response of myeloid cells to foreign materials. The purpose of this study is to create a CD200 immobilized biomaterial surface through polydopamine coating to suppress macrophage cell adhesion and reduce inflammatory cytokine secretion accordingly by macrophages. In this study, tissue-culture treated polystyrene (TCPS) surface was modified with biotin through polydopamine coating. Purified CD200-streptavidin fusion protein was then immobilized onto the biotinylated TCPS surface through the high affinity between biotin and streptavidin. Mouse J774A.1 macrophages were seeded on CD200-immobilized TCPS surface to evaluate the effect of CD200 on preventing macrophage attachment. The effects of CD200-immobilized TCPS surface on pro-inflammatory cytokine secretion from J774A.1 macrophages were measured by enzyme-linked immunosorbent assay. As a result, CD200-immobilized TCPS surface suppressed macrophage attachment for up to 9 hr. The level of IL-6 and TNF-α secreted from J774A.1 macrophages interacted with CD200-coated TCPS surface was reduced by 36.3% and 32.4%, respectively.

Diminution of Phagocytosed Micro/Nanoparticles by Tethering with Immunoregulatory CD200 Protein

CD200 is known as an anti-inflammatory transmembrane glycoprotein in the immunoglobulin superfamily. CD200 interacts with its receptor CD200R which is highly expressed on myeloid cells such as macrophages and neutrophils. CD200-CD200R interaction has known to reduce macrophage activation and chronic inflammation. To harness the immunomodulatory property of CD200 for surface modification, CD200-streptavidin fusion protein was expressed from bacteria transformed with pET20b plasmid encoded with CD200 extracellular domain and core streptavidin. The purified CD200-SA protein was bound to biotin-coated fluorescent polystyrene particles of various sizes ranging from 0.15 to 2 µm. THP-1 macrophages were cultivated with CD200-modified micro/nanoparticles in comparison with controls. Our results showed that both nano- and micro-sized particles decorated with CD200 decreased phagocytosis activities of THP-1 macrophages. Such diminution of phagocytosis was examined to be associated with downregulation of Toll-like receptor 4 (TLR4) expression on the surface of macrophages. Moreover, THP-1 macrophages treated with CD200-coated particles decreased the secretion of tumor necrosis factor-α (TNF-α).

Dual functionality of lamprey VLRB C-terminus (LC) for multimerization and cell surface display

Lamprey, one of the living representatives of jawless vertebrates, uses variable lymphocyte receptors B (VLRB) for antigen recognition, rather than immunoglobulin (Ig) based receptors as used by higher vertebrates. The C-terminus of lamprey VLRB (LC) possess a glycosylphosphatidylinositol (GPI) signal sequence and seven cysteine residues providing dual functionality of the VLRB antibody in the form of a humoral agglutinin and cell membrane receptors. Here, we show that the LC can be either secreted or be membrane anchored as a heterologous fused protein in a multimeric form comprising of eight or ten monomeric units. Using serially truncated LC variants, we showed that the LC, in which the last three amino acid "RKR" were deleted, referred to as LC7, was the most suitable domain for multimeric construction, whereas, the intact LC is more tailored for applications involving membrane anchorage. We show that an antibody specific for viral hemorrhagic septicemia virus (VHSV) (VLR43), displayed on HEK-293F cells using a PiggyBac (PB) transposase system, exhibited a dose-dependent reaction with its antigen, verifying that the LC can be applied in antibody display technology. Therefore, the present report provides valuable insight into the structure of the lamprey VLRB and highlights its potential use as a novel fusion partner for multimerization and membrane anchorage of chimeric proteins.

Generation and characterization of hagfish variable lymphocyte receptor B against glycoprotein of viral hemorrhagic septicemia virus (VHSV)

Variable lymphocyte receptors B (VLRBs) are non-immunoglobulin components of the humoral immune system in jawless vertebrates including hagfish (Eptatretus burgeri) and lamprey (Petromyzon marinus). Hagfish VLRBs consist of leucine rich repeat (LRR) modules with a superhydrophobic C-terminal tail, the latter of which leads to extremely low expression levels in recombinant protein technology. Here, we present an artificially oligomerized VLRB (arVLRB) that conjugates via the C4bp oligomerization domain derived from human C4b-binding protein (hC4bp) rather than the superhydrophobic tail. The resulting arVLRB had a tightly multimerized form with seven monomeric VLRB arms and showed high expression and secretion levels in a mammalian expression system. To isolate antigen-specific arVLRB, we constructed large VLRB libraries from hagfish immunized with the fish pathogen, viral hemorrhagic septicemia virus (VHSV). The selected arVLRBs were found to recognize various types of antigens, including the recombinant target protein, purified viruses, and progeny viruses, with high antigen binding abilities and specificities. We also performed in vitro affinity maturation of the arVLRBs through LRRCT mutagenesis, and found that this enhanced their antigen-binding properties by at least 125-fold. Our epitope mapping analysis revealed that ³⁷DWDTPL⁴², which is located in a region conserved among the glycoproteins of all VHSV isolates, is the recognition epitope of the arVLRBs. Thus, our newly developed arVLRB could prove useful in the development of universal diagnostic tools and/or therapeutic agents for the virus. Together, our novel findings provide valuable insights into hagfish VLRB and its potential use as a novel alternative to conventional antibodies for biotechnological applications.

Evaluation of a Pretargeting Strategy for Molecular Imaging of the Prostate Stem Cell Antigen with a Single Chain Antibody

In pretargeted radio-immunotherapy, the gradual administration of a non-radioactive tumor antigen-addressing antibody-construct and the subsequent application of a radioactive labeled, low molecular weight substance enable a highly effective and selective targeting of tumor tissue. We evaluated this concept in prostate stem cell antigen (PSCA)-positive cancers using the antigen-specific, biotinylated single chain antibody scFv(AM1)-P-BAP conjugated with tetrameric neutravidin. To visualize the systemic biodistribution, a radiolabeled biotin was injected to interact with scFv(AM1)-P-BAP/neutravidin conjugate. Biotin derivatives conjugated with different chelators for complexation of radioactive metal ions and a polyethylene glycol linker (n = 45) were successfully synthesized and evaluated in vitro and in a mouse xenograft model. In vivo, the scFv(AM1)-P-BAP showed highly PSCA-specific tumor retention with a PSCA⁺ tumor/PSCA^- tumor accumulation ratio of ten. PEGylation of radiolabeled biotin resulted in lower liver uptake improving the tumor to background ratio.

Purification of CD47-streptavidin fusion protein from bacterial lysate using biotin-agarose affinity chromatography

CD47 is a widely expressed transmembrane glycoprotein that modulates the activity of a plethora of immune cells via its extracellular domain. Therefore, CD47 plays important roles in the regulation of immune responses and may serve as targets for the development of immunotherapeutic agents. To make sure CD47 functionality is intact under the process of protein conjugation, CD47-streptavidin fusion protein was expressed and purified because it can easily bind to biotin-tagged materials via the unique biotin-streptavidin affinity. In this study, gene sequences of CD47 extracellular domain (CD47ECD) and core streptavidin (coreSA) with a total 834 bp were inserted into pET20b plasmid to construct recombinant plasmid encoding CD47-SA fusion gene. After bacteria transformation, the CD47-SA fusion protein was expressed by isopropyl-β-d-thiogalactopyranoside (IPTG) induction. The collected bacteria lysate was loaded on biotinylated agarose to proceed the purification of CD47-SA fusion protein. Due to the unexpected high affinity between biotin and coreSA, standard washing and elution approaches (e.g., varying pH, using biotin, and applying guanidine hydrochloride) reported for biotin-streptavidin affinity chromatography were not able to separate the target fusion protein. Instead, using low concentration of the non-ionic detergent Triton X-100 followed with alkaline buffer could efficiently weaken the binding between biotin and coreSA, thereby eluting out CD47-SA fusion protein from the biotin agarose column. The purified CD47-SA fusion protein was further characterized by molecular biology methods and its antiphagocytic functionality was confirmed by the phagocytosis assay. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:949-958, 2016.

Twigged streptavidin polymer as a scaffold for protein assembly

Protein assemblies are an emerging tool that is finding many biological and bioengineering applications. We here propose a method for the site-specific assembly of proteins on a twigged streptavidin (SA) polymer using streptavidin as a functional scaffold. SA was genetically appended with a G tag (sortase A recognition sequence) and a Y tag (HRP recognition sequence) on its N- and C-termini, respectively, to provide G-SA-Y. G-SA-Y was polymerized using HPR-mediated tyrosine coupling, then fluorescent proteins were immobilized on the polymer by biotin-SA affinity and sortase A-mediated ligation. Fluorescence measurements showed that the proteins were immobilized in close proximity to each other. Hydrolyzing enzymes were also functionally assembled on the G-SA-Y polymer. The site-specific assembly of proteins on twigged SA polymer may find new applications in various biological and bioengineering fields.

A novel multivalent (99m)Tc-labeled EG2-C4bpα antibody for targeting the epidermal growth factor receptor in tumor xenografts

INTRODUCTION

The C4b binding protein (C4bp) α/β-chain C-terminal effectively induces polymerization during protein synthesis. Using this fragment and the single-domain antibody EG2, which targets the epidermal growth factor receptor (EGFR), we generated the novel multimeric antibody EG2-C4bpα. We radiolabeled EG2-C4bpα with (99m)Tc and evaluated its targeting efficiency and pharmacokinetics in tumor xenografts.

METHODS

EGFR expression and EGFR-EG2-C4bpα binding was evaluated in A431 and OCM-1 cells by Western blotting and flow cytometry, respectively. EG2-C4bpα was radiolabeled with [(99m)Tc(CO)3(OH2)3](+) using a tricarbonyl vial followed by purification on a PD-10 column. In vitro studies with (99m)Tc-EG2-C4bpα were performed in A431 and/or OCM-1 cells. Single photon emission computed tomography (SPECT) imaging and biodistribution studies were carried out in (99m)Tc-EG2-C4bpα-injected mice bearing A431- and OCM-1-derived tumors. EGFR immunofluorescent staining in A431 and OCM-1 tumors was performed.

RESULTS

A431 cells showed higher EGFR expression levels than OCM-1 cells, and flow cytometry confirmed EG2-C4bpα bound more A431 cells than OCM-1 cells. (99m)Tc-EG2-C4bpα was successfully prepared with radiochemical yields of 30.3-50.4%. The binding affinity of (99m)Tc-EG2-C4bpα to A431 cells was approximately 20 nM. (99m)Tc-EG2-C4bpα specifically bound A431 cells and this binding was blocked by 41% in the presence of 50 nM excess unlabeled EG2-C4bpα. In vivo radioactivity uptake in A431 tumors was detected 2h after (99m)Tc-EG2-C4bpα administration and sustained up to 18h. The highest ratio of A431 tumor-to-muscle and tumor-to-blood was 3.69 ± 0.48 at 10h and 0.77 ± 0.14 at 20 h, respectively. Excess unlabeled EG2-C4bpα blocked radioactivity uptake in A431 tumors by 55% at 10h. (99m)Tc-EG2-C4bpα was barely detectable in OCM-1 tumors, and biodistribution analysis confirmed that radioactivity uptake was significantly lower than in A431 tumors.

CONCLUSIONS

(99m)Tc-EG2-C4bpα specifically and efficiently targets EGFR over-expressing tumors suggesting that EG2-C4bpα may be a promising antibody alternative for future diagnostic application and potential radioimmunotherapy. However, the high activity in the blood and liver, and the relative low ratio of tumor-to-blood should be noticed and improved.

Recombinant streptavidin nanopeptamer anti-immunocomplex assay for noncompetitive detection of small analytes

Short peptide loops selected from phage libraries can specifically recognize the formation of hapten-antibody immunocomplexes and can thus be used to develop phage anti-immunocomplex assays (PHAIA) for noncompetitive detection of small molecules. In this study, we generated recombinant chimeras by fusing anti-immunocomplex peptides selected from phage libraries to the N- or C-termini of core streptavidin and used them to setup phage-free noncompetitive assays for the herbicide clomazone (MW 240 Da). The best conditions for refolding were optimized by a high throughput screening allowing to obtain tens of mg of purified protein per liter of culture. The noncompetitive assay developed with these chimeras performed with a 50% saturating concentration (SC50) of 2.2 ± 0.3 ng/mL and limit of detection (LOD) of 0.48 ng/mL. Values that are 13- and 8-fold better that those obtained for the SC50 and LOD of the competitive assay setup with the same antibody. Apart from the first demonstration that recombinant peptide-streptavidin chimeras can be used for sensitive immunodetection of small molecules with a positive readout, this new assay component is a highly standardized reagent with a defined stoichiometry, which can be used in combination with the broad option of existing biotinylated reagents offering a great versatility for the development of conventional immunoassay and biosensors. The utility of the test was demonstrated analyzing the clomazone runoff during the rice growing season in northern Uruguay.

Bioconjugates of rhizavidin with single domain antibodies as bifunctional immunoreagents

Use of the avidin-biotin binding interaction for immunoassay applications is widespread. One advantageous immunoreagent is the recombinant fusion of an antibody fragment with a biotin binding protein. These genetic fusions alleviate the need to prepare chemical conjugates to achieve molecules that combine target recognition with signal transduction or to facilitate the directional immobilization of the binding element. In order for such a fusion protein to be useful, however, it must be able to be produced in good yield. Unfortunately, recombinant production of avidin or streptavidin as well as bioconjugates derived thereof has been problematic. An alternative biotin binding molecule called rhizavidin has been described, which forms a homodimer instead of a tetramer, but it has not been evaluated in genetic fusions with antibody binding domains. Single domain antibodies, the variable domain derived from camelid heavy chain only antibodies, offer binding domains with high affinity, and solubility that are well expressed in Escherichia coli. In this work, we prepared an anti-ricin single domain antibody - rhizavidin bioconjugate and evaluated it on the basis of its production in E. coli and on its activity in comparison to a streptavidin core bioconjugate and unfused single domain antibody. The single domain antibody-rhizavidin bioconjugate produced much better than its streptavidin core counterparts, yielding an average of 14 mg/L, a 20-fold improvement. When used in assays the rhizavidin conjugate provided the same desirable characteristics as the streptavidin core fusion as both capture and detection reagents. Since rhizavidin and single domain antibodies both display impressive thermal stabilities their fusion provides a route to achieve robust bifunctional immunoreagents.

Creation and Evaluation of a Single-chain Antibody Tetramer that Targets Brain Endothelial Cells

Antibodies that target and internalize into blood-brain barrier (BBB) endothelial cells offer promise as drug delivery agents. Previously, we identified a single-chain antibody (scFvA) capable of binding to the BBB. In an attempt to improve the binding and internalization properties of the single chain antibody (scFvA), a biotinylation tag (Avitag) was fused to scFvA and the protein secreted by yeast. The scFvA-Avitag could be biotinylated by yeast-displayed BirA enzyme and biotinylated scFvA-Avitag could be used to create scFv tetramers. Tetramerization of scFvA improved the internalization of scFvA into BBB endothelial cells, and biotinylated scFvA-Avitag could also be used to target streptavidin-coated quantum dots for BBB endothelial cell internalization. Perfusing the rat brain with scFvA-tetramer confirmed that the antigen targeted by scFvA is distributed on blood side of the BBB, suggesting the potential for downstream application of scFvA in brain-targeted drug delivery.

The IgM CH2 domain as covalently linked homodimerization module for the generation of fusion proteins with dual specificity

Dimeric assembly of antibody fragments and other therapeutic molecules can result in increased binding and improved bioactivity. Here, we investigated the use of the IgM heavy chain domain 2 (MHD2) as covalently linked homodimerization module. Fusion of single-chain fragment variable (scFv) molecules directed against epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor 2 to the N- and/or C-terminus of the MHD2, respectively, resulted in molecules with single or dual specificity for tumor cells. Bispecific tetravalent molecules were further generated by fusing a bispecific single-chain diabody directed against EGFR and epithelial cell adhesion molecule to the N-terminus of the MHD2. By combining an anti-EGFR scFv with a single-chain derivative of tumor necrosis factor, a tetravalent bifunctional fusion protein was produced. This fusion protein exhibited improved TNF activity, also mimicking the membrane-bound form of TNF, as shown by the activation of TNFR2-mediated cell killing. Furthermore, the scFv moiety allowed for an antigen-dependent delivery of TNF to EGFR-positive cells and an improved stimulatory TNF action on these cells. Thus, we established the MHD2 as a versatile module for the generation of bispecific and bifunctional fusion proteins.

Use of phage display for the identification of molecular sensors specific for activated Rho

We describe a phage display approach to select active Rho-specific scFv sensors. This in vitro technique allows preserving the antigen conformation stability all along the selection process. We used the GTP locked RhoBQ63L mutant as antigen against the Griffin.1 library composed of a human synthetic V(H) + V(L) scFv cloned in the pHEN2 phagemid vector. The method described here has permitted to identify an scFv that discriminates between the activated and the inactivated form of the Rho subfamily.

Recombinant antibodies and in vitro selection technologies

Over the past decade, the accumulation of detailed knowledge of antibody structure and function has enabled antibody phage display to emerge as a powerful in vitro alternative to hybridoma methods for creating antibodies. Many antibodies produced using phage display technology have unique properties that are not obtainable using traditional hybridoma technologies. In phage display, selections are performed under controlled, in vitro conditions that are tailored to suit demands of the antigen and the sequence encoding the antibody is immediately available. These features obviate many of the limitations of hybridoma methodology, and because the entire process relies on scalable molecular biology techniques, phage display is also suitable for high-throughput applications. Thus, antibody phage display technology is well suited for genome-scale biotechnology and therapeutic applications. This review describes the antibody phage display technology and highlights examples of antibodies with unique properties that cannot easily be obtained by other technologies.

Beyond natural antibodies: the power of in vitro display technologies

In vitro display technologies, best exemplified by phage and yeast display, were first described for the selection of antibodies some 20 years ago. Since then, many antibodies have been selected and improved upon using these methods. Although it is not widely recognized, many of the antibodies derived using in vitro display methods have properties that would be extremely difficult, if not impossible, to obtain by immunizing animals. The first antibodies derived using in vitro display methods are now in the clinic, with many more waiting in the wings. Unlike immunization, in vitro display permits the use of defined selection conditions and provides immediate availability of the sequence encoding the antibody. The amenability of in vitro display to high-throughput applications broadens the prospects for their wider use in basic and applied research.

Isolation of scFv fragments specific to OmpD of Salmonella Typhimurium

Pork meat is one of the major sources for human infections with Salmonella enterica subspecies enterica serovars. Further, zoonoses caused by S. enterica subspecies enterica serovars are responsible for substantial economical losses in industrial countries. Quick and reliable detection of this infection is urgently needed to improve consumer security. Due to its capability to identify infections independent of the species, a competitive ELISA is the preferable method for the detection of anti-Salmonella antibodies in serum. Recombinant antibody fragments (scFvs) were isolated from the naive human antibody gene library HAL7 by phage display. Recombinant produced outer membrane protein D (OmpD) of Salmonella Typhimurium was used as antigen. The characterization of the isolated single chain Fv (scFv) antibodies was done by enzyme-linked immunosorbent assay (ELISA), immunoblot, sequencing, epitope mapping and size exclusion chromatography (SEC). The detection of anti-OmpD IgGs in swine sera by competitive ELISA was shown in a proof of principle concept. Furthermore, the developed competitive ELISA would be compatible to a recently published DIVA vaccine, allow to distinguish between infected and vaccinated pigs.

Antibody vectors for imaging

Noninvasive molecular imaging approaches include nuclear, optical, magnetic resonance imaging, computed tomography, ultrasound, and photoacoustic imaging, which require accumulation of a signal delivered by a probe at the target site. Monoclonal antibodies are high affinity molecules that can be used for specific, high signal delivery to cell surface molecules. However, their long circulation time in blood makes them unsuitable as imaging probes. Efforts to improve antibodies pharmacokinetics without compromising affinity and specificity have been made through protein engineering. Antibody variants that differ in antigen binding sites and size have been generated and evaluated as imaging probes to target tissues of interest. Fast clearing fragments, such as single-chain variable fragment (scFv; 25 kDa), with 1 antigen-binding site (monovalent) demonstrated low accumulation in tumors because of the low exposure time to the target. Using scFv as building block to produce larger, bivalent fragments, such as scFv dimers (diabodies, 50 kDa) and scFv-fusion proteins (80 kDa minibodies and 105 kDa scFv-Fc), resulted in higher tumor accumulation because of their longer residence time in blood. Imaging studies with these fragments after radiolabeling have demonstrated excellent, high-contrast images in gamma cameras and positron emission tomography scanners. Several studies have also investigated antibody fragments conjugated to fluorescence (near infrared dyes), bioluminescence (luciferases), and quantum dots for optical imaging and iron oxides nanoparticles for magnetic resonance imaging. However, these studies indicate that there are several factors that influence successful targeting and imaging. These include stability of the antibody fragment, the labeling chemistry (direct or indirect), whether critical residues are modified, the number of antigen expressed on the cell, and whether the target has a rapid recycling rate or internalizes upon binding. The preclinical data presented are compelling, and it is evident that antibody-based molecular imaging tracers will play an important future role in the diagnosis and management of cancer and other diseases.

Quick method of multimeric protein production for biologically active substances such as human GM-CSF (hGM-CSF)

The C-terminal fragment of C4b-binding protein (C4BP)-based multimerizing system was applied to hGM-CSF to induce dendritic cells (DCs) from peripheral blood monocytes (PBMCs), to see whether the C4BP could stimulate immature DCs, since DCs, equipped with pattern recognition receptors such as toll-like receptors (TLRs), are hypersensitive to various immunologically active molecules like LPS. hGM-CSF gene was merged to the 3'-terminal region of the C4BPalpha-chain gene, and the transfected human 293FT cells produced sufficient amount of octameric hGM-CSF, which resulted in iDCs with the same phenotype and the same response to a TRL4 ligand, LPS and a TLR3 ligand, poly I:C, as those induced with authentic monomeric hGM-CSF. These results suggest that the C4BP-based multimerizing system could facilitate the design of self-associating multimeric recombinant proteins without stimulating iDCs, which might be seen with the other multimerizing systems such as that using Fc fragment of IgM.

Production of multivalent protein binders using a self-trimerizing collagen-like peptide scaffold

A class of multivalent protein binders was designed to overcome the limitations of low-affinity therapeutic antibodies. These binders, termed "collabodies," use a triplex-forming collagen-like peptide to drive the trimerization of a heterologous target-binding domain. Different forms of collabody, consisting of the human single-chain variable fragment (scFv) fused to either the N or C terminus of the collagen-like peptide scaffold (Gly-Pro-Pro)(10), were stably expressed as soluble secretory proteins in mammalian cells. The collabody consisting of scFv fused to the N terminus of collagen scaffold is present as a homotrimer, whereas it exhibited a mixture of trimer and interchain disulfide-bonded hexamer when cysteine residues were introduced and flanked the scaffold. The collagenous motif in collabody is prolyl-hydroxylated, with remarkable thermal and serum stabilities. The collabody erb_scFv-Col bound to the extracellular domain of epidermal growth factor receptor with a binding strength approximately 20- and 1000-fold stronger than the bivalent and monovalent counterparts, respectively. The trimeric collagen scaffold does not compromise the functionality of the binding moieties of parental immunoglobulin G (IgG); therefore, it could be applied to fuse other protein molecules to acquire significantly improved targeting-binding strengths.

Cloning and characterization of a single chain antibody to glucose oxidase from a murine hybridoma

Glucose oxidase (GOD) is an oxidoreductase catalyzing the reaction of glucose and oxygen to peroxide and gluconolacton (EC 1.1.3.4.). GOD is a widely used enzyme in biotechnology. Therefore the production of monoclonal antibodies and antibody fragments to GOD are of interest in bioanalytics and even tumor therapy. We describe here the generation of a panel of monoclonal antibodies to native and heat inactivated GOD. One of the hybridomas, E13BC8, was used for cloning of a single chain antibody (scFv). This scFv was expressed in Escherichia coli XL1-blue with the help of the vector system pOPE101. The scFv was isolated from the periplasmic fraction and detected by western blotting. It reacts specifically with soluble active GOD but does not recognize denatured GOD adsorbed to the solid phase. The same binding properties were also found for the monoclonal antibody E13BC8.

Isolation of Lawsonia intracellularis specific single-chain Fv antibody fragments from phage display library

Single-chain antibodies (scFv) exhibiting specific binding to Lawsonia intracellularis were isolated from a phagemid library expressing scFvs molecules on the surface of filamentous bacteriophages. For scFv selection whole bacterial cells were used and individual clones were tested in ELISA test. The total of seven unique clones with different fingerprint profiles was isolated. All clones were able to bind specifically in immunofluorescence assay. This is the first report of species specific recombinant antibodies against L. intracellularis.

Design of a bifunctional fusion protein for ovarian cancer drug delivery: single-chain anti-CA125 core-streptavidin fusion protein

We have developed a universal ovarian cancer cell targeting vehicle that can deliver biotinylated therapeutic drugs. A single-chain antibody variable domain (scFv) that recognizes the CA125 antigen of ovarian cancer cells was fused with a core-streptavidin domain (core-streptavidin-VL-VH and VL-VH-core-streptavidin orientations) using recombinant DNA technology and then expressed in Escherichia coli using the T7 expression system. The bifunctional fusion protein (bfFp) was expressed in a shaker flask culture, extracted from the periplasmic soluble protein, and affinity purified using an IMAC column. The two distinct activities (biotin binding and anti-CA125) of the bfFp were demonstrated using ELISA, Western blot and confocal laser-scanning microscopy (CLSM). The ELISA method utilized human NIH OVCAR-3 cells along with biotinylated bovine serum albumin (B-BSA) or biotinylated liposomes, whereas, the Western blot involved probing with B-BSA. The CLSM study has shown specificity in binding to the OVCAR-3 cell-line. ELISA and Western blot studies have confirmed the bifunctional activity and specificity. In the presence of bfFp, there was enhanced binding of biotinylated antigen and liposome to OVCAR-3 cells. In contrast, the control EMT6 cells, which do not express the CA125 antigen, showed minimal binding of the bfFp. Consequently, bfFp based targeting of biotinylated therapeutic drugs, proteins, liposomes, or nanoparticles could be an alternative, convenient method to deliver effective therapy to ovarian cancer patients. Peritoneal infusion of the bfFp-therapeutic complex could also be effective in locally targeting the most common site of metastatic spread.

A genetically engineered anti-CD45 single-chain antibody-streptavidin fusion protein for pretargeted radioimmunotherapy of hematologic malignancies

Acute myelogenous leukemia (AML) currently kills the majority of afflicted patients despite combination chemotherapy and hematopoietic cell transplantation (HCT). Our group has documented the promise of radiolabeled anti-CD45 monoclonal antibodies (Ab) administered in the setting of allogeneic HCT for AML, but toxicity remains high, and cure rates are only 25% to 30% for relapsed AML. We now show the superiority of pretargeted radioimmunotherapy (PRIT) compared with conventional radioimmunotherapy using a recombinant tetravalent single-chain Ab-streptavidin (SA) fusion protein (scFv(4)SA) directed against human CD45, administered sequentially with a dendrimeric N-acetylgalactosamine-containing clearing agent and radiolabeled 1,4,7,10-tetraazacyclododecane-N,N',N'',N'''-tetraacetic (DOTA)-biotin. The scFv(4)SA construct was genetically engineered by fusing Fv fragments of the human CD45-specific BC8 Ab to a full-length genomic SA gene and was expressed as a soluble tetramer in the periplasmic space of Escherichia coli. The fusion protein was purified to >95% homogeneity at an overall yield of approximately 50% using iminobiotin affinity chromatography. The immunoreactivity and avidity of the fusion protein were comparable with those of the intact BC8 Ab, and the scFv(4)SA construct bound an average of 3.9 biotin molecules out of four theoretically possible. Mouse lymphoma xenograft experiments showed minimal toxicity, excellent tumor-specific targeting of the fusion protein and radiolabeled DOTA-biotin in vivo, marked inhibition of tumor growth, and cured 100% of mice bearing CD45-expressing tumors. These promising results have prompted large-scale cGMP production of the BC8 fusion protein for clinical trials to be conducted in patients with hematologic malignancies.

Tailoring antibodies for radionuclide delivery

Therapeutic antibodies are well established as an important class of drugs in modern medicine. The exquisite specificity and affinity for a specific target offered by antibodies has also encouraged their development as delivery vehicles for agents such as radionuclides to target tissues, for radioimmunoimaging and radioimmunotherapy. Specifically, in nuclear medicine, radionuclide-conjugated antibody molecules make it possible to image diseased loci with greater sensitivity than other imaging modalities such as magnetic resonance imaging. Furthermore, two radionuclide-conjugated antibodies have recently been approved for the therapy of non-Hodgkin's lymphoma. However, optimal implementation of antibodies has been limited by the extended circulation persistence that is characteristic of native antibodies, which is responsible for increased background activity in radioimmunoimaging applications and dose-related normal organ toxicities in radioimmunotherapy. In this article the current status of radiolabelled intact antibodies is reviewed, focusing on strategies to improve their pharmacokinetic properties to suit a desired application. Examples from the literature that represent different approaches to accomplishing this task in terms of their successes as well as limitations, and perspectives for the future are discussed.

Arming antibodies: prospects and challenges for immunoconjugates

Immunoconjugates--monoclonal antibodies (mAbs) coupled to highly toxic agents, including radioisotopes and toxic drugs (ineffective when administered systemically alone)--are becoming a significant component of anticancer treatments. By combining the exquisite targeting specificity of mAbs with the enhanced tumor-killing power of toxic effector molecules, immunoconjugates permit sensitive discrimination between target and normal tissue, resulting in fewer toxic side effects than most conventional chemotherapeutic drugs. Two radioimmunoconjugates, ibritumomab tiuxetan (Zevalin) and tositumomab-131I (Bexxar), and one drug conjugate, gemtuzumab ozogamicin (Mylotarg), are now on the market. For the next generation of immunoconjugates, advances in protein engineering will permit greater control of mAb targeting, clearance and pharmacokinetics, resulting in significantly improved delivery to tumors of radioisotopes and potent anticancer drugs. Pre-targeting strategies, which separate the two functions of antibody-based localization and delivery or generation of the toxic agent into two steps, also promise to afford superior tumor targeting and therapeutic efficacy. Several challenges in optimizing immunoconjugates remain, however, including poor intratumoral mAb uptake, normal tissue conjugate exposure and issues surrounding drug potency and conditional release from mAb carriers. Nonetheless, highly promising results from preclinical models will continue to drive the clinical development of this therapeutic class.

Reduced phagocytosis of colloidal carriers using soluble CD47

PURPOSE

This study was designed to illustrate the feasibility of using soluble CD47 protein to antagonize phagocytosis of colloidal drug carriers by macrophages.

METHODS

Expression of CD47-streptavidin (CD47-SA) fusion protein was achieved in B21CodonPlus host cells following IPTG induction. Murine macrophage cell line J774A.1, expressing high levels of SIRPalpha, was selected as the biologic model system for phagocytosis. FITC-labeled perfluorocarbon (PFC) emulsions were used as the colloidal carriers to trigger phagocytosis. Microscopy (inverted light and UV-fluorescence) and flow cytometry were used to qualitatively and quantitatively determine the degree of phagocytosis, respectively.

RESULTS

The bacterially expressed, purified CD47-SA had neither cytotoxic nor cytostatic effects when incubated with J774A.1 cells up to a concentration of 400 nM for 24 h. Phagocytosis of FITC-labeled PFC emulsions was significantly diminished when macrophages were pretreated with 100 nM CD47-SA for 1 h.

CONCLUSIONS

We demonstrated that soluble CD47-SA antagonized phagocytosis of colloidal carriers to a significant degree by interaction with macrophage SIRPalpha.

Recombinant Antibodies Against Subcellular Fractions Used to Track Endogenous Golgi Protein Dynamicsin Vivo

Generation of specific antibodies against enriched subcellular fractions is a powerful strategy to identify and characterize cellular components. We show that recombinant antibodies can be selected in vitro by phage display against complex subcellular fractions, namely microtubule-binding proteins and Golgi stacks. This technique has allowed us to overcome many limitations of the classical animal-based approach and generate cell biology-compliant antibodies. In addition, we show that intracellular expression of GFP-tagged recombinant antibodies can reveal the dynamics of endogenous proteins in vivo. Endogenous Giantin is very static and outlines the Golgi in living cells. It accumulates neither onto Golgi-derived tubules upon Brefeldin A treatment before Golgi disappearance, nor onto de novo formed Golgi mini-stacks upon microtubule depolymerization, and remains instead on the 'old' pericentriolar Golgi. This suggests that, in contrast to other Golgi matrix proteins, endogenous Giantin is very stably associated with the Golgi and does not efficiently recycle to the ER. Altogether, we show that the antibody phage display technique represents an efficient alternative to rapidly generate versatile antibodies that represent new tools to study protein function.

Di-diabody: a novel tetravalent bispecific antibody molecule by design

The clinical development of bispecific antibodies (BsAb) as therapeutics has been hampered by the difficulty in preparing the materials in sufficient quantity and quality by traditional methods. In recent years, a variety of recombinant methods have been developed for efficient production of BsAb, both as antibody fragments and as full-length IgG-like molecules. These recombinant antibody molecules possess dual antigen-binding capability with, in most cases, monovalency to each of their target antigens. Here, we describe an efficient approach for the production of a novel tetravalent BsAb, with two antigen-binding sites to each of its target antigens, by genetically fusing a bispecific/divalent diabody to, via the hinge region, the N-terminus of the CH(3) domain of an IgG. The novel BsAb, which we termed "di-diabody", represents a tetravalent diabody dimer resulting from dimerization between the hinge region and the CH(3) domains. A di-diabody was constructed using two antibodies directed against the two tyrosine kinase receptors of vascular endothelial growth factor, expressed both in a single Escherichia coli host and in mammalian cells, and purified to homogeneity by a one-step affinity chromatography. Compared to the bispecific/divalent diabody, the tetravalent di-diabody binds more efficiently to both of its target antigens and is more efficacious in blocking ligand binding to the receptors. The di-diabody retained good antigen-binding activity after incubation at 37 degrees C in mouse serum for 72 h, demonstrating good product stability. Finally, expression of the di-diabody in mammalian cells yielded higher level of production and better antibody activity. This design and expression for BsAb fragments should be applicable to any pair of antigen specificities.

Effect of polyclonal, monoclonal, and recombinant (single-chain variable fragment) antibodies on in vitro morphology, growth, and metabolism of the phytopathogenic mollicute Spiroplasma citri

Antibodies are known to affect the morphology, growth, and metabolism of mollicutes and thus may serve as candidate molecules for a plantibody-based control strategy for plant-pathogenic spiroplasmas and phytoplasmas. Recombinant single-chain variable fragment (scFv) antibodies are easy to engineer and express in plants, but their inhibitory effects on mollicutes have never been evaluated and compared with those of polyclonal and monoclonal antibodies. We describe the morphology, growth, and glucose metabolism of Spiroplasma citri in the presence of polyclonal, monoclonal, and recombinant antibodies directed against the immunodominant membrane protein spiralin. We showed that the scFv antibodies had no effect on S. citri glucose metabolism but were as efficient as polyclonal antibodies in inhibiting S. citri growth in liquid medium. Inhibition of motility was also observed.

Secretory production and purification of functional full-length streptavidin from Bacillus subtilis

Streptavidin is a versatile molecule for many in vitro and in vivo applications. To optimize the production of the full-length streptavidin in a soluble and functional form via secretion using Bacillus subtilis as the expression host, three different strategies were used. These strategies include the construction of a synthetic streptavidin gene, the installation of a transcription terminator, and the use of a sporulation mutant strain. In comparison with the wild-type streptavidin gene in expression studies, a combination of these approaches resulted in a 2.3-fold increase in streptavidin production. The production yields in complex and semidefined media were 94 and 24 mg/liter, respectively. A simple purification scheme which requires only a single ion-exchange matrix was designed to purify streptavidin to homogeneity directly from the culture supernatant. Purified streptavidin was in full length with good biotin binding capacity (3.2 binding sites available per tetramer). A combination of this expression system and purification scheme would be useful for production and purification of high-quality functional streptavidin for characterizations and practical applications.

Molecular immunolabeling with recombinant single-chain variable fragment (scFv) antibodies designed with metal-binding domains

To study the molecular structure and function of gene products in situ, we developed a molecular immunolabeling technology. Starting with cDNA from hybridomas producing monoclonal antibodies against biotin, catalase, and superoxide dismutase, we bioengineered recombinant single-chain variable fragment antibodies (scFv) and their derivatives containing metal-binding domains (scFv:MBD). As tested with surface plasmon resonance and enzyme-linked immunosorbent assay, affinity binding constants of the scFv (5.21 x 10(6) M(-1)) and scFv:MBD (4.17 x 10(6) M(-1)) were close to those of Fab proteolytic fragments (9.78 x 10(6) M(-1)) derived from the parental IgG antibodies. After saturation of MBD with nickel or cobalt, scFv:MBD was imaged with electron spectroscopic imaging at each element's specific energy loss, thus generating the element's map. Immunolabeling with scFv:MBD resulted in a significant improvement of the labeling fidelity over that obtained with Fab or IgG derivatives, as it produced a much heavier specific labeling and label-free background. As determined with radioimmunoassay, labeling effectiveness with scFv:MBD was nearly the same as with scFv, but much higher than with scFv conjugated to colloidal gold, Nanogold, or horseradish peroxidase. This technology opens possibilities for simultaneous imaging of multiple molecules labeled with scFv:MBD at the molecular resolution within the same sample with electron spectroscopic imaging. Moreover, the same scFv:MBD can also be imaged with fluorescence resonance energy transfer and lifetime imaging as well as positron emission tomography and magnetic resonance imaging. Therefore, this technology may serve as an integrative factor in life science endeavors.

Anti-c-myc antibody 9E10: epitope key positions and variability characterized using peptide spot synthesis on cellulose

The 9E10 antibody epitope (EQKLISEEDL) derives from a protein sequence in the human proto-oncogen p62(c-myc) and is widely used as a protein fusion tag. This myc-tag is a powerful tool in protein localization, immunochemistry, ELISA or protein purification. Here, we characterize the myc-tag epitope by substitutional analysis and length variation using peptide spot synthesis on cellulose. The key amino acids of this interaction are the core residues LISE. The shortest peptide with a strong binding signal is KLISEEDL. Dissociation constants of selected peptide variants to the antibody 9E10 were determined. scFv constructs with the shortest possible myc-tags were successfully detected by Western blot and ELISA, giving a signal comparable to that of the original myc-tag.

Cloning and expression in Pichia pastoris of a genetically engineered single chain antibody against the rat transferrin receptor

The present investigation describes the construction of a genetically engineered single chain antibody (scFv) against the rat transferrin receptor (OX26), and demonstrates that this scFv antibody can be fully processed and expressed as a soluble secreted molecule in the methylotrophic yeast Pichia pastoris. Restriction endonuclease sites located at both 5'- and 3'-flanking regions of OX26 coding region in the prokaryote pOPE-OX26 vector were engineered to incorporate yeast compatible restriction endonuclease sites (i.e. EcoRI and SmaI or AvrII). The modified OX26 cDNA was subcloned into the Pichia expression vectors pPIC9 and pHIL-S1. An OX26 scFv high producer clone [GS115 His+ Mut+ (pPIC-OX26 SacI)] was isolated and used for large-scale production and characterization. Because the engineered scFv contains both a c-myc tag and a (His)5 tail, the OX26 scFv was purified to homogeneity by immobilized metal affinity chromatography. The identity of the OX26 scFv was confirmed by Western blot analyses with both anti c-myc and anti poly-His antibodies. Minor immunoreactive bands corresponding to hyperglycosylated and partially processed alpha-factor leader prosequence were also detected in the purified OX26 scFv, and these contaminants were markedly reduced when the expression of the OX26 scFv was performed in minimal methanol medium buffered with phosphate at pH = 7. The present investigation suggests that this expression system may be useful for the production of anti-receptor single chain antibodies that can be used as brain drug delivery vectors.

Immobilization of sugar-non-specific nucleases by utilizing the streptavidin--biotin interaction

Due to their high enzymatic activity, the sugar-non-specific endonucleases from Serratia marcescens and Anabaena can be used for a number of applications, such as the removal of contaminating genetic material from biological preparations, footprinting studies, and the determination of nucleic acids in biochemical samples. These methods would benefit from immobilized nucleases. For this purpose, a single cysteine residue was added at the N-terminus of the Serratia and Anabaena nucleases and subsequently modified with a maleimide-biotin conjugate. Alternatively, a biotin acceptor domain was fused to the Anabaena nuclease, allowing biotinylation during expression in E. coli without a further chemical step. The attachment of biotin-modified nucleases to streptavidin-coated paramagnetic beads and to streptavidin-coated surface plasmon resonance sensor chips (to study interactions with substrate and inhibitor) worked well when aggregates present in the protein preparations were removed by ultrafiltration. These methods should be of general use for similar enzyme systems.

Preclinical evaluation of a humanized NR-LU-10 antibody-streptavidin fusion protein for pretargeted cancer therapy

A humanized single chain Fv antibody fragment specific to the EGP40 antigen was genetically engineered as a streptavidin fusion (scFvSA) for use in pretargeted radioimmunotherapy. The scFvSA construct was expressed as a soluble, tetrameric species in the Escherichia coli periplasm at 110-140 mg/liter. The fusion protein was purified from crude lysates by iminobiotin affinity chromatography with an overall yield of 50-60%. Characterization of the purified protein by SDS-PAGE, light scattering, and size exclusion chromatography demonstrated that the fusion protein was tetrameric with a molecular weight of approximately 172,000. Competitive immunoreactivity assays showed a two-fold greater binding to the antigen than the comparable whole antibody. The purified protein had a biotin disassociation rate identical to recombinant streptavidin and bound an average of three of four possible biotins per molecule. The radiolabeled fusion protein showed a faster blood clearance rate in normal mice than the corresponding whole antibody-streptavidin chemical conjugate. Tumor-specific targeting of a subsequently administered radionuclidechelate/biotin molecule was demonstrated in nude mice bearing SW1222 human colon carcinoma xenografts. A single dose of 800 microCi of 90Y-DOTA-biotin produced cures in mice with established subcutaneous human small cell lung or colon cancer xenografts.

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.