Oligomerization and ligand-binding properties of the ectodomain of the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptor subunit GluRD.

The extracellular part of ionotropic glutamate receptor (iGluR) subunits can be divided into a conserved two-lobed ligand-binding domain ("S1S2") and an N-terminal approximately 400-residue segment of unknown function ("X domain") which shows high sequence variation among subunits. To investigate the structure and properties of the N-terminal domain, we have now produced affinity-tagged recombinant fragments which represent the X domain of the GluRD subunit of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA)-selective glutamate receptors either alone or covalently linked to the ligand-binding domain ("XS1S2"). These fragments were expressed in insect cells as secreted soluble proteins and were recognized by a conformation-specific anti-GluRD monoclonal antibody. A hydrodynamic analysis of the purified fragments revealed them to be dimers, in contrast to the S1S2 ligand-binding domain which is monomeric. The X domain did not bind radiolabeled AMPA or glutamate nor did its presence affect the ligand binding properties of the S1S2 domain. Our findings demonstrate that the N-terminal domain of AMPA receptor can be expressed as a soluble polypeptide and suggest that subunit interactions in iGluR may involve the extracellular domains.     

Phage display and hybridoma generation of antibodies to human CXCR2 yields antibodies with distinct mechanisms and epitopes

Generation of functional antibodies against integral membrane proteins such as the G-protein coupled receptor CXCR2 is technically challenging for several reasons, including limited epitope accessibility, the requirement for a lipid environment to maintain structure and their existence in dynamic conformational states. Antibodies to human CXCR2 were generated by immunization in vivo and by in vitro selection methods. Whole cell immunization of transgenic mice and screening of phage display libraries using CXCR2 magnetic proteoliposomes resulted in the isolation of antibodies with distinct modes of action. The hybridoma-derived antibody fully inhibited IL-8 and Gro-α responses in calcium flux and β-arrestin recruitment assays. The phage-display derived antibodies were allosteric antagonists that showed ligand dependent differences in functional assays. The hybridoma and phage display antibodies did not cross-compete in epitope competition assays and mapping using linear and CLIPS peptides confirmed that they recognized distinct epitopes of human CXCR2. This illustrates the benefits of using parallel antibody isolation approaches with different antigen presentation methods to successfully generate functionally and mechanistically diverse antagonistic antibodies to human CXCR2. The method is likely to be broadly applicable to other complex membrane proteins.     

Kinetic mechanism of channel opening of the GluRDflip AMPA receptor

AMPA-type ionotropic glutamate receptors mediate the majority of fast excitatory neurotransmission in the mammalian central nervous system and are essential for brain functions, such as memory and learning. Dysfunction of these receptors has been implicated in a variety of neurological diseases. Using a laser-pulse photolysis technique, we investigated the channel opening mechanism for GluRD(flip) or GluR4(flip) (i.e., the flip isoform of GluRD), an AMPA receptor subunit. The minimal kinetic mechanism for channel opening is consistent with binding of two glutamate molecules per receptor complex. The GluRD(flip) channel opens with a rate constant of (6.83 +/- 0.74) x 10(4) s(-1) and closes with a rate constant of (3.35 +/- 0.17) x 10(3) s(-1). On the basis of these rate constants, the channel opening probability is calculated to be 0.95 +/- 0.12. Furthermore, the shortest rise time (20-80% of the receptor current response to glutamate) is predicted to be 20 micros, which is approximately 8 times shorter than the previous estimate. These findings suggest that the kinetic property of GluRD(flip) is similar to that of GluR2Q(flip), another fast-activating AMPA receptor subunit.     

Phage display selection of EGFR-specific antibodies by capture-sandwich panning

Sandwich ELISA experiment requires two mutually compatible affinity reagents, typically antibodies, that are highly sensitive and specific to the target analyte in its native conformation, and whose epitopes do not overlap each other’s. Finding or developing a pair of antibodies that meet these requirements can be a challenge and many otherwise useful antibodies fail in sandwich ELISA-format analysis. In order to discover sandwich immunoassay-compatible antibodies for epidermal growth factor receptor (EGFR), we first immobilized cetuximab, a high-affinity monoclonal antibody against EGFR, on a plastic surface and affinity-captured EGFR in A431 cell lysate. Panning of a phage antibody library on the surface-captured antigen yielded a couple of antibody clones that can be paired with cetuximab and also with each other in the sandwich-format immunoassay. This provides a potentially useful strategy for the development of sandwich ELISA reagents or antibodies against antigens that are hard to purify.     

Isolation of a novel neutralizing antibody fragment against human vascular endothelial growth factor from a phage-displayed human antibody repertoire using an epitope disturbing strategy

Following the clinical success of Bevacizumab, a humanized monoclonal antibody that blocks the interaction between vascular endothelial growth factor (VEGF) and its receptors, the search for new neutralizing antibodies targeting this molecule has continued until now. We used a human VEGF variant containing three mutations in the region recognized by Bevacizumab to direct antibody selection towards recognition of other epitopes. A total of seven phage-displayed antibody fragments with diverse binding properties in terms of inter-species cross-reactivity and sensitivity to chemical modifications of the antigen were obtained from a human phage display library. All of them were able to recognize not only the selector mutated antigen, but also native VEGF. One of these phage-displayed antibody fragments, denominated 2H1, was shown to compete with the VEGF receptor 2 for VEGF binding. Purified soluble 2H1 inhibited in a dose dependent manner the ligand-receptor interaction and abolished VEGF-dependent proliferation of human umbilical vein endothelial cells. Our epitope disturbing strategy based on a triple mutant target antigen was successful to focus selection on epitopes different from a known one. Similar approaches could be used to direct phage isolation towards the desired specificity in other antigenic systems.     

Paramagnetic proteoliposomes containing a pure, native, and oriented seven-transmembrane segment protein, CCR5

Seven-transmembrane segment, G protein-coupled receptors play central roles in a wide range of biological processes, but their characterization has been hindered by the difficulty of obtaining homogeneous preparations of native protein. We have created paramagnetic proteoliposomes containing pure and oriented CCR5, a seven-transmembrane segment protein that serves as the principal coreceptor for human immunodeficiency virus (HIV-1). The CCR5 proteoliposomes bind the HIV-1 gp120 envelope glycoprotein and conformation-dependent antibodies against CCR5. The binding of gp120 was enhanced by a soluble form of the other HIV-1 receptor, CD4, but did not require additional cellular proteins. Paramagnetic proteoliposomes are uniform in size, stable in a broad range of salt concentrations and pH, and can be used in FACS and competition assays typically applied to cells. Integral membrane proteins can be inserted in either orientation into the liposomal membrane. The magnetic properties of these proteoliposomes facilitate rapid buffer exchange useful in multiple applications. As an example, the CCR5-proteoliposomes were used to select CCR5-specific antibodies from a recombinant phage display library. Thus, paramagnetic proteoliposomes should be useful tools in the analysis of membrane protein interactions with extracellular and intracellular ligands, particularly in establishing screens for inhibitors.     

A novel synthetic na?ve human antibody library allows the isolation of antibodies against a new epitope of oncofetal fibronectin

Human monoclonal antibodies (mAbs) can routinely be isolated from phage display libraries against virtually any protein available in sufficient purity and quantity, but library design can influence epitope coverage on the target antigen. Here we describe the construction of a novel synthetic human antibody phage display library that incorporates hydrophilic or charged residues at position 52 of the CDR2 loop of the variable heavy chain domain, instead of the serine residue found in the corresponding germline gene. The novel library was used to isolate human mAbs to various antigens, including the alternatively-spliced EDA domain of fibronectin, a marker of tumor angiogenesis. In particular, the mAb 2H7 was proven to bind to a novel epitope on EDA, which does not overlap with the one recognized by the clinical-stage F8 antibody. F8 and 2H7 were used for the construction of chelating recombinant antibodies (CRAbs), whose tumor-targeting properties were assessed in vivo in biodistribution studies in mice bearing F9 teratocarcinoma, revealing a preferential accumulation at the tumor site.Key words: human antibody library, phage display, oncofetal fibronectin, vascular tumor targeting, scFv antibody fragments, chelating recombinant antibody (CRAb)     

Mapping of linear epitopes recognized by monoclonal antibodies with gene-fragment phage display libraries

Epitope mapping with mono- or polyclonal antibodies has so far been done either by dissecting the antigens into overlapping polypeptides in the form of recombinantly expressed fusion proteins, or by synthesizing overlapping short peptides, or by a combination of both methods. Here, we report an alternative method which involves the generation of random gene fragments of approximately 50–200 by in length and cloning these into the 5 terminus of the protein III gene of fd phages. Selection for phages that bind a given monoclonal antibody and sequencing the DNA inserts of immunopositive phages yields derived amino acid sequences containing the desired epitope. A monoclonal antibody (mAb 215) directed against the largest subunit of Drosophila RNA polymerase II (RPB215) was used to map the corresponding epitope in a fUSE5 phage display library made of random DNA fragments from plasmid DNA containing the entire gene. After a single round of panning with this phage library, bacterial colonies were obtained which produced fd phages displaying the mAb 215 epitope. Sequencing of single-stranded phage DNA from a number of positive colonies (recognized by the antibody on colony immunoblots) resulted in overlapping sequences all containing the 15mer epitope determined by mapping with synthetic peptides. Similarly, we have localized the epitopes recognized by a mouse monoclonal antibody directed against the human p53 protein, and by a mouse monoclonal antibody directed against the human cytokeratin 19 protein. Identification of positive colonies after the panning procedure depends on the detection system used (colony immunoblot or ELISA) and there appear to be some restrictions to the use of linker-encoded amino acids for optimal presentation of epitopes. A comparison with epitope mapping by synthetic peptides shows that the phage display method allows one to map linear epitopes down to a size only slightly larger than the true epitope. In general, our phage display method is faster, easier, and cheaper than the construction of overlapping fusion proteins or the use of synthetic peptides, especially in cases where the antigen is a large polypeptide such as the 215 kDa subunit of eukaryotic RNA polymerase II.     

Monoclonal antibody DH12 reacts with a cell surface and a precursor form of the beta subunit of the human fibronectin receptor

Monoclonal and polyclonal antibodies were raised against a placenta plasma membrane protein preparation, which was obtained by fractionation on Blue B dye matrix and by HPLC-anionexchange, and which was shown to contain fibronectin receptors. Immunochemical and functional evidence showed that monoclonal antibody DH12 recognized the beta subunit of the human fibronectin receptor on fibroblasts. This monoclonal antibody reacted with two proteins in Western blots and in double immune precipitations of whole cell preparations. Only the higher Mr protein became labeled by surface iodination of intact fibroblasts. The lower Mr protein is thought to be an intracellular precursor of the beta subunit of the fibronectin receptor.     

Characterization of two monoclonal antibodies reactive with the external domain of the platelet-derived growth factor receptor

Two monoclonal antibodies against the receptor for platelet-derived growth factor (PDGF) were obtained by immunizing mice with pure PDGF receptor preparations derived from porcine uterus. The antibodies, denoted PDGFR-B1 and PDGFR-B2, both bound to the external domain of the receptor, as demonstrated by indirect immunofluorescence and binding of 125I-labeled antibodies to intact human fibroblasts. Both antibodies precipitated pure 175-kDa 32P-labeled autophosphorylated porcine PDGF receptor as well as a Mr 175,000 glycoprotein from metabolically labeled cells. The monoclonal antibodies did not inhibit binding of 125I-PDGF to human fibroblasts and did not stimulate these cells to undergo mitosis. Both antibodies induced clustering and down-regulation of their antigen. However, this resulted in only a partial loss of cell surface binding sites for PDGF itself, consistent with the conclusion that the monoclonals recognized only one of two or several receptors for PDGF. Clustering and down-regulation were not seen when the cells were incubated with monovalent Fab' fragments of the PDGFR-B2 antibody. The antibodies also stimulated autophosphorylation of pure PDGF receptor, and PDGFR-B2 was shown to stimulate phosphorylation of phosphofructokinase, an exogenous substrate for the PDGF receptor kinase. High concentrations of PDGFR-B2 antibody, or Fab' fragments thereof, failed to enhance the PDGF receptor kinase activity, compatible with the possibility that dimerization was of importance in the antibody-stimulated kinase activity of purified PDGF receptors.     

Use of phage display and high-density screening for the isolation of an antibody against the 51-kDa subunit of complex I

Monoclonal antibodies play an increasingly important role in structural biology. In this report, we develop the use of phage display technology for the isolation of an antibody that binds to a specific subunit of a macromolecular assembly. Antibodies that bind to the intact complex are selected from a phage display library and screened with a high-density Western blot to identify a subunit-specific binder. Conventional Western blotting and competition ELISA are then used to confirm the identity of the target subunit and that the antibody binds to the native protein complex and not to an epitope that is only revealed when the antibody is immobilized for phage selection. Using this technique, monoclonal scFv and Fab fragments have been produced that bind to the 51-kDa subunit of bovine complex I, a large integral membrane protein complex from mitochondria.     

Autophosphorylation of the insulin receptor in vitro. Designation of phosphorylation sites and correlation with receptor kinase activation

Chemical degradation and antipeptide antibodies were used to study alterations in the structure and function of the human placental insulin receptor following autophosphorylation in vitro. Antibodies elicited to residues 1143-1162 (P2) of the human insulin proreceptor immunoprecipitated the native, phosphorylated receptor but not the unphosphorylated receptor. Since this antibody recognizes both forms of the receptor on immunoblots, it was concluded that the accessibility of the P2 domain to the antibody is increased by in vitro autophosphorylation. Chemical cleavage at either tryptophan or methionine residues followed by immunoprecipitation with antipeptide antibodies was used to map the in vitro autophosphorylation sites of the beta subunit of the insulin receptor. Two phosphorylated fragments were resolved. One, recognized by antibody elicited to amino acid residues 1328-1343 (P5), is derived from the carboxyl terminus of the beta subunit and includes tyrosine 1316. The other, recognized by antibody to P2, is located in a domain that includes tyrosine 1150. The rate of phosphorylation of this latter site correlates with the rate of activation of the insulin receptor kinase during in vitro autophosphorylation. The results support the following conclusions: autophosphorylation alters the conformation of the beta subunit of the insulin receptor; autophosphorylation in vitro leads to phosphorylation of tyrosine residues near the carboxyl terminus of the protein and in the P2 domain that includes tyrosine 1150; activation of the insulin receptor kinase correlates with autophosphorylation of the domain containing tyrosine 1150.     

Monoclonal antibodies reveal lamB antigenic determinants on both faces of the Escherichia coli outer membrane

LamB protein is involved in the transport of maltose across the outer membrane and constitutes the receptor for phage lambda. In this study we characterized six previously described anti-LamB monoclonal antibodies (mAbs). Four of these, the E-mAbs, recognized determinants that were exposed at the cell surface, whereas the other two, the I-mAbs, recognized determinants which were not exposed. Competition experiments demonstrated that the domains recognized by these two classes of mAbs were completely distinct. In addition, the E-mAbs prevented LamB from neutralizing phage lambda in vitro and protected LamB against proteolytic degradation, whereas the I-mAbs had no such effects. The E-mAbs have been shown previously to constitute two classes: some E-mAbs inhibit maltose transport in vivo, and others do not. Immunoelectron microscopy demonstrated that the I-mAbs also define at least two types of determinants. One of these, which is accessible in membrane fragments from a mutant (lpp) devoid of lipoprotein but not in membrane fragments from an lpp+ strain, probably corresponds to a region of LamB that is involved in the interactions with peptidoglycan. The other determinant, which is fully accessible in LamB-peptidoglycan complexes and in LamB-containing phospholipid vesicles but only slightly accessible in membrane fragments from an lpp mutant, is probably located very close to the inner surface of the outer membrane. LamB also contains at least one additional determinant, which (i) is exposed at the inner surface of the membrane, (ii) is accessible to antibodies in membrane fragments from an lpp+ strain, and (iii) may be involved in the interaction of LamB with the periplasmic maltose-binding protein.     

Structure-function studies of two synthetic anti-vascular endothelial growth factor Fabs and comparison with the Avastin Fab

In the quest to discover new research tools and to develop better agents in the fight against cancer, two antibodies, G6 and B20-4, were isolated from synthetic antibody phage libraries. Unlike the AVASTINtrade mark antibody, a recently approved agent for the treatment of patients with colorectal cancer, B20-4 and G6 bind and block both human and murine vascular endothelial growth factor (VEGF). Here we have analyzed and compared the binding epitopes on VEGF for these three antibodies using alanine-scanning mutagenesis and structural analyses. The epitopes recognized by both synthetic antibodies are conserved between human and mouse VEGF, and they match closely to the receptor epitopes both structurally and functionally. In contrast, the Avastin epitope overlaps minimally with the receptor binding surface and centers around a residue that is not conserved in mouse. Our structural and functional analyses elucidate the cross-species reactivity of all three antibodies and emphasize the potential advantages of antibody generation using phage display as the resulting antibodies do not depend on sequence differences across species and preferentially target natural protein-protein interaction surfaces.     

Characterization of Curaremimetic Neurotoxin Binding Sites on Cellular Membrane Fragments Derived from the Rat Pheochromocytoma PC 12

Studies were conducted on the properties of 125I-labeled alpha-bungarotoxin binding sites on cellular membrane fragments derived from the PC12 rat pheochromocytoma. Two classes of specific toxin binding sites are present at approximately equal densities (50 fmol/mg of membrane protein) and are characterized by apparent dissociation constants of 3 and 60 nM. Nicotine and d-tubocurarine are among the most potent inhibitors of high-affinity toxin binding. The affinity of high-affinity toxin binding sites for nicotinic cholinergic agonists is reversibly or irreversibly decreased, respectively, on treatment with dithiothreitol or dithiothreitol and N-ethylmaleimide. The nicotinic receptor affinity reagent bromoacetylcholine irreversibly blocks high-affinity toxin binding to PC12 cell membranes that have been treated with dithiothreitol. Two polyclonal antisera raised against the nicotinic acetylcholine receptor from Electrophorus electricus inhibit high-affinity toxin binding. These detailed studies confirm that curaremimetic neurotoxin binding sites on the PC12 cell line are comparable to toxin binding sites from neural tissues and to nicotinic acetylcholine receptors from the periphery. Because toxin binding sites are recognized by anti-nicotinic receptor antibodies, the possibility remains that they are functionally analogous to nicotinic receptors.     

Selection and characterization of FcεRI phospho-ITAM specific antibodies

ABSTRACT

Post-translational modifications, such as the phosphorylation of tyrosines, are often the initiation step for intracellular signaling cascades. Pan-reactive antibodies against modified amino acids (e.g., anti-phosphotyrosine), which are often used to assay these changes, require isolation of the specific protein prior to analysis and do not identify the specific residue that has been modified (in the case that multiple amino acids have been modified). Phosphorylation state-specific antibodies (PSSAs) developed to recognize post-translational modifications within a specific amino acid sequence can be used to study the timeline of modifications during a signal cascade. We used the FcεRI receptor as a model system to develop and characterize high-affinity PSSAs using phage and yeast display technologies. We selected three β-subunit antibodies that recognized: 1) phosphorylation of tyrosines Y₂₁₈ or Y₂₂₄; 2) phosphorylation of the Y₂₂₈ tyrosine; and 3) phosphorylation of all three tyrosines. We used these antibodies to study the receptor activation timeline of FcεR1 in rat basophilic leukemia cells (RBL-2H3) upon stimulation with DNP₂₄-BSA. We also selected an antibody recognizing the N-terminal phosphorylation site of the γ-subunit (Y₆₅) of the receptor and applied this antibody to evaluate receptor activation. Recognition patterns of these antibodies show different timelines for phosphorylation of tyrosines in both β and γ subunits. Our methodology provides a strategy to select antibodies specific to post-translational modifications and provides new reagents to study mast cell activation by the high-affinity IgE receptor, FcεRI.     

A peptide isolated from phage display libraries is a structural and functional mimic of an RGD-binding site on integrins

Many integrins recognize short RGD-containing amino acid sequences and such peptide sequences can be identified from phage libraries by panning with an integrin. Here, in a reverse strategy, we have used such libraries to isolate minimal receptor sequences that bind to fibronectin and RGD-containing fibronectin fragments in affinity panning. A predominant cyclic motif, *CWDDG/LWLC*, was obtained (the asterisks denote a potential disulfide bond). Studies using the purified phage and the corresponding synthetic cyclic peptides showed that *CWDDGWLC*-expressing phage binds specifically to fibronectin and to fibronectin fragments containing the RGD sequence. The binding did not require divalent cations and was inhibited by both RGD and *CWDDGWLC*-containing synthetic peptides. Conversely, RGD-expressing phage attached specifically to immobilized *CWDDGWLC*-peptide and the binding could be blocked by the respective synthetic peptides in solution. Moreover, fibronectin bound to a *CWDDGWLC*-peptide affinity column, and could be eluted with an RGD-containing peptide. The *CWDDGWLC*-peptide inhibited RGD-dependent cell attachment to fibronectin and vitronectin, but not to collagen. A region of the beta subunit of RGD-binding integrins that has been previously demonstrated to be involved in ligand binding includes a polypeptide stretch, KDDLW (in beta 3) similar to WDDG/LWL. Synthetic peptides corresponding to this region in beta 3 were found to bind RGD-displaying phage and conversion of its two aspartic residues into alanines greatly reduced the RGD binding. Polyclonal antibodies raised against the *CWDDGWLC*- peptide recognized beta 1 and beta 3 in immunoblots. These data indicate that the *CWDDGWLC*-peptide is a functional mimic of ligand binding sites of RGD-directed integrins, and that the structurally similar site in the integrin beta subunit is a binding site for RGD.     

A nanobody directed to a functional epitope on VEGF,as a novel strategy for cancer treatment

Compelling evidence suggests that vascular endothelial growth factor (VEGF), due to its essential role in angiogenesis, is a critical target for cancer treatment. Neutralizing monoclonal antibodies against VEGF are important class of drugs used in cancer therapy. However, the cost of production, large size, and immunogenicity are main drawbacks of conventional monoclonal therapy. Nanobodies are the smallest antigen-binding antibody fragments, which occur naturally in camelidae. Because of their remarkable features, we decided to use an immune library of nanobody to direct phage display to recognition of novel functional epitopes on VEGF. Four rounds of selection were performed and six phage-displayed nanobodies were obtained from an immune phage library. The most reactive clone in whole-cell ELISA experiments, was purified and assessed in proliferation inhibition assay. Purified ZFR-5 not only blocked interaction of VEGF with its receptor in cell ELISA experiments, but also was able to significantly inhibit proliferation response of human umbilical vein endothelial cells to VEGF in a dose-dependent manner. Taken together, our study demonstrates that by using whole-cell ELISA experiments, nanobodies against antigenic regions included in interaction of VEGF with its receptors can be directed. Because of unique and intrinsic properties of a nanobody and the ability of selected nanobody for blocking the epitope that is important for biological function of VEGF, it represents novel potential drug candidate.     

Analysis of acetylcholine receptor phosphorylation sites using antibodies to synthetic peptides and monoclonal antibodies.

Three peptides corresponding to residues 354-367, 364-374, 373-387 of the acetylcholine receptor (AChR) delta subunit were synthesized. These peptides represent the proposed phosphorylation sites of the cAMP-dependent protein kinase, the tyrosine-specific protein kinase and the calcium/phospholipid-dependent protein kinase respectively. Using these peptides as substrates for phosphorylation by the catalytic subunit of cAMP-dependent protein kinase it was shown that only peptides 354-367 was phosphorylated whereas the other two were not. These results verify the location of the cAMP-dependent protein kinase phosphorylation site within the AChR delta subunit. Antibodies elicited against these peptides reacted with the delta subunit. The antipeptide antibodies and two monoclonal antibodies (7F2, 5.46) specific for the delta subunit were tested for their binding to non-phosphorylated receptor and to receptor phosphorylated by the catalytic subunit of cAMP-dependent protein kinase. Antibodies to peptide 354-367 were found to react preferentially with non-phosphorylated receptor whereas the two other anti-peptide antibodies bound equally to phosphorylated and non-phosphorylated receptors. Monoclonal antibody 7F2 reacted preferentially with the phosphorylated form of the receptor whereas monoclonal antibody 5.46 did not distinguish between the two forms.     

Development and Characterization of Recombinant Antibody Fragments That Recognize and Neutralize In Vitro Stx2 Toxin from Shiga Toxin-Producing Escherichia coli

Background

Stx toxin is a member of the AB₅ family of bacterial toxins: the active A subunit has N-glycosidase activity against 28S rRNA, resulting in inhibition of protein synthesis in eukaryotic cells, and the pentamer ligand B subunits (StxB) bind to globotria(tetra)osylceramide receptors (Gb3/Gb4) on the cell membrane. Shiga toxin-producing Escherichia coli strains (STEC) may produce Stx1 and/or Stx2 and variants. Strains carrying Stx2 are considered more virulent and related to the majority of outbreaks, besides being usually associated with hemolytic uremic syndrome in humans. The development of tools for the detection and/or neutralization of these toxins is a turning point for early diagnosis and therapeutics. Antibodies are an excellent paradigm for the design of high-affinity, protein-based binding reagents used for these purposes.

Methods and Findings

In this work, we developed two recombinant antibodies; scFv fragments from mouse hybridomas and Fab fragments by phage display technology using a human synthetic antibody library. Both fragments showed high binding affinity to Stx2, and they were able to bind specifically to the GKIEFSKYNEDDTF region of the Stx2 B subunit and to neutralize in vitro the cytotoxicity of the toxin up to 80%. Furthermore, the scFv fragments showed 79% sensitivity and 100% specificity in detecting STEC strains by ELISA.

Conclusion

In this work, we developed and characterized two recombinant antibodies against Stx2, as promising tools to be used in diagnosis or therapeutic approaches against STEC, and for the first time, we showed a human monovalent molecule, produced in bacteria, able to neutralize the cytotoxicity of Stx2 in vitro.