Inhibition of signaling from Type 1 receptor tyrosine kinases via intracellular expression of single-chain antibodies

ER intrabody-mediated inhibition of interferon α secretion by mouse macrophages and dendritic cells

Interferon α (IFNα) counteracts viral infections by activating various IFNα-stimulated genes (ISGs). These genes encode proteins that block viral transport into the host cell and inhibit viral replication, gene transcription and translation. Due to the existence of 14 different, highly homologous isoforms of mouse IFNα, an IFNα knockout mouse has not yet been established by genetic knockout strategies. An scFv intrabody for holding back IFNα isoforms in the endoplasmic reticulum (ER) and thus counteracting IFNα secretion is reported. The intrabody was constructed from the variable domains of the anti-mouse IFNα rat monoclonal antibody 4EA1 recognizing the 5 isoforms IFNα1, IFNα2, IFNα4, IFNα5, IFNα6.

A soluble form of the intrabody had a K_D of 39 nM to IFNα4. It could be demonstrated that the anti-IFNα intrabody inhibits clearly recombinant IFNα4 secretion by HEK293T cells. In addition, the secretion of IFNα4 was effectively inhibited in stably transfected intrabody expressing RAW 264.7 macrophages and dendritic D1 cells. Colocalization of the intrabody with IFNα4 and the ER marker calnexin in HEK293T cells indicated complex formation of intrabody and IFNα4 inside the ER. Intracellular binding of intrabody and antigen was confirmed by co-immunoprecipitation. Complexes of endogenous IFNα and intrabody could be visualized in the ER of Poly (I:C) stimulated RAW 264.7 macrophages and D1 dendritic cells. Infection of macrophages and dendritic cells with the vesicular stomatitis virus VSV-AV2 is attenuated by IFNα and IFNβ. The intrabody increased virus proliferation in RAW 264.7 macrophages and D1 dendritic cells under IFNβ-neutralizing conditions. To analyze if all IFNα isoforms are recognized by the intrabody was not in the focus of this study. Provided that binding of the intrabody to all isoforms was confirmed, the establishment of transgenic intrabody mice would be promising for studying the function of IFNα during viral infection and autoimmune diseases.

Production of a germline-humanized cetuximab scFv and evaluation of its activity in recognizing EGFR- overexpressing cancer cells

The aim of this study was to produce a humanized single chain antibody (scFv) as a potential improved product design to target EGFR (Epidermal Growth Factor Receptor) overexpressing cancer cells. To this end, CDR loops of cetuximab (an FDA-approved anti-EGFR antibody) were grafted on framework regions derived from type 3 (VH3 and VL3 kappa) human germline sequences to obtain recombinant VH and VL domainslinked together with a flexible linker [(Gly₄Ser)₃] to form a scFv. Codon optimized synthetic gene encoding the scFv (with NH2-VH-linker-VL-COOH orientation) was expressed in E. coli Origami™ 2(DE3) cells and the resultant scFv purified by using Ni-NTA affinity chromatography. The scFv, called cet.Hum scFv, was evaluated in ELISA and immunoblot to determine whether it can recognize EGFR. The scFv was able to recognize EGFR over-expressing cancer cells (A-431) but failed to detect cancer cells with low levels of EGFR (MCF-7 cells). Although the affinity of the scFv forA-431 cells was 9 fold lower than that of cetuximab, it was strong enough to recognize these cells. Considering its ability to bind EGFR molecules, the scFv may exhibit a potential application for the detection of EGFR-overexpressing cancer cells.

Specific in vivo knockdown of protein function by intrabodies

Intracellular antibodies (intrabodies) are recombinant antibody fragments that bind to target proteins expressed inside of the same living cell producing the antibodies. The molecules are commonly used to study the function of the target proteins (i.e., their antigens). The intrabody technology is an attractive alternative to the generation of gene-targeted knockout animals, and complements knockdown techniques such as RNAi, miRNA and small molecule inhibitors, by-passing various limitations and disadvantages of these methods. The advantages of intrabodies include very high specificity for the target, the possibility to knock down several protein isoforms by one intrabody and targeting of specific splice variants or even post-translational modifications. Different types of intrabodies must be designed to target proteins at different locations, typically either in the cytoplasm, in the nucleus or in the endoplasmic reticulum (ER). Most straightforward is the use of intrabodies retained in the ER (ER intrabodies) to knock down the function of proteins passing the ER, which disturbs the function of members of the membrane or plasma proteomes. More effort is needed to functionally knock down cytoplasmic or nuclear proteins because in this case antibodies need to provide an inhibitory effect and must be able to fold in the reducing milieu of the cytoplasm. In this review, we present a broad overview of intrabody technology, as well as applications both of ER and cytoplasmic intrabodies, which have yielded valuable insights in the biology of many targets relevant for drug development, including α-synuclein, TAU, BCR-ABL, ErbB-2, EGFR, HIV gp120, CCR5, IL-2, IL-6, β-amyloid protein and p75NTR. Strategies for the generation of intrabodies and various designs of their applications are also reviewed.

A bispecific transmembrane antibody simultaneously targeting intra- and extracellular epitopes of the epidermal growth factor receptor inhibits receptor activation and tumor cell growth

Epidermal growth factor receptor (EGFR) plays an important role in essential cellular processes such as proliferation, survival and migration. Aberrant activation of EGFR is frequently found in human cancers of various origins and has been implicated in cancer pathogenesis. The therapeutic antibody cetuximab (Erbitux) inhibits tumor growth by binding to the extracellular domain of EGFR, thereby preventing ligand binding and receptor activation. This activity is shared by the single chain antibody fragment scFv(225) that contains the same antigen binding domain. The unrelated EGFR-specific antibody fragment scFv(30) binds to the intracellular domain of the receptor and retains antigen binding upon expression as an intrabody in the reducing environment of the cytosol. Here, we used scFv(225) and scFv(30) domains to generate a novel type of bispecific transmembrane antibody termed 225.TM.30, that simultaneously targets intra- and extracellular EGFR epitopes. Bispecific 225.TM.30 and related membrane-anchored monospecific 225.TM and TM.30 proteins carrying extracellular scFv(225) or intracellular scFv(30) antibody fragments linked to a transmembrane domain were expressed in EGFR-overexpressing tumor cells using a doxycycline-inducible retroviral system. Induced expression of 225.TM.30 and 225.TM, but not TM.30 reduced EGFR surface levels and ligand-induced EGFR activation, while all three molecules markedly inhibited tumor cell growth. Co-localization of 225.TM with EGFR was predominantly found on the cell surface, while interaction with 225.TM.30 and TM.30 proteins resulted in the redistribution of EGFR to perinuclear compartments. Our data demonstrate functionality of this novel type of membrane-anchored intrabodies in tumor cells and suggest distinct modes of action of mono- and bispecific variants.

Essential function for ErbB3 in breast cancer proliferation

The overexpression of the ErbB family of tyrosine kinase receptors is thought to be important in the development of many breast tumours. To date, most attention has focused on the ErbB2 receptor. Now, in a recent report, it has been shown that ErbB3 is a critical partner for the transforming activity of ErbB2 in breast cancer cells. Importantly, the proliferative signals from this transforming complex appear to act via the PI-3 kinase pathway.

Targeting an extracellular epitope of the human multidrug resistance protein 1 (MRP1) in malignant cells with a novel recombinant single chain Fv antibody

Inherent and acquired multidrug resistance (MDR) is characterized by a simultaneous resistance to diverse anticancer drugs and is a major impediment towards curative chemotherapy of cancer. Hence one important goal is to develop strategies aimed at specific targeting of major anticancer drug efflux transporters of the ATP-binding cassette (ABC) superfamily including multidrug resistance protein 1 -MRP1 (ABCC1). To date, no monoclonal antibody has been isolated that can target an extracellular MRP1 epitope. Using a phage display approach, we have isolated a recombinant single-chain Fv (scFv) antibody that specifically reacts with the extracellular N-terminus of the human MRP1. Flow cytometric analysis revealed that this scFv fragment binds specifically to various viable human tumor cells that display variable MRP1 expression levels but not to MRP1 null cells. Furthermore, this scFv antibody failed to react with tumor cells that overexpress other members of the MRP family that have an extracellular N-terminus (MRP2 and MRP3) as well as with MRP4, MRP5, and breast cancer resistance protein. Flow cytometric analysis also showed a good correlation between the fluorescence intensity of the anti-MRP1 scFv antibody and MRP1 levels in viable tumor cells. These findings constitute the first successful isolation of a small recombinant scFv antibody directed to an extracellular epitope of the MRP1 in viable malignant cells. These novel small Fv-based recombinant antibodies that possess superior tumor penetration capabilities may possibly be used to selectively target drugs or tumor cells that express MRP-1.

Localized expression of an anti-TNF single-chain antibody prevents development of collagen-induced arthritis

Although systemic administration of neutralizing anti-TNF antibodies has been used successfully in treating rheumatoid arthritis, there is a potential for side effects. We transduced a collagen reactive T-cell hybridoma with tissue-specific homing properties to assess therapeutic effects of local delivery to inflamed joints of anti-TNF single-chain antibodies (scFv) by adoptive cellular gene therapy. Cell culture medium conditioned with 1 x 10(6) scFv producer cells/ml had TNF neutralizing capacity in vitro equivalent to 50 ng/ml anti-TNF monoclonal antibody. Adding a kappa chain constant domain to the basic scFv (construct TN3-Ckappa) gave increased in vitro stability and in vivo therapeutic effect. TN3-Ckappa blocked development of collagen-induced arthritis in DBA/1LacJ mice for >60 days. Transgene expression was detected in the paws but not the spleen of treated animals for up to 55 days postinjection. No significant variations in cell proliferation or cytokine secretion were found in splenocytes or peripheral lymphocytes. IL-6 expression was blocked in the diseased paws of mice in the scFv treatment groups compared to controls. In conclusion, we have shown that local expression of an anti-inflammatory agent blocks disease development without causing demonstrable systemic immune function changes. This is encouraging for the potential development of safe adoptive cellular therapies to treat autoimmunity.

Generation and functional characterization of intracellular antibodies interacting with the kinase domain of human EGF receptor

Intracellular expression of single-chain antibodies (scFvs) represents a promising approach for selective interference with cellular proto-oncogenes such as the epidermal growth factor receptor (EGFR). Previously, we have shown that intrabodies targeted to the lumen of the endoplasmic reticulum prevent the transit of EGFR or the related ErbB2 molecule to the cell surface, thereby inactivating their transforming potential. While intramolecular disulfide bridges important for antibody stability are correctly formed during expression in the secretory pathway, scFvs expressed in the reducing environment of the cytosol are often inactive. To overcome this problem and to generate antibody fragments that interact with the intracellular domain of human EGFR in the cytoplasm, here we have chosen a two-step approach combining classical selection of scFvs by phage display with subsequent expression in yeast. After enrichment of EGFR-specific antibody fragments from a combinatorial library by biopanning, a yeast two-hybrid screen was performed using the intracellular domain of EGFR as bait. Screening of 1.5 x 10(5) preselected scFv plasmids under highly stringent conditions yielded 223 colonies that represented at least five independent scFv clones functional in the intracellular milieu of eukaryotic cells. Interaction of selected antibody fragments with the intracellular domain of EGFR was confirmed in GST pull-down and coimmunoprecipitation experiments. Upon cytoplasmic expression in human tumor cells, scFvs colocalized with EGFR at the plasma membrane demonstrating their functionality in vivo.

Monoclonal antibodies to elongation factor-1alpha inhibit in vitro translation in lysates of Sf21 cells

Elongation factor-1alpha (EF-1alpha) is an enzyme that is essential for protein synthesis. Although EF-1alpha offers an excellent target for the disruption of insect metabolism, agents known to interfere with EF-1alpha activity are toxic to humans. In this article, we describe the development of monoclonal antibodies (MAbs) that can disrupt the activity of insect EF-1alpha without cross-reacting with the human enzyme. MAbs were generated to EF-1alpha from Sf21 cells derived from the fall armyworm, Spodoptera frugiperda, by immunizing mice with EF-1alpha eluted from SDS-PAGE gels. The MAbs reacted with EF-1alpha in eggs and first through fifth instars of the fall armyworm in immunoblots of SDS-PAGE gels, but did not recognize EF-1alpha in human carcinoma cells and normal tissues. MAbs with the ability to recognize EF-1alpha in its native conformation, identified through immunoprecipitation experiments, were added to Sf21 cell lysates to determine whether the antibodies could inhibit incorporation of [(35)S]methionine into newly synthesized in vitro translation products. Of the four EF-1alpha-specific MAbs tested, three significantly inhibited protein synthesis when compared to the negative control antibody (P < 0.001, one-way ANOVA; followed by Dunnett's test, P < 0.05).

Targeting multidrug resistant tumor cells with a recombinant single-chain FV fragment directed to P-glycoprotein

The MDR1 gene product P-glycoprotein (Pgp) plays a key role in multidrug resistance of cancer cells. Pgp is an ATP-driven efflux pump that extrudes a variety of dissimilar hydrophobic cytotoxic compounds. P-glycoprotein overexpression results in multidrug resistance (MDR) of tumor cell lines in vitro as well as in cancer patients. To selectively target and eliminate MDR tumor cells, we have isolated a monoclonal antibody that specifically reacts with the first extracellular loop of the human Pgp. We have cloned the variable domain genes of this antibody and assembled a functional single-chain Fv fragment capable of specifically targeting various Pgp-expressing MDR carcinoma cells lines. Targeting and specific elimination of Pgp-dependent MDR human cancer cells was achieved by constructing a single-chain immunotoxin in which the scFv fragment was fused to a truncated form of Pseudomonas exotoxin (PE38). We conclude that recombinant Fv-immunotoxins or other Fv-based molecules armed with potent cytotoxins represent an effective tool in targeted cancer therapy aimed at specific elimination of MDR tumor cell sub-populations. Recombinant antibody fragments targeting MDR proteins such as Pgp may be also used for intracellular expression and consequent phenotypic knockout of MDR.

Tyrosine kinase inhibitors targeted to the epidermal growth factor receptor subfamily: role as anticancer agents

Abnormal cell signal transduction arising from protein tyrosine kinases has been implicated in the initiation and progression of a variety of human cancers. Over the past 2 decades pharmaceutical and university laboratories have been involved in a tremendous effort to develop compounds that can selectively modulate these abnormal signalling pathways. Targeting receptor tyrosine kinases, especially the epidermal growth factor receptor subfamily, has been at the forefront of this effort as a result of strong clinical data correlating over-expression of these receptors with more aggressive cancers. There are a variety of strategies under development for inhibiting the kinase activity of these receptors, targeting both the extracellular and intracellular domains. Antibody-based approaches, immunotoxins and ligand-binding cytotoxic agents use the extracellular domain for targeted tumour therapy. Small molecule inhibitors target the intracellular catalytic region by interfering with ATP binding, while nonphosphorylatable peptides are aimed at the intracellular substrate binding region. Compounds that inhibit subsequent downstream signals from the receptor by interrupting intracellular protein recognition sequences are also being investigated. In the past 5 years enormous progress has been made in developing tyrosine kinase inhibitor compounds with sufficient potency, bioavailability and selectivity against this subfamily of receptor tyrosine kinases. The anti-HER2 monoclonal antibody, trastuzumab, for patients with metastatic breast cancer is the first of these inhibitor compounds to gain FDA approval. However, preclinical and clinical trials are ongoing with a variety of other monoclonal antibodies, immunotoxins, and small molecule quinazoline and pyrimidine-based inhibitors. Although their cytotoxic and cytostatic potential has been proven, they are not likely to replace standard chemotherapy regimens as single-agent, first-line therapeutics. Instead, their promising additive and synergistic antitumour effects in combination with standard chemotherapeutics suggest that these novel agents will find their greatest utility and efficacy in conjunction with existing anticancer agents.

Intracellular expression of the anti-erbB-2 sFv N29 fails to accomplish efficient target modulation

The use of intracellular single chain antibodies has recently emerged as a highly efficient method of down-regulating or ablating protein expression. In this regard, we have demonstrated that a single chain antibody directed against the extracellular domain of the erbB-2 molecule causes a specific toxicity in erbB-2 positive tumor types. To further investigate the mechanism of this effect, we developed a second anti-erbB-2 sFv predicted to recognize an alternate extracellular epitope of the erbB-2 molecule. When produced as a secreted protein from the erbB-2 negative COS-1 cell line, this sFv binds specifically to erbB-2 positive cells, indicating that cellular machinery is able to produce a properly folded and functional sFv protein. However, by several assays, this sFv was shown to be unable to retain the erbB-2 protein within the ER. These negative results have implications for the evaluation and utilization of sFv knockout strategies in experimental contexts.

An antibody diagnostic for hymenopteran parasitism is specific for a homologue of elongation factor-1 alpha

An enzyme-linked immunosorbent assay (ELISA) useful for identifying noctuid pests parasitized by hymenopteran endoparasitoids was recently described. The ELISA employed a monoclonal antibody (MAb 9A5) that appeared highly polyspecific for parasitoid antigens, yielding banding patterns more typical of a polyclonal antiserum than of a monoclonal antibody in immunoblots of parasitoid homogenates subjected to SDS-PAGE. Although MAb 9A5 appeared capable of binding to dozens of parasitoid antigens, no cross-reactivity for noctuid antigens was evident by either immunoblotting or ELISA. In the study described here, immunoprecipitation, SDS-PAGE, and N-terminus amino acid sequencing were used to identify the protein recognized by MAb 9A5 as a homologue of elongation factor-1 alpha (EF-1 alpha). The propensity for EF-1 alpha to bind to cytoskeletal components, the additional subunits of EF-1, and other proteins may account for the apparent polyspecificity of MAb 9A5 in immunoblots of whole-body parasitoid homogenates. The presence of a unique hymenopteran epitope suggests that EF-1 alpha molecules from other insect groups could similarly express novel determinants. These determinants may prove useful not only for insect detection, but also as targets for selective insecticides that act by inhibiting protein synthesis.