Medical applications of single-chain antibodies

Alternative triggering molecules and single chain bispecific antibodies

Although T cell receptors and Fc receptors are the best known cytotoxic triggering molecules, a number of adhesion molecules recently have been identified as alternative triggering molecules. We discuss how CD44, an adhesion molecule found on all leukocytes and many other cell types, becomes a triggering molecule on NK cells following stimulation with IL-2. We also describe a genetically engineered single chain bispecific antibody, produced in mammalian cells and in bacteria, that is capable of redirecting lysis in the nanogram per milliliter range.

Expression of monovalent and bivalent antibody fragments in Escherichia coli

The technology of humanization of rodent antibodies has opened the way for a broad range of therapeutic antibodies with very low immunogenicity, which are, therefore, suitable for repeated dosing. Such intact antibodies have extended serum half-lives and biodistribution profiles very similar to human antibodies. For some applications, however, the ideal therapeutic should have reduced serum half-life and altered biodistribution patterns typical of antibody fragments, such as Fab or single chain Fv. Bispecific antibody fragments offer exciting additional therapeutic possibilities, but their successful manufacture and purification on a large scale require the development of new methods. Antibody fragments often assemble in Escherichia coli as monovalent fragments with reduced affinities. We describe the spontaneous assembly of bivalent antibody fragments in E. coli and methods of purification that yield either bivalent or monovalent molecules as required.

Cloning, expression, and purification of an anti-desipramine single chain antibody in NS/O myeloma cells

Drug-specific monoclonal antibodies and their antigen-binding Fab fragments reverse acute desipramine toxicity in a rat experimental model by inducing a redistribution of drug from cardiac tissue into serum and extracellular fluid. In order to investigate the use of smaller recombinant antibody fragments such as single chain Fv (sFv) as an antidote, an efficient murine NS/O myeloma expression system was developed. The variable light (VL) and variable heavy (VH) domains of a murine anti-desipramine monoclonal antibody were cloned and sequenced. A 270 amino acid VH-(Gly4Ser)3-VL sFv was prepared by overlapping polymerase chain reaction (PCR) amplification of VH with heavy chain leader peptide, VL, and the linker. This construct was subcloned into a mammalian expression vector which utilizes the SR alpha promoter, a hybrid promoter consisting of the SV40 early promoter with portions of the human T-cell leukemia virus type I long terminal repeat and also containing the Escherichia cloi xanthine-guanine phosphoribosyltransferase gene for selection. NS/O myeloma cells were transfected by electroporation. Stable recombinant NS/O clones were screened for expression of sFv using reverse transcriptase-PCR to detect mRNA and an enzyme-linked immunosorbent assay (ELISA) to detect sFv. Secreted sFv from clones capable of growth to a cell density of 2-4 x 10(6) viable cells/mL was purified in a single step using a desipramine affinity column resulting in 12-39 mg/L of purified sFv. Affinity-purified sFv had comparable desipramine binding activity to Fab when evaluated by competitive ELISA.

Clinical issues in antibody design

Antibody genes can now be cloned and expressed in various ways to give new versions of antibodies that possess reduced immunogenicity, improved affinity, altered size, increased avidity and novel effector functions. The task for any clinical application is, first, to define a relevant target, and then to design the optimal antibody-based therapeutic molecule to react with that target. This article reviews these improved antibody-based molecules and examines their role in cancer therapy.

Idiotypic DNA vaccines against B-cell lymphoma

Idiotypic antigens are clearly defined tumor-associated protein antigens, which can induce protective immunity against lymphoma. Because each patient requires an individual vaccine, idiotypic antigens also provide ideal candidates for exploring the feasibility of replacing protein antigens by DNA vaccines. Component idiotypic variable region genes can be identified in patients' tumor biopsies and rapidly assembled as scFv sequences. These can be used to produce recombinant scFv protein in bacteria, or as direct naked DNA vaccines. A preliminary small trial of DNA vaccines for chemotherapy-resistant patients with lymphoma has begun. Intramuscular idiotypic DNA vaccination in a mouse model induces low levels of anti-idiotypic antibody in serum. Levels can be increased dramatically by coinjection of DNA plasmids encoding either IL-2 or GM-CSF, and specific proliferative anti-idiotypic T cells are induced. However protective immunity remains to be demonstrated, and a possible reason for this may lie in the continued secretion of idiotypic scFv antigen which blocks antibody activity by formation of immune complexes. Methods for regulating secretion of antigen are required before this category of tumor antigen can be fully exploited as a vaccine. The power of DNA technology should allow analysis and manipulation of pathways of antigen presentation to induce maximal therapeutic attack on neoplastic B cells. In addition, lymphoma presents a model for application of DNA technology to the wide range of human tumors known to harbor potential tumor antigens.

Redirection of cellular cytotoxicity. A two-step approach using recombinant single-chain Fv molecules

In this article the authors discuss an indirect system for redirecting cellular cytotoxicity, which utilizes a "universal" bispecific antibody to redirect T-cells to kill cells targeted with single-chain Fv (sFv) fusion proteins that carry a peptide tag recognized by the bispecific antibody. This approach has a number of theoretical advantages in the immunotherapy of cancer.

A single chain Fv derived from a filamentous phage library has distinct tumor targeting advantages over one derived from a hybridoma

Single-chain antibodies (scFvs) can be derived from a monoclonal antibody (MAb) or produced directly using filamentous phage technology, where antibodies with desired binding and purification characteristics can be readily selected from libraries. To test the hypothesis that the latter approach is more useful, we compared 2 anti-carcinoembryonic antigen (CEA) scFvs produced by these 2 different approaches. Our study showed that, both in the purification process and in the biodistribution pattern, MFE-23, produced by filamentous phage technology, gave favourable results compared to A5-SC, which is derived from the A5B7 MAb. This indicates the value of the filamentous phage approach for obtaining tumour-targeting scFvs.

Optimization of in vivo tumor targeting in SCID mice with divalent forms of 741F8 anti-c-erbB-2 single-chain Fv: effects of dose escalation and repeated i.v. administration

Single-chain Fv molecules in monovalent (sFv) and divalent [(sFv')2] forms exhibit highly specific tumor targeting in mice as a result of their small size and rapid systemic clearance. As a consequence, there is a rapid reversal of the sFv blood/tumor gradient, resulting in diminished retention of sFv species in tumors. In this report we investigate two distinct strategies, dose escalation and repetitive intravenous (i.v.) dosing, aiming to increase the absolute selective retention of radiolabeled anti-c-erbB-2 125I-741F8 (sFv')2 in c-erbB-2-overexpressing SK-OV-3 tumors in mice with severe combined immunodeficiency (SCID). A dose-escalation strategy was applied to single i.v. injections of 125I-741F8 (sFv')2. Doses from 50 micrograms to 1000 micrograms were administered without a significant decrease in tumor targeting or specificity. High doses resulted in large increases in the absolute retention of 125I-741F8 (sFv')2. For example, raising the administered dose from 50 micrograms to 1000 micrograms increased the tumor retention 24 h after injection from 0.46 microgram/g to 9.5 micrograms/g, and resulted in a net increase of greater than 9 micrograms/g. Over the same dose range, the liver retention rose from 0.06 microgram/g to 1 microgram/g, and resulted in a net increase of less than 1 microgram/g. The retention of 9.5 micrograms/g in tumor 24 h following the 1000-micrograms dose of (sFv')2 was comparable to that seen 24 h after a 50-micrograms dose of 125I-741F8 IgG, indicating that the use of large doses of (sFv')2 may partially offset their rapid clearance. When two doses were administered by i.v. injection 24 h apart, the specificity of delivery to tumor observed after the first dose was maintained following the second injection. Tumor retention of 125I-741F8 (sFv')2 was 0.32 microgram/g at 24 h and 0.22 micrograms/g at 48 h following a single injection of 20 micrograms, while 0.04 microgram/ml and 0.03 microgram/ml were retained in blood at the same assay times. After a second 20-micrograms injection at the 24-h assay time, tumor retention increased to 0.49 micrograms/g, and blood retention was 0.06 microgram/ml, at the 48-h point. These results suggest that multiple high-dose administrations of radiolabeled 741F8 (sFv')2 may lead to the selective tumor localization of therapeutic radiation doses.

Purification of bacterially expressed single chain Fv antibodies for clinical applications using metal chelate chromatography

A new procedure is described for the purification of an anti-carcinoembryonic antigen (CEA) single chain Fv (scFv), referred to as MFE-23, from bacterial supernatant. A simple insertion of a hexa-histidine tail fused at the C-terminus (MFE-23 His) provides an affinity tag which selectively binds to transition metal ions immobilised on an iminodiacetic acid (IDA) derivitised solid phase matrix. This method proved to be superior to standard CEA antigen affinity chromatography in the following ways. (1) A higher yield was produced (10 mg/l as opposed to 2.2 mg/l of bacterial supernatant). The latter figure was largely affected by the limited availability (size of the column) of immobilised CEA antigen. (2) Scale-up was relatively simple and less costly. (3) The risk of tumour derived antigen leaching from the column is eliminated. Results showed that immobilised Cu2+ ions were more effective than Ni2+ and Zn2+ ions in retaining the His tagged product giving a 90% pure product on elution. Clinical grade material was generated using size exclusion chromatography to remove aggregated material, and Detoxi gel to remove bacterial endotoxins. Validation assays to measure DNA, copper and endotoxins were performed to assess the levels of contaminants. MFE-23 His retained 84% antigen binding after 6 months storage at 4 degrees C and > 75% after radiolabelling. Further experiments confirmed that the His tail did not affect biodistribution and tumour localisation in nude mice bearing human colorectal tumour xenografts.

Functional three-domain single-chain T-cell receptors

T-cell receptors (TCRs) are membrane anchored heterodimers structurally related to antibody molecules. Single-chain antibodies can be engineered by linking the two variable domains, which fold properly by themselves. However, proper assembly of the variable domains of a human TCR (V alpha and V beta) that recognize the HLA-DR2b/myelin basic protein-(85-99) peptide complex was critically dependent on the addition of a third domain, the constant region of the TCR beta chain (C beta), to the single-chain construct. Single-chain molecules with the three-domain design, but not those with the two-domain design, expressed in a eukaryotic cell as chimeric molecules linked either to glycosyl phosphatidylinositol or to the transmembrane/cytoplasmic domains of the CD3 zeta chain were recognized by a conformation-sensitive monoclonal antibody. The chimeric three-domain single-chain TCR linked to CD3 zeta chain signaled in response to both the specific HLA-DR/peptide and the HLA-DR/superantigen staphylococcal enterotoxin B complexes. Thus, by using this three-domain design, functional single-chain TCR molecules were expressed with high efficiency. The lipid-linked single-chain TCR was solubilized by enzymatic cleavage and purified by affinity chromatography. The apparent requirement of the constant domain for cooperative folding of the two TCR variable domains may reflect significant structural differences between TCR and antibody molecules.

Mammalian cell expression of single-chain Fv (sFv) antibody proteins and their C-terminal fusions with interleukin-2 and other effector domains

The production of several single-chain Fv (sFv) antibody proteins was examined by three modes of mammalian cell expression. Our primary model was the 741F8 anti-c-erbB-2 sFv, assembled as either the VH-VL or VL-VH, and expressed alone, with C-terminal cysteine for dimerization, or as fusion proteins with carboxyl-terminal effector domains, including interleukin-2, the B domain of staphylococcal protein A, the S-peptide of ribonuclease S, or hexa-histidine metal chelate peptide. Constructs were expressed and secreted transiently in 293 cells and stably in CHO or Sp2/0 cell lines, the latter yielding up to 10 mg per liter. Single-chain constructs of MOPC 315 myeloma and 26-10 monoclonal antibodies were also expressed, as were hybrids comprising unrelated VH and VL regions. Our results suggest that mammalian expression is a practical and valuable complement to the bacterial expression of single-chain antibodies.

Genetically engineered antibodies for diagnostic pathology

Antibody genes can be cloned, genetically manipulated, and expressed in both homologous and heterologous expression systems to produce viable antigen-binding proteins complete with natural effector functions. Manipulation of antibody genes permits the expression of fusion proteins or truncated proteins that retain antigen-binding activity. The new antibody technologies are becoming increasingly sophisticated, permitting the alteration of antigen-binding responses, the transfer of antigen specificity between antibodies, and the expression of minimal-size antigen-binding protein domains. These new molecules have been made mostly for studies on function or to provide molecules suited for in vivo diagnosis and therapy; very few have been specifically designed for, or used for, diagnostic histopathology. We describe here the adaptation of small antibody derivatives for use in immunohistochemistry. Molecules suitable for this purpose need only to possess specific antigen-binding ability and some means of detection of antigen-bound material. Detection could be by recognition of a genetically fused flag or tag epitope, by the fusion of an enzyme whose activity can be assayed, or by fusion with a protein that can interact with pre-existing histopathological reagents.

Antibody engineering

A century ago, in his private laboratory in Berlin, Paul Ehrlich conceived and developed the idea of using antibodies to target toxic molecules. Recent advances in genetic engineering have enormously extended the potential of this concept for the treatment of malignant diseases, by making it possible to isolate genes encoded for the antibody variable regions and to manipulate them to create new molecules.

Monoclonal antibodies in cancer therapy

A review of the clinical trials of antibody based cancer therapies reveals that this approach can, in rare cases, induce complete remissions in individual patients with cancer. Since these trials have usually involved patients with large tumor masses, tumor cell inaccessibility is probably a major reason for the prevailing failures. Minimal residual disease, the stage when tumor cells are few and dispersed, should therefore be a more promising target for therapeutic antibodies. This hypothesis is supported by a prospective randomized trial on patients with resected Dukes C colorectal carcinoma that resulted in increased survival and prolonged recurrence-free intervals. Thus, in addition to strategies designed to produce more effective, human-derived reagents, efforts need to be concentrated on directing passive antibody therapy towards the appropriate target.