Recombinant human monoclonal antibodies. Basic principles of the immune system transferred to E. coli

Can antibodies be "vegan"? A guide through the maze of today's antibody generation methods

There is no doubt that today's life sciences would look very different without the availability of millions of research antibody products. Nevertheless, the use of antibody reagents that are poorly characterized has led to the publication of false or misleading results. The use of laboratory animals to produce research antibodies has also been criticized. Surprisingly, both problems can be addressed with the same technology. This review charts today's maze of different antibody formats and the various methods for antibody production and their interconnections, ultimately concluding that sequence-defined recombinant antibodies offer a clear path to both improved quality of experimental data and reduced use of animals.

A recombinant PSMA-specific single-chain immunotoxin has potent and selective toxicity against prostate cancer cells

Prostate cancer is the most commonly diagnosed form of cancer and the second leading cancer-related death among men in the Western civilization. Since no effective therapy exists for this tumor after progression beyond resectable boundaries, there is an urgent need for new treatment strategies. Prostate specific membrane antigen (PSMA) represents an excellent target on prostate cancer cells, and therefore specific immunotherapy may be a novel therapeutic option for the management of this tumor. We constructed a fully recombinant immunotoxin (A5-PE40) from a single-chain antibody fragment (scFv) against cell-adherent PSMA and a truncated form of Pseudomonas exotoxin A (PE40) lacking its natural binding domain Ia. The scFv A5 was obtained from a mAb elicited with native PSMA by phage display technology and direct selection on cells carrying the antigen. The bacterially expressed and purified immunotoxin A5-PE40 specifically binds to PSMA-positive prostate cancer cells and induces a 50% reduction of viability (IC50) at a concentration of 20 pM, while PSMA-negative cells remain unaffected. Due to its high and specific toxicity this recombinant immunotoxin is a promising candidate for therapeutic applications in patients with prostate cancer.

A new generation of monoclonal and recombinant antibodies against cell-adherent prostate specific membrane antigen for diagnostic and therapeutic targeting of prostate cancer

BACKGROUND

Prostate-specific membrane antigen (PSMA) is an excellent candidate for targeting prostate cancer by virtue of its restricted expression on prostatic epithelial cells and its upregulation on prostatic carcinoma cells. PSMA is expressed on the cell surface displaying a specific three-dimensional structure. Therefore, only antibodies with a high cell binding activity will have an important impact on antibody-based imaging and therapy.

METHODS

Monoclonal antibodies (mAbs) and single chain antibody fragments (scFvs) were prepared from spleen cells of mice that had been immunized either with purified PSMA or a cell lysate of prostate cancer LNCaP cells containing native PSMA. mAbs and scFvs were screened for reactivity with purified PSMA and binding to PSMA-expressing LNCaP cells.

RESULTS

From mice immunized with purified PSMA, we obtained three mAbs (K7, K12, D20) and four scFvs (G0, G1, G2, G4), which were highly reactive with the isolated antigen, but showed weak or no reaction with viable LNCaP cells. From mice immunized with unpurified LNCaP lysate, we obtained three mAbs (3/E7, 3/F11, 3/A12), and one scFv (A5), which were reactive with purified PSMA, also showing a strong and specific binding to viable LNCaP cells and PSMA-transfected cells.

CONCLUSIONS

Our results suggest that only the mAbs and scFvs, that were elicited with unpurified LNCaP lysate and not with purified PSMA will be useful agents for diagnostic imaging and therapeutic applications of prostate cancer.

Imaging and therapy of tumors induced to express somatostatin receptor by gene transfer using radiolabeled peptides and single chain antibody constructs

The fields of radioimmunodetection and radioimmunotherapy began with an initial paradigm that a targeting molecule (eg, antibody) carrying a radioisotope had the potential of selectively imaging and delivering a therapeutic dose of radiation to tumor sites. A second paradigm was developed in which injection of the targeting molecule was separated from injection of a short-lived radioisotope-labeled ligand (so-called "pretargeting strategy"). This strategy has improved radioisotope delivery to tumors in animal models, enhanced radioimmune imaging in man, and therapeutic trials are in an early phase. We proposed a third paradigm to achieve radioisotopic localization at tumor sites by inducing tumor cells to synthesize a membrane expressed receptor with a high affinity for infused radiolabeled ligands. The use of gene transfer technology to induce expression of high affinity membrane receptors can enhance the specificity of radioligand localization, while the use of radioisotopes with the ability to deliver radiation damage across several cell diameters will compensate for less than perfect transduction efficiency. This approach was termed "Genetic Radioisotope Targeting Strategy." Using this strategy, induction of high levels of gastrin releasing peptide receptor or human somatostatin receptor subtype 2 expression and selective tumor uptake of radiolabeled peptides was achieved. The advantages of the genetic transduction approach are (1) constitutive expression of a tumor-associated antigen/receptor is not required; (2) tumor cells are altered to express a new target receptor or increased quantities of an existing receptor at levels that may significantly improve tumor targeting of radiolabeled ligands compared with normal tissues; (3) gene transfer can be achieved by intratumoral or regional injection of gene vectors; (4) it is feasible to target adenovirus vectors to receptors overexpressed on tumor cells by modifying adenoviral tropism (binding) so that the virus will be targeted specifically to the desired tumor; and (5) it is possible to coexpress the receptor gene and a therapeutic gene, such as cytosine deaminase, for molecular prodrug therapy to produce an enhanced therapeutic effect.

An improved helper phage system for efficient isolation of specific antibody molecules in phage display

Phage display technology has been applied in many fields of biological and medical sciences to study molecular interactions and especially in the generation of monoclonal antibodies of human origin. However, extremely low display level of antibody molecules on the surface of phage is an intrinsic problem of a phagemid-based display system resulting in low success rate of isolating specific binding molecules. We show here that display of single-chain antibody fragment (scFv) generated with pIGT3 phagemid can be increased dramatically by using a genetically modified Ex-phage. Ex-phage has a mutant pIII gene that produces a functional wild-type pIII in suppressing Escherichia coli strains but does not make any pIII in non-suppressing E.coli strains. Packaging phagemids encoding antibody-pIII fusion in F+ non-suppressing E.coli strains with Ex-phage enhanced the display level of antibody fragments on the surfaces of recombinant phage particles resulting in an increase of antigen-binding reactivity >100-fold compared to packaging with M13KO7 helper phage. Thus, the Ex-phage and pIGT3 phagemid vector provides a system for the efficient enrichment of specific binding antibodies from a phage display library and, thereby, increases the chance of obtaining more diverse antibodies specific for target antigens.

A two-step selection approach for the identification of ligand-binding determinants in cytokine receptors

We have developed a novel cell-based method for the isolation and selection of mutant cytokine receptors with defects in ligand binding and applied it to the human interleukin-4 receptor. The experimental procedure is based upon the functional heterologous expression of receptor mutants in eukaryotic cells followed by a two-step selection procedure. Positive selection for cells that express receptor variants is achieved by means of an agonistic antibody that mediates cell survival through receptor dimerization. An IL-4-coupled toxin is subsequently used to select against cells expressing wild-type receptors. Cells expressing mutant receptors that are unable to bind the cytotoxic ligand survive and can be amplified. The procedure allows the isolation of rare receptor variants from cell pools containing predominantly wild-type cells. This method, which should be equally applicable to similar receptor systems, was used to demonstrate the importance of a critical charged amino acid residue in the human IL-4 receptor alpha-subunit for IL-4-induced receptor activation.

Generating monoclonal antibodies to neuronal antigens

Methods for generating monoclonal antibodies directed to the functional sites of neuronal antigens are reviewed. These methods include optimal antigen preparation and presentation as well as selective targeting and manipulation of the antigenic response. We describe our use of the immunosuppressant drug, cyclophosphamide, to produce a selective immune response to rare, poorly immunogenic, or actively suppressed antigens. These techniques allow us to generate antibodies to the functional sites of neuronal antigens, such as cell surface molecules. Such antibodies are directed to complex carbohydrates, proteins, protein complexes and glycolipids that form the active site of neuronal antigens. We can use these antibodies in the molecular dissection of functional active sites that are inaccessible to genetic manipulation. These techniques favor the generation of antibodies that can be used to understand and manipulate neuronal cellular activity.

Surface display of antibodies

To screen antibody libraries that contain many millions of different clones, a selection system is required with an efficiency comparable to that of the immune system. This can be achieved by displaying antibodies on the surface of microorganisms containing the antibody's gene, analogous to the expression of the IgM antigen receptor on the surface of unactivated B-lymphocytes. Specific clones can then be selected using immobilized antigens. The minor coat protein of filamentous phages, pIII, which initiates the infection of E.coli by binding to their F-pili, and the major coat protein, pVIII, have been used as carriers for displaying antibodies on the phage surface. Recombinant antibodies have also been targeted to the cell surface of bacteria by fusing them with outer membrane components derived from lipoproteins, OmpA and an IgA protease. However, only the pIII system has been routinely used for screening antibody libraries. Here we describe the various antibody surface display systems and the screening of antibody libraries generated from the gene repertoire of lymphocytes and by gene synthesis. Finally, we have made a short comparison of the bacterial production of Fabs versus single chain antibodies (scFv).

Regulated secretion and purification of recombinant antibodies in E. coli

A plasmid for optimized protein expression of recombinant Fv antibodies (pOPE) in E. coli was used to express the variable domains of the murine monoclonal antibody HD39 specific for the human B-cell surface antigen CD22. The production of Fv antibodies by pOPE can be regulated over a wide range by varying the IPTG concentration. Antibodies that can discriminate between secreted and nonsecreted Fv antibody fragments were used to show that secretion is the limiting step for the production of functional Fv antibodies. IPTG concentrations above 20 microM increased the total antibody production, but did not yield larger amounts of secreted Fv antibodies. The addition of five histidines to the C terminus facilitates an easy single-step enrichment procedure based on immobilized metal affinity chromatography.