A recombinant immunotoxin containing a disulfide-stabilized Fv fragment

Delivery and therapeutic potential of human granzyme B

Granzyme B (GzmB) is used by cytotoxic lymphocytes as a molecular weapon for the defense against virus-infected and malignantly transformed host cells. It belongs to a family of small serine proteases that are stored in secretory vesicles of killer cells. After secretion of these cytolytic granules during killer cell attack, GzmB is translocated into the cytosol of target cells with the help of the pore-forming protein perforin. GzmB has adopted similar protease specificity as caspase-8, and once delivered, it activates major executioner apoptosis pathways. Since GzmB is very effective in killing human tumor cell lines that are otherwise resistant against many cytotoxic drugs and since GzmB of human origin can be recombinantly expressed, its use as part of a 'magic bullet' in tumor therapy is a very tempting idea. In this review, we emphasize the peculiar characteristics of GzmB that make it suited for use as an effector domain in potential immunoconjugates. We discuss what is known about its uptake into target cells and the trials performed with GzmB-armed immunoconjugates, and we assess the prospects of its potential therapeutic value.

Signal sequence as a determinant in expressing disulfide-stabilized single chain antibody variable fragments (sc-dsFv) against human VEGF

Phage-displayed single chain variable fragment (scFv) libraries have been powerful tools in antibody engineering. But the scFv structures are frequently unstable due to the dissociation of the dimeric interface between the two variable domains. One solution is the sc-dsFv construct, where the single chain variable domain fragment is stabilized with an additional interface disulfide bond, leading to stable and homogeneous dimeric interface for the sc-dsFv structure. However, the phagemid system that is capable of effective expression for both sc-dsFv-pIII fusion proteins on phage surface and secreted non-fusion sc-dsFv in bacterial culture medium has not been demonstrated. In this work, a biological combinatorial approach was applied to optimize the signal sequence N-terminal to the sc-dsFv-pIII fusion protein encoded in a phagemid. The optimized sc-dsFv phage display systems were compatible with both the phage-based directed evolution procedure and the high throughput screening of the soluble sc-dsFv. The utility of the phagemid systems was demonstrated in generating anti-VEGF sc-dsFv with VEGF-binding affinity one order of magnitude higher than the corresponding scFv, due only to the interface disulfide bond in the sc-dsFv. Moreover, the protein stability of the sc-dsFv construct was unmatched by the corresponding scFv. These advantages of the sc-dsFv were gained through the interface disulfide bond of the sc-dsFv and the novel signal sequence in the phagemid.

Effector cell recruitment with novel Fv-based dual-affinity re-targeting protein leads to potent tumor cytolysis and in vivo B-cell depletion

Bispecific antibodies capable of redirecting the lytic potential of immune effector cells to kill tumor targets have long been recognized as a potentially potent biological therapeutic intervention. Unfortunately, efforts to produce such molecules have been limited owing to inefficient production and poor stability properties. Here, we describe a novel Fv-derived strategy based on a covalently linked bispecific diabody structure that we term dual-affinity re-targeting (DART). As a model system, we linked an Fv specific for human CD16 (FcgammaRIII) on effector cells to an Fv specific for mouse or human CD32B (FcgammaRIIB), a normal B-cell and tumor target antigen. DART proteins were produced at high levels in mammalian cells, retained the binding activity of the respective parental Fv domains as well as bispecific binding, and showed extended storage and serum stability. Functionally, the DART molecules demonstrated extremely potent, dose-dependent cytotoxicity in retargeting human PBMC against B-lymphoma cell lines as well as in mediating autologous B-cell depletion in culture. In vivo studies in mice demonstrated effective B-cell depletion that was dependent on the transgenic expression of both CD16A on the effector cells and CD32B on the B-cell targets. Furthermore, DART proteins showed potent in vivo protective activity in a human Burkitt's lymphoma cell xenograft model. Thus, DART represents a biologically potent format that provides a versatile platform for generating bispecific antibody fragments for redirected killing and, with the selection of appropriate binding partners, applications outside of tumor cell cytotoxicity.

Improvement of tumor targeting and antitumor activity by a disulphide bond stabilized diabody expressed in Escherichia coli

We have generated an anti-Pgp/anti-CD3 diabody which can effectively inhibit the growth of multidrug-resistant human tumors. However, the two chains of the diabody are associated non-covalently and are therefore capable of dissociation. Cysteine residues were introduced into the V-domains to promote disulphide cross-linking of the dimer as secreted by Escherichia coli. Compared with the parent diabody, the ds-Diabody obtained was more stable in human serum at 37 degrees C, without loss of affinity or cytotoxicity activity in vitro. Furthermore, the ds-Diabody showed improved tumor localization and a twofold improved antitumor activity over the parent diabody in nude mice bearing Pgp-overexpressing K562/A02 xenografts. Our data demonstrate that ds-Diabody may be more useful in therapeutic applications than the parent diabody.

The hinge region fragment of immunoglobulin G improves immunogenicity of recombinant gonadotrophin-releasing hormone conjugated to the T-helper epitope in designing peptide vaccines

In our previous study, the hinge fragment (225-232/225'-232') of human immunoglobulin G1 (IgG1) was used as a space peptide linker for synthesizing the GnRH3-hinge-MVP chimeric peptide, whereby three repeated gonadotrophin-releasing hormone (GnRH) units and a T-cell epitope from measles virus fusion protein (MVP) were amide-bond-linked at the N and C terminus, respectively, to the hinge peptide for producing anti-GnRH antibody responses. To investigate whether or not the hinge region fragment can improve the immunogenicity of GnRH, we further synthesized and purified GnRH3-hinge-MVP, GnRH3-hinge and GnRH3-MVP using recombinant DNA technology. Under high pH conditions, GnRH3-hinge-MVP was capable of forming double-chain structures. Immunization of male mice with the immunogens of GnRH3-hinge-MVP resulted in the generation of high-titre antibodies specific for GnRH. The synthetic GnRH3-hinge and GnRH3-MVP induced a lower titre of anti-GnRH antibody than GnRH3-hinge-MVP. This was followed by a decrease in serum testosterone levels, which resulted in a low level of expression of the relaxin-like factor gene in the testis. Our data suggest that peptide and T-cell epitopes oriented at the N-terminus or C-terminus of hinge peptides simplify the antigenic peptide conjugates and may be considered as potential synthetic immunogens.

Listeriolysin O as cytotoxic component of an immunotoxin

Monoclonal antibodies (mAbs) have been developed over the past years as promising anticancer therapeutics. The conjugation of tumor specific mAbs with cytotoxic molecules has been shown to improve their efficacy dramatically. These bifunctional immunotoxins, consisting of covalently linked antibodies and protein toxins, possess considerable potential in cancer therapy. Many of them are under investigation in clinical trials. As a result of general interest in new toxic components, we describe here the suitability of the bacterial protein Listeriolysin O (LLO) as cytotoxic component of an immunotoxin. Unique characteristics of LLO, such as its acidic pH optimum and the possibility to regulate the cytolytic activity by cysteine-oxidation, make LLO an interesting toxophore. Oxidized LLO shows a substantially decreased cytolytic activity when compared with the reduced protein as analyzed by hemolysis. Both oxidized and reduced LLO exhibit a cell-type-unspecific toxicity in cell culture with a significantly higher toxicity of reduced LLO. For cell-type-specific targeting of LLO to tumor cells, LLO was coupled to the dsFv fragment of the monoclonal antibody B3, which recognizes the tumor-antigen Lewis Y. The coupling of LLO to dsFv-B3 was performed via cysteine-containing polyionic fusion peptides that act as a specific heterodimerization motif. The novel immunotoxin B3-LLO could be shown to specifically eliminate antigen positive MCF7 cells with an EC(50) value of 2.3 nM, whereas antigen negative cell lines were 80- to 250-fold less sensitive towards B3-LLO.

Conserved amino acid networks involved in antibody variable domain interactions

Engineered antibodies are a large and growing class of protein therapeutics comprising both marketed products and many molecules in clinical trials in various disease indications. We investigated naturally conserved networks of amino acids that support antibody V(H) and V(L) function, with the goal of generating information to assist in the engineering of robust antibody or antibody-like therapeutics. We generated a large and diverse sequence alignment of V-class Ig-folds, of which V(H) and V(L) domains are family members. To identify conserved amino acid networks, covariations between residues at all possible position pairs were quantified as correlation coefficients (phi-values). We provide rosters of the key conserved amino acid pairs in antibody V(H) and V(L) domains, for reference and use by the antibody research community. The majority of the most strongly conserved amino acid pairs in V(H) and V(L) are at or adjacent to the V(H)-V(L) interface suggesting that the ability to heterodimerize is a constraining feature of antibody evolution. For the V(H) domain, but not the V(L) domain, residue pairs at the variable-constant domain interface (V(H)-C(H)1 interface) are also strongly conserved. The same network of conserved V(H) positions involved in interactions with both the V(L) and C(H)1 domains is found in camelid V(HH) domains, which have evolved to lack interactions with V(L) and C(H)1 domains in their mature structures; however, the amino acids at these positions are different, reflecting their different function. Overall, the data describe naturally occurring amino acid networks in antibody Fv regions that can be referenced when designing antibodies or antibody-like fragments with the goal of improving their biophysical properties.

Recombinant immunotoxins for the treatment of chemoresistant hematologic malignancies

Recombinant immunotoxins are proteins composed of fragments of monoclonal antibodies fused to truncated protein toxins. No agents of this class are approved yet for medical use, although a related molecule, denileukin diftitox, composed of interleukin-2 fused to truncated diphtheria toxin, is approved for relapsed/refractory cutaneous T-cell lymphoma. Recombinant immunotoxins which have been tested in patients with chemotherapy-pretreated hematologic malignancies include LMB-2 (anti-CD25), BL22 (CAT-3888, anti-CD22) and HA22 (CAT-8015, anti-CD22), each containing an Fv fragment fused to truncated Pseudomonas exotoxin. Major responses were observed with LMB-2 in adult T-cell leukemia, chronic lymphocytic leukemia (CLL), cutaneous T-cell lymphoma, Hodgkin's disease, and hairy cell leukemia (HCL). BL22 resulted in a high complete remission rate in patients with HCL, particularly those without excessive tumor burden. HA22, an improved version of BL22 with higher affinity to CD22, is now undergoing phase I testing in HCL, CLL, non-Hodgkin's lymphoma, and pediatric acute lymphoblastic leukemia.

Recent developments in the use of adenoviruses and immunotoxins in cancer gene therapy

Despite setbacks in the past and apparent hurdles ahead, gene therapy is advancing toward reality. The past several years have witnessed this new field of biomedicine developing rapidly both in breadth and depth, especially for the treatment of cancer, thanks largely to the better understanding of molecular and genetic basis of oncogenesis and the development of new and improved vectors and technologies for gene delivery and targeting. This article is intended to provide a brief review of recent advances in cancer gene therapy using adenoviruses, both as vectors and as oncolytic agents, and some of the recent progress in the development of immunotoxins for use in cancer gene therapy.

Fungal ribotoxins: molecular dissection of a family of natural killers

RNase T1 is the best known representative of a large family of ribonucleolytic proteins secreted by fungi, mostly Aspergillus and Penicillium species. Ribotoxins stand out among them by their cytotoxic character. They exert their toxic action by first entering the cells and then cleaving a single phosphodiester bond located within a universally conserved sequence of the large rRNA gene, known as the sarcin-ricin loop. This cleavage leads to inhibition of protein biosynthesis, followed by cellular death by apoptosis. Although no protein receptor has been found for ribotoxins, they preferentially kill cells showing altered membrane permeability, such as those that are infected with virus or transformed. Many steps of the cytotoxic process have been elucidated at the molecular level by means of a variety of methodological approaches and the construction and purification of different mutant versions of these ribotoxins. Ribotoxins have been used for the construction of immunotoxins, because of their cytotoxicity. Besides this activity, Aspf1, a ribotoxin produced by Aspergillus fumigatus, has been shown to be one of the major allergens involved in allergic aspergillosis-related pathologies. Protein engineering and peptide synthesis have been used in order to understand the basis of these pathogenic mechanisms as well as to produce hypoallergenic proteins with potential diagnostic and immunotherapeutic applications.

Optimizing the generation of recombinant single-chain antibodies against placental alkaline phosphatase

Recombinant technologies to engineer ordinary hybridoma monoclonal antibodies (MAbs) to single-chain fragment variable (scFv) may cause loss of antibody affinity, increased tendency to aggregate, increased temperature sensitivity, and low yield of active protein. In the present investigation, the well-characterized MAb H7 against placental alkaline phosphatase (PLAP), used as a model antibody, was engineered to improve solubility and stability of scFv with retained high affinity. The original procedure to generate single-chain antibodies with a 10-amino acid linker between VH and VL yielded an almost insoluble product. By site-directed mutagenesis, four selective sequence substitutions were made in the VL fragment and one in the VH fragment to improve solubility. The importance of the linker length was investigated, and a 25/30 amino acid linker was found to improve solubility. In order to further increase the stability of the single-chain antibody, an additional covalent -S-S- bond was introduced between amino acid 100 in the VL fragment and amino acid 44 in the VH region, to make a single-chain disulphide stabilized variable fragment (scdsFv). Altogether five different antibody constructs were produced and compared in terms of solubility, stability, affinity, and production properties. Immunospecificity was tested by enzyme-linked immunosorbent assay (ELISA) against the target antigen, temperature sensitivity by exposing the purified scFv to higher temperatures. All the new constructs retained almost equal activity and high affinity for their target antigen, placental alkaline phosphatase (PLAP), compared to the intact MAb H7, up to +42 degrees C as evaluated by ELISA. The overall affinity K(A) > 10(9) (M(1)) of the new antibodies could be maintained in the same order of magnitude as the original one (H7), when evaluated by Biacore technology. The best final single-chain antibody was obtained by performing the specific site-directed mutations and introducing a linker of 30 amino acids, but not by additional stabilizing disulphide bonds. The yield of the final antibody was improved approximately 10-fold by the modifications. This antibody could easily be expressed in a bacterial system using the PET-32a TrxA vector and the Escherichia coli strain BL21 Origami B (DE3). Purified antibody, which could be kept at concentrations up to 0.8 mg/mL, was obtained, which is sufficient for clinical testing of therapeutic applications.

Multi-template approach to modeling engineered disulfide bonds

The key issue for disulfide bond engineering is to select the most appropriate location in the protein. By surveying the structure of experimentally engineered disulfide bonds, we found about half of them that have geometry incompatible with any native disulfide bond geometry. To improve the current prediction methods that tend to apply either ideal geometrical or energetical criteria to single three-dimensional structures, we have combined a novel computational protocol with the usage of multiple protein structures to take into account protein backbone flexibility. The multiple structures can be selected from either independently determined crystal structures for identical proteins, models of nuclear magnetic resonance experiments, or crystal structures of homology-related proteins. We have validated our approach by comparing the predictions with known disulfide bonds. The accuracy of prediction for native disulfide bonds reaches 99.6%. In a more stringent test on the reported engineered disulfide bonds, we have obtained a success rate of 93%. Our protocol also determines the oxido-reduction state of a predicted disulfide bond and the corresponding mutational cost. From the energy ranking, the user can easily choose top predicted sites for mutagenesis experiments. Our method provides information about local stability of the engineered disulfide bond surroundings.

SV40 Pseudovirion gene delivery of a toxin to treat human adenocarcinomas in mice

SV40 vectors packaged in vitro (pseudovirions) are an efficient delivery system for plasmids up to 17.7 kb, with or without SV40 sequences. A truncated Pseudomonas exotoxin gene (PE38) was delivered into various human cells (HeLa, KB-3-1, human lymphoblastoids, and erythroleukemia cells), in vitro using pseudovirions. The number of viable cells was reduced significantly in the PE38-transduced cells. Human KB adenocarcinomas growing in mice were treated with intratumoral injection of PE38 packaged in vitro, and tumor size decreased significantly. Intraperitoneal treatments were as effective in reducing tumor size as intratumoral treatments. To check the viability of mock- or PE38-treated mice, every 4 days they were weighed, their blood was tested, and various tissues were screened for pathology. All parameters showed that the in vitro-packaged vectors, injected into tumors or intraperitoneally, caused no abnormalities in mice. The combined treatment of doxorubicin with in vitro-packaged PE38 reduced tumor size slightly more than each of the treatments separately. However, the combined treatment did not cause the weight loss seen with doxorubicin alone. These results indicate that SV40 in vitro packaging is an effective system for cancer gene delivery using two different routes of injection and in combination with chemotherapy.

Bioinformatics in molecular immunology laboratories demonstrated: Modeling an anti-CMV scFv antibody

A scFv (single chain variable fragment) antibody clone from anti-CMV (anti-cucumber mosaic virus) was successfully constructed from immunized mouse and the DNA sequence was submitted to GenBank (AY337618 and AY337619). The expression of a 32 kDa recombinant antibody in bacteria was verified using ELISA (enzyme-linked immunoassay) and western blot. However, elucidation of specific anti-CMV scFv function requires detailed and time consuming immuno-assays. Alternatively, useful functional information on anti-CMV scFV antibody can be obtained using available Bioinformatics tools and techniques without performing tedious assays. Here, we use the commonly used Bioinformatics tools and databases such as BLAST (basic local alignment search tool), GenBank, PDB (protein databank), KABAT numbering, SWISS-MODEL and Insight II to gain specific functional insights into anti-CMV scFv.

Disulfide-stabilized single-chain antibody-targeted superantigen: Construction of a prokaryotic expression system and its functional analysis

AIM: To construct the expression vector of B3 (scdsFv)-SEA (D227A) and to identify its binding and cytotoxic ability to B3 antigen positive carcinoma cell lines.

METHODS: This fusion protein was produced by a bacterial expression system in this study. It was expressed mainly in the inclusion body. The gene product was solubilized by guanidine hydrochloride, refolded by conventional dilution method, and purified using SP-sepharose cation chromatography.

RESULTS: The expression vector B3 (scdsFv)-SEA-PET was constructed, the expression product existed mainly in the inclusion body, the refolding product retained the binding ability of the single-chain antibody and had cytotoxic effect on HT-29 colon carcinoma cells. The stability assay showed that the resulting protein was stable at 37 °C.

CONCLUSION: This genetically engineered B3 (scdsFv)-SEA fusion protein has bifunction of tumor targeting and tumor cell killing and shows its promises as an effective reagent for tumor-targeted immunotherapy.

Development of a phage displayed disulfide-stabilized Fv fragment vaccine against Vibrio anguillarum

Anti-idiotype monoclonal antibody 1E10 can mimic the protective epitope of Vibrio anguillarum and be used as vaccine to prevent fish infection of V. anguillarum. In this study, the variable heavy (V(H)) domain and variable light (V(L)) domain of mAb1E10 were cloned by RT-PCR and were linked to each other by a disulfide bond engineered at position 44 of V(H) and position 105 of V(L) that lie between structurally conserved framework positions. Mutated V(H) 44 and V(L) 105 were inserted into phagemid pCANTAB5E. When co-transfected by recombinant pCANTAB5E and helper phage M13KO7, the host Escherichia coli cells secreted disulfide-stabilized Fv fragment (dsFv) which displayed on the surface of filamentous phage. The binding specificity of the phage-displayed dsFv was proved by ELISA method. Protection experiment showed that Japanese flounders can develop high titer of antibody against the dsFv and survival ratio of vaccinated group was significantly different from control groups. Thus, this phage-displayed dsFv may be used as vaccine against V. anguillarum in fishery.

Effective lysis of lymphoma cells with a stabilised bispecific single-chain Fv antibody against CD19 and FcgammaRIII (CD16)

A recombinant bispecific single-chain fragment variable antibody (bsscFv), directed against the B-cell antigen CD19 and the low affinity Fc-receptor FcgammaRIII (CD16), was designed for use in the treatment of patients with leukaemias and lymphomas. The Fc-portions of whole antibodies were deliberately eliminated in this construct to avoid undesired effector functions. A stabilised bsscFv, ds[CD19 x CD16], was generated, in which disulphide bonds bridging the respective variable light (VL) and variable heavy (VH) chains were introduced into both component single-chain (sc)Fvs. After production in 293T cells and chromatographic purification, ds[CD19 x CD16] specifically and simultaneously bound both antigens. The serum stability of ds[CD19 x CD16] was increased more than threefold when compared with the unstabilised counterpart, while other biological properties were not affected by these mutations. In antibody-dependent cellular cytotoxicity experiments, ds[CD19 x CD16] mediated specific lysis of both CD19-positive malignant human B-lymphoid cell lines and primary tumour cells from patients with B-cell chronic lymphocytic leukaemia or B-cell acute lymphoblastic leukaemia. Natural killer cells, mononuclear cells (MNCs) from healthy donors and, in some instances, MNCs isolated from patients after allogeneic stem cell transplantation, were used as effectors. Thus, ds[CD19 x CD16] holds promise for the treatment of CD19(+) B-lineage malignancies.

Tailor-made antibody therapeutics

Therapeutic antibodies represent one of the fastest growing areas of the pharmaceutical industry. There are currently 18 monoclonal antibodies in the market that have been approved by the FDA and over 150 in clinical developments. Driven by innovation and technological developments, scientists have gone beyond the traditional antibody molecules. Antibodies have been engineered in a variety of ways to meet the challenges posed by different biological settings. Described in this review is an abridged account of the different ways antibodies have been tailored to make them efficient drug molecules.

Intranasal immunisation using recombinant Lactobacillus johnsonii as a new strategy to prevent allergic disease

We have previously demonstrated the induction of a specific anti-IgE response in vivo by parenteral immunisation of rhesus monkeys using short IgE mimotopes or an anti-idiotypic antibody mimicking an IgE epitope. Such specific anti-IgE responses may be of clinical benefit for atopic patients. In this study, we examined the potential for a more convenient therapy via mucosal immunisation using live recombinant Lactobacillus johnsonii (Lj) as a vaccine delivery vehicle. Either an anti-idiotypic scFv or an IgE mimotope were expressed on the surface of Lj as fusion proteins using the cell wall anchored proteinase PrtB from Lactobacillus delbrueckii subsp. bulgaricus. The recombinant Lj were shown to express the heterologous fusion proteins and were specifically recognised by the corresponding anti-human IgE monoclonal antibody. Subcutaneous and intranasal immunisation of mice with recombinant Lj, expressing these fusion proteins induced a systemic IgG response against human IgE. Our data suggest that recombinant Lactobacilli expressing IgE epitopes may represent a novel means of vaccination to induce a beneficial anti-IgE response.

In Vitro Antibody Evolution Targeting Germline Hot Spots to Increase Activity of an Anti-CD22 Immunotoxin

Recombinant immunotoxin BL22, containing the Fv portion of an anti-CD22 antibody, produced complete remissions in most patients with drug-resistant hairy cell leukemia but had less activity in leukemias with low CD22 expression. Complementarity-determining region (CDR) mutagenesis is used to increase antibody affinity but can be difficult to perform successfully. We previously showed that antibodies with increased affinity and immunotoxins with increased activity could be obtained by directing mutations at specific DNA residues called hot spots. Because hot spots can arise either by somatic mutation or be present in the germline, we examined which type of hot spot is preferred for increasing antibody affinity. Initially, a second generation antibody phage-display library targeting a germline hot spot (Ser(30)-Asn(31)) within CDR1 of the antibody light chain was mutated. Substitution of serine 30 or asparagine 31 with arginine produced mutant immunotoxins with an affinity (0.8 nM) increased 7-fold over BL22 (5.8 nM) and 3-fold over the first generation mutant HA22 (2.3 nM). More importantly, a 10-fold increase in activity over BL22 and a 2-3-fold increase over HA22 were observed in various B lymphoma cell lines including WSU-CLL that contains only 5500 CD22 sites per cell. For comparison, two phage-display libraries targeting non-germline hot spots in heavy chain CDR1 and CDR3 were generated but did not produce Fv with increased affinity. Our results demonstrate that germline hot spots but not non-germline hot spots are effective for in vitro antibody affinity maturation.

Recombinant antibodies for the diagnosis and treatment of cancer

The advent of recombinant antibody technology led to an enormous revival in the use of antibodies as diagnostic and therapeutic tools for fighting cancer. This review provides a brief historical sketch of the development of recombinant antibodies for the diagnosis and immunotherapy of cancer and summarizes the most significant clinical data for the best established reagents to date. It also discusses clinically relevant aspects of the use of recombinant antibodies in cancer patients.

Killing of target cells by redirected granzyme B in the absence of perforin

Granzyme B (GzmB) is a potent apoptosis-inducing serine protease of cytotoxic lymphocytes. Following receptor-mediated endocytosis, GzmB is supposed to enter the cytosol through perforin-mediated membrane disruption. We investigated whether retargeting of GzmB to Lewis Y positive surface receptors could lead to perforin-independent target cell death. We coupled recombinant GzmB to the Lewis Y-binding antibody dsFv-B3. Targeting of GzmB to Lewis Y positive cells triggered cell death with similar efficacy as dsFv-B3 targeted Pseudomonas exotoxin fragment 38 (PE38). Since GzmB was only weakly inhibited by plasma proteins, GzmB-based immunoconjugates should be useful as a new class of immunotoxins with low immunogenicity utilizing programmed cell death for therapeutic purposes.

In vitro and in vivo cytotoxic activities of recombinant immunotoxin 8H9(Fv)-PE38 against breast cancer, osteosarcoma, and neuroblastoma

The 8H9 monoclonal antibody (MAb) is highly reactive with a cell surface glycoprotein expressed on human breast cancers, childhood sarcomas, and neuroblastomas but is not reactive with the cell surface of normal human tissues. This specific reactivity suggests that MAb 8H9 may be useful for targeted cancer therapy. To explore this possibility, we generated two recombinant immunotoxins (ITs) using the single-chain Fv (scFv) of MAb 8H9. Initially the 8H9(scFv) cDNA was fused to a DNA encoding a 38-kDa truncated form of Pseudomonas exotoxin (PE38) to generate the IT 8H9(scFv)-PE38. The fusion gene was expressed in Escherichia coli, and the IT was purified to near homogeneity from inclusion bodies. The purified IT showed specific cytotoxicity on nine different cancer cell lines derived from breast cancer, osteosarcoma, and neuroblastomas, known to react with MAb 8H9. The cytotoxic activity was inhibited by MAb 8H9, showing the cytotoxic activity is specific. The antitumor activity of 8H9(scFv)-PE38 was evaluated in severe combined immunodeficient mice bearing MCF-7 breast cancers or OHS-M1 osteosarcomas. The IT showed a specific dose-dependent antitumor activity at 0.075 and 0.15 mg/kg. Next, a more stable disulfide-linked IT, 8H9(dsFv)-PE38, was constructed. It was produced in high yield (16%) and showed cytotoxic and antitumor activities similar to those of 8H9(scFv)-PE38. 8H9(dsFv)-PE38 was given to two cynomolgus monkeys at doses of 0.1 and 0.2 mg/kg i.v. QOD x 3 and was well tolerated. This shows that a dose that causes significant tumor regressions in mice is well tolerated by monkeys. These results make 8H9(dsFv)-PE38 a candidate for further development as a therapeutic agent for breast cancers, osteosarcomas, and neuroblastomas.

Recombinant immunotoxins for treating cancer

Recombinant immunotoxins are antibody-toxin chimeric molecules that kill cancer cells via binding to a surface antigen, internalization and delivery of the toxin moiety to the cell cytosol. In the cytosol, toxins catalytically inhibit a critical cell function and cause cell death. The antibody portion of the chimera targets antigens that are expressed preferentially on the surface of cancer cells. Truncated versions of either diphtheria toxin (DT) or Pseudomonas exotoxin (PE) can be used to construct fusions with cDNAs encoding antibody fragments or cell-binding ligands. Recombinant immunotoxins are routinely produced in E. coli and purified using standard chromatographic methods. Before they can be evaluated for anticancer activity in humans, recombinant immunotoxins undergo extensive preclinical testing. Immunotoxins must demonstrate cell-killing activity in tissue culture, antitumor activity in an animal model and have favorable pharmacokinetic and toxicity profiles. Candidate molecules with favorable characteristics are then evaluated in clinical trials. Here we report on the initial evaluation of BL22, a recombinant immunotoxin targeted to CD22 expressed on the surface of B-cell malignancies.

Induction of caspase-dependent programmed cell death in B-cell chronic lymphocytic leukemia by anti-CD22 immunotoxins

B cells of chronic lymphocytic leukemia (CLL) are long-lived in vivo, possibly because of defects in apoptosis. We investigated BL22, an immunotoxin composed of the Fv portion of an anti-CD22 antibody fused to a 38-kDa Pseudomonas exotoxin-A fragment. B cells from 22 patients with CLL were immunomagnetically enriched (96% purity) and were cultured with BL22 or an immunotoxin that does not recognize hematopoietic cells. The antileukemic activity of BL22 was correlated with CD22 expression, as determined by flow cytometry. BL22 induced caspase-9 and caspase-3 activation, poly(adenosine diphosphate [ADP]-ribose)polymerase (PARP) cleavage, DNA fragmentation, and membrane flipping. Cell death was associated with the loss of mitochondrial membrane potential and the down-regulation of Mcl-1 and X-chromosomal inhibitor of apoptosis protein (XIAP). Furthermore, BL22 induced a proapoptotic 18-kDa Bax protein and conformational changes of Bax. Z-VAD.fmk abrogated apoptosis, confirming that cell death was executed by caspases. Conversely, interleukin-4, a survival factor, inhibited spontaneous death in culture but failed to prevent immunotoxin-induced apoptosis. BL22 cytotoxicity was markedly enhanced when combined with anticancer drugs including vincristine. We also investigated HA22, a newly engineered immunotoxin, in which BL22 residues are mutated to improve target binding. HA22 was more active than BL22. In conclusion, these immunotoxins induce caspase-mediated apoptosis involving mitochondrial damage. Combination with chemotherapy is expected to improve the efficacy of immunotoxin treatment.

Design of a modular immunotoxin connected by polyionic adapter peptides

Immunotoxins are genetically engineered fusion proteins of an antibody Fv fragment and a toxin from bacteria or plants, which function as anti-cancer therapeutics. Here, we describe a new generation of immunotoxins in which both proteins do not form a single fusion protein but are coupled specifically via cysteine-containing polyionic fusion peptides. The engineered Pseudomonas exotoxin PE38 was N-terminally fused to the peptide E(8)C. In combination with the disulfide-stabilized Fv fragment of the tumor-specific antibody B3, which was extended by the peptide R(8)CP, the fusion peptides ensured a specific and covalent coupling of the Fv fragment and the toxin. The resulting immunotoxin was as active and as specific as an immunotoxin consisting of a fusion protein of the same antibody fragment connected to the toxin.

The production of antibody fragments and antibody fusion proteins by yeasts and filamentous fungi

In this review we will focus on the current status and views concerning the production of antibody fragments and antibody fusion proteins by yeasts and filamentous fungi. We will focus on single-chain antibody fragment production (scFv and VHH) by these lower eukaryotes and the possible applications of these proteins. Also the coupling of fragments to relevant enzymes or other components will be discussed. As an example of the fusion protein strategy, the 'magic bullet' approach for industrial applications, will be highlighted.

Assessment of cell type specific gene transfer of polyoma virus like particles presenting a tumor specific antibody Fv fragment

Application of delivery systems in cancer therapy is restricted as a result of the lack of cell specificity of the respective vectors. Recently, a vector system based on virus-like particles (VLPs) of modified polyoma-VP1 was described which were able to bind specifically a tumor-specific antibody fragment, thus directing the vector system towards tumor cells. The functional gene transfer using the VP1 variant VP1-E8C, coupled with the antibody fragment of the tumor-specific antibody B3 is described in this paper. The specific targeting of the antigen expressing cells was highly efficient as determined by fluorescence microscopy. However, only a low percentage of these cells showed a functional gene transfer. This discrepancy could be accounted for by a rather low capacity of the virus like particles to transport DNA and the mechanism of their internalization by the target cells, which led to a lysosomal degradation of the particles. These limitations could be surmounted partially in cell culture experiments, and the principles suitable for applying this vector system in vivo are discussed.

Sensitization of B-cell chronic lymphocytic leukemia cells to recombinant immunotoxin by immunostimulatory phosphorothioate oligodeoxynucleotides

A recombinant anti-CD25 immunotoxin, LMB-2, has shown clinical efficacy in hairy cell leukemia and T-cell neoplasms. Its activity in B-cell chronic lymphocytic leukemia (B-CLL) is inferior but might be improved if B-CLL cells expressed higher numbers of CD25 binding sites. It was recently reported that DSP30, a phosphorothioate CpG-oligodeoxynucleotide (CpG-ODN) induces immunogenicity of B-CLL cells by up-regulation of CD25 and other antigens. The present study investigated the antitumor activity of LMB-2 in the presence of DSP30. To this end, B-CLL cells from peripheral blood of patients were isolated immunomagnetically to more than 98% purity. Incubation with DSP30 for 48 hours augmented CD25 expression in 14 of 15 B-CLL samples, as assessed by flow cytometry. DSP30 increased LMB-2 cytotoxicity dose dependently whereas a control ODN with no CpG motif did not. LMB-2 displayed no antitumor cell activity in the absence of CpG-ODN as determined colorimetrically with an (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium, inner salt (MTS) assay. In contrast, B-CLL growth was inhibited in 12 of 13 samples with 50% inhibition concentrations (IC50) in the range of LMB-2 plasma levels achieved in clinical studies. Two samples were not evaluable because of spontaneous B-CLL cell death in the presence of DSP30. Control experiments with an immunotoxin that does not recognize hematopoietic cells, and an anti-CD22 immunotoxin, confirmed that sensitization to LMB-2 was specifically due to up-regulation of CD25. LMB-2 was much less toxic to normal B and T lymphocytes compared with B-CLL cells. In summary, immunostimulatory CpG-ODNs efficiently sensitize B-CLL cells to a recombinant immunotoxin by modulation of its target. This new treatment strategy deserves further attention.

Helix-stabilized Fv (hsFv) antibody fragments: substituting the constant domains of a Fab fragment for a heterodimeric coiled-coil domain

Antibody Fv fragments would in principle be useful for a variety of biotechnological applications because of their small size and the possibility to produce them in relatively large amounts in recombinant form; however, their limited stability is a drawback. To solve this problem, both domains are usually fused via a peptide linker to form a single-chain Fv (scFv) fragment, but in some cases this leads to a dimerization. We present an alternative format for stabilizing antibody Fv fragments. The C(H)1 and C(L) domain of the Fab fragment were replaced with a heterodimeric coiled coil (WinZip-A2B1), which had previously been selected using a protein-fragment complementation assay in Escherichia coli. This new antibody format was termed helix-stabilized Fv fragment (hsFv), and was compared to the corresponding Fv, Fab and single-chain Fv format. Bacterial growth and expression of the hsFv was significantly improved compared to the Fab fragment. The hsFv fragment formed a heterodimer of heavy and light chain with the expected molecular mass, also under conditions where the scFv fragment was predominantly dimeric. The hsFv fragment was significantly more stable than the Fv fragment, and nearly as stable as the scFv fragment under the conditions used (80 nM protein concentration). Thus, the format of a helix-stabilized Fv (hsFv) fragment can be a useful alternative to existing recombinant antibody formats, especially in cases where poor expression of Fab fragments or multimerization of scFv fragments is a problem.

Efficient enantioselective separation of drug enantiomers by immobilised antibody fragments

There is an increasing need for methods for efficient enantioselective separation and purification of chiral drugs. Genetic engineering provides the means for generating recombinant antibodies exhibiting extremely high specificity for even small molecular mass compounds. Here, recombinant antibody fragments have been generated for the drug diarylalkyltriazole that contains two chiral centres. Immobilised antibody fragments has been used successfully for efficient, step-wise separation of two enantiomers of the drug. Owing to the antibody specificity, one enantiomer came out in the flow-through, while the bound enantiomer could be specifically eluted. One of the antibodies tolerated solvents required both for dissolving the target molecules and for their elution for extended times and was shown to function over multiple cycles of the separation process.

Recombinant immunotoxins for cancer therapy

Recombinant immunotoxins consist of Fv regions of tumour-selective antibodies fused to toxins found in bacteria, plants or fungi. These toxins must be modified to remove normal-tissue binding sites but to retain all other functions of cytotoxicity. The recombinant antibody fragments target the modified toxin to cancer cells which are killed, either by direct inhibition of protein synthesis, or by concomitant induction of apoptosis. Cells that are not recognised by the antibody fragment because they do not carry the tumour antigen, are spared. Many factors influence the in vivo antitumour activity of recombinant immunotoxins. Among them are considerations of which types of cancer may be the best targets for immunotoxin therapy as well as tumour specificity of the antigen that is targeted by the recombinant antibody. Other relevant issues are the affinity of immunotoxins and their ability to enter and penetrate into tissues and tumours, which in turn is dependent on the size of the protein. A great deal of protein-engineering is required to stabilise the recombinant antibody moiety of immunotoxins, since stability of the molecules is crucial for good clinical efficacy. Excellent activity and specificity can be observed for many recombinant immunotoxins in in vitro assays using cultured cancer cells as well as in animal tumour models. Ongoing clinical trials provide examples where the promising preclinical data correlate with successful results in experimental cancer therapy.

Conjugation of an antibody Fv fragment to a virus coat protein: cell-specific targeting of recombinant polyoma-virus-like particles

The development of cell-type-specific delivery systems is highly desirable for gene-therapeutic applications. Current virus-based vector systems show broad cell specificity, which results in the need to restrict the natural tropism of these viral systems. Here we demonstrate that tumour-cell-specific virus-like particles can be functionally assembled in vitro from recombinant viral coat protein expressed in Escherichia coli. The insertion of a negatively charged peptide in the HI loop of polyoma VP1 interferes with the binding of VP1 to the natural recognition site on mammalian cells and also serves as an adapter for the coupling of antibody fragments that contain complementary charged fusion peptides. A recombinant antibody fragment of the tumour-specific anti-(Lewis Y) antibody B3 could be coupled to the mutant VP1 by engineered polyionic peptides and an additional disulphide bond. With this system an entirely recombinant cell-specific delivery system assembled in vitro could be generated that transfers genes preferentially to cells presenting the tumour-specific antigen on the cell surface.

Cytotoxicity of antiosteosarcoma recombinant immunotoxins composed of TP-3 Fv fragments and a truncated Pseudomonas exotoxin A

Regrowth of drug-resistant tumor cells is responsible for approximately half of an unselected osteosarcoma population still dying of the disease despite aggressive combination therapy. Two monoclonal antibodies, TP-1 (immunoglobulin 2a) and TP-3 (immunoglobulin 2b) are available, which specifically recognize an antigen on osteosarcoma cells. In this work, we have fused the variable (V) genes of TP-3 to a truncated fragment of Pseudomonas exotoxin A, referred to as PE38. Two immunotoxins were made that differed in the Fv portion: TP-3(scFv)-PE38, which contains a peptide linker, and TP-3(dsFv)-PE38, which contains a disulfide bond for stabilization of the association between the V domains. Recombinant TP-3 immunotoxins were expressed in Escherichia coli and purified from inclusion bodies. We describe the design and expression of these immunotoxins, and their properties with regard to antigen binding, stability, and cytotoxicity. Toxicity studies were done in mice. We found that the immunotoxins exhibited very similar in vitro properties, whereas in vivo TP-3(dsFv)-PE38 was much better tolerated than TP-3(scFv)-PE38.

Antibody recognition of a conformational epitope in a peptide antigen: Fv-peptide complex of an antibody fragment specific for the mutant EGF receptor, EGFRvIII

Epitope mapping studies and the determination of the structure to 1.8 A resolution have been carried out for the antigen-binding fragment MR1 in complex with peptide antigen. MR1 is specific for the novel fusion junction of the mutant epidermal growth factor receptor EGFRvIII and has been reported to have a high degree of specificity for the mutant EGFRvIII over the wild-type EGF receptor. The structure of the complex shows that the peptide antigen residue side-chains found by epitope mapping studies to be critical for recognition are accommodated in pockets on the surface of the Fv. However, the most distinctive portion of the peptide antigen, the novel fusion glycine residue, makes no contact to the Fv and does not contribute directly to the epitope. The specificity of MR1 lies in the ability of this glycine residue to assume the restricted conformation needed to form a type II' beta-hairpin turn more easily, and demonstrates that a peptide antigen can be used to generate a conformational epitope.

Stability engineering of antibody single-chain Fv fragments

The application of single-chain Fv fragments (scFv) in medicine and biotechnology places great demands on their stability. Only recently has attention been given to the production of highly stable scFvs, and in a number of examples it was found that such fragments indeed perform better during practical applications. The structural parameters influencing scFv stability are now beginning to be elucidated. This review summarizes progress in rational and evolutionary engineering methods, the structural implications of these results, as well as some examples where stability engineering has been successfully applied.

Expression of a bispecific dsFv-dsFv' antibody fragment in Escherichia coli

A bispecific disulfide-stabilized Fv antibody fragment (dsFv-dsFv') consisting of two different disulfide-stabilized Fv antibody fragments connected by flexible linker peptides was produced by secretion of three polypeptide chains into the periplasm of Escherichia coli. The dsFv-dsFv' molecules were enriched by immobilized metal affinity chromatography and further purified by anion-exchange chromatography. The recombinant antibody constructs retained the two parental antigen binding specificities and were able to cross-link the two different antigens. The described dsFv-dsFv' design might be of particular value for therapeutic in vivo applications since improved stability is expected to be combined with minimal immunogenicity.

The use of phage display for the development of tumour targeting agents

One way to improve the selectivity of therapeutic molecules in clinical oncology would be to target them on the tumour site, thereby sparing normal tissues. The development of targeted therapeutic methodologies relies in most cases on the availability of binding molecules specific for tumour-associated markers. The display of repertoires of polypeptides on the surface of filamentous phage, together with the efficient selection-amplification of the desired binding specificities using affinity capture, represents an efficient route towards the isolation of specific peptides and proteins that could act as vehicles for tumour targeting applications. Most investigations in this area of research have so far been performed with phage derived recombinant antibodies, which have been shown to selectively target tumour-associated markers both in preclinical animal models and in the clinic. However, future developments with other classes of polypeptides (small constrained peptides, small globular proteins) promise to be important for the selective delivery of therapeutic agents to the tumour site.

Targeting of T lymphocytes to melanoma cells through chimeric anti-GD3 immunoglobulin T-cell receptors

Immunoglobulin T-cell receptors (IgTCRs) combine the specificity of antibodies with the potency of cellular killing by grafting antibody recognition domains onto TCR signaling chains. IgTCR-modified T cells are thus redirected to kill tumor cells based on their expression of intact antigen on cell surfaces, bypassing the normal mechanism of activation through TCR-peptide-major histocompatibility complex (MHC) recognition. Melanoma is one of the most immunoresponsive of human cancers and has served as a prototype for the development of a number of immunotherapies. The target antigen for this study is the ganglioside GD3, which is highly expressed on metastatic melanoma with only minor immunologic cross-reaction with normal tissues. To determine an optimal configuration for therapy, four combinations of IgTCRs were prepared and studied: sFv-epsilon, sFv-zeta, Fab-epsilon, Fab-zeta. These were expressed on the surface of human T cells by retroviral transduction. IgTCR successfully redirected T-cell effectors in an MHC-unrestricted manner, in this case against a non-T-dependent antigen, with specific binding, activation, and cytotoxicity against GD3+ melanoma cells. Soluble GD3 in concentrations up to 100 microg/ml did not interfere with recognition and binding of membrane-bound antigen. Based on the outcomes of these structural and functional tests, the sFv-zeta construct was selected for clinical development. These results demonstrate key features that emphasize the potential of anti-GD3 IgTCR-modified autologous T cells for melanoma therapies.

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.

Engineering antibody molecules

Advances in PCR techniques and the increase of the antibody V region sequences in the database have boosted developments in the field of antibody engineering. The V region genes can be amplified from hybridomas (1), preimmunized donors (2), naive donors (3), or from the cells expressing antibodies.

Generation of recombinant antibodies

Recombinant antibody technology is opening new perspectives for the development of novel therapeutic and diagnostic agents. In this review we focus on advances in the generation of both genetically engineered humanized and fully human monoclonal antibodies. Methods for their production in different expression systems are also discussed.

Making artificial antibodies: a format for phage display of combinatorial heterodimeric arrays

The gene VII protein (pVII) and gene IX protein (pIX) are associated closely on the surface of filamentous bacteriophage that is opposite of the end harboring the widely exploited pIII protein. We developed a phagemid format wherein antibody heavy- and light-chain variable regions were fused to the amino termini of pVII and pIX, respectively. Significantly, the fusion proteins interacted to form a functional Fv-binding domain on the phage surface. Our approach will be applicable to the display of generic peptide and protein libraries that can form combinatorial heterodimeric arrays. Consequently, it represents a first step toward artificial antibodies and the selection of novel biological activities.

A bivalent disulfide-stabilized Fv with improved antigen binding to erbB2

We have used protein engineering to generate a stable bivalent Fv molecule of the anti-erbB2 monoclonal antibody e23. The VH and VL domains of the Fv are linked to each other by a disulfide bond and the two Fvs are connected by a flexible 15 amino acid residue (Gly4-Ser)3 linker. The e23 (dsFv)2 molecule is fused to a truncated form of Pseudomonas exotoxin to generate a bivalent disulfide-stabilized, (dsFv)2, immunotoxin. The immunotoxin was expressed in Escherichia coli, refolded in vitro and purified to about 95% purity. Binding studies demonstrated that the (dsFv)2 molecule has a much higher affinity for erbB2 than a monovalent dsFv molecule and a similar binding affinity as the parental antibody e23. The (dsFv)2 immunotoxin was 5 to 20-fold more cytotoxic to two e23 antigen-positive cell lines than the monovalent dsFv immunotoxin. The bivalent dsFv molecule is very stable, retaining 94% of its activity after a 24 hours incubation in human serum at 37 degreesC. Two other molecules with shorter linkers five and ten amino acid residues in length were produced and showed similar activities as the molecule containing a 15 amino acid residue linker. The bivalency, stability and the relative ease of purification makes these e23 (dsFv)2 molecules valuable reagents for cancer immunotherapy and diagnosis.