Expression of an anti-CD3 single-chain immunotoxin with a truncated diphtheria toxin in a mutant CHO cell line

Diphtheria toxin-derived, anti-PD-1 immunotoxin, a potent and practical tool to selectively deplete PD-1+ cells

Programmed death-1 (PD-1), an immune checkpoint receptor, is expressed on activated lymphocytes, macrophages, and some types of tumor cells. While PD-1+ cells have been implicated in outcomes of cancer immunity, autoimmunity, and chronic infections, the exact roles of these cells in various physiological and pathological processes remain elusive. Molecules that target and deplete PD-1+ cells would be instrumental in defining the roles unambiguously. Previously, an immunotoxin has been generated for the depletion of PD-1+ cells though its usage is impeded by its low production yield. Thus, a more practical molecular tool is desired to deplete PD-1+ cells and to examine functions of these cells. We designed and generated a novel anti-PD1 diphtheria immunotoxin, termed PD-1 DIT, targeting PD-1+ cells. PD-1 DIT is comprised of two single chain variable fragments (scFv) derived from an anti-PD-1 antibody, coupled with the catalytic and translocation domains of the diphtheria toxin. PD-1 DIT was produced using a yeast expression system that has been engineered to efficiently produce protein toxins. The yield of PD-1 DIT reached 1-2 mg/L culture, which is 10 times higher than the previously reported immunotoxin. Flow cytometry and confocal microscopy analyses confirmed that PD-1 DIT specifically binds to and enters PD-1+ cells. The binding avidities between PD-1 DIT and two PD-1+ cell lines are approximately 25 nM. Moreover, PD-1 DIT demonstrated potent cytotoxicity toward PD-1+ cells, with a half maximal effective concentration (EC50 ) value of 1 nM. In vivo experiments further showed that PD-1 DIT effectively depleted PD-1+ cells and enabled mice inoculated with PD-1+ tumor cells to survive throughout the study. Our findings using PD-1 DIT revealed the critical role of pancreatic PD-1+ T cells in the development of type-1 diabetes (T1D). Additionally, we observed that PD-1 DIT treatment ameliorated relapsing-remitting experimental autoimmune encephalomyelitis (RR-EAE), a mouse model of relapsing-remitting multiple sclerosis (RR-MS). Lastly, we did not observe significant hepatotoxicity in mice treated with PD-1 DIT, which had been reported for other immunotoxins derived from the diphtheria toxin. With its remarkable selective and potent cytotoxicity toward PD-1+ cells, coupled with its high production yield, PD-1 DIT emerges as a powerful biotechnological tool for elucidating the physiological roles of PD-1+ cells. Furthermore, the potential of PD-1 DIT to be developed into a novel therapeutic agent becomes evident.

Manufacture and evaluation of a HER2-positive breast cancer immunotoxin 4D5Fv-PE25

BACKGROUND

Human epidermal growth factor receptor 2 (HER2) positive breast cancer is an aggressive subtype, accounting for around 20% of all breast cancers. The development of HER2-targeted therapy has substantially improved patient outcomes. Nevertheless, the increasing rate of side effects and resistance to targeted drugs limit their efficacy in clinical practice. In this study, we designed and synthesized a new immunotoxin, 4D5Fv-PE25, which targets HER2-positive breast cancer, and evaluated its effectiveness in vitro and in vivo.

RESULTS

The 4D5Fv-PE25 was expressed in high-density Escherichia coli (E. coli.) using the fermentor method and refined via hydrophobicity, ion exchange, and filtration chromatography, achieving a 56.06% recovery rate. Additionally, the semi-manufactured product with 96% purity was prepared into freeze-dried powder by the lyophilized process. Flow cytometry was used to detect the expression of HER2 in SK-BR-3, BT-474, MDA-MB-231, and MDA-MB-468 breast cancer cell lines. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) method was used for cytotoxicity assay, and the half-maximal inhibitory concentration (IC₅₀) of 4D5Fv-PE25 lyophilized products to HER2-positive cell line SK-BR-3 was 12.53 ng/mL. The 4D5Fv-PE25 was injected into xenograft tumor mice via the tail vein on the 1st, 4th, and 8th day, it indicated that the growth of tumor volume was effectively inhibited for 24 days, although the 4D5Fv-PE25 was metabolized within 60 min by measuring the release of 3 H-Thymidine radiation.

CONCLUSION

we succeeded in producing the 4D5Fv-PE25 freeze-dried powder using the prokaryotic expression method, and it could be employed as a potential drug for treating HER2-positive breast cancer.

Bacteria-derived chimeric toxins as potential anticancer agents

Cancer is one of the major causes of death globally, requiring everlasting efforts to develop novel, specific, effective, and safe treatment strategies. Despite advances in recent years, chemotherapy, as the primary treatment for cancer, still faces limitations such as the lack of specificity, drug resistance, and treatment failure. Bacterial toxins have great potential to be used as anticancer agents and can boost the effectiveness of cancer chemotherapeutics. Bacterial toxins exert anticancer effects by affecting the cell cycle and apoptotic pathways and regulating tumorigenesis. Chimeric toxins, which are recombinant derivatives of bacterial toxins, have been developed to address the low specificity of their conventional peers. Through their targeting moieties, chimeric toxins can specifically and effectively detect and kill cancer cells. This review takes a comprehensive look at the anticancer properties of bacteria-derived toxins and discusses their potential applications as therapeutic options for integrative cancer treatment.

Plant-made immunotoxin building blocks: A roadmap for producing therapeutic antibody-toxin fusions

Molecular farming in plants is an emerging platform for the production of pharmaceutical proteins, and host species such as tobacco are now becoming competitive with commercially established production hosts based on bacteria and mammalian cell lines. The range of recombinant therapeutic proteins produced in plants includes replacement enzymes, vaccines and monoclonal antibodies (mAbs). But plants can also be used to manufacture toxins, such as the mistletoe lectin viscumin, providing an opportunity to express active antibody-toxin fusion proteins, so-called recombinant immunotoxins (RITs). Mammalian production systems are currently used to produce antibody-drug conjugates (ADCs), which require the separate expression and purification of each component followed by a complex and hazardous coupling procedure. In contrast, RITs made in plants are expressed in a single step and could therefore reduce production and purification costs. The costs can be reduced further if subcellular compartments that accumulate large quantities of the stable protein are identified and optimal plant growth conditions are selected. In this review, we first provide an overview of the current state of RIT production in plants before discussing the three key components of RITs in detail. The specificity-defining domain (often an antibody) binds cancer cells, including solid tumors and hematological malignancies. The toxin provides the means to kill target cells. Toxins from different species with different modes of action can be used for this purpose. Finally, the linker spaces the two other components to ensure they adopt a stable, functional conformation, and may also promote toxin release inside the cell. Given the diversity of these components, we extract broad principles that can be used as recommendations for the development of effective RITs. Future research should focus on such proteins to exploit the advantages of plants as efficient production platforms for targeted anti-cancer therapeutics.

A novel diphtheria toxin-based bivalent human EGF fusion toxin for treatment of head and neck squamous cell carcinoma

Epidermal growth factor receptor (EGFR) is often overexpressed in head and neck squamous cell carcinoma (HNSCC) and represents a top candidate for targeted HNSCC therapy. However, the clinical effectiveness of current Food and Drug Administration (FDA)-approved drugs targeting EGFR is moderate, and the overall survival rate for HNSCC patients remains low. Therefore, more effective treatments are urgently needed. In this study, we generated a novel diphtheria toxin-based bivalent human epidermal growth factor fusion toxin (bi-EGF-IT) to treat EGFR-expressing HNSCC. Bi-EGF-IT was tested for in vitro binding affinity, cytotoxicity, and specificity using 14 human EGFR-expressing HNSCC cell lines and three human EGFR-negative cancer cell lines. Bi-EGF-IT had increased binding affinity for EGFR-expressing HNSCC compared with the monovalent version (mono-EGF-IT), and both versions specifically depleted EGFR-positive HNSCC, but not EGFR-negative cell lines, in vitro. Bi-EGF-IT exhibited a comparable potency to that of the FDA-approved EGFR inhibitor, erlotinib, for inhibiting HNSCC tumor growth in vivo using both subcutaneous and orthotopic HNSCC xenograft mouse models. When tested in an experimental metastasis model, survival was significantly longer in the bi-EGF-IT treatment group than the erlotinib treatment group, with a significantly reduced number of metastases compared with mono-EGF-IT. In addition, in vivo off-target toxicities were significantly reduced in the bi-EGF-IT treatment group compared with the mono-EGF-IT group. These results demonstrate that bi-EGF-IT is more effective and markedly less toxic at inhibiting primary HNSCC tumor growth and metastasis than mono-EGF-IT and erlotinib. Thus, the novel bi-EGF-IT is a promising drug candidate for further development.

Critical Issues in the Development of Immunotoxins for Anticancer Therapy

Immunotoxins (ITs) are attractive anticancer modalities aimed at cancer-specific delivery of highly potent cytotoxic protein toxins. An IT consists of a targeting domain (an antibody, cytokine, or another cell-binding protein) chemically conjugated or recombinantly fused to a highly cytotoxic payload (a bacterial and plant toxin or human cytotoxic protein). The mode of action of ITs is killing designated cancer cells through the effector function of toxins in the cytosol after cellular internalization via the targeted cell-specific receptor-mediated endocytosis. Although numerous ITs of diverse structures have been tested in the past decades, only 3 ITs-denileukin diftitox, tagraxofusp, and moxetumomab pasudotox-have been clinically approved for treating hematological cancers. No ITs against solid tumors have been approved for clinical use. In this review, we discuss critical research and development issues associated with ITs that limit their clinical success as well as strategies to overcome these obstacles. The issues include off-target and on-target toxicities, immunogenicity, human cytotoxic proteins, antigen target selection, cytosolic delivery efficacy, solid-tumor targeting, and developability. To realize the therapeutic promise of ITs, novel strategies for safe and effective cytosolic delivery into designated tumors, including solid tumors, are urgently needed.

Targeted Diphtheria Toxin-Based Therapy: A Review Article

Cancer remains one of the leading causes of death worldwide. Conventional therapeutic strategies usually offer limited specificity, resulting in severe side effects and toxicity to normal tissues. Targeted cancer therapy, on the other hand, can improve the therapeutic potential of anti-cancer agents and decrease unwanted side effects. Targeted applications of cytolethal bacterial toxins have been found to be especially useful for the specific eradication of cancer cells. Targeting is either mediated by peptides or by protein-targeting moieties, such as antibodies, antibody fragments, cell-penetrating peptides (CPPs), growth factors, or cytokines. Together with a toxin domain, these molecules are more commonly referred to as immunotoxins. Targeting can also be achieved through gene delivery and cell-specific expression of a toxin. Of the available cytolethal toxins, diphtheria toxin (DT) is one of the most frequently used for these strategies. Of the many DT-based therapeutic strategies investigated to date, two immunotoxins, Ontak^TM and Tagraxofusp^TM, have gained FDA approval for clinical application. Despite some success with immunotoxins, suicide-gene therapy strategies, whereby controlled tumor-specific expression of DT is used for the eradication of malignant cells, are gaining prominence. The first part of this review focuses on DT-based immunotoxins, and it then discusses recent developments in tumor-specific expression of DT.

Hosts for Hostile Protein Production: The Challenge of Recombinant Immunotoxin Expression

For the recombinant expression of toxin-based drugs, a crucial step lies not only in the choice of the production host(s) but also in the accurate design of the protein chimera. These issues are particularly important since such products may be toxic to the expressing host itself. To avoid or limit the toxicity to productive cells while obtaining a consistent yield in chimeric protein, several systems from bacterial to mammalian host cells have been employed. In this review, we will discuss the development of immunotoxin (IT) expression, placing special emphasis on advantages and on potential drawbacks, as one single perfect host for every chimeric protein toxin or ligand does not exist.

Recombinant Immunotoxin Therapy of Glioblastoma: Smart Design, Key Findings, and Specific Challenges

Recombinant immunotoxins (RITs) refer to a group of recombinant protein-based therapeutics, which consists of two components: an antibody variable fragment or a specific ligand that allows RITs to bind specifically to target cells and an engineered toxin fragment that kills the target cells upon internalization. To date, over 1,000 RITs have been generated and significant success has been achieved in the therapy of hematological malignancies. However, the immunogenicity and off-target toxicities of RITs remain as significant barriers for their application to solid tumor therapy. A group of RITs have also been generated for the treatment of glioblastoma multiforme, and some have demonstrated evidence of tumor response and an acceptable profile of toxicity and safety in early clinical trials. Different from other solid tumors, how to efficiently deliver the RITs to intracranial tumors is more critical and needs to be solved urgently. In this article, we first review the design and expression of RITs, then summarize the key findings in the preclinical and clinical development of RIT therapy of glioblastoma multiforme, and lastly discuss the specific issues that still remain to forward RIT therapy to clinical practice.

An anti-PSMA bivalent immunotoxin exhibits specificity and efficacy for prostate cancer imaging and therapy

Prostate specific membrane antigen (PSMA) is overexpressed on prostate tumor cells and the neovascular endothelia various solid tumors. A bivalent immunotoxin generated by fusing a fold-back single-chain diabody derived from the Fv fragments of an anti-PSMA monoclonal antibody with a truncated diphtheria toxin (DT) containing the activity and translocation domains [A-dmDT390-scfbDb(PSMA)] might be suitable for targeted therapy of tumors that overexpress PSMA. In this study, a PSMA-positive and a PSMA-negative prostate cancer cell lines were treated with immunotoxin A-dmDT390-scfbDb(PSMA) in order to study the tumor targeting specificity and therapeutic potential of the immunotoxin. The cellular uptake and selective toxicity of the immunotoxin were evident in monolayer cultures of PSMA-positive LNCaP prostate cancer cells but not in cultures of PSMA-negative PC-3 prostate cancer cells. Cellular accumulation of A-dmDT390-scfbDb(PSMA) increased with increasing incubation times and concentrations in LNCaP cells. The proportion of apoptotic LNCaP cells increased upon incubation with increasing doses of the fold-back immunotoxin. Optical imaging and MRI with the Alexa Fluor 680-labeled A-dmDT390-scfbDb(PSMA) confirmed the specific targeting and therapeutic efficacy of this immunotoxin towards PSMA-positive LNCaP solid tumor xenografts in athymic nude mice.

A truncated diphtheria toxin based recombinant porcine CTLA-4 fusion toxin

Targeted cell therapies are possible through the generation of recombinant fusion proteins that combine a toxin, such as diphtheria toxin (DT), with an antibody or other molecule that confers specificity. Upon binding of the fusion protein to the cell of interest, the diphtheria toxin is internalized which results in protein synthesis inhibition and subsequent cell death. We have recently expressed and purified the recombinant soluble porcine CTLA-4 both with and without N-glycosylation in yeast Pichia pastoris for in vivo use in our preclinical swine model. The glycosylated and non-N-glycosylated versions of this recombinant protein each bind to a porcine CD80 expressing B-cell lymphoma line (LCL13271) with equal affinity (K(D)=13 nM). In this study we have linked each of the glycosylated and non-N-glycosylated soluble porcine CTLA-4 proteins to the truncated diphtheria toxin DT390 through genetic engineering yielding three versions of the porcine CTLA-4 fusion toxins: 1) monovalent glycosylated soluble porcine CTLA-4 fusion toxin; 2) monovalent non-N-glycosylated soluble porcine CTLA-4 fusion toxin and 3) bivalent non-N-glycosylated soluble porcine CTLA-4 fusion toxin. Protein synthesis inhibition analysis demonstrated that while all three fusion toxins are capable of inhibiting protein synthesis in vitro, the non-N-glycosylated porcine CTLA-4 isoforms function most efficiently. Binding analysis using flow cytometry of the porcine CTLA-4 fusion toxins to LCL13271 cells also demonstrated that the non-N-glycosylated porcine CTLA-4 isoforms bind to these cells with higher affinity compared to the glycosylated fusion toxin. The monovalent non-N-glycosylated porcine CTLA-4 fusion toxin was tested in vivo. NSG (NOD/SCID IL-2 receptor γ(-)/(-)) mice were injected with porcine CD80(+) LCL13271 tumor cells. All animals succumbed to tumors and those treated with the monovalent non-N-glycosylated porcine CTLA-4 fusion toxin survived longer based on a symptomatic scoring system compared to the untreated controls. This recombinant protein may therefore provide a novel approach for in vivo depletion of porcine antigen presenting cells (APCs) for studies investigating the induction of transplantation tolerance, autoimmune disease and cancer treatment.

Development of a diphtheria toxin based antiporcine CD3 recombinant immunotoxin

Anti-CD3 immunotoxins, which induce profound but transient T-cell depletion in vivo by inhibiting eukaryotic protein synthesis in CD3+ cells, are effective reagents in large animal models of transplantation tolerance and autoimmune disease therapy. A diphtheria toxin based antiporcine CD3 recombinant immunotoxin was constructed by fusing the truncated diphtheria toxin DT390 with two identical tandem single chain variable fragments (scFv) derived from the antiporcine CD3 monoclonal antibody 898H2-6-15. The recombinant immunotoxin was expressed in a diphtheria-toxin resistant yeast Pichia pastoris strain under the control of the alcohol oxidase promoter. The secreted recombinant immunotoxin was purified sequentially with hydrophobic interaction chromatography (Butyl 650 M) followed by strong anion exchange (Poros 50 HQ). The purified antiporcine CD3 immunotoxin was tested in vivo in four animals; peripheral blood CD3+ T-cell numbers were reduced by 80% and lymph node T-cells decreased from 74% CD3+ cells pretreatment to 24% CD3+ cells remaining in the lymph node following 4 days of immunotoxin treatment. No clinical toxicity was observed in any of the experimental swine. We anticipate that this conjugate will provide an important tool for in vivo depletion of T-cells in swine transplantation models.

Investigation of a plasmid containing a novel immunotoxin VEGF165-PE38 gene for antiangiogenic therapy in a malignant glioma model

Inhibition of tumor neovascularization has profound effects on the growth of solid tumors. Our previous studies have shown the effect of VEGF165-PE38 recombinant immunotoxin on proliferation and apoptosis in human umbilical vein endothelial cells in vitro. In this study, we explored the direct inhibition of angiogenesis in chick chorioallantoic membrane and antiangiogenic therapy in a malignant glioma model. HEK293 cells were transfected with the pVEGF165PE38-IRES2-EGFP plasmid. ELISA was used to confirm the expression of VEGF165-PE38 in the transfected cells. These cells released 1396 + or - 131.9 pg VEGF165-PE38/1x10(4) cells/48 h into the culture medium and the supernatant was capable of inhibiting the growth of capillary-like structures in chick chorioallantoic membrane assay. In a murine malignant glioma model, plasmid was directly administered via multiple local intratumoral delivery. After day 16 the tumor volume in mice treated with pVEGF165PE38-IRES2-EGFP was significantly lower than that in mice in the control groups. Immunohistochemistry studies showed that the treated group had decreased expression of CD31. Quantitative analysis of microvessel density in the treated group was 1.99 + or - 0.69/0.74 mm(2), and was significantly lower than that in the control groups (9.33 + or - 1.99/0.74 mm(2), 8.09 + or - 1.39/0.74 mm(2) and 8.49 + or - 1.69/0.74 mm(2)). Immunohistochemistry analysis indicated that immunotoxin VEGF165-PE38 was distributed in the treated group in malignant glioma tissue. Our findings provide evidence that the in vivo production of VEGF165-PE38 through gene therapy using a eukaryotic expression plasmid had potential antiangiogenic activity in malignant glioma in vivo.

Preclinical studies in rats and squirrel monkeys for safety evaluation of the bivalent anti-human T cell immunotoxin, A-dmDT390-bisFv(UCHT1)

The bivalent anti-human T cell immunotoxin A-dmDT390-bisFv(UCHT1) for treatment of patients with T cell malignancies is a single chain fusion protein composed of the catalytic domain and translocation domains of diphtheria toxin fused to two tandem sFv molecules reactive with human CD3 epsilon. This immunotoxin selectively kills CD3 epsilon positive T cells. To determine the maximum tolerated dose (MTD), pharmacokinetics and immunogenicity of A-dmDT390-bisFv(UCHT1), rat and squirrel monkey studies were performed. In both animal studies, animals received either 0, 2.5 (low), 25 (medium), or 56.25 microg/kg (high) of A-dmDT390-bisFv(UCHT1) intravenously twice daily for four consecutive days. Although transient elevation of liver transaminases in the high groups was observed, the A-dmDT390-bisFv(UCHT1) administration did not affect liver function, renal function, the hemogram, or produce serious organ histopathology. Adverse events included transient lethargy, inappetence and weight loss in high groups. A-dmDT390-bisFv(UCHT1) plasma half life was 26.95 min in rats and 18.33 min in squirrel monkeys. Immune responses to A-dmDT390-bisFv(UCHT1) were minimal in squirrel monkeys and mild in rats. In vitro cytokine release, T cell activation and CD3 epsilon receptor occupancy assays using human PBMC were further performed since rat and squirrel monkey T cells do not react with A-dmDT390-bisFv(UCHT1). A-dmDT390-bisFv(UCHT1) did not induce cytokine release or T cell activation. The A-dmDT390-bisFv(UCHT1) concentration for 50% CD3 epsilon receptor occupancy was 7.4 nM. The MTD of 200 microg/kg total provides a dose level sufficient for anti-tumor activity in vitro and in a rodent model. Therefore, we propose that this agent is a promising drug for patients with surface CD3+ T cell malignancies.

Overexpression of an anti-CD3 immunotoxin increases expression and secretion of molecular chaperone BiP/Kar2p by Pichia pastoris

We previously reported that the secretory capacity of Pichia pastoris is limited with respect to the secretion of a 96.5-kDa bivalent anti-CD3 immunotoxin; double-copy expression generated more translation products than single-copy expression but did not increase the secretion of the immunotoxin. In Saccharomyces cerevisiae heterologous protein secretion has been reported to increase the expression of molecular chaperones, most prominently BiP/Kar2p. We therefore investigated the relationships between immunotoxin secretion and Kar2p expression in P. pastoris. We found that expression of the immunotoxin in P. pastoris increased the expression of Kar2p to levels that surpassed the retrieval capacity of the cell, leading to secretion of Kar2p into the medium. The level of Kar2p secretion was correlated with the copy number of the immunotoxin gene. Intracellular Kar2p was found to bind exclusively to the unprocessed immunotoxin containing the prosequence of alpha-factor in the endoplasmic reticulum. These results show that Kar2p is intimately involved in immunotoxin secretion in P. pastoris. The limited capacity of P. pastoris to retain a sufficiently high level of intracellular Kar2p may be a factor restricting the production of the immunotoxin.

Increasing secretion of a bivalent anti-T-cell immunotoxin by Pichia pastoris

The bivalent anti-T-cell immunotoxin A-dmDT390-bisFv(G(4)S) was developed for treatment of T-cell leukemia and autoimmune diseases and for tolerance induction for transplantation. This immunotoxin was produced extracellularly in toxin-sensitive Pichia pastoris JW102 (Mut(+)) under control of the AOX1 promoter. There were two major barriers to efficient immunotoxin production, the toxicity of the immunotoxin for P. pastoris and the limited capacity of P. pastoris to secrete the immunotoxin. The immunotoxin toxicity resulted in a decrease in the methanol consumption rate, cessation of cell growth, and low immunotoxin productivity after the first 22 h of methanol induction. Continuous cell growth and continuous immunotoxin secretion after the first 22 h of methanol induction were obtained by adding glycerol to the methanol feed by using a 4:1 methanol-glycerol mixed feed as an energy source and by continuously adding a yeast extract solution during methanol induction. The secretory capacity was increased from 22.5 to 37 mg/liter by lowering the induction temperature. A low temperature reduced the methanol consumption rate and protease activity in the supernatant but not cell growth. The effects of adding glycerol and yeast extract to the methanol feed were synergistic. Adding yeast extract primarily enhanced methanol utilization and cell growth, while adding glycerol primarily enhanced immunotoxin production. The synergy was further enhanced by decreasing the induction temperature from 23 to 15 degrees C, which resulted in a robust process with a yield of 37 mg/liter, which was sevenfold greater than the yield previously reported for a toxin-resistant CHO cell expression system. This methodology should be applicable to other toxin-related recombinant proteins in toxin-sensitive P. pastoris.

Targeted introduction of a diphtheria toxin resistant mutation into the chromosomal EF-2 locus of Pichia pastoris and expression of immunotoxin in the EF-2 mutants

In an attempt to increase the production of a diphtheria toxin (DT) based immunotoxin by Pichia pastoris, we have created DT-resistant mutants that contain a substitution of arginine for glycine at position 701 in elongation factor 2 (EF-2). To achieve this, we first cloned and characterized the EF-2 gene (PEF1), and then made a construct pBLURA-Delta5'mutEF-2 that efficiently introduces specific mutations into the chromosomal EF-2 gene in P. pastoris by in vivo homologous recombination. pBLURA-Delta5(')mutEF-2 contains a selection marker URA3 and a 5' truncated form of the P. pastoris PEF1 that had been modified in vitro to carry the nucleotide mutations for the Gly(701) to Arg transition. Unlike the non-mutated strains, the EF-2 mutants are resistant to high-level intracellular expression of DT A chain that can catalyze the ADP-ribosylation. When used to express the secreted bivalent anti-T cell immunotoxin, A-dmDT390-bisFv(G4S), the EF-2 mutant strains showed increased viability compared to the non-mutated strains. However, they did not show an advantage over the non-mutated expressing strain in the production of the immunotoxin. Western blotting analysis revealed that although the EF-2 mutants did not increase the accumulation of intact A-dmDT390-bisFv(G4S) in the culture medium, they generated larger amounts of degraded products found in both the medium and cell pellets compared to the non-mutant expressing clone. In addition, double copy expression resulted in greater amounts of intact immunotoxin being retained within cellular compartments as well as degraded products. Based on these findings, we suggest that the secretory capacity may be rate limiting for divalent immunotoxin production in P. pastoris.

Expression in Pichia pastoris and purification of a membrane-acting immunotoxin based on a synthetic gene coding for the Bacillus thuringiensis Cyt2Aa1 toxin

We explored the production in Pichia pastoris of a membrane-acting immunotoxin (IT) based on the Cyt2Aa1 toxin from the bacterium Bacillus thuringiensis subspecies kyushuensis. Initial attempts at the P. pastoris expression of Cyt2Aa1 were not successful due to the high A+T-content of the native bacterial gene, resulting in premature transcription termination. Accordingly, we designed and constructed a synthetic cyt2Aa1 gene (syncyt2Aa1)(2) that was optimised for expression in this eukaryotic host. This was achieved through a recursive PCR strategy where the overall G+C-content of the cyt2Aa1 DNA sequence was systematically increased to approximately 50% compared to approximately 30% in the native bacterial gene and only the P. pastoris preferred codons were used. A synthetic DNA sequence coding for a soluble and flexible serine/glycine linker was then used to genetically fuse syncyt2Aa1 with the human single-chain antibody fragment (scFv) C6.5 targeting p185(HER-2), a cell-surface glycoprotein overexpressed in 30% of human breast and ovarian cancers. Subsequent expression of the resulting IT construct [scFvC6.5-syncyt2Aa1(mychis(6))](2) led to high-level accumulation of the recombinant protein in yeast membranes. Although the solubilisation of scFvC6.5-syncyt2Aa1(mychis(6)) from P. pastoris membranes necessitated the use of guanidine hydrochloride, the use of subsequent in vitro refolding and immobilised metal affinity chromatography (IMAC) steps allowed purification of the recombinant product at yields as high as approximately 10 mgl(-1) culture. Despite being core N-linked glycosylated and retaining part of the yeast secretion signal, the P. pastoris produced scFvC6.5-syncyt2Aa1(mychis(6)) exhibited significant specific activity for p185(HER-2)-overexpressing SK-BR-3 cells but not p185(HER-2)-negative Swiss 3T3 cells or human erythrocytes.

Secretion of functional anti-CD30-angiogenin immunotoxins into the supernatant of transfected 293T-cells

Immunotoxins consist of a target-cell-specific binding moiety, chemically or recombinantly linked to a cytotoxic component. A number of different immunotoxins (IT) have increasingly been evaluated for immunotherapy. Since these foreign proteins are highly immunogenic in human, we have developed recombinant IT using the human ribonuclease angiogenin. Due to their potential toxic effects on eucaryotic cells, these IT are usually expressed in bacteria. Depending on the structure, size, and sequence of the desired IT, bacterial expression can be limited and the yield after purification be unsatisfactory. Therefore, the expression of IT in eucaryotic cells could provide a promising alternative. For this purpose we genetically fused the anti-CD30 single-chain variable fragment (scFv) Ki4 to the N- and C-termini of recombinant angiogenin. Both IT possess leader sequences, which mediate their secretion into the cell culture supernatant. Using a bicistronic mRNA the IT were simultaneously expressed together with enhanced green fluorescent protein (EGFP). This allows direct monitoring of transfected cells. An additional plasmid encoded Zeocin resistance enhances the cultivation of transfected cells under selection pressure. Three days after transfection of 293T-cells, unpurified IT were analyzed by flow cytometry and competitive cell proliferation assays. This is the first report on the use of eucaryotic cells for the secretion of functionally active IT with a human effector domain.

Gene optimization is necessary to express a bivalent anti-human anti-T cell immunotoxin in Pichia pastoris

The bivalent anti-human anti-T cell immunotoxin A-dmDT390-bisFv(G(4)S) was developed for treatment of T cell leukemia, autoimmune diseases, and tolerance induction for transplantation. The multi-domain structure of the bivalent immunotoxin hinders efficient production in Escherichia coli and most eukaryotes are sensitive to the toxin. However, Pichia pastoris has a tolerance to levels of DT (diphtheria toxin) that were previously observed to intoxicate wild type eukaryotic cells, including Saccharomyces cerevisiae. This tolerance has permitted the optimization of the secreted expression of A-dmDT390-bisFv(G(4)S) in P. pastoris under the control of AOX1 (alcohol oxidase 1) promoter. The original DNA sequence of A-dmDT390-bisFv(G(4)S) was not expressed in P. pastoris because of several AT-rich regions, which induce an early termination of transcription. After DNA rebuilding for abolishing AT-rich regions and codon optimization, the immunotoxin could be expressed up to 10mg/L in the shake flask culture. No differences in the expression levels of immunotoxin were observed by using different secretional signal sequences, Mut(s) (methanol utilization slow phenotype) or Mut(+) (methanol utilization plus phenotype) phenotypes. Buffered complex medium (pH 7.0) having 1% casamino acids provided the highest expression in shake flask culture and PMSF (phenylmethylsulfonyl fluoride) in the range of 1 to 3mM further improved the expression level presumably by inhibiting protein degradation. The immunotoxin was purified by DEAE (diethylaminoethyl) Sepharose ion exchange chromatography and Protein L affinity chromatography. The immunotoxin purified from P. pastoris culture was as fully functional as that expressed in a toxin resistant mutant CHO (Chinese hamster ovary) cell line. Our results demonstrate that P. pastoris is an ideal system for expression of toxin-based fusion proteins.

Improved binding of a bivalent single-chain immunotoxin results in increased efficacy for in vivo T-cell depletion

Anti-CD3 immunotoxins exhibit considerable promise for the induction of transplantation tolerance in pre-clinical large animal models. Recently an anti-human anti-CD3epsilon single-chain immunotoxin based on truncated diphtheria toxin has been described that can be expressed in CHO cells that have been mutated to diphtheria toxin resistance. After the two toxin glycosylation sites were removed, the bioactivity of the expressed immunotoxin was nearly equal to that of the chemically conjugated immunotoxin. This immunotoxin, A-dmDT390-sFv, contains diphtheria toxin to residue 390 at the N-terminus followed by VL and VH domains of antibody UCHT1 linked by a (G(4)S)(3) spacer (sFv). Surprisingly, we now report that this immunotoxin is severely compromised in its binding affinity toward CD3(+) cells as compared with the intact parental UCHT1 antibody, the UCHT1 Fab fragment or the engineered UCHT1 sFv domain alone. Binding was increased 7-fold by adding an additional identical sFv domain to the immunotoxin generating a divalent construct, A-dmDT390-bisFv (G(4)S). In vitro potency increased 10-fold over the chemically conjugated immunotoxin, UCHT1-CRM9 and the monovalent A-dmDT390-sFv. The in vivo potency of the genetically engineered immunotoxins was assayed in the transgenic heterozygote mouse, tgepsilon 600, in which the T-cells express human CD3epsilon as well as murine CD3epsilon. T-cell depletion in the spleen and lymph node observed with the divalent construct was increased 9- and 34-fold, respectively, compared with the monovalent construct. The additional sFv domain appears partially to compensate for steric hindrance of immunotoxin binding due to the large N-terminal toxin domain.

Treatment with immunotoxin

T-cell depletion prior to or beginning at the time of transplantation has been shown to be a valuable adjunct to the induction of immunological unresponsiveness. Both total lymphoid irradiation and anti-lymphocyte globulin have been used for this purpose in experimental models of transplantation as well as in human organ transplant recipients. However, these methods of T-cell depletion are limited in their ability to deplete T cells selectively due to non-specific targeting and limited efficacy. A new anti-CD3 immunotoxin has been developed with a far more potent ability to deplete T cells selectively as measured by flow cytometry analysis of peripheral blood T lymphocytes as well as lymph node lymphocytes. This immunotoxin is well tolerated by rhesus monkeys when administered in vivo. When administered as a single immunosuppressive agent pretransplant, it substantially promotes allograft survival, inducing tolerance in at least one-third of recipients as measured by subsequent acceptance of donor skin grafts and rejection of third-party skin grafts. When administered on the day of transplant in combination with steroid pretreatment and a brief course of deoxyspergualin or mycophenolate mofetil (4 to 14 days), long-term unresponsiveness is also produced and in a more reliable manner than using immunotoxin alone. A new immunotoxin directed at the human CD3epsilon has been developed with excellent potency in T-cell killing and lacking the Fc portion of the CD3 antibody. This construct may be useful for T-cell depletion in humans and has a potential application in tolerance induction in human organ transplantation. Lessons learned from anti-CD3 immunotoxin in the non-human primate model to date include (i) profound (2-3 log) depletion of T-cells can be accomplished safely without inducing lymphoma or infection, (ii) such depletion is a useful adjunct for tolerance induction to allogeneic organ transplants, and (iii) tolerance to both allogeneic renal transplants and xenogeneic islet transplants has been accomplished using such strategies to date in non-human primates and in pigs. Immunotoxin may be useful for the induction of chimerism using strategies that include donor bone marrow infusion. Successful strategies for tolerance induction have also been developed using immunotoxin without the adjunct of donor bone marrow or stem cell infusion. Clinical application of immunotoxin will use a newly engineered construct with the potential for causing cytokine release, less susceptibility to neutralization by anti-diphtheria antibody and not dependent on chemical conjugation of an antibody and toxin. The usefulness of immunotoxin is directly related to its tremendous potency for depleting T cells. Based on results in nonhuman primates, it is anticipated that it will become a useful agent in tolerance induction in humans.

Influence of relative binding affinity on efficacy in a panel of anti-CD3 scFv immunotoxins

The in vitro cell killing potency of an immunotoxin reflects the aggregate of several independent biochemical properties. These include antigen binding affinity; internalization rate, intracellular processing and intrinsic toxin domain potency. This study examines the influence of antigen binding affinity on potency in various immunotoxin fusion proteins where target antigen binding is mediated by single chain antibody variable region fragments (scFv). Firstly, the relationship between affinity and potency was examined in a panel of four scFv immunotoxins generated from different anti-CD3 monoclonal antibodies fused to the 38 kDa fragment of Pseudomonas aeruginosa exotoxin A (PE38). Of these four scFv-PE38 immunotoxins, the one derived from the anti-CD3 monoclonal antibody UCHT1 has highest cell killing potency. Analysis of these four scFv-PE38 immunotoxins indicated a correlation between antigen binding affinity and immunotoxin potency in the cell killing assay with the exception of the scFvPE38 immunotoxin derived from the antibody BC3. However this scFv appeared to suffer a greater drop in affinity ( approximately 100x), relative to the parent Mab than did the other three scFvs used in this study (2-10x). Secondly, the scFv(UCHT1)-PE38 immunotoxin was then compared with a further panel of scFv(UCHT1)-derived immunotoxins including a divalent PE38 version and both monovalent and divalent Corynebacterium diphtheriae toxin (DT389) fusion proteins. When the scFv-UCHT1 domain was amino-terminally positioned relative to the toxin, as in the scFv(UCHT1)-PE38, an approximately 10-fold higher antigen-binding affinity was observed than with the C-terminal fusion, used in the DT389-scFv(UCHT1) molecule. Despite this lower antigen-binding activity, the DT389-scFv immunotoxin had a 60-fold higher potency in the T-cell-killing assay. Thirdly, a divalent form of the DT389-scFv construct, containing tandem scFv domains, had a 10-fold higher binding activity, which was exactly reflected in a 10-fold increase in potency. Therefore, when comparing immunotoxins in which scFvs from different antibodies are fused to the same toxin domain (DT or PE) a broad correlation appears to exist between binding affinity and immunotoxin potency. However, no correlation between affinity and potency appears to exist when different toxin domains are combined with the same scFv antibody domain.