Novel Anti-Mesothelin Nanobodies and Recombinant Immunotoxins with Pseudomonas Exotoxin Catalytic Domain for Cancer Therapeutics

Recombinant immunotoxins (RITs) are fusion proteins consisting of a targeting domain linked to a toxin, offering a highly specific therapeutic strategy for cancer treatment. In this study, we engineered and characterized RITs aimed at mesothelin, a cell surface glycoprotein overexpressed in various malignancies. Through an extensive screening of a large nanobody library, four mesothelin-specific nanobodies were selected and genetically fused to a truncated Pseudomonas exotoxin (PE24B). Various optimizations, including the incorporation of furin cleavage sites, maltose-binding protein tags, and tobacco etch virus protease cleavage sites, were implemented to improve protein expression, solubility, and purification. The RITs were successfully overexpressed in Escherichia coli, achieving high solubility and purity post-purification. In vitro cytotoxicity assays on gastric carcinoma cell lines NCI-N87 and AGS revealed that Meso(Nb2)-PE24B demonstrated the highest cytotoxic efficacy, warranting further characterization. This RIT also displayed selective binding to human and monkey mesothelins but not to mouse mesothelin. The competitive binding assays between different RIT constructs revealed significant alterations in IC50 values, emphasizing the importance of nanobody specificity. Finally, a modification in the endoplasmic reticulum retention signal at the C-terminus further augmented its cytotoxic activity. Our findings offer valuable insights into the design and optimization of RITs, showcasing the potential of Meso(Nb2)-PE24B as a promising therapeutic candidate for targeted cancer treatment.

Hydroquinones Inhibit Biofilm Formation and Virulence Factor Production in Staphylococcus aureus

Staphylococcus aureus is one of the major pathogens responsible for antimicrobial resistance-associated death. S. aureus can secrete various exotoxins, and staphylococcal biofilms play critical roles in antibiotic tolerance and the persistence of chronic infections. Here, we investigated the inhibitory effects of 18 hydroquinones on biofilm formation and virulence factor production by S. aureus. It was found that 2,5-bis(1,1,3,3-tetramethylbutyl) hydroquinone (TBHQ) at 1 µg/mL efficiently inhibits biofilm formation by two methicillin-sensitive and two methicillin-resistant S. aureus strains with MICs of 5 µg/mL, whereas the backbone compound hydroquinone did not (MIC > 400 µg/mL). In addition, 2,3-dimethylhydroquinone and tert-butylhydroquinone at 50 µg/mL also exhibited antibiofilm activity. TBHQ at 1 µg/mL significantly decreased the hemolytic effect and lipase production by S. aureus, and at 5−50 µg/mL was non-toxic to the nematode Caenorhabditis elegans and did not adversely affect Brassica rapa seed germination or growth. Transcriptional analyses showed that TBHQ suppressed the expression of RNAIII (effector of quorum sensing). These results suggest that hydroquinones, particularly TBHQ, are potentially useful for inhibiting S. aureus biofilm formation and virulence.

Computational modeling and druggability assessment of Aggregatibacter actinomycetemcomitans leukotoxin

The leukotoxin (LtxA) of Aggregatibacter actinomycetemcomitans (A. actinomycetemcomitans) is a protein exotoxin belonging to the repeat-in-toxin family (RTX). Numerous studies have demonstrated that LtxA may play a critical role in the pathogenicity of A. actinomycetemcomitans since hyper-leukotoxic strains have been associated with severe disease. Accordingly, considerable effort has been made to elucidate the mechanisms by which LtxA interacts with host cells and induce their death. However, these attempts have been hampered by the unavailability of a tertiary structure of the toxin, which limits the understanding of its molecular properties and mechanisms. In this paper, we used homology and template free modeling algorithms to build the complete tertiary model of LtxA at atomic level in its calcium-bound Holo-state. The resulting model was refined by energy minimization, validated by Molprobity and ProSA tools, and subsequently subjected to a cumulative 600ns of all-atom classical molecular dynamics simulation to evaluate its structural aspects. The druggability of the proposed model was assessed using Fpocket and FTMap tools, resulting in the identification of four putative cavities and fifteen binding hotspots that could be targeted by rational drug design tools to find new ligands to inhibit LtxA activity.

Pseudomonas Exotoxin A-Based Immunotherapy Targeting CCK2R-Expressing Colorectal Malignancies: An In Vitro and In Vivo Evaluation

Cholecystokinin-2 receptor (CCK2R) has been proven to be a specific biomarker for colorectal malignancies. Immunotoxins are a valuable class of immunotherapy agents consisting of a targeting element and a bacterial or plant toxin. Previous work demonstrated that targeting CCK2R is a good therapeutic strategy for the treatment of colorectal cancer (CRC). In the present study, we developed a new version of CCK2R-targeting immunotoxin GD9P using a targeted peptide, GD9, as the binding motif and a truncated Pseudomonas exotoxin A (PE38) as the cytokiller. BALB/c nude mice were treated with different doses of GD9P, and pharmacodynamics, pharmacokinetic, and toxicological data were obtained throughout this study. Compared to the parental immunotoxin rCCK8PE38, GD9P exhibited about 1.5-fold yield, higher fluorescence intensity, and increased antitumor activity against human CRC in vitro and in vivo. The IC50 values of GD9P in vitro ranged from 1.61 to 4.55 nM. Pharmacokinetic studies were conducted in mice with a T1/2 of 69.315 min. When tumor-bearing nude mice were treated with GD9P at doses ≥2 mg/kg for five doses, a rapid shrinkage in tumor volume and, in some cases, complete remission was observed. A preliminary safety evaluation demonstrated a good safety profile of GD9P as a Pseudomonas exotoxin A-based immunotherapy. The therapy in combination with oxaliplatin can increase the antitumor efficacy and reduce the toxic side effects caused by chemotherapy. In conclusion, the data support the use of GD9P as a promising immunotherapy targeting CCK2R-expressing colorectal malignancies.

Modular Conjugation of a Potent Anti-HER2 Immunotoxin Using Coassociating Peptides

Immunotoxins are emerging candidates for cancer therapeutics. These biomolecules consist of a cell-targeting protein combined to a polypeptide toxin. Associations of both entities can be achieved either chemically by covalent bonds or genetically creating fusion proteins. However, chemical agents can affect the activity and/or stability of the conjugate proteins, and additional purification steps are often required to isolate the final conjugate from unwanted byproducts. As for fusion proteins, they often suffer from low solubility and yield. In this report, we describe a straightforward conjugation process to generate an immunotoxin using coassociating peptides (named K3 and E3), originating from the tetramerization domain of p53. To that end, a nanobody targeting the human epidermal growth factor receptor 2 (nano-HER2) and a protein toxin fragment from Pseudomonas aeruginosa exotoxin A (TOX) were genetically fused to the E3 and K3 peptides. Entities were produced separately in Escherichia coli in soluble forms and at high yields. The nano-HER2 fused to the E3 or K3 helixes (nano-HER2-E3 and nano-HER2-K3) and the coassembled immunotoxins (nano-HER2-K3E3-TOX and nano-HER2-E3K3-TOX) presented binding specificity on HER2-overexpressing cells with relative binding constants in the low nanomolar to picomolar range. Both toxin modules (E3-TOX and K3-TOX) and the combined immunotoxins exhibited similar cytotoxicity levels compared to the toxin alone (TOX). Finally, nano-HER2-K3E3-TOX and nano-HER2-E3K3-TOX evaluated on various breast cancer cells were highly potent and specific to killing HER2-overexpressing breast cancer cells with IC50 values in the picomolar range. Altogether, we demonstrate that this noncovalent conjugation method using two coassembling peptides can be easily implemented for the modular engineering of immunotoxins targeting different types of cancers.

Immunotoxin Screening System: A Rapid and Direct Approach to Obtain Functional Antibodies with Internalization Capacities

Toxins, while harmful and potentially lethal, have been engineered to develop potent therapeutics including cytotoxins and immunotoxins (ITs), which are modalities with highly selective targeting capabilities. Currently, three cytotoxins and IT are FDA-approved for treatment of multiple forms of hematological cancer, and additional ITs are tested in the clinical trials or at the preclinical level. For next generation of ITs, as well as antibody-mediated drug delivery systems, specific targeting by monoclonal antibodies is critical to enhance efficacies and reduce side effects, and this methodological field remains open to discover potent therapeutic monoclonal antibodies. Here, we describe our application of engineered toxin termed a cell-based IT screening system. This unique screening strategy offers the following advantages: (1) identification of monoclonal antibodies that recognize cell-surface molecules, (2) selection of the antibodies that are internalized into the cells, (3) selection of the antibodies that induce cytotoxicity since they are linked with toxins, and (4) determination of state-specific activities of the antibodies by differential screening under multiple experimental conditions. Since the functional monoclonal antibodies with internalization capacities have been identified successfully, we have pursued their subsequent modifications beyond antibody drug conjugates, resulting in development of immunoliposomes. Collectively, this screening system by using engineered toxin is a versatile platform, which enables straight-forward and rapid selection for discovery of novel functional antibodies.

Mechanisms of Resistance to Immunotoxins Containing Pseudomonas Exotoxin A in Cancer Therapy

Immunotoxins are a class of targeted cancer therapeutics in which a toxin such as Pseudomonas exotoxin A (PE) is linked to an antibody or cytokine to direct the toxin to a target on cancer cells. While a variety of PE-based immunotoxins have been developed and a few have demonstrated promising clinical and preclinical results, cancer cells frequently have or develop resistance to these immunotoxins. This review presents our current understanding of the mechanism of action of PE-based immunotoxins and discusses cellular mechanisms of resistance that interfere with various steps of the pathway. These steps include binding of the immunotoxin to the target antigen, internalization, intracellular processing and trafficking to reach the cytosol, inhibition of protein synthesis through ADP-ribosylation of elongation factor 2 (EF2), and induction of apoptosis. Combination therapies that increase immunotoxin action and overcome specific mechanisms of resistance are also reviewed.

Engineered Anti-GPC3 Immunotoxin, HN3-ABD-T20, Produces Regression in Mouse Liver Cancer Xenografts Through Prolonged Serum Retention

BACKGROUND AND AIMS

Treatment of hepatocellular carcinomas using our glypican-3 (GPC3)-targeting human nanobody (HN3) immunotoxins causes potent tumor regression by blocking protein synthesis and down-regulating the Wnt signaling pathway. However, immunogenicity and a short serum half-life may limit the ability of immunotoxins to transition to the clinic.

APPROACH AND RESULTS

To address these concerns, we engineered HN3-based immunotoxins to contain various deimmunized Pseudomonas exotoxin (PE) domains. This included HN3-T20, which was modified to remove T-cell epitopes and contains a PE domain II truncation. We compared them to our previously reported B-cell deimmunized immunotoxin (HN3-mPE24) and our original HN3-immunotoxin with a wild-type PE domain (HN3-PE38). All of our immunotoxins displayed high affinity to human GPC3, with HN3-T20 having a KD value of 7.4 nM. HN3-T20 retained 73% enzymatic activity when compared with the wild-type immunotoxin in an adenosine diphosphate-ribosylation assay. Interestingly, a real-time cell growth inhibition assay demonstrated that a single dose of HN3-T20 at 62.5 ng/mL (1.6 nM) was capable of inhibiting nearly all cell proliferation during the 10-day experiment. To enhance HN3-T20's serum retention, we tested the effect of adding a streptococcal albumin-binding domain (ABD) and a llama single-domain antibody fragment specific for mouse and human serum albumin. For the detection of immunotoxin in mouse serum, we developed a highly sensitive enzyme-linked immunosorbent assay and found that HN3-ABD-T20 had a 45-fold higher serum half-life than HN3-T20 (326 minutes vs. 7.3 minutes); consequently, addition of an ABD resulted in HN3-ABD-T20-mediated tumor regression at 1 mg/kg.

CONCLUSION

These data indicate that ABD-containing deimmunized HN3-T20 immunotoxins are high-potency therapeutics ready to be evaluated in clinical trials for the treatment of liver cancer.

Cytotoxic and apoptotic properties of a novel nano-toxin formulation based on biologically synthesized silver nanoparticle loaded with recombinant truncated pseudomonas exotoxin A

Bacterial toxins have received a great deal of attention in the development of antitumor agents. Currently, these protein toxins were used in the immunotoxins as a cancer therapy strategy. Despite the successful use of immunotoxins, immunotherapy strategies are still expensive and limited to hematologic malignancies. In the current study, for the first time, a nano-toxin comprised of truncated pseudomonas exotoxin (PE38) loaded silver nanoparticles (AgNPs) were prepared and their cytotoxicity effect was investigated on human breast cancer cells. The PE38 protein was cloned into pET28a and expressed in Escherichia coli, BL21 (DE3), and purified using metal affinity chromatography and was analyzed by 15% sodium dodecyl sulfate-polyacrylamide gel electrophoresis. AgNPs were biologically prepared using cell-free supernatant of E. Coli K12 strain. Nanoparticle formation was characterized by energy dispersive spectroscopy, transmission electron microscopy, and dynamic light scattering. The PE38 protein was loaded on AgNPs and prepared the PE38-AgNPs nano-toxin. Additionally, in vitro release indicated a partial slow release of toxin in about 100 hr. The nano-toxin exhibited dose-dependent cytotoxicity on MCF-7 cells. Also, real-time polymerase chain reaction results demonstrated the ability of nano-toxin to upregulate Bax/Bcl-2 ratio and caspase-3, -8, -9, and P53 apoptotic genes in the MCF-7 tumor cells. Apoptosis induction was determined by Annexin-V/propidium flow cytometry and caspases activity assay after treatment of cancer cells with the nano-toxin. In general, in the current study, the nano-toxin exhibit an inhibitory effect on the viability of breast cancer cells through apoptosis, which suggests that AgNPs could be used as a delivery system for targeting of toxins to cancer cells.

Engineering a Nanostructured Nucleolin-Binding Peptide for Intracellular Drug Delivery in Triple-Negative Breast Cancer Stem Cells

Five peptide ligands of four different cell surface receptors (nucleolin, CXCR1, CMKLR1, and CD44v6) have been evaluated as targeting moieties for triple-negative human breast cancers. Among them, the peptide F3, derived from phage display, promotes the fast and efficient internalization of a genetically fused green fluorescent protein (GFP) inside MDA-MB-231 cancer stem cells in a specific receptor-dependent fashion. The further engineering of this protein into the modular construct F3-RK-GFP-H6 and the subsequent construct F3-RK-PE24-H6 resulted in self-assembling polypeptides that organize as discrete and regular nanoparticles. These materials, 15-20 nm in size, show enhanced nucleolin-dependent cell penetrability. We show that the F3-RK-PE24-H6, based on the Pseudomonas aeruginosa exotoxin A (PE24) as a core functional domain, is highly cytotoxic over target cells. The combination of F3, the cationic peptide (RK)n, and the toxin domain PE24 in such unusual presentation appears as a promising approach to cell-targeted drug carriers in breast cancers and addresses selective drug delivery in otherwise difficult-to-treat triple-negative breast cancers.

Growth Retardation of Poorly Transfectable Tumor by Multiple Injections of Plasmids Encoding PE40 Based Targeted Toxin Complexed with Polyethylenimine

BACKGROUND

One of the approaches to cancer gene therapy relies on tumor transfection with DNA encoding toxins under the control of tumor-specific promoters.

METHODS

Here, we used DNA plasmids encoding very potent anti-ERBB2 targeted toxin, driven by the human telomerase promoter or by the ubiquitous CAG promoter (pTERT-ETA and pCAG-ETA) and linear polyethylenimine to target cancer cells.

RESULTS

We showed that the selectivity of cancer cell killing by the pTERT-ETA plasmid is highly dependent upon the method of preparation of DNA-polyethylenimine complexes. After adjustment of complex preparation protocol, cell lines with high activity of telomerase promoter can be selectively killed by transfection with the pTERT-ETA plasmid. We also showed that cells transfected with pTERT-ETA and pCAG-ETA plasmids do not exert any detectable bystander effect in vitro.

CONCLUSION

Despite this, three intratumoral injections of a plasmid-polyethylenimine complex resulted in substantial growth retardation of a poorly transfectable D2F2/E2 tumor in mice. There were no significant differences in anti-tumor properties between DNA constructs with telomerase or CAG promoters in vivo.

Preparation of Diphtheria and Pseudomonas Exotoxin A Immunotoxins and Evaluation of Their Cytotoxicity Effect on SK-BR-3, BT-474, and MDA-MB-231 Breast Cancer Cell Lines

Immunotoxin targeted therapy is a promising way of cancer therapy that is made from a toxin attached to an antibody which target a specific protein presented on cancer cells. In this study, we introduce immunotoxins comprising of truncated pseudomonas exotoxin A (PEA) and diphtheria toxin (DT) conjugated to trastuzumab. The effectiveness of 20 and 30 μg/ml immunotoxins and trastuzumab were studied on SK-BR-3 and BT-474 HER2/neu positive breast cancer cell lines by a cell death assay test. The produced immunotoxins have the potential to reduce the therapeutic dose of the trastuzumab and in the same time achieve higher efficiency.

Aggregatibacter actinomycetemcomitans Leukotoxin (LtxA; Leukothera®): Mechanisms of Action and Therapeutic Applications

Aggregatibacter actinomycetemcomitans is an oral pathogen that produces the RTX toxin, leukotoxin (LtxA; Leukothera^®). A. actinomycetemcomitans is strongly associated with the development of localized aggressive periodontitis. LtxA acts as a virulence factor for A. actinomycetemcomitans to subvert the host immune response by binding to the β₂ integrin lymphocyte function-associated antigen-1 (LFA-1; CD11a/CD18) on white blood cells (WBCs), causing cell death. In this paper, we reviewed the state of knowledge on LtxA interaction with WBCs and the subsequent mechanisms of induced cell death. Finally, we touched on the potential therapeutic applications of LtxA (trade name Leukothera^®) toxin therapy for the treatment of hematological malignancies and immune-mediated diseases.

A chemical conjugate between HER2-targeting antibody fragment and Pseudomonas exotoxin A fragment demonstrates cytotoxic effects on HER2-expressing breast cancer cells

Conventionally, immunotoxins have been produced as a single polypeptide from fused genes of an antibody fragment and a toxin. In this study, we adopted a unique approach of chemical conjugation of a toxin protein and an antibody fragment. The two genes were separately expressed in Escherichia coli and purified to high levels of purity. The two purified proteins were conjugated using a chemical linker. The advantage of this approach is its ability to overcome the problem of low recombinant immunotoxin production observed in some immunotoxins. Another advantage is that various combinations of immunotoxins can be prepared with fewer efforts, because the chemical conjugation of components is relatively simpler than the processes involved in cloning, expression, and purification of multiple immunotoxins. As a proof of concept, the scFv of trastuzumab and the PE24 fragment of Pseudomonas exotoxin A were separately produced using E. coli and then chemically crosslinked. The new immunotoxin was tested on four breast cancer cell lines variably expressing HER2. The chemically crosslinked immunotoxin exhibited cytotoxicity in proportion to the expression level of HER2. In conclusion, the present study revealed an alternative method of generating an immunotoxin that could effectively reduce the viability of HER2-expressing breast cancer cells. These results suggest the effectiveness of this method of immunotoxin crosslinking as a suitable alternative for producing immunotoxins. [BMB Reports 2019; 52(8): 496-501].

Evaluating the influence of common antibiotics on the efficacy of a recombinant immunotoxin in tissue culture

OBJECTIVE

Recombinant immunotoxins (RITs) are antibody-toxin fusion proteins that can selectively eliminate populations of cells expressing specific surface receptors. They are in evaluation as therapeutic agents for cancer. RITs based on Pseudomonas exotoxin A (PE) are in use clinically for the treatment of hairy cell leukemia, and under trial for the treatment of other cancers. In an effort to improve the efficacy of PE-based RITs, we evaluated the potential of combination therapy with several common antibiotics (tetracycline, chloramphenicol, streptomycin, linezolid, fusidic acid, and kanamycin) on human cell lines HEK293, OVCAR8, and CA46. Antibiotics were selected based on their potential to inhibit mitochondrial protein synthesis and disrupt energy metabolism in cancer cells.

RESULTS

Tetracycline, chloramphenicol, linezolid, and fusidic acid alone killed cultured human cells at high concentrations. At high but nontoxic concentrations of each antibiotic, only chloramphenicol treatment of the Burkitt's lymphoma cell line CA46 showed enhanced cytotoxicity when paired with an anti-transferrin receptor/PE RIT. This result, however, could not be replicated in additional Burkitt's lymphoma cell lines Ramos and Raji. Although the six antibiotics we tested are not promising candidates for RIT combination therapy, we suggest that fusidic acid could be considered independently as a potential cancer therapeutic.

HER2-Specific Targeted Toxin DARPin-LoPE: Immunogenicity and Antitumor Effect on Intraperitoneal Ovarian Cancer Xenograft Model

High immunogenicity and systemic toxicity are the main obstacles limiting the clinical use of the therapeutic agents based on Pseudomonas aeruginosa exotoxin A. In this work, we studied the immunogenicity, general toxicity and antitumor effect of the targeted toxin DARPin-LoPE composed of HER2-specific DARPin and a low immunogenic exotoxin A fragment lacking immunodominant human B lymphocyte epitopes. The targeted toxin has been shown to effectively inhibit the growth of HER2-positive human ovarian carcinoma xenografts, while exhibiting low non-specific toxicity and side effects, such as vascular leak syndrome and liver tissue degradation, as well as low immunogenicity, as was shown by specific antibody titer. This represents prospects for its use as an agent for targeted therapy of HER2-positive tumors.

Synergistic antitumor effects of 9.2.27-PE38KDEL and ABT-737 in primary and metastatic brain tumors

Standard treatment, unfortunately, yields a poor prognosis for patients with primary or metastatic cancers in the central nervous system, indicating a necessity for novel therapeutic agents. Immunotoxins (ITs) are a class of promising therapeutic candidates produced by fusing antibody fragments with toxin moieties. In this study, we investigated if inherent resistance to IT cytotoxicity can be overcome by rational combination with pro-apoptotic enhancers. Therefore, we combined ITs (9.2.27-PE38KDEL or Mel-14-PE38KDEL) targeting chondroitin sulfate proteoglycan 4 (CSPG4) with a panel of Bcl-2 family inhibitors (ABT-737, ABT-263, ABT-199 [Venetoclax], A-1155463, and S63845) against patient-derived glioblastoma, melanoma, and breast cancer cells/cell lines. In vitro cytotoxicity assays demonstrated that the addition of the ABT compounds, specifically ABT-737, sensitized the different tumors to IT treatment, and improved the IC₅₀ values of 9.2.27-PE38KDEL up to >1,000-fold. Mechanistic studies using 9.2.27-PE38KDEL and ABT-737 revealed that increased levels of intracellular IT, processed (active) exotoxin, and PARP cleavage correlated with the enhanced sensitivity to the combination treatment. Furthermore, we confirmed the synergistic effect of 9.2.27-PE38KDEL and ABT-737 combination therapy in orthotopic GBM xenograft and cerebral melanoma metastasis models in nude mice. Our study defines strategies for overcoming IT resistance and enhancing specific antitumor cytotoxicity in primary and metastatic brain tumors.

Elimination of factor VIII-specific B cells by immunotoxins composed of a single factor VIII domain fused to Pseudomonas exotoxin A

Essentials There is still a need for novel therapeutic approaches for hemophilia A patients with inhibitors. A factor VIII domain was used as the targeting moiety for elimination of FVIII-specific B cells. The immunodominant C2 domain was fused to exotoxin A from Pseudomonas aeruginosa (hC2-ETA). Murine C2 domain-specific B cells were selectively and efficiently eliminated by hC2-ETA ex vivo. SUMMARY: Background Today, the most serious complication for patients with hemophilia A undergoing factor VIII (FVIII) replacement therapy is the development of neutralizing antibodies (inhibitors). Although inhibitors can be eradicated by application of high doses of FVIII, the immune tolerance induction therapy fails in up to 30% of patients. Hence, there is still an urgent need for novel therapeutic approaches for patients with persisting inhibitors. Objectives In the present study, the potential use of immunotoxins containing exotoxin A (ETA) from Pseudomonas aeruginosa for selective elimination of FVIII-specific B cells was explored. Methods The immunodominant C2 domain of human FVIII was used as a targeting moiety instead of the full-length FVIII protein and the resulting human C2 domain-ETA fusion protein (hC2-ETA) was produced in Escherichia coli. Results Binding studies with monoclonal C2 domain-specific antibodies confirmed the conformational integrity of the C2 domain in hC2-ETA. The functionality of hC2-ETA was tested ex vivo by incubation of splenocytes from inhibitor-positive FVIII knockout mice with hC2-ETA and controls. FVIII-specific memory B cells from splenocytes were differentiated by FVIII stimulation in antibody-secreting cells (ASC) and detected by an enzyme-linked immunospot assay. Although the controls showed no effect, incubation of splenocytes with hC2-ETA reduced the number of C2-specific ASC in a dose-dependent fashion, indicating specific and efficient elimination of C2-specific memory B cells. Conclusions Overall, the results of the study support the fact that FVIII domain immunotoxins might be a potential new tool for the elimination of FVIII-specific B cells in patients with hemophilia A and persisting inhibitors.

EGF ligand fused to truncated Pseudomonas aeruginosa exotoxin A specifically targets and inhibits EGFR‑positive cancer cells

Cancer cells have been known to overexpress the epidermal growth factor receptor (EGFR) and hence relevant multiple‑targeted therapies have been developed, with a recent clinical application of the antibody‑mediated inhibition of the EGFR. However, this strategy is not useful in cancer cells with mutations in KRAS; a GTPase downstream of EGFR which constitutively activates the pathway without EGF stimulation. Furthermore, mutations in EGFR also reduce the binding of monoclonal antibodies and thereby render them ineffective. In the present study, we designed a chimeric EGF protein fused to the truncated N‑terminal domain fragment of Pseudomonas aeruginosa exotoxin A (EGF‑ETA), which has ADP‑ribosylation activity and induces apoptosis. The EGF‑ETA protein was expressed in E. coli as a His‑tagged fusion. Our results showed that EGF‑ETA significantly inhibited the proliferation of EGFR‑positive A431 epidermoid carcinoma (IC50 27 ng/ml) and HN5 head and neck squamous cell carcinoma (IC50 36 ng/ml) cells. However, its effect on cancer cells with little or no EGFR expression was limited (A549‑IC50 1,000 ng/ml; MCF‑7‑IC50 >10,000 ng/ml). Compared to cetuximab, EGF‑ETA was highly potent in its killing capacity of HN5 cancer cells at 1,000 ng/ml, while cetuximab had little effect at 1,000 ng/ml. Furthermore, EGF‑ETA was just as potent in HCT116 (KRAS G13D) and SW480 (KRAS G12V) colon cancer cell lines harbouring KRAS hyperactivating mutations when compared to KRAS wild‑type HT29 colon cancer cells. Finally, co‑incubation of EGF‑ETA with an anti‑EGF antibody abrogated its effect on the EGFR‑positive A431 cells. Our results show that the chimeric EGF‑ETA toxin is extremely effective against EGFR‑positive cancers and raises the potential to further develop this chimera for use in targeting EGFR‑positive tumours resistant to monoclonal antibodies.

Generation of Potent Anti-HER1/2 Immunotoxins by Protein Ligation Using Split Inteins

Cell targeting protein toxins have gained increasing interest for cancer therapy aimed at increasing the therapeutic window and reducing systemic toxicity. Because recombinant expression of immunotoxins consisting of a receptor-binding and a cell-killing moiety is hampered by their high toxicity in a eukaryotic production host, most applications rely on recombinant production of fusion proteins consisting of an antibody fragment and a protein toxin in bacterial hosts such as Escherichia coli ( E. coli). These fusions often lack beneficial properties of whole antibodies like extended serum half-life or efficient endocytic uptake via receptor clustering. Here, we describe the production of full-length antibody immunotoxins using self-splicing split inteins. To this end, the short (11 amino acids) N-terminal intein part of the artificially designed split intein M86, a derivative of the Ssp DnaB intein, was recombinantly fused to the heavy chain of trastuzumab, a human epidermal growth factor receptor 2 (HER2) receptor targeting antibody and to a nanobody-Fc fusion targeting the HER1 receptor, respectively. Both antibodies were produced in Expi293F cells. The longer C-terminal counterpart of the intein was genetically fused to the protein toxins gelonin or Pseudomonas Exotoxin A, respectively, and expressed in E. coli via fusion to maltose binding protein. Using optimized in vitro splicing conditions, we were able to generate a set of specific and potent immunotoxins with IC50 values in the mid- to subpicomolar range.

Expression of VGRNb-PE immunotoxin in transplastomic lettuce (Lactuca sativa L.)

KEY MESSAGE

This research has shown, for the first time, that plant chloroplasts are a suitable compartment for synthesizing recombinant immunotoxins and the transgenic immunotoxin efficiently causes the inhibition of VEGFR2 overexpression, cell growth and proliferation. Angiogenesis refers to the formation of new blood vessels, which resulted in the growth, invasion and metastasis of cancer. The vascular endothelial growth factor receptor 2 (VEGFR2) plays a major role in angiogenesis and blocking of its signaling inhibits neovascularization and tumor metastasis. Immunotoxins are promising therapeutics for targeted cancer therapy. They consist of an antibody linked to a protein toxin and are designed to specifically kill the tumor cells. In our previous study, VGRNb-PE immunotoxin protein containing anti-VEGFR2 nanobody fused to the truncated form of Pseudomonas exotoxin A has been established. Here, we expressed this immunotoxin in lettuce chloroplasts. Chloroplast genetic engineering offers several advantages, including high levels of transgene expression, multigene engineering in a single transformation event and maternal inheritance of the transgenes. Site specific integration of transgene into chloroplast genomes, and homoplasmy were confirmed. Immunotoxin levels reached up to 1.1% of total soluble protein or 33.7 µg per 100 mg of leaf tissue (fresh weight). We demonstrated that transgenic immunotoxin efficiently causes the inhibition of VEGFR2 overexpression, cell growth and proliferation. These results indicate that plant chloroplasts are a suitable compartment for synthesizing recombinant immunotoxins.

Construction, expression, and activity of a novel immunotoxin comprising a humanized antiepidermal growth factor receptor scFv and modified Pseudomonas aeruginosa exotoxin A

Overexpression of epidermal growth factor receptor (EGFR) plays a significant role in the development and metastasis of many solid tumors. Strategies based on anti-EGFR immunotoxins have shown promising results in several studies, but immunogenicity of antibody and toxin moieties is a limitation of this type of therapeutics. In the present study, a novel humanized anti-EGFR immunotoxin (huscFv-PE25) was developed by genetic fusing of a humanized anti-EGFR single-chain variable fragment (huscFv) with a modified Pseudomonas aeruginosa exotoxin A (PE25KDEL). The reactivity and toxicity of this immunotoxin with tumor cells were assessed by dot-blot, enzyme-linked immunosorbent assay, and MTT procedures. Results of enzyme-linked immunosorbent assay and dot-blot assay indicated that the immunotoxin recognizes and efficiently binds to EGFR-overexpressing tumor cells. MTT assay showed a specific growth-inhibitory effect of huscFv-PE25 on EGFR-overexpressing A431 cells, without any inhibitory effect on EGFR-negative cells. In conclusion, the results of this study indicated that huscFv-PE25 can recognize and exert an inhibitory effect on EGFR-overexpressing cancer cells, despite its smaller size and lower immunogenicity. This may provide a basis for the development of novel clinical therapeutic agents against EGFR-overexpressing tumor cells.

Targeting a Cancer-Specific Epitope of the Epidermal Growth Factor Receptor in Triple-Negative Breast Cancer

BACKGROUND

Triple-negative breast cancers (TNBCs) are typically more aggressive and result in poorer outcomes than other breast cancers because treatment options are limited due to lack of hormone receptors or amplified human epidermal growth factor receptor 2 (HER2). Many TNBCs overexpress the epidermal growth factor receptor (EGFR) or manifest amplification of theEGFRgene, supporting EGFR as a therapeutic target. While EGFR-directed small molecule inhibitors have shown limited effectiveness in clinical settings, use of EGFR as a mechanism of delivering enzymatic cytotoxins to TNBC has not been demonstrated.

METHODS

Using the single-chain variable fragment (scFv) of the 806 antibody that binds only cells with overexpressed, misfolded, or mutant variants of the EGFR, a recombinant immunotoxin was engineered through gene fusion withPseudomonas aeruginosaExotoxin A (806-PE38). The potency of 806-PE38 on reducing TNBC cell growth in vitro and in xenograft models (n ≥ 6) was examined for six TNBC cell lines. All statistical tests were two-sided.

RESULTS

806-PE38 statistically significantly reduced the viability of all tested TNBC lines, with IC50values below 10 ng/mL for three of six cell lines, while not affecting cells with wild-type EGFR (IC50>300 ng/mL). Systemic treatments with 806-PE38 vs vehicle resulted in statistically significantly reduced tumor burdens (806-PE38 mean = 128 mm(3)[SD = 46 mm(3)] vs vehicle mean = 749 mm(3)[SD = 395 mm(3)], P = .001) and increased median survival (806-PE38 median = 82 days vs vehicle median = 50 days,P= .01) in a MDA-MB-468 TNBC mouse xenograft. Deletion of the catalytic residue eliminated both cytotoxic activity in vitro and the reduction in tumor burden and survival (P= .52).

CONCLUSIONS

These data support the further development of the 806-PE38 immunotoxin as a therapeutic agent for the treatment of patients with EGFR-positive TNBC. Follow-up experiments with combination therapies will be attempted to achieve full remissions.

Protection of the Furin Cleavage Site in Low-Toxicity Immunotoxins Based on Pseudomonas Exotoxin A

Recombinant immunotoxins (RITs) are fusions of an Fv-based targeting moiety and a toxin. Pseudomonas exotoxin A (PE) has been used to make several immunotoxins that have been evaluated in clinical trials. Immunogenicity of the bacterial toxin and off-target toxicity have limited the efficacy of these immunotoxins. To address these issues, we have previously made RITs in which the Fv is connected to domain III (PE24) by a furin cleavage site (FCS), thereby removing unneeded sequences of domain II. However, the PE24 containing RITs do not contain the naturally occurring disulfide bond around the furin cleavage sequence, because it was removed when domain II was deleted. This could potentially allow PE24 containing immunotoxins to be cleaved and inactivated before internalization by cell surface furin or other proteases in the blood stream or tumor microenvironment. Here, we describe five new RITs in which a disulfide bond is engineered to protect the FCS. The most active of these, SS1-Fab-DS3-PE24, shows a longer serum half-life than an RIT without the disulfide bond and has the same anti-tumor activity, despite being less cytotoxic in vitro. These results have significance for the production of de-immunized, low toxicity, PE24-based immunotoxins with a longer serum half-life.

Proinflammatory Effects of Staphylococcus Aureus Exotoxin B on Nasal Epithelial Cells

OBJECTIVE: The aim of this study was to evaluate the effects of Staphylococcus aureus exotoxin B (SE-B) on proinflammatory cytokine/chemokine releases in primary nasal epithelial cell cultures (NECC) of subjects with and without chronic rhinosinusitis (CRS).

STUDY DESIGN AND SETTING: NECC (CRS: n = 14; Controls: n = 11) were stimulated with SE-B. Protein concentrations of interleukin-(IL)-1β, IL-6, and IL-8 levels were measured in NECC supernatants by ELISA before (T0) and after 24 hr stimulation with SE-B (T1).

RESULTS: T0: supernatants of the NECC of CRS patients contained significant lower levels of IL-8 (2.1 ng/ml) compared to Controls (IL-8: 6.2 ng/ml; P < 0.01). T1: SE-B induced a significant increase of IL-6 in NECC ( P < 0.001). IL-1β was not detectable.

CONCLUSIONS: This is the first study evaluating the effects of exotoxins on NECC. SE-B showed proinflammatory effects on NECC.

SIGNIFICANCE: Our data suggest that resident NECC are involved in immunological responses to Staphylococcus aureus toxins, supplementing the so-called “superantigen hypothesis” in CRS.

Anticancer Effects of Mesothelin-Targeted Immunotoxin Therapy Are Regulated by Tyrosine Kinase DDR1

Recombinant immunotoxins (RIT) have been highly successful in cancer therapy due, in part, to the high cancer-specific expression of cell surface antigens such as mesothelin, which is overexpressed in mesothelioma, ovarian, lung, breast, and pancreatic cancers, but is limited in normal cells. RG7787 is a clinically optimized RIT consisting of a humanized anti-mesothelin Fab fused to domain III of Pseudomonas exotoxin A, in which immunogenic B-cell epitopes are silenced. To enhance the therapeutic efficacy of RITs, we conducted a kinome RNAi sensitization screen, which identified discoidin domain receptor 1 (DDR1), a collagen-activated tyrosine kinase, as a potential target. The collagen/DDR1 axis is implicated in tumor-stromal interactions and potentially affects tumor response to therapy. Therefore, we investigated the effects of DDR1 on RIT. Knockdown of DDR1 by siRNA or treatment with inhibitor, 7rh, greatly enhanced the cytotoxic activity of RG7787 in several cancer cell lines. Investigation into the mechanism of action showed DDR1 silencing was associated with decreased expression of several ribosomal proteins and enhanced inhibition of protein synthesis. Conversely, induction of DDR1 expression or collagen-stimulated DDR1 activity protected cancer cells from RG7787 killing. Moreover, the combination of RG7787 and DDR1 inhibitor caused greater shrinkage of tumor xenografts than either agent alone. These data demonstrate that DDR1 is a key modulator of RIT activity and represents a novel therapeutic strategy to improve targeting of mesothelin-expressing cancers.

Increased Susceptibility of Humanized NSG Mice to Panton-Valentine Leukocidin and Staphylococcus aureus Skin Infection

Staphylococcus aureus is a leading cause of skin and soft-tissue infections worldwide. Mice are the most commonly used animals for modeling human staphylococcal infections. However a supra-physiologic S. aureus inoculum is required to establish gross murine skin pathology. Moreover, many staphylococcal factors, including Panton-Valentine leukocidin (PVL) elaborated by community-associated methicillin-resistant S. aureus (CA-MRSA), exhibit selective human tropism and cannot be adequately studied in mice. To overcome these deficiencies, we investigated S. aureus infection in non-obese diabetic (NOD)/severe combined immune deficiency (SCID)/IL2rγ^null (NSG) mice engrafted with human CD34⁺ umbilical cord blood cells. These “humanized” NSG mice require one to two log lower inoculum to induce consistent skin lesions compared with control mice, and exhibit larger cutaneous lesions upon infection with PVL⁺ versus isogenic PVL^-S. aureus. Neutrophils appear important for PVL pathology as adoptive transfer of human neutrophils alone to NSG mice was sufficient to induce dermonecrosis following challenge with PVL⁺S. aureus but not PVL^-S. aureus. PMX53, a human C5aR inhibitor, blocked PVL-induced cellular cytotoxicity in vitro and reduced the size difference of lesions induced by the PVL⁺ and PVL^-S. aureus, but PMX53 also reduced recruitment of neutrophils and exacerbated the infection. Overall, our findings establish humanized mice as an important translational tool for the study of S. aureus infection and provide strong evidence that PVL is a human virulence factor.

S. aureus infection has emerged in the past decade as a major burden to public health and is responsible for a surge in preclinical research. Mice are the most commonly studied animals for modeling of human S. aureus infection. However, it is increasingly evident that available murine models poorly mimic human S. aureus disease. Routinely, a supra-physiologic inoculum is required to establish soft-tissue pathology. Additionally, many S. aureus factors exhibit unique human tropism and cannot be adequately investigated in rodents. Here we investigated S. aureus infection in NSG mice engrafted with human umbilical CD34⁺ cells. We showed that a one to two log lower infectious inoculum of S. aureus establishes consistent skin lesions in humanized NSG mice. This inoculum is comparable to published inocula required to induce infection in humans. In addition, we showed that Panton-Valentine Leucocidin, a human tropic factor secreted by S. aureus, contributes to the development of dermonecrosis in the humanized mice, and its interaction with human neutrophils and human C5a receptor appears to be important for immunopathology. Overall our study suggests that humanized mice are an improved tool for modeling of human S. aureus infection.

Identification, characterization, and targeting of IL-4 receptor by IL-4-Pseudomonas exotoxin in mouse models of anaplastic thyroid cancer

Thyroid cancer is a rapidly increasing endocrine cancer. Since interleukin-4 receptor (IL-4R) is overexpressed in human solid cancer, we examined expression of IL-4R in 50 cases of anaplastic thyroid cancer (ATC), 37 well-differentiated papillary cancer (WDPC), 35 well-differentiated follicular cancer of thyroid (WDFC), and 37 normal thyroid specimens by immunohistochemistry (IHC) and in-situ hybridization (ISH) techniques. We demonstrated that IL-4Rα was overexpressed in 36/50 (72%) ATC, 20/35 (57%) WDFC, and 11/37 (30%) WDPC tumors. Other two subunits of IL-4R, interleukin-13 receptor α1 (IL-13Rα1) and interleukin-2 receptor gamma (IL-2RγC), were either weakly expressed or absent. As ATC is a highly aggressive cancer with higher incidence of IL-4Rα expression, we characterized IL-4R in 3 ATC cell lines. RT-qPCR and IFA results showed that IL-4Rα is overexpressed while IL-13Rα1 is weakly expressed. Control human umbilical vein endothelial cell line (HUVEC) showed weak expression of IL-4Rα. Binding and competition studies with 125I-IL-4 in ATC cell lines demonstrated that IL-4 specifically bound to IL-4Rα on cell surface. ATC cell lines were highly sensitive to a chimeric fusion cytotoxin consisting of circularly permuted IL-4 and truncated Pseudomonas exotoxin (IL-4-PE), which killed them in a concentration dependent manner. IL-4-PE also blocked colony formation of ATC cell lines in clonogenic assays. IL-4-PE mediated a significant antitumor activity in mouse models of ATC. Intratumoral administration of IL-4-PE caused significant regression of established tumors in a dose dependent manner and increased the overall survival without any visible toxicity. Thus, IL-4Rα in ATC may represent a novel therapeutic target and IL-4-PE may serve as an investigational therapeutic option for ATC.

Target-specific cytotoxic effects on HER2-expressing cells by the tripartite fusion toxin ZHER2:2891-ABD-PE38X8, including a targeting affibody molecule and a half-life extension domain

Development of cancer treatment regimens including immunotoxins is partly hampered by their immunogenicity. Recently, deimmunized versions of toxins have been described, potentially being better suited for translation to the clinic. In this study, a recombinant tripartite fusion toxin consisting of a deimmunized version of exotoxin A from Pseudomonas aeruginosa (PE38) genetically fused to an affibody molecule specifically interacting with the human epidermal growth factor receptor 2 (HER2), and also an albumin binding domain (ABD) for half-life extension, has been produced and characterized in terms of functionality of the three moieties. Biosensor based assays showed that the fusion toxin was able to interact with human and mouse serum albumin, but not with bovine serum albumin and that it interacted with HER2 (KD=5 nM). Interestingly, a complex of the fusion toxin and human serum albumin also interacted with HER2 but with a somewhat weaker affinity (KD=12 nM). The IC50-values of the fusion toxin ranged from 6 to 300 pM on SKOV-3, SKBR-3 and A549 cells and was lower for cells with higher surface densities of HER2. The fusion toxin was found specific for HER2 as shown by blocking available HER2 receptors with free affibody molecule before subjecting the cells to the toxin. Analysis of contact time showed that 10 min was sufficient to kill 50% of the cells. In conclusion, all three regions of the fusion toxin were found to be functional.

Designing the furin-cleavable linker in recombinant immunotoxins based on Pseudomonas exotoxin A

Recombinant immunotoxins (RITs) are fusion proteins that join antibodies to protein toxins for targeted cell killing. RITs armed with Pseudomonas exotoxin A (PE) are undergoing clinical trials for the treatment of cancer. The current design of PE-based RITs joins an antibody fragment to the catalytic domain of PE using a polypeptide linker that is cleaved by the protease furin. Intracellular cleavage of native PE by furin is required for cytotoxicity, yet the PE cleavage site has been shown to be a poor furin substrate. Here we describe the rational design of more efficiently cleaved furin linkers in PE-based RITs, and experiments evaluating their effects on cleavage and cytotoxicity. We found that changes to the furin site could greatly influence both cleavage and cytotoxicity, but the two parameters were not directly correlated. Furthermore, the effects of alterations to the furin linker were not universal. Identical mutations in the anti-CD22 RIT HA22-LR often displayed different cytotoxicity from mutations in the anti-mesothelin RIT SS1-LR/GGS, underscoring the prominent role of the target site in their intoxication pathways. Combining several beneficial mutations in HA22-LR resulted in a variant (HA22-LR/FUR) with a remarkably enhanced cleavage rate and improved cytotoxicity against five B cell lines and similar or enhanced cytotoxicity in five out of six hairy cell leukemia patient samples. This result informs the design of protease-sensitive linkers and suggests that HA22-LR/FUR may be a candidate for further preclinical development.

Efficient cell-specific uptake of binding proteins into the cytoplasm through engineered modular transport systems

Through advances in protein scaffold engineering and selection technologies, highly specific binding proteins, which fold under reducing conditions, can be generated against virtually all targets. Despite tremendous therapeutic opportunities, intracellular applications are hindered by difficulties associated with achieving cytosolic delivery, compounded by even correctly measuring it. Here, we addressed cytosolic delivery systematically through the development of a biotin ligase-based assay that objectively quantifies cytosolic delivery in a generic fashion. We developed modular transport systems that consist of a designed ankyrin repeat protein (DARPin) for receptor targeting and a different DARPin for intracellular recognition and a bacterial toxin-derived component for cytosolic translocation. We show that both anthrax pores and the translocation domain of Pseudomonas exotoxin A (ETA) efficiently deliver DARPins into the cytosol. We found that the cargo must not exceed a threshold thermodynamic stability for anthrax pores, which can be addressed by engineering, while the ETA pathway does not appear to have this restriction.

Phage display-based generation of novel internalizing antibody fragments for immunotoxin-based treatment of acute myeloid leukemia

The current standard treatment for acute myeloid leukemia (AML) is chemotherapy based on cytarabine and daunorubicine (7 + 3), but it discriminates poorly between malignant and benign cells. Dose-limiting off‑target effects and intrinsic drug resistance result in the inefficient eradication of leukemic blast cells and their survival beyond remission. This minimal residual disease is the major cause of relapse and is responsible for a 5-year survival rate of only 24%. More specific and efficient approaches are therefore required to eradicate malignant cells while leaving healthy cells unaffected. In this study, we generated scFv antibodies that bind specifically to the surface of AML blast cells and AML bone marrow biopsy specimens. We isolated the antibodies by phage display, using subtractive whole-cell panning with AML M2‑derived Kasumi‑1 cells. By selecting for internalizing scFv antibody fragments, we focused on potentially novel agents for intracellular drug delivery and tumor modulation. Two independent methods showed that 4 binders were internalized by Kasumi-1 cells. Furthermore, we observed the AML‑selective inhibition of cell proliferation and the induction of apoptosis by a recombinant immunotoxin comprising one scFv fused to a truncated form of Pseudomonas exotoxin A (ETA'). This method may therefore be useful for the selection of novel disease-specific internalizing antibody fragments, providing a novel immunotherapeutic strategy for the treatment of AML patients.

Influence of pistachios on performance and exercise-induced inflammation, oxidative stress, immune dysfunction, and metabolite shifts in cyclists: a randomized, crossover trial

OBJECTIVES

Pistachio nut ingestion (3 oz./d, two weeks) was tested for effects on exercise performance and 21-h post-exercise recovery from inflammation, oxidative stress, immune dysfunction, and metabolite shifts.

METHODS

Using a randomized, crossover approach, cyclists (N = 19) engaged in two 75-km time trials after 2-weeks pistachio or no pistachio supplementation, with a 2-week washout period. Subjects came to the lab in an overnight fasted state, and ingested water only or 3 oz. pistachios with water before and during exercise. Blood samples were collected 45 min pre-exercise, and immediately post-, 1.5-h post-, and 21-h post-exercise, and analyzed for plasma cytokines, C-reactive protein (CRP), F2-isoprostanes (F2-IsoP), granulocyte phagocytosis (GPHAG) and oxidative burst activity (GOBA), and shifts in metabolites.

RESULTS

Performance time for the 75-km time trial was 4.8% slower under pistachio conditions (2.84 ± 0.11 and 2.71 ± 0.07 h, respectively, P = 0.034). Significant time effects were shown for plasma cytokines, CRP, F2-IsoP, GPHAG, and GOBA, with few group differences. Metabolomics analysis revealed 423 detectable compounds of known identity, with significant interaction effects for 19 metabolites, especially raffinose, (12Z)-9,10-Dihydroxyoctadec-12-enoate (9,10-DiHOME), and sucrose. Dietary intake of raffinose was 2.19 ± 0.15 and 0.35 ± 0.08 mg/d during the pistachio and no pistachio periods, and metabolomics revealed that colon raffinose and sucrose translocated to the circulation during exercise due to increased gut permeability. The post-exercise increase in plasma raffinose correlated significantly with 9,10-DiHOME and other oxidative stress metabolites.

CONCLUSIONS

In summary, 2-weeks pistachio nut ingestion was associated with reduced 75-km cycling time trial performance and increased post-exercise plasma levels of raffinose, sucrose, and metabolites related to leukotoxic effects and oxidative stress.

TRIAL REGISTRATION

ClinicalTrials.gov NCT01821820.

Effect of antigen shedding on targeted delivery of immunotoxins in solid tumors from a mathematical model

Most cancer-specific antigens used as targets of antibody-drug conjugates and immunotoxins are shed from the cell surface (Zhang & Pastan (2008) Clin. Cancer Res. 14: 7981-7986), although at widely varying rates and by different mechanisms (Dello Sbarba & Rovida (2002) Biol. Chem. 383: 69–83). Why many cancer-specific antigens are shed and how the shedding affects delivery efficiency of antibody-based protein drugs are poorly understood questions at present. Before a detailed numerical study, it was assumed that antigen shedding would reduce the efficacy of antibody-drug conjugates and immunotoxins. However, our previous study using a comprehensive mathematical model showed that antigen shedding can significantly improve the efficacy of the mesothelin-binding immunotoxin, SS1P (anti-mesothelin-Fv-PE38), and suggested that receptor shedding can be a general mechanism for enhancing the effect of inter-cellular signaling molecules. Here, we improved this model and applied it to both SS1P and another recombinant immunotoxin, LMB-2, which targets CD25. We show that the effect of antigen shedding is influenced by a number of factors including the number of antigen molecules on the cell surface and the endocytosis rate. The high shedding rate of mesothelin is beneficial for SS1P, for which the antigen is large in number and endocytosed rapidly. On the other hand, the slow shedding of CD25 is beneficial for LMB-2, for which the antigen is small in number and endocytosed slowly.

A CSPG4-specific immunotoxin kills rhabdomyosarcoma cells and binds to primary tumor tissues

The treatment of rhabdomyosarcoma (RMS) remains challenging, with metastatic and alveolar RMS offering a particularly poor prognosis. Therefore, the identification and evaluation of novel antigens, which are suitable targets for immunotherapy, is one attractive possibility to improve the treatment of this disease. Here we show that chondroitin sulfate proteoglycan 4 (CSPG4) is expressed on RMS cell lines and RMS patient material. We evaluated the immunotoxin (IT) αMCSP-ETA', which specifically recognizes CSPG4 on the RMS cell lines RD, FL-OH1, TE-671 and Rh30. It is internalized rapidly, induces apoptosis and thus kills RMS cells selectively. We also demonstrate the specific binding of this IT to RMS primary tumor material from three different patients.

Tofacitinib suppresses antibody responses to protein therapeutics in murine hosts

Immunogenicity remains the "Achilles' heel" of protein-based therapeutics. Anti-drug Abs produced in response to protein therapeutics can severely limit both the safety and efficacy of this expanding class of agent. In this article, we report that monotherapy of mice with tofacitinib (the JAK inhibitor) quells Ab responses to an immunotoxin derived from the bacterial protein Pseudomonas exotoxin A, as well as to the model Ag keyhole limpet hemocyanin. Thousand-fold reductions in IgG1 titers to both Ags were observed 21 d post immunization. In fact, suppression was evident for all IgG isotypes and IgM. A reduction in IgG3 production was also noted with a thymus-independent type II Ag. Mechanistic investigations revealed that tofacitinib treatment led to reduced numbers of CD127+ pro-B cells. Furthermore, we observed fewer germinal center B cells and the impaired formation of germinal centers of mice treated with tofacitinib. Because normal Ig levels were still present during tofacitinib treatment, this agent specifically reduced anti-drug Abs, thus preserving the potential efficacy of biological therapeutics, including those used as cancer therapeutics.

The novel immunotoxin HM1.24-ETA' induces apoptosis in multiple myeloma cells

Despite new treatment modalities, the clinical outcome in a substantial number of patients with multiple myeloma (MM) has yet to be improved. Antibody-based targeted therapies for myeloma patients could make use of the HM1.24 antigen (CD317), a surface molecule overexpressed on malignant plasma cells and efficiently internalized. Here, a novel immunotoxin, HM1.24-ETA', is described. HM1.24-ETA' was generated by genetic fusion of a CD317-specific single-chain Fv (scFv) antibody and a truncated variant of Pseudomonas aeruginosa exotoxin A (ETA'). HM1.24-ETA' inhibited growth of interleukin 6 (IL-6)-dependent and -independent myeloma cell lines. Half-maximal growth inhibition was observed at concentrations as low as 0.3 nM. Target cell killing occurred via induction of apoptosis and was unaffected in co-culture experiments with bone marrow stromal cells. HM1.24-ETA' efficiently triggered apoptosis of freshly isolated/cryopreserved cells of patients with plasma cell leukemia and MM and was active in a preclinical severe combined immunodeficiency (SCID) mouse xenograft model. Importantly, HM1.24-ETA' was not cytotoxic against CD317-positive cells from healthy tissue (monocytes, human umbilical vein endothelial cells). These results indicate that CD317 may represent a promising target structure for specific and efficient immunotoxin therapy for patients with plasma cell tumors.

Separately or combined, LukG/LukH is functionally unique compared to other staphylococcal bicomponent leukotoxins

Staphylococcus aureus is a major human pathogen that elaborates several exotoxins. Among these are the bicomponent leukotoxins (BCLs), which include γ-hemolysin, Panton-Valentine leukocidin (PVL), and LukDE. The toxin components are classified as either F or S proteins, which are secreted individually and assemble on cell surfaces to form hetero-oligomeric pores resulting in lysis of PMNs and/or erythrocytes. F and S proteins of γ-hemolysin, PVL and LukDE have ∼ 70% sequence homology within the same class and several heterologous combinations of F and S members from these three bicomponent toxin groups are functional. Recently, an additional BCL pair, LukGH (also called LukAB) that has only 30% homology to γ-hemolysin, PVL and LukDE, has been characterized from S. aureus. Our results showed that LukGH was more cytotoxic to human PMNs than PVL. However, LukGH-induced calcium ion influx in PMNs was markedly attenuated and slower than that induced by PVL and other staphylococcal BCLs. In contrast to other heterologous BCL combinations, LukG in combination with heterologous S components, and LukH in combination with heterologous F components did not induce calcium ion entry or cell lysis in human PMNs or rabbit erythrocytes. Like PVL, LukGH induced IL-8 production by PMNs. While individual components LukG and LukH had no cytolytic or calcium influx activity, they each induced high levels of IL-8 transcription and secretion. IL-8 production induced by LukG or LukH was dependent on NF-κB. Therefore, our results indicate LukGH differs functionally from other staphylococcal BCLs.

Enhanced endoplasmic reticulum entry of tumor antigen is crucial for cross-presentation induced by dendritic cell-targeted vaccination

Efficient cross-presentation of protein Ags to CTLs by dendritic cells (DCs) is essential for the success of prophylactic and therapeutic vaccines. In this study, we report a previously underappreciated pathway involving Ag entry into the endoplasmic reticulum (ER) critically needed for T cell cross-priming induced by a DC-targeted vaccine. Directing the clinically relevant, melanoma Ag gp100 to mouse-derived DCs by molecular adjuvant and chaperone Grp170 substantially facilitates Ag access to the ER. Grp170 also strengthens the interaction of internalized protein Ag with molecular components involved in ER-associated protein dislocation and/or degradation, which culminates in cytosolic translocation for proteasome-dependent degradation and processing. Targeted disruption of protein retrotranslocation causes exclusive ER retention of tumor Ag in mouse bone marrow-derived DCs and splenic CD8(+) DCs. This results in the blockade of Ag ubiquitination and processing, which abrogates the priming of Ag-specific CD8(+) T cells in vitro and in vivo. Therefore, the improved ER entry of tumor Ag serves as a molecular basis for the superior cross-presenting capacity of Grp170-based vaccine platform. The ER access and retrotranslocation represents a distinct pathway that operates within DCs for cross-presentation and is required for the activation of Ag-specific CTLs by certain vaccines. These results also reinforce the importance of the ER-associated protein quality control machinery and the mode of the Ag delivery in regulating DC-elicited immune outcomes.

Targeted reduction of CCR4⁺ cells is sufficient to suppress allergic airway inflammation

BACKGROUND

Bronchial asthma is characterized by allergic airway inflammation involving C-C chemokine receptor type 4 (CCR4)-positive Th2 cells. As such, we hypothesize that the disease can be alleviated by targeted-elimination of CCR4⁺ cells. Thymus and activation-regulated chemokine (TARC)-PE38, a TARC fused the exotoxin fragment PE38 from Pseudomonas aeruginosa, has been shown to efficiently kill CCR4⁺ cells by delivering the exotoxin fragment PE38 into CCR4⁺ cells. To test our hypothesis, we examined whether TARC-PE38 could suppress allergic airway inflammation in a mouse model of house dust mite (HDM)-induced allergic airway inflammation.

METHODS

We evaluated the effect of TARC-PE38 on the major characteristics of HDM-induced allergic airway inflammation. Airway hyperresponsiveness, lung histopathology, lung Th1/Th2 cell populations, and concentrations of Th1/Th2 cytokines in the lungs were assessed in HDM-sensitized and challenged mice in the presence and absence of TARC-PE38.

RESULTS

TARC-PE38 efficiently suppressed allergic airway inflammation by significantly reducing airway hyperresponsiveness, the overall area of inflammation, and goblet cell hyperplasia. In HDM-sensitized and challenged mice, TARC-PE38 specifically reduced the numbers of CCR4⁺ cells. This reduction was associated with a significant decrease in the production of Th2 cytokines in the airway,and a decrease in the number of leukocytes, including macrophages, eosinophils and lymphocytes, within the subepithelial area of the lungs and airway lumen. TARC-PE38 had noeffect on Th1 cells.

CONCLUSION

Our data suggest that the elimination of CCR4⁺ cells via TARC-PE38 treatment is sufficient to control allergic airway inflammation and airway hyperresponsiveness.

[Pseudomonas aeruginosa recombinant proteins: effect on mice cytokine profile]

AIM

Study cytokine-mediated immune response in mice vaccinated with Pseudomonas aeruginosa recombinant antigen preparations.

MATERIALS AND METHODS

Cytokine-mediated immune response was studied in mice vaccinated with membrane recombinant proteins OprF, OprL, a hybrid recombinant protein OprF-I consisting of sequences of OprF and OprI proteins and a recombinant atoxic form of exotoxin A with a deletion of 106 amino acid sequences (recombinant anatoxin - aTox) of P. aeruginosa.

RESULTS

An induction of a wide specter of studied cytokines was detected in the mice. The highest level was observed for IL-1 and IL-6 after administration of recombinant proteins OprL, OprF, OprF-1, aTox. OprF-I actively stimulated production of IL-2 that is a factor of growth and differentiation of lymphocytes, natural killers and cytotoxic lymphocytes; as well as IL-5, IL-O10, TNF-alpha and IFN-gamma. Recombinant protein OprF-I facilitated induction of IL-6, IL-17, TNF-alpha and IFN-gamma, whereas aTox - expression of IL-1, IL-2, IFN-gamma. Recombinant protein OprL induced IL-17 synthesis to the most extent and TNF-alpha and IL-10 - moderately.

CONCLUSION

The P. aeruginosa recombinant proteins obtained during intraperitoneal administration to mice facilitated formation of immune response with the direction of induction in both Thl and Th2 pathways.

Effects of Aggregatibacter actinomycetemcomitans leukotoxin on endothelial cells

Aggregatibacter actinomycetemcomitans is a human pathogen that produces leukotoxin (LtxA) as a major virulence factor. In this study the effect of LtxA on microvascular endothelial cell viability and phenotype was studied. High doses of single LtxA treatment (500 ng/ml to 5 μg/ml) significantly and irreversibly decreased cell proliferation and induced apoptosis, as assessed by tetrazolium salt and annexin V assay, respectively. Apoptosis was partially inhibited by the pan-caspase inhibitor, z-VAD-fmk. LtxA caused a cell cycle arrest in the G2/M phase after 72 h. Between 500 ng/ml and 5 μg/ml, after long- or short-term stimulation LtxA increased the expression of ICAM-1 and VCAM-1, as well as the percentages of endothelial cells expressing these adhesion molecules. Thus, A. actinomycetemcomitans LtxA has substantial pro-inflammatory effects on human brain endothelial cells by upregulation of ICAM-1 and VCAM-1. Furthermore, LtxA in higher concentration was found to decrease proliferation and induces apoptosis in microvascular endothelial cells.

Determining antibody-binding site of streptococcal pyrogenic exotoxin B to protect mice from group a streptococcus infection

Streptococcal pyrogenic exotoxin B (SPE B), a cysteine protease, is an important virulence factor in group A streptococcal (GAS) infection. SPE B binds and cleaves antibody isotypes and further impairs the immune system by inhibiting complement activation. In this study, we examined the antibody-binding site of SPE B and used it to block SPE B actions during GAS infection. We constructed different segments of the spe B gene and induced them to express different recombinant fragments of SPE B. Using an enzyme-linked immunosorbent assay (ELISA), we found that residues 345–398 of the C-terminal domain of SPE B (rSPE B_345–398), but not the N-terminal domain, was the major binding site for antibody isotypes. Using a competitive ELISA, we also found that rSPE B_345–398 bound to the Fc portion of IgG. The in vitro functional assays indicate that rSPE B_345–398 not only interfered with cleavage of antibody isotypes but also interfered with SPE B-induced inhibition of complement activation. Immunization of BALB/c mice using rSPE B_345–398 was able to induce production of a high titer of anti-rSPE B_345–398 antibodies and efficiently protected mice from GAS-induced death. These findings suggest that SPE B uses its C-terminal domain to bind the Fc portion of IgG and that immunization of mice with this binding domain (rSPE B_345–398) could protect mice from GAS infection.

Development of VEGFR2-specific Nanobody Pseudomonas exotoxin A conjugated to provide efficient inhibition of tumor cell growth

Angiogenesis targeting is an attractive approach for cancer treatment. Vascular endothelial growth factor receptor 2 (VEGFR2) is such an important target that is overexpressed in tumor vasculature compared to the endothelium cells of resting blood vessels and blocking of its signaling inhibits neovascularization and tumor metastasis. Immunotoxins represent a promising group of targeted therapeutics to combat tumors. They consist of an antibody linked to a toxin and are designed to kill specifically the tumor cells. In this study, we fused a VEGFR2-specific Nanobody, the antigen-binding single-domain fragment derived from functional Heavy-chain antibody of Camelidae, to the truncated form of Pseudomonas exotoxin A and evaluated its ability to bind the VEGFR2 molecule on the cell surface. We demonstrate that this immunotoxin inhibits the proliferation of VEGFR2-expressing cells in vitro. This finding is considered to be a significant achievement in tumor therapy and it forms a basis for further studies in animal models.

Construction, expression, and characterization of an anti-tumor immunotoxin containing the human anti-c-Met single-chain antibody and PE38KDEL

Recombinant immunotoxins consisting of small antibody fragments fused to cytotoxic moieties are being evaluated for use in prospective antibody-targeted cancer therapies. A receptor tyrosine kinase known as c-Met is overexpressed in a vast range of human malignancies, making it an ideal target for antibody-mediated delivery of numerous cytotoxic agents. A single Fab molecule capable of binding to human c-Met with high affinity and specificity was previously identified using antibody phage-display technology. In order to develop a molecule to increase both the cytotoxicity and anti-tumor activity of the anti-c-Met molecule, a recombinant immunotoxin anti-c-Met/PE38KDEL was constructed and expressed by fusing the human anti-c-Met single-chain variable fragment (ScFv) with a modified Pseudomonas exotoxin A (PE38KDEL). Purified anti-c-Met/PE38KDEL was demonstrated to specifically bind to cells of c-Met-positive human hepatoma cell lines, causing a proliferation defect by inducing caspase-3/8-mediated apoptosis, as observed by in vitro assays. Furthermore, anti-c-Met/PE38KDEL administration was shown to inhibit the growth of hepatocellular carcinoma xenografts in vivo through suppression of Ki-67 expression and enhancement of tumor cell apoptosis rates. Cumulatively, the current findings demonstrate the successful construction of a recombinant immunotoxin capable of accurately targeting c-Met-positive human hepatoma cell lines both in vitro and in vivo, providing a novel compound with potential for applications as an alternative therapy for c-Met-positive cancer management.

Immunotoxin resistance via reversible methylation of the DPH4 promoter is a unique survival strategy

HA22 is a recombinant immunotoxin composed of an anti-CD22 Fv fused to a portion of Pseudomonas exotoxin A. HA22 produced a high rate of complete remissions in drug-resistant hairy cell leukemia and has a lower response rate in pediatric acute lymphoblastic leukemia (ALL). To understand why patients with ALL have poorer responses, we isolated an ALL cell line that is resistant to killing by HA22. The resistance is unstable; without HA22 the cells revert to HA22 sensitivity in 4 mo. We showed that in the resistant cell line, HA22 is unable to ADP ribosylate and inactivate elongation factor-2 (EF2), owing to a low level of DPH4 mRNA and protein, which prevents diphthamide biosynthesis and renders EF2 refractory to HA22. Analysis of the promoter region of the DPH4 gene shows that the CpG island was hypomethylated in the HA22-sensitive cells, heavily methylated in the resistant cells, and reverted to low methylation in the revertant cells. Our data show that immunotoxin resistance is associated with reversible CpG island methylation and silencing of DPH4 gene transcription. Incubation of sensitive cells with the methylation inhibitor 5-azacytidine prevented the emergence of resistant cells, suggesting that this agent in combination with HA22 may be useful in the treatment of some cases of ALL.

Staphylococcal Panton-Valentine leukocidin induces pro-inflammatory cytokine production and nuclear factor-kappa B activation in neutrophils

Panton-Valentine leukocidin (PVL) is a cytotoxin secreted by Staphylococcus aureus and associated with severe necrotizing infections. PVL targets polymorphonuclear leukocytes, especially neutrophils, which are the first line of defense against infections. Although PVL can induce neutrophil death by necrosis or apoptosis, the specific inflammatory responses of neutrophils to this toxin are unclear. In this study, both in vivo and in vitro studies demonstrated that recombinant PVL has an important cytotoxic role in human neutrophils, leading to apoptosis at low concentrations and necrosis at high concentrations. Recombinant PVL also increased the levels of pro-inflammatory cytokine secretion from neutrophils. The up-regulation of pro-inflammatory cytokines was due to nuclear factor-kappa B (NF-κB) activation induced by PVL. Moreover, blocking NF-κB inhibited the production of inflammatory cytokines. To test the role of neutrophil immune responses during the pathogenesis of PVL-induced acute lung injury, we used immunocompetent or neutropenic rabbits to develop a model of necrotizing pneumonia. Immunocompetent rabbits challenged with PVL demonstrated increased inflammation containing neutrophilic infiltrates. In addition, there were elevated levels of inflammatory cytokines (IL-6, IL-8, TNF-α and IL-10) and NF-κB in the lung homogenate. In contrast, the lung tissues from neutropenic rabbits contained mild or moderate inflammation, and the levels of inflammatory cytokines and NF-κB increased only slightly. Data from the current study support growing evidence that neutrophils play an important role in the pathogenesis of PVL-induced tissue injury and inflammation. PVL can stimulate neutrophils to release pro-inflammatory mediators, thereby causing an acute inflammatory response. The ability of PVL to induce inflammatory cytokine release may be associated with the activation of NF-κB or its pore-forming properties.

Reductive activation of type 2 ribosome-inactivating proteins is promoted by transmembrane thioredoxin-related protein

Members of the type 2 ribosome-inactivating proteins (RIPs) family (e.g. ricin, abrin) are potent cytotoxins showing a strong lethal activity toward eukaryotic cells. Type 2 RIPs contain two polypeptide chains (usually named A, for "activity", and B, for "binding") linked by a disulfide bond. The intoxication of the cell is a consequence of a reductive process in which the toxic domain is cleaved from the binding domain by oxidoreductases located in the lumen of the endoplasmic reticulum (ER). The best known example of type 2 RIPs is ricin. Protein disulfide isomerase (PDI) was demonstrated to be involved in the process of ricin reduction; however, when PDI is depleted from cell fraction preparations ricin reduction can still take place, indicating that also other oxidoreductases might be implicated in this process. We have investigated the role of TMX, a transmembrane thioredoxin-related protein member of the PDI family, in the cell intoxication operated by type 2 RIPs ricin and abrin. Overexpressing TMX in A549 cells resulted in a dramatic increase of ricin or abrin cytotoxicity compared with control mock-treated cells. Conversely, no difference in cytotoxicity was observed after treatment of A549 cells or control cells with saporin or Pseudomonas exotoxin A whose intracellular mechanism of activation is not dependent upon reduction (saporin) or only partially dependent upon it (Pseudomonas exotoxin A). Moreover, the silencing of TMX in the prostatic cell line DU145 reduced the sensitivity of the cells to ricin intoxication further confirming a role for this enzyme in intracellular ricin activation.

Injection of Pseudomonas aeruginosa Exo toxins into host cells can be modulated by host factors at the level of translocon assembly and/or activity

Pseudomonas aeruginosa type III secretion apparatus exports and translocates four exotoxins into the cytoplasm of the host cell. The translocation requires two hydrophobic bacterial proteins, PopB and PopD, that are found associated with host cell membranes following infection. In this work we examined the influence of host cell elements on exotoxin translocation efficiency. We developed a quantitative flow cytometry based assay of translocation that used protein fusions between either ExoS or ExoY and the ß-lactamase reporter enzyme. In parallel, association of translocon proteins with host plasma membranes was evaluated by immunodetection of PopB/D following sucrose gradient fractionation of membranes. A pro-myelocytic cell line (HL-60) and a pro-monocytic cell line (U937) were found resistant to toxin injection even though PopB/D associated with host cell plasma membranes. Differentiation of these cells to either macrophage- or neutrophil-like cell lines resulted in injection-sensitive phenotype without significantly changing the level of membrane-inserted translocon proteins. As previous in vitro studies have indicated that the lysis of liposomes by PopB and PopD requires both cholesterol and phosphatidyl-serine, we first examined the role of cholesterol in translocation efficiency. Treatment of sensitive HL-60 cells with methyl-ß-cyclodextrine, a cholesterol-depleting agent, resulted in a diminished injection of ExoS-Bla. Moreover, the PopB translocator was found in the membrane fraction, obtained from sucrose-gradient purifications, containing the lipid-raft marker flotillin. Examination of components of signalling pathways influencing the toxin injection was further assayed through a pharmacological approach. A systematic detection of translocon proteins within host membranes showed that, in addition to membrane composition, some general signalling pathways involved in actin polymerization may be critical for the formation of a functional pore. In conclusion, we provide new insights in regulation of translocation process and suggest possible cross-talks between eukaryotic cell and the pathogen at the level of exotoxin translocation.