Analysis of IgG heavy chain to light chain ratio with mutant Encephalomyocarditis virus internal ribosome entry site

Manipulating gene expression levels in mammalian cell factories: An outline of synthetic molecular toolboxes to achieve multiplexed control

Mammalian cells have developed dedicated molecular mechanisms to tightly control expression levels of their genes where the specific transcriptomic signature across all genes eventually determines the cell's phenotype. Modulating cellular phenotypes is of major interest to study their role in disease or to reprogram cells for the manufacturing of recombinant products, such as biopharmaceuticals. Cells of mammalian origin, for example Chinese hamster ovary (CHO) and Human embryonic kidney 293 (HEK293) cells, are most commonly employed to produce therapeutic proteins. Early genetic engineering approaches to alter their phenotype have often been attempted by "uncontrolled" overexpression or knock-down/-out of specific genetic factors. Many studies in the past years, however, highlight that rationally regulating and fine-tuning the strength of overexpression or knock-down to an optimum level, can adjust phenotypic traits with much more precision than such "uncontrolled" approaches. To this end, synthetic biology tools have been generated that enable (fine-)tunable and/or inducible control of gene expression. In this review, we discuss various molecular tools used in mammalian cell lines and group them by their mode of action: transcriptional, post-transcriptional, translational and post-translational regulation. We discuss the advantages and disadvantages of using these tools for each cell regulatory layer and with respect to cell line engineering approaches. This review highlights the plethora of synthetic toolboxes that could be employed, alone or in combination, to optimize cellular systems and eventually gain enhanced control over the cellular phenotype to equip mammalian cell factories with the tools required for efficient production of emerging, more difficult-to-express biologics formats.

Optimization of recombinant antibody production based on the vector design and the level of metabolites for generation of Ig- producing stable cell lines

BACKGROUND

The biopharmaceutical industry is significantly growing worldwide, and the Chinese hamster ovary (CHO) cells are used as a main expression host for the production of recombinant monoclonal antibodies. Various metabolic engineering approaches have been investigated to generate cell lines with improved metabolic characteristics for increasing longevity and mAb production. A novel cell culture method based on the 2-stage selection makes it possible to develop a stable cell line with high-quality mAb production.

RESULTS

We have constructed several design options of mammalian expression vectors for the high production of recombinant human IgG antibodies. Versions for bipromoter and bicistronic expression plasmids different in promoter orientation and cistron arrangements were generated. The aim of the work presented here was to assess a high-throughput mAb production system that integrates the advantages of high-efficiency cloning and stable cell clones to stage strategy selection reducing the time and effort required to express therapeutic monoclonal mAbs. Development of a stable cell line using bicistronic construct with EMCV IRES-long link gave an advantage in high mAb expression and long-term stability. Two-stage selection strategies allowed the elimination of low-producer clones by using metabolic level intensity to estimate the IgG production in the early steps of selection. The practical application of the new method allows to reduce time and costs during stable cell line development.

Synthetic polycistronic sequences in eukaryotes

The need for co-ordinate, high-level, and stable expression of multiple genes is essential for the engineering of biosynthetic circuits and metabolic pathways. This work outlines the functionality and design of IRES- and 2 A-peptide-based constructs by comparing different strategies for co-expression in polycistronic vectors. In particular, 2 A sequences are small peptides, mostly derived from viral polyproteins, that mediate a ribosome-skipping event such that several, different, separate proteins can be generated from a single open reading frame. When applied to metabolic engineering and synthetic gene circuits, 2 A peptides permit to achieve co-regulated and reliable expression of various genes in eukaryotic cells.

Idiotype vaccines produced with a non-cytopathic alphavirus self-amplifying RNA vector induce antitumor responses in a murine model of B-cell lymphoma

A promising therapy for patients with B-cell lymphoma is based on vaccination with idiotype monoclonal antibodies (mAbs). Since idiotypes are different in each tumor, a personalized vaccine has to be produced for each patient. Expression of immunoglobulins with appropriate post-translational modifications for human use often requires the use of stable mammalian cells that can be scaled-up to reach the desired level of production. We have used a noncytopathic self-amplifying RNA vector derived from Semliki Forest virus (ncSFV) to generate BHK cell lines expressing murine follicular lymphoma-derived idiotype A20 mAb. ncSFV/BHK cell lines expressed approximately 2 mg/L/24 h of A20 mAb with proper quaternary structure and a glycosylation pattern similar to that of A20 mAb produced by hybridoma cells. A20 mAb purified from the supernatant of a ncSFV cell line, or from the hybridoma, was conjugated to keyhole limpet hemocyanin and used to immunize Balb/c mice by administration of four weekly doses of 25 µg of mAb. Both idiotype mAbs were able to induce a similar antitumor protection and longer survival compared to non-immunized mice. These results indicate that the ncSFV RNA vector could represent a quick and efficient system to produce patient-specific idiotypes with potential application as lymphoma vaccines.

Current Practice in Bicistronic IRES Reporter Use: A Systematic Review

Bicistronic reporter assays have been instrumental for transgene expression, understanding of internal ribosomal entry site (IRES) translation, and identification of novel cap-independent translational elements (CITE). We observed a large methodological variability in the use of bicistronic reporter assays and data presentation or normalization procedures. Therefore, we systematically searched the literature for bicistronic IRES reporter studies and analyzed methodological details, data visualization, and normalization procedures. Two hundred fifty-seven publications were identified using our search strategy (published 1994-2020). Experimental studies on eukaryotic adherent cell systems and the cell-free translation assay were included for further analysis. We evaluated the following methodological details for 176 full text articles: the bicistronic reporter design, the cell line or type, transfection methods, and time point of analyses post-transfection. For the cell-free translation assay, we focused on methods of in vitro transcription, type of translation lysate, and incubation times and assay temperature. Data can be presented in multiple ways: raw data from individual cistrons, a ratio of the two, or fold changes thereof. In addition, many different control experiments have been suggested when studying IRES-mediated translation. In addition, many different normalization and control experiments have been suggested when studying IRES-mediated translation. Therefore, we also categorized and summarized their use. Our unbiased analyses provide a representative overview of bicistronic IRES reporter use. We identified parameters that were reported inconsistently or incompletely, which could hamper data reproduction and interpretation. On the basis of our analyses, we encourage adhering to a number of practices that should improve transparency of bicistronic reporter data presentation and improve methodological descriptions to facilitate data replication.

Intramuscular Delivery of Replicon RNA Encoding ZIKV-117 Human Monoclonal Antibody Protects against Zika Virus Infection

Monoclonal antibody (mAb) therapeutics are an effective modality for the treatment of infectious, autoimmune, and cancer-related diseases. However, the discovery, development, and manufacturing processes are complex, resource-consuming activities that preclude the rapid deployment of mAbs in outbreaks of emerging infectious diseases. Given recent advances in nucleic acid delivery technology, it is now possible to deliver exogenous mRNA encoding mAbs for in situ expression following intravenous (i.v.) infusion of lipid nanoparticle-encapsulated mRNA. However, the requirement for i.v. administration limits the application to settings where infusion is an option, increasing the cost of treatment. As an alternative strategy, and to enable intramuscular (IM) administration of mRNA-encoded mAbs, we describe a nanostructured lipid carrier for delivery of an alphavirus replicon encoding a previously described highly neutralizing human mAb, ZIKV-117. Using a lethal Zika virus challenge model in mice, our studies show robust protection following alphavirus-driven expression of ZIKV-117 mRNA when given by IM administration as pre-exposure prophylaxis or post-exposure therapy.

Rapid production of a chimeric antibody-antigen fusion protein based on 2A-peptide cleavage and green fluorescent protein expression in CHO cells

To enable large-scale antibody production, the creation of a stable, high producer cell line is essential. This process often takes longer than 6 months using standard limited dilution techniques and is very labor intensive. The use of a tri-cistronic vector expressing green fluorescent protein (GFP) and both antibody chains, separated by a GT2A peptide sequence, allows expression of all proteins under a single promotor in equimolar ratios. By combining the advantages of 2A peptide cleavage and single cell sorting, a chimeric antibody-antigen fusion protein that contained the variable domains of mouse IgG with a porcine IgA constant domain fused to the FedF antigen could be produced in CHO-K1 cells. After transfection, a strong correlation was found between antibody production and GFP expression (r = 0.69) using image analysis of formed monolayer patches. This enables the rapid selection of GFP-positive clones using automated image analysis for the selection of high producer clones. This vector design allowed the rapid selection of high producer clones within a time-frame of 4 weeks after transfection. The highest producing clone had a specific antibody productivity of 2.32 pg/cell/day. Concentrations of 34 mg/L were obtained using shake-flask batch culture. The produced recombinant antibody showed stable expression, binding and minimal degradation. In the future, this antibody will be assessed for its effectiveness as an oral vaccine antigen.

Targeting Aspergillus fumigatus Crf Transglycosylases With Neutralizing Antibody Is Relevant but Not Sufficient to Erase Fungal Burden in a Neutropenic Rat Model

Aspergillus fumigatus is an airborne opportunistic fungal pathogen responsible for severe infections. Among them, invasive pulmonary aspergillosis has become a major concern as mortality rates exceed 50% in immunocompromised hosts. In parallel, allergic bronchopulmonary aspergillosis frequently encountered in cystic fibrosis patients, is also a comorbidity factor. Current treatments suffer from high toxicity which prevents their use in weakened subjects, resulting in impaired prognostic. Because of their low toxicity and high specificity, anti-infectious therapeutic antibodies could be a new alternative to conventional therapeutics. In this study, we investigated the potential of Chitin Ring Formation cell wall transglycosylases of A. fumigatus to be therapeutic targets for therapeutic antibodies. We demonstrated that the Crf target was highly conserved, regardless of the pathophysiological context; whereas the CRF1 gene was found to be 100% conserved in 92% of the isolates studied, Crf proteins were expressed in 98% of the strains. In addition, we highlighted the role of Crf proteins in fungal growth, using a deletion mutant for CRF1 gene, for which a growth decrease of 23.6% was observed after 48 h. It was demonstrated that anti-Crf antibodies neutralized the enzymatic activity of recombinant Crf protein, and delayed fungal growth by 12.3% in vitro when added to spores. In a neutropenic rat model of invasive pulmonary aspergillosis, anti-Crf antibodies elicited a significant recruitment of neutrophils, macrophages and T CD4 lymphocytes but it was not correlated with a decrease of fungal burden in lungs and improvement in survival. Overall, our study highlighted the potential relevance of targeting Crf cell wall protein (CWP) with therapeutic antibodies.

Viruses, IRESs, and a universal translation initiation mechanism

Internal ribosome entry sites (IRESs) are cis-acting RNA elements capable of recruiting ribosomes and initiating translation on an internal portion of an mRNA. This is divergent from canonical eukaryotic translation initiation, where the 5' cap is recognized by initiation factors (IFs) that recruit the ribosome to initiate translation of the encoded peptide. All known IRESs are capable of initiating translation in a cap-independent manner, and are therefore not constrained by the absence or presence of a 5' m⁷G cap. In addition to being cap-independent, IRES-mediated translation often uses only a subset of IFs allowing them to function independently of canonical initiation. The ability to function independently of the canonical translation initiation pathway allows IRESs to mediate gene expression when cap-dependent translation has been inhibited. Recent reports of viral IRESs capable of initiating translation across taxonomic domains (Eukarya and Bacteria) have sparked interest in designing gene expression systems compatible with multiple organisms. The ability to drive translation independent of cellular context using a common mechanism would have a wide range of applications ranging from agriculture biotechnology to the development of antiviral drugs. Here we discuss IRES-mediated translation and critically compare the available mechanistic and structural information. A particular focus will be on IRES-meditated translation across domains of life (viral and cellular IRESs) , IRES bioengineering and the possibility of an evolutionary conserved translation initiation mechanism.

An IRES-Mediated Tricistronic Vector for Efficient Generation of Stable, High-Level Monoclonal Antibody Producing CHO DG44 Cell Lines

The generation of stable, high-level monoclonal antibody (mAb) producing cell lines remains a major challenge in biopharmaceutical industry. The commonly used plasmid vectors for mAb expression, which express light chain (LC), heavy chain (HC), and selection marker genes on separate vectors or via multiple promoters on a single vector, are not able to accurately control the ratio of LC over HC expression and tend to result in non-expressing clones. To overcome these issues, we have developed a tricistronic vector using two internal ribosome entry sites (IRES) to express the LC, HC, and dihydrofolate reductase (DHFR) selection marker genes in one transcript. In this tricistronic vector, the three genes are under the control of a hapten-modified human cytomegalovirus (hCMV) promoter containing a core CpG island element (IE) to enhance the production stability. The LC gene is arranged as the first cistron followed by a wild-type IRES to control the HC expression. Such design expresses excess LC polypeptides which enhance mAb expression level and reduce aggregate. A mutated IRES with attenuated strength is applied on DHFR to reduce its expression for enhancing the stringency of selection for high producers. This vector allows easy generation of stable, high mAb producing CHO DG44 pools and clones for antibody development and manufacturing.

Cell-free synthesis of functional antibodies using a coupled in vitro transcription-translation system based on CHO cell lysates

Antibodies are indispensable tools for basic research as well as diagnostic and therapeutic applications. Consequently, the development of alternative manufacturing strategies which circumvent the hurdles connected to conventional antibody production technologies is of enormous interest. To address this issue, we demonstrate the synthesis of complex antibody formats, in particular immunoglobulin G (IgG) and single-chain variable fragment Fc fusion (scFv-Fc), in a microsome-containing cell-free system based on translationally active chinese hamster ovary (CHO) cell lysates. To mimic the environment for antibody folding and assembly present in living cells, antibody genes were fused to an endoplasmic reticulum (ER)-specific signal sequence. Signal-peptide induced translocation of antibody polypeptide chains into the lumen of ER microsomes was found to be the prerequisite for antibody chain assembly and functionality. In this context, we show the rapid synthesis of antibody molecules in different reaction formats, including batch and continuous-exchange cell-free (CECF) reactions, depending on the amount of protein needed for further analysis. In addition, we demonstrate site-specific and residue-specific labeling of antibodies with fluorescent non-canonical amino acids. In summary, our study describes a novel antibody production platform which combines the highly efficient mammalian protein folding machinery of CHO cells with the benefits of cell-free protein synthesis.

PhiC31 integrase can improve the efficiency of different construct designs for monoclonal antibody expression in CHO cells

OBJECTIVES

Several types of expression vectors have been used for recombinant protein expression in Chinese hamster ovary cells (CHO) which usually result in variable and unstable levels of expression.

METHODS AND RESULTS

In this study, we have compared the mAb0014 expression level of single ORF/IRES vector and dual ORF vector in the presence and absence of phiC31 integrase targeting system. Both expression vectors contain an elongation factor 1α (EF1α) promoter upstream of LC and harboring an attB site. CHO-S cells were co-transfected with single ORF/IRES or dual ORF vectors along with a phiC31 integrase expression vector which can catalyze recombination between attB site and pseudo-attP sites presented in the mammalian genome. Our results demonstrated that dual ORF vector in the presence of phiC31 integrase expression vectors (+FC31 2P) generated more recombinant antibody in comparison to its negative control (-FC31 2P). Moreover, both of +FC31 2P and -FC31 2P cell pools yield higher recombinant protein in comparison to single ORF/IRES vector (FC31 IRES) cell pools. Stability of expression in phiC31 co-transfected cell pools (+FC31 2P and +FC31 IRES) had no considerable changes.

CONCLUSIONS

Our results indicated that the dual ORF vector using integrase can support the generation of cell lines with stable transgene expression at an elevated mAb relative to single ORF/IRES vector.

A simple method to determine IgG light chain to heavy chain polypeptide ratios expressed by CHO cells

OBJECTIVES

To establish a high-throughput method for determination of antibodies intra- and extracellular light chain (LC) to heavy chain (HC) polypeptide ratio as screening parameter during cell line development.

RESULTS

Chinese Hamster Ovary (CHO) TurboCell pools containing different designed vectors supposed to result in different LC:HC polypeptide ratios were generated by targeted integration. Cell culture supernatants and cell lysates of a fed batch experiment were purified by combined Protein A and anti-kappa affinity batch purification in 96-well format. Capture of all antibodies and their fragments allowed the determination of the intra- and extracellular LC:HC peptide ratios by reduced SDS capillary electrophoresis. Results demonstrate that the method is suitable to show the significant impact of the vector design on the intra- and extracellular LC:HC polypeptide ratios.

CONCLUSION

Determination of LC:HC polypeptide ratios can give important information in vector design optimization leading to CHO cell lines with optimized antibody assembly and preferred product quality.

A Human Anti-Toll Like Receptor 4 Fab Fragment Inhibits Lipopolysaccharide-Induced Pro-Inflammatory Cytokines Production in Macrophages

The results of clinical and experimental studies suggest that endotoxin/toll-like receptor 4 (TLR4)-mediated proinflammatory and profibrotic signaling activation is critical in the development of hepatic fibrosis. However, studies examining the role of specific TLR4 inhibitor are still lacking. The present study was aimed to prepare a human anti-TLR4 Fab fragment, named hTLR4-Fab01, and to explore its immune activity. We screened the positive clone of anti-human TLR4 phagemid from a human phage-display antibody library using recombinant TLR4 protein, which was used as template cDNA for the amplification of variable regions of the heavy (V_H) chain and light chain (V_L), then coupled with highly conserved regions of the heavy chain domain 1 (C_H1) and the light chain (C_L), respectively. Thus, the prokaryotic expression vector pETDuet-1 of hTLR4-Fab01 was constructed and transformed into Escherichia coli (E. coli) BL21. The characteristic of hTLR4-Fab01 was examined by SDS-PAGE, Western blotting, ELISA, affinity and kinetics assay. Further, our data demonstrate that hTLR4-Fab01 could specifically bind to TLR4, and its treatment obviously attenuated the proinflammatory effect, characterized by less LPS-induced TNF-α, IL-1, IL-6 and IL-8 production in human macrophages. In conclusion, we have successfully prepared the hTLR4-Fab01 with efficient activity for blocking LPS-induced proinflammatory cytokines production, suggesting that the hTLR4-Fab01 may be a potential candidate for the treatment of hepatic fibrosis.

Cleavage efficient 2A peptides for high level monoclonal antibody expression in CHO cells

Linking the heavy chain (HC) and light chain (LC) genes required for monoclonal antibodies (mAb) production on a single cassette using 2A peptides allows control of LC and HC ratio and reduces non-expressing cells. Four 2A peptides derived from the foot-and-mouth disease virus (F2A), equine rhinitis A virus (E2A), porcine teschovirus-1 (P2A) and Thosea asigna virus (T2A), respectively, were compared for expression of 3 biosimilar IgG1 mAbs in Chinese hamster ovary (CHO) cell lines. HC and LC were linked by different 2A peptides both in the absence and presence of GSG linkers. Insertion of a furin recognition site upstream of 2A allowed removal of 2A residues that would otherwise be attached to the HC. Different 2A peptides exhibited different cleavage efficiencies that correlated to the mAb expression level. The relative cleavage efficiency of each 2A peptide remains similar for expression of different IgG1 mAbs in different CHO cells. While complete cleavage was not observed for any of the 2A peptides, GSG linkers did enhance the cleavage efficiency and thus the mAb expression level. T2A with the GSG linker (GT2A) exhibited the highest cleavage efficiency and mAb expression level. Stably amplified CHO DG44 pools generated using GT2A had titers 357, 416 and 600 mg/L for the 3 mAbs in shake flask batch cultures. Incomplete cleavage likely resulted in incorrectly processed mAb species and aggregates, which were removed with a chromatin-directed clarification method and protein A purification. The vector and methods presented provide an easy process beneficial for both mAb development and manufacturing.

Dynamics of unfolded protein response in recombinant CHO cells

Genes in the protein secretion pathway have been targeted to increase productivity of monoclonal antibodies in Chinese hamster ovary cells. The results have been highly variable depending on the cell type and the relative amount of recombinant and target proteins. This paper presents a comprehensive study encompassing major components of the protein processing pathway in the endoplasmic reticulum (ER) to elucidate its role in recombinant cells. mRNA profiles of all major ER chaperones and unfolded protein response (UPR) pathway genes are measured at a series of time points in a high-producing cell line under the dynamic environment of a batch culture. An initial increase in IgG heavy chain mRNA levels correlates with an increase in productivity. We observe a parallel increase in the expression levels of majority of chaperones. The chaperone levels continue to increase until the end of the batch culture. In contrast, calreticulin and ERO1-L alpha, two of the lowest expressed genes exhibit transient time profiles, with peak induction on day 3. In response to increased ER stress, both the GCN2/PKR-like ER kinase and inositol-requiring enzyme-1alpha (Ire1α) signalling branch of the UPR are upregulated. Interestingly, spliced X-Box binding protein 1 (XBP1s) transcription factor from Ire1α pathway is detected from the beginning of the batch culture. Comparison with the expression levels in a low producer, show much lower induction at the end of the exponential growth phase. Thus, the unfolded protein response strongly correlates with the magnitude and timing of stress in the course of the batch culture.

Scalable transient protein expression

Transient transfection is a well-established method to rapidly express recombinant proteins from mammalian cells. Accelerating activity in biotherapeutic drug development, demand for protein-based reagents, vaccine research, and large initiatives in structural and functional studies of proteins have propelled the need to generate moderate to high amounts of recombinant proteins and other macromolecules in a flexible and rapid manner. Progress over the last 10-15 years has demonstrated that transient transfections can be reliably and readily scaled up to handle milliliters to tens of liters of cells in suspension culture and obtain milligrams to grams of recombinant protein in a process that requires only days to weeks. This review will summarize developments in this field, properties of the components of a transient expression system that enable maximal protein production, and detailed protocols for this application.

An internal ribosome entry site (IRES) mutant library for tuning expression level of multiple genes in mammalian cells

A set of mutated Encephalomyocarditis virus (EMCV) internal ribosome entry site (IRES) elements with varying strengths is generated by mutating the translation initiation codons of 10^th, 11^th, and 12^th AUG to non-AUG triplets. They are able to control the relative expression of multiple genes over a wide range in mammalian cells in both transient and stable transfections. The relative strength of each IRES mutant remains similar in different mammalian cell lines and is not gene specific. The expressed proteins have correct molecular weights. Optimization of light chain over heavy chain expression by these IRES mutants enhances monoclonal antibody expression level and quality in stable transfections. Uses of this set of IRES mutants can be extended to other applications such as synthetic biology, investigating interactions between proteins and its complexes, cell engineering, multi-subunit protein production, gene therapy, and reprogramming of somatic cells into stem cells.

Expression of recombinant antibodies

Recombinant antibodies are highly specific detection probes in research, diagnostics, and have emerged over the last two decades as the fastest growing class of therapeutic proteins. Antibody generation has been dramatically accelerated by in vitro selection systems, particularly phage display. An increasing variety of recombinant production systems have been developed, ranging from Gram-negative and positive bacteria, yeasts and filamentous fungi, insect cell lines, mammalian cells to transgenic plants and animals. Currently, almost all therapeutic antibodies are still produced in mammalian cell lines in order to reduce the risk of immunogenicity due to altered, non-human glycosylation patterns. However, recent developments of glycosylation-engineered yeast, insect cell lines, and transgenic plants are promising to obtain antibodies with "human-like" post-translational modifications. Furthermore, smaller antibody fragments including bispecific antibodies without any glycosylation are successfully produced in bacteria and have advanced to clinical testing. The first therapeutic antibody products from a non-mammalian source can be expected in coming next years. In this review, we focus on current antibody production systems including their usability for different applications.

Comparison of internal ribosome entry site (IRES) and Furin-2A (F2A) for monoclonal antibody expression level and quality in CHO cells

Four versions of tricistronic vectors expressing IgG1 light chain (LC), IgG1 heavy chain (HC), and dihydrofolate reductase (DHFR) in one transcript were designed to compare internal ribosome entry site (IRES) and furin-2A (F2A) for their influence on monoclonal antibody (mAb) expression level and quality in CHO DG44 cells. LC and HC genes are arranged as either the first or the second cistron. When using mAb quantification methods based on the detection antibodies against HC Fc region, F2A-mediated tricistronic vectors appeared to express mAb at higher levels than the IRES-mediated tricistronic vectors in both transient and stable transfections. Further analysis revealed that more than 40% of products detected in stably transfected pools generated using the two F2A-mediated tricistronic vectors were aggregates. LC and HC from the F2A stably transfected pools were not properly processed, giving rise to LC+F2A+HC or HC+F2A+LC fusion proteins, LC and HC polypeptides with F2A remnants, and incorrectly cleaved signal peptides. Both IRES-mediated tricistronic vectors express mAb with correct sizes and signal peptide cleavage. Arrangement of LC as the first cistron in the IRES-mediated tricistronic vectors exhibits increased mAb expression level, better growth, and minimized product aggregation, while arrangement of HC as first cistron results in low expression, slower growth, and high aggregation. The results obtained will be beneficial for designing vectors that enhance mAb expression level and quality in mammalian cells.

Design, expression and characterization of a novel coexpression system of two antiarthritic molecules

The complexity of rheumatoid arthritis (RA) pathogenesis makes combined blockade of multiple targets an attractive therapeutic strategy. The combination therapy with anti-TNF plus anti-T-cell has been mostly reported to provide greater efficacy than anti-TNF alone. TNFR (p75)-Fc fusion protein, which has been proven effective in clinics, is chosen as the TNF antagonist in this study. CTLA4-FasL fusion molecule, which has been well characterized in our previous studies for its suppressive effect in rat arthritis model, is chosen as the T-cell antagonist. In this study, furin cleavage site and 2A self-processing sequence were introduced to link upstream TNFR-Fc and downstream CTLA4-FasL and mediate separate coexpression of the two fusion proteins in a single recombinant adeno-associated virus (rAAV) vector. Using this expression system, we generated two fusion proteins with same size as their individual counterparts in vitro and in vivo, and the proteins desirably retained their parent biological activities. In vivo results demonstrated that furin-2A technology is able to regulate separate coexpression of these proteins under arthritic inflammatory conditions. This study describes a single rAAV vector for production of two antiarthritic molecules antagonizing both TNF and T cells, which may serve as an attractive expression system for RA gene therapy.

Suppression of p75 neurotrophin receptor surface expression with intrabodies influences Bcl-xL mRNA expression and neurite outgrowth in PC12 cells

BACKGROUND

Although p75 neurotrophin receptor (p75NTR) is the first neurotrophin receptor isolated, its diverse physiological functions and signaling have remained elusive for many years. Loss-of-function phenotypic analyses for p75NTR were mainly focused at the genetic level; however these approaches were impacted by off-target effect, insufficient stability, unspecific stress response or alternative active splicing products. In this study, p75NTR surface expression was suppressed for the first time at the protein level by endoplasmic reticulum (ER) retained intrabodies.

RESULTS

Three monoclonal recombinant antibody fragments (scFv) with affinities in the low nanomolar range to murine p75NTR were isolated by antibody phage display. To suppress p75NTR cell surface expression, the encoding genes of these scFvs extended by the ER retention peptide KDEL were transiently transfected into the neuron-like rat pheochromocytoma cell line PC12 and the mouse neuroblastoma x mouse spinal cord hybrid cell line NSC19. The ER retained intrabody construct, SH325-G7-KDEL, mediated a downregulation of p75NTR cell surface expression as shown by flow cytometry. This effect was maintained over a period of at least eight days without activating an unfolded protein response (UPR). Moreover, the ER retention of p75NTR resulted in downregulation of mRNA levels of the anti-apoptotic protein Bcl-xL as well as in strong inhibition of NGF-induced neurite outgrowth in PC12 cells.

CONCLUSION

The ER retained intrabody SH325-G7-KDEL not only induces phenotypic knockdown of this p75NTR but also p75NTR-associated cellular responses in PC12 cells.

Mammalian stable expression of biotherapeutics

Many therapeutically relevant proteins, like IgG antibodies, are highly complex, multimeric glycoproteins that are difficult to express in microbial systems and thus usually produced in mammalian host cells. During the past two decades, stable mammalian expression technologies have made huge progress resulting in highly increased speed of cell line development and yield of manufacturing processes. Here, we give an overview of technologies that are applied at different stages of state-of-the-art cell line development processes for biomanufacturing.

Impact of gene vector design on the control of recombinant monoclonal antibody production by Chinese hamster ovary cells

In this study, we systematically compare two vector design strategies for recombinant monoclonal antibody (Mab) synthesis by Chinese hamster ovary (CHO) cells; a dual open reading frame (ORF) expression vector utilizing separate cytomegalovirus (CMV) promoters to drive heavy chain (HC) and light chain (LC) expression independently, and a single ORF vector design employing a single CMV promoter to drive HC and LC polypeptide expression joined by a foot and mouth disease virus F2A polypeptide self-cleaving linker sequence. Initial analysis of stable transfectants showed that transfectants utilizing the single ORF vector designs exhibited significantly reduced Mab production. We employed an empirical modeling strategy to quantitatively describe the cellular constraints on recombinant Mab synthesis in all stable transfectants. In all transfectants, an intracellular molar excess of LC polypeptide over HC polypeptide was observed. For CHO cells transfected with the single ORF vectors, model-predicted, and empirical intracellular intermediate levels could only be reconciled by inclusion of nascent HC polypeptide degradation. Whilst a local sensitivity analysis showed that qMab of all transfectants was primarily constrained by recombinant mRNA translation rate, our data indicated that all single ORF transfectants exhibited a reduced level of recombinant gene transcription and that Mab folding and assembly reactions generically exerted greater control over qMab. We infer that the productivity of single ORF transfectants is limited by ER processing/degradation "capacity" which sets a limit on transcriptional input. We conclude that gene vector design for oligomeric recombinant proteins should be based on an understanding of protein-specific synthetic kinetics rather than polypeptide stoichiometry.

Stable expression and purification of a functional processed Fab' fragment from a single nascent polypeptide in CHO cells expressing the mCAT-1 retroviral receptor

Monoclonal antibodies and derivative formats such as Fab' fragments are used in a broad range of therapeutic, diagnostic and research applications. New systems and methodologies that can improve the production of these proteins are consequently of much interest. Here we present a novel approach for the rapid production of processed Fab' fragments in a CHO cell line that has been engineered to express the mouse cationic amino acid transporter receptor 1 (mCAT-1). This facilitated the introduction of the target antibody gene through retroviral transfection, rapidly producing stable expression. Using this system, we designed a single retroviral vector construct for the expression of a target Fab' fragment as a single polypeptide with a furin cleavage site and a FMDV 2A self-cleaving peptide introduced to bridge the light and truncated heavy chain regions. The introduction of these cleavage motifs ensured equimolar expression and processing of the heavy and light domains as exemplified by the production of an active chimeric Fab' fragment against the Fas receptor, routinely expressed in 1-2mg/L yield in spinner-flask cell cultures. These results demonstrate that this method could have application in the facile production of bioactive Fab' fragments.

Combination of the 2A/furin technology with an animal component free cell line development platform process

The recently described 2A/furin technology combines both chains of the antibody in a single open reading frame. Upon translation and secretion, the peptide is processed by the cell to generate native fully functional IgG antibodies. Here, we describe the results of an evaluation study of this technology for an industrial CHO cell line development process. The 2A/furin expression cassette setup was combined with a Novartis vector system. A transfection, selection, and cloning procedure in chemically defined media was established at Novartis and applied for a monoclonal test antibody. The productivity of 2A/furin-vector-derived clones in non-optimized generic shake flask fed-batch models was in a comparable range with clones derived from the reference control vector. Higher clonal production stability was seen for the majority of clones generated with the 2A/furin technology compared to the clones generated with the reference control vector. Product quality was analyzed by SDS-PAGE and no significant difference was detected between the two systems. Thus, it was shown that the 2A/furin technology can be successfully combined with a Novartis CHO expression system and platform. Due to the single ORF setup, the 2A/furin technology may therefore offer a suitable approach to reduce vector size and complexity.

Oligomeric forms of single chain immunoglobulin (scIgG)

Assembly of immunoglobulin G (IgG) molecules from two heavy and two light chains can be facilitated by connecting the light chain to the heavy chain by an oligopeptide linker. Production of the anti-lysozyme D1.3-single chain (sc) IgG1 in HEK293T cells yielded up to 8 mg/L functional scIgG polypeptide. Size exclusion chromatography of material purified by protein-A affinity chromatography revealed that the majority of the D1.3-scIgG1 molecules were 150 kDa monomers, with a K(D) of 1.8 x 10(-10) M measured by surface plasmon resonance; however, significant fractions of scIgG dimers and oligomers with molecular masses of 300 kDa and >600 kDa, respectively, were identified. The oligomerization resulted in an increased avidity. The observed oligomerization capability may allow new approaches for the generation of bispecific/multivalent antibodies.

Designed auto-assembly of nanostreptabodies for rapid tissue-specific targeting in vivo

Molecular medicine can benefit greatly from antibodies that deliver therapeutic and imaging agents to select organs and diseased tissues. Yet the development of complex and defined composite nanostructures remains a challenge that requires both designed stoichiometric assembly and superior in vivo testing ability. Here, we generate nanostructures called nanostreptabodies by controlled sequential assembly of biotin-engineered antibody fragments on a streptavidin scaffold with a defined capacity for additional biotinylated payloads such as other antibodies to create bispecific antibodies as well as organic and non-organic moieties. When injected intravenously, these novel and stable nanostructures exhibit exquisite targeting with tissue-specific imaging and delivery, including rapid transendothelial transport that enhances tissue penetration. This "tinkertoy construction" strategy provides a very flexible and efficient way to link targeting vectors with reporter and/or effector agents, thereby providing virtually endless combinations potentially useful for multipurpose molecular and functional imaging in vivo as well as therapies.

Alternative-splicing-based bicistronic vectors for ratio-controlled protein expression and application to recombinant antibody production

In the last decade polycistronic vectors have become essential tools for both basic science and gene therapy applications. In order to co-express heterologous polypeptides, different systems have been developed from Internal Ribosome Entry Site (IRES) based vectors to the use of the 2A peptide. Unfortunately, these methods are not fully suitable for the efficient and reproducible modulation of the ratio between the proteins of interest. Here we describe a novel bicistronic vector type based on the use of alternative splicing. By modifying the consensus sequence that governs splicing, we demonstrate that the ratio between the synthesized proteins could easily vary from 1 : 10 to 10 : 1. We have established this system with luciferase genes and we extended its application to the production of recombinant monoclonal antibodies. We have shown that these vectors could be used in several typical cell lines with similar efficiencies. We also present an adaptation of these vectors to hybrid alternative splicing/IRES constructs that allow a ratio-controlled expression of proteins of interest in stably transfected cell lines.