Engineering novel Lec1 glycosylation mutants in CHO-DUKX cells: Molecular insights and effector modulation of N-acetylglucosaminyltransferase I

From Efficiency to Yield: Exploring Recent Advances in CHO Cell Line Development for Monoclonal Antibodies

The increasing demand for biosimilar monoclonal antibodies (mAbs) has prompted the development of stable high-producing cell lines while simultaneously decreasing the time required for screening. Existing platforms have proven inefficient, resulting in inconsistencies in yields, growth characteristics, and quality features in the final mAb products. Selecting a suitable expression host, designing an effective gene expression system, developing a streamlined cell line generation approach, optimizing culture conditions, and defining scaling-up and purification strategies are all critical steps in the production of recombinant proteins, particularly monoclonal antibodies, in mammalian cells. As a result, an active area of study is dedicated to expression and optimizing recombinant protein production. This review explores recent breakthroughs and approaches targeted at accelerating cell line development to attain efficiency and consistency in the synthesis of therapeutic proteins, specifically monoclonal antibodies. The primary goal is to bridge the gap between rising demand and consistent, high-quality mAb production, thereby benefiting the healthcare and pharmaceutical industries.

Sialylation as an Important Regulator of Antibody Function

Antibodies play a critical role in linking the adaptive immune response to the innate immune system. In humans, antibodies are categorized into five classes, IgG, IgM, IgA, IgE, and IgD, based on constant region sequence, structure, and tropism. In serum, IgG is the most abundant antibody, comprising 75% of antibodies in circulation, followed by IgA at 15%, IgM at 10%, and IgD and IgE are the least abundant. All human antibody classes are post-translationally modified by sugars. The resulting glycans take on many divergent structures and can be attached in an N-linked or O-linked manner, and are distinct by antibody class, and by position on each antibody. Many of these glycan structures on antibodies are capped by sialic acid. It is well established that the composition of the N-linked glycans on IgG exert a profound influence on its effector functions. However, recent studies have described the influence of glycans, particularly sialic acid for other antibody classes. Here, we discuss the role of glycosylation, with a focus on terminal sialylation, in the biology and function across all antibody classes. Sialylation has been shown to influence not only IgG, but IgE, IgM, and IgA biology, making it an important and unappreciated regulator of antibody function.

New Opportunities in Glycan Engineering for Therapeutic Proteins

Glycans as sugar polymers are important metabolic, structural, and physiological regulators for cellular and biological functions. They are often classified as critical quality attributes to antibodies and recombinant fusion proteins, given their impacts on the efficacy and safety of biologics drugs. Recent reports on the conjugates of N-acetyl-galactosamine and mannose-6-phosphate for lysosomal degradation, Fab glycans for antibody diversification, as well as sialylation therapeutic modulations and O-linked applications, have been fueling the continued interest in glycoengineering. The current advancements of the human glycome and the development of a comprehensive network in glycosylation pathways have presented new opportunities in designing next-generation therapeutic proteins.

Discovery of a Series of Pyrimidine Carboxamides as Inhibitors of Vanin-1

A diaryl ketone series was identified as vanin-1 inhibitors from a high-throughput screening campaign. While this novel scaffold provided valuable probe 2 that was used to build target confidence, concerns over the ketone moiety led to the replacement of this group. The successful replacement of this moiety was achieved with pyrimidine carboxamides derived from cyclic secondary amines that were extensively characterized using biophysical and crystallographic methods as competitive inhibitors of vanin-1. Through optimization of potency and physicochemical and ADME properties, and guided by co-crystal structures with vanin-1, 3 was identified with a suitable profile for advancement into preclinical development.

A new insect cell line engineered to produce recombinant glycoproteins with cleavable N-glycans

Glycoproteins are difficult to crystallize because they have heterogeneous glycans composed of multiple monosaccharides with considerable rotational freedom about their O-glycosidic linkages. Crystallographers studying N-glycoproteins often circumvent this problem by using β1,2-N-acetylglucosaminyltransferase I (MGAT1)–deficient mammalian cell lines, which produce recombinant glycoproteins with immature N-glycans. These glycans support protein folding and quality control but can be removed using endo-β-N-acetylglucosaminidase H (Endo H). Many crystallographers also use the baculovirus-insect cell system (BICS) to produce recombinant proteins for their work but have no access to an MGAT1-deficient insect cell line to facilitate glycoprotein crystallization in this system. Thus, we used BICS-specific CRISPR–Cas9 vectors to edit the Mgat1 gene of a rhabdovirus-negative Spodoptera frugiperda cell line (Sf-RVN) and isolated a subclone with multiple Mgat1 deletions, which we named Sf-RVN^Lec1. We found that Sf-RVN and Sf-RVN^Lec1 cells had identical growth properties and served equally well as hosts for baculovirus-mediated recombinant glycoprotein production. N-glycan profiling showed that a total endogenous glycoprotein fraction isolated from Sf-RVN^Lec1 cells had only immature and high mannose-type N-glycans. Finally, N-glycan profiling and endoglycosidase analyses showed that the vast majority of the N-glycans on three recombinant glycoproteins produced by Sf-RVN^Lec1 cells were Endo H-cleavable Man₅GlcNAc₂ structures. Thus, this study yielded a new insect cell line for the BICS that can be used to produce recombinant glycoproteins with Endo H-cleavable N-glycans. This will enable researchers to combine the high productivity of the BICS with the ability to deglycosylate recombinant glycoproteins, which will facilitate efforts to determine glycoprotein structures by X-ray crystallography.

Modulation of high mannose levels in N-linked glycosylation through cell culture process conditions to increase antibody-dependent cell-mediated cytotoxicity activity for an antibody biosimilar

The regulatory approval of a biosimilar product is contingent on the favorable comparability of its safety and efficacy to that of the innovator product. As such, it is important to match the critical quality attributes of the biosimilar product to that of the innovator product. The N-glycosylation profile of a monoclonal antibody (mAb) can influence effector function activities such as antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity. In this study, we describe efforts to modulate the high-mannose (HM) levels of a biosimilar mAb produced in a Chinese hamster ovary cell fed-batch process. Because the HM level of the mAb was observed to impact ADCC activity, it was desirable to match it to the innovator mAb's levels. Several cell culture process related factors known to modulate the HM content of N-glycosylation were investigated, including osmolality, ammonium chloride (NH₄ Cl) addition, glutamine concentration, monensin addition, and the addition of alternate sugars and amino sugars to the feed medium. The process conditions evaluated varied in impact on HM levels, process performance and product quality. One condition, the addition of alternate sugars and amino sugars to feed medium, was identified as the preferred method for increasing HM levels with minimal disruptions to process performance or other product quality attributes. Interestingly, a secondary interaction between sugar and amino sugar supplemented feeds and osmolality was observed during process scale-up. These studies demonstrate sugar and amino sugar concentrations and osmolality are critical variables to evaluate to match HM content in biosimilar and their innovator mAbs.

Point mutations that inactivate MGAT4D-L, an inhibitor of MGAT1 and complex N-glycan synthesis

The membrane-bound, long form of MGAT4D, termed MGAT4D-L, inhibits MGAT1 activity in transfected cells and reduces the generation of complex N-glycans. MGAT1 is the GlcNAc-transferase that initiates complex and hybrid N-glycan synthesis. We show here that Drosophila MGAT1 was also inhibited by MGAT4D-L in S2 cells. In mammalian cells, expression of MGAT4D-L causes the substrate of MGAT1 (Man₅GlcNAc₂Asn) to accumulate on glycoproteins, a change that is detected by the lectin Galanthus nivalis agglutinin (GNA). Using GNA binding as an assay for the inhibition of MGAT1 in MGAT4D-L transfectants, we performed site-directed mutagenesis to determine requirements for MGAT1 inhibition. Deletion of 25 amino acids (aa) from the C terminus inactivated MGAT4D-L, but deletion of 20 aa did not. Conversion of the five key amino acids (PSLFQ) to Ala, or deletion of PSLFQ in the context of full-length MGAT4D-L, also inactivated MGAT1 inhibitory activity. Nevertheless, mutant, inactive MGAT4D-L interacted with MGAT1 in co-immuno-precipitation experiments. The PSLFQ sequence also occurs in MGAT4A and MGAT4B GlcNAc-transferases. However, neither inhibited MGAT1 in transfected CHO cells. MGAT4D-L inhibitory activity could be partially transferred by attaching PSLFQ or the 25-aa C terminus of MGAT4D-L to the C terminus of MGAT1. Mutation of each amino acid in PSLFQ to Ala identified both Leu and Phe as independently essential for MGAT4D-L activity. Thus, replacement of either Leu-395 or Phe-396 with Ala led to inactivation of MGAT4D-L inhibitory activity. These findings provide new insights into the mechanism of inhibition of MGAT1 by MGAT4D-L, and for the development of small molecule inhibitors of MGAT1.

A Detailed Protocol for Generation of Therapeutic Antibodies with Galactosylated Glycovariants at Laboratory Scale Using In-Vitro Glycoengineering Technology

N-linked glycosylation is an important post translational modification that occurs on Asparagine 297 residue or a homologous position on the Fc portion of monoclonal antibodies (mAbs). mAb Fc glycans play important roles in antibody structure, stability, and function including effector function and pharmacokinetics. The Fc glycans are made up of a wide variety of sugars including galactose, mannose, and sialic acid. The role of galactose in mediating antibody effector functions is not well understood. Hence, there is widespread interest in the antibody research community to understand the role of galactose in antibody effector functions as galactose is a major constituent of antibody glycans. This requires generation of highly enriched galactosylated variants that has been very challenging via cell culture process. To tackle this challenge, we developed a laboratory scale biochemical process to produce highly enriched galactosylated variants. In this article, we report optimized lab-scale workflows and detailed protocols for generation of deglycosylated, hypo-galactosylated and hyper-galactosylated variants of IgG therapeutic antibodies using the in-vitro glycoengineering technology. The optimized workflows offer short turnaround time and produce highly enriched deglycosylated/hypo-galactosylated/hyper-galactosylated IgG glycovariants, with high purity & molecular integrity as demonstrated by data from an example IgG.

Structure-function relationships of the soluble form of the antiaging protein Klotho have therapeutic implications for managing kidney disease

The fortuitously discovered antiaging membrane protein αKlotho (Klotho) is highly expressed in the kidney, and deletion of the Klotho gene in mice causes a phenotype strikingly similar to that of chronic kidney disease (CKD). Klotho functions as a co-receptor for fibroblast growth factor 23 (FGF23) signaling, whereas its shed extracellular domain, soluble Klotho (sKlotho), carrying glycosidase activity, is a humoral factor that regulates renal health. Low sKlotho in CKD is associated with disease progression, and sKlotho supplementation has emerged as a potential therapeutic strategy for managing CKD. Here, we explored the structure-function relationship and post-translational modifications of sKlotho variants to guide the future design of sKlotho-based therapeutics. Chinese hamster ovary (CHO)- and human embryonic kidney (HEK)-derived WT sKlotho proteins had varied activities in FGF23 co-receptor and β-glucuronidase assays in vitro and distinct properties in vivo Sialidase treatment of heavily sialylated CHO-sKlotho increased its co-receptor activity 3-fold, yet it remained less active than hyposialylated HEK-sKlotho. MS and glycopeptide-mapping analyses revealed that HEK-sKlotho is uniquely modified with an unusual N-glycan structure consisting of N,N'-di-N-acetyllactose diamine at multiple N-linked sites, one of which at Asn-126 was adjacent to a putative GalNAc transfer motif. Site-directed mutagenesis and structural modeling analyses directly implicated N-glycans in Klotho's protein folding and function. Moreover, the introduction of two catalytic glutamate residues conserved across glycosidases into sKlotho enhanced its glucuronidase activity but decreased its FGF23 co-receptor activity, suggesting that these two functions might be structurally divergent. These findings open up opportunities for rational engineering of pharmacologically enhanced sKlotho therapeutics for managing kidney disease.

New Mammalian Expression Systems

There are an increasing number of recombinant antibodies and proteins in preclinical and clinical development for therapeutic applications. Mammalian expression systems are key to enabling the production of these molecules, and Chinese hamster ovary (CHO) cell platforms continue to be central to delivery of the stable cell lines required for large-scale production. Increasing pressure on timelines and efficiency, further innovation of molecular formats and the shift to new production systems are driving developments of these CHO cell line platforms. The availability of genome and transcriptome data coupled with advancing gene editing tools are increasing the ability to design and engineer CHO cell lines to meet these challenges. This chapter aims to give an overview of the developments in CHO expression systems and some of the associated technologies over the past few years.

Main Quality Attributes of Monoclonal Antibodies and Effect of Cell Culture Components

The culture media optimization is an inevitable part of upstream process development in therapeutic monoclonal antibodies (mAbs) production. The quality by design (QbD) approach defines the assured quality of the final product through the development stage. An important step in QbD is determination of the main quality attributes. During the media optimization, some of the main quality attributes such as glycosylation pattern, charge variants, aggregates, and low-molecular-weight species, could be significantly altered. Here, we provide an overview of how cell culture medium components affects the main quality attributes of the mAbs. Knowing the relationship between the culture media components and the main quality attributes could be successfully utilized for a rational optimization of mammalian cell culture media for industrial mAbs production.

Engineered N-acetylhexosamine-active enzymes in glycoscience

BACKGROUND

In recent years, enzymes modifying N-acetylhexosamine substrates have emerged in numerous theoretical studies as well as practical applications from biology, biomedicine, and biotechnology. Advanced enzyme engineering techniques converted them into potent synthetic instruments affording a variety of valuable glycosides.

SCOPE OF REVIEW

This review presents the diversity of engineered enzymes active with N-acetylhexosamine carbohydrates: from popular glycoside hydrolases and glycosyltransferases to less known oxidases, epimerases, kinases, sulfotransferases, and acetylases. Though hydrolases in natura, engineered chitinases, β-N-acetylhexosaminidases, and endo-β-N-acetylglucosaminidases were successfully employed in the synthesis of defined natural and derivatized chitooligomers and in the remodeling of N-glycosylation patterns of therapeutic antibodies. The genes of various N-acetylhexosaminyltransferases were cloned into metabolically engineered microorganisms for producing human milk oligosaccharides, Lewis X structures, and human-like glycoproteins. Moreover, mutant N-acetylhexosamine-active glycosyltransferases were applied, e.g., in the construction of glycomimetics and complex glycostructures, industrial production of low-lactose milk, and metabolic labeling of glycans. In the synthesis of biotechnologically important compounds, several innovative glycoengineered systems are presented for an efficient bioproduction of GlcNAc, UDP-GlcNAc, N-acetylneuraminic acid, and of defined glycosaminoglycans.

MAJOR CONCLUSIONS

The above examples demonstrate that engineering of N-acetylhexosamine-active enzymes was able to solve complex issues such as synthesis of tailored human-like glycoproteins or industrial-scale production of desired oligosaccharides. Due to the specific catalytic mechanism, mutagenesis of these catalysts was often realized through rational solutions.

GENERAL SIGNIFICANCE

Specific N-acetylhexosamine glycosylation is crucial in biological, biomedical and biotechnological applications and a good understanding of its details opens new possibilities in this fast developing area of glycoscience.

Mechanistic understanding of the cysteine capping modifications of antibodies enables selective chemical engineering in live mammalian cells

Protein modifications by intricate cellular machineries often redesign the structure and function of existing proteins to impact biological networks. Disulfide bond formation between cysteine (Cys) pairs is one of the most common modifications found in extracellularly-destined proteins, key to maintaining protein structure. Unpaired surface cysteines on secreted mammalian proteins are also frequently found disulfide-bonded with free Cys or glutathione (GSH) in circulation or culture, the mechanism for which remains unknown. Here we report that these so-called Cys-capping modifications take place outside mammalian cells, not in the endoplasmic reticulum (ER) where oxidoreductase-mediated protein disulfide formation occurs. Unpaired surface cysteines of extracellularly-arrived proteins such as antibodies are uncapped upon secretion before undergoing disulfide exchange with cystine or oxidized GSH in culture medium. This observation has led to a feasible way to selectively modify the nucleophilic thiol side-chain of cell-surface or extracellular proteins in live mammalian cells, by applying electrophiles with a chemical handle directly into culture medium. These findings provide potentially an effective approach for improving therapeutic conjugates and probing biological systems.

Antitumor Efficacy of Anti-GD2 IgG1 Is Enhanced by Fc Glyco-Engineering

The affinity of therapeutic antibodies for Fcγ receptors (FcγRs) strongly influences their antitumor potency. To generate antibodies with optimal binding and immunologic efficacy, we compared the affinities of different versions of an IgG1 Fc region that had an altered peptide backbone, altered glycans, or both. To produce IgG1 with glycans that lacked α1,6-fucose, we used CHO cells that were deficient in the enzyme UDP-N-acetylglucosamine: α-3-d-mannoside-β-1,2-N-acetylglucosaminyltransferase I (GnT1), encoded by the MGAT1 gene. Mature N-linked glycans require this enzyme, and without it, CHO cells synthesize antibodies carrying only Man5-GlcNAc2, which were more effective in antibody-dependent cell-mediated cytotoxicity (ADCC). Our engineered IgG1, hu3F8-IgG1, is specific for GD2, a neuroendocrine tumor ganglioside. Its peptide mutant is IgG1-DEL (S239D/I332E/A330L), both produced in wild-type CHO cells. When produced in GnT1-deficient CHO cells, we refer to them as IgG1n and IgG1n-DEL, respectively. Affinities for human FcγRs were measured using Biacore T-100 (on CD16 and CD32 polymorphic alleles), their immunologic properties compared for ADCC and complement-mediated cytotoxicity (CMC) in vitro, and pharmacokinetics and antitumor effects were compared in vivo in humanized mice. IgG1n and IgG1n-DEL contained only mannose and acetylglucosamine and had preferential affinity for activating CD16s, over inhibitory CD32B, receptors. In vivo, the antitumor effects of IgG1, IgG1-DEL, and IgG1n-DEL were similar but modest, whereas IgG1n was significantly more effective (P < 0.05). Thus, IgG1n antibodies produced in GnT1-deficient CHO cells may have potential as improved anticancer therapeutics. Cancer Immunol Res; 4(7); 631-8. ©2016 AACR.

Producing recombinant therapeutic glycoproteins with enhanced sialylation using CHO-gmt4 glycosylation mutant cells

Recombinant glycoprotein drugs require proper glycosylation for optimal therapeutic efficacy. Glycoprotein therapeutics are rapidly removed from circulation and have reduced efficacy if they are poorly sialylated. Ricinus communis agglutinin-I (RCA-I) was found highly toxic to wild-type CHO-K1 cells and all the mutants that survived RCA-I treatment contained a dysfunctional N-acetylglucosaminyltransferase I (GnT I) gene. These mutants are named CHO-gmt4 cells. Interestingly, upon restoration of GnT I, the sialylation of a model glycoprotein, erythropoietin, produced in CHO-gmt4 cells was shown to be superior to that produced in wild-type CHO-K1 cells. This addendum summarizes the applicability of this cell line, from transient to stable expression of the recombinant protein, and from a lab scale to an industrial scale perfusion bioreactor. In addition, CHO-gmt4 cells can be used to produce glycoproteins with mannose-terminated N-glycans. Recombinant glucocerebrosidase produced by CHO-gmt4 cells will not require glycan remodeling and may be directly used to treat patients with Gaucher disease. CHO-gmt4 cells can also be used to produce other glycoprotein therapeutics which target cells expressing mannose receptors.

A novel baculovirus vector for the production of nonfucosylated recombinant glycoproteins in insect cells

Glycosylation is an important attribute of baculovirus-insect cell expression systems, but some insect cell lines produce core α1,3-fucosylated N-glycans, which are highly immunogenic and render recombinant glycoproteins unsuitable for human use. To address this problem, we exploited a bacterial enzyme, guanosine-5'-diphospho (GDP)-4-dehydro-6-deoxy-d-mannose reductase (Rmd), which consumes the GDP-l-fucose precursor. We expected this enzyme to block glycoprotein fucosylation by blocking the production of GDP-l-fucose, the donor substrate required for this process. Initially, we engineered two different insect cell lines to constitutively express Rmd and isolated subclones with fucosylation-negative phenotypes. However, we found the fucosylation-negative phenotypes induced by Rmd expression were unstable, indicating that this host cell engineering approach is ineffective in insect systems. Thus, we constructed a baculovirus vector designed to express Rmd immediately after infection and facilitate the insertion of genes encoding any glycoprotein of interest for expression later after infection. We used this vector to produce a daughter encoding rituximab and found, in contrast to an Rmd-negative control, that insect cells infected with this virus produced a nonfucosylated form of this therapeutic antibody. These results indicate that our Rmd(+) baculoviral vector can be used to solve the immunogenic core α1,3-fucosylation problem associated with the baculovirus-insect cell system. In conjunction with existing glycoengineered insect cell lines, this vector extends the utility of the baculovirus-insect cell system to include therapeutic glycoprotein production. This new vector also extends the utility of the baculovirus-insect cell system to include the production of recombinant antibodies with enhanced effector functions, due to its ability to block core α1,6-fucosylation.

Biological Insights into Therapeutic Protein Modifications throughout Trafficking and Their Biopharmaceutical Applications

Over the lifespan of therapeutic proteins, from the point of biosynthesis to the complete clearance from tested subjects, they undergo various biological modifications. Therapeutic influences and molecular mechanisms of these modifications have been well appreciated for some while remained less understood for many. This paper has classified these modifications into multiple categories, according to their processing locations and enzymatic involvement during the trafficking events. It also focuses on the underlying mechanisms and structural-functional relationship between modifications and therapeutic properties. In addition, recent advances in protein engineering, cell line engineering, and process engineering, by exploring these complex cellular processes, are discussed and summarized, for improving functional characteristics and attributes of protein-based biopharmaceutical products.