Russell body inducing threshold depends on the variable domain sequences of individual human IgG clones and the cellular protein homeostasis

Temperature-dependent intracellular crystallization of firefly luciferase in mammalian cells is suppressed by D-luciferin and stabilizing inhibitors

Firefly luciferase (Fluc) from Photinus pyralis is one of the most widely used reporter proteins in biomedical research. Despite its widespread use, Fluc's protein phase transition behaviors and phase separation characteristics have not received much attention. Current research uncovers Fluc's intrinsic property to phase separate in mammalian cells upon a simple cell culture temperature change. Specifically, Fluc spontaneously produced needle-shaped crystal-like inclusion bodies upon temperature shift to the hypothermic temperatures ranging from 25 °C to 31 °C. The crystal-like inclusion bodies were not associated with or surrounded by membranous organelles and were likely built from the cytosolic pool of Fluc. Furthermore, the crystal-like inclusion formation was suppressed when cells were cultured in the presence of D-luciferin and its synthetic analog, as well as the benzothiazole family of so-called stabilizing inhibitors. These two classes of compounds inhibited intracellular Fluc crystallization by different modes of action as they had contrasting effects on steady-state luciferase protein accumulation levels. This study suggests that, under substrate insufficient conditions, the excess Fluc phase separates into a crystal-like state that can modulate intracellular soluble enzyme availability and protein turnover rate.

Development of a 10 g/L process for a difficult-to-express multispecific antibody format using a holistic process development approach

Ichnos has developed a multi-specific antibody platform based on the BEAT® (Bispecific engagement by antibodies based on the T-cell receptor) interface. The increased complexity of the bi- and multi-specific formats generated with this platform makes these molecules difficult-to-express proteins compared to standard monoclonal antibodies (mAbs). This report describes how expression limitations of a bi-specific bi-paratopic BEAT antibody were improved in a holistic approach. An initial investigation allowed identification of a misbalance in the subunits composing the BEAT antibody as the potential root cause. This misbalance was then addressed by a signal peptide optimization, and the overall expression level was increased by the combination of two vector design elements on a single gene vector. Further improvements were made in the selection of cell populations and an upstream (USP) platform process was applied in combination with a cell culture temperature shift. This allowed titer levels of up to 6 g/L to be reached with these difficult-to-express proteins. Furthermore, a high-density seeding process was developed that allowed titers of around 11 g/L for the BEAT antibody, increasing the initial titer by a factor of 10. The approach was successfully applied to a tri-specific antibody with titer levels reaching 10 g/L. In summary, a platform process for difficult-to-express proteins was developed using molecular biology tools, cell line development, upstream process optimization and process intensification.

Understanding the biosynthesis of human IgM SAM-6 through a combinatorial expression of mutant subunits that affect product assembly and secretion

Polymeric IgMs are secreted from plasma cells abundantly despite their structural complexity and intricate multimerization steps. To gain insights into IgM's assembly mechanics that underwrite such high-level secretion, we characterized the biosynthetic process of a natural human IgM, SAM-6, using a heterologous HEK293(6E) cell platform that allowed the production of IgMs both in hexameric and pentameric forms in a controlled fashion. By creating a series of mutant subunits that differentially disrupt secretion, folding, and specific inter-chain disulfide bond formation, we assessed their effects on various aspects of IgM biosynthesis in 57 different subunit chain combinations, both in hexameric and pentameric formats. The mutations caused a spectrum of changes in steady-state subcellular subunit distribution, ER-associated inclusion body formation, intracellular subunit detergent solubility, covalent assembly, secreted IgM product quality, and secretion output. Some mutations produced differential effects on product quality depending on whether the mutation was introduced to hexameric IgM or pentameric IgM. Through this systematic combinatorial approach, we consolidate diverse overlapping knowledge on IgM biosynthesis for both hexamers and pentamers, while unexpectedly revealing that the loss of certain inter-chain disulfide bonds, including the one between μHC and λLC, is tolerated in polymeric IgM assembly and secretion. The findings highlight the differential roles of underlying non-covalent protein-protein interactions in hexamers and pentamers when orchestrating the initial subunit interactions and maintaining the polymeric IgM product integrity during ER quality control steps, secretory pathway trafficking, and secretion.

The secretory pathway - the key for unlocking the potential of Chinese hamster ovary cell factories for manufacturing therapeutic proteins

Mammalian cell factories (in particular the CHO cell system) have been crucial in the rise of biopharmaceuticals. Mammalian cells have compartmentalized organelles where intricate networks of proteins manufacture highly sophisticated biopharmaceuticals in a specialized production pipeline - the secretory pathway. In the bioproduction context, the secretory pathway functioning is key for the effectiveness of cell factories to manufacture these life-changing medicines. This review describes the molecular components and events involved in the secretory pathway, and provides a comprehensive summary of the intracellular steps limiting the production of therapeutic proteins as well as the achievements in engineering CHO cell secretory machinery. We also consider antibody-producing plasma cells (so called "professional" secretory cells) to explore the mechanisms underpinning their unique secretory function/features. Such understandings offer the potential to further enhancement of the current CHO cell production platforms for manufacturing next generation of biopharmaceuticals.

Expression liabilities in a four-chain bispecific molecule

Multispecific antibodies, often composed of three to five polypeptide chains, have become increasingly relevant in the development of biotherapeutics. These molecules have mechanisms of action that include redirecting T cells to tumors and blocking multiple pathogenic mediators simultaneously. One of the major challenges for asymmetric multispecific antibodies is generating a high proportion of the correctly paired antibody during production. To understand the causes and effects of chain mispairing impurities in a difficult to express multispecific hetero-IgG, we investigated consequences of individual and pairwise chain expression in mammalian transient expression hosts. We found that one of the two light chains (LC) was not secretion competent when transfected individually or cotransfected with the noncognate heavy chain (HC). Overexpression of this secretion impaired LC reduced cell growth while inducing endoplasmic reticulum stress and CCAAT/enhancer-binding protein homologous protein (CHOP) expression. The majority of this LC was observed as monomer with incomplete intrachain disulfide bonds when expressed individually. Russell bodies (RB) were induced when this LC was co-expressed with the cognate HC. Moreover, one HC paired promiscuously with noncognate LC. These results identify the causes for the low product quality observed from stable cell lines expressing this heteroIgG and suggest mitigation strategies to improve overall process productivity of the correctly paired multispecific antibody. The approach described here provides a general strategy for identifying the molecular and cellular liabilities associated with difficult to express multispecific antibodies.

Light chain subunit of a poorly soluble human IgG2λ crystallizes in physiological pH environment both in cellulo and in vitro

Prominent inclusion bodies can develop in the endoplasmic reticulum (ER) when overexpressed antibodies possess intrinsically high condensation propensities. These observations suggest that antibodies deemed to show notable solubility problems may reveal such characteristics preemptively in the form of ER-associated inclusion bodies during antibody overexpression. To define the relationships between solubility problems and inclusion body phenotypes, we investigated the biosynthesis of a model human IgG2λ that shows severe opalescence in an acidic formulation buffer yet retains high solubility at physiological pH. Consistent with the pH-dependent solubility characteristics, the model antibody did not induce notable inclusion body in the physiological pH environment of the ER lumen. However, when individual subunit chains of the antibody were expressed separately, the light chain (LC) spontaneously induced notable crystal-like inclusion bodies in the ER. The LC crystallization event was readily reproducible in vitro by simply concentrating the purified LC protein at physiological pH. Two independent structural determinants for the LC crystallization were identified through rational mutagenesis approach by monitoring the effect of amino acid substitutions on intracellular LC crystallogenesis. The effect of mutations on crystallization was also recapitulated in vitro using purified LC proteins. Importantly, when introduced directly into the model antibody, a mutation that prevents the LC crystallization remediated the antibody's solubility problem without compromising the secretory output or antigen binding. These results illustrate that the ER can serve as a "physiological test tube" that not only reports secretory cargo's high condensation propensity at physiological pH, but also provides an orthogonal method that guides antibody engineering strategy.

NHDL, a recombinant VL/VH hybrid antibody control for IgG2/4 antibodies

The mechanism of action of recombinant IgG2/4 antibodies involves blocking of their target without the induction of effector functions. Examples are eculizumab (Soliris®), which is used clinically to block complement factor C5, as well as anti-human CD14 (r18D11) and anti-porcine CD14 (rMIL2) produced in our laboratory. So far, no proper IgG2/4 control antibody has been available for controlled validation of IgG2/4 antibody functions. Here, we describe the design of a recombinant control antibody (NHDL), which was generated by combining the variable light (V_L) and heavy (V_H) chains from two unrelated specificities. NHDL was readily expressed and purified as a stable IgG2/4 antibody, and showed no detectable specificity toward any putative antigen present in human or porcine blood. The approach of artificial V_L/V_H combination may be adopted for the design of other recombinant control antibodies.

I-152, a supplier of N-acetyl-cysteine and cysteamine, inhibits immunoglobulin secretion and plasma cell maturation in LP-BM5 murine leukemia retrovirus-infected mice by affecting the unfolded protein response

Excessive production of immunoglobulins (Ig) causes endoplasmic reticulum (ER) stress and triggers the unfolded protein response (UPR). Hypergammaglobulinemia and lymphadenopathy are hallmarks of murine AIDS that develops in mice infected with the LP-BM5 murine leukemia retrovirus complex. In these mice, Th2 polarization and aberrant humoral response have been previously correlated to altered intracellular redox homeostasis. Our goal was to understand the role of the cell's redox state in Ig secretion and plasma cell (PC) maturation. To this aim, LP-BM5-infected mice were treated with I-152, an N-acetyl-cysteine and cysteamine supplier. Intraperitoneal I-152 administration (30 μmol/mouse three times a week for 9 weeks) decreased plasma IgG and increased IgG/Syndecan 1 ratio in the lymph nodes where IgG were in part accumulated within the ER. PC containing cytoplasmic inclusions filled with IgG were present in all animals, with fewer mature PC in those treated with I-152. Infection induced up-regulation of signaling molecules involved in the UPR, i.e. CHAC1, BiP, sXBP-1 and PDI, that were generally unaffected by I-152 treatment except for PDI and sXBP-1, which have a key role in protein folding and PC maturation, respectively. Our data suggest that one of the mechanisms through which I-152 can limit hypergammaglobulinemia in LP-BM5-infected mice is by influencing IgG folding/assembly as well as secretion and affecting PC maturation.

Germinality does not necessarily define mAb expression and thermal stability

The production potential of recombinant monoclonal antibody (mAb) expressing cell lines depends, among other factors, on the intrinsic antibody structure determined by the amino acid sequence. In this study, we investigated the influence of somatic mutations in the V(D)J sequence of four individual, mature model mAbs on the expression potential. Therefore, we defined four couples, each consisting of one naturally occurring mAb (2G12, Ustekinumab, 4B3, and 2F5) and the corresponding germline-derived cognate mAb (353/11, 554/12, 136/63, and 236/14). For all eight mAb variants, recombinant Chinese hamster ovary (CHO) cell lines were developed with mAbs expressed from a defined chromosomal locus. The presented workflow investigates critical parameters including productivity, intra- and extracellular product profile, XBP1 splicing, thermal stability, and in silico hydrophobicity. Significant differences in productivity were even observed between the germline-derived mAbs which did not undergo somatic mutagenesis. Accordingly, back-to-germline mutations of mature mAbs are not necessarily reflecting improved expression and stability but indicate opportunities and limits of mAb engineering. From our studies, we conclude that germinalization represents a potential to improve mAb properties depending on the antibody’s germline family, highlighting the fact that mAbs should be treated individually.

Secretion analysis of intracellular "difficult-to-express" immunoglobulin G (IgG) in Chinese hamster ovary (CHO) cells

The Chinese hamster ovary (CHO) cell line is the most widely used host cell for therapeutic antibody production. Although its productivity has been improved by various strategies to satisfy the growing global demand, some difficult-to-express (DTE) antibodies remain at low secretion levels. To improve the production of various therapeutic antibodies, it is necessary to determine possible rate-limiting steps in DTE antibody secretion in comparison with other high IgG producers. Here, we analyzed the protein secretion process in CHO cells producing the DTE immunoglobulin G (IgG) infliximab. The results from chase assays using a translation inhibitor revealed that infliximab secretion could be nearly completed within 2 h, at which time the cells still retained about 40% of heavy chains and 65% of light chains. Using fluorescent microscopy, we observed that these IgG chains remained in the endoplasmic reticulum and Golgi apparatus. The cells inefficiently form fully assembled heterodimer IgG by making LC aggregates, which may be the most serious bottleneck in the production of DTE infliximab compared with other IgG high producers. Our study could contribute to establish the common strategy for constructing DTE high-producer cells on the basis of rate-limiting step analysis.

Secretory leakage of IgG1 aggregates from recombinant Chinese hamster ovary cells

Aggregation of therapeutic antibodies is one of the most important issues to be resolved in manufacturing processes because of reduced efficacy and immunogenicity. Despite aggregation studies in vitro, little is known about the aggregation mechanism in cell culture processes. In this study, we investigated the process of aggregate formation of IgG1 antibodies during the culture of Chinese hamster ovary (CHO) cells to determine how aggregation occurs. A recombinant CHO cell line was cultivated in a bioreactor, and purified IgG1 from daily culture supernatants was analyzed by size exclusion chromatography. We found a linear correlation between the peak plots of IgG1 by-products, dimeric and aggregated IgG1, and integrated viable cell density, indicating that these by-products were secreted from CHO cells at a constant secretion rate. In addition, aggregate formation was not reproduced in pseudo-culture experiments, and the solution structures of intracellular and extracellular IgG1 aggregates were similar. These results support the concept of secretory leakage of IgG1 by-products. Secreted aggregates appeared to be in an alternatively folded state, which can pass through the protein quality control system in mammalian cells.

Membrane cholesterol modulates STEAP2 conformation during dynamic intracellular trafficking processes leading to broad subcellular distribution

STEAP2 is a member of the Six-Transmembrane Epithelial Antigen of the Prostate (STEAP) protein family that is proposed to function as metalloreductase. While STEAP2 shows a complex subcellular distribution pattern localizing to both secretory and endocytic pathway organelles, how such broad steady-state distribution is maintained is unknown. Similarly, whether STEAP2 undergoes any compartment-specific modulation during intracellular trafficking has not been reported. Leveraging a newly-identified monoclonal antibody that recognizes a conformation-sensitive epitope nested in the second extracellular loop of STEAP2, we demonstrate that the epitope formation was dependent on the cholesterol content of the membrane in which STEAP2 was embedded. Monitoring the STEAP2-dependent internalization of this antibody uncovered STEAP2's rapid internalization from the cell surface and their subsequence trafficking to the Golgi region and endosome-like puncta. Acute inhibition of endocytosis also increased the detectable amount of STEAP2 at the plasma membrane. Collectively, these experiments demonstrate that an intricate balance of membrane flux between the secretory and endocytic pathways underlies the characteristic broad subcellular localization of STEAP2. By using a cell-based assay that detects the metalloreductase functions of cell surface-localizing STEAP4, STEAP2's metalloreductase activities were not detectable, suggesting that its enzymatic function is suppressed at the plasma membrane. The conformational modulation of STEAP2 by the local membrane cholesterol content can therefore serve as a potential mechanism to modulate STEAP2 function in a compartment-restricted manner, by coupling a pre-existing difference in cholesterol content among different cellular membranes to a dynamic trafficking process leading to broad subcellular distribution.

A systematic dissection of sequence elements determining β-Klotho and FGF interaction and signaling

Endocrine fibroblast growth factors (FGFs) require Klotho transmembrane proteins as necessary co-receptors to activate FGF receptor (FGFR) signaling. In particular, FGF19 and FGF21 function through β-Klotho to regulate glucose and lipid metabolism. Recent research has focused on elucidating how these two FGFs interact with β-Klotho and FGFRs to activate downstream signaling. In this study, using hydrogen deuterium exchange coupled to mass spectrometry (HDX-MS), we identified regions on the β-Klotho protein that likely participate in ligand interaction, and vice versa. Alanine and arginine mutagenesis were carried out to further probe the contributions of individual residues to receptor/ligand interactions. Using biochemical and cell-based signaling assays with full-length proteins, we show that both the KL1 and KL2 domains of β-Klotho participate in ligand interaction, and these binding sites on β-Klotho are shared by FGF19 and FGF21. In addition, we show that two highly conserved regions in the C-terminal tail of FGF19 and FGF21 are responsible for interaction with the co-receptor. Our results are consistent with recent publications on the crystal structures of the Klotho proteins and provide insight into how endocrine FGFs interact with co-receptors for signal transduction.

A protein chimera strategy supports production of a model "difficult-to-express" recombinant target

Due in part to the needs of the biopharmaceutical industry, there has been an increased drive to generate high quality recombinant proteins in large amounts. However, achieving high yields can be a challenge as the novelty and increased complexity of new targets often makes them 'difficult-to-express'. This study aimed to define the molecular features that restrict the production of a model 'difficult-to-express' recombinant protein, Tissue Inhibitor Metalloproteinase-3 (TIMP-3). Building from experimental data, computational approaches were used to rationalize the redesign of this recombinant target to generate a chimera with enhanced secretion. The results highlight the importance of early identification of unfavourable sequence attributes, enabling the generation of engineered protein forms that bypass 'secretory' bottlenecks and result in efficient recombinant protein production.

Intermolecular interactions involving an acidic patch on immunoglobulin variable domain and the γ2 constant region mediate crystalline inclusion body formation in the endoplasmic reticulum

Full-length immunoglobulins (Igs) are widely considered difficult to crystallize because of their large size, N-linked glycosylation, and flexible hinge region. However, numerous cases of intracellular Ig crystallization are reported in plasma cell dyscrasias. What makes some Ig clones more prone to crystallize during biosynthesis as well as the biochemical and cell biological requirements for this cryptic event are poorly understood. To investigate the underlying process of intracellular Ig crystallization we searched for model IgGs that can induce crystalline inclusions during recombinant overexpression. By testing various subunit combinations through mixing and matching of individual subunit chains derived from a panel of human IgG clones, we identified one secretion competent IgG2λ that induced needle-like crystalline inclusions in transfected HEK293 cells. Ig crystallization rarely occurred at steady-state cell growth conditions but was easily induced when ER-to-Golgi transport was pharmacologically blocked. Homology modeling revealed the presence of a prominent negatively-charged patch on the variable domain surface. The patch was composed of eight aspartic acids, of which five were in the heavy chain variable region and three were in the light chain. Crystallization occurred only when the two subunits were co-transfected and the intracellular crystals co-localized with ER resident proteins. Furthermore, subtype switching from IgG2 to IgG1 and stepwise neutralization of the acidic patch independently abrogated Ig crystallization events. The evidence supported that the formation of needle-like crystalline inclusions in the ER was underscored by multivalent intermolecular interactions between the acidic patch and undefined determinants present on the γ2 subunit constant region.

Single amino acid substitution in LC-CDR1 induces Russell body phenotype that attenuates cellular protein synthesis through eIF2α phosphorylation and thereby downregulates IgG secretion despite operational secretory pathway traffic

Amino acid sequence differences in the variable region of immunoglobulin (Ig) cause wide variations in secretion outputs. To address how a primary sequence difference comes to modulate Ig secretion, we investigated the biosynthetic process of 2 human IgG2κ monoclonal antibodies (mAbs) that differ only by one amino acid in the light chain complementarity-determining region 1 while showing ∼20-fold variance in secretion titer. Although poorly secreted, the lower-secreting mAb of the 2 was by no means defective in terms of its folding stability, antigen binding, and in vitro biologic activity. However, upon overexpression in HEK293 cells, the low-secreting mAb revealed a high propensity to aggregate into enlarged globular structures called Russell bodies (RBs) in the endoplasmic reticulum. While Golgi morphology was affected by the formation of RBs, secretory pathway membrane traffic remained operational in those cells. Importantly, cellular protein synthesis was severely suppressed in RB-positive cells through the phosphorylation of eIF2α. PERK-dependent signaling was implicated in this event, given the upregulation and nuclear accumulation of downstream effectors such as ATF4 and CHOP. These findings illustrated that the underlining process of poor Ig secretion in RB-positive cells was due to downregulation of Ig synthesis instead of a disruption or blockade of secretory pathway trafficking. Therefore, RB formation signifies an end of active Ig production at the protein translation level. Consequently, depending on how soon and how severely an antibody-expressing cell develops the RB phenotype, the productive window of Ig secretion can vary widely among the cells expressing different mAbs.

A rare IL33 loss-of-function mutation reduces blood eosinophil counts and protects from asthma

IL-33 is a tissue-derived cytokine that induces and amplifies eosinophilic inflammation and has emerged as a promising new drug target for asthma and allergic disease. Common variants at IL33 and IL1RL1, encoding the IL-33 receptor ST2, associate with eosinophil counts and asthma. Through whole-genome sequencing and imputation into the Icelandic population, we found a rare variant in IL33 (NM_001199640:exon7:c.487-1G>C (rs146597587-C), allele frequency = 0.65%) that disrupts a canonical splice acceptor site before the last coding exon. It is also found at low frequency in European populations. rs146597587-C associates with lower eosinophil counts (β = -0.21 SD, P = 2.5×10^–16, N = 103,104), and reduced risk of asthma in Europeans (OR = 0.47; 95%CI: 0.32, 0.70, P = 1.8×10^–4, N cases = 6,465, N controls = 302,977). Heterozygotes have about 40% lower total IL33 mRNA expression than non-carriers and allele-specific analysis based on RNA sequencing and phased genotypes shows that only 20% of the total expression is from the mutated chromosome. In half of those transcripts the mutation causes retention of the last intron, predicted to result in a premature stop codon that leads to truncation of 66 amino acids. The truncated IL-33 has normal intracellular localization but neither binds IL-33R/ST2 nor activates ST2-expressing cells. Together these data demonstrate that rs146597587-C is a loss of function mutation and support the hypothesis that IL-33 haploinsufficiency protects against asthma.

Only a few genes have been found to play a role in asthma. These include the genes IL33 and IL1RL1, and sequence variants in the human genome close to these genes were initially found to affect the number of eosinophils, cells that play a role in inflammation of the airways in asthma. Based on this knowledge, we decided to use high resolution sequencing technology to search for variants in these genes that cause changes in structure and function of the proteins they encode. We found a rare (0.65%) sequence variant in the IL33 gene, that causes less production of the IL33 protein and some of the protein formed lacks the capacity to bind to its receptor on cells and promote inflammation. This rare mutation causes reduced number of eosinophils in blood and protects against asthma. These results suggest that drugs that could interfere with the inflammatory activity of the IL33 protein may be beneficial for treatment of asthma.

Strategies and Considerations for Improving Expression of "Difficult to Express" Proteins in CHO Cells

Despite substantial advances in the field of mammalian expression, there are still proteins that are characterized as difficult to express. Determining the expression bottleneck requires troubleshooting techniques specific for the given molecule and host. The complex array of intracellular processes involved in protein expression includes transcription, protein folding, post-translation processing, and secretion. Challenges in any of these steps could result in low protein expression, while the inherent properties of the molecule itself may limit its production via mechanisms such as cytotoxicity or inherent instability. Strategies to identify the rate-limiting step and subsequently improve expression and production are discussed here.

Topogenesis and cell surface trafficking of GPR34 are facilitated by positive-inside rule that effects through a tri-basic motif in the first intracellular loop

Protein folding, topogenesis and intracellular targeting of G protein-coupled receptors (GPCRs) must be precisely coordinated to ensure correct receptor localization. To elucidate how different steps of GPCR biosynthesis work together, we investigated the process of membrane topology determination and how it relates to the acquisition of cell surface trafficking competence in human GPR34. By monitoring a fused FLAG-tag and a conformation-sensitive native epitope during the expression of GPR34 mutant panel, a tri-basic motif in the first intracellular loop was identified as the key topogenic signal that dictates the orientation of transmembrane domain-1 (TM1). Charge disruption of the motif perturbed topogenic processes and resulted in the conformational epitope loss, post-translational processing alteration, and trafficking arrest in the Golgi. The placement of a cleavable N-terminal signal sequence as a surrogate topogenic determinant overcame the effects of tri-basic motif mutations and rectified the TM1 orientation; thereby restored the conformational epitope, post-translational modifications, and cell surface trafficking altogether. Progressive N-tail truncation and site-directed mutagenesis revealed that a proline-rich segment of the N-tail and all four cysteines individually located in the four separate extracellular regions must simultaneously reside in the ER lumen to muster the conformational epitope. Oxidation of all four cysteines was necessary for the epitope formation, but the cysteine residues themselves were not required for the trafficking event. The underlying biochemical properties of the conformational epitope was therefore the key to understand mechanistic processes propelled by positive-inside rule that simultaneously regulate the topogenesis and intracellular trafficking of GPR34.

Russell body phenotype is preferentially induced by IgG mAb clones with high intrinsic condensation propensity: relations between the biosynthetic events in the ER and solution behaviors in vitro

The underlying reasons for why some mAb (monoclonal antibody) clones are much more inclined to induce a Russell body (RB) phenotype during immunoglobulin biosynthesis remain elusive. Although RBs are morphologically understood as enlarged globular aggregates of immunoglobulins deposited in the endoplasmic reticulum (ER), little is known about the properties of the RB-inducing mAb clones as secretory cargo and their physical behaviors in the extracellular space. To elucidate how RB-inducing propensities, secretion outputs, and the intrinsic physicochemical properties of individual mAb clones are interrelated, we used HEK293 cells to study the biosynthesis of 5 human IgG mAbs for which prominent solution behavior problems were known a priori. All 5 model mAbs with inherently high condensation propensities induced RB phenotypes both at steady state and under ER-to-Golgi transport block, and resulted in low secretion titer. By contrast, one reference mAb that readily crystallized at neutral pH in vitro produced rod-shaped crystalline bodies in the ER without inducing RBs. Another reference mAb without notable solution behavior issues did not induce RBs and was secreted abundantly. Intrinsic physicochemical properties of individual IgG clones thus directly affected the biosynthetic steps in the ER, and thereby produced distinctive cellular phenotypes and influenced IgG secretion output. The findings implicated that RB formation represents a phase separation event or a loss of colloidal stability in the secretory pathway organelles. The process of RB induction allows the cell to preemptively reduce the extracellular concentration of potentially pathogenic, highly aggregation-prone IgG clones by selectively storing them in the ER.

Uncovering methods for the prevention of protein aggregation and improvement of product quality in a transient expression system

Mammalian expression systems are used routinely for the production of recombinant proteins as therapeutic molecules as well as research tools. Transient expression has become increasingly popular in recent years due to its rapid timeline and improvements in expression level. While improvements to transient expression systems have focused mainly on the level of protein expression, the aspect of protein quality has received little attention. The removal of undesirable products, such as aggregation, depends primarily on purification, requiring additional cumbersome steps, which can lead to a lower product yield and longer timelines. In this study, we show that reducing the level of transcription by transfecting at a lower gene dose improves the quality of secreted molecules prone to aggregation. For gene dosing to have this effect, it is critical for the carrier DNA to be an empty vector containing the same elements as the gene containing plasmid. This approach can be used in combination with a temperature shift to hypothermic conditions during production to enhance the effect. The observed improvements not only minimized aggregation levels, but also generated products with overall superior quality, including more homogeneous signal peptide cleavage and N-linked glycosylation profiles. These techniques have produced a similar improvement in product quality with a variety of other molecules, suggesting that this may be a general approach to enhance product quality from transient expression systems.

The endoplasmic reticulum and unfolded protein response in the control of mammalian recombinant protein production

The endoplasmic reticulum (ER) of eukaryotic cells is involved in the synthesis and processing of proteins and lipids in the secretory pathway. These processing events that proteins undergo in the ER may present major limiting steps for recombinant protein production. Increased protein synthesis, accumulation of improperly processed or mis-folded protein can induce ER stress. To cope with ER stress, the ER has quality control mechanisms, such as the unfolded protein response (UPR) and ER-associated degradation to restore homeostasis. ER stress and UPR activation trigger multiple physiological cellular changes. Here we review cellular mechanisms that cope with ER stress and illustrate how this knowledge can be applied to increase the efficiency of recombinant protein expression.

In search of expression bottlenecks in recombinant CHO cell lines--a case study

The efficient production of recombinant proteins such as antibodies typically involves the screening of an extravagant number of clones in order to finally select a stable and high-producing cell line. Thereby, the underlying principles of a powerful protein machinery, but also potential expression limitations, often remain poorly understood. To shed more light on this topic, we applied several different techniques to investigate a previously generated cell line (4B3-IgA), which expressed recombinant immunoglobulin A (IgA) with an unusually low specific productivity. Results were compared to the host cell line and to another recombinant CHO cell line (3D6-IgA) expressing another IgA that binds to an overlapping epitope. The low specific productivity of clone 4B3-IgA could not be explained by GCN or mRNA levels, but insufficiencies in protein maturation and/or secretion were determined. Despite the presence of free light chain polypeptides, they occasionally failed to associate with their heavy chain partners. Consequently, heavy chains were misassembled and accumulated to form intracellular aggregates, so-called Russell bodies. These protein deposits evoked the expression of increased amounts of ER-resident chaperones to combat the induced stress. Despite bottlenecks in protein processing, the cells' quality checkpoints remained intact, and predominantly correctly processed IgA was exported into the culture medium. The results of our study demonstrated that recombinant protein expression was impaired by heavy chain aggregation despite the presence of a disposable light chain and revealed elevated chaperone formation in combination with limited antibody assembly. Our studies suggest that the primary amino acid sequence and consequently the resulting structure of an expressed protein need to be considered as a factor influencing a cell's productivity.

Proteins improving recombinant antibody production in mammalian cells

Mammalian cells have been successfully used for the industrial manufacture of antibodies due to their ability to synthesize antibodies correctly. Nascent polypeptides must be subjected to protein folding and assembly in the ER and the Golgi to be secreted as mature proteins. If these reactions do not proceed appropriately, unfolded or misfolded proteins are degraded by the ER-associated degradation (ERAD) pathway. The accumulation of unfolded proteins or intracellular antibody crystals accompanied by this failure triggers the unfolded protein response (UPR), which can considerably attenuate the levels of translation, folding, assembly, and secretion, resulting in reduction of antibody productivity. Accumulating studies by omics-based analysis of recombinant mammalian cells suggest that not only protein secretion processes including protein folding and assembly but also translation are likely to be the rate-limiting factors for increasing antibody production. Here, this review describes the mechanism of antibody folding and assembly and recent advantages which could improve recombinant antibody production in mammalian cells by utilizing proteins such as ER chaperones or UPR-related proteins.

Aggregates, crystals, gels, and amyloids: intracellular and extracellular phenotypes at the crossroads of immunoglobulin physicochemical property and cell physiology

Recombinant immunoglobulins comprise an important class of human therapeutics. Although specific immunoglobulins can be purposefully raised against desired antigen targets by various methods, identifying an immunoglobulin clone that simultaneously possesses potent therapeutic activities and desirable manufacturing-related attributes often turns out to be challenging. The variable domains of individual immunoglobulins primarily define the unique antigen specificities and binding affinities inherent to each clone. The primary sequence of the variable domains also specifies the unique physicochemical properties that modulate various aspects of individual immunoglobulin life cycle, starting from the biosynthetic steps in the endoplasmic reticulum, secretory pathway trafficking, secretion, and the fate in the extracellular space and in the endosome-lysosome system. Because of the diverse repertoire of immunoglobulin physicochemical properties, some immunoglobulin clones' intrinsic properties may manifest as intriguing cellular phenotypes, unusual solution behaviors, and serious pathologic outcomes that are of scientific and clinical importance. To gain renewed insights into identifying manufacturable therapeutic antibodies, this paper catalogs important intracellular and extracellular phenotypes induced by various subsets of immunoglobulin clones occupying different niches of diverse physicochemical repertoire space. Both intrinsic and extrinsic factors that make certain immunoglobulin clones desirable or undesirable for large-scale manufacturing and therapeutic use are summarized.