Aglycosylated full-length IgG antibodies: steps toward next-generation immunotherapeutics

Escherichia coli and Pichia pastoris: microbial cell-factory platform for -full-length IgG production

Owing to the unmet demand, the pharmaceutical industry is investigating an alternative host to mammalian cells to produce antibodies for a variety of therapeutic and research applications. Regardless of some disadvantages, Escherichia coli and Pichia pastoris are the preferred microbial hosts for antibody production. Despite the fact that the production of full-length antibodies has been successfully demonstrated in E. coli, which has mostly been used to produce antibody fragments, such as: antigen-binding fragments (Fab), single-chain fragment variable (scFv), and nanobodies. In contrast, Pichia, a eukaryotic microbial host, is mostly used to produce glycosylated full-length antibodies, though hypermannosylated glycan is a major challenge. Advanced strategies, such as the introduction of human-like glycosylation in endotoxin-edited E. coli and cell-free system-based glycosylation, are making progress in creating human-like glycosylation profiles of antibodies in these microbes. This review begins by explaining the structural and functional requirements of antibodies and continues by describing and analyzing the potential of E. coli and P. pastoris as hosts for providing a favorable environment to create a fully functional antibody. In addition, authors compare these microbes on certain features and predict their future in antibody production. Briefly, this review analyzes, compares, and highlights E. coli and P. pastoris as potential hosts for antibody production.

Functional studies with IgM and IgA immunoglobulins: binding to pIgR, FcαμR, FcμR, and CDC activities

IgMs are the first antibodies produced by the immune system upon encounter of a possible pathogen and are one of five antibody subclasses in humans. For IgG, the most intensively studied antibody class, the N-linked glycosylation site located in the Fc-domain is directly involved in high affinity binding to the respective receptors and initiation of corresponding immune response. IgM molecules have five N-glycosylation sites and one N-glycosylation site in the J-chain, which can be incorporated in IgM or IgA molecules. There is only limited knowledge available concerning the function of these N-glycosylations in IgMs. To address this question, we produced IgM molecules lacking a particular N-glycosylation site and tested these variants as well as IgA molecules for binding to the known receptors: the polymeric immunoglobulin receptor (pIgR), the dual receptor for IgA and IgM, FcαμR, and the specific receptor for IgM, FcμR. The single glycosylation sites did not show an impact on expression and multimerization, except for variant N402Q, which could not be expressed. In SPR measurements, no major impact on the binding to the receptors by particular glycosylation sites could be detected. In cellular assays, deglycosylated variants showed some alterations in induction of CDC activity. Most strikingly, we observed also binding of IgA to the FcμR in the same affinity range as IgM, suggesting that this might have a physiological role. To further substantiate the binding of IgA to FcμR we used IgA from different origins and were able to confirm binding of IgA preparations to the FcμR.

In vivo deglycosylation of recombinant glycoproteins in tobacco BY-2 cells

Production of recombinant pharmaceutical glycoproteins has been carried out in multiple expression systems. However, N-glycosylation, which increases heterogeneity and raises safety concerns due to the presence of non-human residues, is usually not controlled. The presence and composition of N-glycans are also susceptible to affect protein stability, function and immunogenicity. To tackle these issues, we are developing glycoengineered Nicotiana tabacum Bright Yellow-2 (BY-2) cell lines through knock out and ectopic expression of genes involved in the N-glycosylation pathway. Here, we report on the generation of BY-2 cell lines producing deglycosylated proteins. To this end, endoglycosidase T was co-expressed with an immunoglobulin G or glycoprotein B of human cytomegalovirus in BY-2 cell lines producing only high mannose N-glycans. Endoglycosidase T cleaves high mannose N-glycans to generate single, asparagine-linked, N-acetylglucosamine residues. The N-glycosylation profile of the secreted antibody was determined by mass spectrometry analysis. More than 90% of the N-glycans at the conserved Asn297 site were deglycosylated. Likewise, extensive deglycosylation of glycoprotein B, which possesses 18 N-glycosylation sites, was observed. N-glycan composition of gB glycovariants was assessed by in vitro enzymatic mobility shift assay and proven to be consistent with the expected glycoforms. Comparison of IgG glycovariants by differential scanning fluorimetry revealed a significant impact of the N-glycosylation pattern on the thermal stability. Production of deglycosylated pharmaceutical proteins in BY-2 cells expands the set of glycoengineered BY-2 cell lines.

An oligosaccharyltransferase from Leishmania donovani increases the N-glycan occupancy on plant-produced IgG1

N-Glycosylation of immunoglobulin G1 (IgG1) at the heavy chain Fc domain (Asn297) plays an important role for antibody structure and effector functions. While numerous recombinant IgG1 antibodies have been successfully expressed in plants, they frequently display a considerable amount (up to 50%) of unglycosylated Fc domain. To overcome this limitation, we tested a single-subunit oligosaccharyltransferase from the protozoan Leishmania donovani (LdOST) for its ability to improve IgG1 Fc glycosylation. LdOST fused to a fluorescent protein was transiently expressed in Nicotiana benthamiana and confocal microscopy confirmed the subcellular location at the endoplasmic reticulum. Transient co-expression of LdOST with two different IgG1 antibodies resulted in a significant increase (up to 97%) of Fc glycosylation while leaving the overall N-glycan composition unmodified, as determined by different mass spectrometry approaches. While biochemical and functional features of "glycosylation improved" antibodies remained unchanged, a slight increase in FcγRIIIa binding and thermal stability was observed. Collectively, our results reveal that LdOST expression is suitable to reduce the heterogeneity of plant-produced antibodies and can contribute to improving their stability and effector functions.

Innovative Metrics for Reporting and Comparing the Glycan Structural Profile in Biotherapeutics

Glycosylation is a critical quality attribute in biotherapeutics, impacting properties such as protein stability, solubility, clearance rate, efficacy, immunogenicity, and safety. Due to the heterogenic and complex nature of protein glycosylation, comprehensive characterization is demanding. Moreover, the lack of standardized metrics for evaluating and comparing glycosylation profiles hinders comparability studies and the establishment of manufacturing control strategies. To address both challenges, we propose a standardized approach based on novel metrics for a comprehensive glycosylation fingerprint which greatly facilitates the reporting and objective comparison of glycosylation profiles. The analytical workflow is based on a liquid chromatography-mass spectrometry-based multi-attribute method. Based on the analytical data, a matrix of glycosylation-related quality attributes, both at site-specific and whole molecule level, are computed, which provide metrics for a comprehensive product glycosylation fingerprint. Two case studies illustrate the applicability of the proposed indices as a standardized and versatile approach for reporting all dimensions of the glycosylation profile. The proposed approach further facilitates the assessments of risks associated with changes in the glycosylation profile that may affect efficacy, clearance, and immunogenicity.

Functional Expression of the Recombinant Spike Receptor Binding Domain of SARS-CoV-2 Omicron in the Periplasm of Escherichia coli

A new severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) variant known as Omicron has caused a rapid increase in recent global patients with coronavirus infectious disease 2019 (COVID-19). To overcome the COVID-19 Omicron variant, production of a recombinant spike receptor binding domain (RBD) is vital for developing a subunit vaccine or a neutralizing antibody. Although bacterial expression has many advantages in the production of recombinant proteins, the spike RBD expressed in a bacterial system experiences a folding problem related to disulfide bond formation. In this study, the soluble Omicron RBD was obtained by a disulfide isomerase-assisted periplasmic expression system in Escherichia coli. The Omicron RBD purified from E. coli was very well recognized by anti-SARS-CoV-2 antibodies, sotrovimab (S309), and CR3022, which were previously reported to bind to various SARS-CoV-2 variants. In addition, the kinetic parameters of the purified Omicron RBD upon binding to the human angiotensin-converting enzyme 2 (ACE2) were similar to those of the Omicron RBD produced in the mammalian expression system. These results suggest that an E. coli expression system would be suitable to produce functional and correctly folded spike RBDs of the next emerging SARS-CoV-2 variants quickly and inexpensively.

Glycosylation of immunoglobin G in tumors: Function, regulation and clinical implications

Immunoglobulin G (IgG) is the predominant component in humoral immunity and the major effector of neutralizing heterogeneous antigens. Glycosylation, as excessive posttranscriptional modification, can modulate IgG immune function. Glycosylated IgG has been reported to correlate with tumor progression, presenting several characteristic modifications, including the core fucose, galactose, sialic acid, and the bisect N-acetylglucosamine (GlcNAc). Meanwhile, IgG glycosylation regulates tumor immunity involved in tumor progression and is thus a potential target. Herein, we summarized the research progression to provide novel insight into the application of IgG glycosylation in tumor diagnosis and treatment.

GlyConnect-Ugi: site-selective, multi-component glycoprotein conjugations through GlycoDelete expressed glycans

Recently, the GlyConnect-oxime (GC) protein conjugation strategy was developed to provide a site-selective glycan-based conjugation strategy as an extension to the in-house developed GlycoDelete (GD) technology. GD gives access to glycoproteins with single GlcNAc, LacNAc, or LacNAc-Sia type glycans on their N-glycosylation sites. We have previously shown that these glycans provide a unique handle for site-selective conjugation as they provide a short, homogeneous and hydrophilic link to the protein backbone. GC focused on the use of chemical and chemo-enzymatic pathways for conjugation of a single molecule of interest via oxime formation or reductive amination. In the current work, we explore multicomponent reactions (MCR), namely Ugi and Passerini reactions, for GlycoDelete glycan directed, site-specific protein conjugation (MC-GC). The use of the Ugi and Passerini multicomponent reactions holds the potential of introducing multiple groups of interest in a single reaction step while creating a hydrophilic peptide-like linker.

Full-length recombinant antibodies from Escherichia coli: production, characterization, effector function (Fc) engineering, and clinical evaluation

Although several antibody fragments and antibody fragment-fusion proteins produced in Escherichia coli (E. coli) are approved as therapeutics for various human diseases, a full-length monoclonal or a bispecific antibody produced in E. coli has not yet been approved. The past decade witnessed substantial progress in expression of full-length antibodies in the E. coli cytoplasm and periplasm, as well as in cell-free expression systems. The equivalency of E. coli-produced aglycosylated antibodies and their mammalian cell-produced counterparts, with respect to biochemical and biophysical properties, including antigen binding, in vitro and in vivo serum stability, pharmacokinetics, and in vivo serum half-life, has been demonstrated. Extensive engineering of the Fc domain of aglycosylated antibodies enables recruitment of various effector functions, despite the lack of N-linked glycans. This review summarizes recent research, preclinical advancements, and clinical development of E. coli-produced aglycosylated therapeutic antibodies as monoclonal, bispecific, and antibody-drug conjugates for use in autoimmune, oncology, and immuno-oncology areas.Abbreviations: ADA Anti-drug antibody; ADCC Antibody-dependent cellular cytotoxicity; ADCP Antibody-dependent cellular phagocytosis; ADC Antibody-drug conjugate; aFc Aglycosylated Fc; AMD Age-related macular degeneration aTTP Acquired thrombotic thrombocytopenic purpura; BCMA B-cell maturation antigen; BLA Biologics license application; BsAb Bispecific antibody; C1q Complement protein C1q; CDC Complement-dependent cytotoxicity; CDCC Complement-dependent cellular cytotoxicity; CDCP Complement-dependent cellular phagocytosis; CEX Cation exchange chromatography; CFPS Cell-free protein expression; CHO Chinese Hamster Ovary; CH1-3 Constant heavy chain 1-3; CL Constant light chain; DLBCL Diffuse large B-cell lymphoma; DAR Drug antibody ratio; DC Dendritic cell; dsFv Disulfide-stabilized Fv; EU European Union; EGFR Epidermal growth factor receptor; E. coli Escherichia coli; EpCAM Epithelial cell adhesion molecule; Fab Fragment antigen binding; FACS Fluorescence activated cell sorting; Fc Fragment crystallizable; FcRn Neonatal Fc receptor; FcɣRs Fc gamma receptors; FDA Food and Drug Administration; FL-IgG Full-length immunoglobulin; Fv Fragment variable; FolRαa Folate receptor alpha; gFc Glycosylated Fc; GM-CSF Granulocyte macrophage-colony stimulating factor; GPx7 Human peroxidase 7; HCL Hairy cell leukemia; HIV Human immunodeficiency virusl; HER2 Human epidermal growth factor receptor 2; HGF Hepatocyte growth factor; HIC Hydrophobic interaction chromatography; HLA Human leukocyte antigen; IBs Inclusion bodies; IgG1-4 Immunoglobulin 1-4; IP Intraperitoneal; ITC Isothermal titration calorimetry; ITP Immune thrombocytopenia; IV Intravenous; kDa Kilodalton; KiH Knob-into-Hole; mAb Monoclonal antibody; MAC Membrane-attack complex; mCRC Metastatic colorectal cancer; MM Multipl myeloma; MOA Mechanism of action; MS Mass spectrometry; MUC1 Mucin 1; MG Myasthenia gravis; NB Nanobody; NK Natural killer; nsAA Nonstandard amino acid; NSCLC Non-small cell lung cancer; P. aeruginosa Pseudomonas aeruginosa; PD-1 Programmed cell death 1; PD-L1 Programmed cell death-ligand 1; PDI Protein disulfide isomerase; PECS Periplasmic expression cytometric screening; PK Pharmacokinetics; P. pastoris Pichia pastoris; PTM Post-translational modification; Rg Radius of gyration; RA Rheumatoid arthritis; RT-PCR Reverse transcription polymerase chain reaction; SAXS Small angle X-ray scattering; scF Single chain Fv; SCLC Small cell lung cancer; SDS-PAGE Sodium dodecyl sulfate-polyacrylamide gel electrophoresis; SEC Size exclusion chromatography; SEED Strand-exchange engineered domain; sRNA Small regulatory RNA; SRP Signal recognition particle; T1/2 Half-life; Tagg Aggregation temperature; TCR T cell receptor; TDB T cell-dependent bispecific; TF Tissue factor; TIR Translation initiation region; Tm Melting temperature; TNBC Triple-negative breast cancer; TNF Tumor necrosis factor; TPO Thrombopoietin; VEGF Vascular endothelial growth factor; vH Variable heavy chain; vL Variable light chain; vWF von Willebrand factor; WT Wild type.

Production of antibodies in SHuffle Escherichia coli strains

Antibodies are globally important macromolecules, used for research, diagnostics, and as therapeutics. The common mammalian antibody immunoglobulin G (IgG) is a complex glycosylated macromolecule, composed of two heavy chains and two light chains held together by multiple disulfide bonds. For this reason, IgG and related antibody fragments are usually produced through secretion from mammalian cell lines, such as Chinese Hamster Ovary cells. However, there is growing interest in production of antibodies in prokaryotic systems due to the potential for rapid and cheap production in a highly genetically manipulable system. Research on oxidative protein folding in prokaryotes has enabled engineering of Escherichia coli strains capable of producing IgG and other disulfide bonded proteins in the cytoplasm, known as SHuffle. In this protocol, we provide a review of research on prokaryotic antibody production, guidelines on cloning of antibody expression constructs, conditions for an initial expression and purification experiment, and parameters which may be optimized for increased purification yields. Last, we discuss the limitations of prokaryotic antibody production, and highlight potential future avenues for research on antibody expression and folding.

Engineered Fc-glycosylation switch to eliminate antibody effector function

Antibodies mediate effector functions through Fcγ receptor (FcγR) interactions and complement activation, causing cytokine release, degranulation, phagocytosis, and cell death. They are often undesired for development of therapeutic antibodies where only antigen binding or neutralization would be ideal. Effector elimination has been successful with extensive mutagenesis, but these approaches can potentially lead to manufacturability and immunogenicity issues. By switching the native glycosylation site from position 297 to 298, we created alternative antibody glycosylation variants in the receptor interaction interface as a novel strategy to eliminate the effector functions. The engineered glycosylation site at Asn298 was confirmed by SDS-PAGE, mass spectrometry, and X-ray crystallography (PDB code 6X3I). The lead NNAS mutant (S298N/T299A/Y300S) shows no detectable binding to mouse or human FcγRs by surface plasmon resonance analyses. The effector functions of the mutant are completely eliminated when measured in antibody-dependent cell-meditated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) assays. In vivo, the NNAS mutant made on an antibody against a human lymphocyte antigen does not deplete T cells or B cells in transgenic mice, in contrast to wild-type antibody. Structural study confirms the successful glycosylation switch to the engineered Asn298 site. The engineered glycosylation would clash with approaching FcγRs based on reported Fc-FcγR co-crystal structures. In addition, the NNAS mutants of multiple antibodies retain binding to antigens and neonatal Fc receptor, exhibit comparable purification yields and thermal stability, and display normal circulation half-life in mice and non-human primate. Our work provides a novel approach for generating therapeutic antibodies devoid of any ADCC and CDC activities with potentially lower immunogenicity.

Glycosylation of Therapeutic Proteins: A Critical Quality Attribute

Glycosylation is a common posttranslational modification of therapeutic proteins. The glycosylation pattern is dependent on many parameters such as the host cell line or the culture conditions. N- and O-linked glycans usually play a great role on the stability, safety, and efficacy of the drug. For this reason, glycosylation is considered as a critical quality attribute of therapeutic glycoproteins, and a thorough characterization should be performed, as well as a systematic control for each batch produced. This chapter gives a short presentation of the structure of glycans commonly found on recombinant therapeutic proteins, and their role on the properties of the drug, in terms of stability, pharmacokinetics, safety, and efficacy. Lastly, the use of mass spectrometry for the analysis of glycoproteins is briefly described.

A general Fc engineering platform for the next generation of antibody therapeutics

Rationale: Fc engineering has become the focus of antibody drug development. The current mutagenesis and in silico protein design methods are confined by the limited throughput and high cost, while the high-throughput phage display and yeast display technologies are not suitable for screening glycosylated Fc variants. Here we developed a mammalian cell display-based Fc engineering platform. Methods: By using mammalian cell display and next generation sequencing, we screened millions of Fc variants for optimized affinity and specificity for FcγRIIIa or FcγRIIb. The identified Fc variants with improved binding to FcγRIIIa were substituted into trastuzumab and rituximab and the effector function of antibodies were examined in the PBMC-based assay. On the other hand, the identified Fc variants with selectively enhanced FcγRIIb binding were applied to CD40 agonist antibody and the activities of the antibodies were measured on different cell assays. The immunostimulatory activity of CD40 antibodies was also evaluated by OVA-specific CD8+ T cell response model in FcγR/CD40-humanized mice. Results: Using this approach, we screened millions of Fc variant and successfully identified several novel Fc variants with enhanced FcγRIIIa or FcγRIIb binding. These identified Fc variants displayed a dramatic increase in antibody-dependent cellular cytotoxicity in PBMC-based assay. Novel variants with selectively enhanced FcγRIIb binding were also identified. CD40 agonist antibodies substituted with these Fc variants displayed activity more potent than the parental antibody in the in vitro and in vivo models.Conclusions: This approach increased the throughput of Fc variant screening from thousands to millions magnitude, enabled screening variants containing multiple mutations and could be integrated with glycoengineering technology, represents an ideal platform for Fc engineering. The initial efforts demonstrated the capability of the platform and the novel Fc variants could be substituted into nearly any antibody for the next generation of antibody therapeutics.

Taking the Hinge off: An Approach to Effector-Less Monoclonal Antibodies

A variety of Fc domain engineering approaches for abrogating the effector functions of mAbs exists. To address some of the limitations of the current Fc domain silencing approaches, we are exploring a less commonly considered option which relies on the deletion of the hinge. Removal of the hinge domain in humanized IgG1 and IgG4 mAbs obliterates their ability to bind to activating human Fc gamma receptors I and IIIA, while leaving their ability to engage their target antigen intact. Deletion of the hinge also reduces binding to the Fc neonatal receptor, although Fc engineering allows partial recovery of affinity. Engineering of the CH3 domain, stabilizes hinge deleted IgG4s and prevents Fab arm exchange. The faster clearing properties together with the pacified Fc make modality of the hinge deleted mAb an appealing solution for therapeutic and diagnostic applications.

BTN3A1 governs antitumor responses by coordinating αβ and γδ T cells

Gamma delta (γδ) T cells infiltrate most human tumors, but current immunotherapies fail to exploit their in situ major histocompatibility complex-independent tumoricidal potential. Activation of γδ T cells can be elicited by butyrophilin and butyrophilin-like molecules that are structurally similar to the immunosuppressive B7 family members, yet how they regulate and coordinate αβ and γδ T cell responses remains unknown. Here, we report that the butyrophilin BTN3A1 inhibits tumor-reactive αβ T cell receptor activation by preventing segregation of N-glycosylated CD45 from the immune synapse. Notably, CD277-specific antibodies elicit coordinated restoration of αβ T cell effector activity and BTN2A1-dependent γδ lymphocyte cytotoxicity against BTN3A1⁺ cancer cells, abrogating malignant progression. Targeting BTN3A1 therefore orchestrates cooperative killing of established tumors by αβ and γδ T cells and may present a treatment strategy for tumors resistant to existing immunotherapies.

sRNA-Based Screening Chromosomal Gene Targets and Modular Designing Escherichia coli for High-Titer Production of Aglycosylated Immunoglobulin G

The production of the aglycosylated immunoglobulin G (IgG) in Escherichia coli has received wide interest for its analytical and therapeutic applications. To enhance the production titer of IgG, we first used synthetic sRNAs to perform a systematical analysis of the gene expression in the translational level in the glycolytic pathway (module 1) and the tricarboxylic acid (TCA) cycle (module 2) to reveal the critical genes for the efficient IgG production. Second, to provide sufficient amino acid precursors for the protein biosynthesis, amino acid biosynthesis pathways (module 3) were enhanced to facilitate the IgG production. Upon integrated engineering of these genes in the three modules (module 1, aceF; module 2, gltA and acnA; module 3, serB) and optimization of fermentation conditions, the recombinant E. coli enabled a titer of the full-assembled IgG of 4.5 ± 0.6 mg/L in flask cultures and 184 ± 9.2 mg/L in the 5 L high cell density fed-batch fermenter, which is, as far as we know, the highest reported titer of IgG production in recombinant E. coli.

Differentiated agonistic antibody targeting CD137 eradicates large tumors without hepatotoxicity

CD137 (4-1BB) is a member of the TNFR superfamily that represents a promising target for cancer immunotherapy. Recent insights into the function of TNFR agonist antibodies implicate epitope, affinity, and IgG subclass as critical features, and these observations help explain the limited activity and toxicity seen with clinically tested CD137 agonists. Here, we describe the preclinical characterization of CTX-471, a fully human IgG4 agonist of CD137 that engages a unique epitope that is shared by human, cynomolgus monkey, and mouse and is associated with a differentiated pharmacology and toxicology profile. In vitro, CTX-471 increased IFN-γ production by human T cells in an Fcγ receptor-dependent (FcγR-dependent) manner, displaying an intermediate level of activity between 2 clinical-stage anti-CD137 antibodies. In mice, CTX-471 exhibited curative monotherapy activity in various syngeneic tumor models and showed a unique ability to cure mice of very large (~500 mm3) tumors compared with validated antibodies against checkpoints and TNFR superfamily members. Extremely high doses of CTX-471 were well tolerated, with no signs of hepatic toxicity. Collectively, these data demonstrate that CTX-471 is a unique CD137 agonist that displays an excellent safety profile and an unprecedented level of monotherapy efficacy against very large tumors.

Synthetic sRNA-Based Engineering of Escherichia coli for Enhanced Production of Full-Length Immunoglobulin G

Production of monoclonal antibodies (mAbs) receives considerable attention in the pharmaceutical industry. There has been an increasing interest in the expression of mAbs in Escherichia coli for analytical and therapeutic applications in recent years. Here, a modular synthetic biology approach is developed to rationally engineer E. coli by designing three functional modules to facilitate high-titer production of immunoglobulin G (IgG). First, a bicistronic expression system is constructed and the expression of the key genes in the pyruvate metabolism is tuned by the technologies of synthetic sRNA translational repression and gene overexpression, thus enhancing the cellular material and energy metabolism of E. coli for IgG biosynthesis (module 1). Second, to prevent the IgG biodegradation by proteases, the expression of a number of key proteases is identified and inhibited via synthetic sRNAs (module 2). Third, molecular chaperones are co-expressed to promote the secretion and folding of IgG (module 3). Synergistic integration of the three modules into the resulting recombinant E. coli results in a yield of the full-length IgG ≈150 mg L^-1 in a 5L fed-batch bioreactor. The modular synthetic biology approach could be of general use in the production of recombinant mAbs.

Considerations for the Design of Antibody-Based Therapeutics

Antibody-based proteins have become an important class of biologic therapeutics, due in large part to the stability, specificity, and adaptability of the antibody framework. Indeed, antibodies not only have the inherent ability to bind both antigens and endogenous immune receptors but also have proven extremely amenable to protein engineering. Thus, several derivatives of the monoclonal antibody format, including bispecific antibodies, antibody-drug conjugates, and antibody fragments, have demonstrated efficacy for treating human disease, particularly in the fields of immunology and oncology. Reviewed here are considerations for the design of antibody-based therapeutics, including immunological context, therapeutic mechanisms, and engineering strategies. First, characteristics of antibodies are introduced, with emphasis on structural domains, functionally important receptors, isotypic and allotypic differences, and modifications such as glycosylation. Then, aspects of therapeutic antibody design are discussed, including identification of antigen-specific variable regions, choice of expression system, use of multispecific formats, and design of antibody derivatives based on fragmentation, oligomerization, or conjugation to other functional moieties. Finally, strategies to enhance antibody function through protein engineering are reviewed while highlighting the impact of fundamental biophysical properties on protein developability.

Optimal combination of beneficial mutations for improved ADCC effector function of aglycosylated antibodies

The Fc region of IgG antibodies is crucial for binding to Fc receptors expressed on the surfaces of various immune leukocytes and eliciting therapeutic effector functions such as clearance of antibody-opsonized tumor cells. Despite abrogated Fc gamma receptor (FcγR) binding and therapeutic effector function in the absence of N-linked glycosylation at Asn297, the aglycosylated Fc region of IgG antibodies has bioprocessing advantages such as the absence of glycan heterogeneity and simple bacterial antibody production. Therefore, these antibodies have been comprehensively engineered as effector functional units for human therapy. In this work, we constructed a huge library of Fc variants with combinations of 25 beneficial mutations that were previously identified to improve binding of glycosylated or aglycosylated Fc regions to human FcγRs in previous studies. High-throughput screening of the resulting library led to the identification of an aglycosylated Fc variant that exhibited almost double the antibody-dependent cell-mediated cytotoxicity than wild-type glycosylated Fc. All mutations in this aglycosylated Fc variant were derived from previously identified beneficial mutations for engineered aglycosylated Fc variants as opposed to glycosylated variants, suggesting that significantly different sets of beneficial mutations are necessary to improve the effector function of aglycosylated Fc.

CH2 domain orientation of human immunoglobulin G in solution: Structural comparison of glycosylated and aglycosylated Fc regions using small-angle X-ray scattering

The N-linked glycan in immunoglobulin G is critical for the stability and function of the crystallizable fragment (Fc) region. Alteration of these protein properties upon the removal of the N-linked glycan has often been explained by the alteration of the CH2 domain orientation in the Fc region. To confirm this hypothesis, we examined the small-angle X-ray scattering (SAXS) profile of the glycosylated Fc region (gFc) and aglycosylated Fc region (aFc) in solution. Conformational characteristics of the CH2 domain orientation were validated by comparison with SAXS profiles theoretically calculated from multiple crystal structures of the Fc region with different CH2 domain orientations. The reduced chi-square values from the fitting analyses of gFc and aFc associated with the degree of openness or closure of each crystal structure, as determined from the first principal component that partially governed the variation of the CH2 domain orientation extracted by a singular value decomposition analysis. For both gFc and aFc, the best-fitted SAXS profiles corresponded to ones calculated based on the crystal structure of gFc that formed a "semi-closed" CH2 domain orientation. Collectively, the data indicated that the removal of the N-linked glycan only negligibly affected the CH2 domain orientation in solution. These findings will guide the development of methodology for the production of highly refined functional Fc variants.

Invited Commentary on Preventive Anti-Migraine Therapy (PAMT)

Migraine is a chronic paroxysmal neurological disorder characterized by multiphase attacks of head pain and a myriad of neurological symptoms. Chronic migraine causes a great personal and societal burden. Many patients are poorly responsive to, or non-compliant with, conventional migraine preventive therapies. For this reason, physicians are constantly looking for effective migraine prevention strategies. The recent introduction of an innovative pharmacological class useful for migraine prevention, namely monoclonal antibodies towards calcitonin gene-related peptide or its receptors, opens a new, immense therapeutic scenario. In this commentary, the development and efficacy of this novel class of preventive anti-migraine therapy have been discussed and compared with the conventional therapies of migraine prevention.

Escherichia coli inner membrane display system for high-throughput screening of dimeric proteins

Multimer formation is indispensable to the intrinsicbiologicalfunctions of many natural proteins. For example, the human immunoglobulin G (IgG) antibody has two variable regions (heavy chain variable domain [VH] and light chain variable domain [VL]) that must be assembled for specific antigen binding, and homodimerization of the antibody's Fc domain is essential for eliciting therapeutic effector functions. For the more efficient high-throughput directed evolution of multimeric proteins with ease of cultivation and handling, here we report a membrane protein drift and assembly (MPDA) system, in which a multimeric protein is displayed on a bacterial inner membrane by drifting and auto-assembling membrane-anchored subunit polypeptides. This system enabled the auto-assembly of membrane-tethered Fv domains (VH and VL) or the monomeric Fc domain into a functional hetero- or homodimeric protein complex on the bacterial inner membrane. This system could also be used to enrich a desired engineered Fc variant from a mixture containing a million-fold excess of wild-type Fc domain, indicating the applicability of the MPDA system for the high-throughput directed evolution of a variety of multimeric proteins, such as cytokines, enzymes, or structural proteins.

Understanding the in vivo fate of radioimmunoconjugates for nuclear imaging

Over the past 25 years, antibodies have emerged as extraordinarily promising vectors for the delivery of radionuclides to tumors for nuclear imaging. While radioimmunoconjugates often produce very high activity concentrations in target tissues, they also are frequently characterized by elevated activity concentrations in healthy organs as well. The root of this background uptake lies in the complex network of biological interactions between the radioimmunoconjugate and the subject. In this review, we seek to provide an overview of these interactions and thus paint a general picture of the in vivo fate of radioimmunoconjugates. To cover the entire story, we have divided our discussion into 2 parts. First, we will address the path of the entire radioimmunoconjugate as it travels through the body. And second, we will cover the fate of the radionuclide itself, as its course can diverge from the antibody under certain circumstances. Ultimately, our goal is to provide the nuclear imaging field with a resource covering these important-yet often underestimated-pathways.

Glycosylation engineering of therapeutic IgG antibodies: challenges for the safety, functionality and efficacy

Glycosylation of the Fc region of IgG has a profound impact on the safety and clinical efficacy of therapeutic antibodies. While the biantennary complex-type oligosaccharide attached to Asn297 of the Fc is essential for antibody effector functions, fucose and outer-arm sugars attached to the core heptasaccharide that generate structural heterogeneity (glycoforms) exhibit unique biological activities. Hence, efficient and quantitative glycan analysis techniques have been increasingly important for the development and quality control of therapeutic antibodies, and glycan profiles of the Fc are recognized as critical quality attributes. In the past decade our understanding of the influence of glycosylation on the structure/function of IgG-Fc has grown rapidly through X-ray crystallographic and nuclear magnetic resonance studies, which provides possibilities for the design of novel antibody therapeutics. Furthermore, the chemoenzymatic glycoengineering approach using endoglycosidase-based glycosynthases may facilitate the development of homogeneous IgG glycoforms with desirable functionality as next-generation therapeutic antibodies. Thus, the Fc glycans are fertile ground for the improvement of the safety, functionality, and efficacy of therapeutic IgG antibodies in the era of precision medicine.

Engineered aglycosylated full-length IgG Fc variants exhibiting improved FcγRIIIa binding and tumor cell clearance

FcγRIIIa, which is predominantly expressed on the surface of natural killer cells, plays a key role in antibody-dependent cell-mediated cytotoxicity (ADCC), a major effector function of therapeutic IgG antibodies that results in the death of aberrant cells. Despite the potential uses of aglycosylated IgG antibodies, which can be easily produced in bacteria and do not have complicated glycan heterogeneity issues, they show negligible binding to FcγRIIIa and abolish the activation of immune leukocytes for tumor cell clearance, in sharp contrast to most glycosylated IgG antibodies used in the clinical setting. For directed evolution of aglycosylated Fc variants that bind to FcγRIIIa and, in turn, exert potent ADCC effector function, we randomized the aglycosylated Fc region of full-length IgG expressed on the inner membrane of Escherichia coli. Multiple rounds of high-throughput screening using flow cytometry facilitated the isolation of aglycosylated IgG Fc variants that exhibited higher binding affinity to FcγRIIIa-158V and FcγRIIIa-158F compared with clinical-grade trastuzumab (Herceptin®). The resulting aglycosylated trastuzumab IgG antibody Fc variants could elicit strong peripheral blood mononuclear cell-mediated ADCC without glycosylation in the Fc region.

Crystallizable Fragment Glycoengineering for Therapeutic Antibodies Development

Monoclonal antibody (mAb)-based therapeutics are the fastest growing class of human pharmaceuticals. They are typically IgG1 molecules with N-glycans attached to the N297 residue on crystallizable fragment (Fc). Different Fc glycoforms impact their effector function, pharmacokinetics, stability, aggregation, safety, and immunogenicity. Fc glycoforms affect mAbs effector functions including antibody-dependent cell-mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC) by modulating the Fc-FcγRs and Fc-C1q interactions. While the terminal galactose enhances CDC activity, the fucose significantly decreases ADCC. Defucosylated immunoglobulin Gs (IgGs) are thus highly pursued as next-generation therapeutic mAbs with potent ADCC at reduced doses. A plethora of cell glycoengineering and chemoenzymatic glycoengineering strategies is emerging to produce IgGs with homogenous glycoforms especially without core fucose. The chemoenzymatic glycosylation remodeling also offers useful avenues for site-specific conjugations of small molecule drugs onto mAbs. Herein, we review the current progress of IgG-Fc glycoengineering. We begin with the discussion of the structures of IgG N-glycans and biosynthesis followed by reviewing the impact of IgG glycoforms on antibody effector functions and the current Fc glycoengineering strategies with emphasis on Fc defucosylation. Furthermore, we briefly discuss two novel therapeutic mAbs formats: aglycosylated mAbs and Fc glycan specific antibody-drug conjugates (ADCs). The advances in the understanding of Fc glycobiology and development of novel glycoengineering technologies have facilitated the generation of therapeutic mAbs with homogenous glycoforms and improved therapeutic efficacy.

Library-based display technologies: where do we stand?

Over the past two decades, library-based display technologies have been staggeringly optimized since their appearance in order to mimic the process of natural molecular evolution. Display technologies are essential for the isolation of specific high-affinity binding molecules (proteins, polypeptides, nucleic acids and others) for diagnostic and therapeutic applications in cancer, infectious diseases, autoimmune, neurodegenerative, inflammatory pathologies etc. Applications extend to other fields such as antibody and enzyme engineering, cell-free protein synthesis and the discovery of protein-protein interactions. Phage display technology is the most established of these methods but more recent fully in vitro alternatives, such as ribosome display, mRNA display, cis-activity based (CIS) display and covalent antibody display (CAD), as well as aptamer display and in vitro compartmentalization, offer advantages over phage in library size, speed and the display of unnatural amino acids and nucleotides. Altogether, they have produced several molecules currently approved or in diverse stages of clinical or preclinical testing and have provided researchers with tools to address some of the disadvantages of peptides and nucleotides such as their low affinity, low stability, high immunogenicity and difficulty to cross membranes. In this review we assess the fundamental technological features and point out some recent advances and applications of display technologies.

Biological Characterization of a Stable Effector Functionless (SEFL) Monoclonal Antibody Scaffold in Vitro

The stable effector functionLess (SEFL) antibody was designed as an IgG1 antibody with a constant region that lacks the ability to interact with Fcγ receptors. The engineering and stability and pharmacokinetic assessments of the SEFL scaffold is described in the accompanying article (Jacobsen, F. W., Stevenson, R., Li, C., Salimi-Moosavi, H., Liu, L., Wen, J., Luo, Q., Daris, K., Buck, L., Miller, S., Ho, S-Y., Wang, W., Chen, Q., Walker, K., Wypych, J., Narhi, L., and Gunasekaran, K. (2017) J. Biol. Chem 292). The biological properties of these SEFL antibodies were assessed in a variety of human and cynomolgus monkey in vitro assays. Binding of parent molecules and their SEFL variants to human and cynomolgus monkey FcγRs were evaluated using flow cytometry-based binding assays. The SEFL variants tested showed decreased binding affinity to human and cynomolgus FcγRs compared with the wild-type IgG1 antibody. In addition, SEFL variants demonstrated no antibody-dependent cell-mediated cytotoxicity in vitro against Daudi cells with cynomolgus monkey peripheral blood mononuclear cells, and had minimal complement-dependent cytotoxicity activity similar to that of the negative control IgG2 in a CD20⁺ human Raji lymphoma cell line. SEFL mutations eliminated off-target antibody-dependent monocyte phagocytosis of cynomolgus monkey platelets, and cynomolgus platelet activation in vitro These experiments demonstrate that the SEFL modifications successfully eliminated Fc-associated effector binding and functions.

Advances in Therapeutic Fc Engineering - Modulation of IgG-Associated Effector Functions and Serum Half-life

Today, monoclonal immunoglobulin gamma (IgG) antibodies have become a major option in cancer therapy especially for the patients with advanced or metastatic cancers. Efficacy of monoclonal antibodies (mAbs) is achieved through both its antigen-binding fragment (Fab) and crystallizable fragment (Fc). Fab can specifically recognize tumor-associated antigen (TAA) and thus modulate TAA-linked downstream signaling pathways that may lead to the inhibition of tumor growth, induction of tumor apoptosis, and differentiation. The Fc region can further improve mAbs’ efficacy by mediating effector functions such as antibody-dependent cellular cytotoxicity, complement-dependent cytotoxicity, and antibody-dependent cell-mediated phagocytosis. Moreover, Fc is the region interacting with the neonatal Fc receptor in a pH-dependent manner that can slow down IgG’s degradation and extend its serum half-life. Loss of the antibody Fc region dramatically shortens its serum half-life and weakens its anticancer effects. Given the essential roles that the Fc region plays in the modulation of the efficacy of mAb in cancer treatment, Fc engineering has been extensively studied in the past years. This review focuses on the recent advances in therapeutic Fc engineering that modulates its related effector functions and serum half-life. We also discuss the progress made in aglycosylated mAb development that may substantially reduce the cost of manufacture but maintain similar efficacies as conventional glycosylated mAb. Finally, we highlight several Fc engineering-based mAbs under clinical trials.

Site-Specific Conjugation of Monomethyl Auristatin E to Anti-CD30 Antibodies Improves Their Pharmacokinetics and Therapeutic Index in Rodent Models

Antibody-drug conjugates (ADCs) have demonstrated clinical benefits that have led to the recent FDA approval of KADCYLA and ADCETRIS. Most ADCs that are currently in clinical use or development, including ADCETRIS, are produced by chemical conjugation of a toxin via either lysine or cysteine residues, inevitably leading to heterogeneous products with variable drug-to-antibody ratios (DARs). Here, we describe the in vitro and in vivo characterization of four novel ADCs that are based on the anti-CD30 antibody cAC10, which has the same polypeptide backbone as ADCETRIS, and compare the results with the latter. Bacterial transglutaminase (BTG) was exploited to site-specifically conjugate derivatives of monomethyl auristatin E (all comprising a cleavable linker) to the glutamine at positions 295 and 297 of cAC10, thereby yielding homogeneous ADCs with a DAR of 4. In vitro cell toxicity experiments using two different CD30-positive cell lines (Karpas 299 and Raji-CD30(+)) revealed comparable EC50 values for ADCETRIS (1.8 ± 0.4 and 3.6 ± 0.6 ng/mL, respectively) and the four cAC10-based ADCs (2.0 ± 0.4 to 4.9 ± 1.0 ng/mL). Quantitative time-dependent in vivo biodistribution studies (3-96 h p.i.) in normal and xenografted (Karpas 299 cells) SCID mice were performed with a selected (125)I-radioiodinated cAC10 ADC and compared with that of (125)I-ADCETRIS. The chemo-enzymatically conjugated, radioiodinated ADC showed higher tumor uptake (17.84 ± 2.2% ID/g 24 h p.i.) than (125)I-ADCETRIS (10.5 ± 1.8% ID/g 24 h p.i.). Moreover, (125)I-ADCETRIS exhibited higher nontargeted liver and spleen uptake. In line with these results, the maximum tolerated dose of the BTG-coupled ADC (>60 mg/kg) was significantly higher than that of ADCETRIS (18 mg/kg) in rats. These results suggest that homogeneous ADCs display improved pharmacokinetics and better therapeutic indexes compared to those of chemically modified ADCs with variable DARs.