Comprehensive characterization of higher order structure changes in methionine oxidized monoclonal antibodies via NMR chemometric analysis and biophysical approaches.

The higher order structure (HOS) of monoclonal antibodies (mAbs) is an important quality attribute with strong contribution to clinically relevant biological functions and drug safety. Due to the multi-faceted nature of HOS, the synergy of multiple complementary analytical approaches can substantially improve the understanding, accuracy, and resolution of HOS characterization. In this study, we applied one- and two-dimensional (1D and 2D) nuclear magnetic resonance (NMR) spectroscopy coupled with chemometric analysis, as well as circular dichroism (CD), differential scanning calorimetry (DSC), and fluorescence spectroscopy as orthogonal methods, to characterize the impact of methionine (Met) oxidation on the HOS of an IgG1 mAb. We used a forced degradation method involving concentration-dependent oxidation by peracetic acid, in which Met oxidation is site-specifically quantified by liquid chromatography-mass spectrometry. Conventional biophysical techniques report nuanced results, in which CD detects no change to the secondary structure and little change in the tertiary structure. Yet, DSC measurements show the destabilization of Fab and Fc domains due to Met oxidation. More importantly, our study demonstrates that 1D and 2D NMR and chemometric analysis can provide semi-quantitative analysis of chemical modifications and resolve localized conformational changes with high sensitivity. Furthermore, we leveraged a novel 15N-Met labeling technique of the antibody to directly observe structural perturbations at the oxidation sites. The NMR methods described here to probe HOS changes are highly reliable and practical in biopharmaceutical characterization.

Characterization of Higher Order Structural Changes of a Thermally Stressed Monoclonal Antibody via Mass Spectrometry Footprinting and Other Biophysical Approaches.

Characterizing changes in the higher order structure (HOS) of monoclonal antibodies upon stressed conditions is critical to gaining a better understanding of the product and process. One single biophysical approach may not be best suited to assess HOS comprehensively; thus, the synergy from multiple, complementary approaches improves characterization accuracy and resolution. In this study, we employed two mass spectrometry (MS )-based footprinting techniques, namely, fast photochemical oxidation of proteins (FPOP)-MS and hydrogen-deuterium exchange (HDX)-MS, supported by dynamic light scattering (DLS), differential scanning calorimetry (DSC), circular dichroism (CD), and nuclear magnetic resonance (NMR) to study changes to the HOS of a mAb upon thermal stress. The biophysical techniques report a nuanced characterization of the HOS in which CD detects no changes to the secondary or tertiary structure, yet DLS measurements show an increase in the hydrodynamic radius. DSC indicates that the stability decreases, and chemical or conformational changes accumulate with incubation time according to NMR. Furthermore, whereas HDX-MS does not indicate HOS changes, FPOP-MS footprinting reveals conformational changes at residue resolution for some amino acids. The local phenomena observed with FPOP-MS indicate that several residues show various patterns of degradation during thermal stress: no change, an increase in solvent exposure, and a biphasic response to solvent exposure. All evidences show that FPOP-MS efficiently resolves subtle structural changes and novel degradation pathways upon thermal stress treatment at residue-level resolution.

Elucidation of lipid nanoparticle surface structure in mRNA vaccines.

Lipid nanoparticles (LNPs) have been used as a carrier for messenger RNA (mRNA) vaccines. Surface properties of LNPs are important to the stability and function of mRNA vaccines. Polyethylene-glycol (PEG) is a functional lipid at the surface of LNPs that improves colloidal stability, increases circulation time, and impacts cellular uptake. In this study, we explore in-depth lipid composition at the surface of mRNA-LNPs using high-field nuclear magnetic resonance (NMR) spectroscopy. Our results provide a unique surface lipid profile of intact LNPs identifying PEG chains and partial ionizable lipids are present with quantification capability. The surface PEG density is determined to reveal the brush-like conformation on the surface of mRNA-LNPs. Furthermore, we implement a diffusion NMR strategy for routine testing of formulated drug products during drug development. Comparative NMR analysis of different vaccine preparations and stability samples provides a global view of the mRNA-LNP surface structure for enhanced product knowledge.

Application of NMR and Chemometric Analyses to Better Understand the Quality Attributes in pH and Thermally Degraded Monoclonal Antibodies.

PURPOSE: Nuclear magnetic resonance (NMR) spectroscopy provides the sensitivity and specificity to probe the higher order structure (HOS) of monoclonal antibodies (mAbs) for potential changes. This study demonstrates an application of chemometric tools to measure differences in the NMR spectra of mAbs after forced degradation relative to the respective unstressed starting materials. METHODS: Samples of adalimumab (Humira, ADL-REF) and trastuzumab (Herceptin, TRA-REF) were incubated in three buffer-pH conditions at 40°C for 4 weeks to compare to a control sample that was left unstressed. Replicate 1D 1H and 2D 1H-13C HMQC NMR spectra were collected on all samples. Chemometric analyses such as Easy Comparability of HOS (ECHOS), PROtein FIngerprinting by Lineshape Enhancement (PROFILE), and Principal Component Analysis (PCA) were applied to capture and quantitate differences between the spectra. RESULTS: Visual and statistical inspection of the 2D 1H-13C HMQC spectra of adalimumab and trastuzumab after forced degradation conditions shows no changes in the spectra relative to the unstressed material. Chemometric analysis of the 1D 1H NMR spectra shows only minor changes in the spectra of adalimumab after forced degradation, but significant differences in trastuzumab. CONCLUSION: The chemometric analyses support the lack of statistical differences in the structure of pH-thermal stressed adalimumab, however, it reveals conformational changes or chemical modifications in trastuzumab after forced degradation. Application of chemometrics in comparative NMR studies enables HOS characterization and showcases the sensitivity and specificity in detecting differences in the spectra of mAbs after pH-thermal forced degradation with respect to local and global protein structure.

Oligonucleotide mapping via mass spectrometry to enable comprehensive primary structure characterization of an mRNA vaccine against SARS-CoV-2.

Oligonucleotide mapping via liquid chromatography with UV detection coupled to tandem mass spectrometry (LC-UV-MS/MS) was recently developed to support development of Comirnaty, the world's first commercial mRNA vaccine which immunizes against the SARS-CoV-2 virus. Analogous to peptide mapping of therapeutic protein modalities, oligonucleotide mapping described here provides direct primary structure characterization of mRNA, through enzymatic digestion, accurate mass determinations, and optimized collisionally-induced fragmentation. Sample preparation for oligonucleotide mapping is a rapid, one-pot, one-enzyme digestion. The digest is analyzed via LC-MS/MS with an extended gradient and resulting data analysis employs semi-automated software. In a single method, oligonucleotide mapping readouts include a highly reproducible and completely annotated UV chromatogram with 100% maximum sequence coverage, and a microheterogeneity assessment of 5' terminus capping and 3' terminus poly(A)-tail length. Oligonucleotide mapping was pivotal to ensure the quality, safety, and efficacy of mRNA vaccines by providing: confirmation of construct identity and primary structure and assessment of product comparability following manufacturing process changes. More broadly, this technique may be used to directly interrogate the primary structure of RNA molecules in general.

A genetic off-target event in a site-specific integration cell line expressing monoclonal antibody has no impact on commercial suitability.

Site-specific integration (SSI) cell line systems are gaining popularity for biotherapeutic development and production. Despite the proven advantages for these expression hosts, the SSI system is still susceptible to rare off-target events and potential vector rearrangements. Here we describe the development process of an SSI cell line for production of an IgG1 monoclonal antibody (mAb-086). During cell line generational studies to assess suitability of clone C10 for commercial purposes, restriction fragment lengths of genomic DNA harboring the light chain (LC) were not in agreement with the predicted size. We first confirmed that the SSI landing-pad achieved occupancy of the desired expression plasmid. Additional investigation revealed that random integration had occurred, resulting in the acquisition of a partial copy of the LC and a full-length copy of the heavy chain (HC) at a different locus in the host genome. This off-target event had no impact on the genotypic consistency and phenotypic stability of the cell line, the production process, or the drug substance product quality. Given the genetic, phenotypic, and process consistency of the cell line, clone C10 was deemed suitable as a manufacturing cell line.

Cysteine metabolic engineering and selective disulfide reduction produce superior antibody-drug-conjugates.

Next-generation site-specific cysteine-based antibody-drug-conjugates (ADCs) broaden therapeutic index by precise drug-antibody attachments. However, manufacturing such ADCs for clinical validation requires complex full reduction and reoxidation processes, impacting product quality. To overcome this technical challenge, we developed a novel antibody manufacturing process through cysteine (Cys) metabolic engineering in Chinese hamster ovary cells implementing a unique cysteine-capping technology. This development enabled a direct conjugation of drugs after chemoselective-reduction with mild reductant tris(3-sulfonatophenyl)phosphine. This innovative platform produces clinical ADC products with superior quality through a simplified manufacturing process. This technology also has the potential to integrate Cys-based site-specific conjugation with other site-specific conjugation methodologies to develop multi-drug ADCs and exploit multi-mechanisms of action for effective cancer treatments.

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.

Scientific Best Practices for Primary Sequence Confirmation and Sequence Variant Analysis in the Development of Therapeutic Proteins.

In this commentary, we will provide a high-level introduction into LC-MS product characterization methodologies deployed throughout biopharmaceutical development. The ICH guidelines for early and late phase filings is broad so that it is applicable to diverse biotherapeutic products in the clinic and industry pipelines. This commentary is meant to address areas of protein primary sequence confirmation and sequence variant analysis where ambiguity exists in industry on the specific scope of work that is needed to fulfill the general guidance that is given in sections Q5b and Q6b. This commentary highlights the discussion and outcomes of two recent workshops centering on the application of LC-MS to primary structure confirmation and sequence variant analysis (SVA) that were held at the 2018 and 2019 CASSS Practical Applications of Mass Spectrometry in the Biotechnology Industry Symposia in San Francisco, CA and Chicago, IL, respectively. Recommendations from the conferences fall into two distinct but related areas; 1) consolidation of opinions amongst industry stakeholders on the specific definitions of peptide mapping and peptide sequencing for primary structure confirmation and the technologies used for both, as they relate to regulatory expectations and submissions and 2) development of fit-for-purpose strategy to define appropriate assay controls in SVA experiments.

Automated, High-Throughput Infrared Spectroscopy for Secondary Structure Analysis of Protein Biopharmaceuticals.

Protein higher order structure (HOS) is an important product quality attribute that governs the structure-function characteristics, safety, and efficacy of therapeutic proteins. Infrared (IR) spectroscopy has long been recognized as a powerful biophysical tool in determining protein secondary structure and monitoring the dynamic structural changes. Such biophysics analyses help establish process and product knowledge, understand the impact of upstream (cell culture) and downstream (purification) process conditions, create stable formulations, monitor product stability, and assess product comparability when process improvements are implemented (or establish biosimilarity to originator products). This paper provides an overview of a novel automated mid-IR spectroscopic technique called microfluidic modulation spectroscopy (MMS) for the characterization of protein secondary structure. The study demonstrates that MMS secondary structure analysis of therapeutic monoclonal antibodies (mAb) is comparable with a conventional Fourier transform infrared (FTIR) method. More importantly the study shows MMS exhibits higher sensitivity and repeatability for low concentration samples over FTIR, as well as provides automated operation and superior robustness with simplified data analysis, increasing the utility of the instrument in determination of mAb secondary structure. Therefore, we propose that the MMS method can be widely applied in characterization and comparability/biosimilarity studies for biopharmaceutical process and product development.

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 ß-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.

Crystal Structures of PF-06438179/GP1111, an Infliximab Biosimilar.

BACKGROUND: Higher-order structure (HOS) assessment is an important component of biosimilarity evaluations. While established spectroscopic methods are routinely used to characterize structure and evaluate similarity, the addition of X-ray crystallographic analysis to these biophysical methods enables orthogonal elucidation of HOS at higher resolution. METHODS: Crystal structures of the infliximab biosimilar PF-06438179/GP1111 and the reference product Remicade®, sourced from US and European Union markets, were determined and compared to evaluate HOS similarity. Analytical ultracentrifugation studies were conducted to understand reversible self-association. RESULTS: In contrast to more routine spectroscopic methods, the crystal structures enable three-dimensional assessment of complementarity-determining regions and other local regions at near-atomic resolution. The biosimilar structures are highly similar to those of the reference product, as demonstrated visually and though all-atom root-mean-squared deviation measurements. CONCLUSION: The structures provide new insights into the physicochemical properties of the proposed biosimilar and the reference product, further strengthening the 'totality of evidence' in the evaluation of similarity.

Utility of High Resolution NMR Methods to Probe the Impact of Chemical Modifications on Higher Order Structure of Monoclonal Antibodies in Relation to Antigen Binding.

PURPOSE: An understanding of higher order structure (HOS) of monoclonal antibodies (mAbs) could be critical to predicting its function. Amongst the various factors that can potentially affect HOS of mAbs, chemical modifications that are routinely encountered during production and long-term storage are of significant interest. METHODS: To this end, two Pfizer mAbs were subjected to forced deamidation stress for a period of eight weeks. Samples were aliquoted at various time points and high resolution accurate mass liquid chromatography-mass spectrometry (LC-MS/MS) was performed using low-artifact trypsin digestion (LATD) peptide mapping to identify and quantify chemical modifications. 2D backbone amide and sidechain methyl NMR spectra were acquired to gauge the effect of HOS changes upon chemical modification. Differential scanning calorimetry was also performed to assess the effect of thermal stability of mAbs upon modification. Finally, functional studies via target-binding based ELISA were performed to connect HOS changes to any loss of potency. RESULTS: The extent of deamidation in the mAb domains were quantified by LC-MS/MS. The HOS changes as obtained from 2D NMR were mostly localized around the affected sites leaving the overall structure relatively unchanged. The antigen-antibody binding of the mAbs, in spite of deamidation in the Fab region, remains unchanged. CONCLUSION: This case study provides an integrated approach of relating chemical modifications in mAb domains with possible changes in HOS. This can be potentially used to assess a possible loss of potency within the structure-function paradigm of proteins in an orthogonal manner.

Interlaboratory Comparison of Hydrogen-Deuterium Exchange Mass Spectrometry Measurements of the Fab Fragment of NISTmAb.

Hydrogen-deuterium exchange mass spectrometry (HDX-MS) is an established, powerful tool for investigating protein-ligand interactions, protein folding, and protein dynamics. However, HDX-MS is still an emergent tool for quality control of biopharmaceuticals and for establishing dynamic similarity between a biosimilar and an innovator therapeutic. Because industry will conduct quality control and similarity measurements over a product lifetime and in multiple locations, an understanding of HDX-MS reproducibility is critical. To determine the reproducibility of continuous-labeling, bottom-up HDX-MS measurements, the present interlaboratory comparison project evaluated deuterium uptake data from the Fab fragment of NISTmAb reference material (PDB: 5K8A ) from 15 laboratories. Laboratories reported ∼89 800 centroid measurements for 430 proteolytic peptide sequences of the Fab fragment (∼78 900 centroids), giving ∼100% coverage, and ∼10 900 centroid measurements for 77 peptide sequences of the Fc fragment. Nearly half of peptide sequences are unique to the reporting laboratory, and only two sequences are reported by all laboratories. The majority of the laboratories (87%) exhibited centroid mass laboratory repeatability precisions of ⟨ sLab⟩ ≤ (0.15 ± 0.01) Da (1σx̅). All laboratories achieved ⟨sLab⟩ ≤ 0.4 Da. For immersions of protein at THDX = (3.6 to 25) °C and for D2O exchange times of tHDX = (30 s to 4 h) the reproducibility of back-exchange corrected, deuterium uptake measurements for the 15 laboratories is σreproducibility15 Laboratories( tHDX) = (9.0 ± 0.9) % (1σ). A nine laboratory cohort that immersed samples at THDX = 25 °C exhibited reproducibility of σreproducibility25C cohort( tHDX) = (6.5 ± 0.6) % for back-exchange corrected, deuterium uptake measurements.

Evolution of a comprehensive, orthogonal approach to sequence variant analysis for biotherapeutics.

Amino acid sequence variation in protein therapeutics requires close monitoring during cell line and cell culture process development. A cross-functional team of Pfizer colleagues from the Analytical and Bioprocess Development departments worked closely together for over 6 years to formulate and communicate a practical, reliable sequence variant (SV) testing strategy with state-of-the-art techniques that did not necessitate more resources or lengthen project timelines. The final Pfizer SV screening strategy relies on next-generation sequencing (NGS) and amino acid analysis (AAA) as frontline techniques to identify mammalian cell clones with genetic mutations and recognize cell culture process media/feed conditions that induce misincorporations, respectively. Mass spectrometry (MS)-based techniques had previously been used to monitor secreted therapeutic products for SVs, but we found NGS and AAA to be equally informative, faster, less cumbersome screening approaches. MS resources could then be used for other purposes, such as the in-depth characterization of product quality in the final stages of commercial-ready cell line and culture process development. Once an industry-wide challenge, sequence variation is now routinely monitored and controlled at Pfizer (and other biopharmaceutical companies) through increased awareness, dedicated cross-line efforts, smart comprehensive strategies, and advances in instrumentation/software, resulting in even higher product quality standards for biopharmaceutical products.

Practical approaches for overcoming challenges in heightened characterization of antibody-drug conjugates with new methodologies and ultrahigh-resolution mass spectrometry.

Antibody-drug conjugation strategies are continuously evolving as researchers work to improve the safety and efficacy of the molecules. However, as a part of process and product development, confirmation of the resulting innovative structures requires new, specialized mass spectrometry (MS) approaches and methods, as compared to those already established for antibody-drug conjugates (ADCs) and the heightened characterization practices used for monoclonal antibodies (mAbs), in order to accurately elucidate the resulting conjugate forms, which can sometimes have labile chemical bonds and more extreme chemical properties like hydrophobic patches. Here, we discuss practical approaches for characterization of ADCs using new methodologies and ultrahigh-resolution MS, and provide specific examples of these approaches. Denaturing conditions of typical liquid chromatography (LC)/MS analyses impede the successful detection of intact, 4-chain ADCs generated via cysteine site-directed chemistry approaches where hinge region disulfide bonds are partially reduced. However, this class of ADCs is detected intact reliably under non-denaturing size-exclusion chromatography/MS conditions, also referred to as native MS. For ADCs with acid labile linkers such as one used for conjugation of calicheamicin, careful selection of mobile phase composition is critical to the retention of intact linker-payload during LC/MS analysis. Increasing the pH of the mobile phase prevented cleavage of a labile bond in the linker moiety, and resulted in retention of the intact linker-payload. In-source fragmentation also was observed with typical electrospray ionization (ESI) source parameters during intact ADC mass analysis for a particular surface-accessible linker-payload moiety conjugated to the heavy chain C-terminal tag, LLQGA (via transglutaminase chemistry). Optimization of additional ESI source parameters such as cone voltages, gas pressures and ion transfer parameters led to minimal fragmentation and optimal sensitivity. Ultrahigh-resolution (UHR) MS, combined with reversed phase-ultrahigh performance (RP-UHP)LC and use of the FabRICATOR® enzyme, provides a highly resolving, antibody subunit-domain mapping method that allows rapid confirmation of integrity and the extent of conjugation. For some ADCs, the hydrophobic nature of the linker-payload hinders chromatographic separation of the modified subunit/domains or causes very late elution/poor recovery. As an alternative to the traditionally used C4 UHPLC column chemistry, a diphenyl column resulted in the complete recovery of modified subunit/domains. For ADCs based on maleimide chemistry, control of pH during proteolytic digestion is critical to minimize ring-opening. The optimum pH to balance digestion efficiency and one that does not cause ring opening needed to be established for successful peptide mapping.

Biopharmaceutical Informatics: supporting biologic drug development via molecular modelling and informatics.

OBJECTIVES: The purpose of this article is to introduce an emerging field called 'Biopharmaceutical Informatics'. It describes how tools from Information technology and Molecular Biophysics can be adapted, developed and gainfully employed in discovery and development of biologic drugs. KEY FINDINGS: The findings described here are based on literature surveys and the authors' collective experiences in the field of biologic drug product development. A strategic framework to forecast early the hurdles faced during drug product development is weaved together and elucidated using chemical degradation as an example. Efficiency of translating biologic drug discoveries into drug products can be significantly improved by combining learnings from experimental biophysical and analytical data on the drug candidates with molecular properties computed from their sequences and structures via molecular modeling and simulations. SUMMARY: Biopharmaceutical Informatics seeks to promote applications of computational tools towards discovery and development of biologic drugs. When fully implemented, industry-wide, it will enable rapid materials-free developability assessments of biologic drug candidates at early stages as well as streamline drug product development activities such as commercial scale production, purification, formulation, analytical characterization, safety and in vivo performance.

A View on the Importance of "Multi-Attribute Method" for Measuring Purity of Biopharmaceuticals and Improving Overall Control Strategy.

Today, we are experiencing unprecedented growth and innovation within the pharmaceutical industry. Established protein therapeutic modalities, such as recombinant human proteins, monoclonal antibodies (mAbs), and fusion proteins, are being used to treat previously unmet medical needs. Novel therapies such as bispecific T cell engagers (BiTEs), chimeric antigen T cell receptors (CARTs), siRNA, and gene therapies are paving the path towards increasingly personalized medicine. This advancement of new indications and therapeutic modalities is paralleled by development of new analytical technologies and methods that provide enhanced information content in a more efficient manner. Recently, a liquid chromatography-mass spectrometry (LC-MS) multi-attribute method (MAM) has been developed and designed for improved simultaneous detection, identification, quantitation, and quality control (monitoring) of molecular attributes (Rogers et al. MAbs 7(5):881-90, 2015). Based on peptide mapping principles, this powerful tool represents a true advancement in testing methodology that can be utilized not only during product characterization, formulation development, stability testing, and development of the manufacturing process, but also as a platform quality control method in dispositioning clinical materials for both innovative biotherapeutics and biosimilars.

Evidence for intermolecular domain exchange in the Fab domains of dimer and oligomers of an IgG1 monoclonal antibody.

Recombinant protein therapeutics have become increasingly useful in combating human diseases, such as cancer and those of genetic origin. One quality concern for protein therapeutics is the content and the structure of the aggregated proteins in the product, due to the potential immunogenicity of these aggregates. Collective efforts have led to a better understanding of some types of protein aggregates, and have revealed the diversity in the structure and cause of protein aggregation. In this work we used a broad range of analytical techniques to characterize the quinary structure (complexes in which each composing unit maintains native quaternary structure) of the stable non-covalent dimer and oligomers of a monoclonal IgG1λ antibody. The results supported a mechanism of intermolecular domain exchange involving the Fab domains of 2 or more IgG molecules. This mechanism can account for the native-like higher order (secondary, tertiary and disulfide bonding) structure, the stability of the non-covalent multimers, and the previously observed partial loss of the antigen-binding sites without changing the antigen-binding affinity and kinetics of the remaining sites (Luo et al., 2009, mAbs 1:491). Furthermore, the previously observed increase in the apparent affinity to various Fcγ receptors (ibid), which may potentially promote immunogenicity, was also explained by the quinary structure proposed here. Several lines of evidence indicated that the formation of multimers by the mechanism of intermolecular domain exchange took place mostly during expression, not in the purified materials. The findings in this work will advance our knowledge of the mechanisms for aggregation in therapeutic monoclonal antibodies.