Native SEC and Reversed-Phase LC-MS Reveal Impact of Fab Glycosylation of Anti-SARS-COV-2 Antibodies on Binding to the Receptor Binding Domain.

The binding affinity of monoclonal antibodies (mAbs) for their intended therapeutic targets is often affected by chemical and post-translational modifications in the antigen binding (Fab) domains. A new two-dimensional analytical approach is described here utilizing native size exclusion chromatography (SEC) to separate populations of antibodies and bound antibody-antigen complexes for subsequent characterization of these modifications by reversed-phase (RP) liquid chromatography-mass spectrometry (LC-MS) at the intact antibody level. Previously, we utilized peptide mapping to measure modifications impacting binding. However, in this study, the large size of the modification (N-glycosylation) allowed assessing its impact from small amounts (∼20 ug) of intact antibody, without the need for peptide mapping. Here, we apply the native SEC-based competitive binding assay to quickly and qualitatively investigate the effects of Fab glycosylation of four antispike protein mAbs that were developed for use in the treatment of COVID-19 disease. Three of the mAbs were observed to have consensus N-glycosylation sites (N-X-T/S) in the Fab domains, a relatively rare occurrence in therapeutic mAbs. The goal of the study was to characterize the levels of Fab glycosylation present, as well as determine the impact of glycosylation on binding to the spike protein receptor binding domain (RBD) and the ability of the mAbs to inhibit RBD-ACE2 interaction at the intact antibody level, with minimal sample treatment and preparation. The three mAbs with Fab N-glycans were found to have glycosylation profiles ranging from full occupancy at each Fab (in one mAb) to partially glycosylated with mixed populations of two, one, or no glycan moieties. Competitive SEC analysis of mAb-RBD revealed that the glycosylated antibody populations outcompete their nonglycosylated counterparts for the available RBD molecules. This competitive SEC binding analysis was applied to investigate the three-body interaction of a glycosylated mAb blocking the interaction between endogenous binding partners RBD-ACE2, finding that both glycosylated and nonglycosylated mAb populations bound to RBD with high enough affinity to block RBD-ACE2 binding.

Impact of antibody subclass and disulfide isoform differences on the biological activity of CD200R and ßklotho agonist antibodies.

Agonism of cell surface receptors by monoclonal antibodies is dependent not only on its ability to bind the target, but also to deliver a biological signal through receptors to the cell. Immunoglobulin G2 antibodies (IgG2s) are made up of a mixture of distinct isoforms (IgG2-A, -B and A/B), which differ by the disulfide connectivity at the hinge region. When evaluating panels of agonistic antibodies against CD200 receptor (CD200R) or ßklotho receptor (ßklotho), we noticed striking activity differences of IgG1 or IgG2 antibodies with the same variable domains. For the CD200R antibody, the IgG2 antibody demonstrated higher activity than the IgG1 or IgG4 antibody. More significantly, for ßklotho, agonist antibodies with higher biological activity as either IgG2 or IgG1 were identified. In both cases, ion exchange chromatography was able to isolate the bioactivity to the IgG2-B isoform from the IgG2 parental mixture. The subclass-related increase in agonist activity was not correlated with antibody aggregation or binding affinity, but was driven by enhanced avidity for the CD200R antibody. These results add to the growing body of evidence that show that conformational differences in the antibody hinge region can have a dramatic impact on the antibody activity and must be considered when screening and engineering therapeutic antibody candidates. The results also demonstrate that the IgG1 (IgG2-A like) or the IgG2-B form may provide the most active form of agonist antibodies for different antibodies and targets.

Non-targeted characterization of attributes affecting antibody-FcγRIIIa V158 (CD16a) binding via online affinity chromatography-mass spectrometry.

Antibodies facilitate targeted cell killing by engaging with immune cells such as natural killer cells through weak binding interactions with Fcγ receptors on the cell surface. Here, we evaluate the binding affinity of the receptor FcγRIIIa V158 (CD16a) for several therapeutic antibody classes, isoforms, and Fc-fusion proteins using an immobilized receptor affinity liquid chromatography (LC) approach coupled with online mass spectrometry (MS) detection. Aglycosylated FcγRIIIa was used in the affinity chromatography and compared with published affinities using glycosylated receptors. Affinity LC-MS differentiated the IgG1 antibodies primarily according to their Fc glycosylation patterns, with highly galactosylated species having greater affinity for the immobilized receptors and thus eluting later from the column (M5< G0F < G0 afucosylated ≅ G1F < G2F). Sialylated species bound weaker to their asialylated counterparts as reported previously. High mannose glycoforms bound weaker than G0F, contrary to previously published studies using glycosylated receptors. Also, increased receptor binding affinity associated with afucosylated antibodies was not observed with the aglycosylated FcγRIIIa. This apparent difference from previous findings highlighted the importance of the glycans on the receptors for mediating stronger binding interactions. Characterization of temperature-stressed samples by LC-MS peptide mapping revealed over 200 chemical and post-translational modifications, but only the Fc glycans, deamidation of EU N325, and an unknown modification to either proline or cysteine residues of the hinge region were found to have a statistically significant impact on binding.Abbreviations: Antibody-dependent cell-mediated cytotoxicity (ADCC), chimeric antigen receptor (CAR), Chinese hamster ovary (CHO), dithiothreitol (DTT), electrospray ionization (ESI), hydrogen-deuterium exchange (HDX), filter aided-sample preparation (FASP), Fcγ receptor (FcγR), fragment crystallizable (Fc), high-pressure liquid chromatography (HPLC), immunoglobulin G (IgG), liquid chromatography (LC), monoclonal antibody (mAb), mass spectrometry (MS), natural killer (NK), N-glycolylneuraminic acid (NGNA), N-acetylneuraminic acid (NANA), principal component analysis (PCA), surface plasmon resonance (SPR), trifluoroacetic acid (TFA), and extracted mass chromatogram (XMC).

Field flow fractionation for assessing neonatal Fc receptor and Fcγ receptor binding to monoclonal antibodies in solution.

Analysis of the strength and stoichiometry of immunoglobulin G (IgG) binding to neonatal Fc receptor (FcRn) and Fcγ receptor (FcγR) is important for evaluating the pharmacokinetics and effector functions of therapeutic monoclonal antibody (mAb) products, respectively. The current standard for assessing FcγR and FcRn binding is composed of cell-based and surface plasmon resonance (SPR) assays. In this work, asymmetrical flow field flow fractionation (AF4) was evaluated to establish the true stoichiometry of IgG binding in solution. AF4 and liquid chromatography-mass spectrometry (LC-MS) were applied to directly observe IgG/FcγR and IgG/FcRn complexes, which were not observed using nonequilibrium size exclusion chromatography (SEC) analysis. Human serum albumin (HSA), an abundant component of human blood and capable of binding FcRn, was studied in combination with FcRn and IgG. AF4 demonstrated that the majority of large complexes of IgG/FcRn/HSA were at an approximate 1:2:1 molar ratio. In addition, affinity measurements of the complex were performed in the sub-micromolar affinity range. A significant decrease in binding was detected for IgG molecules with increased oxidation in the Fc region. AF4 was useful in detecting weak binding between full-length IgG/Fc fragments and Fc receptors and the effect of chemical modifications on binding. AF4 is a useful technique in the assessment of mAb product quality attributes.

Identification of critical chemical modifications by size exclusion chromatography of stressed antibody-target complexes with competitive binding.

Chemical modifications (attributes) in the binding regions of stressed therapeutic proteins may affect binding to target and efficacy of therapeutic proteins. The method presented here describes the criticality assessment of therapeutic antibody modifications by size-exclusion chromatography (SEC) of competitive binding between a stressed antibody and its target, human epidermal growth factor receptor-2 (HER2), followed by SEC fractionation and peptide mapping characterization of bound and unbound antibodies. When stressed antibody and its target were mixed at a stoichiometric molar ratio of 1:2, only antibody-receptor complex eluted from SEC, indicating that binding was not decreased to break the complex. When a smaller amount of the receptor was provided (1:1), the antibody species with modifications reducing binding eluted as unbound from SEC, while the antibody-receptor complex eluted as the bound fraction. Peptide mapping revealed ratios of modifications between unbound and bound fractions. Statistical analysis after triplicate measurements (n = 3) indicated that heavy chain (HC) D102 isomerization and light chain (LC) N30 deamidation were four-fold higher in unbound fraction with high statistical significance. Although HC N55 deamidation and M107 oxidation were also abundant, they were not statistically different between unbound and bound. Our findings agree with previously published potency measurements of collected CEX fractions and the crystal structure of antibody and HER2. Overall, competitive SEC of stressed antibody-receptor mixture followed by peptide mapping is a useful tool in revealing critical residues and modifications involved in the antibody-target binding, even if they elute as a complex from SEC when mixed at 1:2 stoichiometric ratio.

Identification of critical chemical modifications and paratope mapping by size exclusion chromatography of stressed antibody-target complexes.

Therapeutic proteins including antibodies and Fc-fusion proteins undergo a large number of chemical modifications during cell culture, purification, storage and in human circulation. They are also exposed to harsh conditions during stress studies, including elevated temperature, extremes of pH, forced oxidation, physiological pH, UV light to assess the possible degradation pathways and suitability of methods for detecting them. Some of these modifications are located on residues in binding regions, leading to loss of binding and potency and classified as critical quality attributes. Currently, criticality of modifications is assessed by a laborious process of collecting antibody fractions from the soft chromatography techniques ion exchange and hydrophobic interaction chromatography and characterizing the fractions one-by-one for potency and chemical modifications. Here, we describe a method for large-scale, parallel identification of all critical chemical modifications in one experiment. In the first step, the antibody is stressed by one or several stress methods. It is then mixed with target protein and separated by size-exclusion chromatography (SEC) on bound antibody-target complex and unbound antibody. Peptide mapping of fractions and statistical analysis are performed to identify modifications on amino acid residues that affect binding. To identify the modifications leading to slight decreases in binding, competitive SEC of antibody and antigen mixtures was developed and described in a companion study by Shi et al, where target protein is provided at lower level, below the stoichiometry. The newly described method was successfully correlated to crystallography for assessing criticality of chemical modifications and paratope mapping. It is more sensitive to low-level modifications, better streamlined and platform ready.

Characterization of therapeutic proteins by cation exchange chromatography-mass spectrometry and top-down analysis.

Recently, cation exchange chromatography (CEX) using aqueous volatile buffers was directly coupled with mass spectrometry (MS) and applied for intact analysis of therapeutic proteins and antibodies. In our study, chemical modifications responsible for charge variants were identified by CEX-UV-MS for a monoclonal antibody (mAb), a bispecific antibody, and an Fc-fusion protein. We also report post-CEX column addition of organic solvent and acid followed by mixing at elevated temperatures, which unfolded proteins, increased ion intensity (sensitivity) and facilitated top-down analysis. mAb stressed by hydrogen peroxide oxidation was used as a model system, which produced additional CEX peaks. The on-line CEX-UV-MS top-down analysis produced gas-phase fragments containing one or two methionine residues. Oxidation of some methionine residues contributed to earlier (acidic), some to later (basic) eluting peaks, while oxidation of other residues did not change CEX elution. The abundance of the oxidized and non-oxidized fragment ions also allowed estimation of the oxidation percentage of different methionine residues in stressed mAb. CEX-UV-MS measurement revealed a new intact antibody proteoform at 5% that eluted as a basic peak and included paired modifications: high-mannose glycosylation and remaining C-terminal lysine residue (M5/M5 + K). This finding was confirmed by peptide mapping and on-column disulfide reduction coupled with reversed-phase liquid chromatography - top-down MS analysis of the collected basic peak. Overall, our results demonstrate the utility of the on-line method in providing site-specific structural information of charge modifications without fraction collection and laborious peptide mapping.

Interchain disulfide bonding in human IgG2 antibodies probed by site-directed mutagenesis.

Human IgG2 exists as a mixture of disulfide-linked structural isoforms that can show different activities. To probe the contribution of specific cysteine residues to the formation of structural isoforms, we characterized a series of Cys-->Ser mutant IgG2 recombinant monoclonal antibodies, focused on the first C(H)1 cysteine and the first two hinge cysteines. These residues participate in the formation of structural isoforms that have been noted by nonreduced capillary sodium dodecyl sulfate polyacrylamide gel electrophoresis, reversed-phase high-performance liquid chromatography, and cation exchange chromatography. We show that single Cys-->Ser mutants can greatly reduce heterogeneous disulfide bonding in human IgG2 and maintain in vitro activity. The data demonstrate the feasibility of applying site-directed mutagenesis to reduce disulfide bond heterogeneity in human IgG2 while preserving the activity of this therapeutically important class of human antibodies.

Removal of cysteinylation from an unpaired sulfhydryl in the variable region of a recombinant monoclonal IgG1 antibody improves homogeneity, stability, and biological activity.

The antibody MAB007 was recently shown to be cysteinylated on an unpaired cysteine residue in the CDR3 variable region. Cysteinylation at this position was not complete and resulted in heterogeneous lots of MAB007 with respect to this posttranslational modification. In this report, a mild redox step was used that effectively removed cysteinylation while keeping native inter and intra-molecular disulfide bonds intact. Biophysical methods were employed to determine what consequences cysteinylation of the variable region had by directly comparing cysteinylated and de-cysteinylated MAB007 antibodies. No differences were detected in secondary structure; however, several pieces of evidence indicated that cysteinylation may result in tertiary or quaternary structural perturbations. These included differences in the cation-exchange chromatography and fluorescence-emission spectra of the cysteinylated and de-cysteinylated antibodies as well as differences in the solvent accessibility of the unpaired cysteine residue determined by labeling experiments. Such structural changes induced by cysteinylation were shown to increase the rate of MAB007 aggregation and to decrease the melting temperature of the Fab region by as much as 6 degrees C. The bioactivity of MAB007 was also shown to be adversely affected by cysteinylation and a direct correlation was made between the percent cysteinylation and biological activity.

Automated tryptic digestion procedure for HPLC/MS/MS peptide mapping of immunoglobulin gamma antibodies in pharmaceutics.

The rapid growth of antibody drugs and drug candidates in the biopharmaceutical industry has created a demand for automated proteolytic digestion to assist in pharmaceutical stability studies, identity assays and quality control of these therapeutic proteins. Here, we describe the development of a fully automated proteolytic digestion procedure for monoclonal antibodies in solution, which requires a high concentration of denaturants for unfolding. The antibody samples were placed in a 96-well plate or in 0.5-mL Eppendorf tubes. The proteins were then reduced and alkylated in a denaturing solution of 6M guanidine HCl. The denaturing solution was replaced with a digestion buffer using a custom-designed 96-well size-exclusion plate for desalting. The sample was digested for 5 h with two additions of trypsin. The completeness and reproducibility of digestion were verified by reversed-phase high-performance liquid chromatography tandem mass spectrometry (HPLC/MS) analysis of the digestion products. The performance of the automatic digestion was comparable to the currently used manual digestion procedure, but saved time, reduced manual labor, and increased the reproducibility of the tryptic digests. Our method should be useful not only for high-throughput analysis of antibodies, but for other therapeutic protein samples as well. Other applications like gel-free proteomics, where the analysis of a large number of samples is often needed and the completeness of the liquid digestion is critical for the identification of a large number of different proteins, should also benefit from this fully automated liquid proteolytic digestion procedure.

Improving mass accuracy of high performance liquid chromatography/electrospray ionization time-of-flight mass spectrometry of intact antibodies.

The glycosylation profile of intact antibody due to the galactose and fucose heterogeneity in the N-linked sugars was determined with instrument resolution of 5000 and 10,000. After deconvolution of electrospray ionization mass spectra to complete convergence, several extra peaks appeared in addition to the peaks observed in the original mass spectra. The artificial peaks were avoided if deconvolution was stopped after a smaller number of iterations. A standard antibody was used as an external calibrant to minimize mass measurement errors during long-period experiments. Precision of four consecutive LC/MS measurements of the same antibody was 10 ppm (+/-1.5 Da). By using this approach, the masses of 11 intact antibodies were measured. All antibodies containing N-terminal glutamines had a negative mass shift due to the formation of pyroglutamate (-17 Da). Although the pyroglutamate variant of intact antibody was not resolved from the unmodified variant, this modification led to a mass shift proportional to the percentage of N-terminal pyroglutamate. By accurately measuring the mass shift we were able to quantify the abundance of pyroglutamic acid on intact antibodies. Mass accuracy in measuring different antibodies was below 30 ppm (+/-4 Da). The accurate mass measurement can be an effective tool for monitoring chemical degradations in therapeutic antibodies.

Reversed-phase liquid chromatography/mass spectrometry analysis of reduced monoclonal antibodies in pharmaceutics.

A reversed-phase LC/MS method was developed for reduced antibodies that provides efficient separation of light chain and two variants of heavy chain containing N-terminal glutamine and pyroglutamic acid. The best separation was achieved on Zorbax CN and Varian Pursuit DiPhenyl columns eluted with increasing percentage of n-propanol and acetonitrile in 0.1% trifluoroacetic acid. Although glutamine was genetically coded for the N-terminal residue of heavy chain of a monoclonal antibody used in this study, we found that most of it (70%) was converted to pyroglutamate during production. The conversion process continued in vitro and was monitored by the method. Deconvoluted electrospray ionization mass spectrum of the heavy chain revealed the glycosylation profile of a single N-linked sugar including a-, mono-, and di-galactosylated biantennary glycans and a 5-mannose sugar form.

Optimization of a reversed-phase high-performance liquid chromatography/mass spectrometry method for characterizing recombinant antibody heterogeneity and stability.

An enhanced analytical RP-HPLC/MS method was developed for monitoring the stability and production of intact and fragmented monoclonal antibodies (MAbs). The use of high column temperatures (70-80 degrees C), organic solvents with high eluotropic strength coefficients (isopropyl and n-propyl alcohols), and Zorbax StableBond columns, were critical for good recovery and resolution of immunoglobulin G1 (IgG1) and IgG2 monoclonal antibodies. Using this method, cleavage products of a degraded IgG1 antibody were clearly separated and identified by in-line electrospray ionization time-of-flight (ESI-TOF) mass spectrometry generating exact masses and unique terminal ladder sequences. The glycosylation profile, including mapping of the terminal galactose and fucose heterogeneity of the N-linked sugars, was determined by mass spectrometry of intact MAbs. In addition, we discovered that several IgG2 MAbs exhibited greater structural heterogeneity compared to IgG1s. Mass spectral characterization data and reduction data suggested that the heterogeneity is disulfide related. This reversed-phase LC/MS method represents a key advancement in monitoring intact MAb production and stability.

Development of an analytical reversed-phase high-performance liquid chromatography-electrospray ionization mass spectrometry method for characterization of recombinant antibodies.

Analytical characterization of monoclonal antibodies has been hindered by the lack of appropriate chromatographic methods to be used in conjunction with high-resolution MS. Current methodologies for standard RP-HPLC are incompatible with antibodies due to irreproducibility, low recovery, short column lifetimes, and poor resolution of degradation products. An analytical RP-HPLC-MS method was developed for monitoring and characterizing intact IgG1antibodies. Key parameters required for improved chromatographic resolution included long alkyl chains of the stationary phase (Zorbax SB300 C8), column temperatures elevated to 65-70 degrees C and combination of trifluoroacetic acid and heptafluorobutyric acid ion-pairing agents. RP chromatographic separation of degradation species and C-terminal lysine variants along with the characterization of glycosylation profile by mass spectrometry demonstrates the capability of this method for whole antibody analysis.

Protected hinge in the immunoglobulin G2-A2 disulfide isoform.

Human IgG2 consists of disulfide-mediated structural isoforms, classified by the number of Fab arms disulfide-linked to the heavy chain hinge. In the IgG2-B isoform, both Fab arms are linked to the hinge region, and in IgG2-A, neither Fab arm are linked to the hinge. IgG2-A/B is a hybrid between these two forms, with only one Fab arm disulfide-linked to the hinge. Within each of these isoform types are subtypes, with subtle disulfide-linkage differences. Here we explored the structural basis for the A1 and A2 isoform subtypes. Whereas A1 isoform converts into the A/B and B isoforms under mild redox conditions, A2 does not. Characterization of the disulfide connectivities of A2 isoform revealed a similar structure to A1 isoform, with parallel inter heavy chain disulfide linkages in the hinge region. However, the hinge disulfides in A2 isoform were resistant to reduction under conditions where A1 isoform hinge disulfides became reduced and they required thermal treatment (>55 °C) to obtain thiol-dependent disulfide reduction. Structural analysis of the hinge region indicated that the protected disulfides were restricted to cysteines 219 and 220 of the upper hinge. Disruption of the upper hinge through insertion mutagenesis eliminated A2 isoform behavior. (1)H NMR studies showed that the A1 isoform Fc glycan was more dynamic than that on A2 isoform and showed some other conformational differences. Results point to an IgG2-A2 upper hinge region that is more akin to the interior of a globular protein than the flexible hinge region expected on an IgG.

IgG2 disulfide isoform conversion kinetics.

Human IgG2 antibodies contain three types of disulfide isoforms, classified by the number of Fab arms having disulfide links to the heavy chain hinge region. In the IgG2-B form, both Fab arms have interchain disulfide bonds to the hinge region, and in IgG2-A, neither Fab arm are disulfide linked to the hinge. The IgG2-A/B is a hybrid between these two forms, with only one Fab arm disulfide linked to the hinge. Changes in the relative levels of these forms over time are observed while IgG2 circulates in humans, suggesting IgG2-A→IgG2-A/B→IgG2-B conversion. Using a flow-through dialysis system, we studied the conversion kinetics of these forms in vitro under physiological conditions. For two IgG2κ antibodies, in vivo results closely matched the kinetics observed in vitro, indicating that the changes observed in vivo were solely conversions between isoforms, not differential clearance of specific forms. Moreover, the combined results validate the accuracy of the physiological model for the study of blood redox reactions. Further exploration of the conversion kinetics using material enriched in the IgG2-A forms revealed that the IgG2-A→IgG2-A/B rate was similar between IgG2κ and IgG2λ antibodies. In IgG2κ antibodies, conversion of IgG2-A/B→IgG2-B was slower than the IgG2-A→IgG2-A/B reaction. However, in IgG2λ antibodies, little IgG2-A/B→IgG2-B conversion was detected under physiological conditions. Thus, small differences in the C-terminus of the light chain sequences affect the disulfide conversion kinetics and impact the IgG2 disulfide isoforms produced in vivo.

Conformational implications of an inversed pH-dependent antibody aggregation.

Antibody formulation development relies on accelerated stability data at elevated temperatures to optimize formulation parameters. However, the pH- and temperature-dependence of aggregation is complicated for antibody formulations. In this study, a human monoclonal IgG2 antibody exhibited typical pH-dependent dimer formation under normal storage conditions (4 and/or 29 degrees C). However, an inversed pH-dependence was discovered for high molecular weight aggregate formation at elevated temperatures (37 degrees C). The different stability profiles exhibited at the various storage conditions resulted in nonlinearity of the Arrhenius kinetics. Thermal unfolding at or below 37 degrees C was not evident by differential scanning calorimetry. Enriched populations of the structural isoforms of the IgG2 subclass were tested for their unique temperature and pH-dependence of aggregation. The Arrhenius kinetics of aggregation for each of the individual IgG2 isoforms was also nonlinear. However, the temperature-dependence of clipping suggested that clip-mediated aggregation was responsible for the increased higher order aggregates at low pH and elevated temperatures. Unique clip species resulting from the conformational differences between the IgG2 isoforms lead to increased aggregation. These results have implications on the mechanisms of antibody aggregation and on the validity of accelerated data to predict shelf-life accurately.

Characterization of IgG1 immunoglobulins and peptide-Fc fusion proteins by limited proteolysis in conjunction with LC-MS.

A combinatory approach for the characterization of post-translational and chemical modifications in high molecular weight therapeutic proteins like antibodies and peptide-Fc fusion proteins (MW > or = 50 000 Da) is presented. In this approach, well-established techniques such as limited proteolysis, reversed-phase (RP) high-performance liquid chromatography (HPLC), and in-line mass spectrometry (MS) were combined for the characterization of a monoclonal IgG1 antibody and three different peptide-Fc fusion proteins. The one commonality of these molecules is the presence of a similarly accessible lysine residue either located in the flexible hinge region of the antibody or in the flexible linker of the peptide-Fc fusion proteins. Applying limited proteolysis using endoproteinase Lys-C resulted in the predominant cleavage C-terminal of this lysine residue. The created fragments, two identical Fab domain fragments and one Fc domain fragment derived from the IgG1 antibody and one Fc domain fragment and each of the three individual peptide moieties generated from the peptide-Fc fusion proteins, were readily accessible for complete separation by RP-HPLC and detailed characterization by in-line MS analysis. This approach facilitated rapid detection of a variety of chemical modifications such as methionine oxidation, disulfide bond scrambling, and reduction as well as the characterization of various carbohydrate chains. We found limited proteolysis followed by RP-HPLC-MS to be less time-consuming for sample preparation, analysis, and data interpretation than traditional peptide mapping procedures. At the same time, the reduced sample complexity provided superior chromatographic and mass spectral resolution than the analysis of the corresponding intact molecules or a large number of enzymatically generated fragments.

Human IgG2 antibody disulfide rearrangement in vivo.

Proteins destined to circulate in the blood are first folded and assembled in the endoplasmic reticulum of secretory cells. For antibodies, like many other serum proteins, the folding and assembly steps involve the formation of disulfide bonds. Such bonds have been thought to be static features of proteins, stabilizing domains, and linking polypeptide chains, although some cases of extracellular disulfide bond cleavage have been noted. Recently, the human IgG2 antibody subclass was found to possess multiple structures differing in specific disulfide linkages. These structures are naturally occurring and can, in some cases, affect the activity of the antibody. Here we show that these IgG2 disulfide linkages interconvert while circulating in humans. Secretory cells initially produce primarily one form (IgG2-A), which is rapidly converted to a second form (IgG2-A/B) while circulating in the blood, followed by a slower conversion to a third form (IgG2-B). This work demonstrates that the disulfide structure of the IgG2 antibody is dynamic in vivo, on a time scale similar to that of the protein's lifetime. Thus, changes to the IgG2 disulfide structure provide a marker of the protein's age and may alter its activity over its lifetime.

Isomerization of a single aspartyl residue of anti-epidermal growth factor receptor immunoglobulin gamma2 antibody highlights the role avidity plays in antibody activity.

A new isoform of the light chain of a fully human monoclonal immunoglobulin gamma2 (IgG2) antibody panitumumab against human epidermal growth factor receptor (EGFR) was generated by in vitro aging. The isoform was attributed to the isomerization of aspartate 92 located between phenylalanine 91 and histidine 93 residues in the antigen-binding region. The isomerization rate increased with increased temperature and decreased pH. A size-exclusion chromatography binding assay was used to show that one antibody molecule was able to bind two soluble extracellular EGFR molecules in solution, and isomerization of one or both Asp-92 residues deactivated one or both antigen-binding regions, respectively. In addition, isomerization of Asp-92 showed a decrease in in vitro potency as measured by a cell proliferation assay with a 32D cell line that expressed the full-length human EGFR. The data indicate that antibodies containing either one or two isomerized residues were not effective in inhibiting EGFR-mediated cell proliferation, and that two unmodified antigen binding regions were needed to achieve full efficacy. For comparison, the potency of an intact IgG1 antibody cetuximab against the same receptor was correlated with the bioactivity of its individual antigen-binding fragments. The intact IgG1 antibody with two antigen-binding fragments was also much more active in suppressing cell proliferation than the individual fragments, similar to the IgG2 results. These results indicated that avidity played a key role in the inhibition of cell proliferation by these antibodies against the human EGFR, suggesting that their mechanisms of action are similar.