Theresa Martinez

Human IgG2 Antibodies Display Disulfide-mediated Structural Isoforms

In this work, we present studies of the covalent structure of human IgG2 molecules. Detailed analysis showed that recombinant human IgG2 monoclonal antibody could be partially resolved into structurally distinct forms caused by multiple disulfide bond structures. In addition to the presently accepted structure for the human IgG2 subclass, we also found major structures that differ from those documented in the current literature. These novel structural isoforms are defined by the light chain constant domain (CL) and the heavy chain CH1 domain covalently linked via disulfide bonds to the hinge region of the molecule. Our results demonstrate the presence of three main types of structures within the human IgG2 subclass, and we have named these structures IgG2-A, -B, and -A/B. IgG2-A is the known classic structure for the IgG2 subclass defined by structurally independent Fab domains and hinge region. IgG2-B is a structure defined by a symmetrical arrangement of a (CH1-CL-hinge)2 complex with both Fab regions covalently linked to the hinge. IgG2-A/B represents an intermediate form, defined by an asymmetrical arrangement involving one Fab arm covalently linked to the hinge through disulfide bonds. The newly discovered structural isoforms are present in native human IgG2 antibodies isolated from myeloma plasma and from normal serum. Furthermore, the isoforms are present in native human IgG2 with either κ or λ light chains, although the ratios differ between the light chain classes. These findings indicate that disulfide structural heterogeneity is a naturally occurring feature of antibodies belonging to the human IgG2 subclass.

Disulfide Connectivity of Human Immunoglobulin G2 Structural Isoforms

In this communication we present the detailed disulfide structure of IgG2 molecules. The consensus structural model of human IgGs represents the hinge region positioned as a flexible linker connecting structurally isolated Fc and Fab domains. IgG2 molecules are organized differently from that model and exhibit multiple structural isoforms composed of (heavy chain-light chain-hinge) covalent complexes. We describe the precise connection of all the disulfide bridges and show that the IgG2 C H1 and C-terminal C L cysteine residues are either linked to each other or to the two upper hinge cysteine residues specific to the IgG2 subclass. A defined arrangement of these disulfide bridges is unique to each isoform. Mutation of a single cysteine residue in the hinge region eliminates these natural complexes. These results show that IgG2 structure is significantly different from the conventionally accepted immunoglobulin structural model and may help to explain some of the unique biological activity attributed only to this subclass.

Electrophoretic evidence for the presence of structural isoforms specific for the IgG2 isotype.

Recombinant monoclonal antibodies of therapeutic interest were analyzed by a nonreduced CE‐SDS (nrCE‐SDS) method developed for the evaluation of size‐based variants. We found that immunoglobulins analyzed by this technique exhibited different behavior depending on their subclasses. Under nrCE‐SDS conditions, IgG1 molecules were separated in a well‐resolved, single peak, whereas IgG2 molecules were consistently separated as a doublet. Investigation of these isoforms showed that they were structurally different, and that the difference was not caused by cell culture condition, glycosylation structure, or recombinant expression system. Commercially available IgG2 affinity‐purified from human plasma also showed the presence of structural isoforms. The structural isoforms remained present under pH‐ and temperature‐stressed conditions. Application of a mild cysteine/cystine redox potential converted the main peak doublet into a single peak, indicating that these isoforms were disulfide bond‐related species. Bioactivity measured before and after application of a redox potential gave similar values, indicating that the structural isoforms have comparable potency. The nrCE‐SDS technique described here demonstrated a unique capability to resolve IgGs, leading to the discovery of novel structural isoforms specific to the IgG2 isotype.

Molecular mass analysis of antibodies by on-line SEC-MS

Mass analysis of recombinant protein therapeutics is an important assay for product characterization. Intact mass analysis is used to provide confirmation of proper translation of the DNA sequence and to detect the presence of post-translational modifications such as amino acid processing and glycosylation. We present here a method for the rapid mass analysis of antibodies using a polyhydroxyethyl aspartamide column operated in size-exclusion mode and coupled with ESI-MS. This method allows extremely efficient desalting of proteins under acidic conditions that are optimal for subsequent mass analysis using standard ESI conditions. Furthermore, this technique is significantly faster and more sensitive than rpHPLC methods, typically considered the standard chromatography approach for mass analysis of proteins. This method is flexible and robust, and should prove useful for applications where a combination of speed and sensitivity are required.

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.

Characterization of Minor N-linked Glycans on Antibodies Using Endo H Release and MALDI–Mass Spectrometry

Abstract A MALDI mass spectrometry method using Bruker Daltonics LIFT technology for MS/MS analysis has been developed for profiling and characterizing low abundant N-glycans from recombinant immunoglobulin G (IgG) antibodies. In this method, Endoglycosidase H (Endo H) released N-glycans are derivatized at their reducing end with 2-aminobenzamide (2-AB) and separated by normal phase chromatography. Endo H hydrolyses the bond between the two GlcNAc residues of the trimannosyl core of high mannose and hybrid N-linked glycans, leaving the core GlcNAc attached to the protein. High mannose and hybrid type N-glycans are released from the glycoprotein whereas the more abundant, complex biantennary type oligosaccharide structures are unaffected. Analysis of Endo H treated glycan moieties by MALDI mass spectrometry identified several minor species of high mannose and hybrid type glycans. Subsequent MALDI TOF MS/MS analysis of the resulting products yielded information about structural features of the high mannose and hybrid type glycans. This study involving Endo H treatment followed by MALDI mass spectrometry coupled with LIFT technology for MS/MS analysis offers a specific and sensitive technique for visualizing, and characterizing minor glycan species.

Optimized UV detection of high-concentration antibody formulations using high-throughput SE-HPLC.

High-concentration antibody solutions (>100 mg/mL) present significant challenges for formulation and process development, including formulation attributes such as increased solution viscosity, and the propensity for self-association. An additional challenge comes from the adaptation of analytical methods designed for low-concentration formulations to the high-concentration regime. The oligomeric state is a good example: it is a quality attribute monitored during pharmaceutical development and is one that can be affected by dilution; a typical first step in the analysis of high-concentration solutions. The objective of this work was to develop a size-exclusion HPLC (SE-HPLC) method that would allow the injection of high-concentration antibody formulations without the need for dilution prior to injection and their analysis in a high-throughput manner that does not create a bottleneck for the execution of complex formulation development studies. It was found that changing the UV detection wavelength from 215 to 235 nm simplified sample preparation by allowing for an approximately fivefold increase in injection load while maintaining the signal within the linear range of detection. In addition, the chromatographic peak properties (i.e., peak symmetry, resolution, and sensitivity) were determined to be consistent when compared with analytical methods developed for formulations with lower antibody concentrations.