Antibody–receptor interactions mediate antibody-dependent cellular cytotoxicity

Binding of antibodies to their receptors is a core component of the innate immune system. Understanding the precise interactions between antibodies and their Fc receptors has led to the engineering of novel mAb biotherapeutics with tailored biological activities. One of the most significant findings is that afucosylated monoclonal antibodies demonstrate increased affinity toward the receptor FcγRIIIa, with a commensurate increase in antibody-dependent cellular cytotoxicity. Crystal structure analysis has led to the hypothesis that afucosylation in the Fc region results in reduced steric hindrance between antibody–receptor intermolecular glycan interactions, enhancing receptor affinity; however, solution-phase data have yet to corroborate this hypothesis. In addition, recent work has shown that the fragment antigen-binding (Fab) region may directly interact with Fc receptors; however, the biological consequences of these interactions remain unclear. By probing differences in solvent accessibility between native and afucosylated immunoglobulin G1 (IgG1) using hydroxyl radical footprinting–MS, we provide the first solution-phase evidence that an IgG1 bearing an afucosylated Fc region appears to require fewer conformational changes for FcγRIIIa binding. In addition, we performed extensive molecular dynamics (MD) simulations to understand the molecular mechanism behind the effects of afucosylation. The combination of these techniques provides molecular insight into the steric hindrance from the core Fc fucose in IgG1 and corroborates previously proposed Fab–receptor interactions. Furthermore, MD-guided rational mutagenesis enabled us to demonstrate that Fab–receptor interactions directly contribute to the modulation of antibody-dependent cellular cytotoxicity activity. This work demonstrates that in addition to Fc–polypeptide and glycan-mediated interactions, the Fab provides a third component that influences IgG–Fc receptor biology.     

Characterizing monoclonal antibody structure by carbodiimide/GEE footprinting

Amino acid-specific covalent labeling is well suited to probe protein structure and macromolecular interactions, especially for macromolecules and their complexes that are difficult to examine by alternative means, due to size, complexity, or instability. Here we present a detailed account of carbodiimide-based covalent labeling (with GEE tagging) applied to a glycosylated monoclonal antibody therapeutic, which represents an important class of biologic drugs. Characterization of such proteins and their antigen complexes is essential to development of new biologic-based medicines. In this study, the experiments were optimized to preserve the structural integrity of the protein, and experimental conditions were varied and replicated to establish the reproducibility and precision of the technique. Homology-based models were generated and used to compare the solvent accessibility of the labeled residues, which include D, E, and the C-terminus, against the experimental surface accessibility data in order to understand the accuracy of the approach in providing an unbiased assessment of structure. Data from the protein were also compared to reactivity measures of several model peptides to explain sequence or structure-based variations in reactivity. The results highlight several advantages of this approach. These include: the ease of use at the bench top, the linearity of the dose response plots at high levels of labeling (indicating that the label does not significantly perturb the structure of the protein), the high reproducibility of replicate experiments (<2 % variation in modification extent), the similar reactivity of the 3 target probe residues (as suggested by analysis of model peptides), and the overall positive and significant correlation of reactivity and solvent accessible surface area (the latter values predicted by the homology modeling). Attenuation of reactivity, in otherwise solvent accessible probes, is documented as arising from the effects of positive charge or bond formation between adjacent amine and carboxyl groups, the latter accompanied by observed water loss. The results are also compared with data from hydroxyl radical-mediated oxidative footprinting on the same protein, showing that complementary information is gained from the 2 approaches, although the number of target residues in carbodiimide/GEE labeling is fewer. Overall, this approach is an accurate and precise method for assessing protein structure of biologic drugs.     

Characterizing monoclonal antibody structure by carboxyl group footprinting

Structural characterization of proteins and their antigen complexes is essential to the development of new biologic-based medicines. Amino acid-specific covalent labeling (CL) is well suited to probe such structures, especially for cases that are difficult to examine by alternative means due to size, complexity, or instability. We present here a detailed account of carboxyl group labeling (with glycine ethyl ester (GEE) tagging) applied to a glycosylated monoclonal antibody therapeutic (mAb). The experiments were optimized to preserve the structural integrity of the mAb, and experimental conditions were varied and replicated to establish the reproducibility of the technique. Homology-based models were generated and used to compare the solvent accessibility of the labeled residues, which include aspartic acid (D), glutamic acid (E), and the C-terminus (i.e., the target probes), with the experimental data in order to understand the accuracy of the approach. Data from the mAb were compared to reactivity measures of several model peptides to explain observed variations in reactivity. Attenuation of reactivity in otherwise solvent accessible probes is documented as arising from the effects of positive charge or bond formation between adjacent amine and carboxyl groups, the latter accompanied by observed water loss. A comparison of results with previously published data by Deperalta et al using hydroxyl radical footprinting showed that 55% (32/58) of target residues were GEE labeled in this study whereas the previous study reported 21% of the targets were labeled. Although the number of target residues in GEE labeling is fewer, the two approaches provide complementary information. The results highlight advantages of this approach, such as the ease of use at the bench top, the linearity of the dose response plots at high levels of labeling, reproducibility of replicate experiments (<2% variation in modification extent), the similar reactivity of the three target probes, and significant correlation of reactivity and solvent accessible surface area.