Local disorder of the C-terminal segment of the heavy chain as a common sign of stressed antibodies evidenced with a peptide affinity probe specific to non-native IgG

Biosensing-based quality control monitoring of the higher-order structures of therapeutic antibody domains

Advanced biopharmaceutical manufacturing requires novel process analytical technologies for the rapid and sensitive assessment of the higher-order structures of therapeutic proteins. However, conventional physicochemical analyses of denatured proteins have limitations in terms of sensitivity, throughput, analytical resolution, and real-time monitoring capacity. Although probe-based sensing can overcome these limitations, typical non-specific probes lack analytical resolution and provide little to no information regarding which parts of the protein structure have been collapsed. To meet these analytical demands, we generated biosensing probes derived from artificial proteins that could specifically recognize the higher-order structural changes in antibodies at the protein domain level. Biopanning of phage-displayed protein libraries generated artificial proteins that bound to a denatured antibody domain, but not its natively folded structure, with nanomolar affinity. The protein probes not only recognized the higher-order structural changes in intact IgGs but also distinguished between the denatured antibody domains. These domain-specific probes were used to generate response contour plots to visualize the antibody denaturation caused by various process parameters, such as pH, temperature, and holding time for acid elution and virus inactivation. These protein probes can be combined with established analytical techniques, such as surface plasmon resonance for real-time monitoring or plate-based assays for high-throughput analysis, to aid in the development of new analytical technologies for the process optimization and monitoring of antibody manufacturing.

Getting Smaller by Denaturation: Acid-Induced Compaction of Antibodies

Protein denaturation is a ubiquitous process that occurs both in vitro and in vivo. While our molecular understanding of the denatured structures of proteins is limited, it is commonly accepted that the loss of unique intramolecular contacts makes proteins larger. Herein, we report compaction of the immunoglobulin G1 (IgG1) protein upon acid denaturation. Small-angle X-ray scattering coupled with size exclusion chromatography revealed that IgG1 radii of gyration at pH 2 were ∼75% of those at a neutral pH. Scattering profiles showed a compact globular shape, supported by analytical ultracentrifugation. The acid denaturation of proteins with a decrease in size is energetically costly, and acid-induced compaction requires an attractive force for domain reorientation. Such intramolecular aggregation may be widespread in immunoglobulin proteins as noncanonical structures. Herein, we discuss the potential biological significance of these noncanonical structures of antibodies.

Live-cell imaging to analyze intracellular aggregation of recombinant IgG in CHO cells

Recombinant immunoglobulin G (IgG) aggregates are formed during their production. However, the process underlying intracellular/extracellular aggregation in cell culture conditions is not well understood, and no effective method exists to assess IgG aggregates. Here, we establish an approach to detect intracellular aggregates using AF.2A1, a small artificial protein that binds to non-native IgG conformers and aggregates. Fluorescent-labeled AF.2A1 is prepared via conjugation and transfected into antibody-producing Chinese hamster ovary (CHO) cells. Micrographic images show intracellular IgG aggregates in CHO cells. The relative amount of intracellular aggregates (versus total intracellular IgG) differed depending on the type of additives used during cell culture. Interestingly, the relative amount of intracellular aggregates moderately correlates with that of in vitro extracellular IgG aggregates, suggesting they are secreted. This method will allow the investigation of antibody aggregation in cells, and may guide the production of therapeutic antibodies with high yield/quality.