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[Display omitted] Detailed evaluations of the antigen and antibody interaction rate and strength of the immune complex formed are very important for medical and bioanalytical applications. These data are crucial for the development of sensitive and fast immunosensors suitable for continuous measurements. Therefore, combined spectroscopic ellipsometry (SE) and quartz crystal microbalance with dissipation (QCM-D) technique (SE/QCM-D) was used for the evaluation: (i) of covalent immobilization of SARS-CoV-2 nucleocapsid protein (SCoV2-N) on QCM-D sensor disc modified by self-assembled monolayer based on 11-mercaptoundecanoic acid and (ii) interaction of immobilized SCoV2-N with specific polyclonal anti-SCoV2-N antibodies followed by immune complex formation process. The results show that the SCoV2-N monolayer is rigid due to the low energy dissipation registered during the QCM-D measurement. In contrast, the anti-SCoV2-N layer produced after interaction with the immobilized SCoV2-N formed a soft and viscous layer. It was determined, that the sparse distribution of SCoV2-N on the surface affected the spatial arrangement of the antibody during the formation of immune complexes. The hinge-mediated flexibility of the antibody Fab fragments allows them to reach the more distantly located SCoV2-N and establish a bivalent binding between proteins in the formed SCoV2-N/anti-SCoV2-N complex. It was noted that the SE/QCM-D method can provide more precise quantitative information about the flexibility and conformational changes of antibody during the formation of the immune complex on the surface over time. [ABSTRACT FROM AUTHOR]

Antibodies are large protein assemblies capable of both specifically recognising antigens and engaging with other proteins and receptors to coordinate immune action. Traditionally, structural studies have been dedicated to antibody variable regions, but efforts to determine and model full-length antibody structures are emerging. Here we review the current knowledge on modelling the structures of antibody assemblies, focusing on their conformational flexibility and the challenge this poses to obtaining and evaluating structural models. Integrative modelling approaches, combining experiments (cryo-electron microscopy, mass spectrometry, etc.) and computational methods (molecular dynamics simulations, deep-learning based approaches, etc.), hold the promise to map the complex conformational landscape of full-length antibody structures. • Conformational flexibility of antibodies affects their function. • Currently available experimental data pose challenges to the modelling of full-length antibody assemblies. • Integrative modelling combining experiments and computational methods can address this challenge. [ABSTRACT FROM AUTHOR]

Both conformational and colloidal stability of therapeutic proteins must be closely monitored and thoroughly characterized to assess the long-term viability of drug products. We characterized the IgG1 NISTmAb reference material in its histidine formulation buffer and report our findings on the higher order structure and interactions of NISTmAb under a range of conditions. In this paper we present the analysis of experimental small-angle scattering data with atomistic molecular simulations to characterize the monodisperse dilute solution of NISTmAb. In part II we describe the characterization of the NISTmAb at high protein concentration (Castellanos et al. 2018). The NISTmAb was found to be a flexible protein with a radius of gyration of 49.0 ± 1.2 Å in histidine formulation buffer using a variety of neutron and X-ray scattering measurements. Scattering data were then modeled using molecular simulation. After building and validating a starting NISTmAb structure from the Fc and Fab crystallographic coordinates, molecular dynamics and torsion-angle Monte Carlo simulations were performed to explore the configuration space sampled in the NISTmAb and obtain ensembles of structures with atomistic detail that are consistent with the experimental data. Our results indicate that the small-angle scattering profiles of the NISTmAb can be modeled using ensembles of flexible structures that explore a wide configuration space. The NISTmAb is flexible in solution with no single preferred orientation of Fc and Fab domains, but with some regions of configuration space that are more consistent with measured scattering profiles. Analysis of inter-domain atomistic contacts indicated that all ensembles contained configurations where residues between domains are ≤ 4 Å, although few contacts were observed for variable and CH3 regions.Heavy atom self contact maps of the NISTmAb indicate a highly-flexible structure. [ABSTRACT FROM AUTHOR]

Polyreactivity is the ability of a single antibody to bind to multiple molecularly distinct antigens and is a common feature of antibodies induced upon pathogen exposure. However, little is known about the role of polyreactivity during anti-influenza virus antibody responses. By analyzing more than 500 monoclonal antibodies (mAbs) derived from B cells induced by numerous influenza virus vaccines and infections, we found mAbs targeting conserved neutralizing influenza virus hemagglutinin epitopes were polyreactive. Polyreactive mAbs were preferentially induced by novel viral exposures due to their broad viral binding breadth. Polyreactivity augmented mAb viral binding strength by increasing antibody flexibility, allowing for adaption to imperfectly conserved epitopes. Lastly, we found affinity-matured polyreactive B cells were typically derived from germline polyreactive B cells that were preferentially selected to participate in B cell responses over time. Together, our data reveal that polyreactivity is a beneficial feature of antibodies targeting conserved epitopes. • Antibodies targeting conserved hemagglutinin epitopes are polyreactive • Polyreactive antibodies are preferentially induced by novel influenza viruses • Polyreactivity increases antibody flexibility, affinity, and neutralization potency • Polyreactive naive B cells are selected into the memory B cell pool Polyreactivity is a common feature of antibodies, but little is known about the selection, function, and role of polyreactive B cells against pathogens. Guthmiller et al. demonstrate polyreactive B cells are selected against broadly neutralizing epitopes of influenza viruses and are linked to increased viral-binding breadth, affinity, and antibody flexibility. [ABSTRACT FROM AUTHOR]