Determinants of the assembly and function of antibody variable domains.

The antibody Fv module which binds antigen consists of the variable domains VL and VH. These exhibit a conserved ß-sheet structure and comprise highly variable loops (CDRs). Little is known about the contributions of the framework residues and CDRs to their association. We exchanged conserved interface residues as well as CDR loops and tested the effects on two Fvs interacting with moderate affinities (KDs of ~2.5 µM and ~6 µM). While for the rather instable domains, almost all mutations had a negative effect, the more stable domains tolerated a number of mutations of conserved interface residues. Of particular importance for Fv association are VLP44 and VHL45. In general, the exchange of conserved residues in the VL/VH interface did not have uniform effects on domain stability. Furthermore, the effects on association and antigen binding do not strictly correlate. In addition to the interface, the CDRs modulate the variable domain framework to a significant extent as shown by swap experiments. Our study reveals a complex interplay of domain stability, association and antigen binding including an unexpected strong mutual influence of the domain framework and the CDRs on stability/association on the one side and antigen binding on the other side.

High-throughput analysis of sub-visible mAb aggregate particles using automated fluorescence microscopy imaging.

Aggregation of therapeutic proteins is a major concern as aggregates lower the yield and can impact the efficacy of the drug as well as the patient's safety. It can occur in all production stages; thus, it is essential to perform a detailed analysis for protein aggregates. Several methods such as size exclusion high-performance liquid chromatography (SE-HPLC), light scattering, turbidity, light obscuration, and microscopy-based approaches are used to analyze aggregates. None of these methods allows determination of all types of higher molecular weight (HMW) species due to a limited size range. Furthermore, quantification and specification of different HMW species are often not possible. Moreover, automation is a perspective challenge coming up with automated robotic laboratory systems. Hence, there is a need for a fast, high-throughput-compatible method, which can detect a broad size range and enable quantification and classification. We describe a novel approach for the detection of aggregates in the size range 1 to 1000 µm combining fluorescent dyes for protein aggregate labelling and automated fluorescence microscope imaging (aFMI). After appropriate selection of the dye and method optimization, our method enabled us to detect various types of HMW species of monoclonal antibodies (mAbs). Using 10 µmol L-1 4,4'-dianilino-1,1'-binaphthyl-5,5'-disulfonate (Bis-ANS) in combination with aFMI allowed the analysis of mAb aggregates induced by different stresses occurring during downstream processing, storage, and administration. Validation of our results was performed by SE-HPLC, UV-Vis spectroscopy, and dynamic light scattering. With this new approach, we could not only reliably detect different HMW species but also quantify and classify them in an automated approach. Our method achieves high-throughput requirements and the selection of various fluorescent dyes enables a broad range of applications.

Reversible NaCl-induced aggregation of a monoclonal antibody at low pH: Characterization of aggregates and factors affecting aggregation.

We investigated the influence of pH and sodium chloride concentration on aggregation kinetics of a monoclonal antibody. Aggregation was induced by sodium chloride addition at low pH. Protein conformation before and after salt addition was determined as well as the reversibility of aggregation. Aggregation was monitored at pH values between 2 and 7 with NaCl up to 1.5M by turbidity measurement and size-exclusion chromatography. Particle size distribution was assessed by using size-exclusion chromatography as well as nanoparticle tracking analysis and flow imaging microscopy. Structural changes were monitored by circular dichroism, Fourier transform infrared and fluorescence spectroscopy. Thermal stability was measured by differential scanning fluorimetry. Aggregation propensity was maximal at low pH and high ionic strength. While thermal stability decreased with pH, the secondary structure remained unchanged down to pH 3.5 and up to 1.5M NaCl. Precipitated protein could be largely reverted to monomers by dilution into salt-free buffer. The re-solubilized antibody was indistinguishable in structure, solubility and monodispersity from the unstressed protein. Also, binding to Protein A was steady. Aggregation could be reduced in the presence of trehalose. The results suggest a reversible aggregation mechanism characterized by a limited change in tertiary structure at low pH and a subsequent loss of colloidal stability resulting from electrostatic repulsion once salt is added to the sample. The experimental setup is robust and allows high-throughput quantification of the effect of additives on aggregation kinetics.

The influence of bisphenol A on mammalian cell cultivation.

Bisphenol A (BPA) plays a substantial role in industry, as it is used for polycarbonate (PC) plastics and epoxy resins which are required for various plastic consumer products. However, BPA is known to be an endocrine disruptor, and its influence on humans, animals, and various cell lines was addressed in diverse studies. As the burden of BPA can be increased by using disposable plastic articles and single-use technologies for cultivation, it is essential to examine the consequences of BPA presence on mammalian cells, as they are a contributing factor in the production of complex pharmaceutical therapeutics. We selected three industrially relevant cell lines and analyzed systemic effects of BPA by comparing cell culture performance in BPA-free poly-ethylene terephthalate glycol (PETG) and in PC shaking flasks. We focused on the influence of BPA on cellular growth, viability, and several metabolic parameters. In addition, we determined the product concentration and aggregation behavior of the recombinant proteins expressed by these cell lines and the BPA concentration within the medium caused by leaching. Moreover, we performed EC50 studies to determine the toxic concentration of BPA. Our results indicated that leached BPA had no effect on specific growth rates and viability and toxicity appeared at about 10(4) times higher concentrations; however, it influenced the specific productivity rate and metabolic activity parameters of our Chinese hamster ovary (CHO) cell line. Consequently, one can neglect BPA from leaching in the culture as long as the selected cell line is BPA tolerant. Otherwise, BPA can be a hurdle for pharmaceutical production, as it can influence the specific productivity of recombinant proteins.

Inhibition of BMP activity protects epithelial barrier function in lung injury.

Epithelial injury is a central finding in pulmonary disease and is accompanied by disruption of epithelial barrier function, leading to pulmonary oedema and inflammation. Injured epithelial cells lose their properties and gain mesenchymal characteristics, a phenotypic switch that contributes to lung remodelling after injury. Here we studied bone morphogenetic protein (BMP) signalling and, in particular, the role of BMP2 and the BMP modulator BMPER in injured lung epithelium. Increased BMP activity, reflected by up-regulation of the Smad1/5-Id1 axis, is detected after injury of lung epithelium in vitro and in vivo. Two members of the BMP family, BMP2 and BMPER, have opposing effects. BMP2 is up-regulated after epithelial injury and causes epithelial dysfunction and hyperpermeability, mediated by the Smad1/5-Id1-dependent down-regulation of E-cadherin. In contrast, BMPER expression is decreased following injury, which in turn impairs epithelial integrity, characterized by reduction of E-cadherin and epithelial leakage in vitro and in vivo. High levels of BMPER antagonized BMP2-Smad5-Id1 signalling and prevented BMP2-mediated decrease of E-cadherin and hyperpermeability, suggesting that BMPER restores epithelial homeostasis. Supporting this notion, pharmacological inhibition of BMP signalling by LDN193189 prevented reduction of E-cadherin and disruption of epithelial barrier function. Inhibition of excessive BMP activation could be a new approach to restore epithelial integrity and prevent disruption of epithelial barrier function after lung injury.

Dissecting the alternatively folded state of the antibody Fab fragment.

Intact antibodies and antigen binding fragments (Fab) have been previously shown to form an alternatively folded state (AFS) at low pH. This state consists primarily of secondary structure interactions, with reduced tertiary structure content. The AFS can be distinguished from the molten globule state by the formation of nonnative structure and, in particular, its high stability. In this study, the isolated domains of the MAK33 (murine monoclonal antibody of the subtype kappa/IgG1) Fab fragment were investigated under conditions that have been reported to induce the AFS. Surprising differences in the ability of individual domains to form the AFS were observed, despite the similarities in their native structures. All Fab domains were able to adopt the AFS, but only for V(H) (variable domain of the heavy chain) could a significant amount of tertiary structure be detected and different conditions were needed to induce the AFS. V(H), the least stable of the domains under physiological conditions, was the most stable in the AFS, yet all domains showed significant stability against thermal and chemical unfolding in their AFS. Formation of the AFS was found to generally proceed via the unfolded state, with similar rates for most of the domains. Taken together, our data reveal striking differences in the biophysical properties of the AFS of individual antibody domains that reflect the variation possible for domains of highly homologous native structures. Furthermore, they allow individual domain contributions to be dissected from specific oligomer effects in the AFS of the antibody Fab fragment.

The folding pathway of the antibody V(L) domain.

Antibodies are modular proteins consisting of domains that exhibit a beta-sandwich structure, the so-called immunoglobulin fold. Despite structural similarity, differences in folding and stability exist between different domains. In particular, the variable domain of the light chain V(L) is unusual as it is associated with misfolding diseases, including the pathologic assembly of the protein into fibrillar structures. Here, we have analysed the folding pathway of a V(L) domain with a view to determine features that may influence the relationship between productive folding and fibril formation. The V(L) domain from MAK33 (murine monoclonal antibody of the subtype kappa/IgG1) has not previously been associated with fibrillisation but is shown here to be capable of forming fibrils. The folding pathway of this V(L) domain is complex, involving two intermediates in different pathways. An obligatory early molten globule-like intermediate with secondary structure but only loose tertiary interactions is inferred. The native state can then be formed directly from this intermediate in a phase that can be accelerated by the addition of prolyl isomerases. However, an alternative pathway involving a second, more native-like intermediate is also significantly populated. Thus, the protein can reach the native state via two distinct folding pathways. Comparisons to the folding pathways of other antibody domains reveal similarities in the folding pathways; however, in detail, the folding of the V(L) domain is striking, with two intermediates populated on different branches of the folding pathway, one of which could provide an entry point for molecules diverted into the amyloid pathway.