Fast and robust recombinant protein production utilizing episomal stable pools in WAVE bioreactors.

Protein reagents are essential resources for several stages of drug discovery projects from structural biology and assay development through lead optimization. Depending on the aim of the project different amounts of pure protein are required. Small-scale expressions are initially used to determine the reachable levels of production and quality before scaling up protein reagent supply. Commonly, amounts of several hundreds of milligrams to grams are needed for different experiments, including structural investigations and activity evaluations, which require rather large cultivation volumes. This implies that cultivation of large volumes of either transiently transfected cells or stable pools/stable cell lines is needed. Hence, a production process that is scalable, speeds up the development projects, and increases the robustness of protein reagent quality throughout scales. Here we present a protein production pipeline with high scalability. We show that our protocols for protein production in Chinese hamster ovary cells allow for a seamless and efficient scale-up with robust product quality and high performance. The flexible scale of the production process, as shown here, allows for shorter lead times in drug discovery projects where there is a reagent demand for a specific protein or a set of target proteins.

CaRA - A multi-purpose phage display library for selection of calcium-regulated affinity proteins.

Protein activity regulated by interactions with metal ions can be utilized for many different purposes, including biological therapies and bioprocessing, among others. Calcium ions are known to interact with the frequently occurring EF-hand motif, which can alter protein activity upon binding through an induced conformational change. The calcium-binding loop of the EF-hand motif has previously been introduced into a small protein domain derived from staphylococcal Protein A in a successful effort to render antibody binding dependent on calcium. Presented here, is a combinatorial library for calcium-regulated affinity, CaRA, based on this domain. CaRA is the first alternative scaffold library designed to achieve novel target specificities with metal-dependent binding. From this library, several calcium-dependent binders could be isolated through phage display campaigns towards a set of unrelated target proteins (IgE Cε3-Cε4, TNFα, IL23, scFv, tPA, PCSK9 and HER3) useful for distinct applications. Overall, these monomeric CaRA variants showed high stability and target affinities within the nanomolar range. They displayed considerably higher melting temperatures in the presence of 1 mM calcium compared to without calcium. Further, all discovered binders proved to be calcium-dependent, with the great majority showing complete lack of target binding in the absence of calcium. As demonstrated, the CaRA library is highly capable of providing protein-binding domains with calcium-dependent behavior, independent of the type of target protein. These binding domains could subsequently be of great use in gentle protein purification or as novel therapeutic modalities.

Affinity-Based Methods for Site-Specific Conjugation of Antibodies.

Conjugation of various reagents to antibodies has long been an elegant way to combine the superior binding features of the antibody with other desired but non-natural functions. Applications range from labels for detection in different analytical assays to the creation of new drugs by conjugation to molecules which improves the pharmaceutical effect. In many of these applications, it has been proven advantageous to control both the site and the stoichiometry of the conjugation to achieve a homogeneous product with predictable, and often also improved, characteristics. For this purpose, many research groups have, during the latest decade, reported novel methods and techniques, based on small molecules, peptides, and proteins with inherent affinity for the antibody, for site-specific conjugation of antibodies. This review provides a comprehensive overview of these methods and their applications and also describes a historical perspective of the field.

Systematic evaluation of SARS-CoV-2 antigens enables a highly specific and sensitive multiplex serological COVID-19 assay.

OBJECTIVE: The COVID-19 pandemic poses an immense need for accurate, sensitive and high-throughput clinical tests, and serological assays are needed for both overarching epidemiological studies and evaluating vaccines. Here, we present the development and validation of a high-throughput multiplex bead-based serological assay. METHODS: More than 100 representations of SARS-CoV-2 proteins were included for initial evaluation, including antigens produced in bacterial and mammalian hosts as well as synthetic peptides. The five best-performing antigens, three representing the spike glycoprotein and two representing the nucleocapsid protein, were further evaluated for detection of IgG antibodies in samples from 331 COVID-19 patients and convalescents, and in 2090 negative controls sampled before 2020. RESULTS: Three antigens were finally selected, represented by a soluble trimeric form and the S1-domain of the spike glycoprotein as well as by the C-terminal domain of the nucleocapsid. The sensitivity for these three antigens individually was found to be 99.7%, 99.1% and 99.7%, and the specificity was found to be 98.1%, 98.7% and 95.7%. The best assay performance was although achieved when utilising two antigens in combination, enabling a sensitivity of up to 99.7% combined with a specificity of 100%. Requiring any two of the three antigens resulted in a sensitivity of 99.7% and a specificity of 99.4%. CONCLUSION: These observations demonstrate that a serological test based on a combination of several SARS-CoV-2 antigens enables a highly specific and sensitive multiplex serological COVID-19 assay.

ZCa: A Protein A-Derived Domain with Calcium-Dependent Affinity for Mild Antibody Purification.

Therapeutic antibodies are at the forefront of modern medicine where high purity, which is typically obtained by Protein A-based affinity purification, is of utmost importance. In this chapter, we present a method for neutral and selective purification of antibodies by utilizing an engineered affinity ligand, ZCa, derived from Protein A. This domain displays a calcium-dependent binding of antibodies and has been multimerized and immobilized to a chromatography resin to achieve an affinity matrix with high binding capacity. IgG antibodies can be eluted from the tetrameric ZCa ligand at pH 7 with the addition of EDTA, or at pH 5.5 with EDTA for purification of monoclonal IgG1, which is significantly milder than the low pH (3-4) required in conventional Protein A affinity chromatography. Here, a protocol for selective capture of IgG with elution at neutral pH from a ZCa tetramer ligand immobilized on a chromatography resin is described.

Small Bispecific Affinity Proteins for Simultaneous Target Binding and Albumin-Associated Half-Life Extension.

Albumin-binding fusion partners are frequently used as a means for the in vivo half-life extension of small therapeutic molecules that would normally be cleared very rapidly from circulation. However, in applications where small size is key, fusion to an additional molecule can be disadvantageous. Albumin-derived affinity proteins (ADAPTs) are a new type of scaffold proteins based on one of the albumin-binding domains of streptococcal protein G, with engineered binding specificities against numerous targets. Here, we engineered this scaffold further and showed that this domain, as small as 6 kDa, can harbor two distinct binding surfaces and utilize them to interact with two targets simultaneously. These novel ADAPTs were developed to possess affinity toward both serum albumin as well as another clinically relevant target, thus circumventing the need for an albumin-binding fusion partner. To accomplish this, we designed a phage display library and used it to successfully select for single-domain bispecific binders toward a panel of targets: TNFα, prostate-specific antigen (PSA), C-reactive protein (CRP), renin, angiogenin, myeloid-derived growth factor (MYDGF), and insulin. Apart from successfully identifying bispecific binders for all targets, we also demonstrated the formation of the ternary complex consisting of the ADAPT together with albumin and each of the five targets, TNFα, PSA, angiogenin, MYDGF, and insulin. This simultaneous binding of albumin and other targets presents an opportunity to combine the advantages of small molecules with those of larger ones allowing for lower cost of goods and noninvasive administration routes while still maintaining a sufficient in vivo half-life.

Improvements of a high-throughput protein purification process using a calcium-dependent setup.

The Human Secretome Project aims to produce and purify all human secreted proteins as full-length. In order to enable this, a robust, gentle and effective purification process is needed, where multiple proteins can be purified in parallel. For this reason, a purification system based on a Protein C-tag and the HPC4 antibody with high affinity to the tag was chosen for purification. The strong binding between the tag and the antibody is specific and calcium-dependent, which allows for mild elution with EDTA. Presented here is a study comparing different protein purification base matrices coupled with the HPC4 antibody, aiming to increase the yield of purified protein and reduce the time for purification. Among the different tested matrices, Capto XP showed a high coupling degree and increased the amount of eluted protein as compared to the control matrix. By moving from batch incubation to direct sample loading and by performing the purification on the ÄKTAxpress, an automated protein purification process and a high reduction of hands-on sample handling was achieved. This new method also integrates the desalting step in the purification process, and the time for purification and analysis of each sample was decreased from five to three days. Moreover, a new mild method for matrix regeneration was developed using 50 mM EDTA pH 7.5 instead of 0.1 M glycine pH 2. This method was proven to be efficient for regeneration while maintaining the column binding performance even after nine rounds of regeneration.

High throughput generation of a resource of the human secretome in mammalian cells.

The proteins secreted by human tissues and blood cells, the secretome, are important both for the basic understanding of human biology and for identification of potential targets for future diagnosis and therapy. Here, a high-throughput mammalian cell factory is presented that was established to create a resource of recombinant full-length proteins covering the majority of those annotated as 'secreted' in humans. The full-length DNA sequences of each of the predicted secreted proteins were generated by gene synthesis, the constructs were transfected into Chinese hamster ovary (CHO) cells and the recombinant proteins were produced, purified and analyzed. Almost 1,300 proteins were successfully generated and proteins predicted to be secreted into the blood were produced with a success rate of 65%, while the success rates for the other categories of secreted proteins were somewhat lower giving an overall one-pass success rate of ca. 58%. The proteins were used to generate targeted proteomics assays and several of the proteins were shown to be active in a phenotypic assay involving pancreatic ß-cell dedifferentiation. Many of the proteins that failed during production in CHO cells could be rescued in human embryonic kidney (HEK 293) cells suggesting that a cell factory of human origin can be an attractive alternative for production in mammalian cells. In conclusion, a high-throughput protein production and purification system has been successfully established to create a unique resource of the human secretome.

The human secretome.

The proteins secreted by human cells (collectively referred to as the secretome) are important not only for the basic understanding of human biology but also for the identification of potential targets for future diagnostics and therapies. Here, we present a comprehensive analysis of proteins predicted to be secreted in human cells, which provides information about their final localization in the human body, including the proteins actively secreted to peripheral blood. The analysis suggests that a large number of the proteins of the secretome are not secreted out of the cell, but instead are retained intracellularly, whereas another large group of proteins were identified that are predicted to be retained locally at the tissue of expression and not secreted into the blood. Proteins detected in the human blood by mass spectrometry-based proteomics and antibody-based immunoassays are also presented with estimates of their concentrations in the blood. The results are presented in an updated version 19 of the Human Protein Atlas in which each gene encoding a secretome protein is annotated to provide an open-access knowledge resource of the human secretome, including body-wide expression data, spatial localization data down to the single-cell and subcellular levels, and data about the presence of proteins that are detectable in the blood.

Optimization of a calcium-dependent Protein A-derived domain for mild antibody purification.

As reported here, we developed and optimized a purification matrix based on a Protein A-derived domain, ZCa, displaying calcium-dependent antibody binding. It provides an alternative to the acidic elution conditions of conventional Protein A affinity chromatography for purification of sensitive antibodies and other Fc-based molecules. We describe the multimerization of ZCa to generate a chromatography resin with higher binding capacity. The highest order multimeric variant, ZCaTetraCys, demonstrated a considerably high dynamic binding capacity (35 mg IgG/ml resin) while preserving the specificity for IgG. High recovery was obtained and host cell protein and DNA content in purified fractions proved to be comparable to commercial MabSelect SuRe and MabSelect PrismA. Various elution conditions for use of this domain in antibody purification were investigated. The purification data presented here revealed variations in the interaction of different subclasses of human IgG with ZCaTetraCys. This resulted in diverse elution properties for the different IgGs, where complete elution of all captured antibody for IgG2 and IgG4 was possible at neutral pH. This optimized protein ligand and the proposed purification method offer a unique strategy for effective and mild purification of antibodies and Fc-fusion proteins that cannot be purified under conventional acidic elution conditions due to aggregation formation or loss of function.

Protein Engineering Allows for Mild Affinity-based Elution of Therapeutic Antibodies.

Presented here is an engineered protein domain, based on Protein A, that displays a calcium-dependent binding to antibodies. This protein, ZCa, is shown to efficiently function as an affinity ligand for mild purification of antibodies through elution with ethylenediaminetetraacetic acid. Antibodies are commonly used tools in the area of biological sciences and as therapeutics, and the most commonly used approach for antibody purification is based on Protein A using acidic elution. Although this affinity-based method is robust and efficient, the requirement for low pH elution can be detrimental to the protein being purified. By introducing a calcium-binding loop in the Protein A-derived Z domain, it has been re-engineered to provide efficient antibody purification under mild conditions. Through comprehensive analyses of the domain as well as the ZCa-Fc complex, the features of this domain are well understood. This novel protein domain provides a very valuable tool for effective and gentle antibody and Fc-fusion protein purification.

Site-Specific Photolabeling of the IgG Fab Fragment Using a Small Protein G Derived Domain.

Antibodies are widely used reagents for recognition in both clinic and research laboratories all over the world. For many applications, antibodies are labeled through conjugation to different reporter molecules or therapeutic agents. Traditionally, antibodies are covalently conjugated to reporter molecules via primary amines on lysines or thiols on cysteines. While efficient, such labeling is variable and nonstoichiometric and may affect an antibody's binding to its target. Moreover, an emerging field for therapeutics is antibody-drug conjugates, where a toxin or drug is conjugated to an antibody in order to increase or incorporate a therapeutic effect. It has been shown that homogeneity and controlled conjugation are crucial in these therapeutic applications. Here we present two novel protein domains developed from an IgG-binding domain of Streptococcal Protein G. These domains show obligate Fab binding and can be used for site-specific and covalent attachment exclusively to the constant part of the Fab fragment of an antibody. The two different domains can covalently label IgG of mouse and human descent. The labeled antibodies were shown to be functional in both an ELISA and in an NK-cell antibody-dependent cellular cytotoxicity assay. These engineered protein domains provide novel tools for controlled labeling of Fab fragments and full-length IgG.

In vivo biotinylation and incorporation of a photo-inducible unnatural amino acid to an antibody-binding domain improve site-specific labeling of antibodies.

Antibodies are important molecules in many research fields, where they play a key role in various assays. Antibody labeling is therefore of great importance. Currently, most labeling techniques take advantage of certain amino acid side chains that commonly appear throughout proteins. This makes it hard to control the position and exact degree of labeling of each antibody. Hence, labeling of the antibody may affect the antibody-binding site. This paper presents a novel protein domain based on the IgG-binding domain C2 of streptococcal protein G, containing the unnatural amino acid BPA, that can cross-link other molecules. This novel domain can, with improved efficiency compared to previously reported similar domains, site-specifically cross-link to IgG at the Fc region. An efficient method for simultaneous in vivo incorporation of BPA and specific biotinylation in a flask cultivation of Escherichia coli is described. In comparison to a traditionally labeled antibody sample, the C2-labeled counterpart proved to have a higher proportion of functional antibodies when immobilized on a solid surface and the same limit of detection in an ELISA. This method of labeling is, due to its efficiency and simplicity, of high interest for all antibody-based assays where it is important that labeling does not interfere with the antibody-binding site.