Mechanism of antibodies purification by protein A

Method for B Cell Receptor Enrichment in Malignant B Cells

B cells are central to the adaptive immune response and provide long-lasting immunity after infection. B cell activation is mediated by the surface membrane-bound B cell receptor (BCR) following recognition of a specific antigen. The BCR has been challenging to analyse using mass spectrometry (MS) due to the difficulty of isolating and enriching this membrane-bound protein complex. There are approximately 120,000 BCRs on the B cell surface; however, depending on the B cell activation state, there may be hundreds-of-millions to billions of proteins in a B cell. Consequently, advanced proteomic techniques such as MS workflows that use purified proteins to yield structural and protein-interaction information have not been published for the BCR complex. This paper describes a method for enriching the BCR complex that is MS-compatible. The method involves a Protein G pull down on agarose beads using an intermediary antibody to each of the BCR complex subcomponents (CD79a, CD79b, and membrane immunoglobulin). The enrichment process is shown to pull down the entire BCR complex and has the advantage of being readily compatible with further proteomic study including MS analysis. Using intermediary antibodies has the potential to enrich all isotypes of the BCR, unlike previous methods described in the literature that use protein G-coated beads to directly pull down the membrane IgG (mIgG) but cannot be used for other mIg isotypes.

Direct Affinity Ligand Immobilization onto Bare Iron Oxide Nanoparticles Enables Efficient Magnetic Separation of Antibodies

Magnetic separation is a promising alternative to chromatography for enhancing the downstream processing (DSP) of monoclonal antibodies (mAbs). However, there is a lack of efficient magnetic particles for successful application. Aiming to fill this gap, we demonstrate the suitability of bare iron oxide nanoparticles (BION) with physical site-directed immobilization of an engineered Protein A affinity ligand (rSpA) as an innovative magnetic material. The rSpA ligand contains a short peptide tag that enables the direct and stable immobilization onto the uncoated BION surface without commonly required laborious particle activation. The resulting BION@rSpA have beneficial characteristics outperforming conventional Protein A-functionalized magnetic particles: a simple, fast, low-cost synthesis, a particle size in the nanometer range with a large effective specific surface area enabling large immunoglobulin G (IgG) binding capacity, and a high magnetophoretic velocity advantageous for fast processing. We further show rapid interactions of IgG with the easily accessible rSpA ligands. The binding of IgG to BION@rSpA is thereby highly selective and not impeded by impurity molecules in perfusion cell culture supernatant. Regarding the subsequent acidic IgG elution from BION@rSpA@IgG, we observed a hampering pH increase caused by the protonation of large iron oxide surfaces after concentrating the particles in 100 mM sodium acetate buffer. However, the pH can be stabilized by adding 50 mM glycine to the elution buffer, resulting in recoveries above 85% even at high particle concentrations. Our work shows that BION@rSpA enable efficient magnetic mAb separation and could help to overcome emerging bottlenecks in DSP.

Affinity purification of mAb from serum-containing hybridoma culture supernatant through a novel nanobody that discriminates mouse IgG from bovine IgG by recognizing the mouse kappa constant region (mCK)

When purifying mAb from serum-containing hybridoma culture supernatant, it is essential that mouse IgG remains free from contaminations of bovine IgG. However, the broadly used Protein A resin cannot achieve this goal due to binding between both mouse and bovine IgG. Here, a novel nanobody-based affinity purification magnetic beads that discriminates mouse IgG from bovine IgG was developed. To bind all subtypes of mouse IgG (IgG1, IgG2a, IgG2b and IgG3) that contain the kappa light chain, mCK (mouse kappa constant region)-specific nanobody binders were selected from an immune phage display VHH library; this library was constructed with peripheral blood mononuclear cells (PBMCs), which were collected from Bactrian camels immunized with a mix of intact mouse IgGs (IgG1, IgG2a, IgG2b and IgG3). A novel clone that exhibited a higher expression level and a higher binding affinity was selected (4E6). Then, the 4E6 nanobody in the format of VHH-hFC (human Fc) was conjugated on magnetic beads with a maximal binding capacity of 15.41±0.69 mg mouse IgG/mL beads. Furthermore, no bovine IgG could be copurified from hybridoma culture supernatant with immunomagnetic beads. This approach is valuable for the large-scale in vitro production of highly pure antibodies by hybridoma cells.

Progress of ligand-modified agarose microspheres for protein isolation and purification

Proteins are the material basis of life and the primary carriers of life activities, containing various impurities that must be removed before use. To keep pace with the increasing complexity of protein samples, it is essential to constantly work on developing new purification technologies for downstream processes. While traditional downstream purification methods rely heavily on protein A affinity chromatography, there is still a lot of interest in finding safer and more cost-effective alternatives to protein A. Many non-affinity ligands and technologies have also been developed in biological purification recently. Here, the current status of biotechnology and the progress of protein separation technology from 2018 to 2023 are reviewed from the aspects of new preparation methods and new composite materials of commonly used separation media. The research status of new ligands with different mechanisms of action was reviewed, including the expanded application of affinity ligands, the development prospect of biotechnology such as polymer grafting, continuous column technology, and its new applications.

Study of the reusability and stability of nylon nanofibres as an antibody immobilisation surface

In the case of a biological threat, early, rapid, and specific detection is critical. In addition, ease of handling, use in the field, and low-cost production are important considerations. Immunological devices are able to respond to these needs. In the design of these immunological devices, surface antibody immobilisation is crucial. Nylon nanofibres have been described as a very good option because they allow for an increase in the surface-to-volume ratio, leading to an increase in immunocapture efficiency. In this paper, we want to deepen the study of other key points, such as the reuse and stability of these nanofibres, in order to assess their profitability. On the one hand, the reusability of nanofibres has been studied using different stripping treatments at different pH values on the nylon nanofibres with well-oriented antibodies anchored by protein A/G. Our study shows that stripping with glycine buffer pH 2.5 allows the nanofibres to be reused as long as protein A/G has been previously anchored, leaving both nanofibre and protein A/G unchanged. On the other hand, we investigated the stability of the nylon nanofibres. To achieve this, we analysed any loss of immunocapture ability of well-oriented antibodies anchored both to the nylon nanofibres and to a specialised surface with high protein binding capacity. The nanofibre immunocapture system maintained an unchanged immunocapture ability for a longer time than the specialised planar surface. In conclusion, nylon nanofibres seem to be a very good choice as an antibody immobilisation surface, offering not only higher immunocapture efficiency, but also more cost efficiency as they are reusable and stable.

Comparison of oriented and non-oriented antibody conjugation with AIE fluorescence microsphere for the immunochromatographic detection of enrofloxacin

Antibodies and labels were typically non-oriented conjugated in conventional immunochromatographic assays (ICAs). In this work, a C-terminal cysteine-tagged recombinant protein A (rPA) was conjugated in an oriented manner onto aggregation-induced emission fluorescence microsphere (AIEFM). The Fc fragment of anti-enrofloxacin monoclonal antibody (anti-ENR mAb) was then conjugated onto the rPA. The resulting oriented mAb-AIEFM probe was used in an ENR-ICA for the rapid detection of ENR, a widely abused animal drug. The ENR-ICA with the oriented probe saved 66.7% of anti-ENR mAb and 25% of ENR-bovine serum albumin, and had a limit of detection of 0.035 ng/mL, compared with 0.079 ng/mL for the non-oriented probe. The corresponding linear ranges of the ENR-ICA based on the oriented and non-oriented probes were 0.25-10 ng/mL and 0.1-2.5 ng/mL, respectively. This novel ICA based on the oriented probe has the potential to be used for sensitive and rapid detection in food safety.

Functional integration of protein A binding ability to antibody fragments for convenient and tag-free purification

Although the development of small therapeutic antibodies is important, the affinity tags used for their purification often result in heterogeneous production and immunogenicity. In this study, we integrated Staphylococcus aureus protein A (SpA) binding ability into antibody fragments for convenient and tag-free purification. SpA affinity chromatography is used as a global standard purification method for conventional antibodies owing to its high binding affinity to the Fc region. SpA also has a binding affinity for some variable heavy domains (VH) classified in the VH3 subfamily. Through mutagenesis based on alignment and structural modeling results using the SpA-VH3 cocrystal structure, we integrated the SpA-binding ability into the anti-CD3 single-chain Fv. Furthermore, we applied this mutagenesis approach to more complicated small bispecific antibodies and successfully purified the antibodies using SpA affinity chromatography. The antibodies retained their biological function after purification. Integration of SpA-binding ability into conventional antibody fragments simplifies the purification and monitoring of the production processes and, thus, is an ideal strategy for accelerating the development of small therapeutic antibodies. Furthermore, because of its immunoactivity, the anti-CD3 variable region with SpA-binding ability is an effective building block for developing engineered cancer therapeutic antibodies without the Fc region.

Profiling the Biophysical Developability Properties of Common IgG1 Fc Effector Silencing Variants

Therapeutic antibodies represent the most significant modality in biologics, with around 150 approved drugs on the market. In addition to specific target binding mediated by the variable fragments (Fvs) of the heavy and light chains, antibodies possess effector functions through binding of the constant region (Fc) to Fcγ receptors (FcγR), which allow immune cells to attack and kill target cells using a variety of mechanisms. However, for some applications, including T-cell-engaging bispecifics, this effector function is typically undesired. Mutations within the lower hinge and the second constant domain (CH2) of IgG1 that comprise the FcγR binding interface reduce or eliminate effector function ("Fc silencing") while retaining binding to the neonatal Fc receptor (FcRn), important for normal antibody pharmacokinetics (PKs). Comprehensive profiling of biophysical developability properties would benefit the choice of constant region variants for development. Here, we produce a large panel of representative mutations previously described in the literature and in many cases in clinical or approved molecules, generate select combinations thereof, and characterize their binding and biophysical properties. We find that some commonly used CH2 mutations, including D265A and P331S, are effective in reducing binding to FcγR but significantly reduce stability, promoting aggregation, particularly under acidic conditions commonly employed in manufacturing. We highlight mutation sets that are particularly effective for eliminating Fc effector function with the retention of WT-like stability, including L234A, L235A, and S267K (LALA-S267K), L234A, L235E, and S267K (LALE-S267K), L234A, L235A, and P329A (LALA-P329A), and L234A, L235E, and P329G (LALE-P329G).

Staphylococcal protein A-modified hydrogel facilitates in situ immunomodulation by capturing anti-HMGB1 for islet grafts

Islet transplantation is regarded as the most promising therapy for type 1 diabetes. However, both hypoxia and immune attack impair the grafted islets after transplantation, eventually failing the islet graft. Although many studies showed that biomaterials with nanoscale pores, like hydrogels, could protect islets from immune cells, the pores on biomaterials inhibited vascular endothelial cells (VECs) to creep in, which resulted in poor revascularization. Thus, a hydrogel device that can facilitate in situ immune modulations without the cost of poor revascularization should be put forward. Accordingly, we designed a spA-modified hydrogel capturing anti-HMGB1 mAB (mAB-spA Gel): the Staphylococcus aureus protein A (spA) was conjugated on the network of hydrogel to capture anti-HMGB1mAB which can inactivate immune cells, while the pore sizes of the hydrogel were more than 100μm which allows vascular endothelial cells (VECs) to creep in. In this study, we screened the optimal spA concentration in mAB-spA Gel according to the physical properties and antibody binding capability, then demonstrated that it could facilitate in situ immunomodulation without decreasing the vessel reconstruction in vitro. Further, we transplanted islet graft in vivo and showed that the survival of islets was elongated. In conclusion, mAB-spA Gel provided an alternative islet encapsulation strategy for type 1 diabetes. STATEMENT OF SIGNIFICANCE: Although various studies have shown that the backbone of the hydrogels can isolate islets grafts from immune cells and the survival of the islets can be prolonged by this way, it is also reported that when the pore size of the backbone is too small the revascularization will be adversely affected. According to this point, it is hard to adjust hydrogel's pore size to protect the islets from the immune attack while allowing endothelial vascular cells to creep in. To solve this dilemma, we designed an immunomodulatory hydrogel inhibiting the activation of T cells by immunosuppressive IgGs instead of the backbone network, so the hydrogel can prolong the survival of islets without the sacrifice of revascularization.

Selective and High-Capacity Binding of Immunoglobulin G to Zirconia Nanoparticles Modified with Phosphate Groups

High-performance and cost-effective purification is necessary for the development of antibody drugs. This study found that nanoparticles of zirconia modified with phosphate groups selectively adsorb immunoglobulin G (IgG) antibodies against serum proteins with high adsorption capacity. The IgG antibodies collected from the zirconia nanoparticle surfaces retain their molecular conformation. Importantly, zirconia nanoparticles have the highest affinity for human IgG antibodies among tested mammalian IgG antibodies. The affinity for human IgG subclasses is in the order IgG3 > IgG1 > IgG2, which contrasts with a conventional ligand (Protein A) that has a lower affinity for IgG3. Because zirconia nanoparticles are chemically and mechanically stable, they can be utilized for the purification of antibody drugs not only in batch methods but also in chromatography as a process upstream or downstream of Protein A chromatography and even as an alternative process.

A luciferase based automated assay for rapid and sensitive detection of SARS-CoV-2 antibodies

The Coronavirus disease 2019 (COVID-19) pandemic brings great challenges to the public health and social economics around the world. As the pandemic continues and the mass vaccination goes on, monitoring the antibodies is particularly important for the epidemiological survey and vaccine assessment. Here, we developed a luciferase immunoprecipitation assay combined with an automated platform to detect anti-Receptor Binding Domain (RBD) antibody, where protein A and protein G modified magnetic beads were used to capture antibodies in serum samples and SARS-CoV-2 RBD was fused with Gaussia luciferase to label the captured target antibodies. The whole detection procedure can be completed within 20 min. The developed assay has proven up to 32 times more sensitive than ELISA for the detection of RBD antibodies. Furthermore, the results of the antibody detection of sera from vaccination as well as convalescence displayed good performance. The automated platform may provide a powerful tool for the control of COVID-19 pandemic by vaccination and the research of SARS-CoV-2 seroconversion.

Magnetic Nanoparticles for Protein Separation and Purification

Proteins are essential for various functions such as brain activity and muscle contraction in humans. Even though food is a source of proteins, the bioavailability of proteins in most foods is usually limited due to matrix interaction with other biomolecules. Thus, it is essential to extract these proteins and provide them as a nutraceutical supplement to maintain protein levels and avoid protein deficiency. Hence, protein purification and extraction from natural sources are highly significant in biomedical applications. Chromatography, crude mechanical disruption, use of extractive chemicals, and electrophoresis are some of the methods applied to isolate specific proteins. Even though these methods possess several advantages, they are unable to extract specific proteins with high purity. A suitable alternative is the use of nanoparticles, which can be beneficial in protein purification and extraction. Notably, magnetic iron and iron-based nanoparticles have been employed in protein extraction processes and can be reused via demagnetization due to their magnetic property, smaller size, morphology, high surface-to-volume ratio, and surface charge-mediated property. This chapter is a summary of various magnetic nanoparticles (MNPs) that can be used for the biomolecular separation of proteins.

Development of an Antibody Delivery Method for Cancer Treatment by Combining Ultrasound with Therapeutic Antibody-Modified Nanobubbles Using Fc-Binding Polypeptide

A key challenge in treating solid tumors is that the tumor microenvironment often inhibits the penetration of therapeutic antibodies into the tumor, leading to reduced therapeutic efficiency. It has been reported that the combination of ultrasound-responsive micro/nanobubble and therapeutic ultrasound (TUS) enhances the tissue permeability and increases the efficiency of delivery of macromolecular drugs to target tissues. In this study, to facilitate efficient therapeutic antibody delivery to tumors using this combination system, we developed therapeutic antibody-modified nanobubble (NBs) using an Fc-binding polypeptide that can quickly load antibodies to nanocarriers; since the polypeptide was derived from Protein G. TUS exposure to this Herceptin^®-modified NBs (Her-NBs) was followed by evaluation of the antibody's own ADCC activity, resulting the retained activity. Moreover, the utility of combining therapeutic antibody-modified NBs and TUS exposure as an antibody delivery system for cancer therapy was assessed in vivo. The Her-NBs + TUS group had a higher inhibitory effect than the Herceptin and Her-NBs groups. Overall, these results suggest that the combination of therapeutic antibody-modified NBs and TUS exposure can enable efficient antibody drug delivery to tumors, while retaining the original antibody activity. Hence, this system has the potential to maximize the therapeutic effects in antibody therapy for solid cancers.

Characterization of FcγRIa (CD64) as a Ligand Molecule for Site-Specific IgG1 Capture: A Side-By-Side Comparison with Protein A

Fc γ receptors (FcγRs) are one of the structures that can initiate effector function for monoclonal antibodies. FcγRIa has the highest affinity toward IgG1-type monoclonal antibodies among all FcγRs. In this study, a comprehensive characterization was performed for FcγRIa as a potential affinity ligand for IgG1-type monoclonal antibody binding. The binding interactions were assessed with the SPR technique using different immobilization techniques such as EDC-NHS coupling, streptavidin-biotin interaction, and His-tagged FcγRIa capture. The His-tagged FcγRIa capture was the most convenient method based on assay repeatability. Next, a crude IgG1 sample and its fractions with different monomer contents obtained from protein A affinity chromatography were used to evaluate FcγRIa protein in terms of monoclonal antibody binding capacity. The samples were also compared with a protein A-immobilized chip (a frequently used affinity ligand) for IgG1 binding responses. The antibody binding capacity of the protein A-immobilized chip surface was significantly better than that of the FcγRIa-immobilized chip surface due to its 5 Ig binding domains. The antibody binding responses changed similarly with protein A depending on the monomer content of the sample. Finally, a different configuration was used to assess the binding affinity of free FcγRs (FcγRIa, FcγRIIa, and FcγRIIIa) to three different immobilized IgGs by immobilizing protein L to the chip surface. Unlike previous immobilization techniques tested where the FcγRIa was utilized as a ligand, nonimmobilized or free FcγRIa resulted in a significantly higher antibody binding response than free protein A. In this configuration, kinetics data of FcγRI revealed that the association rate (k_a 50-80 × 10⁵ M^-1 s^-1) increased in comparison to His capture method (1.9-2.4 × 10⁵ M^-1 s^-1). In addition, the dissociation rate (k_d 10^-5 s^-1) seemed slower over the His capture method (10^-4 s^-1) and provided stability on the chip surface during the dissociation phase. The K_D values for FcγRIa were found in the picomolar range (2.1-10.33 pM from steady-state affinity analysis and 37.5-46.2 pM from kinetic analysis) for IgG1-type antibodies. FcγRIa possesses comparable ligand potential as well as protein A. Even though the protein A-immobilized surface bound more antibodies than the FcγRIa-captured surface, FcγRIa presented a significant antibody binding capacity in protein L configuration. The results suggest FcγRIa protein as a potential ligand for site-oriented immobilization of IgG1-type monoclonal antibodies, and it needs further performance investigation on different surfaces and interfaces for applications such as sensing and antibody purification.

Real-time kinetics and affinity analysis of the interaction between protein A and immunoglobulins G derived from different species on silica colloidal crystal films

Kinetic and affinity analysis of protein interactions reveals information on their related activities in biological processes. Herein, we established a system for evaluating the kinetics and affinity of the interaction between protein A and various IgG species on the surface of silica spheres of silica colloidal crystal (SCC) films by the extraordinary optical interference capabilities of 190 nm silica spheres after self-assembly. The equilibrium association constant (KA) was calculated by the equilibrium Langmuir model and nonlinear least-squares analysis of time-dependent data. The relative protein A/IgG binding affinity is human > rabbit >cow >goat. In addition, the competitive interaction of distinct species of IgG with protein A at the interface of SCC films was studied and performed. These findings may help with the use of protein A and other recognition components in a number of sensor types. Furthermore, this research might offer a novel approach to determining the kinetics and affinity of proteins on the surface of spheres particles, which may contribute to the development of the application of spheres particles in pharmaceutical science, biomedical engineering, and other techniques.

Surface Passivation with a Perfluoroalkane Brush Improves the Precision of Single-Molecule Measurements

Single-molecule imaging is invaluable for investigating the heterogeneous behavior and interactions of biological molecules. However, an impediment to precise sampling of single molecules is the irreversible adsorption of components onto the surfaces of cover glasses. This causes continuous changes in the concentrations of different molecules dissolved or suspended in the aqueous phase from the moment a sample is dispensed, which will shift, over time, the position of chemical equilibria between monomeric and multimeric components. Interferometric scattering microscopy (iSCAT) is a technique in the single-molecule toolkit that has the capability to detect unlabeled proteins and protein complexes both as they adsorb onto and desorb from a glass surface. Here, we examine the reversible and irreversible interactions between a number of different proteins and glass via analysis of the adsorption and desorption of protein at the single-molecule level. Furthermore, we present a method for surface passivation that virtually eliminates irreversible adsorption while still ensuring the residence time of molecules on surfaces is sufficient for detection of adsorption by iSCAT. By grafting high-density perfluoroalkane brushes on cover-glass surfaces, we observe approximately equal numbers of adsorption and desorption events for proteins at the measurement surface (±1%). The fluorous-aqueous interface also prevents the kinetic trapping of protein complexes and assists in establishing a thermodynamic equilibrium between monomeric and multimeric components. This surface passivation approach is valuable for in vitro single-molecule experiments using iSCAT microscopy because it allows for continuous monitoring of adsorption and desorption of protein without either a decline in detection events or a change in sample composition due to the irreversible binding of protein to surfaces.

When smartphone enters food safety: A review in on-site analysis for foodborne pathogens using smartphone-assisted biosensors

Pathogens are one of the supreme threats for the public health around the world in food supply chain. The on-site monitoring is an emerging trend for screening pathogens during the food processing and preserving. Traditional analytical tools have been unable to satisfy the current demands. Smartphones have enormous potentials for achieving on-site detection of foodborne pathogens, with intrinsic advantages such as small size, high accessibility, fast processing speed, and powerful imaging capacity. This review aims to synthesize the current advances in smartphone-assisted biosensors (SABs) for sensing foodborne pathogens, and briefly put forward the problem that consist in the research. We present the role of nanotechnology and recognition modes targeting foodborne pathogens in SABs, and discuss the signal conversion platforms coupling with smartphone. The challenges and perspectives in SABs are also proposed. The smartphone analytics area is moving forward, and it much be subject to careful quality standards and validation.

Real-time monitoring of immunoglobulin G levels in milk using an ordered porous layer interferometric optical sensor

Immunoglobulin G (IgG) is a significant ingredient of immunological activity in milk and colostrum, the activity and content of which is easily disturbed by potentially conditional variant during sterilization. Therefore, developing robust methods for the detection of IgG levels in milk is especially important. Herein, protein A from the Staphylococcus aureus functionalized silica colloidal crystalline film (SCC@SPA) sensing unit combined with ordered porous layer interferometry (OPLI) for IgG detection in untreated bovine milk was developed. Calibration curves in milk and buffer were set up by the variations of the optical thickness (OT) of the sensing unit after the IgG association and dissociation phases. The influence of temperature on the level of IgG was evaluated. Furthermore, the identification of IgG levels with pasteurized milk and ultrahigh temperature (UHT) sterilized milk from the market randomly was successfully carried out without any sample pretreatment. More importantly, compared with other methods, this novel method has the advantages of convenient operation, low cost, and suitability for point-of-care (POC) testing.

The first report on the sortase-mediated display of bioactive protein A from Staphylococcus aureus (SpA) on the surface of the vegetative form of Bacillus subtilis

Protein A (SpA) is one of the most important Staphylococcus aureus cell wall proteins. It includes five immunoglobulin (Ig)-binding domains which can bind to immune complexes through the Fc region of immunoglobulins. The binding of SpA to the polymeric supports can be used to prepare affinity chromatography resins, which are useful for immunoprecipitation (IP) of antibodies. Protein A is also used to purify many anti-cancer antibodies. In this study, SpA was displayed on the surface of Bacillus subtilis cells using a sortase-mediated system to display the target protein to the B. subtilis cell wall. A series of plasmids consisting of cassettes for cell wall-directed protein A as well as negative controls were constructed and transformed into B. subtilis WASD (wprA sigD) cells. SDS-PAGE, western blot, flow cytometry, functional IgG purification assay, and a modified ELISA assay were used to confirm the surface display of SpA and evaluate its function. Semi-quantitative ELISA results showed that the binding capacity of lyophilized Bs-SpA is 100 μg IgG from rabbit serum per 1 mg of cells under optimal experimental conditions. Low production costs, optimal performance, and the use of a harmless strain compared to a similar commercial product predict the possible use of SpA immobilization technology in the future.

A versatile platform for the tumor-targeted delivery of immune checkpoint-blocking immunoglobin G

Immunotherapies based on immune checkpoint-blocking antibodies have been considered the most attractive cancer treatments in recent years. However, the systemic administration of immune checkpoint-blocking antibodies is limited by low response rates and high risk of inducing immune-related adverse events (irAEs), which might be overcome by the tumor-targeted delivery of these antibodies. To achieve tumor-targeted delivery, immune checkpoint-blocking antibodies are usually modified with tumor-homing ligands through difficult genetic fusion or chemical conjugation. As most immune checkpoint-blocking antibodies are immunoglobin G (IgG) antibodies, we hypothesize that these IgG antibodies might be noncovalently modified with a tumor-homing ligand fused to an IgG-binding domain (IgBD). To test this hypothesis, the tumor-homing Z_PDGFRβ affibody, which targets platelet-derived growth factor receptor β (PDGFRβ), was fused to the Fab-selective IgBD in a trimeric format. After mixing Z_PDGFRβ fused to the IgBD with immune checkpoint-blocking IgG against programmed death-ligand 1 (αPD-L1), a novel homogenous complex was formed, indicating that αPD-L1 had been successfully modified with Z_PDGFRβ fused to the IgBD. Z_PDGFRβ-modified αPD-L1 bound to both PDGFRβ and PD-L1, thus leading to greater tumor uptake and antitumor effects in mice bearing PDGFRβ⁺PD-L1⁺ tumor grafts. In addition, due to the broad spectrum of IgBD for IgG, immune checkpoint-blocking IgG antibodies against cytotoxic T-lymphocyte-associated protein 4 (αCTLA-4) and signal regulatory protein alpha (αSIRPα) were also modified with Z_PDGFRβ fused to the IgBD. These results demonstrated that a tumor-homing ligand fused to the IgBD might be developed as a versatile platform for the modification of immune checkpoint-blocking IgG antibodies to achieve tumor-targeted delivery.

Interactions between Amino Acids and Zirconia Modified with Ethylenediaminetetra(methylenephosphonic acid): Mechanistic Insights into the Selective Binding of Antibodies

Zirconia modified with ethylenediaminetetra(methylenephosphonic acid) (EDTMP) has an affinity for antibodies, including immunoglobulin G (IgG) and immunoglobulin M (IgM). However, little is known about the mechanism underlying antibody selectivity. In this study, we examined the interactions of EDTMP-modified zirconia with proteinogenic amino acids using chromatographic and batch methods to gain mechanistic insights into antibody selectivity at the amino acid level. We demonstrated that EDTMP-modified zirconia has an affinity for amino acids with a positively charged side chain, especially lysine. Similar trends were observed for oligopeptides. This affinity was reduced by the addition of sodium phosphate or sodium polyphosphates. Thus, the antibody selectivity of EDTMP-modified zirconia is primarily ascribable to electrostatic attractions between the EDTMP moieties of the zirconia surfaces and the constant region of antibodies that are rich in lysine residues. Consistent with this, the human IgG antibody has a higher adsorption ability onto EDTMP-modified zirconia than the rabbit IgG antibody, which has fewer lysine residues in the constant region. These findings are useful not only for improving antibody purification but also for developing new applications, including purification of proteins tagged with positively charged amino acid residues.