A Cell-Based Internalization and Degradation Assay with an Activatable Fluorescence-Quencher Probe as a Tool for Functional Antibody Screening

Antibodies internalization mechanisms by dendritic cells and their role in therapeutic antibody immunogenicity

Internalization and processing by antigen-presenting cells such as dendritic cells (DCs) are critical steps for initiating a T-cell response to therapeutic antibodies. Consequences are the production of neutralizing antidrug antibodies altering the clinical response, the presence of immune complexes, and, in some rare cases, hypersensitivity reactions. In recent years, significant progress has been made in the knowledge of cellular uptake mechanisms of antibodies in DCs. The uptake of antibodies could be directly related to their immunogenicity by regulating the quantity of materials entering the DCs in relation to antibody structure. Here, we summarize the latest insights into cellular uptake mechanisms and pathways in DCs. We highlight the approaches to study endocytosis, the impact of endocytosis routes on T-cell response, and discuss the link between how DCs internalize therapeutic antibodies and the potential mechanisms that could give rise to immunogenicity. Understanding these processes could help in developing assays to evaluate the immunogenicity potential of biotherapeutics.

Long-term retention of antigens in germinal centers is controlled by the spatial organization of the follicular dendritic cell network

Germinal centers (GCs) require sustained availability of antigens to promote antibody affinity maturation against pathogens and vaccines. A key source of antigens for GC B cells are immune complexes (ICs) displayed on follicular dendritic cells (FDCs). Here we show that FDC spatial organization regulates antigen dynamics in the GC. We identify heterogeneity within the FDC network. While the entire light zone (LZ) FDC network captures ICs initially, only the central cells of the network function as the antigen reservoir, where different antigens arriving from subsequent immunizations colocalize. Mechanistically, central LZ FDCs constitutively express subtly higher CR2 membrane densities than peripheral LZ FDCs, which strongly increases the IC retention half-life. Even though repeated immunizations gradually saturate central FDCs, B cell responses remain efficient because new antigens partially displace old ones. These results reveal the principles shaping antigen display on FDCs during the GC reaction.

The Dawn of a New Era: Targeting the "Undruggables" with Antibody-Based Therapeutics

The high selectivity and affinity of antibodies toward their antigens have made them a highly valuable tool in disease therapy, diagnosis, and basic research. A plethora of chemical and genetic approaches have been devised to make antibodies accessible to more "undruggable" targets and equipped with new functions of illustrating or regulating biological processes more precisely. In this Review, in addition to introducing how naked antibodies and various antibody conjugates (such as antibody-drug conjugates, antibody-oligonucleotide conjugates, antibody-enzyme conjugates, etc.) work in therapeutic applications, special attention has been paid to how chemistry tools have helped to optimize the therapeutic outcome (i.e., with enhanced efficacy and reduced side effects) or facilitate the multifunctionalization of antibodies, with a focus on emerging fields such as targeted protein degradation, real-time live-cell imaging, catalytic labeling or decaging with spatiotemporal control as well as the engagement of antibodies inside cells. With advances in modern chemistry and biotechnology, well-designed antibodies and their derivatives via size miniaturization or multifunctionalization together with efficient delivery systems have emerged, which have gradually improved our understanding of important biological processes and paved the way to pursue novel targets for potential treatments of various diseases.

An internalizing antibody targeting of cell surface GRP94 effectively suppresses tumor angiogenesis of colorectal cancer

Colorectal cancer (CRC) is one of the life-threatening malignancies worldwide. Thus, novel potential therapeutic targets and therapeutics for the treatment of CRC need to be identified to improve the clinical outcomes of patients with CRC. In this study, we found that glucose-regulated protein 94 (GRP94) is overexpressed in CRC tissues, and its high expression is correlated with increased microvessel density. Next, through phage display technology and consecutive in vitro functional isolations, we generated a novel human monoclonal antibody that specifically targets cell surface GRP94 and shows superior internalizing activity comparable to trastuzumab. We found that this antibody specifically inhibits endothelial cell tube formation and simultaneously promotes the downregulation of GRP94 expression on the endothelial cell surface. Finally, we demonstrated that this antibody effectively suppresses tumor growth and angiogenesis of HCT116 human CRC cells without causing severe toxicity in vivo. Collectively, these findings suggest that cell surface GRP94 is a novel potential anti-angiogenic target in CRC and that antibody targeting of GRP94 on the endothelial cell surface is an effective strategy to suppress CRC tumor angiogenesis.

Targeted Mass Spectrometry-Based Approach for the Determination of Intrinsic Internalization Kinetics of Cell-Surface Membrane Protein Targets

Successful development of targeted therapeutics aimed at the elimination of diseased cells relies on the target properties and the therapeutics that target them. Currently, target properties have been evaluated through antibody-dependent semiquantitative approaches such as flow cytometry, Western blotting, or microscopy. Since antibodies can alter target properties following binding, antibody-dependent approaches provide at best skewed measurements for target intrinsic properties. To circumvent, here we attempted to develop an antibody-free targeted mass spectrometry-based (ATM) strategy to measure the surface densities and the intrinsic rates (K_int) of CD38 internalization in multiple myeloma cell lines. Using cell-surface biotinylation in conjunction with differential mass tagging to separate inward CD38 molecules from the outbound and nascent ones, the ATM approach revealed diversities in measured CD38 K_int values of 0.239 min^-1 S.E. ± 0.076, 0.109 min^-1 S.E. ± 0.032, and 0.058 min^-1 S.E. ± 0.001 for LP1, NCIH929, and MOLP8 cell lines, respectively. Together with CD38 surface densities, intrinsic K_int values aligned well with the tumor penetration model and supported the outcomes for tumor regression in mouse xenografts upon drug treatment. Additionally, the ATM approach can evaluate molecules with fast K_int as we determined for CTLA4 protein. We believe that the ATM approach has the potential to evaluate diverse cell-surface targets as part of the pharmacological assessment in drug discovery.

Quantification of Antibody Persistence for Cell Surface Protein Labeling

Introduction

Antibodies are an essential research tool for labeling surface proteins but can potentially influence the behavior of proteins and cells to which they bind. Because of this, researchers and clinicians are interested in the persistence of these antibodies, particularly for live-cell applications. We developed an easily adoptable method for researchers to characterize antibody removal timelines for any cell-antibody combination, with the benefit of studying broad, hypothesized mechanisms of antibody removal.

Methods

We developed a method using four experimental conditions to elucidate the contributions of possible factors influencing antibody removal: cell proliferation, internalization, permanent dissociation, and environmental perturbation. This method was tested on adipose-derived stem cells and a human lung fibroblast cell line with anti-CD44, CD90, and CD105 antibodies. The persistence of the primary antibody was probed using a fluorescent secondary antibody daily over 10 days. Relative contributions by the antibody removal mechanisms were quantified based on differences between the four culture conditions.

Results

Greater than 90% of each antibody tested was no longer present on the surface of the two cell types after 5 days, with removal observed in as little as 1 day post-labeling. Anti-CD90 antibody was primarily removed by environmental perturbation, anti-CD105 antibody by internalization, and anti-CD44 antibody by a combination of all four factors.

Conclusions

Antibody removal mechanism depended on the specific antibody tested, while removal timelines for the same antibody depended more on cell type. This method should be broadly relevant to researchers interested in quantifying an initial timeframe for uninhibited use of antibody-labeled cells.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12195-021-00670-3.

Immunotoxin Screening System: A Rapid and Direct Approach to Obtain Functional Antibodies with Internalization Capacities

Toxins, while harmful and potentially lethal, have been engineered to develop potent therapeutics including cytotoxins and immunotoxins (ITs), which are modalities with highly selective targeting capabilities. Currently, three cytotoxins and IT are FDA-approved for treatment of multiple forms of hematological cancer, and additional ITs are tested in the clinical trials or at the preclinical level. For next generation of ITs, as well as antibody-mediated drug delivery systems, specific targeting by monoclonal antibodies is critical to enhance efficacies and reduce side effects, and this methodological field remains open to discover potent therapeutic monoclonal antibodies. Here, we describe our application of engineered toxin termed a cell-based IT screening system. This unique screening strategy offers the following advantages: (1) identification of monoclonal antibodies that recognize cell-surface molecules, (2) selection of the antibodies that are internalized into the cells, (3) selection of the antibodies that induce cytotoxicity since they are linked with toxins, and (4) determination of state-specific activities of the antibodies by differential screening under multiple experimental conditions. Since the functional monoclonal antibodies with internalization capacities have been identified successfully, we have pursued their subsequent modifications beyond antibody drug conjugates, resulting in development of immunoliposomes. Collectively, this screening system by using engineered toxin is a versatile platform, which enables straight-forward and rapid selection for discovery of novel functional antibodies.

Recent advances in high-throughput flow cytometry for drug discovery

INTRODUCTION

High-throughput flow cytometry (HTFC) has proven to be an important technology in drug discovery. The use of HTFC enables multi-parametric screening of suspension cells containing heterogenous cell populations and coated particles for screening proteins of interest. Novel targets, novel cell markers and compound clusters for drug development have been identified from HTFC screens.

AREAS COVERED

In this article, the authors focus on reviewing the recent HTFC applications reported during the last 5-6 years, including drug discovery screens and studies for immune, immune-oncology, infectious and inflammatory diseases. The main HTFC approaches, development of HTFC systems, and automated sample preparation systems for HTFC are also discussed.

EXPERT OPINION

The advance of HTFC technology coupled with automated sample acquisition and sample preparation has demonstrated its utility in screening large numbers of compounds using suspension cells, facilitated screening of disease-relevant human primary cells, and enabled deep understanding of mechanism of action by analyzing multiple parameters. The authors see HTFC as a very valuable tool in immune, immune-oncology, infectious and inflammatory diseases where immune cells play essential roles.

Recent progress in the molecular imaging of therapeutic monoclonal antibodies

Therapeutic monoclonal antibodies have become one of the central components of the healthcare system and continuous efforts are made to bring innovative antibody therapeutics to patients in need. It is equally critical to acquire sufficient knowledge of their molecular structure and biological functions to ensure the efficacy and safety by incorporating new detection approaches since new challenges like individual differences and resistance are presented. Conventional techniques for determining antibody disposition including plasma drug concentration measurements using LC-MS or ELISA, and tissue distribution using immunohistochemistry and immunofluorescence are now complemented with molecular imaging modalities like positron emission tomography and near-infrared fluorescence imaging to obtain more dynamic information, while methods for characterization of antibody's interaction with the target antigen as well as visualization of its cellular and intercellular behavior are still under development. Recent progress in detecting therapeutic antibodies, in particular, the development of methods suitable for illustrating the molecular dynamics, is described here.

Dose-Dependent Internalization and Externalization Integrity Study of Newly Synthesized 99mTc-Thymoquinone Radiopharmaceutical as Cancer Theranostic Agent

Thymoquinone (TQ) is a bioactive phytochemical isolated from Nigella sativa and has been investigated for biochemical and biological activities in both in vitro and in vivo models. It is best known for its anticancer activities. Thymoquinone accomplishes anticancer activities through targeting multiple cancer markers including PPAR-γ, PTEN, P53, P73, STAT3, and generation of reactive oxygen species at the cancer cell surface. The radiolabeling of TQ with γ- and β-emitter radionuclide could be used as cancer diagnostic or therapeutic radiopharmaceutical, respectively. In this study, we are reporting the radiolabeling of TQ with technetium-99m (^99mTc), stability in saline and blood serum, internalization and externalization of ^99mTc-TQ using rhabdomyosarcoma cancer cells line. The quality control study revealed more than 95% labeling yield and stable in blood serum up to 4 hours. In vitro internalization rate was recorded 27.08% ± 0.95% at 1 hour post 2 hours internalization period and comparatively slow externalization. The results of this study are quite encourging and could be investigated for further key preclinical parameters to enter phase I clinical trials.

Novel Antibody Drug Conjugates Targeting Tumor-Associated Receptor Tyrosine Kinase ROR2 by Functional Screening of Fully Human Antibody Libraries Using Transpo-mAb Display on Progenitor B Cells

Receptor tyrosine kinase-like orphan receptor 2 (ROR2) has been identified as a highly relevant tumor-associated antigen in a variety of cancer indications of high unmet medical need, including renal cell carcinoma and osteosarcoma, making it an attractive target for targeted cancer therapy. Here, we describe the de novo discovery of fully human ROR2-specific antibodies and potent antibody drug conjugates (ADCs) derived thereof by combining antibody discovery from immune libraries of human immunoglobulin transgenic animals using the Transpo-mAb mammalian cell-based IgG display platform with functional screening for internalizing antibodies using a secondary ADC assay. The discovery strategy entailed immunization of transgenic mice with the cancer antigen ROR2, harboring transgenic IgH and IgL chain gene loci with limited number of fully human V, D, and J gene segments. This was followed by recovering antibody repertoires from the immunized animals, expressing and screening them as full-length human IgG libraries by transposon-mediated display in progenitor B lymphocytes ("Transpo-mAb Display") for ROR2 binding. Individual cellular "Transpo-mAb" clones isolated by single cell sorting and capable of expressing membrane-bound as well as secreted human IgG were directly screened during antibody discovery, not only for high affinity binding to human ROR2, but also functionally as ADCs using a cytotoxicity assay with a secondary anti-human IgG-toxin-conjugate. Using this strategy, we identified and validated 12 fully human, monoclonal anti-human ROR2 antibodies with nanomolar affinities that are highly potent as ADCs and could be promising candidates for the therapy of human cancer. The screening for functional and internalizing antibodies during the early phase of antibody discovery demonstrates the utility of the mammalian cell-based Transpo-mAb Display platform to select for functional binders and as a powerful tool to improve the efficiency for the development of therapeutically relevant ADCs.

Expediting Antibody Discovery with a Cell and Bead Multiplexed Competition Assay

With advances in molecular engineering and humanization, monoclonal antibodies are one of the fastest-growing classes of biopharmaceuticals. During antibody discovery, antibody from hybridoma or primary B-cell supernatants is screened for the desired binding characteristics, and secondary screens measure antibody function and concentration, identify immunoglobulin G (IgG) isotype, and assess cell health. In order to expedite the antibody discovery process, we developed a high-throughput, multiplexed cell and bead-based competition assay that identifies and quantitates mouse IgG isotypes and assesses cell health. No differences in assay performance were observed between single and multiplex formats. The linear range of the assay was from 0.5 to 50 µg/mL, and washing was not required, decreasing assay time and variability. Slight modifications to the protocol allowed quantification of dilute antibody supernatants (0.1–5 µg/mL). Using hybridoma cultures, we showed that cell viability measurements in the assay did not interfere with the bead-based IgG measurements. The assay described here is a simple mix-and-read, no-dilution screen that can reduce the time to antibody cloning and production. The high-content data can differentiate monoclonal and polyclonal wells, determine IgG quantity for downstream functional assays, provide isotype information, and monitor cell proliferation and viability.

A red-emissive antibody-AIEgen conjugate for turn-on and wash-free imaging of specific cancer cells

An antibody-AIEgen conjugate is designed and developed as a "turn-on" fluorescent probe for wash-free specific cancer cell imaging. The cetuximab-conjugated AIEgen shows red fluorescence only when it is internalized and accumulated in cancer cells with overexpressed epidermal growth factor receptor through endocytosis. The probe first lights up the lysosomes. After hydrolysis, its residue is accumulated in mitochondria, making them highly emissive with a long cell retention time. Compared with conventional "always-on" fluorescent probes, the antibody-AIEgen conjugate exhibits a very good image contrast during wash-free cancer cell imaging and less interference from normal cells. To the best of our knowledge, this is the first time "turn-on" antibody-AIEgen conjugates have been reported. This new strategy can be further extended to many proteins and water-soluble AIEgens, and many of their potential applications such as real-time tracking of cell dynamics and cancer theranostics will be explored. The present work is expected to inspire more marvellous research in the fields of AIE and cancer imaging.