Development of a quantitative, cell-line based assay to measure ADCC activity mediated by therapeutic antibodies

NK Cytotoxicity Mediated by NK-92 Cell Lines Expressing Combinations of Two Allelic Variants for FCGR3

Natural killer (NK) cells play an important role in the surveillance of viral infections and cancer. NK cell antibody-dependent cellular cytotoxicity (ADCC) and direct cytotoxicity are mediated by the recognition of antibody-coated target cells through the Fc gamma receptor IIIA (FcγRIIIa/CD16) and by ligands of activating/inhibitory NK receptors, respectively. Allelic variants of the FCGR3A gene include the high-affinity single-nucleotide polymorphism (SNP) rs396991 (V176F), which is associated with the efficacy of monoclonal antibody (mAb) therapies, and the SNP rs10127939 (L66H/R). The contribution of FCGR3A SNPs to NK cell effector functions remains controversial; therefore, we generated a panel of eight NK-92 cell lines expressing specific combinations of these SNPs and tested their cytotoxicities. NK-92 cells were stably transfected with plasmids containing different combinations of FCGR3A SNPs. Messenger RNA and FcγRIIIa/CD16 cell surface expressions were detected using new generation sequencing (NGS) and flow cytometry, respectively. All FcγRIIIa/CD16-transfected NK-92 cell lines exhibited robust ADCC against three different target cell lines with minor differences. In addition, enhanced direct NK cytotoxicity against K562 target cells was observed, suggesting a mechanistic role of FcγRIIIa/CD16 in direct NK cytotoxicity. In conclusion, we generated eight FcγRIIIa/CD16-transfected NK-92 cell lines carrying different combinations of two of the most studied FCGR3A SNPs, representing the major genotypes described in the European population. The functional characterization of these cell lines revealed differences in ADCC and direct NK cytotoxicity that may have implications for the design of adoptive cancer immunotherapies using NK cells and tumor antigen-directed mAbs.

An Antibody-Dependent Cellular Cytotoxicity Assay for Detecting Ocrelizumab Neutralizing Antibody

Ocrelizumab (OCREVUS®) is a humanized anti-CD20 monoclonal antibody approved for the treatment of adult patients with relapsing multiple sclerosis (RMS) and primary progressive multiple sclerosis (PPMS). Here, we discuss the strategic and technical considerations needed to develop a robust antibody-dependent cellular cytotoxicity (ADCC)-based neutralizing antibody (NAb) assay to detect anti-ocrelizumab NAb in patients enrolled in the ocrelizumab registered clinical trials. The NAb detection assay consisted of a two-tier assay that included a screening assay and a confirmation assay. In the screening assay, patient samples were analyzed in the presence of ocrelizumab. Samples that tested positive in the screening assay were subsequently analyzed in the confirmatory assay where another anti-CD20 mAb, obinutuzumab, was replaced by ocrelizumab, to verify NAb specificity. Both assays utilized MEC-2 cells, a chronic B cell leukemia cell line, pre-labeled with calcein AM as the target cells, and natural killer (NK) cells engineered to stably express Fc gamma receptor IIIa_ F158 as effector cells. Both cell lines were prepared to be thaw-and-use cells. The NAb assay measures fluorescence from the calcein AM released into the assay media upon the lysis of target cells by ADCC in the presence of ocrelizumab or obinutuzumab. Our validated NAb assay showed a relative sensitivity of 743 ng/mL and can detect 1500 ng/mL of a surrogate positive control antibody in the presence of 1500 ng/mL ocrelizumab. This ADCC assay is the first reported NAb assay that directly measures target cell lysis by using thaw-and-use target and effector cells simultaneously.

Assessment of Functional Characterization and Comparability of Biotherapeutics: a Review

The development of monoclonal antibody (mAb) biosimilars is a complex process. The key to their successful development and commercialization is an in-depth understanding of the key product attributes that impact safety and efficacy and the strategies to control them. Functional assessment of mAb is a crucial part of the comparability of biopharmaceutical drugs. The development of a relevant and robust functional assay requires an interdisciplinary approach and sufficient flexibility to balance regulatory concerns as well as dynamics and variability during the manufacturing process. Although many advanced tools are available to study and compare the potency and bioactivity of the protein, most of these techniques suffer from major shortcomings that limit their routine use. These include the complexity of the task, establishment of the relevance of the chosen method with the mechanism of action (MOA) of the biosimilar, cost and extended time of analysis, and often the ambiguity in interpretation of the resulting data. To overcome or to address these challenges, the use of multiple orthogonal state-of-the-art techniques is a necessary prerequisite.

Determining ADCC Activity of Antibody-Based Therapeutic Molecules using Two Bioluminescent Reporter-Based Bioassays

Antibody Fc effector function is one of the main mechanisms of action (MoA) for therapeutic monoclonal antibodies. Measurement of antibody-dependent cellular cytotoxicity (ADCC) is critical for understanding the Fc effector function during monoclonal antibody development. This article covers two cell-based ADCC bioassays which can quantitatively measure the antibody potency in ADCC. Basic Protocol 1 describes the ADCC reporter bioassay using engineered ADCC effector cells which measures the FcγRIIIa-mediated luciferase reporter activation upon the binding of antibody-coated target cells. Basic Protocol 2 describes the PBMC ADCC bioassay using primary peripheral blood mononuclear cells (PBMC) as effector cells and engineered HiBiT target cells in an assay that measures the release of HiBiT from target cells upon antibody-mediated target lysis. Optimization of several key assay parameters including cell handling, effector:target (E:T) ratios, assay plate, and plate reader requirement, and how these parameters impact assay performance are discussed. © 2021 Promega Corporation. Current Protocols published by Wiley Periodicals LLC. Basic Protocol 1: ADCC reporter bioassay using engineered ADCC bioassay effector cells Basic Protocol 2: PBMC ADCC bioassay using primary PBMC and engineered HiBiT target cells.

The Role of Fc Receptors on the Effectiveness of Therapeutic Monoclonal Antibodies

Since the approval of the first monoclonal antibody (mAb) in 1986, a huge effort has been made to guarantee safety and efficacy of therapeutic mAbs. As of July 2021, 118 mAbs are approved for the European market for a broad range of clinical indications. In order to ensure clinical efficacy and safety aspects, (pre-)clinical experimental approaches evaluate the respective modes of action (MoA). In addition to antigen-specificity including binding affinity and -avidity, MoA comprise Fc-mediated effector functions such as antibody dependent cellular cytotoxicity (ADCC) and the closely related antibody dependent cellular phagocytosis (ADCP). For this reason, a variety of cell-based assays have been established investigating effector functions of therapeutic mAbs with different effector/target-cell combinations and several readouts including Fcγ receptor (FcγR)-mediated lysis, fluorescence, or luminescence. Optimized FcγR-mediated effector functions regarding clinical safety and efficacy are addressed with modification strategies such as point mutations, altered glycosylation patterns, combination of different Fc subclasses (cross isotypes), and Fc-truncation of the mAb. These strategies opened the field for a next generation of therapeutic mAbs. In conclusion, it is of major importance to consider FcγR-mediated effector functions for the efficacy of therapeutic mAbs.

Validation of a commercially available indirect assay for SARS-CoV-2 neutralising antibodies using a pseudotyped virus assay

OBJECTIVES

To assess whether a commercially available CE-IVD, ELISA-based surrogate neutralisation assay (cPass, Genscript) provides a genuine measure of SARS-CoV-2 neutralisation by human sera, and further to establish whether measuring responses against the RBD of S was a diagnostically useful proxy for responses against the whole S protein.

METHODS

Serum samples from 30 patients were assayed for anti-NP responses, for 'neutralisation' by the surrogate neutralisation assay and for neutralisation by SARS-CoV-2 S pseudotyped virus assays utilising two target cell lines. Correlation between assays was measured using linear regression.

RESULTS

The responses observed within the surrogate neutralisation assay demonstrated an extremely strong, highly significant positive correlation with those observed in both pseudotyped virus assays.

CONCLUSIONS

The tested ELISA-based surrogate assay provides an immunologically useful measure of functional immune responses in a much quicker and highly automatable fashion. It also reinforces that detection of anti-RBD neutralising antibodies alone is a powerful measure of the capacity to neutralise viral infection.

Toward a Better Understanding of Bioassays for the Development of Biopharmaceuticals by Exploring the Structure-Antibody-Dependent Cellular Cytotoxicity Relationship in Human Primary Cells

Pharmaceutical manufacturing relies on rigorous methods of quality control of drugs and in particular of the physico-chemical and functional characterizations of monoclonal antibodies. To that end, robust bioassays are very often limited to reporter gene assays and the use of immortalized cell lines that are supposed to mimic immune cells such as natural killer (NK) cells to the detriment of primary materials, which are appreciated for their biological validity but are also difficult to exploit due to the great diversity between individuals. Here, we characterized the phenotype of the peripheral blood circulating cytotoxic cells of 30 healthy donors, in particular the repertoire of cytotoxic markers, using flow cytometry. In parallel, we characterized the antibody-dependent cellular cytotoxicity (ADCC) effector functions of these primary cells by measuring their cytolytic activity against a cancer cell-line expressing HER2 in the presence of trastuzumab and with regards to FCGR3A genotype. We could not establish a correlation or grouping of individuals using the data generated from whole peripheral blood mononuclear cells, however the isolation of the CD56-positive population, which is composed not only of NK cells but also of natural killer T (NKT) and γδ-T cells, as well as subsets of activated cytotoxic T cells, monocytes and dendritic cells, made it possible to standardize the parameters of the ADCC and enhance the overall functional avidity without however eliminating the inter-individual diversity. Finally, the use of primary CD56+ cells in ADCC experiments comparing glycoengineered variants of trastuzumab was conclusive to test the limits of this type of ex vivo system. Although the effector functions of CD56+ cells reflected to some extent the in vitro receptor binding properties and cytolytic activity data using NK92 cells, as previously published, reaching a functional avidity plateau could limit their use in a quality control framework.

Effects of sialic acid linkage on antibody-fragment crystallizable receptor binding and antibody dependent cytotoxicity depend on levels of fucosylation/bisecting

Aim: Fragment crystallizable (Fc) glycosylation of immunoglobulin G-type monoclonal antibodies applied to therapeutic applications is regarded a critical quality attribute and can influence bioactivity, pharmacokinetics and/or immunogenicity/safety. Investigating the impact of certain Fc N-glycans is therefore of importance to assess its criticality for a therapeutic product. This has been done for N-glycan types like fucosylation, galactosylation or sialylation. There were contradictory results reported for functionality especially with regard to sialylation. Material & methods: We elucidated the effect of terminal sialic acid residues on Fcγ receptor binding and antibody dependent cytotoxicity activity of two immunoglobulin G1 antibodies with different levels of fucosylation/bi-secting. Conclusion: We found the impact to be specific to the sialylation linkage type, in other words, α2,3- versus α2,6-linked sialic acid attached to the terminal galactose residues.

Best practices in bioassay development to support registration of biopharmaceuticals

Biological activity is a critical quality attribute for biopharmaceuticals, which is accurately measured using an appropriate relative potency bioassay. Developing a bioassay is a complex, rigorous undertaking that needs to address several challenges including modelling all of the mechanisms of action associated with the biotherapeutic. Bioassay development is also an exciting and fast evolving field, not only from a scientific, medical and technological point of view, but also in terms of statistical approaches and regulatory expectations. This has led to an industry-wide discussion on the most appropriate ways to develop, validate and control the bioassays throughout the drug lifecycle.

Development of a kinetic antibody-dependent cellular cytotoxicity assay

Antibody-dependent cellular cytotoxicity (ADCC) is an important mechanism of action (MOA) of monoclonal antibody (mAb) therapeutics. Target cells opsonized with therapeutic antibody bind and activate FcγR-bearing immune effector cells, resulting in target cell lysis. A key step in mAb drug development is the characterization of ADCC activity for its potential to inform mAb efficacy and safety. A number of in vitro assays are commonly used to assess ADCC. Most are endpoint assays that measure a surrogate marker of cell lysis. Newer imaging technologies allow direct measurement of ADCC-mediated cell lysis over time. In this study, we detail the development and characterization of a kinetic ADCC assay applicable to multiple target and effector cell types. This kinetic assay shows comparable sensitivity to an endpoint fluorescence release ADCC assay, while offering the advantages of a simpler set up and shorter assay time. Our results demonstrate that kinetic ADCC activity is a valid alternative assay format for measuring in vitro ADCC of mAbs.

Sensitive Detection of the Natural Killer Cell-Mediated Cytotoxicity of Anti-CD20 Antibodies and Its Impairment by B-Cell Receptor Pathway Inhibitors

The antibody-dependent cell-mediated cytotoxicity (ADCC) of the anti-CD20 monoclonal antibodies (mAbs) rituximab and obinutuzumab against the cell line Raji and isolated CLL cells and its potential impairment by kinase inhibitors (KI) was determined via lactate dehydrogenase release or calcein retention, respectively, using genetically modified NK92 cells expressing CD16-176V as effector cells. Compared to peripheral blood mononuclear cells, recombinant effector cell lines showed substantial alloreactivity-related cytotoxicity without addition of mAbs but afforded determination of ADCC with reduced interassay variability. The cytotoxicity owing to alloreactivity was less susceptible to interference by KI than the ADCC of anti-CD20 mAbs, which was markedly diminished by ibrutinib, but not by idelalisib. Compared to rituximab, the ADCC of obinutuzumab against primary CLL cells showed approximately 30% higher efficacy and less interference with KI. Irreversible BTK inhibitors at a clinically relevant concentration of 1 μM only weakly impaired the ADCC of anti-CD20 mAbs, with less influence in combinations with obinutuzumab than with rituximab and by acalabrutinib than by ibrutinib or tirabrutinib. In summary, NK cell line-based assays permitted the sensitive detection of ADCC of therapeutic anti-CD20 mAbs against CLL cells and of the interference of KI with this important killing mechanism.

Glycosylation profile and biological activity of Remicade® compared with Flixabi® and Remsima®

As biosimilars enter the market, comparisons of product quality are needed. Manufacturing differences may lead to differences in critical quality attributes, which affect efficacy. Therefore, critical quality attributes (structure and biological activity) of Remicade® and of 2 biosimilar products (Flixabi®/Renflexis® and Remsima®/Inflectra®) were determined. We assessed binding to tumor necrosis factor in a fluorescence competitive binding assay; potency in a luciferase reporter gene assay; percentages of galactosylated glycan, afucose plus high mannosylated glycans, and charged glycan; FcγRIIIa (CD16) binding (assessed by 3 methods); and antibody-dependent cell-mediated cytotoxicity (ADCC) in the NK92-CD16a cell line and in peripheral blood mononuclear cells (PBMC). The results of Fab-related activity were similar for all products. Compared with Remicade®, Flixabi® had a lower percentage of charged glycan, and Remsima® had a higher percentage of galactosylated glycan and a lower percentage of afucose plus high mannosylated glycans. Whereas Remsima® and Remicade® are expressed in a Sp2/0 cell line, Flixabi® is expressed in a CHO cell line. Despite this difference, galactosylated glycans from the 3 products were not correlated with the expression system. The results of all 3 methods used in this study indicated that FcγRIIIa binding was lower with Remsima® than with Remicade®. The percentage of ADCC in NK92-CD16a cells was lower with Remsima® and higher with Flixabi® compared with Remicade®, but was similar for all 3 products in PBMC. Surface expression of CD16 was 5.7-fold greater on NK92-CD16a cells than on PBMC. Combined percentages of afucosylated and high mannosylated glycans were positively correlated with FcγRIIIa binding and ADCC in NK92-CD16 cells, while no correlation was observed in PBMC.

Analysis of carbohydrates and glycoconjugates by matrix-assisted laser desorption/ionization mass spectrometry: An update for 2011-2012

This review is the seventh update of the original article published in 1999 on the application of MALDI mass spectrometry to the analysis of carbohydrates and glycoconjugates and brings coverage of the literature to the end of 2012. General aspects such as theory of the MALDI process, matrices, derivatization, MALDI imaging, and fragmentation are covered in the first part of the review and applications to various structural types constitute the remainder. The main groups of compound are oligo- and poly-saccharides, glycoproteins, glycolipids, glycosides, and biopharmaceuticals. Much of this material is presented in tabular form. Also discussed are medical and industrial applications of the technique, studies of enzyme reactions, and applications to chemical synthesis. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:255-422, 2017.

Thaw-and-use target cells pre-labeled with calcein AM for antibody-dependent cell-mediated cytotoxicity assays

In vitro antibody-dependent cell-mediated cytotoxicity (ADCC) assays are routinely performed to support the research and development of therapeutic antibodies. In ADCC assays, target cells bound by the antibodies are lysed by activated effector cells following interactions between the Fc region of the bound antibody and Fcγ receptors on effector cells. Target cell lysis is typically measured by quantification of released endogenous enzymes, e.g., lactate dehydrogenase, or measurement of released exogenous labels, e.g., ⁵¹Cr, europium or calcein. ADCC assays based on the detection of exogenous labels released from lysed target cells generally show higher sensitivity and require shorter incubation times. However, target cells are usually labeled immediately prior to assay, which inadvertently introduces additional assay variations due to differences in target cell conditions and labeling/handling processes. In this report, we describe the use of thaw-and-use pre-labeled target cells for ADCC assays. Thaw-and-use target cells in our experiments were pre-labeled with the fluorescent dye calcein AM, cryopreserved in single-use aliquots and used directly in assays after thawing. Upon thaw, the pre-labeled cells displayed viability and label retention comparable to freshly labeled cells, responded to ADCC mediated by both peripheral blood mononuclear cells and engineered natural killer cells, performed stably for at least 3 years and provided favorable precision and accuracy to ADCC assays. Implementation of thaw-and-use pre-labeled target cells in ADCC assays can help to alleviate both cell culture and dye labeling derived variability, increase the flexibility of assay scheduling and improve assay consistency and robustness.

Characterization of FcγRIIIA effector cells used in in vitro ADCC bioassay: Comparison of primary NK cells with engineered NK-92 and Jurkat T cells

Antibody-dependent cell-mediated cytotoxicity (ADCC) is an important mechanism of action (MOA) of several therapeutic antibody drugs and evaluation in ADCC bioassays is important in antibody drug development and maintenance. Three types of effector cells now routinely used in bioassay evaluation of ADCC are natural killer cells from human donors (FcγRIIIA+primary NK), FcγRIIIA engineered NK-92 cells and FcγRIIIA/NFAT-RE/luc2 engineered Jurkat T cells. Engineered effector cells were developed to address need for improved precision and accuracy of classic NK cell ADCC bioassays. The main purpose of our study was to rationalize which of these ADCC effector cells best simulate the expected response in human subjects and to identify which effector cells and assays best fit ADCC bioassay needs during antibody drug development. We characterized differences between the effector cells and compared ADCC biological activities using the well-known humanized IgG1 antibody drug, trastuzumab. The three effector cell types studied expressed either V-158 or F-158 allotype of FcγRIIIA, hence six cell preparations were compared. Our results demonstrate highest surface expression of FcγRIIIA in primary NK and engineered NK-92 (V-158) cells with nearly all expressed on the cell surface. In contrast, expression in engineered Jurkat T cells was low with only a small percentage expressed on the cell surface. Studies evaluating binding of trastuzumab to effector cells demonstrated the highest affinity of FcγRIIIA in primary NK and NK-92 (V-158) cells. ADCC cytotoxicity studies showed greatest trastuzumab potency in primary NK and engineered NK-92 (V-158) cells and negligible cell lysis obtained using engineered Jurkat T cells. In contrast, the engineered Jurkat T (V-158) cells responded as effectively as primary NK (V/V) cells to nuclear factor of activated T cells (NFAT2) activation upon binding of trastuzumab to FcγRIIIA, demonstrating similar ADCC pathway activation in these cells despite the low surface expression of FcγRIIIA and its low affinity for trastuzumab. Dose-response range of trastuzumab in activation of NFAT2 (measured as pNFAT2 dephosphorylation) was very similar to response in classic ADCC assay for primary and NK-92 cells and to response in ADCC reporter assay for Jurkat T effector cells, bridging the assays. Trastuzumab potency in ADCC reporter assay using the engineered Jurkat T cells was close to that seen using either primary NK or engineered NK-92 cells in classic ADCC assay. In summary, all three effector cell systems differentially express FcγRIIIA and provide dose-dependent ADCC pathway activity, yet only primary NK and engineered NK-92 cells are capable of inducing ADCC-mediated cell lysis. Engineered Jurkat T effector cells have value to assure antibody manufacturing consistency and in other applications where accuracy and precision are important. For functional assessment of ADCC activity, primary NK or NK-92 (V-158) cells better reflect the physiologically relevant ADCC mechanism of action. As an engineered cell line, NK-92 cells may behave more reproducibly than primary NK, but this must be balanced with the objective for biological relevance in decisions on which NK cells to use in assay.

HA Antibody-Mediated FcγRIIIa Activity Is Both Dependent on FcR Engagement and Interactions between HA and Sialic Acids

Interactions with receptors for the Fc region of IgG (FcγRs) have been shown to contribute to the in vivo protection against influenza A viruses provided by broadly neutralizing antibodies (bnAbs) that bind to the viral hemagglutinin (HA) stem. In particular, Fc-mediated antibody-dependent cellular cytotoxicity (ADCC) has been shown to contribute to protection by stem-binding bnAbs. Fc-mediated effector functions appear not to contribute to protection provided by strain-specific HA head-binding antibodies. We used a panel of anti-stem and anti-head influenza A and B monoclonal antibodies with identical human IgG1 Fc domains and investigated their ability to mediate ADCC-associated FcγRIIIa activation. Antibodies which do not interfere with sialic acid binding of HA can mediate FcγRIIIa activation. However, the FcγRIIIa activation was inhibited when a mutant HA, unable to bind sialic acids, was used. Antibodies which block sialic acid receptor interactions of HA interfered with FcγRIIIa activation. The inhibition of FcγRIIIa activation by HA head-binding and sialic acid receptor-blocking antibodies was confirmed in plasma samples of H5N1 vaccinated human subjects. Together, these results suggest that in addition to Fc–FcγR binding, interactions between HA and sialic acids on immune cells are required for optimal Fc-mediated effector functions by anti-HA antibodies.

Prospective Evaluation of Cetuximab-Mediated Antibody-Dependent Cell Cytotoxicity in Metastatic Colorectal Cancer Patients Predicts Treatment Efficacy

Cetuximab is a monoclonal antibody to the EGFR that induces antibody-dependent cell cytotoxicity (ADCC) through Fcγ receptors on immune cells. Although SNPs in genes encoding Fcγ receptors are functionally relevant to cetuximab-mediated ADCC in colorectal cancer, a direct correlation between in vitro ADCC and clinical response to cetuximab is not defined. We therefore enrolled 96 consecutive metastatic colorectal cancer (mCRC) patients at diagnosis in a study that assessed FcγR status and cetuximab-mediated ADCC. Patients carrying the FcγRIIa H alleles 131H/Hand 131H/R had significantly higher ADCC compared with patients with the 131R/R alleles (P= 0.013). Patients carrying FcγRIIIa genotypes with the V alleles 158V/V and 158V/F displayed higher ADCC compared with patients carrying the 158F/F genotype (P= 0.001). Progression-free survival of patients with an FcγRIIIa 158V allele was significantly longer compared with patients carrying 158F/F (P= 0.05), whereas no significant difference was observed for overall survival. Twenty-eight of 50 mCRC patients with wild-type KRAS received cetuximab. The average ADCC-mediated killing was 30% of assay targets for patients who experienced cetuximab complete or partial response, 21% in patients with stable disease and 9% in patients with progressive disease. To characterize basal natural killer (NK) activity, cytotoxicity was evaluated in 39 of 96 mCRC patients. Patients who responded to first-line treatment had higher NK-cell cytotoxicity. Thus, although limited to this cohort of patients, in vitro cetuximab-mediated ADCC correlated with FcγR polymorphisms and predicted cetuximab responsiveness.

A stable trimeric influenza hemagglutinin stem as a broadly protective immunogen

The identification of human broadly neutralizing antibodies (bnAbs) targeting the hemagglutinin (HA) stem revitalized hopes of developing a universal influenza vaccine. Using a rational design and library approach, we engineered stable HA stem antigens ("mini-HAs") based on an H1 subtype sequence. Our most advanced candidate exhibits structural and bnAb binding properties comparable to those of full-length HA, completely protects mice in lethal heterologous and heterosubtypic challenge models, and reduces fever after sublethal challenge in cynomolgus monkeys. Antibodies elicited by this mini-HA in mice and nonhuman primates bound a wide range of HAs, competed with human bnAbs for HA stem binding, neutralized H5N1 viruses, and mediated antibody-dependent effector activity. These results represent a proof of concept for the design of HA stem mimics that elicit bnAbs against influenza A group 1 viruses.

In vitro glycoengineering of IgG1 and its effect on Fc receptor binding and ADCC activity

The importance and effect of Fc glycosylation of monoclonal antibodies with regard to biological activity is widely discussed and has been investigated in numerous studies. Fc glycosylation of monoclonal antibodies from current production systems is subject to batch-to-batch variability. If there are glycosylation changes between different batches, these changes are observed not only for one but multiple glycan species. Therefore, studying the effect of distinct Fc glycan species such as galactosylated and sialylated structures is challenging due to the lack of well-defined differences in glycan patterns of samples used. In this study, the influence of IgG1 Fc galactosylation and sialylation on its effector functions has been investigated using five different samples which were produced from one single drug substance batch by in vitro glycoengineering. This sample set comprises preparations with minimal and maximal galactosylation and different levels of sialylation of fully galactosylated Fc glycans. Among others, Roche developed the glycosyltransferase enzyme sialyltransferase which was used for the in vitro glycoengineering activities at medium scale. A variety of analytical assays, including Surface Plasmon Resonance and recently developed FcγR affinity chromatography, as well as an optimized cell-based ADCC assay were applied to investigate the effect of Fc galactosylation and sialylation on the in vitro FcγRI, IIa, and IIIa receptor binding and ADCC activity of IgG1. The results of our studies do not show an impact, neither positive nor negative, of sialic acid- containing Fc glycans of IgG1 on ADCC activity, FcγRI, and RIIIa receptors, but a slightly improved binding to FcγRIIa. Furthermore, we demonstrate a galactosylation-induced positive impact on the binding activity of the IgG1 to FcγRIIa and FcγRIIIa receptors and ADCC activity.

Natural killer cells can inhibit the transmission of human cytomegalovirus in cell culture by using mechanisms from innate and adaptive immune responses

Human cytomegalovirus (HCMV) transmission within the host is important for the pathogenesis of HCMV diseases. Natural killer (NK) cells are well known to provide a first line of host defense against virus infections. However, the role of NK cells in the control of HCMV transmission is still unknown. Here, we provide the first experimental evidence that NK cells can efficiently control HCMV transmission in different cell types. NK cells engage different mechanisms to control the HCMV transmission both via soluble factors and by cell contact. NK cell-produced interferon gamma (IFN-γ) suppresses HCMV production and induces resistance of bystander cells to HCMV infection. The UL16 viral gene contributes to an immune evasion from the NK cell-mediated control of HCMV transmission. Furthermore, the efficacy of the antibody-dependent NK cell-mediated control of HCMV transmission is dependent on a CD16-158V/F polymorphism. Our findings indicate that NK cells may have a clinical relevance in HCMV infection and highlight the need to consider potential therapeutic strategies based on the manipulation of NK cells.

IMPORTANCE

Human cytomegalovirus (HCMV) infects 40% to 100% of the human population worldwide. After primary infection, mainly in childhood, the virus establishes a lifelong persistence with possible reactivations. Most infections remain asymptomatic; however, HCMV represents a major health problem since it is the most frequent cause of infection-induced birth defects and is responsible for high morbidity and mortality in immunocompromised patients. The immune system normally controls the infection by antibodies and immune effector cells. One type of effector cells are the natural killer (NK) cells, which provide a rapid response to virus-infected cells. NK cells participate in viral clearance by inducing the death of infected cells. NK cells also secrete antiviral cytokines as a consequence of the interaction with an infected cell. In this study, we investigated the mechanisms by which NK cells control HCMV transmission, from the perspectives of immune surveillance and immune evasion.

Methods for measuring antibody-dependent cell-mediated cytotoxicity in vitro

Antibody-dependent cell-mediated cytotoxicity (ADCC) is a relevant characteristic to measure for a number of therapeutic monoclonal antibodies (mAbs) under development. ADCC is a mechanism by which antibody-opsonized, infected, or cancerous cells are destroyed by FcγRIII (CD16)-expressing effector cells. Here we describe three methods that can be used to quantify the ADCC activity of mAbs by measuring distinct aspects of the ADCC mechanism.

Development of a cell-based assay measuring the activation of FcγRIIa for the characterization of therapeutic monoclonal antibodies

Antibody-dependent cellular cytotoxicity (ADCC) is one of the important mechanisms of action of the targeting of tumor cells by therapeutic monoclonal antibodies (mAbs). Among the human Fcγ receptors (FcγRs), FcγRIIIa is well known as the only receptor expressed in natural killer (NK) cells, and it plays a pivotal role in ADCC by IgG1-subclass mAbs. In addition, the contributions of FcγRIIa to mAb-mediated cytotoxicity have been reported. FcγRIIa is expressed in myeloid effector cells including neutrophils and macrophages, and it is involved in the activation of these effector cells. However, the measurement of the cytotoxicity via FcγRIIa-expressing effector cells is complicated and inconvenient for the characterization of therapeutic mAbs. Here we report the development of a cell-based assay using a human FcγRIIa-expressing reporter cell line. The FcγRIIa reporter cell assay was able to estimate the activation of FcγRIIa by antigen-bound mAbs by a very simple method in vitro. The usefulness of this assay for evaluating the activity of mAbs with different abilities to activate FcγRIIa was confirmed by the examples including the comparison of the activity of the anti-CD20 mAb rituximab and its Fc-engineered variants, and two anti-EGFR mAbs with different IgG subclasses, cetuximab (IgG1) and panitumumab (IgG2). We also applied this assay to the characterization of a force-oxidized mAb, and we observed that oxidation significantly decreased the FcγRIIa activation by EGFR-bound cetuximab. These results suggest that our FcγRIIa reporter assay is a promising tool for the characterization of therapeutic mAbs, including Fc-engineered mAbs, IgG2-subclass mAbs, and their product-related variants.

Characterization of in vitro antibody-dependent cell-mediated cytotoxicity activity of therapeutic antibodies - impact of effector cells

Antibody-dependent cell-mediated cytotoxicity (ADCC) is an important mechanism of action implicated in the clinical efficacy of several therapeutic antibodies. In vitro ADCC assays employing effector cells capable of inducing lysis of target cells bound by antibodies are routinely performed to support the research and development of therapeutic antibodies. ADCC assays are commonly performed using peripheral blood mononuclear cells (PBMCs), natural killer (NK) cells or engineered cell lines as effector cells. In this study we evaluated the impact of different effector cell types including primary PBMCs, primary NK cells, engineered NK cell lines, and an engineered reporter cell line, on the in vitro ADCC activity of two glycoforms of a humanized IgG1 antibody. The results of this study show the differential effects on both the efficacy and potency of the antibodies by different effector cells and the finding that both the allotype and the expression level of CD16a affect the potency of effector cells in ADCC assays. Our results also show that engineered NK or reporter cell lines provide reduced variability compared to primary effector cells for in vitro ADCC assays.

Knobs-into-holes antibody production in mammalian cell lines reveals that asymmetric afucosylation is sufficient for full antibody-dependent cellular cytotoxicity

Knobs-into-holes is a well-validated heterodimerization technology for the third constant domain of an antibody. This technology has been used to produce a monovalent IgG for clinical development (onartuzumab) and multiple bispecific antibodies.^1,2 The most advanced uses of this approach, however, have been limited to E. coli as an expression host to produce non-glycosylated antibodies. Here, we applied the technology to mammalian host expression systems to produce glycosylated, effector-function competent heterodimeric antibodies. In our mammalian host system, each arm is secreted as a heavy chain-light chain (H-L) fragment with either the knob or hole mutations to allow for preferential heterodimer formation in vitro with low levels of homodimer contaminants. Like full antibodies, the secreted H-L fragments undergo Fc glycosylation in the endoplasmic reticulum. Using a monospecific anti-CD20 antibody, we show that full antibody-dependent cell-mediated cytotoxicity (ADCC) activity can be retained in the context of a knobs-into-holes heterodimer. Because the knobs-into-holes mutations convert the Fc into an asymmetric heterodimer, this technology was further used to systematically explore asymmetric recognition of the Fc. Our results indicate that afucosylation of half the heterodimer is sufficient to produce ADCC-enhancement similar to that observed for a fully afucosylated antibody with wild-type Fc. However, the most dramatic effect on ADCC activity is observed when two carbohydrate chains are present rather than one, regardless of afucosylation state.

Quantitative evaluation of fucose reducing effects in a humanized antibody on Fcγ receptor binding and antibody-dependent cell-mediated cytotoxicity activities

The presence or absence of core fucose in the Fc region N-linked glycans of antibodies affects their binding affinity toward FcγRIIIa as well as their antibody-dependent cell-mediated cytotoxicity (ADCC) activity. However, the quantitative nature of this structure-function relationship remains unclear. In this study, the in vitro biological activity of an afucosylated anti-CD20 antibody was fully characterized. Further, the effect of fucose reduction on Fc effector functions was quantitatively evaluated using the afucosylated antibody, its "regular" fucosylated counterpart and a series of mixtures containing varying proportions of "regular" and afucosylated materials. Compared with the "regular" fucosylated antibody, the afucosylated antibody demonstrated similar binding interactions with the target antigen (CD20), C1q and FcγRIa, moderate increases in binding to FcγRIIa and IIb, and substantially increased binding to FcγRIIIa. The afucosylated antibodies also showed comparable complement-dependent cytotoxicity activity but markedly increased ADCC activity. Based on EC 50 values derived from dose-response curves, our results indicate that the amount of afucosylated glycan in antibody samples correlate with both FcγRIIIa binding activity and ADCC activity in a linear fashion. Furthermore, the extent of ADCC enhancement due to fucose depletion was not affected by the FcγRIIIa genotype of the effector cells.