Development of a robust reporter-based ADCC assay with frozen, thaw-and-use cells to measure Fc effector function of therapeutic antibodies

Abstract

Antibody-dependent cell-mediated cytotoxicity (ADCC) is one of the main mechanisms of action for many therapeutic antibodies. Classic ADCC assays measure antibody-dependent target cell cytotoxicity induced by primary effector cells that are isolated from human blood. They suffer from high assay variability due to the genetic and immune-status-mediated variation from blood donors. Here we report the development of a robust reporter-based ADCC assay that uses an engineered Jurkat stable cell line as the source of effector cells. These engineered effector cells were further developed as frozen, thaw-and-use format that can be plated for assay immediately after thaw. We demonstrate that frozen, thaw-and-use Jurkat effector cells showed appropriate Fc effector function similar to fresh cells from continuous culture, with added benefits of convenience and consistency. This robust assay is able to measure antibody potency for several therapeutic antibodies targeted to hematopoietic or solid tumors. The assay can distinguish effector functions for different antibody IgG isotypes in two antibody model systems: anti-CD20 and anti-EGFR. It is able to detect changes in ADCC biological activity for heat-stressed rituximab and trastuzumab, demonstrating that it possesses proper stability-indicting property. When compared with a classic PBMC-based ADCC assay, the ADCC reporter assay showed better assay precision and similar correlation of antibody glycosylation with ADCC biological activity for a panel of glyco-modified trastuzumab mixtures. Together these data demonstrate that this robust ADCC reporter assay meets the requirement of a potency bioassay that can quantify antibody Fc effector function in ADCC mechanism of action during drug discovery and development.

Introduction

Therapeutic antibodies are used in targeted therapy for many human diseases. To date, more than 30 recombinant therapeutic antibodies are approved for human therapy, and several hundred candidates are in clinical development worldwide (Chan and Carter, 2010, Jiang et al., 2011, Leader et al., 2008). There are two classes of clinical applications of therapeutic antibodies that are governed by antibody structures and their binding partners: blocking or stimulation of receptor signaling by target-specific binding via antibody Fab domain; and stimulation of immune-mediated effector function via the antibody Fc domain, such as via complement-dependent cytotoxicity (CDC) and antibody-dependent cell-mediated cytotoxicity (ADCC) (Chan and Carter, 2010, Jiang et al., 2011, Nimmerjahn and Ravetch, 2008). ADCC is an important mechanism of action for many therapeutic antibodies in oncology and autoimmune disease, including rituximab, trastuzumab and cetuximab (Reff et al., 1994, Carter et al., 1992, Vermorken et al., 2008, Prang et al., 2005). In ADCC, the antibody recognizes the antigen receptors on the target cells via its Fab domain, while the Fc portion of the antibody binds to the Fc receptors on immune cells such as natural killer (NK) cells or monocytes. This is followed by the crosslinking and activation of the Fc receptors, the release of cytokines, and the formation of cytotoxic granules containing perforin and granzyme which ultimately leads to the lysis of the target cells (Leibson, 1997, Azzoni et al., 1992, Lyubchenko et al., 2001). The extent of ADCC response depends on many factors, including the effector cell types (Fanger et al., 1989a, Fanger et al., 1989b, Graziano et al., 1989), different Fc receptors and polymorphism of Fc receptors on effector cells (Nimmerjahn and Ravetch, 2008, Koene et al., 1997, Dall'Ozzo et al., 2004, Siberil et al., 2007), antibody IgG isotypes (Nimmerjahn and Ravetch, 2008, Siberil et al., 2007, Patel et al., 2010), and antigen expression level on target cells (Prang et al., 2005, Patel et al., 2010).

Despite the tremendous interest in the development of therapeutic antibodies, it has been highly challenging to measure the Fc effector function in ADCC for therapeutic antibodies in a reproducible and quantitative manner. Classic ADCC assays measure the short-term cytotoxicity of target cells after exposure to antibody-bound primary effector cells such as PBMCs or the subset NK cells. The target cells are pre-loaded with radioactive chromium (⁵¹Cr) in the earliest version of the ADCC assay, or loaded with the lanthanide Europium, or a fluorescent dye, typically calcein-AM, in later modified ADCC assays (Brunner et al., 1968, Roden et al., 1999, Patel and Boyd, 1995, von Zons et al., 1997). Alternatively, the enzyme release of lactate dehydrogenase (LDH), glyceraldehyde 3-phosphate dehydrogenase (GAPDH), or a specific dead cell protease in target cells can be measured to quantify cytotoxicity without the need of a labeling process (Korzeniewski and Callewaert, 1983, Stewart et al., 2011, Sergeeva et al., 2011). These assays bring the benefit of convenience by eliminating the handling of radioactive isotopes and the variability of cell labeling. Still, the need for primary effector cells isolated from fresh donor blood has made classic ADCC assays vulnerable to high variability. This variability has multiple sources, including tedious steps needed for the isolation of primary cells, and the inherent donor-to-donor variation due to differences in genetic background and the immune status of blood donors (Brunner et al., 1968, Roden et al., 1999, Patel and Boyd, 1995, von Zons et al., 1997). To overcome this, NK cell lines expressing endogenous FcγRIIIa or engineered NK cell lines expressing exogenous FcγRIIIa have been developed and used to replace primary PBMCs as effector cells in ADCC assays (Schnueriger et al., 2011, Binyamin et al., 2008). These NK cell-line based ADCC assays showed improved precision and reproducibility. However, NK cell lines are challenging to use in potency bioassays due to their tendency to form large cell aggregates, being difficult to handle, and genetic instability of endogenous or engineered Fc receptors (Gong et al., 1994, Robertson et al., 1996). As an alternative, the Jurkat T-cell line has been used as a model system to study FcγRIIIa receptor function in ADCC since the cells are easy to handle in cell culture and engineering while sharing similar immune cell background with NK cells (Vivier et al., 1992, Wirthmueller et al., 1992). Recently, Parekh et al. reported the development of a reporter gene assay as a surrogate ADCC assay, by building a Jurkat stable cell line expressing human FcγRIIIa, human CD3γ, and an NFAT-response element regulating a luciferase reporter (Parekh et al., 2012). The assay was successfully validated as a potency bioassay in an anti-CD20 antibody model system with CD20⁺ B cell line WIL2-S cells. Since other target cells and non-CD20 antibodies were not evaluated in that study, it would be of great value if a reporter-based surrogate ADCC assay can be demonstrated to broadly apply to many other target systems.

Low CV and high precision are considered critical requirements of a cell-based potency bioassay. Reproducibility is extremely important during assay transfer. Cells are considered as critical reagents in cell-based bioassays. Many factors during cell culture and preparation can cause run-to-run assay variation, and make assay transfer very challenging (Buttner, 2006). Therefore, a well-controlled, consistent source of cell reagents should significantly decrease assay variability, improve assay consistency, and be highly desired for bioassay development. Frozen, single-use cells are now widely used in the high-throughput screening community for small molecule drug discovery because they offer convenience, uniformity and efficiency (Fursov et al., 2005, Zaman et al., 2007). In the case of biologic drug discovery, there was an early report on frozen primary PBMCs in ADCC assay (Strong et al., 1982). In recent years, frozen cells have been applied in a few quality control laboratories for biologic drug development (TerWee et al., 2011), and in detection of neutralizing antibodies against biologic drugs in clinical research (Lallemand et al., 2011). However, the use of frozen cells is still limited and not well adopted for potency bioassays for biologics, despite the benefits it could bring. For previously reported ADCC bioassays that use effector cell lines that exogenously express FcγRIIIa (Azzoni et al., 1992, Schnueriger et al., 2011, Binyamin et al., 2008, Parekh et al., 2012), the assays are performed using fresh cells from culture. Thus, like many other cell-based bioassays, they have stringent training requirements for laboratory analysts on how to consistently handle fresh cells from culture in assay. Therefore, fresh cells in culture share potential challenges to achieve high assay precision especially in order to maintain good assay reproducibility during assay transfer to different sites in house or to contract laboratories.

In the current study, we describe construction of a quantitative surrogate ADCC reporter bioassay, by applying a similar assay design as that used by Parekh et al. (2012), through the establishment of an engineered Jurkat reporter cell line and their use as effector cells in assay. We further developed the engineered effector cells in frozen, thaw-and-use format without the need of daily cell culture and cell washing after thaw. We demonstrated that the assay using these thaw-and-use cells showed specific Fc effector function, antibody IgG isotype specificity, good precision and sensitivity to antibody activity changes due to heat stress, when tested in multiple therapeutic antibody model systems. Using a panel of glyco-modified antibodies in studies comparing this assay to a classic PBMC-based ADCC assay, we showed the biological relevance of the ADCC reporter assay response, and similar correlation of biological activity with the degree of antibody glycosylation by both types of assays. These results demonstrate that, the ADCC reporter assay using frozen, thaw-and-use Jurkat effector cells can serve as a quantitative surrogate ADCC potency bioassay for therapeutic antibodies. This is the first report on the application of thaw-and-use effector cells in an ADCC bioassay. The assay has high potential for antibody characterization, lot release, and stability studies during therapeutic antibody drug development.