Assessment of the role of afucosylated glycoforms on the in vitro antibody-dependent phagocytosis activity of an antibody to Aβ aggregates

Function-structure approach reveals novel insights on the interplay of Immunoglobulin G 1 proteoforms and Fc gamma receptor IIa allotypes

Human Fc gamma receptor IIa (FcγRIIa) or CD32a has two major allotypes with a single amino acid difference at position 131 (histidine or arginine). Differences in FcγRIIa allotypes are known to impact immunological responses such as the clinical outcome of therapeutic monoclonal antibodies (mAbs). FcγRIIa is involved in antibody-dependent cellular phagocytosis (ADCP), which is an important contributor to the mechanism-of-action of mAbs by driving phagocytic clearance of cancer cells. Hence, understanding the impact of individual mAb proteoforms on the binding to FcγRIIa, and its different allotypes, is crucial for defining meaningful critical quality attributes (CQAs). Here, we report a function-structure based approach guided by novel FcγRIIa affinity chromatography-mass spectrometry (AC-MS) assays to assess individual IgG1 proteoforms. This allowed to unravel allotype-specific differences of IgG1 proteoforms on FcγRIIa binding. FcγRIIa AC-MS confirmed and refined structure-function relationships of IgG1 glycoform interactions. For example, the positive impact of afucosylation was higher than galactosylation for FcγRIIa Arg compared to FcγRIIa His. Moreover, we observed FcγRIIa allotype-opposing and IgG1 proteoform integrity-dependent differences in the binding response of stress-induced IgG1 proteoforms comprising asparagine 325 deamidation. The FcγRIIa-allotype dependent binding differences resolved by AC-MS were in line with functional ADCP-surrogate bioassay models. The molecular basis of the observed allotype specificity and proteoform selectivity upon asparagine 325 deamidation was elucidated using molecular dynamics. The observed differences were attributed to the contributions of an inter-molecular salt bridge between IgG1 and FcγRIIa Arg and the contribution of an intra-molecular hydrophobic pocket in IgG1. Our work highlights the unprecedented structural and functional resolution of AC-MS approaches along with predictive biological significance of observed affinity differences within relevant cell-based methods. This makes FcγRIIa AC-MS an invaluable tool to streamline the CQA assessment of therapeutic mAbs.

Development of a novel reporter gene assay to evaluate antibody-dependent cellular phagocytosis for anti-CD20 therapeutic antibodies

More than 100 monoclonal antibodies (mAbs) have been approved by FDA. The mechanism of action (MoA) involves in neutralization of a specific target via the Fab region and Fc effector functions through Fc region, while the latter include complement-dependent cytotoxicity (CDC), antibody-dependent cell-mediated cytotoxicity (ADCC) and antibody-dependent cellular phagocytosis (ADCP). ADCP has been recognized one of the most important MoAs, especially for anti-cancer mAbs in recent years. However, traditional bioassays measuring ADCP always introduced primary macrophages and flow cytometry, which are difficult to handle and highly variable. In this study, we engineered a monoclonal Jurkat/NFAT/CD32a-FcεRIγ effector cell line that stably expresses CD32a-FcεRIγ chimeric receptor and NFAT-controlled luciferase. The corresponding mAb could bind with the membrane antigens on the target cells with its Fab fragment and CD32a-FcεRIγ on the effector cells with its Fc fragment, leading to the crosslinking of CD32a-FcεRIγ and the resultant expression of subsequent NFAT-controlled luciferase, which represents the bioactivity of ADCP based on the MoA of the mAb. With rituximab as the model mAb, Raji cells as the target cells, and Jurkat/NFAT/CD32a-FcεRIγ cells as the effector cells, we adopted the strategy of Design of Experiment (DoE) to optimize the bioassay. Then we fully validated the established bioassay according to ICH-Q2(R1), which proved the good assay performance characteristics of the bioassay, including specificity, accuracy, precision, linearity, stability and robustness. This RGA can be applied to evaluate the -ADCP bioactivity for anti-CD20 mAbs in lot release, stability testing as well as biosimilar comparability. The engineered cells may also potentially be used to evaluate the ADCP bioactivity of mAbs with other targets.