The membrane distal domain of CD16a controls NK cell ADCC activity independent of IgG binding 2309237

Introduction Antibody dependent cellular cytotoxicity (ADCC) is a crucial immune function mediated by natural killer (NK) cells to destroy malignant or virally infected cells. ADCC occurs when IgG opsonized antigens on target cell surfaces bind to the membrane proximal domain 2 (D2) of FcgRIIIa (CD16a). However, the first membrane distal domain 1 (D1) of CD16a has no known function. Methods Here we used cryo-electron microscopy (EM), super-resolution nanoscopy, and functional assays to study the molecular basis of CD16a D1 activity in the context of ADCC. Results We show that the nanobody C28 potently inhibits NK cell ADCC independently of IgG binding while a similar nanobody, C21, does not. C28 does not alter natural cytotoxicity but quenches phosphorylation of the CD16a co-receptor CD3-zeta and alters synapse structure. Cryo-EM analysis of these nanobodies at better than 4Å resolution defines a new CD16a D1 epitope that specifically influences NK cell ADCC. We are currently determining how perturbation to this epitope alters CD16a orchestration during ADCC using MINFLUX nanoscopy with single nanometer localization precision. Conclusion Here we describe a new function of the IgG receptor in self-regulating NK cell ADCC. Future studies aim to identify possible cis-acting proteins ligands underlying this novel function. Using our structure as a blueprint, we are engineering new variants of CD16a D1 that we predict will enhance ADCC activity without affecting natural cytolytic propensity. Such “super-ADCC” NK cells could augment monoclonal antibody therapies for patients with poor response to treatment. Funding Source NIH/NIAID R01 AI167646 Topic Categories Innate Immune Responses and Host Defense: Molecular Mechanisms (INM)