Comprehensive genetic and functional analysis of FcγRs in rituximab therapy for autoimmunity reveals a key role for FcγRIIIa on NK cells

B cell depletion using rituximab is widely used to treat autoimmune diseases, but patient response varies. The efficacy of rituximab is limited by the efficiency of depletion. Strategies to improve response include altering rituximab dosing, switching anti-CD20-mAb, alternative B cell targets, or non-B cell targeted therapies. Implementing an appropriate strategy requires understanding of the mechanism(s) of resistance to depletion and, if this varies between individuals, a means to test for it. Rituximab kills B cells via a variety of Fc{gamma} receptor (Fc{gamma}R)-dependent mechanisms, including antibody-dependent cellular cytotoxicity (ADCC), as well as non-Fc{gamma}R mechanisms. We conducted a longitudinal cohort study in rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE) using two national registries. Qualitative and quantitative FCGR functional variants were measured using multiplexed ligation-dependent probe amplification, supplemented by novel FCGR2C assays. We provide consistent evidence that FCGR3A, specifically increased number of copies of the FCGR3A-158V allele, was the major Fc{gamma}R gene associated with rituximab response, including clinical response in RA and SLE and depth of B cell depletion in the combined cohort. In SLE, we provide preliminary data suggesting increased FCGR2C ORF copies were also associated with improved clinical response. Furthermore, we demonstrated the impact of disease status and concomitant therapies on both natural killer cell Fc{gamma}RIIIa expression and rituximab-induced ADCC; demonstrating increased Fc{gamma}RIIIa expression and FCGR3A genotype were independently associated with clinical response and B cell depletion. Our findings highlight the importance of enhancing Fc{gamma}R-effector functions, may help stratify patients, and support ongoing development of next-generation CD20 depleting therapeutics. One Sentence SummaryThe high affinity Fc{gamma}RIIIa allotype on NK cells explains depth of B cell depletion and clinical response in rituximab therapy for autoimmune disease