Non-Hodgkin lymphoma (NHL) is the sixth most common malignancy in the U.S., with ~70,000 new cases this year. Rituximab, a monoclonal antibody targeting the CD20 antigen on B-cells, is used to treat the majority of NHL patients, and has improved survival rates significantly. However, not all patients respond to rituximab initially, and many other patients eventually develop resistance. Because of this, a plethora of biosimilar and next-generation anti-CD20 antibodies are in various stages of clinical development, necessitating better predictors of antibody response to optimize choice of therapy. Our long-term objective is to develop genetic strategies to tailor therapy choices to individual patients. This genetic approach is particularly pertinent for immunotherapies, with a complex and genetically variable immune system mediating drug response. Two mechanisms of rituximab-mediated cell death are antibody-dependent cell mediated cytotoxicity (ADCC) and complement-dependent cytotoxicity (CDC). Inherited polymorphisms in genes mediating ADCC and CDC, FCGR3A and C1QA, predict outcomes after rituximab treatment. We have discovered a third gene, CBLB, which mediates rituximab response in vitro, although its significance in vivo is untested. In vivo experiments to test mechanism of action of humanized antibodies are not easily addressed with animal models, since the intact immune system that is required recognizes the humanized antibody itself as foreign. Therefore, we employ a novel genetic approach using annotated human samples to address the potential for linking in vivo and in vitro responses to characterize and predict response to rituximab in patients. We hypothesize that inherited polymorphisms in three genes, FCGR3A, C1QA, and CBLB, determine rituximab response in vitro, and accurately predict rituximab response in vivo. The specific aims of this proposal are to test the effect of alterations in these three genes in vitro, using CDC and ADCC assays, to measure the effect of each gene on rituximab response. We will then test a large clinical trial cohort of diffuse large B-cell lymphoma (DLBCL) patients to measure the effect of polymorphisms in the three genes in vivo. We will compare in vitro and in vivo results to validate the in vitro system. Finally, we will test newer anti-CD20 antibodies using our in vitro system, to measure effects of FCGR3A, C1QA, and CBLB polymorphisms compared with rituximab. In this way, future clinical use of these agents can be personalized based on patient genotype, optimizing use of anti-CD20 antibodies for improved clinical outcomes.