Background:For patients with relapsed/refractory large B-cell lymphomas (rrLBCL), CD19-directed chimeric antigen receptor T cells (CAR19) improve survival compared to autologous hematopoietic cell transplantation (HCT). However, major toxicities of CAR19 therapy include prolonged cytopenias and associated infections. To better understand the impact of CAR19 on such toxicities, we studied a cohort of LBCL patients achieving durable remissions to assess immune recovery after CAR19 treatment. We (1) compared immune reconstitution in HCT vs CAR19 treated patients, (2) performed molecular analysis of post-CAR19 cytopenic marrow specimens, and (3) profiled emergent mutations in cell free DNA (cfDNA) before and after CAR19.