CAR19 therapy drives expansion of clonal hematopoiesis and associated cytopenias.

CD19-directed chimeric antigen receptor T-cell therapy (CAR19) improves survival in patients with relapsed/refractory large B-cell lymphoma (rrLBCL) compared to immunochemotherapy with intent for autologous hematopoietic cell transplantation (HCT). However, major toxicities of CAR19 therapy include prolonged cytopenias, infection, and secondary hematologic malignancies. To investigate the mechanisms underlying these toxicities we studied a cohort of lymphoma patients receiving CAR19. CAR19-treated patients exhibited impaired immune reconstitution and increased infection compared to propensity-matched HCT-treated controls. Bone marrow analysis revealed prolonged post-CAR cytopenias is associated with clonal cytopenias of undetermined significance (CCUS) and is characterized by interferon-mediated inflammation. Despite durable lymphoma remissions, clonal hematopoiesis (CH) commonly expanded following CAR19 infusion and was associated with impaired immune reconstitution and the development of treatment related myeloid malignancy (tMN). The molecular composition and clinical outcomes of post-CAR tMN were comparable to those of post-HCT tMN. Single-cell DNA analysis revealed that most post-CAR CH clones harbored a single independent mutation and that CAR integration into T cells with CH mutations may drive persistence. These findings broadly implicate CH mutation burden and CH expansion in the development of post-CAR cytopenias and malignancies as well as mechanistically suggest these expansions occur in a background of marrow inflammation. Together, our results provide insight into the origins of key CAR19-associated toxicities, including infection and tMN.