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Most relapsed/refractory large B cell lymphoma (r/rLBCL) patients receiving anti-CD19 chimeric antigen receptor (CAR19) T cells relapse. To characterize determinants of resistance, we profiled over 700 longitudinal specimens from two independent cohorts (n = 65 and n = 73) of r/rLBCL patients treated with axicabtagene ciloleucel. A method for simultaneous profiling of circulating tumor DNA (ctDNA), cell-free CAR19 (cfCAR19) retroviral fragments, and cell-free T cell receptor rearrangements (cfTCR) enabled integration of tumor and both engineered and non-engineered T cell effector-mediated factors for assessing treatment failure and predicting outcomes. Alterations in multiple classes of genes are associated with resistance, including B cell identity (PAX5 and IRF8), immune checkpoints (CD274), and those affecting the microenvironment (TMEM30A). Somatic tumor alterations affect CAR19 therapy at multiple levels, including CAR19 T cell expansion, persistence, and tumor microenvironment. Further, CAR19 T cells play a reciprocal role in shaping tumor genotype and phenotype. We envision these findings will facilitate improved chimeric antigen receptor (CAR) T cells and personalized therapeutic approaches. [Display omitted] • Tumors and CAR T cell effectors can be simultaneously profiled using cell-free DNA • Alterations in multiple classes of genes influence outcomes after CAR19 therapy • Tumor genotype and phenotype influence CAR19 T cell expansion, and vice versa • A multivariable model incorporating tumor and effector features predicts outcomes Sworder et al. develop and apply a tool to simultaneously profile tumor and effector-mediated determinants of resistance to anti-CD19 CAR T cells using cell-free DNA. The authors profile two independent cohorts of patients with relapsed/refractory large B cell lymphoma and identify genomic alterations, molecular thresholds, and microenvironmental changes associated with resistance. [ABSTRACT FROM AUTHOR]