A Multicenter Study to Evaluate Next-Generation Sequencing (NGS) Testing and Monitoring of B-Cell Recovery to Guide Management Following Chimeric Antigen Receptor T-cell (CART) Induced Remission in Children and Young Adults With B Lineage Acute Lymph...

Background: * Given the dismal outcomes for patients who experience a relapse following CD19 CART and the potential for using Hematopoietic cell transplantation (HCT) for post-CART remission consolidation for relapse prevention, there is a clear opportunity to improve outcomes for patients with B-ALL who proceed to CART. With a goal of improving overall survival, it remains critically important to be able to predict which patients are at high risk of relapse in whom an HCT would be indicated for remission consolidation. Distinguishing this high-risk cohort from low-risk patients who are able to maintain durable remission following CART and in whom HCT-associated toxicities could be avoided is equally important. * Thus, in the context of this biomarker-based study, we propose a systematic approach utilizing the best-known biomarkers for remission monitoring which assess both functional CART persistence and incorporates antigen immunophenotype agnostic approach for disease detection will improve LFS post CD19 CART. Objective: -To assess efficacy of a novel biomarker-guided risk-based strategy to monitor remission, both by assessing functional CART persistence and incorporating antigen immunophenotype agnostic approach (NGS monitoring) for disease detection, to inform decisions regarding post-CART HCT needed intervention, and to successfully collect biomarker samples at the scheduled times in enrolled HCT naive B-ALL participants receiving CD19 CART. Eligibility: * Age \>=1 year and \<= 25 years old at the time of CD19 CART infusion * Diagnosis of CD19+ B-ALL in a bone marrow morphologic complete remission and are flow cytometry measurable residual disease (MRD) negative within 42 days post CD19 CART infusion. * Must have an allogeneic HCT donor identified for potential HCT * B-cell aplasia post CD19 CART persisting until the time of the first on-study intervention Design: -This single-arm multicenter study will enroll pediatric and young adult participants to evaluate the feasibility, and potential efficacy, of a risk-based, biomarker-driven, consolidation HCT strategy following CD19 CART.