B cell progenitor acute lymphoblastic leukemia (B-ALL) treatment has been revolutionized by T cell-based immunotherapies—including chimeric antigen receptor T cell therapy (CAR-T) and the bispecific T cell engager therapeutic, blinatumomab—targeting surface glycoprotein CD19. Unfortunately, many patients with B-ALL will fail immunotherapy due to ‘antigen escape’—the loss or absence of leukemic CD19 targeted by anti-leukemic T cells. In the present study, we utilized a genome-wide CRISPR–Cas9 screening approach to identify modulators of CD19 abundance on human B-ALL blasts. These studies identified a critical role for the transcriptional activator ZNF143 in CD19 promoter activation. Conversely, the RNA-binding protein, NUDT21, limited expression of CD19 by regulating CD19 messenger RNA polyadenylation and stability. NUDT21 deletion in B-ALL cells increased the expression of CD19 and the sensitivity to CD19-specific CAR-T and blinatumomab. In human B-ALL patients treated with CAR-T and blinatumomab, upregulation of NUDT21 mRNA coincided with CD19 loss at disease relapse. Together, these studies identify new CD19 modulators in human B-ALL.