CD19-negative B-cell precursors in bone marrow: A potential mimicker for CD19-negative B-lymphoblastic leukemia by flow cytometry in patients with anti-CD19 treatment

Novel therapies such as monoclonal antibody or chimeric antigen receptor (CAR) T cell therapy (CAR T) have shown promising results in the management of relapsed/refractory B-lymphoblastic leukemia (B-ALL). CD19, a transmembrane glycoprotein almost always expressed in B-ALL, is a commonly used target by these therapies. Blinatumomab, a bispecific monoclonal antibody T cell engager (BiTE), induces antibody-dependent cellular cytotoxicity against the leukemic cells by binding to CD19 on leukemic cells and CD3 on T cells simultaneously. Tisagenlecleucel (Kymriah), a CAR T targeting CD19, uses patients' own modified T cells to attack CD19+ leukemic cells. The superior efficacy of blinatumomab and tisagenlecleucel has been well demonstrated in relapsed/refractory B-ALL compared with high-dose chemotherapy. Multiparametric flow cytometry (FCM) is commonly used to monitor the therapeutic responses to these treatments. The key strategy for FCM to detect residual or relapsed B-ALL is to find an immature B-cell population phenotypically different from normal B-cell precursors (BCPs, also known as hematogones) in the bone marrow (BM). To investigate if these two anti-CD19 therapies could alter the phenotype of normal BCPs, we retrospectively studied the BCPs in the BM specimens from patients treated with blinatumomab and tisagenlecleucel. We also studied the BCPs from the patients after conventional chemotherapy and hematopoietic stem cell transplantation (HSCT) as a comparison.