Deciphering stage 0 hematogones by flow cytometry in follow‐up bone marrow samples of pediatric B—Acute lymphoblastic leukemia cases: A potential mimicker of residual disease after anti CD19 therapy

CD19 is frequently targeted for immunotherapy in B cell malignancies, which may result in loss of CD19 expression in leukemic cells as an escape mechanism. Stage 0 hematogones (Hgs) are normal CD19‐negative very early B cell precursors that can be potentially mistaken for CD19 negative residual leukemic cells by flow cytometry (FCM) in B cell acute lymphoblastic leukemia (BCP‐ALL) cases treated with anti CD19 therapy. Our main objective was to characterize and study the incidence of stage 0 hematogones in follow‐up bone marrow samples of pediatric BCP‐ALL cases. We analyzed the flow cytometry standard files of 61 pediatric BCP‐ALL cases treated with conventional chemotherapy and targeted anti‐CD19 therapy, for identifying the residual disease and normal B cell precursors including stage 0 Hgs. A non‐CD19 alternate gating strategy was used to isolate the B cells for detecting the residual disease and stage 0 Hgs. The stage 0 Hgs were seen in 95% of marrow samples containing CD19+ Hgs. When compared with controls and posttransplant marrow samples, the fraction of stage 0 Hgs was higher in patients receiving anti CD19 therapy (p = 0.0048), but it was not significant when compared with patients receiving chemotherapy (p = 0.1788). Isolated stage 0 Hgs are found in samples treated with anti‐CD19 therapy simulating CD19 negative residual illness. Our findings aid in understanding the stage 0 Hgs and its association with CD19+ Hgs in anti CD19 therapy and conventional chemotherapy. This is crucial as it can be potentially mistaken for residual disease in patients treated with anti CD19 therapy.