Increased immature T-cells detected by flow cytometry in post chemotherapeutic patients with acute myeloid leukemia, a case report and small series study.

Detection of minimal/measurable residual disease (MRD) in bone marrow specimens by flow cytometry is widely used in patients with T cell acute lymphoblastic leukemia (T-ALL). It plays a central role in guiding treatment and assessing prognosis. However, the occurrence of a normal physiologic reactive immature T-cell population in treated bone marrow is unknown. To investigate this, we examined 14 post chemotherapeutic bone marrow specimens with a T-ALL MRD flow cytometry panel. This included 9 acute myeloid leukemia (AML) and 5 T-ALL cases. Immature T-cells are defined as surface CD3 negative cells that coexpress cytoplasmic CD3 (cyCD3) and terminal deoxynucleotidyl transferase (TdT), or as cells that express CD34 with coexpression of multiple T-cell markers. Immature T-cells were present in 1 of 9 AML cases (11%), between day 20-31 post chemotherapy. Follow-up of this patient who had 4.00% cyCD3+ TdT+ immature T-cells, showed the population gradually decreased to 0.50% at day 31, 0.15% at day 46, and was undetectable (0.00%) at day 116. This population remained undetectable at the most current follow-up on day 147. This pilot study shows that a low level of cyCD3+ TdT+ immature T-cells may be present in post chemotherapeutic regenerating bone marrow and can be detectable by flow cytometry. Thus, extra caution should be taken when interpreting T-ALL MRD results, especially between days 20-31 post chemotherapy.