Increased myeloid precursors in regenerating bone marrow; implications for detection of minimal residual disease in acute myeloid leukemia
Abstract:
Presented study is focused on exact immunophenotypic definition of myeloid precursors and their following stages in regenerating bone marrow during treatment of ALL/AML for correct interpretation of the immunophenotype results and proper distinction from minimal residual disease (MRD) by multiparameter flow cytometry. This study includes bone marrow samples from 36 controls, 27 patients with AML, 39 patients with B-ALL undergoing therapy who remained in complete remission after treatment and also 30 B-ALL patients one year after the end of therapy. We observed substantial expansion of immature bone marrow populations in the regenerating bone marrows, which were identified by expression of CD34 and/or CD117 markers by 4-color flow cytometry. Myeloid precursors were significantly increased after cessation of induction therapy cycle of B-ALL (1.27±2.04%, p=0.0064) and also AML patients (0.87±0.77%, p=0.001), but also during follow-up of B-ALL patients (1.42±2.36%, p=0.0001) when compared with non-treated controls (0.38±0.29%). Some cases where their frequencies achieved up to 12% reflect the massive regeneration of myeloid lineage in bone marrow after chemotherapy cycles. Especially in these cases accurate interpretation of such a high frequency of immature myeloid cells as myeloid precursors was very important to exclude incoming relapse or secondary leukemia. The myeloid precursors represented by CD34+ in regenerating bone marrow expressed CD45 (94.8±5.5%), CD117 (38.3±26.2%), CD38 (91.4±5.7%), HLA-DR (90.6±7.6%), CD13 (73.0±20.8%) and CD33 (85.2±15.6%), while CD90 (2.7±2.5%), CD133 (10.0±8.2%) and T or B lymphocyte markers were negative. Comparing immunophenotypes with control bone marrows, only difference in expression of CD33 marker was found (85.2±15.6% versus 63.0±17.4% p=0.024). In addition, according to expression of these markers three different subsets of myeloid precursor cells were identified in regenerating bone marrow samples: CD34+ CD117- HLA-DR+, CD34+ CD117+ HLA-DR+ and CD34- CD117+ HLA-DR-/+ without aberrant marker expression. In conclusion for the correct discrimination of MRD in acute leukemia it is indispensable to define the range of normality in myeloid differentiation by extensive studies of bone marrows not only from healthy donors but also from regenerating bone marrow of patients undergoing therapy.