Unusual Immunophenotypes in the Bone Marrow of Infants with Acute Leukemia: Minimal Residual Disease or ATRA-Mediated Regeneration?.

Minimal Residual Disease (MRD) monitoring is essential to predict early outcome and further optimize treatment, especially when new approaches to therapy are used. A promising new treatment is the combination of chemotherapy with all trans-retinoic acid (ATRA) for infant acute leukemias. ATRA-mediated maturation of bone marrow cells can lead to the appearance of unusual undifferentiated cells with new immunological characteristics. We investigated the immunophenotypic features of bone marrow cells in infant acute leukemias treated with traditional chemotherapy combined with ATRA. From May 2006 to July 2007, we performed initial and consecutive multicolor flow cytometry assays of bone marrow samples from 4 infants with primary ALL or AML (except M3) treated at our institution. Patient I had BII-ALL, co-expressing CD15, but the majority of his blast cells were CD10(-). Conventional cytogenetics revealed t(4;11) and MLL/AF4 was detected. Early after ATRA administration, we detected MPO(+)TdT(+)-cells and subsequently, CD19, CD10, CD34, CD99 positive cells were found. Moreover, those cells occupied the “blast region” (SSC(low)CD45(dim)) and were seen as an autonomous population on the FSC/SSC dot plot. There was no correlation with the number of CD19(+)MPO(+)-cells and either MRD, measured by multicolor flow cytometry, or the level of the fusion gene transcript performed by the quantitative real-time polymerase chain reaction. In patient II with primary AML-M2, the cells with the same phenotype were also detected after several ATRA courses. Patient III, with primary biphenotypic leukemia, demonstrated total clearance of tumor blasts after the beginning of ATRA treatment with the later appearance of two cells populations: one similar to the phenotype previously described in patients I and II, but additionally expressing CD79a. The second cell population was positive for the cortical thymocytes markers CD7, CD5, CD2, CD4, CD8 and CD1a. This phenotype was observed shortly after ATRA initiation and disappeared with further chemotherapy. Patient IV with primary AML-M7 was switched to BII-ALL after the 3^rd ATRA course. Simultaneously, a minor population of biphenotypic cells was observed and later comprised 2 groups: TdT(−)CD99(−)CD45(bright) and TdT(+)CD99(+)CD45(dim) cells. We infer from this observation that the biphenotypic cells had matured. Cortical thymocytes were also detected in this patient for a short period. In all 4 patients we observed an equal distribution of biphenotypic cells on dot plots despite the differences in the primary leukemia phenotypes. This raises the question of a tumor versus an ATRA-mediated origin of the biphenotypic cells that requires further investigation in the patients treated with ATRA. Moreover, we conclude that a more extensive and specific panel of monoclonal antibodies is also required for the full immunophenotypic characterization of infant leukemia.