Detection of Cells with Low Side Scatter and Dimmer CD45 Expression after Allogeneic Hematopoietic Stem Cell Transplantation Predicts Good Survival

<Introduction> Hematogones are lymphoblast-like cells that increase transiently in the bone marrow and have a characteristic profile of normal B cell precursors co-expressing CD10 and CD19. A cluster of hematogones is often found in the region of low side scatter and dimmer CD45 expression (SSC low CD45 dim), which also includes lymphoid and myeloid precursors, basophils, and partial components of natural killer cells. However, in steady state healthy adult bone marrow, SSC low CD45 dim populations are hardly detectable. In the bone marrow of patients recovering from chemotherapy or bone marrow transplantation, SSC low CD45 dim populations occasionally appear, with the majority of the population consisting of hematogones, especially in patients in complete remission (CR). Studies have shown that increased hematogones in patients with high-risk hematological malignancies after allogeneic hematopoietic stem cell transplantation (HSCT) improved survival. The specific detection of hematogones is difficult because multicolor flow cytometry is necessary to exclude myeloid/lymphoid blasts in active leukemia patients. However, after allogeneic HSCT in patients with leukemia and confirmed CR or malignant lymphoma without bone marrow invasion, hematogones can be easily detected as cells with SSC low CD45 dim populations.

<Patients and methods> This retrospective case analysis included 88 patients treated with allogeneic HSCT at our hospitals from October 2010 to November 2014. The cases included acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL) in CR at the time of allogeneic HSCT, chemo-naïve or azacitidine-treated myelodysplastic syndrome (MDS) and malignant lymphoma (ML) or adult T cell leukemia/lymphoma (ATL) with marrow CR or without bone marrow invasion. We excluded 8 patients without engraftment. The remaining 80 patients underwent bone marrow aspiration around 28 days after HSCT and were confirmed to be in CR. Bone marrow cells were stained with APC-Cy7-conjugated CD45 and analyzed using FACSCanto. Patients were defined as being simply detected hematogones (SHG)-positive if 0.6%, which is the median proportion of SSC low CD45 dim cells in this study, or more of total nuclear cells were SSC low CD45 dim; the others were defined as SHG-negative.

<Results> The median follow-up of survivors of our cohort was 773 (81–1593) days. Primary diagnoses were AML (n=36), ALL (n=21), MDS (n=8), and ML or ATL (n=15). They were treated with bone marrow transplantation (n=51), peripheral blood stem cell transplantation (n=6), and cord blood transplantation (n=23). Among them, 40 were SHG-positive and 40 were SHG-negative. Overall survivals (OS) at 2 years was 94.8% and 58.2% (p<0.001), event free survival (EFS) at 2 years was 94.9% and 46.5% (p<0.001). Cumulative incidence of relapse at 2 years was 5.1% and 34.0% (p<0.001), and cumulative incidence of treatment related mortality (TRM) at 2 years was 0% and 20.6% (p=0.0059) in SHG-positive and SHG-negative groups, respectively. Cumulative incidence of acute graft-versus-host disease (aGVHD) at day 100 was 68.3% and 65.5% (p=0.31) and for severe aGVHD (grade II–IV) was 31.8% and 36.3% (p=0.87) in SHG-positive and SHG-negative groups, respectively. Age ≥50 years, sex, hematopoietic cell transplant-comorbidity index over 2, disease risk index (DRI), myeloablative conditioning regimen and donor source were entered into multivariate analysis, which identified SHG-positive and a low or intermediate DRI as independently associated with a good prognosis.

<Conclusion> Thus, the presence of SHG at day 28 predicts improved OS and EFS, and a lower incidence of relapse and TRM. This population of cells is easily detectable, providing useful information for outcome predictions. Patients without SSC low CD45 dim populations should be carefully observed after allogeneic HSCT.