Hematogones are B-lymphocyte precursors found in large frequencies after chemotherapies. In this study, the frequency of CD10+CD19+ hematogones was analysed routinely prior and post allogeneic stem cell transplantation and compared with moelculargenetic data for donor chimerism and for clonal translocations. Because of similarities in morphology and immunophenotype with frequent expression of CD19, CD10 and TdT they may undistinguishable from malignant B-cell lymphoblasts. As one example underlying the diagnostic difficulties, one patient with thrombopenia day +60 after allogeneic stem cell transplantation is presented. A relapse of Richter’s syndrome was suspected. Investigations of bone marrow specimens revealed a mixed chimerism, the frequency of cells coexpressing CD10 and CD19 was 28%. However, a CD19-sorted chimerism revealed an almost complete donor chimerism. Donor-lymphocytes were administered to improve graft function. Afterwards, donor chimerism reached 100% and platelets reached normal values, but hematogones continued to exceed 5% in the following specimens. As a result of such cases, bone marrow specimens after allogeneic stem cell transplantation were systematically analyzed for the presence of hematogones.

METHODS: Hematogones were analyzed by routine 2-color flow cytometry. Cells coexpressing CD10 and CD19 with lymphocytic light scatter properties were regarded as hematogones. Percentage of cells was determined on the basis on total events. 133 patients undergoing allogeneic stem cell transplantation for AML, ALL, CLL, MM, NHL or aplastic anemia from 2003 to 2008 and surviving more than 60 days after transplantation were included in the analysis. During follow-up, bone marrow specimens were collected 1, 2, 3 and 12 months and in patients with suspected relapse. In total, 446 bone marrow specimens prior (186 specimen) and after (260 specimen) transplantation were collected and reevaluated for the frequencies of hematogones.

RESULTS: The frequency of hematogones exceeded 5% in 8 of 186 specimens prior but in 62 of 260 specimens after transplantation (4.3% and 23.8%, respectively; p<0.001 Chi-Square). During follow-up, the median frequency of hematogones of patients in remission increased from 0.21% prior transplantion to 1.9% at two months after transplantation (range 0% to 13% and 0% to 32.0%, respectively; p=0.001 Mann-Whitney test) with no significant decrease of hematogones 3 and 12 months after transplantation. In contrast, the frequency of hematogones was significantly decreased (median 0.17%; range 0% to 16.7%; p=0.02 Mann-Whitney-test compared to day+60) in patients with relapses other than ALL. Patients with more than 5% hematogones at any time after transplantation were significantly younger (median 41 vs. 54 years; p=0.01 Mann-Whitney test) and received more often myeloablative conditioning therapies (p=0.016 Chi-square) compared to patients with less 5% hematogones. In contrast, the relapse rate or the overall survival, the underlying disease, source of stem cells, immunoglobulin levels prior and one year after transplantation as well as the number of transfused stem cells were not correlated with the maximal frequencies of hematogones.

DISCUSSION: In this study, varying patterns of early B-cell recovery after allogeneic stem cell transplantation were found. Presence of large numbers of hematogones may be misinterpreted as a relapse in patients with B-cell malignancies. Presence of cells coexpressing CD10 and CD19 should be regarded with caution and always be interpreted with moleculargenetic data. The physiological and clinical effects of early B-cell recovery after allogeneic stem cell transplantation remain to be investigated in more detail.

Disclosures: No relevant conflicts of interest to declare.

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