Significance of Increased Blastic-Appearing Cells in Bone Marrow Following Myeloablative Unrelated Cord Blood Transplantation in Adult Patients

An abnormal increase of nonleukemic blastic-appearing lymphocytes in bone marrow (BM) specimens has been reported after unrelated cord blood transplantation (UCBT). This study analyzed the incidence, chronology, biological features, and clinical significance of elevated numbers of these cells in a series of 165 consecutive adult patients demonstrating myeloid engraftment after myeloablative UCBT in a single institution. The patients’ BM samples were routinely evaluated by cytomorphology at different time points after UCBT. When ≥5% of blastic-appearing cells were detected by cytomorphology in the BM, samples were also evaluated by multiparametric flow cytometry to characterize these cells. Systematic chimerism analyses of BM samples using PCR amplification of short tandem repeat markers were performed. Forty-three patients (cumulative incidence, 26.1%) demonstrated ≥5% of nonmalignant blastic-appearing cells in BM after a median of 101 days after UCBT (range, 28-377 days). All of these patients had full-donor chimerism and a clinical course without leukemic relapse. Multiparametric flow cytometry analyses performed in 36 of the 43 patients showed a polyclonal expansion of B lymphocytes with a broad spectrum of maturation stages. An increased number of nonmalignant blastic-appearing cells was significantly associated with a high number of lymphocytes infused at the time of UCBT and with low rates of acute and chronic extensive graft-versus-host disease, suggesting a potential immunoregulatory role of these cells. The observation of ≥5% nonmalignant blastic-appearing cells in BM samples after myeloablative UCBT is frequent, and these should be distinguished from malignant blasts.