Expression of a Surface Antigen (MA6) by Peripheral Blood CD34+ Cells Is Correlated with Improved Platelet Engraftment and May Explain Delayed Platelet Engraftment Following Cord Blood Transplantation

CD34 + cell dose provides a measure of hematopoietic tissue that predicts the rate of engraftment upon transplant. It is positively correlated with multiple measures of hematopoietic recovery, including platelet engraftment. Here we identify a subpopulation of CD34 + cells that coexpress a surface antigen—MA6, which is more positively correlated with platelet engraftment in a clinical setting than CD34 + alone. The specific identity and function of MA6 remain to be determined, however, it is expressed by primitive megakaryocyte (MK) progenitors, but is lost with differentiation and is not expressed by platelets. Commitment of CD34 + MA6 + cells to the MK lineage was confirmed by in vitro assays and their significance in hematopoietic transplantation explored by flow cytometric analysis of cryopreserved samples of granulocyte colony stimulating factor-mobilized peripheral blood progenitor cell (PBPC) products along with a retrospective analysis of platelet engraftment data. Platelet engraftment by day 21 was predicted by receipt of ≥6×10 6 CD34 + cells/kg or ≥0.3×10 6 CD34 + MA6 + cells/kg. Subsequent analysis of cord blood (CB) CD34 + cells revealed