Absolute Number of Transplanted CD34⁺ Cells Expressing c-mpl (CD110) Correlates with Speed of Platelet Engraftment Following Autologous Stem Cell Transplantation.

Recovery of neutrophil numbers after peripheral blood stem cell transplantation (PBSCT) is closely associated with graft CD34⁺ cell dose. Predicting the speed of platelet recovery is more difficult but would be of value given that a significant minority of patients experience delayed platelet recovery and bleeding complications after transplantation.

In this study we retrospectively analysed the graft composition of 29 patients who underwent autologous transplantation, using blood stem cells mobilized with cyclophoshamide and G-CSF, to assess the utility of c-mpl expression on CD34⁺ cells as a predictor of platelet engraftment (ie, time to platelet count greater than 20 x 10⁹/L for three consecutive days without the need for platelet transfusion). Absolute CD34⁺ cells and CD34 subsets expressing c-mpl were enumerated using a published single platform viable CD34 flow cytometry assay (

Of the 29 patients, 7 required at least 21 days for platelet engraftment. These patients received a median graft dose of 5.7 x 10⁴ CD34⁺CD110⁺ cells/kg compared with a median dose of 13.4 x 10⁴ cells/kg received by patients who experienced platelet engraftment within 21 days of transplant (p=0.013). In contrast, there was no difference in the number of CD34⁺ cells/kg infused (4.0 v 4.9 x 10⁶/kg for > or < 21 days for platelet engraftment respectively, p=0.23). There was a poor correlation between the absolute number of CD34⁺ cells and the number of CD34⁺CD110⁺ cells in the graft (r² = 0.48). Similarly there was no correlation between the percentage of CD34⁺ cells expressing c-mpl and the speed of platelet engraftment (8.1 v 5.8%) for > or < 21 days for platelet engraftment respectively, p=0.39). Patients with >21 days for platelet engraftment received platelet transfusions more often than those with <21 days for platelet engraftment (median 9 v 2 transfusions, p <0.001).

The absolute number of CD34⁺/CD110⁺ cells/kg infused at time of transplantation appears to be an important factor identifying patients at risk of delayed (>21 days) platelet engraftment. Those with <6 x 10⁴ CD34⁺/CD110⁺ cells/kg are at particularly high risk of delayed platelet engraftment, requiring multiple transfusions after transplantation.