Analysis of CD34+ cell subsets in stem cell harvests can more reliably predict rapidity and durability of engraftment than total CD34+ cell dose, but steady state levels do not correlate with bone marrow reserve

In peripheral blood stem cell transplantation (PBSCT), the number of CD34+ cells transplanted has been shown to correlate well with both rapidity and durability of engraftment. However, it is clear that engraftment does not necessarily correlate with total CD34+ cell numbers in some patients. Consequently, there is increasing interest in evaluating the role of CD34+ subsets in haemopoietic recovery as a more accurate marker of harvest quality. We analysed the numbers of CD34+ cell subsets, namely Thy‐1+, L‐Selectin+ and CD38−, and correlated this with engraftment in 86 patients undergoing PBSCT. Adequate engraftment was defined as being a platelet count greater than 50 × 109/l and a neutrophil count greater than 1·0 × 109/l. CD34+L‐Selectin+ provided the best prediction of engraftment rapidity, although the improvement over total CD34+ cell dose was minor. Only the dose of CD34+Thy‐1+ cells transplanted correlated with durable engraftment. The probability of adequate 3‐month engraftment increased with the dose of CD34+ cells transplanted, but 10% of patients receiving > 5 × 106/kg still showed poor engraftment at 3 months. However, all patients receiving > 2·5 × 105/kg CD34+Thy‐1+ showed adequate engraftment at this time point. We also demonstrated that CD34+Thy‐1+ progenitors were restricted to the bone marrow under normal conditions and, during stem cell mobilization, their kinetics generally paralleled total CD34+ numbers.