Boosts with Highly Purified Stem Cells Can Improve Hematopoietic Regeneration after Allogeneic Transplantation from Alternative Donors.

In allogeneic transplantation it is a major goal to maintain a stable long term engraftment. However, several factors like viral infections, drug toxicity or low stem cell numbers may hamper hematopoietic recovery or may even cause a late graft failure after successful initial engraftment. Hematopoietic growth factors or additional stem cell donations (boosts) may help to overcome this problem. In this retrospective analysis, the efficacy of boosts with highly purified peripheral stem cells either from closely matched unrelated or from haploidentical related donors was evaluated in pediatric patients with stagnant hematopoiesis (<1500 leucocytes or <50 000 platelets between 30–90 days posttransplant, n=11) or with initially increasing and then decreasing hematopoiesis (n=3). Patients with Graft rejection or hemophagocytosis were excluded. Diagnoses were: acute leukemias (n=10), nonmalignatn diseases (n=4). Most boosts were taken from the original graft: if the grafts contained >10x10⁶ (unrelated donors) or >20x10⁶/kg BW CD34+ cells, surplus stem cells were purified with an immunomagnetic selection method using antiCD34 or antiCD133 coated microbeads and cryopreserved. Three haploidentical donors were asked for a second donation. Our aim was to maintain stable and complete hematopoiesis after transplantation without inducing acute or chronic GvHD.

A total of 18 boosts were given between 30 and 290 days after transplantation with a median number of 5 (0.6–34) x 10⁶ CD34+/CD133+ progenitor cells/kg and only 2200 (100–25000) residual T-cells/kg. All patients had a complete donor chimerism and no reconditioning or posttransplant pharmacological immunosuppression was administered. Patients who received G-CSF were excluded. Due to the low number of T cells, acute GvHD >grade I and also chronic GvHD could be completely avoided even in haploidentical donors.

Leukocyte counts, neutrophile counts, lymphocyte counts (and platelet counts in patients with thrombocytopenia) within the week before infusion of the boosts were compared with cell counts 4 and 8 weeks after infusion, using the paired T-test. A significant increase of all cell counts was observed (medina numbers pre boosting: 900/μl, 312/μl, 142/μl; 4 weeks post boosting: 2065/μl, 1264/μl, 317/μl; 8 weeks post boosting:2570/μl, 1780/μl, 385/μl; p<0.05 for all comparisons). Furthermore, independence from platelet transfusion could be achieved.

Conclusions: Boosts with purified stem cells and low T-cell contamination (<10000 T-cells/kg) can be administered after transplantation without the risk of inducing GvHD, even in mismatched donors. No pharmacological immunosuppression was necessary. Thus, the method represents a safe tool and can improve donor derived hematopoiesis.