Intrabone Injection of T-Cell Depleted Peripheral Blood Stem Cells from HLA-Haploidentical Donors to Reduce the Risk of Graft Rejection in Children

![Graphic][1] Background. Haploidentical hematopoietic stem cell transplantation (haplo-HSCT) is an effective treatment option for patients with malignant and non-malignant hematologic disorders lacking an HLA-compatible donor. Strategies for T-cell depletion (TCD) of the graft, such as positive selection of CD34+ cells, offer the potential to prevent acute and chronic graft-versus-host disease (GVHD). The risk of graft rejection associated with the extensive depletion of both T lymphocytes and accessory cells can be overcome by infusing a very high number (megadose) of granulocyte colony-stimulating factor (G-CSF)-mobilized peripheral blood stem cells (PBSC) (exceeding 10x106/kg recipient body weight) to overcome the HLA barrier (Aversa F. et al . Blood 1994). Moreover, the infusion of a megadose of CD34+ cells (higher than 20x106/kg and 12.4x106/kg, respectively) has been shown to result in faster immunological recovery and improved leukemia-free survival probability in children (Handgretinger R. et al . Bone Marrow Transplant 2001; Klingebiel T. et al. Blood 2010). Nevertheless, in the case of donors considered “poor mobilizers” (10-30% of cases), the threshold dose of CD34+ cells needed to ensure the inoculum of a megadose of stem cells might not be achieved. In the setting of cord blood (CB) transplantation, one of the strategies aimed at overcoming the problem of low cellularity is represented by the intrabone injection of CB stem cells, with good engraftment rates even in adult patients. We explored the same strategy in the context of T-cell depleted haplo-HSCT and low graft cellularity due to poor donor mobilization, ensuing in inadequate dose of CD34+cells available after positive selection TCD. Patients and methods. From September 2009 to April 2013, 11 pediatric patients affected by malignant or non-malignant hematological disorders (5 acute lymphoblastic leukemias, 1 acute myeloid leukemia, 1 myelodysplastic syndrome, 2 dyskeratosis congenita, 1 Fanconi anemia) received a T-cell depleted CD34+positively selected PBSC allograft from an HLA-haploidentical related donor. Due to the failure to achieve a target cell dose higher than 12x106 purified CD34+ cells/kg, part of the stem cell inoculum was infused as intrabone injection. The procedure was carried out at the patient bedside by multiple intrabone injections in the superior-posterior iliac crests under sedoanalgesia, as previously described (Frassoni F. et al . Lancet Oncol 2008). The median dose of CD34+ cells infused was 9x106/kg (range, 5-12) while the median number of CD3+ lymphocytes was 0.7x104/kg recipient body weight (range, 0.3-11). About one third of the stem cell inoculum, corresponding to a total volume of 20-40 ml, was given intrabone, while the remaining stem cell portion was infused intravenously. Results. No complication occurred during, or immediately after, the intrabone injection. Nine out of the 11 patients achieved a complete donor engraftment, while graft rejection occurred in 2 patients. The median time for neutrophil engraftment was 13.5 days (range, 12-20), while the median time for platelet recovery was 14 days (range, 13-24). One patient developed grade II acute GVHD and only 1 case of limited chronic GVHD was observed. No transplant-related deaths were observed. Conclusions. Our data suggest that, in the haplo-HSCT setting, the intrabone injection of positively selected CD34+ cells, can be safely used in cases of low graft cellularity due to poor donor mobilization, with the aim of minimizing the risk of graft rejection or poor engraftment. Our preliminary data need to be confirmed in larger series of patients and compared with those obtained with conventional intravenous administration of comparable dose of CD34+ cells. Disclosures No relevant conflicts of interest to declare. [1]: /embed/inline-graphic-2.gif