Transplantation of Positive Selected Peripheral Stem Cells with Add-Back of T Cells from Unrelated Donors in Children: Favourable Survival and Low Incidence of GvHD.

There is still a significant risk of GvHD in transplantation of peripheral stem cells from matched unrelated donors, despite the use of ATG and pharmacological prophylaxis. Especially in the case of young pediatric patients and adult donors, grafts with high numbers of stem cells but also with extreme amounts of T cells can be obtained. We present an update of a clinical study with T cell reduced grafts in 22 patients, which offer both large stem cell numbers and clearly defined amounts of T cells. Positive selection of peripheral stem cells was done with CD34 or CD133 coated magnetic microbeads and the CliniMACS™ device. Afterwards, an unselected aliquot containing 10x10⁶/kg T cells was added to the purified stem cells and infused on day 0, resulting in a median T cell depletion of 2 log. The diagnoses were: acute lymphatic leukemia (n=9; CR1=4, CR2=5), NHL (n=1) acute (n=5; CR1=1, CR2=4) and chronic myeloic leukemias (n=3), MDS (n=3) and Wiskott-Aldrich (n=1). Median age was 10 years (0.5–18). The donors were matched for HLA-A/B (medium resolution typing) and DRB1/DQB1 (high resolution). A short course of MTX (2–3x10 mg/m²) and CSA (2–3 mg/kg, adjusted to blood levels) until day 100 were given. 21/22 patients had primary engraftment with a median time to ANC >500 of 18 days (12–60). No G-CSF was given. One patient rejected his graft and was successfully retransplanted from another MUD. Platelet recovery was fast (median time to reach independence from substitution: 20 days). Mean numbers of CD3+, CD3+CD4+ and CD3+CD8+ on day 180 (365) were 259/μl (868/μl), 100/μl (400/μl) and 146/μl (456/μl). 19/22 patients had GvHD grade 0–I (86%), 1 and 2 patients had grade II and III, (5%, 10%) respectively. Chronic GvHD occurred in 2 patients (10%). 15/22 patients are alive with a median follow up of 3 years (1.5–5). 3 year EFS was 64% (all patients), 50% (ALL/NHL) and 73% (myeloic leukemias/MDS). Relapse probability at 2 years was 25%, probability of TRM at 2 years was 15%.Causes of death were relapse (n=4) and infections (n=3). Thus, stable and favourable survival rates with a low incidence of GvHD were achieved. The method allows to administer clearly defined T cell numbers independent from the size of the grafts. This may be of advantage in particular in small children and if the donor does not accept a bone marrow harvest: all available stem cells can be infused without the limitation of unacceptable high T cell numbers.