CD3/CD19 Depleted Grafts for Haploidentical Stem Cell Transplantation in Children: Results of a Pilot Study.

Profound depletion of T and B cells is a fundamental prerequisite for haploidentical transplantation. We present our preliminary results with a direct depletion procedure using antiCD3/antiCD19 coated magnetic microbeads and the CliniMACS™ device. Only patients with high risk malignancies (n=25, most of them had active disease or relapsed after previous trp.) or with nonmalignant diseases and high risk of graft failure (n=2) were included. The diagnoses were: AML/MDS/CML (n=13), ALL(n=6), relapsed Neuroblastoma/Ewing-/Rhabdomyosarcoma (n=6), SAA, PNH (n=2); Remission status: NR=8, 2^nd transplantation=10, CR/CP1=3, CR2–4=6.

The patients received either TBI or Bu based standard conditioning regimens (n=9) or a toxicity reduced protocol (Flud, TT, Mel, OKT3, MMF, n=18). The grafts comprised a median number of 16x10⁶/kg (7–41) stem cells as well as high amounts of NK-cells (137x10⁶ (9–550)) and monocytes/granulocytes (6x10⁸ (0.6–13)) with 49x10³/kg (7–200) residual CD3+ cells and <0.01% CD20+ cells. Primary engraftment occurred in 89% of patients (after reconditioning and second stem cell donation:100%). Median time to reach >500 neutrophiles/μl and independence from platelet substitution was 10 and 9 days respectively. Recovery of CD3+ cells was favorable (d90: 320/-l, d180: 600/μl). Acute GvHD grade 0–1 occurred in 74%, 26% had GvHD grade II. Limited chronic GvHD occurred in 4 patients. No transplant related mortality (TRM) and, in particular, no lethal infections were observed. 13/27 patients (49%) were alive with a median follow up of 0.8 years (3 months – 2.1 years). Single cause of death was relapse. Disease status was predictive: 8/9 patients with leukemias and active disease relapsed, whereas only 2/10 patients in CR relapsed. 2/6 patients with solid tumors are alive.

Recovery of platelets was faster in patients with CD3/CD19 depleted grafts than in a historical control group of patients (n=53) transplanted with haploidentical positive CD34+ selected standard grafts (23 vs. 9 days, p<0.01). No lethal viral infections occurred in the study group due to the fast T cell recovery, whereas in patients with positive selected grafts a cumulative incidence of 18% was observed. The toxicity-reduced conditioning regimen helped to avoid any other TRM, despite intensive pretreatment (including previous transplantation) of the patients.

Conclusions: our preliminary results indicate that CD3/CD19 selected grafts can improve immunoreconstitution and TRM. Engraftment rates similar to that of patients with myeloablative standard conditioning regimens and positive selected stem cells could be achieved, although most patients of the study group received an intensity reduced regimen. However, the outcome was poor in patients with active disease. Thus, further options have to be investigated to increase the potential antileukemic activity of donor derived effector cells.