10056 Background: Pediatric patients with relapsed metastatic neuroblastomas have a poor prognosis and additional therapeutic strategies are needed. We present results of a phase I/II-trial with subsequent immunotherapy with an anti-GD2mAb (CH14.18/CHO) after HLA mismatched, haploidentical stem cell transplantation (SCT). Methods: T- and B-cell depleted stem cells from parental donors were used in combination with Melphalan140mg/m², Thiotepa10mg/kg, Fludarabin160mg/m² and ATG-F. Infusions with CH14.18/CHOmAb were started on day 60-180 posttransplant: 6 cycles with 20mg/m²/day x 5 days; in cycles 4-6, 1x106 U/m² Interleukin 2 (IL2) was given additionally. The disease status was evaluated with whole body MRI, MIBG scan and bone marrow aspirates. Results: 34 patients with 1st or 2nd metastatic relapse were enrolled. During antibody infusions, endogenous secretion of IL2 was increased (928U/ml prior vs. 1690U/ml post, p < 0.001), which resulted in significantly increased numbers of activated CD69+ Natural Killer (NK) cells (3 vs. 13% p < 0.01). In 5/7 investigated patients, effective ADCC and complement mediated (CDC) anti-tumor effects against neuroblastoma cells were detectable in vitro (85% specific lysis, E:T-ratio = 20:1, BATDA-release). 14/34 patients did not reach the end of the protocol (due to side effects, n = 2; TRM, n = 1; progression or relapse, n = 11). 8/34 patients could maintain a CR, 9/34 patients improved their partial remission and achieved CR, 3/34 patients had stable disease. Thus, success of treatment defined as stable disease or improvement was shown in 59%. Progression free survival at 2 and 3 years was 55% and 38% (median follow up: 550 days). Frequent side effects were pain, fever and CRP elevation; rare side effects comprised SIRS/capillary leak syndrome, seizures, and accommodation disturbances. Only 1 patient developed transient acute GvHD grade II. Conclusions: CH14.18/CHO infusions after haploidentical stem cell transplantation appear to be feasible without increased risk of inducing GvHD. Results of our study also suggest an anti tumor effect of the new, donor-derived immune system in vitro and in vivo. Clinical trial information: NCT02258815.