Comparison of Early Post-Transplant Outcomes in Ex Vivo αβ⁺ T Cell-Depleted Haploidentical HSCT with or without Pharmacologic Gvhd Prophylaxis

Background and purpose: One of the key obstacles to successful haploidenitcal hematopoietic cell transplantation (HHCT) is a development of fatal GVHD. Although much progress in immunosuppressant (IS) has effectively prevented the development of acute GVHD, they have many serious toxicity and drug interactions requiring serial monitoring of drug levels. Recent advances in ex vivo depletion technique enabled to effectively reduce T cells or their subset, αβ⁺ T cells, leading to residual αβ⁺ T cells in grafts well below 5×10⁴/kg of recipient weight. We eliminated post-transplant pharmacologic GVHD prophylaxis along with targeting αβ⁺ T cell dose ≤ 5×10⁴/kg since November 2015. In this study, we compared early post-transplant outcomes between with (IS+) or without (IS-) post-transplant immunosuppressants after ex vivo αβ⁺ T cell-depleted HHCT.

Methods: Between May 2012 and July 2016, 69 pediatric patients received HHCT using TCRαβ-depleted grafts from haploidentical family donors at Asan Medical Center Children's Hospital. Fifty patients received tacrolimus and mycophenolate mofetil to prevent acute GVHD, while 19 did not receive any immunosuppressant after transplant. All donors received G-CSF for 4 consecutive days and peripheral blood stem cells were collected on days -1 and 0. The αβ⁺ T cells were depleted by negative selection using the CliniMACS^® system (Miltenyi-BioTec, Bergisch-Gladbach, Germany) according to manufacturer's instruction. In the earlier trial of IS+, the final doses of αβ⁺ T cells were adjusted to 1-5×10⁵ cells/kg by add-back from the raw bag. Since November 2015, the cell dose was targeted at ≤ 5×10⁴ αβ⁺T cells/kg with no post-transplant immunosuppressants (IS-).

Results: The median infused CD34⁺ cells, αβ⁺ T cells, γδ⁺ T cells and CD3^-CD56⁺ NK cells per kg of recipient weight were 8.9×10⁶, 33.8×10⁴, 20.0×10⁶, 45.9×10⁶ in IS+ group and 6.1×10⁶, 4.6×10⁴, 17.5×10⁶, 24.6×10⁶ in IS- group, respectively. All 69 patients achieved neutrophil engraftment at a median of 10 days (range, 9-17). Three patients out of 50 in IS+ group experience graft rejection (GR), while no GR occurred in IS- group. The cumulative incidences of acute GVHD grade II-IV were similar (31% vs 33%). Severe acute GVHD ≥ grade III developed in 7 in IS+ group, while none in IS- group developed ≥ grade III. As of July 2016, the median follow-ups were 24 months (range 9.5-50.8) for IS+ group and 5 months (0.5-9.1) for IS- group. Two out of 50 patients in IS+ group died of TRM leading to 2.2% at 6 months and 4.9% at 1 year after HHCT, while no patients in IS- group died of TRM during the follow-up period. The mean time from transplant to discharge were longer in IS+ group compared to IS- group (32 days versus 21 days, P=0.049). While the mean time of hospital stay within 100 days post-HHCT for patients who survived more than 100 days was not different between two groups (47 days versus 34 days, P>0.05).

Conclusions: The major findings of our study were less severe acute GVHD and shorter hospital stay from HHCT to discharge in IS- group, even with less T cell dose, compared to IS+ group. Therefore, this HHCT using ex vivo αβ-depleted graft containing αβ⁺ T cells ≤ 5×10⁴/kg is an effective treatment strategy to prevent acute GVHD without post-transplant IS. In addition, the early clinical outcomes were comparable between with and without post-transplant IS.