Advances in ex vivo T cell-depleted haploidentical hematopoietic stem cell transplantation. The ex vivo techniques to remove T cells have evolved from the selection of CD34⁺ hematopoietic stem cell progenitors to the depletion of CD3⁺ cells, and more recently, the depletion of αβ⁺ T cells. Early attempts with haploidentical HSCT using CD34-selected stem cells, even with a megadose, were complicated by a high rate of infections likely related to delayed immune recovery. To overcome the limitation of CD34⁺ selection, the concept of direct depletion of T cells using anti-CD3 monoclonal antibody was introduced with the advantage of increasing the number of natural killer (NK) cells, and other immunomodulating cells. Depleting CD3⁺ cells was superior to selecting CD34⁺ cells in terms of engraftment speed and immune reconstitution. Although haploidentical HSCT using CD3-depleted grafts successfully reduce lethal infection rates, delayed immune recovery and the high rate of relapse were still problematic. The most recently developed approach using the negative depletion of αβ⁺ T cells improved the outcomes of T cell-depleted haploidentical transplant. Although recent advances in haploidentical HSCT, delayed immune reconstitution with subsequent infections and relapse for malignant disease are current major causes of treatment failure. New depletion technique to deplete naïve T cells or adoptive transfer of immune effector cells and cellular therapy based on γδ T cells or other immune cells could further improve the outcomes of haploidetical HSCT.

Major progress in ex vivo T cell-depleted haploidentical HSCT at AMCCH. In 2008, allogenetic HSCT from haploidentical family donor was initiated at our center using CD3-depleted grafts after reduced-intensity conditioning (RIC) with cacineurin inhibitors (CI) and mycophenolate mofetil (MMF) for the prevention of graft versus host disease (GVHD). Our early experience with CD3-depleted haploidentical HSCT showed a high incidence of graft failure (GF); therefore, low-dose total body irradiation (LD-TBI) was added to the conditioning regimen in an attempt to decrease GF in early 2011. In addition, the infused cell dose was targeted after add-back of T cells from negative selection product. The targeted dose of CD3⁺ T cells was gradually reduced from 1-5×10⁶/kg to 6-8×10⁵/kg to decrease the risk of severe GVHD and ensure stable engraftment. At the end of 2012, the ex vivo αβ⁺ T cell depletion technique with targeted dose of αβ cells at 1-5×10⁵/kg by add-back was introduced. The depletion efficacy improved with the use of anti-TCRαβ monoclonal antibody instead of anti-CD3 monoclonal antibody for depletion, leading to ≤5×10⁴/kg of recipient body weight of the residual αβ⁺ T cells. At the end of 2015, immunosuppressive drugs to prevent GVHD were eliminated for αβ-depleted haploidentical HSCT.

Current haploidentical HSCT strategy for pediatric patients at AMCCH. The donor will receive G-CSF for a minimum of four consecutive days and peripheral blood mononuclear cells (PBMCs) will be collected on days -1 and 0. The αβ⁺ T cells will be depleted by negative depletion using the CliniMACS system (Miltenyi-BioTec, Bergisch-Gladbach, Germany). The final dose of αβ⁺ T cells is targeted ≤5×10⁴/kg by adding back αβ⁺ T cells from the negative selection product. The patient will receive conditioning regimen consisting of fludarabine (FLU), cyclophosphamide (CY), rabbit ATG (r-ATG), and low-dose total body irradiation (LD-TBI). After that, stem cells will be infused on day 0 without any post-transplant immunosuppressants. The patient will also receive rituximab post-transplant to deplete B cells at approximately day +28 or earlier if EBV was detected with PCR. For cytomegalovirus (CMV) prophylaxis, the CMV-seropositive patient will receive ganciclovir prior to transplant and foscarnet after transplantation up until engraftment. After engraftment, ganciclovir or valganciclovir will be administered until 100 days post-transplantation with CD4⁺ cells at >100/µL.

Abbreviations: HSC, hematopoietic stem cells; αβ, αβ⁺ T cells; γδ, γδ⁺ T cells; DC, dendritic cells; B, B cells; HR, high-risk.