HLA-Haploidentical Hematopoietic Cell Transplantation of Ex Vivo Tcrαβ-Depleted Grafts with CD45RA-Depleted Memory T Cell Add-Back in Adults and Children with Hematological Malignancies: 4-Year Follow-up of Multicenter Study in Singapore

Background: Haploidentical hematopoietic cell transplantation (HCT) provides an alternative option for patients without HLA-matched donors. Graft-versus-host disease (GVHD), engraftment failure, and infectious complications are main causes of non-relapse mortality (NRM). We hypothesized that selective depletion of TCRαβ+ and CD45RA+ naïve T cells will permit hematopoietic engraftment while effectively reducing GVHD and providing donor immune reconstitution through adoptive transfer of donor's mature NK cells, γδ T cells and CD45RO+ memory T cells. Methods: Mobilized PBSC products were divided into two fractions in 9:1 ratio and depleted using CliniMACS device after labelling with TCRαβ and CD45RA reagents (Miltenyi Biotec, Bergish-Gladbach, Germany) respectively. All except 6 patients received the standard conditioning regimen of fludarabine 160mg/m 2 divided daily over 4 days, thiotepa 10mg/kg divided twice daily for 1 day, and melphalan 70 - 140mg/m 2 for 1 day, in combination with either total lymphoid irradiation 6Gy (n=53) or 7.5Gy (n=12) over 3 equal fractions, or total body irradiation of 2Gy (n=17), or thymoglobulin (n=2). Short term GVHD prophylaxis was given for 30 days to 1 patient using MMF, 73 using tacrolimus, and 2 using sirolimus. Results: Between January 2017 and July 2021, we transplanted 85 patients, including 78 adults (median age, 48 years; range 20 - 70) and 7 children (median age, 13 years, range 7 - 17), with high risk AML (n=44), ALL (n=19), MDS (n=9), plasma cell neoplasm (n=4), mast cell leukemia (n=1), acute undifferentiated leukemia (n=1), CMMoL (n=2), CML (n=1) and lymphoma (n=2). Patients were infused with TCRαβ and CD45RA depleted grafts containing median of 6.19 x 10 6 (range 3.54 - 20.78) CD34+ cells/kg, 0.00 x 10 4 (range 0 - 0.97) CD45RA+CD3+ cells/kg, and 1.10 x 10 6 (range 0.15 - 11.67) CD45RO+CD3+ cells/kg. TCRαβ depleted graft fraction contained a median of 0.42 x 10 4 (range 0 - 11.30) TCRαβ+ cells/kg, and 7.61 x 10 6 (range 0.62 - 31.84) TCRγδ+ cells/kg. Only 1 patient experienced primary graft failure with no secondary graft failures. All others had engraftment of ANC > 500 cells/µL at median of 12 days (range 8 - 22) and PLT > 20,000 cells/µL at median of 11 days (range 7 - 17). 6 patients with high donor-specific HLA antibodies (DSA) titres engrafted successfully after desensitisation with plasma exchange, rituximab, and immunoglobulin. 29 patients (34%) developed acute GVHD of grade II - IV (Gd II, n=20; Gd III