Haploidentical Hematopoietic Cell Transplantation of Ex Vivo Tcrαβ-Depleted Grafts with CD45RA-Depleted Memory T Cell Add-Back Reduces Chronic Graft-Versus-Host Disease and Results in Favourable Gvhd-Relapse Free Survival: 5-Year Follow-up of 107 Patients Treated in a 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 8-9 : 1-2 ratio and depleted using CliniMACS device after labelling with TCRαβ and CD45RA reagents (Miltenyi Biotec, Bergish-Gladbach, Germany) respectively. All except 13 patients received the standard conditioning regimen of fludarabine 160mg/m² divided daily over 4 days, thiotepa 10mg/kg divided twice daily for 1 day, and melphalan 70 - 140 mg/m² for 1 day, in combination with either total lymphoid irradiation 6 Gy (n=58) or 7.5 Gy (n=12) over 3 equal fractions, total body irradiation of 2 Gy (n=24), or thymoglobuline (n=2). Short term GVHD prophylaxis was given for 30 days to 1 patient using MMF, 95 using tacrolimus, and 2 using sirolimus.

Results: Between January 2017 and July 2022, we transplanted 107 patients, including 100 adults (median age, 52 years; range 19- 70) and 7 children (median age, 13 years, range 7 - 17), with high risk AML (n=60), ALL (n=22), MDS (n=10), plasma cell neoplasm (n=3), mast cell leukemia (n=1), acute undifferentiated leukemia (n=1), CMMoL (n=2), CML (n=2), MPN (n=1) and lymphoma (n=5). Patients were infused with TCRαβ and CD45RA depleted grafts containing a median of 6.18 x 10⁶ (range 2.13 - 20.78) CD34+ cells/kg, 0.00 x 10⁴ (range 0 - 0.97) CD45RA+CD3+ cells/kg, and 1.21 x 10⁶ (range 0.15 - 11.67) CD45RO+CD3+ cells/kg. TCRαβ depleted graft fraction contained a median of 0.37 x 10⁴ (range 0 - 11.30) TCRαβ+ cells/kg, and 7.72 x 10⁶ (range 0.62 - 42.00) 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 - 25) and PLT > 20,000 cells/µL at median of 11 days (range 7 - 17). 10 patients with high donor-specific HLA antibodies (DSA) titres engrafted successfully after desensitisation with plasma exchange, rituximab, immunoglobulin, and irradiated donor buffy coat infusion (n=1).

38 patients (35%) developed acute GVHD of grade II - IV, with a cumulative incidence (CI) of grade II-IV and grade III-IV of 34%, and 12% respectively, at 180 days. Chronic GVHD was seen in only 4 patients at a 2-year CI of 5 % .

1-year CI of non-relapse mortality (NRM) and relapse were 18.4 % and 17.2%, respectively. 4 of the 22 NRM were attributed to aGVHD. Viral reactivation included CMV (n=39), HHV-6 (n=28), EBV (n=19), and adenovirus (n=8). 11 patients (11%) out of the 15 infection-related death (15%) occurred within 180 days. At a median follow up of 23 months (range 0.3- 67.1) in surviving patients, 2-year overall (OS), event-free (EFS), and GVHD-free/relapse-free (GRFS) survival were 69.9 %, 58.7 %, and 57.3 %, respectively (Figure 1).

In multivariate analysis, HCT-comorbidity index (HCT-CI) has shown significant impact on OS (p=0.008), EFS (p=0.03), and NRM (p=0.002). Disease risk index (DRI) also showed significant impact on OS (p=0.042), EFS (p=0.02) and relapse (p=0.04). In contrast, NRM was only influenced by HCT-CI (p=0.002).

In the subset analysis of 63 patients (adults, n= 58; children, n=5) with favorable HCT-CI score of 0/1 and low/intermediate risk DRI, the 2-year OS, EFS, and GRFS were 75.0 %, 68.6%, and 68.0 %, respectively (Figure 2).

Conclusions: Our preliminary results suggest that RIC haplo-HCT with TCRαβ and CD45RA+ depleted grafts allow successful allograft in patients with high-risk haematological malignancies lacking suitable matched donors, including patients with high levels of DSA. Acute GVHD was generally abortive, leading to low incidence of chronic GVHD and favorable GRFS. Best outcomes are seen in patients with favourable HCT-CI and low/intermediate risk DRI. Further efforts are needed to reduce the risk of infection-related death in patients with high risk HCT-CI, and relapse risk in patients with high risk DRI, through optimization of conditioning regimen intensity, anti-microbial prophylaxis or prophylactic infusion of memory-cell DLI.

No relevant conflicts of interest to declare.