A Delayed Innate T Cell Reconstitution Is Associated with Epstein-Barr Virus Reactivation after Haploidentical Hematopoietic Stem Cell Transplantation Using Anti-Thymoglobulin and High-Dose Post-Transplant Cyclophosphamide

Background: Allogeneic hematopoietic stem cell transplantation (allo-HCT) using a haploidentical donor (haplo-HCT) using post-transplant high-dose cyclophosphamide (PTCy) is increasingly used for patients lacking a matched related or unrelated donor. We recently noticed a relatively high incidence of infectious complications, especially Epstein-Barr virus (EBV) reactivation after haplo-HCT. However, the mechanisms underlying the increased incidence of this complication in the haplo-HCT setting are unknown. We hypothesized that the use of PTCy may be associated with a deficit in innate-like effector T cells essential for preventing EBV reactivation.This study aimed to analyze immune reconstitution following haplo-HCT using peripheral blood stem cells (PBSC) grafts and to evaluate the correlations with EBV reactivation and outcomes.

Patients and Methods: One hundred and twenty-three consecutive patients who underwent allo-HCT for hematological malignancies between 2013 and 2016 were included in this single-center retrospective study. All patients received G-CSF mobilized PBSC grafts and ATG 2.5-5 mg/Kg total dose. All patients received a combination of cyclosporine A and mycophenolate mofetil for GvHD prophylaxis, except for patients with a matched related donor (MRD) who received cyclosporine A alone and patients with an haploidentical donor who received PTCy (50mg/kg/d at d3+/-d5). α/β T cells (CD3+, CD4+ and CD8+), γ/δ T cells (pan γ/δ and Vδ2+), mucosal-associated T cells wich express a highly restricted TCR comprising a semi-variant Vα7.2-Jα33 (MAIT), invariant NK T cells, NK cells, B cells, Tregs, monocytes subsets and dendritic cells (myeloid DC, plasmacytoid DC and Slan-DC) were analyzed by multi-color flow cytometry at months (M) 1, 3, 6 and 12 following allo-HCT. Clinical data [acute GvHD (aGvHD), relapse incidence, chronic GvHD (cGvHD), viral reactivations, bacterial and fungal infections, non-relapse mortality (NRM), progression-free survival (PFS), refined GvHD-free and progression-free survival (GPFS), overall survival (OS)] were assessed together with sequential quantitative evaluation of immune subsets.

Results: Median age was 55 (range, 17-70) years, with 32 male patients (26%) receiving a graft from a female donor. Diagnoses were myeloid malignancies (66%) or lymphoid malignancies (34%). Thirty-three patients (27%) received a graft from a MRD, 65 from an unrelated donor (MUD, 53%), and 25 from a haplo-identical donor (20%). Thirty patients (24%) with refractory disease received a sequential conditioning regimen while the remaining (n=93, 76%) received a RIC/RTC regimen based on fludarabine, busulfan +/- thiotepa. At d180, the cumulative incidences (CIs) of grade II-IV and grade III-IV aGvHD were 34% and 5%, respectively. The 2 years CIs of cGvHD, extensive cGvHD and relapse were 23%, 8% and 17%, respectively. At 2 years, NRM was 7%, PFS was 77%, GPFS was 66% and OS was 83%. The rate of EBV reactivation was significantly increased in haplo-HCT recipients as compared to fully-matched donor recipients (respectively, 68% versus 26%, P< .001).

At one month after allo-HCT, the median counts of all immune cells subsets (except monocytes) was significantly lower in haplo-HCT recipients as compared to MRD or MUD recipients. At 3 months, α/β T cells, iNK T cells, NK cells, B cells, Tregs and Slan-DC reached similar median counts in haplo-HCT recipients as compared to MRD or MUD recipients. In contrast, Vδ2+ T cells and MAIT cells median counts remained significantly lower at 3, 6 and 12 months in haplo-HCT recipients compared to MRD or MUD recipients (at M1, M3, M6, M12, the median Vδ2+ T cells counts were 0.05/µL, 0.24/µL, 1.38/µL and 2.97/µL, and MAIT cells were 0.07/µL, 0.70/µL, 1.00/µL and 1.21/µL, respectively). Lower Vδ2+ T-cells and MAIT cells counts at one month was associated with a significantly increased CI of EBV reactivation (respectively, P=.04 and P<.0001) in landmark study.

Conclusion: Immunological reconstitution of innate T cells is significantly delayed after haplo-HCT and low-dose ATG and PTCy. This prolonged deficiency is associated with an increased risk of EBV reactivation. Development of new strategies for innate-T cells expansion are necessary after haplo-HCT.