High-dose post-transplant cyclophosphamide impairs γδ T-cell reconstitution after haploidentical haematopoietic stem cell transplantation using low-dose antithymocyte globulin and peripheral blood stem cell graft.

Objectives: Haploidentical haematopoietic cell transplantation (Haplo-HCT) using peripheral blood stem cell (PBSC) grafts and post-transplant cyclophosphamide (PTCy) is being increasingly used; however, data on immunological reconstitution (IR) are still scarce.

Methods: This retrospective study evaluated T-cell immunological reconstitution in 106 adult patients who underwent allogeneic haematopoietic cell transplantation for haematologic malignancies between 2013 and 2016.

Results: At D30, while conventional T cells reached similar median counts in Haplo-HCT recipients ( = 19) and controls ( = 87), γδ and Vδ2 T-cell median counts were significantly lower in Haplo-HCT recipients and it persists at least until D360 for Vδ2 T cells. PTCy induces a significant reduction in early γδ and Vδ2 T-cell proliferation at D  7. At one year, the rate of increase in Epstein-Barr virus (EBV) viral load was significantly higher in Haplo-HCT recipients as compared to controls (61% versus 34%,  = 0.02). In multivariate analysis, a higher γδ T-cell count (> 4.63 μL) at D30 was the only independent parameter significantly associated with a reduced risk of increase in EBV viral load (RR 0.34; 95% CI, 0.15-0.76,  = 0.009).

Conclusion: Immunological reconstitution of γδ T cells is significantly delayed after Haplo-HCT using PTCy and low-dose ATG and is associated with an increased risk of increase in EBV viral load.