Enhanced Immune Reconstitution of γδ T Cells after Allogeneic Stem Cell Transplantation Overcomes the Negative Impact of Pretransplantation Minimal Residual Disease-Positive Status in Patients with Acute Myelogenous Leukemia

Minimal/measurable residual disease (MRD) before allogeneic stem cell transplantation (allo-SCT) in patients with acute myelogenous leukemia (AML) is a poor risk factor for outcome. γδ T cells represent a unique minority lymphocyte population that is preferentially located in peripheral tissues, can recognize antigens in a non-MHC-restricted manner, and plays a "bridging" role between the innate and adaptive immune systems. In this study, we investigated a potential graft-versus-leukemia effect of γδ T cell reconstitution post-transplantation in AML patients with pretransplantation positive MRD status (MRD+). MRD assessment was performed in 202 patients using multicolored flow cytometry ("different from normal" strategy); 100 patients were deemed MRD+. Analysis for absolute concentrations of CD3+, CD4+, CD8+, natural killer, and γδ T cells were performed by flow cytometry according to an internal protocol at day +30 and +100 post-transplantation. Differences between categorical and continuous variables were determined using the chi-square and Student t test, respectively. The Mann-Whitney U test was used to compare medians of continuous variables. Spearman's correlation was used for nonparametric assessment of correlation between different cell subsets during immune reconstitution. Kaplan-Meier survival analysis and Cox regression analysis were used to investigate the associations between immune reconstitution and survival outcomes. Gray's analysis was used to compute incidences of relapse, nonrelapse mortality, and graft-versus-host disease (GVHD). The median follow-up of survivors was 28 months (range 3 to 59 months). Younger age (≤58 years) of recipient and donor (<30 years), sex mismatch, use of a matched donor, cytomegalovirus reactivation, and administration of antithymocyte globulin were associated with a faster γδ T cell reconstitution. In multivariable analysis for MRD+ patients, a higher than median level of γδ T cells on days +30 and +100 resulted in significantly improved leukemia-free survival (hazard ratio [HR], 0.42 [P = .007] and 0.42 [P = .011], respectively) and overall survival (HR, 0.44 [P = .038] and 0.33 [P = .009], respectively). Furthermore, a higher γδ T cell level on day +30 was associated with a significantly reduced risk of relapse (HR, 0.36; P = .019). No impact of γδ T cell level on relapse at days +30 and +100 could be seen in MRD-negative patients, and no correlation with occurrence of GVHD was observed. Our data indicate that enhanced immune reconstitution of γδ T cells post-transplantation may overcome the higher relapse risk of pretransplantation MRD+ status in patients with AML.