Early CMV Replication-Associated Anti-Leukemia Effect Is Related to the Reconstitution of Specific Subsets of Natural Killer Cells after Haploidentical Stem Cell Transplantation in Acute Leukemia

Background: Cytomegalovirus (CMV) infection is a major infectious complication after allogeneic hematopoietic cell transplantation. Recently, it has been reproducibly demonstrated that CMV reactivation is associated with decreased relapse rate in patients with acute myeloid leukemia (AML). The aim of this study is to evaluate the impact of early CMV reactivation on the incidence of disease relapse in relation to the reconstitution of subsets of natural killer (NK) cells after haploidentical stem cell transplantation (haploSCT) in acute leukemia.

Methods: Clinical data from 47 adult patients diagnosed with acute leukemia who underwent their first haploSCT between September 2009 and December 2017 was retrospectively analyzed. All patients were unable to find a suitable HLA-matched donor in their families or donor registries. Patient blood samples were collected prior to conditioning therapy and following haploSCT at day 30 and day 90. Peripheral blood mononuclear cells obtained from 28 patients abundant cells at every time points were analyzed for flow cytometric immunophenotyping. Expression of specific receptors (NKG2A, NKG2C, NKG2D, DNAM1 and NKp46) on the NK cells (CD56^brightCD16^- or CD56^dim/-CD16⁺ cells) were serially quantified by multiparametric flow cytometry using appropriate monoclonal antibodies.

Results: Median age was 38 years (range, 21-62 years), and 28 (54%) patients were male. Thirty-six (69%) patients received a transplant in their first complete remission (CR) status. Median follow-up duration was 54 months (range, 6.6-83.3 months), and all patients were CMV seropositive before receiving a transplant. Early CMV replication occurred at a median of 23 days after haploSCT in 40 of 47 patients (85%). Among these patients, 14 had more than two episodes of CMV replication throughout the follow-up period. The median peak viral load during CMV replication was 54,000 copies/mL. In univariate analysis, early CMV replication (P < 0.001), older donor age (P = 0.018), high dose of infused T cells (P = 0.022) and chronic graft-versus-host disease (GVHD, P = 0.001) were significantly associated with lower 3-year cumulative incidence of relapse after haploSCT. Early CMV replication was correlated with higher leukemia-free survival (LFS, P < 0.001). In multivariate analysis, early CMV replication (hazard ratio [HR], 0.24; 95% confidence interval [CI], 0.060 to 0.930, P = 0.039) and chronic GHVD (HR, 0.25; CI, 0.089 to 0.695; P = 0.008) were identified as independent factors for higher LFS rate. CMV reactivation was associated with higher overall survival (OS) rates and increased nonrelapse mortality, although there was no statistical significance. The viral load at the initiation of CMV-specific treatment was significantly associated with OS rates. Patients with CMV viral load higher than 45,000 copies/mL had lower OS rate compared to those with lower CMV load (34.5% versus 89.5%, P = 0.022). Longitudinal analysis of NK cell reconstitution after haploSCT showed that the CMV infection was associated with the increased expansion of CD56^brightCD16^dim/- NK cell, particularly in DNAM1⁺ NK cell subset. The rate of CD56^brightCD16^dim/-DNAM1⁺ NK cells increment was significantly higher in the patients with CMV infection compared with patients without CMV infection (P = 0.022). Importantly, we observed that the cumulative relapse rate was significantly decreased in patients with an increased CD56^brightCD16^dim/- DNAM1⁺ NK cells compared to patients with low CD56^brightCD16^dim/- DNAM1⁺ NK cells (20.4% versus 63.6% , P = 0.016).

Conclusion: Early CMV replication was identified as an independent prognostic factor for LFS in acute leukemia in the haploSCT setting. Increased reconstitution of CD56^brightCD16^dim/- DNAM1⁺ NK cells was associated with CMV infection-related reduction in the relapse rate. Further studies are required to elucidate the anti-leukemia effects of these NK cell subsets associated with CMV infection in haploSCT.