Quantity and Quality Reconstitution of NKG2A⁺ NK Cells Are Associated with Graft-Versus-Host Disease after Allogeneic Hematopoietic Cell Transplantation

Background: Allogeneic hematopoietic stem cell transplantation (allo-HSCT) is an effective treatment strategy for hematological malignancies. However, graft-versus-host disease (GVHD) is a common complication after allo-HSCT resulting from the activation, amplification and secretion of numerous inflammatory factors related to donor alloreactive T cells that damage host tissues and organs, mainly in the gastrointestinal tract, liver and skin. Notably, natural killer (NK) cells represent the first donor-derived lymphocyte population to recover after allo-HSCT and generally are observed within the first month after allo-HSCT. NK cells express a series of immune receptors that identify relevant ligands on target cells and maintain the immune balance between NK cell activation and tolerance. Previous studies have shown that the number of NKG2A⁺ cells is decreased in patients with chronic GHVD after HSCT. However, the relationship between NKG2A⁺ NK cells and aGVHD has not been characterized. In addition, the role of NKG2A⁺ cells in aGVHD disease progression and the mechanism underlying NKG2A⁺ cell immunoregulation have not been clearly explained.

Objective: In this study, we used peripheral blood from GVHD, non-GVHD paired specimens and healthy donors to address the underlying mechanism by which NKG2A⁺ NK cells regulate T cells after HSCT.

Methods: We detected the specimens using flow cytometry from two independent cohorts, which were prospective cohort and paired cohort. Futher, we performed experiments in vitro to investigate the potential role of NKG2A⁺ NK cells on T cells.

Results: Here, we found that, compared with non-GVHD subset, NKG2A⁺ NK cells percentage and absolute cell counts were significantly reduced in GVHD patients after HSCT. Moreover, the reduction of NKG2A⁺ NK cells in GVHD patients was ascribed to its increased apoptosis and decreased proliferation capacity, while retaining a strong graft-versus-leukemia (GVL) effect. In vitro assay showed that when co-cultured T cells with NKG2A⁺ NK cells, the T cells secreted IFN-r level was significantly reduced, while the IL-4 level was increased. Moreover, CD25 expression level was decreased, while the CD4⁺CD25⁺FOXP3⁺ cells number was increased. In addition, the NKG2A⁺ NK cells induced T cell apoptosis and decreased T cell proliferation during the coculture process. Significantly, NKG2A⁺ mainly regulated activated but not resting T cells. In vivo assay showed that serological IL-10 level in GVHD subset was evidently lower than those of non-GVHD subgroup, the IL-1β, IFN-r and TNF-a level was however higher in the GVHD subgroup. Furthermore, the percentage of NKG2A⁺ NK cells from GVHD patients was markedly increased by the presence of exogenous IL-10, but not by other cytokines. However, this phenotype was not observed at non-GVHD patients. Together, the GVHD might be ascribed to lower IL-10 induced NKG2A⁺ NK cells reduction, which further overactivate T cells after HSCT.

Discussion and Conclusion: Overall, we herein observed reduced proportions and absolute cell counts of NK cells and NKG2A⁺ subsets in patients with acute GVHD after allo-HSCT. The causative association between NK cell numbers, NKG2A⁺ subsets and GVHD remains debatable. Based on our results, speculating that reduced proportions of NKG2A⁺ subsets in patients after allo-HSCT are associated with acute GVHD due to their interplay with the patient's donor-derived alloreactive T cells is tempting.