HLA-E polymorphisms and NK cell reconstitution after allogeneic hematopoietic stem cell transplantation

Background: Natural killer (NK) cells are an important component of the innate immune response. The study presented here suggests that NK cells can significantly influence the outcome after allogeneic hematopoietic stem cell transplantation (HSCT) - such as the incidence and severity of acute and chronic graft-versus-host disease (GvHD) and importantly the incidence of relapse. This study also discusses phenotypic features and functional activity of these cells after HSCT - in particular the importance of human leukocyte antigen (HLA)-E polymorphisms and NK cell reconstitution to clinical outcomes after HSCT. Methods: The study group included 56 patients undergoing allogeneic HSCT with one-year follow-up investigation. All patient-donor pairs were genotyped for HLA-E locus using a sequence-specific primer polymerase chain reaction (SSP-PCR) strategy. Analyses relevant to the association of HLA-E polymorphisms and post transplant outcomes including incidence and severity of acute and chronic GvHD, risk of infection, transplant-related mortality (TRM), relapse and survival rate were performed. Phenotypic and functional experiments were carried out in 27 patients with either acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL) in complete remission (CR) pre-HSCT and their relevant donors. The samples were obtained prior to HSCT and at various time points during the first year post-HSCT. The expression of NK cell receptors such as natural cytotoxicity receptors (NCRs), NKG2D and CD16 as well as inhibitory killer Immunoglobulin-like receptors (KIRs) and NKG2A was examined using flow cytometry. The cytolytic activity of NK cells was also evaluated using a standard 51 chromium (Cr) release assay. Results: Phenotypic analyses of pre-HSCT NK cells compared to healthy donors showed similar numbers and frequencies of NK cells as well as the normal expression of activatory and inhibitory receptors. The cytotoxic analyses of NK cells prior to HSCT also exhibited comparable results in patients and donors. Analyses of post-HSCT NK cells compared to relevant donors demonstrated an elevated expression of NCRs and NKG2A in addition to a decreased expression of CD16 and inhibitory KIRs. Furthermore, an impaired cytotoxic activity of NK cells was also detected after HSCT. All these phenotypic and functional abnormalities normalized by the 12th month after HSCT. The results also revealed a delayed recovery of CD3+CD56- T cell frequency and number compared to those of CD3-CD56+ NK cells. The effect of HLA-E polymorphisms on transplant outcomes was also investigated. There was a lower frequency of acute GvHD (grade II or more) (p=0.03) and a lower frequency of extensive chronic GvHD (p=0.01) in the patients who had the HLA-E*0103/0103 genotype. There was also a lower incidence of transplant-related mortality (TRM; p=0.02) and better overall survival (p=0.04) in the patients with this genotype. Further experiments demonstrated, for the first time, a higher expression of activatory NKp30 (p<0.01), NKp46 (p<0.001) receptors and a lower expression of inhibitory KIRs (p<0.001) on NK cells as well as an increased NK cell cytotoxicity (p<0.001) in the patients with the HLA-E*0103/0103 genotype compared to the HLA-E*0101/0101 and HLA-E*0101/0103 genotypes. The impaired cytotoxicity and abnormal phenotypic characteristics of NK cells normalized much earlier in the patients with the HLA-E*0103/0103 genotype compared to those with the other genotypes. Conclusions: The study presented here showed for the first time a direct association between the HLA-E genotypes and phenotypic reconstitution of NK cell receptors as well as their cytolytic activity against MNCs of leukemic patients. These findings also supported other observations made here which exhibited a relation between HLA-E polymorphisms and transplant outcomes suggesting a protective role for the HLA-E*0103/0103 genotype. Further studies are required to investigate the exact molecular mechanisms involved in the association between the HLA-E*0103/0103 genotype and better outcomes of HSCT. These investigations will be relevant to a better understanding of the anti-leukemic effect of NK cells after HSCT that may have considerable clinical importance.