Impact of HLA-B Dimorphism in the Leader Peptide on Clinical Outcome in AML

NK cells are negatively regulated by inhibitory receptors, such as the family of Killer Immunoglobulin-like Receptors (KIR) and the NKG2A/CD94 heterodimer. These receptors recognize cognate HLA class I molecules, and recent studies suggest that an HLA-B dimorphism in the leader peptide at position -21 dictates whether NK cell regulation mainly relies on the KIRs or the NKG2A/CD94 receptor. Thus, haplotypes with -21M delivers functional peptides to HLA-E, but will rarely harbor genes encoding the KIR ligand HLA-C2. However, little is known regarding the clinical implications of this dimorphism. For this study, we performed HLA typing and an extensive flow cytometry analysis of the NK cell repertoire in 80 AML patients who received immunotherapy with histamine dihydrochloride (HDC) and IL-2 for the prevention of relapse in the post-consolidation phase. Below median levels of HLA class I expression, reflecting low-grade NK cell inhibition, were found to correlate with significantly better outcome in HDC/IL-2-treated patients, indicating that the efficacy of the immunotherapy is NK cell-dependent. In contrast to healthy controls, the majority of NK cells in AML patients in complete remission displayed expression of NKG2A, which was even more prominent in patients after 3 weeks of HDC/IL-2 treatment, or after in vitro culture of NK cells with IL-2. Thirty-eight patients (47.5 %) carried the M/x genotype in HLA B -21 (predicting NKG2A dependence) and 42 patients (52.5 %) carried the T/T genotype (KIR dependence). Despite the high percentage of NKG2A⁺ NK cells, M/x patients showed significantly improved leukemia-free survival (p=0.04) and overall survival (p=0.007) compared with T/T patients. Only patients with M/x genotype benefited from low expression of HLA class I. The IL-2-induced NKG2A expression was associated with enhanced NKG2A-mediated inhibition as pure populations of NKG2A-expressing NK cells were inhibited by a leukemic cell line transfected to express the NKG2A ligand HLA E, which was reverted by antibody blockade of NKG2A. Our results suggest that the outcome of AML immunotherapy is affected by (i) expression levels of HLA class I, and (ii) the HLA B-21 dimorphism and, hence, whether the KIR or NKG2A-CD94 are the dominant inhibitory pathway for NK cells. Furthermore, the pronounced expression of NKG2A in HDC/IL-2-treated patients suggests that combination strategies, including checkpoint inhibitors targeting NKG2A together with HDC/IL-2, may be feasible for relapse prevention in AML.