Molecular specificities of NK cell-mediated recognition of human tumor cells

Abstract

Natural killer (NK) cells have been implicated in tumor immune surveillance and can reject transformed cells expressing ligands for activating NK cell receptors and low levels of HLA class I. Although NK cells are well known for their ability to kill tumor cells, relatively few studies have addressed the molecular specificity of NK cell-mediated recognition of freshly isolated human tumor cells. The rational for conducting such studies is based on the fact that tumor cell lines display altered molecular expression compared to their origin.

In this thesis, we have assessed the role for NK cells in solid and hematological malignancies. We show that freshly isolated metastatic ovarian carcinoma (OC) cells express low levels of HLA class I. In one patient, we identified a genomic HLA class I haplotype loss that was associated with a HLA-A2 restricted Her2/neu specific T cell response. The low HLA class I levels, in combination with the presence of ligands for activating NK cell receptors, resulted in a significant killing of the metastatic OC cells by allogeneic NK cells, while sparing normal cells. Experiments masking activating NK cell receptors revealed a dominant role for the DNAM-1 receptor with a minor contribution from the NKG2D receptor. Studies of the receptor repertoire and functional integrity of NK cells associated to the tumor in vivo substantiated a role for DNAM-1 since a marked loss of DNAM-1 as well as 2B4 and CD16 were observed and resulted in significantly reduced natural cytotoxicity and antibody-dependent cellular cytotoxicity (ADCC) against autologous carcinoma cells. The DNAM-1 loss was likely caused by chronic ligand exposure, since physical interactions between the receptor and its ligand CD155 induced down-regulation. Suppressed NK cell function due to loss of DNAM-1 and NKG2D expression was also identified in the bone marrow and blood of patients with myelodysplastic syndromes (MDS). Relative to NK cells in peripheral blood, bone marrow-derived NK cells associated to the tumor cells displayed a more severe loss of the two receptors as well as a reduced effector cell function. The receptor loss was most prominent in patients with more than 5% blasts in the bone marrow, suggesting that poor NK cell function may be associated with an increased risk of progression to acute myeloid leukemia (AML). Tumor cells may also evade NK cell-mediated lysis by up-regulation of HLA-E that inhibits NK cell activity through signaling via the CD94/NKG2A receptor. Drugs have been used to manipulate the NK cell receptor ligand repertoire on tumor cells to render them more susceptible to NK cells. Selenite, a highly reactive oxidative agent, is known to selectively kill tumor cells when used in high concentrations. We show that selenite also reduced the expression of HLA-E and rendered the tumor cells more susceptible to killing by CD94/NKG2A expressing NK cells.

Given the emerging evidence for NK cell-mediated tumor immune surveillance, our data indicate that tumor progression may be promoted by perturbed activating NK cell receptor repertoires and poor function of tumor-associated NK cells. The data imply that OC could be targeted by NK cell-based immunotherapy and that MDS patients having more than 5% blasts in the bone marrow could be considered as potential candidates for NK cell-based immunotherapy. Data also indicate that selenite may be used to improve the results of NK cell-based immunotherapies by rendering HLA-E expressing tumor cells more susceptible to NK cells. Thus, a better comprehension of the molecular specificity of NK cells targeting fresh human tumor cells and the role for combinatorial treatments can hopefully advance NK cell-based immunotherapies.