Upregulation of HLA-E Drives Defective Natural Killer Cell Targeting in Venetoclax-Resistant Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is the most common leukemia in adults and is primarily diagnosed in older patients. The combination of the BCL2 inhibitor, venetoclax, with a hypomethylating agent has recently become standard of care front-line therapy for patients who are not able to tolerate high intensity induction chemotherapy and allogeneic stem cell transplantation. However, a significant number of patients experience disease refractoriness/relapse and second line treatment options are limited in efficacy. Natural killer (NK) cells have been shown to have potent anti-tumor effects in hematologic malignancies and may represent a potential therapeutic option for AML patients with progression after venetoclax-based therapy. Our group recently showed, however, that venetoclax-resistant (VR) AML blasts are less susceptible to NK cell-mediated killing (Figure 1), and the mechanisms underlying this phenomenon are not yet defined. HLA-E is a non-classical class I MHC molecule known to inhibit NK cell function through binding NKG2A/CD94 on the surface of NK cells. We hypothesized that defective NK cell killing in venetoclax-resistant AML could be mediated by upregulation of HLA-E on AML blasts. We investigated the sensitivity of AML blasts to NK cell cytotoxicity through in vitro co-culture assays with two different human AML cell lines and NK cells derived from a healthy human donor. We found that the venetoclax-resistant version of the OCI-AML-2 cell line was less susceptible to NK cell killing compared to the wild-type (WT) counterpart (Figure 1). In order to understand why VR AML blasts are resistant to NK cell-mediated killing, we investigated differences in NK cell ligands between WT and VR OCI-AML-2 cells and found that VR cells upregulated HLA-E (Figure 2). Given prior evidence showing that HLA-E upregulation in relapsed AML is mediated by interferon-γ, we stimulated WT AML blasts with interferon-γ and found that not only did this lead to upregulation of HLA-E, but it also led to decreased NK cell mediated lysis (Figure 3). We plan to conduct future studies examining the impact of inhibiting the JAK/STAT pathway in AML blasts and utilizing a blocking antibody against NKG2A and HLA-E to verify the role of HLA-E as a primary driver of defective NK cell killing. Overall, our findings suggest that the HLA-E and interferon-γ axis is a potential therapeutic target for enhancing NK cell responses in venetoclax-resistant AML.