Targeting the Tumor Evasion Interaction of NKG2A and Its Ligand HLA-E Increases Natural-Killer Cell Activity in Chronic Lymphocytic Leukemia

Chronic Lymphocytic Leukemia (CLL) represents the most frequent adult leukemia, and remains incurable with current standard therapies. Natural Killer (NK) cell count is predictive of CLL disease progression and their dysfunction in mediating cytokine release and direct or antibody dependent cellular cytotoxicity (ADCC) against CLL B-cells is well documented. Detailed mechanistic insight into the etiology of NK-cell dysfunction in CLL patients is currently lacking. CLL B-cells overexpress HLA-E, the natural ligand for heterodimer CD94/NKG2A receptor complex that is expressed on the surface of NK cells, and this interaction suppresses NK cell activation. While NKG2A/CD94/HLA-E interaction is known to assist NK cells in recognizing "self", tumor cells utilize this mechanism to evade effector cell killing. Utilizing a novel anti-NKG2A monoclonal blocking antibody (mab) we explored the in vitro preclinical activity of targeting the NKG2A receptor, and the NKG2A/HLA-E interaction as a mechanism of tumor evasion in patients with CLL. We hypothesized that limiting the interaction of HLA-E/NKG2A will reverse NK cell anergy and result in increased direct cytotoxicity of CLL cells. Our results confirm the over expression of HLA-E on CLL B-cells and demonstrate NKG2A expression on CD16+ NK cells from CLL patients. Next, we examined the effect of anti-NKG2A mab on NK cell direct cytotoxicity. Treatment of NK cells, from both healthy donor and CLL patients, with anti-NKG2A mab increased direct cytotoxicity over isotype control on targets at various effector to target ratios of 25:1 (54% vs. 46%, p< 0.05, n= 12), 12:1 (43% vs. 35%, p<0.05, n=14), and 6:1 (31% vs. 23%, p<0.05, n= 12, for anti-NKG2A mediated cytotoxicity vs isotype mediated cytotoxicity respectively). These results were also validated with HLA-E over and underexpressing target cells. Fc-gamma receptor blocking experiments were also performed to confirm the specificity of the interaction. Further studies are being performed to confirm the specific activity of the antibody including its ability to modulate NK cell activation, enhance ADCC, and the impact of anti-NKG2A therapy for reversing ibrutinib mediated NK-cell dysfunction. This work has laid the foundation for the clinical utility of this reagent in patients with relapsed CLL in combination with ibrutinib.