Engineered Natural Killer Cells Expressing Chimeric Ilt Receptors (CIR) Effectively Target HLA-G Positive AML Tumor Cells

Background

Human leukocyte antigen-G (HLA-G), the immune checkpoint of pregnancy, directs immunosuppression by binding to Ig-like transcript (ILT) receptors (ILT2 in lymphoid cells and ILT4 in myeloid cells) via immunoreceptor tyrosine-based inhibitory motif (ITIM) signaling domains. HLA-G is expressed in more than 50% of solid tumors and leukemias as a mechanism to evade immune attack. As a result, HLA-G is an excellent tumor-specific antigen since its expression in normal tissues is low except for cytotrophoblasts in the placenta where it protects the fetus from the mother's immune system. We present here a novel approach to target HLA-G-expressing Acute Myeloid Leukemia (AML) cells by utilizing the natural receptors of ILT2 and ILT4 coupled to positive intracellular signaling domains. Such chimeric ILT receptors, or CIRs drive activation of Natural Killer (NK) cells. Further, we have identified unique intracellular activation modules that function to robustly enhance cytotoxicity and expansion of NK cells upon engagement with tumor cells.

Methods

CD56 ⁺ NK cells were isolated from PBMCs, cultured with IL-15 and activated with a feeder-free cocktail of an immobilized cytokine and activating ligand. Activated NK cells were transduced with γ-retroviruses directing expression of CIR proteins (CIR.4-1BB.CD3ζ or CIR.X.Y where X and Y are candidate activation and coactivation moieties), soluble IL-15 and a ΔCD19 marker or CAR control constructs directed to HLA-G or CD33. After 8 days of expansion, CIR- or CAR-NK cells were cocultured with GFP ffluc-expressing AML target cell lines with or without transgenic HLA-G (G1, G2 and G5) isoforms. Cytotoxicity of CIR-NK cells was determined by reduction of GFP fluorescence in Incucyte or Celligo instruments or luciferase activity and supernatants were analyzed for IFN-γ and TNF-α release by ELISA. Transduced NK cells were labeled with nuclear-RFP for measurement of expansion in coculture. In comparative experiments, T cells from PBMCs were transduced to produce CIR-T cells by standard methods.

Results

CIR (ILT2 or ILT4.4-1BB.ζ) constructs were efficiently and stably expressed in T and NK cells (>80% ΔCD19 ⁺ 8- and 14-days post activation). Mock-transduced NK cells displayed innate killing activity against HLA-G negative AML cells (THP1 and KG1) that was not augmented by CIR constructs (mock = 6.16E6 ± 0.77E6 vs. CIR = 6.43E6 ± 0.37E6 in coculture against KG1 cells). In cocultures against THP1 and KG1 target cells expressing exogenous G1, G2 or G5 isoforms, CIR-NK cells or CIR-T cells displayed enhanced target cell killing in 2 and 7-day assays relative to mock-transduced effector cells (mock = 1.47E6 ± 0.49E6 vs. CIR = 0.57E6 ± 0.6 in coculture against KG1-HLA-G1). Further, while mock NK cells transduced with RFP alone could kill one-third of AML cells expressing endogenous HLA-G1 and G5 (Molm13 and Kasumi1), CIR-NK cells eliminated more than 90% of these AMLs and their activivation resulted in 4-fold higher IFN-γ secretion. Notably, robust cytotoxicity of CIR-NK cells was maintained even at 28 days post activation (mock = 36.95E6 ± 21.85E6 vs. CIR = 7.38E6 ± 1.92 in coculture against Kasumi1). To further enhance the functionality of the CIR constructs, a screen of novel costimulatory molecules to replace 4-1BB and/or CD3ζ resulted in several potent candidates ultilizing MyD88 signaling that greatly improved the cytotoxicity (> 80% tumor elimination), proliferation (> 2-fold expansion) and proinflammatory cytokine production (IFN-γ and TNF-α) of NK cells in cocultures. A subset is displayed in Figure 1.

Conclusions

The use of natural receptors ILT2 and ILT4 to target HLA-G takes advantage of the physiological pairing of the ILT receptors with the multiple immunosuppressive HLA-G isoforms thereby circumventing the possibility of antigen escape often observed in CAR-directed therapy. We demonstrated that engineered CIR-NK cells can convert HLA-G expression in AML from a potent inhibitor of leukocyte activity into a target for anti-tumor control.