Abstract 6707: Inhibition of proteolytic cleavage of NKG2D ligands increases immune cell-mediated cell killing of tumor cells in vitro

Abstract

Abstract Natural Killer Group 2, member D (NKG2D) is an activating receptor that is expressed on the key immune cells involved in the cytotoxic arm of the immune system. Ligands of NKG2D, such as MICA and MICB, are not normally expressed but are upregulated in response to stress, such as oxygen or nutrient deprivation, viral infection or oncogenic transformation. One mechanism that tumor cells, that express MICA and MICB, have adopted to evade the cytotoxic potential of NK and T cells, that express the NKG2D receptor, is to cleave MICA/B from the cell surface. Proteolytic cleavage of MICA/B occurs within the α-3 domain of MICA/B. In order to evaluate the therapeutic potential of blocking cleavage of MICA/B we have generated multiple antibodies that bind to the α-3 domain of MICA/B. The lead antibody, PDI-01, was found to bind to multiple recombinant human MICA alleles and human MICB and to recombinant MIC proteins derived from non-human primate sequences with low nM affinity. PDI-01 bound to target antigen on the surface of multiple cancer cell lines including HCT-116 colon carcinoma, HCC1534 lung carcinoma and PLC/PRF/5 liver cancer cell lines. Each of those tumor cell lines were positive for MICA and MICB expression. Low expression of other NKG2D ligands, known as UL-16 binding proteins 1-6 (ULBP 1-6), has been shown in those cell lines. PDI-01 was found to prevent cleavage of MICA from the surface of HCT-116, HCC1534 and PLC/PRF/5 cells via ELISA quantification of soluble MICA levels. Exposure of HCT-116, HCC1534 and PLC/PRF/5 tumor cell lines to PDI-01 increased surface MICA levels as measured by flow cytometry analysis. An increase in immune cell mediated killing of HCT-116, HCC1534 and PLC/PRF/5 cells was observed when tumor cells were incubated with NK-92 cells (NKG2D positive) in the presence of PDI-01. PDI-01 also enhanced antibody-dependent cell-mediated cytotoxicity (ADCC) against these three tumor cell lines. Co-culture of PBMCs, or purified NK cells, with PLC/PRF/5 cells in the presence of PDI-01 led to an increase in TNF-α, IFN-γ, CCL4, CCL5 and IL-6. PDI-01 is being advanced into clinical development and the Phase 1 program is scheduled to be initiated in the 2Q, 2020. Citation Format: Safak Yalcin, Vanessa Cicchini, Xin Du, Xin Wu, Tom Kaever, Neil Gibson. Inhibition of proteolytic cleavage of NKG2D ligands increases immune cell-mediated cell killing of tumor cells in vitro [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 6707.