Abatacept Augments Natural Killer Cell Cytotoxicity in Haploidentical HCT: A Novel Approach for Separating GVL and Gvhd.

Background Abatacept (Abata, CTLA4Ig), was originally explored for blockade of CD28-B7 costimulation pathway in organ transplantation. However, Abata had sparing effect on NK cells, resulting in NK-mediated rejection in preclinical models. We explored this caveat of abatacept to augment NK cell mediated anti-leukemia effect without invoking T cell mediated GVHD, where unmodified DLI were administered 6 hours following administration of Abata in-vivo (Abata-DLI) to patients with R/R leukemia undergoing haploidentical (Haplo)-HCT, on days +7, +21 and +35, resulting in proliferation of mature NK with reduction in both relapse and GVHD. To explore this further, we studied the effect of Abata on cytotoxicity and cytokine profile of NK cells both in-vitro as well as in-vivo (CTRI:2021/08/046552). Methods NK cells and monocytes were isolated from PBMC of 11 healthy donors and 20 haplo-HCT recipients. NK cells ± monocytes, were incubated with IL2, IL15 and IL12/15/18 with or without Abata. They were assessed for cytotoxicity and IFN-g and perforin expression against K562 cell lines at E:T ratio of 5:1 and 1:1 at 72 hrs & 7 days. In HCT recipients, PBMC were obtained at day +20 (baseline), +30, +45, +60, and 6, 12 and 18 months. Cytokine profile was analysed in NKG2A+ and NKG2C+ subsets. Results NK cells from healthy donors showed significant increase in cytotoxicity at 72 hours in presence of Abata+ IL2 and IL15 (Figure1, p<0.01) at all E:T ratios and time points, but not with IL12/15/18, independent of monocytes. Perforin tended to be upregulated with Abata+ IL15.In patients receiving Abata-DLI, cytotoxicity of NK cells was significantly increased compared to baseline at both E:T ratios (Fig 2, P < 0.01), as well as the corresponding donor samples (p<0.05). This was particularly so in 11 patients without any event compared to 4 patients with relapse/GVHD. Cytotoxicity at D +45 and +60 was significantly greater in the Abata-DLI group than the Abata-only group (p<0.01).In Abata-DLI group, IFN-g was significantly upregulated in the NKG2C+ subset at day +60 at 1:1 ratio. Perforin expression was significantly higher at both ratios in the Abata-DLI cohort, for both NKG2C+ and NKG2A+ subsets and IFN-g and perforin, both were upregulated in the NKG2C+ subset at all time points. 3 patients transplanted with active leukemia, assessed beyond day +60, showed greater cytotoxicity compared to the corresponding donors at 6-18 months. Conclusion In this study, Abata was shown to increase cytotoxicity of NK cells, independent of monocytes, in the presence of IL2 and IL15, but not with IL12/15/18. In Haplo-HCT with Abata-DLI, there was marked increase in NK-cytotoxicity with upregulation of both IFN-g and Perforin in the NKG2C+ subset. This provides the rationale for exploring Abata-augmented NK cell therapy as a novel approach to augment NK-mediated GVL effect without invoking T cell mediated GVHD.