Abstract B27: Kit inhibition decreases tumoral MHC class I expression in gastrointestinal stromal tumors through reduction of type I interferon signaling
Authors: Liu Mengyuan , Medina Benjamin D , Vitiello Geraldo A , Etherington Mark S , Param Nesteene J. , DiLolle Mia E , Bowler Timothy G. , Rossi Ferdinando , DeMatteo Ronald P. 
Published: June 2021

Abstract
Introduction: MHC class I (MHC I) expression is necessary for the antitumoral lymphocyte response but its relationship to oncogene signaling is not well characterized. Gastrointestinal stromal tumor (GIST) is a common sarcoma caused by an activating Kit mutation and is sensitive to the specific Kit inhibitor imatinib. Using KitV558Î”/+ mice, a murine model of GIST, we found imatinib reduced MHC I expression on tumor cells, which we hypothesized was due to attenuation of type I interferon (IFN) signaling in the setting of oncogene inhibition.
Methods: Using flow cytometry and Western blot, we studied tumoral MHC I expression in human GIST cell lines T1 and HG129 under conditions of IFN stimulation and imatinib inhibition. We applied the same methods in vivo in KitV558Î”/+ mice. To show that IFN signaling was responsible for tumoral MHC I expression, we used a blocking anti-IFNAR antibody to abrogate IFN signaling in KitV558Î”/+ mice. Finally, we validated our findings with RNA seq of 35 Kit exon 11 human GIST.
Results: In human GIST cell lines, IFNα (1000U/mL) stimulated MHC I expression at 24h as measured on cell surface by mean florescence intensity (MFI) and by Western blot. This was accompanied by increased Stat1 and pStat1 protein. Imatinib (100nM) inhibited Stat1 activity and reduced MHC I expression by 12-34% on MFI, with and without IFNα stimulation. When Stat1 was knocked down with SiRNA, imatinib did not further decrease MHC I. In vivo, KitV558Δ/+ mice treated with imatinib had 50% reduction in tumoral MHC I expression MFI by flow (n=5/group). Concurrently, imatinib diminished mRNA expression of IFNb1 in bulk KitV558Δ/+ tumors by 8-fold. To demonstrate that IFN signaling contributed to GIST MHC I expression, we administered a blocking anti-IFNAR antibody in KitV558Δ/+ mice and found MHC I decreased by 34% on MFI, along with decreased pStat1 on western blot. In human GIST, imatinib significantly decreased MHC I gene expression on RNA seq (p = 0.03) and reduced IFNb1 mRNA by 77% (p = 0.04). Furthermore, there was significant correlation between MHC I genes and Stat1 expression (p&lt; 0.0001). Taken together, this suggests that oncogene-dependent IFN signaling is responsible for MHC I expression in GIST.
Conclusion: Kit oncogene inhibition reduced MHC I expression in GIST through decreased type I interferon production and signaling via Stat1. Thus, targeted therapies may reduce the antitumor immune response.
Citation Format: Mengyuan Liu, Benjamin D Medina, Geraldo A Vitiello, Mark S Etherington, Nesteene J. Param, Mia E DiLolle, Timothy G. Bowler, Ferdinando Rossi, Ronald P. DeMatteo. Kit inhibition decreases tumoral MHC class I expression in gastrointestinal stromal tumors through reduction of type I interferon signaling [abstract]. In: Proceedings of the AACR Special Conference on Tumor Immunology and Immunotherapy; 2018 Nov 27-30; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Immunol Res 2020;8(4 Suppl):Abstract nr B27.