Loss of tumor cell MHC class II drives MAPK inhibitor insensitivity of BRAF-mutant anaplastic thyroid cancers

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Abstract

Cancer cells present neoantigens dominantly through MHC class I (MHCI) to drive tumor rejection through cytotoxic CD8+ T cells. There is growing recognition that a subset of tumors express MHC class II (MHCII), causing recognition of antigens by TCRs of CD4+ T cells that contribute to the antitumor response. We found that mouse BrafV600E-driven anaplastic thyroid cancers (ATCs) responded markedly to the RAF plus MEK inhibitors dabrafenib and trametinib (dab/tram) and that this was associated with upregulation of MhcII in cancer cells and increased CD4+ T cell infiltration. A subset of recurrent tumors lost MhcII expression due to silencing of Ciita, the master transcriptional regulator of MhcII, despite preserved IFN-γ signal transduction, which could be rescued by EZH2 inhibition. Orthotopically implanted Ciita–/– and H2-Ab1–/– ATC cells into immune-competent mice became unresponsive to the MAPK inhibitors. Moreover, depletion of CD4+, but not CD8+, T cells also abrogated the response to dab/tram. These findings implicate MHCII-driven CD4+ T cell activation as a key determinant of the response of Braf-mutant ATCs to MAPK inhibition.

Authors

Vera Tiedje, Jillian Greenberg, Tianyue Qin, Soo-Yeon Im, Gnana P. Krishnamoorthy, Laura Boucai, Bin Xu, Jena D. French, Eric J. Sherman, Alan L. Ho, Elisa de Stanchina, Nicholas D. Socci, Jian Jin, Ronald A. Ghossein, Jeffrey A. Knauf, Richard P. Koche, James A. Fagin