Exhaustion of CD8+ T cells determines Trametinib efficacy in RAS-mutated head and neck cancers

Acquisition of resistance to anti-cancer therapies is associated immune-escape from the anti-tumor machinery. Here, we investigated the immune-escape from MEK inhibitors in three KRAS-mutated head and neck cancer (HNC) cell lines models These KRAS-driven cell-lines are sensitive to MEK1/2 inhibitor, Trametinib, in vitro and in vivo. In vivo, Trametinib treatment induced a stable-disease, but over time resistance was developed and tumor progression was detected. We demonstrated that Trametinib-efficacy is dependent on the adaptive-immune system, as depletion of CD8+ T-cells was sufficient to limit Trametinib-efficacy. Mechanistically, we showed that Trametinib treatment induces a temporary-immune-activation that includes infiltration of activated CD8+ T-cells into the tumor, but following chronic treatment with Trametinib a re-establishment of immuno-suppressive environment appeared and CD8+ T-cells become exhausted with a high expression of PD-1. The heterogeneity of the immune-cell in the tumor-ecosystem was fully mapped by CyTOF analysis. In addition, we showed that Trametinib-treatment induces upregulation of PD-L1 by the tumor-cells, but knockout of PD-L1 expression did not affect Trametinib efficacy. However, blocking PD1/PD-L1 axis using anti-PD-1 together with Trametinib induced tumor-shrinkage that was associated with massive infiltration of CD8+ T-cells in the tumor site. The schedule of drug treatment was explored, and all tumor-bearing mice were cured when treated with chronic or pulsive of Trametinib and continuesly with anti-PD1. These cured-mice showed a long-term immune memory. Altogether, we suggest that co-blocking the PD1/PD-L1 axis with MAPK-targeted drugs may be a new alternative treatment for HNC patients with mutations in KRAS.