Activation of CD8+ T cells contributes to anti-tumor effects of CDK4/6 inhibitors plus MEK inhibitors.

ConcurrentMEK and CDK4/6 inhibition shows promise in clinical trials for patients with advanced-stage mutant BRAF/NRAS solid tumors. The effects of CDK4/6 inhibitor (CDK4/6i) in combination with BRAF/MEK-targeting agents on the tumor immune microenvironment are unclear, especially in melanoma, for which immune checkpoint inhibitors are effective in approximately 50% of patients. Here, we show that patients progressing on CDK4/6i/MEK pathway inhibitor combinations exhibit T-cell exclusion. We found that MEK and CDK4/6 targeting was more effective at delaying regrowth of mutant BRAF melanoma in immunocompetent versus immune-deficient mice. Although MEK inhibitor (MEKi) treatment increased tumor immunogenicity and intratumoral recruitment ofCD8(+) Tcells, the main effect of CDK4/6i alone and in combination with MEKi was increased expression of CD137L, a T-cell costimulatory molecule on immune cells. Depletion of CD8thorn T cells or blockade of the CD137 ligand-receptor interaction reduced time to regrowth of melanomas in the context of treatment with CDK4/6i plus MEKi treatment in vivo. Together, our data outline an antitumor immunebased mechanism and show the efficacy of targeting both the MEK pathway and CDK4/6.