Microenvironment and Immunology Natural Killer Cells Are Essential for the Ability of BRAF Inhibitors to Control BRAF-Mutant Metastatic Melanoma

BRAF is a major oncogenic mutation found in approximately 50% of human melanoma that confers constitutive activation of the MAPK pathway and increased melanoma growth. Inhibition of BRAF by oncogene targeting therapy increases overall survival of patients with melanoma, but is unable to produce many durable responses. Adaptive drug resistance remains the main limitation to BRAF inhibitor clinical efficacy and immune-based strategies could be useful to overcome disease relapse. Tumor microenvironment greatly differs between visceral metastasis and primary cutaneous melanoma, and the mechanisms involved in the antimetastatic efficacy of BRAF inhibitors remain to be determined. To address this question, we developed a metastatic BRAF-mutant melanoma cell line and demonstrated that the antimetastatic properties of BRAF inhibitor PLX4720 (a research analogue of vemurafenib) require host natural killer (NK) cells and perforin. Indeed, PLX4720 not only directly limited BRAF-induced tumor cell proliferation, but also affected NK cell functions. We showed that PLX4720 increases the phosphorylation of ERK1/2, CD69 expression, and proliferation of mouse NK cells in vitro. NK cell frequencies were significantly enhanced by PLX4720 specifically in the lungs of mice with BRAF lung metastases. Furthermore, PLX4720 also increased human NK cell pERK1/2, CD69 expression, and IFNg release in the context of anti-NKp30 and IL2 stimulation. Overall, this study supports the idea that additional NK cell-based immunotherapy (by checkpoint blockade or agonists or cytokines) may combine well with BRAF inhibitor therapy to promote more durable responses in melanoma. Cancer Res; 74(24); 1–11. 2014 AACR.