Host immunity contributes to the anti-melanoma activity of BRAF inhibitors

The BRAF mutant, BRAF(V600E), is expressed in nearly half of melanomas, and oral BRAF inhibitors induce substantial tumor regression in patients with BRAF(V600E) metastatic melanoma. The inhibitors are believed to work primarily by inhibiting BRAF(V600E)-induced oncogenic MAPK signaling; however, some patients treated with BRAF inhibitors exhibit increased tumor immune infiltration, suggesting that a combination of BRAF inhibitors and immunotherapy may be beneficial. We used two relatively resistant variants of Braf(V600E)-driven mouse melanoma (SM1 and SM1WT1) and melanoma-prone mice to determine the role of host immunity in type I BRAF inhibitor PLX4720 antitumor activity. We found that PLX4720 treatment downregulated tumor Ccl2 gene expression and decreased tumor CCL2 expression in both Braf(V600E) mouse melanoma transplants and in de novo melanomas in a manner that was coincident with reduced tumor growth. While PLX4720 did not directly increase tumor immunogenicity, analysis of SM1 tumor-infiltrating leukocytes in PLX4720-treated mice demonstrated a robust increase in CD8(+) T/FoxP3(+)CD4(+) T cell ratio and NK cells. Combination therapy with PLX4720 and anti-CCL2 or agonistic anti-CD137 antibodies demonstrated significant antitumor activity in mouse transplant and de novo tumorigenesis models. These data elucidate a role for host CCR2 in the mechanism of action of type I BRAF inhibitors and support the therapeutic potential of combining BRAF inhibitors with immunotherapy.