Abstract
A growing body of evidence suggests that BRAF inhibitors, in addition to their acute tumor growth-inhibitory effects, can also promote immune responses to melanoma. The present study aimed to define the immunologic basis of BRAF-inhibitor therapy using the Braf/Pten model of inducible, autochthonous melanoma on a pure C57BL/6 background. In the tumor microenvironment, BRAF inhibitor PLX4720 functioned by on-target mechanisms to selectively decrease both the proportions and absolute numbers of CD4(+)Foxp3(+) regulatory T cells (Treg) and CD11b(+)Gr1(+) myeloid-derived suppressor cells (MDSC), while preserving numbers of CD8(+) effector T cells. In PLX4720-treated mice, the intratumoral Treg populations decreased significantly, demonstrating enhanced apopotosis. CD11b(+) myeloid cells from PLX4720-treated tumors also exhibited decreased immunosuppressive function on a per-cell basis. In accordance with a reversion of tumor immune suppression, tumors that had been treated with PLX4720 grew with reduced kinetics after treatment was discontinued, and this growth delay was dependent on CD8 T cells. These findings demonstrate that BRAF inhibition selectively reverses two major mechanisms of immunosuppression in melanoma and liberates host-adaptive antitumor immunity.