BRAF Inhibition Alleviates Immune Suppression in Murine Autochthonous Melanoma

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 studies 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.

Co-inhibition of colony stimulating factor-1 receptor and BRAF oncogene in mouse models of BRAFV600E melanoma

“…Consistent with published findings, 14,29,30 PLX4720 suppressed the growth of de novo TBP melanomas, prolonging the survival of tumor-bearing mice (Fig. 1A and Fig.…”

Requirement of Treg-intrinsic CTLA4/PKCη signaling pathway for suppressing tumor immunity

“…Tumor infiltration by Tregs and the resulting increase in intratumoral Treg/CD8 + T cell ratios predicts poor prognosis and negatively correlates with survival in a majority of solid tumors (2,3,37,38). Successful therapeutic strategies often correlate with a reduction of the number of intratumoral Tregs (39,40). Indeed, depletion or blockade of Tregs in experimental cancer models and cancer patients can enhance tumor immunity and clearance by the immune system (41)(42)(43)(44)(45)(46)(47)(48).…”

BRAF-inhibition and tumor immune suppression

“…Our studies, highlighted herein, recently addressed the downstream consequences of BRAF-inhibition on T regs and MDSC populations within tumors, and the resulting implications for T cell-mediated tumor control. 8 To answer these questions we selected the Tyr-CreER T Braf CA Pten lox/lox (Braf/ Pten) model of BRAF V600E -driven melanoma, 9 which we bred onto a C57BL/6 background. 8 Tumors in these mice contained large populations of both MDSCs and T regs , and had a dearth of CD8 C T cells, as has been previously described for human melanomas.…”

“…8 To answer these questions we selected the Tyr-CreER T Braf CA Pten lox/lox (Braf/ Pten) model of BRAF V600E -driven melanoma, 9 which we bred onto a C57BL/6 background. 8 Tumors in these mice contained large populations of both MDSCs and T regs , and had a dearth of CD8 C T cells, as has been previously described for human melanomas. 3 Importantly, these tumors underwent stable growth arrest upon treatment with the BRAF-inhibitor PLX4720, a research analog of vemurafenib.…”

“…Accordingly, we showed that T regs undergo apoptosis at an increased rate following BRAF-inhibition, although MDSC apoptosis was unchanged. 8 BRAF-inhibition may also block the ability of tumor cells to secrete T reg and MDSC chemoattractants. It has been shown that the C-C chemokine ligand 2 (CCL2), a chemokine associated with recruitment of both T Regs and MDSCs, is decreased in the TME following BRAFinhibition.…”