Combinatorial TGF-β signaling blockade and anti-CTLA-4 antibody immunotherapy in a murine BRAF^V600E-PTEN-/- transgenic model of melanoma.

Abstract

3011 Background: T cell-targeted checkpoint inhibitor immunotherapy for melanoma and other solid tumor malignancies only benefits a subpopulation of patients. Our work has shown that melanoma-expressed factors including TGF-β can generate an immune privileged site by suppressing the function of critical dendritic cell populations within the local immune microenvironment. We hypothesized that inhibiting TGF-β signaling in the tumor microenvironment could augment the efficacy of anti-CTLA-4 antibody therapy in a murine transgenic model of melanoma. Methods: Upon primary melanoma development in Tyr::CreER;BrafCA/+;PTENlox/loxmice, the LY2157299 type I TGF-β receptor serine/threonine kinase inhibitor was administered daily by oral gavage accompanied by intra-peritoneal delivery of anti-CTLA-4 monoclonal antibody (mAb) every three days. Tumor development was monitored by caliper and photodocumentation/imaging analysis. After 14 days, tumor tissue, tumor-draining lymph node (TDLN) tissue, and lung tissue was resected for whole tissue Western blot analysis, T cell flow cytometry, immunohistochemistry, and indoleamine 2,3-dioxygenase (IDO) enzymatic activity assays. Results: While LY2157299 and anti-CTLA-4 mAb monotherapy failed to suppress melanoma progression, LY2152799-anti-CTLA-4 mAb combination therapy synergistically suppressed both primary melanoma tumor growth as well as melanoma metastasis in this physiologically-relevant transgenic melanoma model. These observations correlated with significant increases in the CD8+ T cell/CD4+FoxP3+regulatory T (Treg) cell ratio in melanoma tissues. In addition, LY2152799 effectively suppressed TDLN-derived dendritic cell IDO enzyme activity and this was found to correlate with diminished levels of Tregs in both TDLN and primary melanoma tissues. Conclusions: TGF-β signaling inhibition is a promising strategy for augmenting the anti-tumor efficacy of immune checkpoint blockade in melanoma. The pharmacological manipulation of the tumor microenvironment to reverse local immune evasion mechanisms and enhance anti-tumor immune responses represents an important avenue for future studies.