Stromal Fibroblasts Mediate Anti-PD-1 Resistance via MMP-9 and Dictate TGF-β Inhibitor Sequencing in Melanoma

Abstract

Although anti-PD-1 therapy has improved clinical outcomes for select patients with advanced cancer, many patients exhibit either primary or adaptive resistance to checkpoint inhibitor immunotherapy. The role of the tumor stroma in the development of these mechanisms of resistance to checkpoint inhibitors remains unclear. We demonstrated that pharmacological inhibition of the TGF-β signaling pathway synergistically enhanced the efficacy of anti-CTLA-4 immunotherapy but failed to augment anti-PD-1/PD-L1 responses in an autochthonous model of BRAF(V600E) melanoma. Additional mechanistic studies revealed that TGF-β pathway inhibition promoted the proliferative expansion of stromal fibroblasts, thereby, facilitating MMP-9-dependent cleavage of PD-L1 surface expression, leading to anti-PD-1 resistance in this model. Further work demonstrated that melanomas escaping anti-PD-1 therapy exhibited a mesenchymal phenotype associated with enhanced TGF-β signaling activity. Delayed TGF-β inhibitor therapy, following anti-PD-1 escape, better served to control further disease progression and was superior to a continuous combination of anti-PD-1 and TGF-β inhibition. This work illustrates that formulating immunotherapy combination regimens to enhance the efficacy of checkpoint blockade requires an in-depth understanding of the impact of these agents on the tumor microenvironment. These data indicated that stromal fibroblast MMP-9 may desensitize tumors to anti-PD-1 and suggests that TGF-β inhibition may generate greater immunologic efficacy when administered following the development of acquired anti-PD-1 resistance.