Tumor microenvironment changes leading to resistance of immune checkpoint inhibitors in metastatic melanoma and strategies to overcome resistance

Immunotherapy with checkpoint inhibitors targeting CTLA-4 and/or PD-1 receptors independent of the BRAF mutational status and targeted therapy with BRAF and MEK inhibitors in BRAF V600 mutated patients have taken the forefront of advanced melanoma treatment. The main advantage of immunotherapy is its ability to provide durable responses in a subset of patients. However, significant proportions of patients either do not respond or have progression after initial response to immunotherapies. Multiple changes in the tumor microenvironment, such as down regulation of immune checkpoint ligands by tumor, alteration in interferon signaling, and activation of alternate immune suppressive pathways, have been identified as possible reasons for failure of immune checkpoint therapy. Here, we review the resistance mechanisms adopted by cancer cells to checkpoint inhibitor therapy and targeted therapy. In addition, we focus on the available and emerging evidence on tumor microenvironment modulation by BRAF/MEK inhibitor therapy and its role in improving responses to checkpoint inhibitor therapy.