PDCD1 and melanoma: Resistance to BRAF/MEKi is associated with upregulation of PD-L1 on both melanoma cells and immune cells [21, 22], a reduction in the ratio of CD8 + T-cell to T-regulatory cells [14, 15, 17] and an increase in T-cell exhaustion markers such as TIM-3 and PD-1 [17].