Tumor cells evade the immune surveillance by up-regulating surface expression of PD-L1, which interacts with PD-1 on T cells to elicit the immune checkpoint response1,2.,Anti-PD-1 antibodies have shown remarkable promise in treating tumors, including metastatic melanoma2-4.,However, patient response rate is low4,5.,A better understanding of PD-L1-mediated immune evasion is needed to predict patient response and improve treatment efficacy.,Here we report that metastatic melanoma releases a high level of extracellular vesicles (EVs), mostly in the form of exosomes, that carry PD-L1 on their surface.,Interferon-γ (IFN-γ) up-regulates PD-L1 on these vesicles, which suppresses the function of CD8 T cells and facilitates tumor growth.,In patients with metastatic melanoma, the level of circulating exosomal PD-L1 positively correlates with that of IFN-γ, and changes during the course of anti-PD-1 therapy.,The magnitudes of the early on-treatment increase in circulating exosomal PD-L1, as an indicator of the adaptive response of the tumor cells to T cell re-invigoration, stratifies clinical responders from non-responders.,Our study unveils a mechanism by which tumor cells systemically suppress the immune system, and provides a rationale for the application of exosomal PD-L1 as a predictor for anti-PD-1 therapy.
Therapies targeting signaling molecules mutated in cancers can often have striking short-term effects, but the emergence of resistant cancer cells is a major barrier to full cures1,2.,Resistance can result from a secondary mutations3,4, but other times there is no clear genetic cause, raising the possibility of non-genetic rare cell variability5-11.,Here, we show that melanoma cells can display profound transcriptional variability at the single cell level that predicts which cells will ultimately resist drug treatment.,This variability involves infrequent, semi-coordinated transcription of a number of resistance markers at high levels in a very small percentage of cells.,The addition of drug then induces epigenetic reprogramming in these cells, converting the transient transcriptional state to a stably resistant state.,This reprogramming begins with a loss of SOX10-mediated differentiation followed by activation of new signaling pathways, partially mediated by activity of Jun-AP-1 and TEAD.,Our work reveals the multistage nature of the acquisition of drug resistance and provides a framework for understanding resistance dynamics in single cells.,We find that other cell types also exhibit sporadic expression of many of these same marker genes, suggesting the existence of a general rare-cell expression program.