Abstract 3155: Interferon-gamma-induced melanoma plasticity and response to PD-1 blockade therapy

Melanoma is a cancer of melanocytes, which develop from neural crest cells during embryogenesis. One of the ways melanoma can resist targeted therapy or adoptive T cell transfer therapy (ACT) is by dedifferentiation, whereby the tumor becomes less melanocytic and more neural crest-like. ACT-induced dedifferentiation and therapy resistance was a result of tumor necrosis factor (TNF) from antitumor T cells (Landsberg, Nature 2012), which can serve as a positive control for inflammatory cytokine-induced melanoma plasticity. To study whether immune checkpoint blockade induces melanoma dedifferentiation, we analyzed RNA-seq data from paired biopsies of 68 patients before and during anti-PD-1 therapy. Surprisingly, the tumors of patients with complete or partial response dedifferentiate following therapy (paired p= 0.00015), but those of stable or progressive disease do not (paired p= 0.31 and 0.32, respectively). As the presence of interferon-gamma signatures in biopsies is best correlated with response to anti-PD-1 therapy, we hypothesized that interferon-gamma mediates dedifferentiation. We dedifferentiated four human melanoma cell lines by in vitro exposure to long-term (three to five weeks) interferon-gamma and compared it to three days of TNF as positive control. Although TNF and long-term interferon-gamma induced similar phenotypic changes based on flow cytometry, ATAC-seq showed that they each drive different global chromatin remodeling. Motif enrichment analyses revealed STAT6, TFAP2C, and SOX proteins are key transcription factors involved in interferon-gamma-induced dedifferentiation. Inferred protein activity analysis of the matching RNA-seq data showed that these regulators, as well as IRF3 and HMGA1, alter in activity selectively with dedifferentiation. In conclusion, dedifferentiation may be a marker of positive response to anti-PD-1 therapy, mediated by chronic exposure to T cells producing interferon-gamma. Citation Format: Yeon Joo Kim, Katherine M. Sheu, Jennifer Tsoi, Gabriel Abril-Rodriguez, Catherine Grasso, Alexander Hoffmann, Stephen T. Smale, Thomas G. Graeber, Cristina Puig-Saus, Antoni Ribas. Interferon-gamma-induced melanoma plasticity and response to PD-1 blockade therapy [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 3155.