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3086 Background: Immunotherapies blocking the interaction of CTLA-4 or Programmed Death 1 (PD-1) with their ligands are the standard of care for advanced MEL although many pts fail to benefit from immunotherapy. Herein, we seek to identify epigenetic and genetic signatures associated with lack of response in patients treated with immunotherapy . Methods: We performed whole exome sequencing of 580 cancer-related genes and whole-genome DNA methylation arrays targeting > 850k CpG sites covering promoters, enhancers, and transcription factor sites in 28 MEL samples from patients treated with PD-1 +/- CTLA-4 inhibitors. Findings were correlated with collected clinical history. Results: Findings are summarized in the table . Unsupervised clustering and multi-parametric analysis showed a distinct methylation signature independent of age, sex, stage, site of metastasis, or type of treatment (adj. p

3076 Background: Immune checkpoint inhibitors (ICIs; anti-CTLA-4 and anti-PD-1) have shown clinical success in many cancers, but may cause rare irSAE. The molecular features of irSAE have not been extensively explored. Therefore, we characterized the immune composition of tissue affected by ICI-mediated inflammation with a focus on colitis and neurologic toxicity. Methods: We performed retrospective T-cell receptor (TCRβ) sequencing, RNA-sequencing (HTG EdgeSeq; > 2500 immune-related genes), and digital spatial profiling (NanoString) for 20 protein markers in 10 regions across tissues representing ICI-induced colitis, autoimmune inflammation (e.g. Crohn’s) and normal colon. Matched tumors were also included in a subset. We also analyzed the encephalitic and healthy brain of a unique presentation of anti-PD-1-induced encephalitis. Results: Patient-matched melanoma and colitis biopsies (n = 3) demonstrated shared T cell clones in all samples ranging from several shared clones to several hundred (0.4%, 2.7%, and 3% of rearrangements, respectively), including high-frequency clones. Shared TCRβ sequences were also identified among and between colon-irSAE and Crohn’s specimens. Gene expression patterns of inflammation in colon-irSAE resembled that of Crohn’s disease in principle components and clustering analysis, highlighting likeness between these diseases/SAEs. NanoString digital spatial profiling of regions of inflammation across samples showed higher CD68 and PD-L1 positivity in colon-irSAE specimens versus normal colon or Crohn’s specimens and reduced beta-catenin levels in both Crohn’s and colon-irSAE specimens relative to normal controls. Finally, we detected a high degree of TCR clonality in the encephalitic brain, including a single sequence present in ~20% of > 12,000 T cells, suggesting a distinct antigen-specific response. Conclusions: We report the molecular characteristics of irSAE in colon and brain specimens from patients receiving ICIs. Highly clonal TCRβ sequences were frequently detected, particularly in a unique case of encephalitis-irSAE. Furthermore, we identify molecular distinctions and similarities between autoimmune and colon-irSAEs at the gene expression and proteomic levels.

9522Background: Although cutaneous cBCC and cSCC are the most frequent primary human cancers, primary treatment is nearly always curative, and metastatic disease (mcBCC and mcSCC) rarely develops. ...