CONCLUSIONS Glucocorticoid excess affects significantly more genes in ACC than other hormones. In GC+ ACC, expression of steroid synthesis genes was elevated while immune-related genes were suppressed. o Normal adrenal cells express steroid synthesis genes but not immune-related genes. o Steroid synthesis genes were hypomethylated in GC+ cases, while no difference in methylation between GC+ and GC- cases was found for immune genes. o Furthermore, fewer infiltrating immune cells (T cells and NKT cells) were found in GC+ cases compared to GC-, suggesting that immune effects are due to changes in immune cell infiltrate rather than transcriptional changes. Tumor mutation burden was higher in GC+ ACC cases, which may be caused by the observed immune suppression or immune cell exclusion that may, in turn, be related to higher tolerance of non-self-antigens in GC+ cases. A published GR activity score showed no difference between GC+ and GC- cases, which may be due to locally high concentrations of GC in the adrenal gland regardless of systemic GC levels. o In contrast, immune infiltration into ACC tumors may be negatively affected by the exposure of lymph nodes to elevated GC activity. A newly derived gene signature predicts the highest frequency of GC+-like tumors in uveal and skin cutaneous melanomas. The observed reduced abundance of immune cells and immune-related transcripts in GC+ ACC provides insight into the mechanisms by which GC may limit response to ICI therapy. o GR antagonism may increase immune related transcripts or immune cell infiltration, thus promoting tumor immune response in GC+ ACC and other malignancies with elevated GC activity. o This hypothesis will be tested in a phase 1 oncology trial of relacorilant combined with ICI (CORT125134-551, NCT04373265).