Lower frequencies of circulating suppressive regulatory T cells and higher frequencies of CD4⁺ na?ve T cells at baseline are associated with severe immune-related adverse events in immune checkpoint inhibitor-treated melanoma

Background Immune-related adverse events (irAEs) are major barriers of clinical management and further development of immune checkpoint inhibitors (ICIs) for cancer therapy. Therefore, biomarkers associated with the onset of severe irAEs are needed. In this study, we aimed to identify immune features detectable in peripheral blood and associated with the development of severe irAEs that required clinical intervention. Methods We used a 43-marker mass cytometry panel to characterize peripheral blood mononuclear cells from 28 unique patients with melanoma across 29 lines of ICI therapy before treatment (baseline), before the onset of irAEs (pre-irAE) and at the peak of irAEs (irAE-max). In the 29 lines of ICI therapy, 18 resulted in severe irAEs and 11 did not. Results Unsupervised and gated population analysis showed that patients with severe irAEs had a higher frequency of CD4(+) naive T cells and lower frequency of CD16(+) natural killer (NK) cells at all time points. Gated population analysis additionally showed that patients with severe irAEs had fewer T cell immunoreceptor with Ig and ITIM domain (TIGIT(+)) regulatory T cells at baseline and more activated CD38(+) CD4(+) central memory T cells (TCM) and CD39(+) and Human Leukocyte Antigen-DR Isotype (HLA-DR)(+) CD8(+) TCM at peak of irAEs. The differentiating immune features at baseline were predominantly seen in patients with gastrointestinal and cutaneous irAEs and type 1 diabetes. Higher frequencies of CD4(+) naive T cells and lower frequencies of CD16(+) NK cells were also associated with clinical benefit to ICI therapy. Conclusions This study demonstrates that high-dimensional immune profiling can reveal novel blood-based immune signatures associated with risk and mechanism of severe irAEs. Development of severe irAEs in melanoma could be the result of reduced immune inhibitory capacity pre-ICI treatment, resulting in more activated TCM cells after treatment.