Baseline peripheral immune signatures associate with toxicity to immunotherapy in melanoma

Background: Adjuvant immune checkpoint inhibitors (ICIs) improve recurrence-free survival in high-risk melanoma. Despite this, a subset of patients do not respond to ICIs while other experience immune-related adverse events (irAEs) without therapeutic benefits. Predictive biomarkers remain undefined, and grouping patients without accounting for irAE status may obscure meaningful immune correlates. We hypothesized that comprehensive immune profiling of baseline peripheral blood mononuclear cells (PBMCs) may help identify predictive immune signatures associated with both response and toxicity to adjuvant ICI. Methods: Blood samples were collected from a total of 22 melanoma patients undergoing adjuvant ICI therapies at baseline and on-treatment. PBMCs were isolated using standard density gradient centrifugation and stained using two comprehensive immunophenotyping panels. Data was acquired on FACS symphony A5. FlowJo's built-in UMAP plugin was used to visualize immune cell clustering and further characterized using Marker Enrichment Modeling. Response to therapy was determined retrospectively. Immune subsets were compared between responders and non-responders at the two time points and was further stratified by occurrence of irAEs. Results: At baseline, responders demonstrated enrichment of central memory T cells and terminally differentiated effector memory T cells, consistent with a primed yet regulated immune state poised for rapid activation. In contrast, non-responders had elevated CD16+ MDSCs, which further expanded on treatment, indicating a suppressive peripheral milieu. Among responders, those developing irAEs demonstrated increased memory-like T cells and CD14+CD62P+ myeloid populations compared to those with no irAEs. Among non-responders, irAE-positive patients displayed higher CD45RA+CD4+CD62L+ memory and CD8+CD57+ effector T cell populations. Within the myeloid compartment, CD62P+ cells were increased in both responders and non-responders with irAEs highlighting platelet-myeloid activation. Non-responders without irAEs showed globally low activation, consistent with an immune quiescent state. No significant treatment induced changes distinguished response categories, underscoring the importance of baseline immune context. Conclusions: Baseline peripheral immune composition reflects distinct states of immune readiness in melanoma patients receiving adjuvant ICI. Responders display a primed immune profile whereas non-responders exhibit myeloid driven suppression. These findings highlight peripheral immune features that may predict therapeutic efficacy and toxicity in melanoma patients in the adjuvant ICI setting. Importantly, stratifying patients by irAE status is essential to reveal true immune correlates of efficacy and toxicity, as pooled analyses risk conflating biologically distinct activation and suppression states.