Circulating B cell and T cell activation states predict clinical outcomes in melanoma and reveal dynamic immune reinvigoration with checkpoint inhibitor immunotherapy

Background: Nearly half of patients with melanoma do not respond to immune checkpoint inhibitors (CPIs) and many develop immune-related adverse events (irAEs), often forcing treatment discontinuation, and underscoring the need to predict and monitor outcomes. Responses may depend on both B cell and T cell activity. Methods: We performed high-dimensional mass cytometry profiling of coexisting peripheral B cell and key T cell states in treatment-naïve patients and healthy individuals, and paired longitudinal samples from CPI-treated patients, with clinical annotations to define immune correlates of outcomes. Results: CPI-naive patients exhibited reduced CD19+ B cells, reduced B cell (CD21, IL-2, CXCR5) and T cell (CD38, CD27) activation markers, alongside enriched naïve (CD21lo) and double-negative (DN2)-like B cells, CD95+IL-10+plasmablasts, consistent with extrafollicular responses. Concurrently, programmed cell death protein 1 (PD-1)+ and proliferation marker protein-67 (Ki67)+ T cell expansion indicates ongoing activation with features of proliferative exhaustion. Active disease featured increased regulatory CD95 expression on B cells and expanded T follicular helper-like and activated DN (CD4-CD8-) T cells, indicating sustained antigen stimulation. Pretreatment, elevated PD-1+ T cells predicted irAEs, whereas VEGF (vascular endothelial growth factor)+TGF-β (transforming growth factor-β)+ DN T cells were enriched in patients without subsequent toxicity. Pretreatment, plasmablasts, transitional B cells, Forkhead box protein P3 (FoxP3)+ and central memory-like CD8+ T cells correlated with worse overall survival; naïve CD21lo B cells, PD-1+CD8+ T cells and CD4+follicular helper-like T cells predicted shorter event-free survival; CD4+ memory T cells predicted better prognosis, implicating dysregulated differentiation and sustained activation in adverse outcomes. On-treatment, naïve CD21hi B cells, plasmablasts, activated CD8+ and central memory-like CD4+ T cells expanded, indicating de novo humoral responses and cytotoxic T cell invigoration. On-treatment, increased class-switched memory (IgG2+) B and activated T cells predicted improved survival, while persistent naïve and DN B cells were associated with poorer outcomes. Anti-PD-1 monotherapy expanded naïve (CD21hi) B cells and global T cells. Anti-PD-1/anti-LAG-3 (lymphocyte-activation gene 3) combination contracted memory B cells. Conclusions: Melanoma displays aberrant peripheral B and T cell activation, maturation and exhaustion, prominent in active disease. Treatment-induced class-switched B cells and T cell invigoration predict clinical benefit and naïve/DN B cells signify resistance. Coordinated B and T cell responses, especially recurrent extrafollicular B cell and exhausted/regulatory T cell states emerge as candidate indicators of outcome.