FP11 Circulating immunoregulatory B- and T-cell profiles in patients with melanoma modulated with checkpoint inhibitor immunotherapy

Introduction and aims We conducted deep immunophenotyping of circulating B- and T-cell repertoires of patients with melanoma at baseline and longitudinally during checkpoint inhibitor (CPI) immunotherapy to assess phenotypic changes and to identify predictive biomarkers of response. Methods A 45-marker mass cytometry panel was utilized to profile circulating adaptive immune features in patients and healthy participants. Paired pretreatment and on-treatment blood samples assessed CPI-induced shifts in B- and T-cell phenotypes, and baseline signatures were used as predictive biomarkers. Results Circulating IL-10+ plasmablasts, double negative (DN), CD21lo B-cell frequencies, and downregulation of key B-cell activation markers (CD21, CXCR5, IL2, CD38) are enriched on B cells from patients with melanoma compared with healthy participants. We observed higher abundance of activated cytotoxic and exhausted (CD8+ PD-1+) and DN (CD4−CD8−) T cells, and higher FoxP3 and transforming growth factor (TGF)β expression by T cells in melanoma. In a patient cohort who received anti-PD1 monotherapy or in combination with anti-LAG3 or anti-CTLA4, we observed a proportional increase in total B-cell frequency, and reduction in memory B cells. We also found expanded activated naïve (CD21hi) B cells and rapidly responding plasmablasts alongside higher expression of B-cell activation markers (CD21, IgD, IgM, CD38, HLA-DR) on the overall B-cell population, suggesting a de novo B-cell response to treatment. Similarly, the T-cell phenotype is altered on treatment favouring activated cytotoxic, immunosuppressive (CD8+ TGFβ+ VEGF+) and exhausted (CD8+ PD-1+) T cells, resting central memory (CD4+) T cells and increased expression of activation markers (CD38, HLA-DR and CD28) by T cells. Baseline enrichment of transitional B cells and plasmablasts, alongside resting CD8+, regulatory (CD4+FoxP3+) and immunosuppressive (VEGF+TGFβ+) T cells were associated with reduced overall survival. Conclusions We report distinct immunosuppressive B- and T-cell signatures in the circulation of patients with melanoma that directly correlate with reduced patient prognosis. Alongside enriched levels of stimulated T cells, we report a de novo B-cell response in patients receiving CPI.