First-week expansion of activated peripheral B-cell subsets is associated with grade 3 or higher immune-related adverse events after nivolumab plus ipilimumab

Background: Although immune checkpoint inhibitors (ICIs) cause grade 3 or higher immune-related adverse events (irAEs), validated biomarkers for identifying patients at risk of these events are lacking. Given the potential contribution of B-cells to irAE development, we evaluated whether early changes in circulating B-cell subsets after nivolumab plus ipilimumab were associated with subsequent severe irAEs. Materials and Methods: This prospective observational study included 12 patients who received nivolumab plus ipilimumab, and an exploratory cohort of five patients who received anti-PD-1 monotherapy. Blood was sampled before treatment, at week 1, and before cycle 2. Flow cytometry quantified CD21low B-cells, CD27+IgD- class-switched B-cells, plasmablasts, and CD19+CD138+ plasma cells. irAEs were assessed for 6 months, and fold changes at week 1 were compared between patients with and without severe irAEs. Paired immunoglobulin G B-cell receptor (BCR) repertoires were analyzed in four patients with grade 3 or higher irAEs. One patient developed myasthenia gravis, and anti-acetylcholine receptor (AChR) antibody was measured in serial serum samples. Results: In the nivolumab plus ipilimumab patients, grade 3 or higher irAEs occurred in five patients. Week-1 increases in all four activated B-cell subsets were significantly greater in these patients, whereas total CD19+ B-cell changes did not differ. Rarefied IgG BCR diversity generally increased in repertoires derived from peripheral blood mononuclear cells, while Gini coefficients decreased in all paired repertoires. In the myasthenia gravis case, anti-AChR antibody exceeded the reference range by Day 11, before symptoms and diagnosis. Conclusion: First-week expansion of activated peripheral B-cell subsets after nivolumab plus ipilimumab was associated with subsequent severe irAEs. These findings support future studies of early B-cell monitoring within integrated multimarker assessments of irAE risk.