(A–C) T/B cocultures. Purified B cells were cultured alone or with flow-sorted ICOS⁺CD4⁺ and ICOS^–CD4⁺ T cells from patients with melanoma after CCB treatment (Supplemental Methods). (A) Representative flow plot of CD38⁺CD27⁺ plasmablast. (B) Human IgG levels by ELISA (n = 9). Kruskal-Wallis with Dunn’s multiple comparisons correction. (C) Effect of B cell subsets. Flow-sorted posttreatment T cells were cultured with memory(n = 5) or naive B cells (n = 6). Bar graphs indicate fold change compared with cultures with B cells alone. One-way ANOVA with Tukey’s multiple test correction. (D–F) Human T/B interactions in vivo. Sorted ICOS⁺CD4⁺ or ICOS^–CD4⁺ T cells from patients with melanoma (n = 5) were injected into MISTRG6 mice along with purified human B cells (Supplemental Methods). (D) Representative flow plots of plasmablast frequency in the spleen at 2 weeks after injection, as quantified in E. One-way ANOVA with Dunn’s multiple correction. (F) Human IgG levels detected by ELISA in mice from E. Wilcoxon’s matched-pairs signed-rank test. (G) Bar graph shows expansion of ICOS⁺CD4⁺ T cells after CCB therapy in patients with no HG-irAEs and those with HG-irAEs. Wilcoxon’s matched-pairs rank test. (H) B cells were cultured with flow-sorted ICOS⁺CD4⁺ and ICOS^–CD4⁺ T cells from patients with HG-irAEs or no HG-irAEs. Bar graph shows fold change in plasmablasts in ICOS⁺ versus ICOS^– cocultures. Mann-Whitney. (I) FlowSOM performed on ICOS⁺CD4⁺ T cells obtained following 1 cycle of combination therapy identified 10 metaclusters (MCs). Bar graph shows proportions of these MCs in patients with HG-irAEs or no HG-irAEs. Mann-Whitney. (J) Depletion of Tregs in patients with no HG-irAEs leads to enhanced B cell activation by ICOS⁺CD4⁺ T cells. Purified B cells were cultured with ICOS^–, bulk ICOS⁺, or Treg-depleted ICOS⁺CD4⁺ T cells from patients with no HG-irAEs. Dot plot shows plasmablast frequency as percentage of B cells. Bar graph shows data for 7 different experiments. Wilcoxon’s matched-pairs rank test. *P < 0.05, **P < 0.01, ***P < 0.001.