(A) Schematic and quantification of islet-infiltrating, CD44⁺CD4⁺ T cells among mice treated with Iso + vehicle (n = 6), anti–PD-1 + vehicle (n = 8), and anti–PD-1 + ruxolitinib (n = 4), via flow cytometry analysis. (B) Quantification of islet-infiltrating, ICOS⁺PD-1^hiCXCR5⁺CD4⁺ Tfh (left) and IL-21⁺IFN-γ⁺ Tfh cells (right), among mice treated with Iso + vehicle (n = 9–12), anti–PD-1 + vehicle (n = 13–15), and anti–PD-1 + ruxolitinib (n = 5), via flow cytometry. (C) Schematic of proposed action of JAKi on CD4⁺ Tfh cells through blockade of autocrine IL-21 signaling (left). STAT3 phosphorylation in murine CD4⁺ T cells in response to IL-21 (100 ng/mL), ruxolitinib (10 μM), or vehicle in vitro, assessed by flow cytometry (right). (D) Effect of IL-21 receptor genetic deletion in CD4⁺ T cells on Tfh cell induction in vitro, following a 3-day Tfh skew of naive CD4⁺ T cells with anti–PD-1. (E) Frequency of CD4⁺ T cells expressing a Tfh cell phenotype (CD4⁺ICOS⁺PD-1^hiCXCR5⁺) following a 3-day Tfh skew of naive CD4⁺ T cells in the presence of ruxolitinib (10 μM) or vehicle in vitro, assessed by flow cytometric analysis. (F) Expression of canonical Tfh transcription factors Bcl6 and cMAF in murine CD4⁺ T cells following a 3-day Tfh skew in the presence ruxolitinib (Ruxo, 10 μM) or vehicle. (G) Comparison of Tfh cell response in PBMC specimens from ICI-treated individuals at baseline and following a 3-day Tfh skew with JAKi ruxolitinib (Ruxo, 10 μM) or vehicle, measured by flow cytometry. Each point represents data from one replicate (C–F); experiments repeated at least twice or animal (A and B), and data are presented as mean ± SD. For human studies (G), connected points represent data from 1 individual. Comparisons by Brown-Forsythe and Welch ANOVA (A and B), 1-way ANOVA (C and D) with subsequent pairwise comparisons, Welch’s t test (E and F), or 1-way ANOVA for paired samples (G). *P < 0.05; **P < 0.01, ****P < 0.0001.