An experimental murine model of cGVHD shows expansion of Tph cells in the blood. Irradiated BALB/c recipients were engrafted with TCD-BM alone (no GVHD) or plus splenic Thy1.2⁺ T cells (2 × 10⁵) to induce cGVHD, and approximately 40% of cGVHD recipients survived at 60 days after HCT. (A) PD1 versus CXCR5 expression among CD4⁺T cells at day 60 after HCT in the PBL from recipients with no GVHD or with cGVHD. (B, C) Cluster identification was visualized using UMAP to analyze peripheral blood CD4⁺T cells among 4 recipients with no GVHD and 8 recipients with cGVHD. Fractions of Tph cells were also calculated. (D) Heatmap shows protein expression levels among the clusters in (B). (E) Intracellular expression levels of BCL6, MAF, BLIMP1, and IRF4 were examined among Tph cells from recipients with no GVHD or cGVHD and naïve CD4⁺ T cells from no-GVHD recipients. MFI was also calculated. (F) PBMCs were stimulated with PMA and ionomycin, and subsequent IL-21 expression levels were measured. (G) Exhaustion-related markers PD1, ICOS, TIGIT, CD160, LAG3, and 2B4 were also compared. Data represent mean ± SEM. P values were calculated by 2-way ANOVA with Holm-Sidak test (A, E) and unpaired Student t test (C, F). ∗P < .05; ∗∗P < .01; ∗∗∗P < .001; ∗∗∗∗P < .0001.