The cytokine signaling deficit in FL TILs is restricted to PD-1^hiCD4⁺ T cells and includes T_FH cells and non-T_FH cells. FL LN specimens were cultured with or without IL-4 for 15 minutes and then assayed for IL-4–induced phosphorylation of p-STAT6 by combining CD3-, CD5-, CD4-, and PD-1-specific Abs with p-STAT6 Ab in the phospho-flow cytometric assay. (A) IL-4–induced p-STAT6 was determined in CD4⁺ TILs gated on different expression levels of PD-1. (B) Data are shown as mean FC ± SEM; FL, n = 7. (C) IL-4–induced p-STAT6 was determined in CD4⁺ FL TILs, based on expression of ICOS and CXCR5. Shown is 1 representative case, and (D) mean FC ± SEM; FL, n = 3. (E) IL-4–induced p-STAT6 was determined in CD4⁺ tonsil T cells, based on expression of ICOS and CXCR5. Mean FC ± SEM; tonsil, n = 3. Statistical difference between groups was determined by paired 2-tailed Student t test.