DL4 expression on Ccl19⁺ LN fibroblasts is required for T_FH and GC differentiation. (A) Representative FACS profiles of CD4⁺CXCR5⁺PD1⁺ T_FH cells (top) and GL7⁺CD95⁺ GC B cells (bottom) from control, DL1^Δ/ΔCcl19, DL4^Δ/ΔCcl19, and DL1^Δ/ΔDL4^Δ/ΔCcl19 mice 14 d after immunization with NP-CGG. Percentages in gates represent the frequency of T_FH cells within CD4⁺ T cells and GC B cells within B220⁺ B cells, respectively. (B) Bar diagrams show relative (top) and absolute cell numbers (bottom) of T_FH and GC cells in LNs of mice immunized 14 d earlier with NP-CGG (mean ± SD of 4–5 animals per group). (C) Total (NP₂₃-binding) and high-affinity (NP₄-binding) NP-specific IgG1 antibody levels were measured by ELISA in the sera of control, DL1^Δ/ΔCcl19, DL4^Δ/ΔCcl19, and DL1Δ^Δ/ΔDL4^Δ/ΔCcl19 mice 14 d after immunization (mean ± SD of 4–5 animals per group). (D) Representative FACS profiles of CD4⁺CXCR5⁺PD1⁺ T_FH cells (top; pregated on CD4⁺ cells) and GL7⁺CD95⁺ GC B cells (bottom; pregated on B220⁺ cells) from control and DL4^Δ/ΔCcl19 mice 21 d after immunization with S. mansoni eggs. (E) Bar diagrams represent relative (top) and absolute cell numbers (bottom) of T_FH and GC B cells in LNs of S. mansoni egg-immunized mice (mean ± SD of 4–5 animals per group). (F) SEA-specific IgG1 antibody levels as measured by ELISA and optical diffraction (OD) in the diluted sera of control and DL4^Δ/ΔCcl19 mice at day 21 after immunization with S. mansoni eggs (mean ± SEM of 4–5 animals per group). *, P < 0.05; **, P < 0.01; ***, P < 0.001. Unpaired Student’s t test was used to determine significance. Results are a composite of two independent experiments.