DL1-expressing Ccl19-Cre⁺ splenic stromal cells are the essential niche cells for MZ B cell differentiation. (A) Representative FACS profile displaying the frequency of splenic MZ B cells (gated as CD21^hiCD23^int) among B220⁺IgM^hi cells in control, DL1^+/ΔCcl19, and DL1^Δ/ΔCcl19 mice. (B) Bar diagrams show relative (left) and absolute numbers (right) of splenic MZ B cells (means ± SD of 5–6 animals per group). (C) Spleen sections of control and DL1^Δ/ΔCcl19 mice were stained for IgM, IgD, and Laminin, as indicated. MS, marginal sinus (laminin⁺); FB, follicular B cells (IgM⁺D⁺); MZB, MZ B cells (IgM⁺D⁻). (D) Representative FACS profiles displaying the frequency of splenic MZ B cells (CD21^hiCD23^int) among B220⁺IgM^hi cells in control, DL1^+/ΔCcl19 and DL1^Δ/ΔCcl19 reverse BM chimeras. (E) Bar diagrams show relative (left) and absolute numbers (right) of splenic MZ B cells in the reverse BM chimeras (means ± SD of 6–7 animals per group). **, P < 0.01; ***, P < 0.001. Unpaired Student’s t test was used to determine significance. Results are a composite of three independent experiments.