Inactivation of DL1 in PDGFβ⁺ BECs or CD11c⁺ DCs does not perturb MZ B cell development. (A) Spleen sections of control (DL1^lox/lox) and induced DL1^{PDGFβiCreERT} mice were stained for Collagen IV and GFP. Collagen IV stains basement membranes, most notably around blood vessels and in the marginal sinus. GFP is expressed bicistronically by the PDGFβ-promoter transgene construct. The dashed boxes are represented as a higher magnification on the right or below. B, B cell follicle; T, T cell zone; CA, central arteriole; MS, marginal sinus. Arrows indicate BECs and arrowheads the marginal sinus. (B) DL1 deletion PCR on sorted CD45⁻Ter119⁻CD31⁺ endothelial cells from spleens of control and tamoxifen-injected DL1^Δ/ΔPDGFβ mice. (C) Representative flow cytometry profile of MZ B cells (gated as CD21^hiCD23^int), pregated on B220⁺IgM^hi cells, in spleens of control and tamoxifen-injected DL1^Δ/ΔPDGFβ mice. (D) Bar diagrams show relative (left) and absolute cell number (right) of MZ B cells (MZB) in spleens of control and TAM-injected DL1^Δ/ΔPDGFβ mice (means ± SD of 4–5 animals per group). One of two representative experiments is shown. (E) Bar diagrams show relative (left) and absolute number (right) of splenic MZ B cells of control and DL1^Δ/ΔCD11c mice (means ± SD of 4–5 animals per group).