FDC ablation leads to overlap between B cell–rich areas and CCL21-expressing cells. Unimmunized CD21-DTR chimeras were control (Ctrl) or DTx treated for 2 d, and spleen and pLN sections analyzed. (A) In situ hybridization for Ccl21 mRNA (dark blue), detected alone or in combination with staining for B cells (B220; brown). Red dotted lines trace the interface between B cell– and T cell–rich areas, determined based on the adjacent section. (B) Immunohistochemical analysis of CCL21 and B220 in spleen and pLN. Data are representative of at least three mice of each type analyzed in two independent experiments. Bars: (A) 200 µm; (B) 100 µm.