The protocol for immunohistochemical staining was as described previously30 (link). Staining reagents included biotinylated PNA (Vector), AlexaFluor 647 anti-B220, biotinylated anti-IgD, AlexaFluor 488 anti-CD45.2 (Biolegend), and streptavidin AlexaFluor 568 (Invitrogen). All stained slides were mounted with ProlongGold antifade reagents (Invitrogen) and examined with an Olympus FV1000 confocal system.