Increased adhesion of p100^−/− MZ B cells to VCAM-1. (A) Adhesion of MZ B cells to plate-bound recombinant Fc-VCAM-1 was measured as described in “Cell culture, detection of apoptotic cells, B-cell migration, and adhesion asays.” (B) Increased mRNA expression of cell-adhesion molecules in spleens from p100^−/− mice. Total RNA from wild-type and p100^−/− spleens was analyzed for VCAM-1, ICAM-1, and MAdCAM-1 expression by QRT-PCR. Relative mRNA expression corrected for β-actin levels is shown. Error bars indicate SD from 3 to 4 experiments. Significant differences are indicated (Student t test, *P < .05; **P < .01). Immunohistochemical staining of spleen sections from wild-type and p100^−/− mice for (C) VCAM-1 (DAB brown), (D) MZMs (MARCO⁺; DAB brown), and (E) MAdCAM-1 (MECA-367⁺; Vector Blue). Sections in panels C and D were counterstained with hematoxylin. (F) Immunohistochemical staining of spleen sections (MARCO⁺, DAB brown; MECA-367⁺, Vector Blue) from lethally irradiated C57BL/6 mice that were reconstituted with wild-type (left panel) or p100^−/− bone marrow (right panel). Images were acquired as in Figure 2E, with 20×/0.70 objective (C), 10×/0.40 objective (D,E), and 40×/0.75 objective (F).