Expression of mouse CD99 on endothelial cells. (A) Immunoperoxidase staining of cryostat sections of mouse heart, kidney, and peripheral lymph nodes with affinity-purified antibodies against mouse CD99, mouse ESAM, and control rabbit IgG (co-IgG) from preimmune serum (as indicated). Sections of heart and kidney were incubated with first antibodies, followed by washing and incubation with secondary and tertiary reagent. In order to avoid lymphocyte staining in the lymph nodes, mice were injected intravenously with first antibody, anesthetized 15 minutes later, and perfused with PBS to remove unbound antibody and then with PFA to fix bound antibody. Cryostat sections of lymph nodes were then incubated with only secondary and tertiary reagent. Arrowheads point to high endothelial venules (HEVs), and arrows to capillaries. Bar = 70 μm. (B) Immunofluorescence staining of mouse bEnd.5 endothelioma cells with affinity-purified antibodies against CD99, mAb against VE-cadherin, and negative control IgG from the respective preimmune serum (as indicated). First antibodies were detected with Cy3-conjugated secondary antibodies and visualized by fluorescence microscopy. Bar = 20 μm.