JAM-C mediates adhesion of U937 cells to RA synovium. Stamper-Woodruff in situ assays were performed using frozen RA ST sections and fluorescence-labeled U937 cells. Left, Mean and SEM percentage of maximal binding, defined as the number of adherent cells on the test sections divided by the number of adherent cells on the control sections, of U937 cells by IgG, F26, H33, and anti–P-selectin. U937 cell adhesion was inhibited to a similar degree by both of the anti–JAM-C antibodies, F26 (49% of maximal binding) and H33 (60% of maximal binding), and by anti–P-selectin (41% of maximal binding). Right, Most U937 cells adhered to the blood vessels and synovial lining (arrows) (original magnification × 400). See Figure 1 for definitions.