Adhesion of red cells and ICAM-4 transfectants to CD11c/CD18 is mediated by ICAM-4. Adhesion assays were as described in “Materials and methods.” (A) The binding of erythrocytes to 1 μmg coated β₂ integrins; (B) the inhibitory effect of anti–ICAM-4 (BS46), anti-CD18 (7E4), and anti-CD11c (3.9) monoclonal antibodies on the adhesion of red cells to purified CD11c/CD18. (C) Wells were coated with 1 μmg purified CD11c/CD18, and the adhesion of parental L cells and different ICAM L-cell transfectants was measured. (D) The effect of antibodies on binding of ICAM-4 L-cell transfectants to 1 μmg coated CD11c/CD18 was studied. The data in panels A and C are presented as a percentage of attached cells (amount of bound cells divided by input of cells). The results in panels B and D are expressed as a relative percentage of bound cells, where 100% is calculated from the total number of cells bound to the CD11c/CD18 in the absence of pretreatment with MAbs. The significance was determined by unpaired Student t test. Controls included unrelated mouse IgG antibody and wells with coated control protein (GPA) or without coated protein (BSA only). Background binding of cells to GPA or BSA was subtracted. The experiments were repeated 3 times with similar results. Data are expressed as mean ± SD, and statistical significances are shown, ***P< .005.