Catalase reversed the suppressive effects of LFA-1 ligation on VLA-4 activation. The effects of catalase on VLA-4-mediated adhesion were examined with cell surface analysis of activated VLA-4. U937 cells were treated with (A) 500 μg/mL ICAM-1-FC protein and (B) 200 ng/mL TS1-18 antibody in the presence or absence of catalase (0.3 U/μL) and then stained with (A) a ligand-induced binding site-specific VLA-4 antibody (HUTS-4) or (B) the soluble VCAM-1 protein. U937 cells were then analyzed with flow cytometry. The MFIs of the ligand-induced binding site-specific antibody staining for control FC, ICAM-1-FC, and ICAM-1-FC plus catalase-treated cells are 46.81, 12.52, and 46.67 respectively (A). The MFIs of the VCAM-1-FC protein binding for isotype control antibody-treated, TS1-18 plus catalase-treated, and TS1-18-treated cells are 20.33, 8.78, and 15.58, respectively (B). This experiment was repeated twice with similar results.