Ligation of LFA-1 on U937 cells decreased the expression of VLA-4 with activated conformation. U937 cells were cultured with plain medium (black profile) or treated with 500 ng/mL, 250 ng/mL, or 50 ng/mL soluble ICAM-1-FC, and 500 ng/mL soluble FC protein or Mn²⁺ (1 mM) and then stained with (A) a conformation-independent anti-VLA-4 antibody (HP1/2) or (B) a ligand-induced binding site-specific VLA-4 antibody (HUTS-4). U937 cells were then analyzed with flow cytometry. The mean fluorescence intensities (MFIs) of the ligand-induced binding site-specific antibody staining for Mn²⁺, 500 ng/mL FC, 50, 250, and 500 ng/mL ICAM-1-FC-treated cells are 61.51, 46.81, 19.94, 14.78, and 12.52, respectively. (C) U937 cells were cultured with plain medium (black profile) or treated with 50 ng/mL, 100 ng/mL, or 200 ng/mL TS1-18, and 200 ng/mL isotype-matched control mAb or Mn²⁺ (1 mM) and then incubated with VCAM-1-FC protein and a FITC-labeled anti-FC antibody. U937 cells were then analyzed with flow cytometry. The MFIs of the VCAM-1-FC protein binding for Mn²⁺ and 50, 100, or 200 ng/mL TS1-18-treated cells are 34.30, 20.33, 15.62, 9.67, and 8.78, respectively. This experiment was repeated 3 times with similar results.