U937 cells expressed both very late antigen (VLA) 4 (α4β1 heterodimer) and VLA-5 (α5β1 heterodimer) integrin but adhered to FN primarily by means of VLA-5 integrin.

(A) Cell-surface expression of integrin subunits was measured using flow cytometry. The mean fluorescence of 10 000 events for each subunit (3 independent experiments) is shown. (B) Blocking antibodies were used to determine the specificity of binding of U937 cells to FN. The presence of the α5 or the β1 blocking antibody reduced adhesion of U937 cells to levels comparable to those with BSA. In contrast, the α4 blocking antibody did not reduce cellular adhesion to FN.