Association between receptor density, cellular activation, and transformation of adhesive behavior of flowing lymphocytes binding to VCAM‐1

We investigated the ability of purified vascular cell adhesion molecule‐1 (VCAM‐1), adsorbed on plastic, to capture and immobilize flowing lymphocytes, and the dependence of adhesive behavior on activation of the counter‐receptor, α4β1 integrin. This integrin/immunoglobulin interaction bound lymphocytes at a wall shear stress at which the β2‐integrin family has previously been found ineffective (> 0.1 Pa), and whereas lymphocytes rolled on lower concentrations of VCAM‐1 (10 μg/ml), they were stationary at high concentrations (100 μg/ml). Activation of α4β1 integrin by Mn2+ or by antibody 12G10 or treatment of lymphocytes with phorbol ester caused transformation to stationary adhesion, and increased binding significantly only at the lower concentrations of VCAM‐1. We thus hypothesized that formation of a high density of ligand between VCAM‐1 and α4β1 integrin actively transformed lymphocyte behavior. This concept was supported by the finding that the proportion of lymphocytes rolling on the higher concentrations of VCAM‐1 increased if cells were pretreated with azide to block energy‐dependent responses, or if intracellular Ca2+ was chelated. However, not all activation responses were equivalent: only phorbol ester induced marked spreading of immobilized cells, and if pretreatment was prolonged, this agent even reduced the efficiency of initial attachment of flowing lymphocytes. Azide treatment had no effect on transformation to stationary adhesion caused by Mn2+ or activating antibody. Thus, different forms of lymphocyte activation were identifiable: external modification of integrin converted rolling to stationary attachment, did not require ATP, and was reversible; high‐density ligand binding induced an energy‐dependent signal for conversion from rolling to stationary attachment, but not spreading; and protein kinase C activation promoted stationary attachment and spreading, but not necessarily capture. VCAM‐1 is thus a versatile adhesion receptor capable of supporting all stages of leukocyte attachment, i.e. rolling, stationary, and spreading, and of ligand‐induced transformation of adhesion, although an additional signal appears necessary to promote lymphocyte spreading and migration.