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 molecul e-1 (VCAM-1), adsorbed on plastic, to capture and immobilize flowing l ymphocytes, and the dependence of adhesive behavior on activation of t he counter-receptor, alpha(4) beta(1) integrin. This integrin/immunogl obulin interaction bound lymphocytes at a wall shear stress at which t he beta(2)-integrin family has previously been found ineffective (>0.1 Pa), and whereas lymphocytes rolled on lower concentrations of VCAM-1 (10 mu g/ml), they were stationary at high concentrations (100 mu g/m l). Activation of alpha(4) beta(1) integrin by Mn2+ or by antibody 12G 10 or treatment of lymphocytes with phorbol ester caused transformatio n to stationary adhesion, and increased binding significantly only at the lower concentrations of VCAM-1. We thus hypothesized that formatio n of a high density of Ligand between VCAM-1 and alpha(4) beta(1) inte grin actively transformed lymphocyte behavior. This concept was suppor ted by the finding that the proportion of lymphocytes rolling on the h igher concentrations of VCAM-1 increased if cells were pretreated with azide to block energy-dependent responses, or if intracellular Ca2+ w as chelated. However, not all activation responses were equivalent: on ly 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 eff ect on transformation to stationary adhesion caused by Mn2+ or activat ing antibody. Thus, different forms of lymphocyte activation were iden tifiable: external modification of integrin converted rolling to stati onary attachment, did not require ATP, and was reversible; high-densit y ligand binding induced an energy-dependent signal for conversion fro m rolling to stationary attachment, but not spreading; and protein kin ase C activation promoted stationary attachment and spreading, but not necessarily capture. VCAM-1 is thus a versatile adhesion receptor cap able of supporting all stages of leukocyte attachment, i.e. rolling, s tationary, and spreading, and of ligand-induced transformation of adhe sion, although an additional signal appears necessary to promote lymph ocyte spreading and migration.