Mechanisms of Lymphocyte Activation

The interactions of phytohemagglutinin (PHA) with normal human lymphocytes were studied utilizing radioiodinated leukoagglutinin (“$I-LPHA) over a concentration spectrum encompassing the entire range of lymphocyte metabolic responses. “jI-LPHA binding was temperature-, pH-, and time-dependent. Ligand association was rapid with at,,* of 3 to 5 min, reaching steady state in 30 min at 22”. Receptor specificity was demonstrated by the high receptor affinity for ‘Z”I-LPHA and by quantitative inhibition of lz51-LPHA binding with LPHA and 9-LPHA but not with concanavalin A or bovine serum albumin. Under our experimental conditions there was no measurable degradation of 12jILPHA and no detectable shedding of lZSI-LPHA receptors or receptor. ““I-LPHA complexes. Equilibrium studies of YLPHA interactions with specific lymphocyte membrane receptors generated a complex curvilinear Scatchard plot. This, added to progressive deceleration of the dissociation reaction inversely proportional to receptor occupancy by “jI-LPHA, reflects changing receptor affinity for the ligand and suggests site-site interactions of the negative cooperativity type. These interactions which appear to be common to all lymphocyte subpopulations, preclude accurate calculation of lymphocyte binding capacity for lz51-LPHA and of physically meaningful affinity constants. Although the fate and role of a small fraction of apparently nondissociable ““I-LPHA remains to be elucidated, occupancy-dependent receptor affinity for lZ51-LPHA, dissociation of receptor. lz51-LPHA complexes, retention of binding properties by cell-exposed ““I-LPHA, and the large numbers of spare surface receptors for lzSI-LPHA might represent important mechanisms for modulating cell activation by ““I-LPHA.