STUDIES OF MECHANISM OF HUMAN PLATELET-RELEASE REACTION INDUCED BY IMMUNOLOGICAL STIMULI .3. RELATIONSHIP BETWEEN BINDING OF SOLUBLE IGG AGGREGATES TO FC RECEPTOR AND CELL RESPONSE IN PRESENCE AND ABSENCE OF PLASMA

  • 1 January 1977
    • journal article
    • research article
    • Vol. 118  (2) , 514-524
Abstract
Aggregated Ig[immunoglobulin]G coupled covalently with bis-diazobenzidine (BDB-IgG) and labeled with 3H-diazobenzene (3H-BDB-IgG) was used to study the binding of soluble IgG aggregates to human platelets in relationship to the release of the contents of intracellular granules (e.g., serotonin). In washed cell suspensions a minimum of 0.14-0.2 .mu.g 3H-BDB-IgG/5 .times. 108 platelets (40-70 aggregates/cell) was required for the triggering of the release reaction and cell aggregation. Binding was independent of divalent cations. The Arrhenius plot gave a straight line between 0-37.degree. C and a Q10 of 1.6. Inhibitors of the release reaction, energy metabolism or formaldehyde fixation of the platelets did not affect binding. Bound 3H-BDB-IgG was not significantly eluted by IgG, bovine serum albumin (BSA), buffer or plasma. Binding to washed platelets was more strongly inhibited by human IgG than by (F(ab'')2, bovine IgG, human albumin (HSA) or BSA. Plasma was an even more effective inhibitor of binding and release. Plasma deficient in IgG or depleted of complement retained its inhibitory capacity. In the presence of plasma, at physiologic ratios of plasma and platelets, no release of serotonin was observed. Binding, although inhibited in rate, nevertheless occurred. It was enhanced by divalent cation chelation and had a Q10 of 2.5. The release reaction of washed platelets to which 3H-BDB-IgG was bound in the presence of HSA or BSA was inhibited by the subsequent addition of plasma or plasma proteins (human IgG being more effective than bovine IgG, F(ab'')2, HSA or BSA). 3H-BDB-IgG bound in the presence of plasma or human IgG did not induce release when the platelets were subsequently suspended in media lacking these proteins. The platelet Fc receptor binds 3H-BDB-IgG by a process which is effectively inhibited by plasma, or by free IgG with an intact Fc, and to some extent by high concentrations of other proteins. The effects of bound IgG aggregates are dependent on the other proteins present during binding and those subsequently added. The mechanism of such receptor modulation and its implications in vivo are discussed.