The quantitation of platelet-associated IgG on cohorts of platelets separated from healthy individuals by buoyant density centrifugation.

Evidence suggests that as platelets age in the circulation they become progressively smaller and less dense through the loss of protein. The smallest, least dense platelets have a significantly shortened survival, but the mechanism of clearance of these platelets is not known. To evaluate whether the binding of IgG could play a role in the clearance of senescent platelets, we measured platelet size, total protein, and platelet-associated IgG on subpopulations of platelets isolated from 6 healthy individuals using a discontinuous iso-osmotic arabinogalactan (stractan) gradient. There was a close correlation between density, size, and total protein content (r greater than 0.9) for all platelet fractions. There was also a relationship between the amount of platelet-associated IgG (PAIgG), total protein, and platelet size (r greater than 0.9) for the first 3 progressively less dense platelet factions. However, the fourth platelet fraction containing the smallest, least dense, and on current evidence, oldest platelets had very elevated amounts of IgG. This amount was approximately 10 times higher than the mean platelet IgG for the same individual and was similar to the amount of PAIgG found on platelets from patients with immune thrombocytopenia. A progressive increase in the ratio of PAIgG measured after platelet solubilization to PAIgG measured on intact platelets was also noted for the first three populations, indirectly suggesting that platelets clear IgG from their surface during aging. Increased binding of IgG to senescent platelets may mediate their destruction.