INFLUENCE OF PLATELET MEMBRANE SIALIC-ACID AND PLATELET-ASSOCIATED IGG ON AGING AND SEQUESTRATION OF BLOOD-PLATELETS IN BABOONS

Platelets were isolated from blood of baboons and treated with neurami nidase to remove platelet membrane sialic acid, a process which artifi cially ages the platelets. The platelets were then labelled with In-11 1 and their mean life span, in vivo distribution and sites of sequestr ation were measured. The effect of removal of sialic acid on the attac hment of immunoglobulin to platelets were investigated and related to the sequestration of the platelets by the spleen, liver, and bone marr ow. Removal of sialic acid by neuraminidase did not affect the aggrega tion of platelets by agonists in vitro, nor their sites of sequestrati on. The removal of 0.51 (median, range 0.01 to 2.10) nmol sialic acid/ 10(8) platelets shortened their life span by 75 h (median, range 0 to 132) h (n = 19, p <0.001), and there was an exponential correlation be tween the shortening of the mean platelet life span and the amount of sialic acid removed. The increase in platelet-associated IgG was 0.112 (median, range 0.007 to 0.309) fg/platelet (n = 25, p <0.001) after 0 .79 (median, range 0.00 to 6.70) nmol sialic acid/10(8) platelets was removed (p <0.001). There was an exponential correlation between the s hortening of mean platelet life span after the removal of sialic acid and the increase in platelet-associated IgG. The results suggest that platelet membrane sialic acid influences ageing of circulating platele ts, and that the loss of sialic acid may have exposed a senescent cell antigen that binds IgG on the platelet membrane. The antibody-antigen complex may then provide a signal to the macrophages that the platele t is old, and can be phagocytosed and destroyed.