Effects of immune complex formation and complement activation on circulating platelets in the primate

Primate platelets are different from rodent and rabbit platelets in that th ey do not express receptors for C3a or C5a or immune adherence receptors. T his study assessed the effects of immune complex (IC)-induced complement ac tivation on primate platelets in the circulation. Cynomolgus monkeys (CYN, N = 4) immunized to bovine gamma globulin (BGG) were infused with EGG over 5 min to induce acute intravascular IC formation and complement activation. The studies were carried out under normal complement conditions (N = 12), partial complement inhibition (CAB-2 treated, N = 3), or total complement i nhibition (CVF treated, N = 1). Under normal complement conditions, EGG; in fusion increased C3a levels from undetectable to an average of 11.9 +/- 2.6 mu g/ml. At this time, decreases occurring in both circulating neutrophils (85 +/- 6%) and monocytes (78 +/- 6%) were significantly greater than decr eases in circulating platelets (13 +/- 3%, p < 0.001). Partial complement i nhibition had an equivocal effect on the EGG-induced changes in circulating leukocytes, while total complement inhibition abrogated these changes. In contrast, platelet changes were unaffected by complement inhibition. We con clude that, compared to circulating leukocytes, circulating platelets are i nsensitive to intravascular complement activation induced by IC in the nonh uman primate. These results contrast with previous studies in rodents which demonstrate strong effects of IC-induced intravascular complement activati on on both circulatingneutrophils and platelets. (C) 1999 Academic Press.