Requirement for Complement Components and Fibrinogen in the Zymosan-Induced Release Reaction of Human Blood Platelets

Abstract

As shown earlier, zymosan particles incubated with human citrated plasma develop the ability to induce a release reaction in platelets. Thus when incubated zymosan is shaken for 4 to 15 min with human citrated platelet-rich plasma, ADP, ATP, and ¹⁴C-serotonin are released and the ADP causes platelet aggregation. Plasma depleted in vitro of C3 or GBG failed to generate activity on zymosan unless at least 65 µg of C3 or 100 µg of glycine-rich β-glycoprotein (GBG) were added/ml plasma. Plasmas from individuals with congenital deficiency of C3 or C3 inactivator (and hence GBG) were also inactive unless either purified C3 or hydrazine-treated plasma was added. Afibrinogenemic plasma was inactive unless at least 80 µg of fibrinogen was added/milliliter, a requirement previously reported for serum. Zymosan particles washed after incubation with normal plasma bore more fibrinogen (determined immunologically) than particles incubated with afibrinogenemic or ethylenediaminetetraacetic acid plasma and washed. The amount of zymosan-associated fibrinogen was not necessarily less on particles incubated with C3- or GBG-deficient plasma than on those incubated with normal plasma but was greater when purified C3 or GBG was added to the appropriate deficient plasma before incubation. Properdin-depleted plasma could not generate release-promoting activity on zymosan; activity was not restored by purified active properdin. When plasma from two patients with C̄1 inhibitor deficiency was depleted of C4 by incubation, one sample generated activity on zymosan normally. Serum from a guinea pig with C4 deficiency generated activity on zymosan when fibrinogen was added. Inulin and endotoxin did not develop release-promoting activity when incubated with normal plasma although they caused as much conversion of C3 and GBG as zymosan. Plasma incubated with inulin and endotoxin generated activity when subsequently incubated with zymosan, whereas plasma incubated with zymosan lost its ability to generate activity when incubated with fresh zymosan. We conclude that the alternate pathway of complement activation and fibrinogen are involved in generating release-promoting, platelet-aggregating activity on zymosan, and that activity may develop only during activation of properdin.