Staphylococcus aureus alpha-toxin attack on human platelets promotes assembly of the prothrombinase complex

alpha-Toxin, the major cytolysin of Staphylococcus aureus, promotes blood coagulation by its attack on human platelets (Bhakdi S., Muhly, M., Mannhardt, U., Hugo, F., Klapettek, K., Mueller-Eckhardt, C., and Roka, L. (1988) J. Exp. Med. 168, 527-542). In the present study we demonstrate that toxin attack on gel-filtered human platelets initiates the assembly of prothrombinase complexes at rates up to 10-fold of controls. Treatment of platelets with 0.1 microgram/ml alpha-toxin resulted in generation of 1.4 units of thrombin/10(8) platelets. A similar rate of thrombin generation was noted when platelets were subjected to three cycles of freezing and thawing. However, the alpha-toxin-induced prothrombinase activity was not due to platelet lysis, since less than 1% of total cellular lactate dehydrogenase was released by this alpha-toxin concentration. Two distinct and dissociable processes contributed to enhanced prothrombinase assembly. First, alpha-toxin promoted the exocytotic release of factor V from alpha-granules, which was accompanied by co-secretion of platelet factor 4. This process was calcium-dependent. Second, toxin-treated platelets exhibited an enhanced capacity to bind external factor V(a), a phenomenon that was not linked to Ca2(+)-dependent factor V secretion. Assembly of prothrombinase complexes via these two mechanisms together accounts for the procoagulant action of S. aureus alpha-toxin.