Sheila Timmons

Staphylococci-induced Human Platelet Injury Mediated by Protein A and Immunoglobulin G Fc Fragment Receptor

Bloodstream infections with staphylococci are accompanied by thromboembolic complications. We have studied the mechanism of the interaction of staphylococci with human blood platelets. Staphylococci that possess protein A, a bacterial receptor for the Fc fragment of immunoglobulin G (IgG), caused aggregation of human platelets in whole plasma accompanied by release of [(3)H]serotonin. These reactions were time and concentration dependent, requiring two or more staphylococci per platelet to give maximal response within 5 min. The interaction between staphylococci and platelets required the presence of cell wall-bound protein A and of IgG with an intact Fc fragment. It did not require an intact complement system. Cell wall-bound protein A (solid phase) was capable of aggregating human platelets in whole plasma. In contrast, free, solubilized protein A (fluid phase) did not cause measurable aggregation, and release of [(3)H]serotonin was reduced. An excess of free, solubilized protein A blocked aggregation of human platelets induced by staphylococci in whole plasma. The role of the Fc fragment of IgG in the staphylococci-human platelet interaction was demonstrated by an experiment in which free, isolated Fc fragment blocked aggregation of platelets in whole plasma induced by staphylococci. Furthermore, binding of (125)I-protein A to human platelets was demonstrated in the presence of complete IgG with intact Fc fragment but not in the presence of the F(ab)(2) fragment. Binding of the protein A-IgG complex to the human platelet Fc receptor was paralleled by the release of [(3)H]serotonin. These results represent a novel example of the interaction of two phylogenetically different Fc receptors, one on prokaryotic staphylococci and the other on human platelets. Their common ligand, IgG, is amplified by one Fc receptor (protein A) to react with another Fc receptor present on human platelets, which results in membrane-mediated aggregation and release reaction occurring in whole plasma. This mechanism can be of significance in the pathomechanism of thromboembolic complications at the site(s) of intravascular staphylococcal infection.

Plasma components are required for platelet activation by the toxin of Loxosceles reclusa

We have used a partially purified toxin from the venom of the brown recluse spider, Loxosceles reclusa, to study its effects on human platelets isolated from plasma proteins. This toxin, which produced skin necrosis in rabbits, contained sphingomyelinase D activity. The toxin induced platelet aggregation and secretion of [3H]serotonin in human plasma but not in buffer or in human neonate plasma. Ca2+ was required for the interaction of toxin, platelets, and plasma factor(s). The addition of C-reactive protein restored aggregation and serotonin release of platelets incubated in human neonate plasma. The ADP-degrading enzyme, apyrase, and the non-steroidal, anti-inflammatory drug, indomethacin, inhibited platelet aggregation, suggesting that ADP secreted from platelet storage granules and indomethacin-sensitive pathway(s) are involve in the toxin-induced human platelet activation (aggregation and serotonin release). Generation of platelet activating factor (PAF) from the platelet by brown recluse toxin is not likely since the PAF receptor antagonist, BN 52021, did not inhibit platelet aggregation induced by the brown recluse toxin.