No abstract

Summary. Background: There is increasing evidence that both chronic and acute infections play a role in the development and progression of atherothrombotic disorders. One potential mechanism is the direct activation of platelets by bacteria. A wide range of bacterial species activate platelets through heterogeneous mechanisms. The oral micro-organism S. sanguinis stimulates platelet aggregation in vitro in a strain-dependent manner, although there are no reports of associated cytokine production. Objective: The aim of the present study was to determine whether platelet activation by S. sanguinis involved the release of pro-inflammatory and immune modulating factors, and whether activation was enhanced by epinephrine. Methods and results: Four strains of S. sanguinis and one of S. gordonii stimulated the release of RANTES, PF4, sCD40L and PDGF-AB, whereas only one S. sanguinis strain caused the release of sCD62p. Epinephrine enhanced S. sanguinis-induced platelet aggregation and phosphorylation of phospholipase Cc2 and Erk, but inhibited RANTES, PF4, sCD40L and PDGF-AB release. Wortmannin inhibited S. sanguinis-induced aggregation and release; however, only aggregation was partially reversed by epinephrine. Conclusions: The present study demonstrates that platelets respond to S. sanguinis with both prothrombotic and pro-inflammatory/immune-modulating responses. Epinephrine, potentially released in response to infection and/or stress, can significantly enhance the prothrombotic response, thereby providing a putative link between bacteraemia and acute coronary events during stress. In contrast, epinephrine inhibited the pro-inflammatory/immune-modulating response by an undetermined mechanism.

There is increasing evidence for a relationship between bacterial infections and several cardiovascular disorders. Although the precise mechanism(s) underlying this association is unknown, the direct activation of platelets by bacteria is one possibility. Individual strains of S. sanguis activate platelets in a non-uniform, donor-dependent manner. In the current study, platelet aggregation profiles were obtained for fourteen donors in response to four strains of S. sanguis (2017-78, 133-79, SK112, SK108a) and one of S. gordonii (SK8) . The platelets from all donors responded to strains 2017-78 and 133-79, whereas strains SK112, SK8 and SK108a caused aggregation in one, five and twelve donors, respectively. Immunoglobulin G (IgG) binding to strains 2017-78, 133-79 and SK108a were significantly greater than to strains SK112 and SK8. Absorption of IgG by strain 2017-78 caused significant decreases in IgG binding, and platelet aggregation in response, to all strains. Single-strand conformational polymorphisms were observed in the Fcgamma RIIA gene from four donors. Sequencing revealed two known and two novel point mutations, none of which correlated with the aggregation profile. Thus, platelet activation to the various strains depends on a common IgG and, while in most cases the level of IgG binding to S. sanguis determines platelet responsiveness, neither the levels of IgG nor FcgammaRIIA polymorphisms can fully account for donor variability.

No abstract

Human platelet activation by collagen occurs in a dose-dependent manner. High concentrations of collagen bind to a pair of receptors, the a2b1 integrin and glycoprotein (GP)VI/Fc-receptor g-chain (FcRg), which stimulate a cascade of events including Syk, LAT, Btk, Gads, and phospholipase Cg2, leading to calcium release and protein kinase C (PKC) activation. Calcium and PKC are responsible for a range of platelet responses including exocytosis and aggregation, as well as the cytosolic phospholipase A 2 (cPLA 2 )-mediated release of arachidonic acid, which is converted to thromboxane (Tx)A 2 . In contrast, low concentrations of collagen are acutely aspirin-sensitive, and calcium release and aggregation are TxA 2 -dependent. Under these conditions, cPLA 2 is not involved and it has been suggested that phospholipase C generates 1,2-diacylglycerol (DG) from which arachidonic acid is liberated by diglyceride lipase (DGL). Here a novel DGL blocker (OMDM-188) inhibited collagen-, but not arachidonic acid-induced aggregation and TxA 2 synthesis. Furthermore, OMDM-188 inhibited collagen-induced arachidonic acid release. Finally OMDM-188 inhibited collagen-induced p38 MAPK phosphorylation, but not extracellular signal-regulated kinase (ERK) phosphorylation, with no effect on the phosphorylation of either enzyme in response to arachidonic acid. Taken together, these data suggest a role for a pathway involving phospholipase C liberating DG from membrane phospholipids in response to minimally activating concentrations of collagen. The DG serves as a substrate for DGL, potentially under the regulations of p38 MAPK , to release arachidonic acid, which is subsequently converted to TxA 2 , which mediates the final platelet response.