FcgRIIa activation, as a result of IgG binding and by action of phosphatidylinositol 3-kinase (PI 3-kinase) and phospholipase C-g2 (PLCg2), leads to release of diacylglycerol (DAG) and inositol triphosphate (IP3), mobilization of internal calcium stores, and, subsequently, platelet aggregation (Anderson and Anderson, 1990). Initially, IgG binding leads to FcgRIIa clustering, resulting in the phosphorylation of FcgRIIa ITAMs by Src family protein tyrosine kinases (Chacko et al., 1994;
FIGURE 1 Electron microscopy of negatively stained platelets activated in situ with HIT serum. Platelets were allowed to settle on bovine serum albumin-coated Formvar grids and then incubated with (A) serum testing negative for HIT antibodies or heparin (not shown) or (B) HIT serum in the presence of heparin, 0.1 U/mL. Platelets were then fixed with 2% glutaraldehyde and negatively stained with 2% phosphotungstic acid. Whereas unactivated platelets demonstrated round or discoid shapes (see A), platelets activated by HIT serum demonstrated numerous surrounding microparticles ranging in size from <0.1 to 1.0 mm in diameter. (Original magnification x13,000.) Abbreviation: HIT, heparin-induced thrombocytopenia. Source: From Hughes et al., 2000.
Huang et al., 1992). Following FcyRIIa phosphorylation, tyrosine kinase activity (e.g., p72syk) and PI 3-kinase activity increase through the noncovalent interaction of their SH2 domains with phosphorylated FcyRIIa ITAMs (Greinacher et al., 1994b; Yanaga et al., 1995; Chacko et al., 1996). Subsequently, PLCy2 is phos-phorylated by p72syk (Blake et al., 1994), which is dependent on phosphatidylino-sitol-trisphosphate (PtdIns[3,4,5]P3) (Gratacap et al., 1998). PLCy2 activation is crucial for the generation of DAG and IP3.
More recently, Gratacap and coworkers (2000) showed that FcyRIIa activation alone does not produce sufficient levels of PtdIns(3,4,5)P3 by PI 3-kinase to cause PLCy2 activation, platelet release, and aggregation. Additionally, ADP receptor activation by Gi-protein signaling is required to generate PtdIns(3,4,5)P3 via PI 3-kinase, which combined with activation by FcyRIIa generates optimal levels of PtdIns(3,4,5)P3, leading to efficient PLCy2 phosphorylation. Activated PLCy2 then generates DAG and IP3 from PtdIns(4,5)P2, mobilizing calcium and effecting platelet aggregation (Fig. 2). Moreover, lipid rafts appear to play an important role in the organization of the FcyRIIa/ADP receptor/PLCy2 signaling pathway (Bodin et al., 2003). These findings help to explain the previous observations that ADP scavengers (e.g., apyrase) fully inhibit platelet aggregation by HIT-IgG (Polgar et al., 1998).
Although an association between heparin treatment and paradoxical thrombosis was first suspected about 40 yr ago (Weismann and Tobin, 1958; Roberts et al., 1964), it was Rhodes and colleagues (1973) who first provided evidence that serum from HIT patients contained a substance, most likely IgG, that aggregated normal platelets in the presence of heparin. This observation was confirmed by Fratantoni et al. (1975) who reported a simple indirect aggregation method for detecting HIT antibodies. In 1986, Sheridan and coworkers (1986) reported a washed platelet
PF4 - heparin - IgG complex
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