Effect of gram-negative bacterial lipopolysaccharide-derived polysaccharides, glycolipids, and lipopolysaccharides on rabbit and human platelets in vitro

Limulus amebocyte lysate (LAL) detection of endotoxin in human blood

Limulus amebocyte lysate (LAL) has been applied to the detection of endotoxin in human serum, plasma and blood since the early 1970s. Although the diagnostic potential of LAL for endotoxemia was recognized immediately, the assay's modest sensitivity and specificity (for Gram-negative sepsis/bacteremia) were perceived as limiting the clinical usefulness of LAL. In an attempt to overcome these drawbacks, many studies have been conducted since the initial work by Levin and his colleagues. Numerous attempts have been made to improve the sensitivity of the assay by changing the formulation of the LAL and assay methodology. The original gel-clot method has for the most part been replaced with turbidimetric or chromogenic methods. The amount of endotoxin detectable within a 1 h incubation period has gone from the nanogram to the picogram range. Since blood (plasma) components interfere with the test, various methods to remove inhibition and/or enhancement have been developed. The chloroform extraction technique of Levin and co-workers has been replaced with acid extraction or with dilution and heating. Partitioning of endotoxin in blood may also influence the assay (recovery). Many recent investigators use platelet-rich plasma instead of ordinary plasma, while a few studies have used whole blood. Even with all the improvements, the specificity and related diagnostic usefulness of the LAL assay for Gram-negative sepsis remain an obstacle for regulatory acceptance. This may have more to do with our understanding of the septic process than with the ability of LAL to detect endotoxin. Although a recent study indicates that the type of Gram-negative bacteremia may be a critical determinant for clinical utility of the LAL test, the presence of endotoxin is not highly predictive of Gram-negative sepsis and vice versa. However, with the potential availability of anti-endotoxin therapy, the diagnosis of endotoxemia, with or without bacteremia, may be extremely important for timely and effective treatment modalities. It is concluded that the LAL test and accompanying sample preparation has evolved into a clinically useful test for the detection of circulating endotoxins and even its modest predictability for sepsis may have some clinical utility.

Lipopolysaccharide signaling without a nucleus: kinase cascades stimulate platelet shedding of proinflammatory IL-1β-rich microparticles

Platelets contain unspliced heteronuclear IL-1β RNA, which is rapidly spliced and translated upon activation. LPS is a superior agonist for this atypical platelet response, but how LPS induces proinflammatory cytokine production in anucleate cells lacking NF-κB is unknown. Platelets express functional TLR4, and stimulation by LPS induced rapid splicing, translation, and secretion of mature IL-1β after caspase-1 processing. LPS stimulated microparticle shedding, and secreted IL-1β was exclusively present in these particles. Microparticles from LPS-stimulated platelets induced VCAM-1 production by cultured human endothelial cells, and blockade of endothelial IL-1β receptor with IL-1 receptor antagonist completely suppressed endothelial activation. Splicing was posttranscriptional as the SR kinase inhibitor TG003 blocked IL-1β RNA production by platelets, but not by monocytes, and was dependent on exogenous CD14--a property of platelets. We used a combination of small-molecule inhibitors, cell-penetrating chimeric peptide inhibitors, and gene-targeted animals to show splicing required MyD88 and TIRAP, and IRAK1/4, Akt, and JNK phosphorylation and activation. Traf6 couples MyD88 to the Akt pathway and, remarkably, a Traf6 interacting peptide-antennapedia chimera was more effective than LPS in stimulating IL-1β splicing. The Traf6 chimera did not, however, stimulate microparticle shedding, nor was IL-1β released. We conclude LPS-induced kinase cascades are sufficient to alter cellular responses, that three signals emanate from platelet TLR4, and that Akt and JNK activation are sufficient to initiate posttranscriptional splicing while another event couples microparticle shedding to TLR4 activation. Platelets contribute to the inflammatory response to LPS through production of microparticles that promote endothelial cell activation.

Lipopolysaccharide stimulates platelet secretion and potentiates platelet aggregation via TLR4/MyD88 and the cGMP-dependent protein kinase pathway

Bacterial LPS induces rapid thrombocytopenia, hypotension, and sepsis. Although growing evidence suggests that platelet activation plays a critical role in LPS-induced thrombocytopenia and tissue damage, the mechanism of LPS-mediated platelet activation is unclear. In this study, we show that LPS stimulates platelet secretion of dense and alpha granules as indicated by ATP release and P-selectin expression, and thus enhances platelet activation induced by low concentrations of platelet agonists. Platelets express components of the LPS receptor-signaling complex, including TLR (TLR4), CD14, MD2, and MyD88, and the effect of LPS on platelet activation was abolished by an anti-TLR4-blocking Ab or TLR4 knockout, suggesting that the effect of LPS on platelet aggregation requires the TLR4 pathway. Furthermore, LPS-potentiated thrombin- and collagen-induced platelet aggregation and FeCl(3)-induced thrombus formation were abolished in MyD88 knockout mice. LPS also induced cGMP elevation and the stimulatory effect of LPS on platelet aggregation was abolished by inhibitors of NO synthase and the cGMP-dependent protein kinase (PKG). LPS-induced cGMP elevation was inhibited by an anti-TLR4 Ab or by TLR4 deficiency, suggesting that activation of the cGMP/protein kinase G pathway by LPS involves the TLR4 pathway. Taken together, our data indicate that LPS stimulates platelet secretion and potentiates platelet aggregation through a TLR4/MyD88- and cGMP/PKG-dependent pathway.