Immune versus thrombotic stimulation of platelets differentially regulates signalling pathways, intracellular protein-protein interactions, and alpha-granule release

Platelet-TLR7 mediates host survival and platelet count during viral infection in the absence of platelet-dependent thrombosis

Viral infections have been associated with reduced platelet counts, the biological significance of which has remained elusive. Here, we show that infection with encephalomyocarditis virus (EMCV) rapidly reduces platelet count, and this response is attributed to platelet Toll-like receptor 7 (TLR7). Platelet-TLR7 stimulation mediates formation of large platelet-neutrophil aggregates, both in mouse and human blood. Intriguingly, this process results in internalization of platelet CD41-fragments by neutrophils, as assessed biochemically and visualized by microscopy, with no influence on platelet prothrombotic properties. The mechanism includes TLR7-mediated platelet granule release, translocation of P-selectin to the cell surface, and a consequent increase in platelet-neutrophil adhesion. Viral infection of platelet-depleted mice also led to increased mortality. Transfusion of wild-type, TLR7-expressing platelets into TLR7-deficient mice caused a drop in platelet count and increased survival post EMCV infection. Thus, this study identifies a new link between platelets and their response to single-stranded RNA viruses that involves activation of TLR7. Finally, platelet-TLR7 stimulation is independent of thrombosis and has implications to the host immune response and survival.

Interleukin 1 receptor 1 and interleukin 1β regulate megakaryocyte maturation, platelet activation, and transcript profile during inflammation in mice and humans

OBJECTIVE

Interleukin 1 Receptor 1 (IL1R1) and its ligand, IL1β, are upregulated in cardiovascular disease, obesity, and infection. Previously, we reported a higher level of IL1R1 transcripts in platelets from obese individuals of the Framingham Heart Study (FHS), but its functional effect in platelets has never been described. Additionally, IL1β levels are increased in atherosclerotic plaques and in bacterial infections. The aim of this work is to determine whether IL1β, through IL1R1, can activate platelets and megakaryocytes to promote atherothrombosis.

APPROACH AND RESULTS

We found that IL1β-related genes from platelets, as measured in 1819 FHS participants, were associated with increased body mass index, and a direct relationship was shown in wild-type mice fed a high-fat diet. Mechanistically, IL1β activated nuclear factor-κB and mitogen-activated protein kinase signaling pathways in megakaryocytes. IL1β, through IL1R1, increased ploidy of megakaryocytes to 64+ N by 2-fold over control. IL1β increased agonist-induced platelet aggregation by 1.2-fold with thrombin and 4.2-fold with collagen. IL1β increased adhesion to both collagen and fibrinogen, and heterotypic aggregation by 1.9-fold over resting. High fat diet-enhanced platelet adhesion was absent in IL1R1(-/-) mice. Wild-type mice infected with Porphyromonas gingivalis had circulating heterotypic aggregates (1.5-fold more than control at 24 hours and 6.2-fold more at 6 weeks) that were absent in infected IL1R1(-/-) and IL1β(-/-) mice.

CONCLUSIONS

In summary, IL1R1- and IL1β-related transcripts are elevated in the setting of obesity. IL1R1/IL1β augment both megakaryocyte and platelet functions, thereby promoting a prothrombotic environment during infection and obesity; potentially contributing to the development of atherothrombotic disease.

Stimulation of platelet apoptosis by peptidoglycan from Staphylococcus aureus 113

Peptidoglycan (PGN), a component of bacterial cell wall and belonging to "Microbe-Associated Molecular Patterns" (MAMP) triggers host reactions contributing to the pathophysiology of infectious disease. Host cell responses to PGN exposure include apoptosis. Bacterial infections may result in activation of blood platelets and thrombocytopenia. The present study explored, whether HPLC-purified fractions of PGNs from Staphylococcus aureus 113 triggers apoptosis of platelets. To this end platelets were exposed to PGN fractions and annexin-V binding determined to depict cell membrane scrambling, DiOC6 fluorescence to estimate depolarization of mitochondrial potential, Fluo-3AM staining for intracellular Ca(2+) activity ([Ca(2+)](i)) and immunofluorescence to quantify protein abundance of active caspase-3. As a result, a 30 min exposure to monomeric fraction (mPGN) (≥50 ng/ml) was followed by annexin-V binding, paralleled by increase of [Ca(2+)](i), mitochondrial depolarization, caspase-3 activation and integrin α(IIb)β(3) upregulation. The annexin-V binding was significantly blunted by anti-TLR-2 antibodies, in absence of extracellular Ca(2+), and by pancaspase inhibitor zVAD-FMK (1 μM). In conclusion, PGN triggers apoptosis of platelets in activation-dependent manner, characterized by mitochondrial depolarization, caspase-3 activation and cell membrane scrambling.

Altered expression of platelet proteins and calpain activity mediate hypoxia-induced prothrombotic phenotype

Oxygen-compromised environments, such as high altitude, air travel, and sports, and pathological conditions, such as solid tumors, have been suggested to be prothrombotic. Despite the indispensable role of platelets in thrombus formation, the studies linking hypoxia, platelet reactivity, and thrombus formation are limited. In the present study, platelet proteome/reactivity was analyzed to elucidate the acute hypoxia-induced prothrombotic phenotype. Rats exposed to acute simulated hypoxia (282 torr/8% oxygen) demonstrated a decreased bleeding propensity and increased platelet reactivity. Proteomic analysis of hypoxic platelets revealed 27 differentially expressed proteins, including those involved in coagulation. Among these proteins, calpain small subunit 1, a 28-kDa regulatory component for calpain function, was significantly upregulated under hypoxic conditions. Moreover, intraplatelet Ca(2+) level and platelet calpain activity were also found to be in accordance with calpain small subunit 1 expression. The inhibition of calpain activity demonstrated reversal of hypoxia-induced platelet hyperreactivity. The prothrombotic role for calpain was further confirmed by an in vivo model of hypoxia-induced thrombosis. Interestingly, patients who developed thrombosis while at extreme altitude had elevated plasma calpain activities and increased soluble P-selectin level. In summary, this study suggests that augmented calpain activity is associated with increased incidence of thrombosis under hypoxic environments.

Regulation of platelet responses triggered by Toll-like receptor 2 and 4 ligands is another non-genomic role of nuclear factor-kappaB

INTRODUCTION

Platelets express Toll-like receptors (TLRs) that recognise molecular components of pathogens and, in nucleated cells, elicit immune responses through nuclear factor-kappaB (NF-κB) activation. We have shown that NF-κB mediates platelet activation in response to classical agonists, suggesting that this transcription factor exerts non-genomic functions in platelets. The aim of this study was to determine whether NF-κB activation is a downstream signal involved in TLR2 and 4-mediated platelet responses.

MATERIAL AND METHODS

Aggregation and ATP release were measured with a Lumi-aggregometer. Fibrinogen binding, P-selectin and CD40 ligand (CD40L) levels and platelet-neutrophil aggregates were measured by cytometry. I kappa B alpha (IκBα) degradation and p65 phosphorylation were determined by Western blot and von Willebrand factor (vWF) by ELISA.

RESULTS

Platelet stimulation with Pam3CSK4 or LPS resulted in IκBα degradation and p65 phosphorylation. These responses were suppressed by TLR2 and 4 blocking and synergised by thrombin. Aggregation, fibrinogen binding and ATP and vWF release were triggered by Pam3CSK4. LPS did not induce platelet responses per se, except for vWF release, but it did potentiate thrombin-induced aggregation, fibrinogen binding and ATP secretion. Pam3CSK4, but not LPS, induced P-selectin and CD40L expression and mixed aggregate formation. All of these responses, except for CD40L expression, were inhibited in platelets treated with the NF-κB inhibitors BAY 11-7082 or Ro 106-9920.

CONCLUSION

TLR2 and 4 agonists trigger platelet activation responses through NF-κB. These data show another non-genomic function of NF-κB in platelets and highlight this molecule as a potential target to prevent platelet activation in inflammatory or infectious diseases.

Platelets as cellular effectors of inflammation in vascular diseases

Platelets are chief effector cells in hemostasis. In addition, they are multifaceted inflammatory cells with functions that span the continuum from innate immune responses to adaptive immunity. Activated platelets have key thromboinflammatory activities in a variety of vascular disorders and vasculopathies. Recently identified inflammatory and immune activities provide insights into the biology of these versatile blood cells that are directly relevant to human vascular diseases.

Oxidative Versus Thrombotic Stimulation of Platelets Differentially activates Signalling Pathways

INTRODUCTION

Atherosclerosis is one of the inflammatory underlying disease associated by oxidative stress and thrombotic agents. This study aimed to evaluate the potential role of cupper oxidized low-density lipoprotein (OxLDL) and thrombin for inducing mitogen activated protein kinases (MAPKs) in platelets.

METHODS

Phosphorylation of P38MAPK, Jun N-terminal Kinase (JNK), and Extracellular signal-regulated kinases (ERK1/2) and P-selectin expression were determined in lysates of washed human platelets pretreated with low doses of thrombin and cu2+-OxLDL By Enzyme-linked immunosorbent assay (ELISA). Pharmacological inhibition was performed by SB203580, PD980559 and SP6000125 for P38MAPK, ERK1/2 and JNK activity, respectively. The ratio of phosphorylated to total protein was used for normalizing the phospho proteins contents of cells.

RESULTS

OxLDL and thrombin significantly and differentially increased P-selectin expression (P<0.05), P38MAPK (P<0.05) and c-JNK (P<0.05) and ERK1/2 (P<0.05) phosphorylation in platelets. SB 203580 and SP6000125 significantly decreased P-selectin expression in both oxidative (P<0.05) and thrombotic (P<0.05) activated platelets.

CONCLUSION

Our results indicated that MAPK inhibitors can reduce atherothrombotic events via alterations in P-selectin expression suggesting that these inhibitors may be useful in the inhibition of atheroma development.

Human platelets can discriminate between various bacterial LPS isoforms via TLR4 signaling and differential cytokine secretion

Platelets are currently acknowledged as cells of innate immunity and inflammation and play a complex role in sepsis. We examined whether different types of LPS have different effects on the release of soluble signaling/effective molecules from platelets. We used platelet-rich plasma from healthy volunteers and LPS from two strains of gram-negative bacteria with disparate LPS structures. We combined LPS-stimulated platelet supernatants with reporter cells and measured the PBMC cytokine secretion profiles. Upon stimulation of platelets with both Escherichia coli O111 and Salmonella minnesota LPS, the platelet LPS::TLR4 interaction activated pathways to trigger the production of a large number of molecules. The different platelet supernatants caused differential PBMC secretion of IL-6, TNFα, and IL-8. Our data demonstrate that platelets have the capacity to sense external signals differentially through a single type of pathogen recognition receptor and adjust the innate immune response appropriately for pathogens exhibiting different types of 'danger' signals.

Ginkgolide B reduces atherogenesis and vascular inflammation in ApoE(-/-) mice

Aims

To investigate whether ginkgolide B (a platelet-activating factor inhibitor) affects vascular inflammation in atherosclerosis-prone apolipoprotein E-deficient (ApoE^−/−) mice.

Methods and Results

Human platelets were used to evaluate the effects of ginkgolide B on platelet aggregation and signal transduction. Ginkgolide B attenuated platelet aggregation and inhibited phosphatidylinositol 3 kinase (PI3K) activation and Akt phosphorylation in thrombin- and collagen-activated platelets. ApoE^−/− mice were administered a high-cholesterol diet for 8 weeks. Plasma platelet factor 4 (PF4) and RANTES (regulated upon activation, normal T-cell expressed, and secreted protein) were then measured using an enzyme-linked immunosorbent assay. Scanning electron microscopy and immunohistochemistry were used to determine atherosclerotic lesions. Ginkgolide B decreased plasma PF4 and RANTES levels in ApoE^−/− mice. Scanning electron microscopic examination showed that ginkgolide B reduced aortic plaque in ApoE^−/− mice. Immunohistochemistry analysis demonstrated that ginkgolide B diminished P-selectin, PF4, RANTES, and CD40L expression in aortic plaque in ApoE^−/− mice. Moreover, ginkgolide B suppressed macrophage and vascular cell adhesion protein 1 (VCAM-1) expression in aorta lesions in ApoE^−/− mice. Similar effects were observed in aspirin-treated ApoE^−/− mice.

Conclusion

Ginkgolide B significantly reduced atherosclerotic lesions and P-selectin, PF4, RANTES, and CD40L expression in aortic plaque in ApoE−/− mice. The efficacy of ginkgolide B was similar to aspirin. These results provide direct evidence that ginkgolide B inhibits atherosclerosis, which may be associated with inhibition of the PI3K/Akt pathway in activated platelets.

Factor XIII is a key molecule at the intersection of coagulation and fibrinolysis as well as inflammation and infection control

Factor XIII (FXIII) is a transglutaminase consisting of two catalytic A subunits (FXIII-A) and two non-catalytic B subunits (FXIII-B) in plasma. FXIII-B protects FXIII-A from its clearance. FXIII-A is also present as a homodimer inside megakaryocytes/platelets and monocytes/macrophages. Although possible functions of intracellular FXIII-A have been proposed, these remain to be established. Intra- and extra-cellular FXIIIs support platelet adhesion and spreading as well as clot retraction, suggesting that FXIII is important for the stabilization of platelet-fibrin clots. Intra- and extra-cellular FXIIIs also support immobilization and killing of bacteria as well as phagocytosis by macrophages. Thus, FXIII may function in innate immunity. Congenital FXIII deficiency due to defective F13-A genes manifests as a life-long bleeding tendency, abnormal wound healing, and recurrent miscarriage. Although congenital FXIII-B deficiency used to be thought rare, reports of such cases have increased recently. As the bleeding tendency is often mild, patients with FXIII-B deficiency may be overlooked by physicians. Patients with acquired FXIII deficiency, in particular those with autoimmune hemorrhaphilia due to anti-FXIII antibodies, are on the increase, at least in Japan. It is important to diagnose such cases as early as possible, and to treat them with immunosuppression in combination with FXIII replacement therapy as their bleeding symptoms can be life-threatening.

Platelet Ca(2+) responses coupled to glycoprotein VI and Toll-like receptors persist in the presence of endothelial-derived inhibitors: roles for secondary activation of P2X1 receptors and release from intracellular Ca(2+) stores

Inhibition of Ca(2+) mobilization by cyclic nucleotides is central to the mechanism whereby endothelial-derived prostacyclin and nitric oxide limit platelet activation in the intact circulation. However, we show that ∼ 50% of the Ca(2+) response after stimulation of glycoprotein VI (GPVI) by collagen, or of Toll-like 2/1 receptors by Pam(3)Cys-Ser-(Lys)(4) (Pam(3)CSK(4)), is resistant to prostacyclin. At low agonist concentrations, the prostacyclin-resistant Ca(2+) response was predominantly because of P2X1 receptors activated by ATP release via a phospholipase-C-coupled secretory pathway requiring both protein kinase C and cytosolic Ca(2+) elevation. At higher agonist concentrations, an additional pathway was observed because of intracellular Ca(2+) release that also depended on activation of phospholipase C and, for TLR 2/1, PI3-kinase. Secondary activation of P2X1-dependent Ca(2+) influx also persisted in the presence of nitric oxide, delivered from spermine NONOate, or increased ectonucleotidase levels (apyrase). Surprisingly, apyrase was more effective than prostacyclin and NO at limiting secondary P2X1 activation. Dilution of platelets reduced the average extracellular ATP level without affecting the percentage contribution of P2X1 receptors to collagen-evoked Ca(2+) responses, indicating a highly efficient activation mechanism by local ATP. In conclusion, platelets possess inhibitor-resistant Ca(2+) mobilization pathways, including P2X1 receptors, that may be particularly important during early thrombotic or immune-dependent platelet activation.

Thrombin as a multi-functional enzyme. Focus on in vitro and in vivo effects

Thrombin is the central protease in the coagulation cascade and one of the most extensively studied of all enzymes. In addition to its recognised role in the coagulation cascade and haemostasis, thrombin is known to have multiple pleiotropic effects, which mostly have been shown only in in vitro studies: it plays a role in inflammation and cellular proliferation and displays a mitogen activity on smooth muscle cells and endothelial cells, predominantly by activation of angiogenesis. In vivo , thrombin effects were examined in animal models of intravenous or intraarterial thrombin infusion. An extensive literature search regarding in vivo data showed that i) thrombin administered as a bolus causes microembolism, ii) thrombin infused slowly at steady-state conditions (up to 1.6 U/kg/min) leads to bleeds but not to intravascular clotting, iii) large quantity of thrombin infused at low rates (0.05 U/kg/min) does not have any measurable effect, and iv) thrombin increases vascular permeability leading to tissue damage. Although several decades of research on thrombin functions have provided a framework for understanding the biology of thrombin, animal and human studies with use of newer laboratory techniques are still needed to confirm the pleiotropic thrombin functions shown in in vitro studies.

Streptococcus sanguinis-induced cytokine release from platelets

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 Cγ2 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.

Factor XIII: a coagulation factor with multiple plasmatic and cellular functions

Factor XIII (FXIII) is unique among clotting factors for a number of reasons: 1) it is a protransglutaminase, which becomes activated in the last stage of coagulation; 2) it works on an insoluble substrate; 3) its potentially active subunit is also present in the cytoplasm of platelets, monocytes, monocyte-derived macrophages, dendritic cells, chondrocytes, osteoblasts, and osteocytes; and 4) in addition to its contribution to hemostasis, it has multiple extra- and intracellular functions. This review gives a general overview on the structure and activation of FXIII as well as on the biochemical function and downregulation of activated FXIII with emphasis on new developments in the last decade. New aspects of the traditional functions of FXIII, stabilization of fibrin clot, and protection of fibrin against fibrinolysis are summarized. The role of FXIII in maintaining pregnancy, its contribution to the wound healing process, and its proangiogenic function are reviewed in details. Special attention is given to new, less explored, but promising fields of FXIII research that include inhibition of vascular permeability, cardioprotection, and its role in cartilage and bone development. FXIII is also considered as an intracellular enzyme; a separate section is devoted to its intracellular activation, intracellular action, and involvement in platelet, monocyte/macrophage, and dendritic cell functions.

Rethinking platelet function: thrombocytopenia induced immunodeficiency in critical illness

Thrombocytopenia in critical illness predicts a poor clinical outcome. Apart from its role in microvascular thrombus formation, it is widely anticipated that this association is indirect rather than causal. Emerging evidence however indicates that platelets are also immune competent cells. Like traditional innate immune cells, platelets are recruited immediately into injured and inflamed tissue, they release immune mediators, express and shed immunologically active membrane receptors, they interact with other immune cells and they recognize and clear pathogens. We hypothesize that thrombocytopenia per se results in immunodeficiency through loss of platelet-mediated immune functions, and propose that thrombocytopenia induced immunodeficiency in critical illness in part explain the negative predictive value of low or declining platelet count. We propose that rethinking the risks of thrombocytopenia to include not only bleeding but also immunodeficiency and immune dysregulation along with the conduct of studies investigating mechanisms contributing to thrombocytopenia induced poor non-hemorrhagic outcome in critical illness, may be means to improve outcome in these patients through development of new treatment modalities.

Platelets: versatile effector cells in hemostasis, inflammation, and the immune continuum

Platelets are chief effector cells in hemostasis. In addition, however, their specializations include activities and intercellular interactions that make them key effectors in inflammation and in the continuum of innate and adaptive immunity. This review focuses on the immune features of human platelets and platelets from experimental animals and on interactions between inflammatory, immune, and hemostatic activities of these anucleate but complex and versatile cells. The experimental findings and evidence for physiologic immune functions include previously unrecognized biologic characteristics of platelets and are paralleled by new evidence for unique roles of platelets in inflammatory, immune, and thrombotic diseases.

Efficient phagocytosis of periodontopathogens by neutrophils requires plasma factors, platelets and TLR2

BACKGROUND

Periodontitis represents a chronic infection of supportive dental tissues by distinct gram-negative bacteria. It is characterized by chronic and local inflammation as well as transient bacteremia with frequently occurring infections at distant sites.

OBJECTIVES

The present work aimed to clarify the role of platelets and plasma factors in neutrophil interactions with the periodontopathogens A. actinomycetemcomitans and P. gingivalis.

METHODS

Phagocytosis, cell-cell interactions and activation of platelets and neutrophils in response to periodontopathogens were analyzed by flow cytometry, confocal microscopy and bacteria survival assay. Plasma factors, platelet signaling pathways and receptors involved in platelet-neutrophil-bacteria interactions were determined. The role of platelet and neutrophil TLR2 in phagocytosis was further evaluated in a murine TLR2 knockout model.

RESULTS

In the presence of plasma neutrophil-mediated clearance of periodontopathogens is doubled due to opsonisation of bacteria. Platelets, which become activated by periodontopathogens, further enhance clearance of bacteria by 20%, via direct interaction with neutrophils. Plasma factors (e.g. CD14) are required for platelet activation, which is mainly TLR2 dependent and results in PI3K/Akt activation. In a murine TLR2 knockout model we prove that platelet TLR2 is important for formation of platelet-neutrophil aggregates and enhanced phagocytosis of periodontopathogens. In contrast, neutrophil TLR2 is not involved in platelet-neutrophil aggregate formation but is required for efficient phagocytosis.

CONCLUSIONS

These data indicate that efficient elimination of periodontopathogens by neutrophils involves a complex interplay of plasma factors as well as platelets and requires functional TLR2. By enhancing neutrophil activation platelets contribute to immune defense but may also foster inflammation.

Regulatory effects of TLR2 on megakaryocytic cell function

TLR2, a functional, inflammatory-related receptor, is known to be expressed on megakaryocytes and platelets and to lead to infection and immune-mediated activation of platelets; however, the role of this receptor in megakaryocytes is not understood. Using Meg-01 cells and mouse megakaryocytes, we found that NFκB, ERK-MAPK, and PI3K/Akt pathways, known downstream pathways of TLRs, are activated by Pam3CSK4, a TLR2-specific ligand. In addition, transcription factors associated with megakaryocyte maturation, GATA-1, NF-E2, and mammalian target of rapamycin (mTOR), are all increased in the presence of Pam3CSK4. The effect of Pam3CSK4 on megakaryocyte maturation was verified by the increase in DNA content and adhesion to extracellular matrix proteins by TLR2-dependent stimulation. In addition, TLR2 stimulation resulted in an increase in reactive oxygen species (ROS) production. Gene expression and protein levels of GP1b, CD41, MCP-1, COX2, NFκB1, and TLR2 were up-regulated in megakaryocytes after TLR2 stimulation through NFκB, PI3K/Akt, and ERK-MAPK pathways. Treatment of wild-type mice with Pam3CSK4 resulted in a return to normal platelet levels and an increase in megakaryocyte maturation, which did not occur in the TLR2(-/-) mice. Therefore, inflammation, through TLR2, can increase maturation and modulate the phenotype of megakaryocytes, contributing to the interrelationship between inflammation and hemostasis.

[In vitro platelet production]

This review aims at presenting a state of the art on platelet functions, not only in well-characterized hemostasis and thrombosis, but also in various domains such as inflammation, immunity, angiogenesis, source of growth factors, metastasis and vascular remodelling. This multivalent phenotype of platelets suggests new potential applications of platelets. The second objective is to present new advances in platelet formation from megakaryocytes and direct platelet release, as initially shown by our group and more recently by others.

Effect of platelet additive solution on bacterial dynamics and their influence on platelet quality in stored platelet concentrates

BACKGROUND

Platelet additive solutions (PASs) are an alternative to plasma for the storage of platelet concentrates (PCs). However, little is known about the effect of PAS on the growth dynamics of contaminant bacteria. Conversely, there have been no studies on the influence of bacteria on platelet (PLT) quality indicators when suspended in PAS.

STUDY DESIGN AND METHODS

Eight buffy coats were pooled, split, and processed into PCs suspended in either plasma or PAS (SSP+, MacoPharma). PCs were inoculated with 10 and 100 colony-forming units (CFUs)/bag of either Serratia liquefaciens or Staphylococcus epidermidis. Bacterial growth was measured over 5 days by colony counts and bacterial biofilm formation was assayed by scanning electron microscopy and crystal violet staining. Concurrently, PLT markers were measured by an assay panel and flow cytometry.

RESULTS

S. liquefaciens exhibited an apparent slower doubling time in plasma-suspended PCs (plasma-PCs). Biofilm formation by S. liquefaciens and S. epidermidis was significantly greater in PCs stored in plasma than in PAS. Although S. liquefaciens altered several PLT quality markers by Days 3 to 4 postinoculation in both PAS- and plasma-PCs, S. epidermidis contamination did not produce measurable PLT changes.

CONCLUSIONS

S. liquefaciens can be detected more quickly in PAS-suspended PCs (PAS-PCs) than in plasma-PCs by colony counting. Furthermore, reduced biofilm formation by S. liquefaciens and S. epidermidis during storage in PAS-PCs increases bacteria availability for sampling detection. Culture-based detection remains the earliest indicator of bacterial presence in PAS-PCs, while changes of PLT quality can herald S. liquefaciens contamination when in excess of 10(8) CFUs/mL.

Pathogen sensing, subsequent signalling, and signalosome in human platelets

Beyond haemostasis, platelets exert a potent role in innate immunity and particularly in its inflammatory arm. The extent of this action remains however debatable, despite clear - and old - evidence of a link between platelets and infection. Platelets can sense infectious pathogens by pathogen recognition receptors and they can even discriminate between various types of infectious signatures. In reply, they can shape their capacity to respond by activating a signalosome and by producing different profiles of pro-inflammatory cytokines and related products. The links between pathogen sensing, signalosome activation and protein production, and their finely tuned regulation are still under investigation since platelets lack a nucleus and thus, canonical molecular biology and genomics apparati.

Bacterial-platelet interactions: virulence meets host defense

Platelets have historically been viewed as cell fragments that only mediate blood coagulation. Yet, platelets have as - or perhaps even more - important roles in tissue remodeling, modulation of inflammation and antimicrobial host defense. It is evident that platelets interact with prokaryotes directly and indirectly through multiple molecular and cellular mechanisms. The important roles of platelets in antibacterial host defense can be exemplified through contemporary themes in platelet immunobiology. Platelets have unambiguous structures and functions of host defense effector cells. Recent discoveries reveal platelet expression of toll-like and purinonergic receptors, which enable detection and response to bacterial infection, degranulation of an array of microbicidal peptides and coordination of other molecular and cellular host defenses. From multiple perspectives, platelets are now increasingly recognized as critical innate immune effector cells that also bridge and facilitate optimization of adaptive immunity. It follows that clinical deficiencies in platelet quantity or quality are now recognized correlates of increased risk and severity of bacterial and other infections. Along these lines, new evidence suggests that certain prokaryotic organisms may be capable of exploiting platelet interactions to gain a virulence advantage. Indeed, certain bacterial pathogens appear to have evolved highly coordinated means by which to seize opportunities to bind to surfaces of activated platelets, and exploit them to establish or propagate infection. Hence, it is conceivable that certain bacterial pathogens subvert platelet functions. From these perspectives, the net consequences of bacterial virulence versus platelet host defenses likely decide initial steps towards the ultimate result of infection versus immunity.

The role of inflammation in regulating platelet production and function: Toll-like receptors in platelets and megakaryocytes

Platelets have been extensively studied as hemostatic regulators, stopping uncontrolled flow of blood from an injured vessel and allowing for repair. However, multiple studies have shown that platelets can interact with bacterial proteins, particularly seen during sepsis and inflammation. Immune cells recognize pathogens through Toll-like Receptors (TLRs). These same receptors allow platelets to recognize bacterial proteins and regulate platelet immunity and function. This review examines the TLRs expressed on platelets and megakaryocytes and how these receptors affect the function of these cells. Through TLRs, platelets go beyond hemostatic regulation and play a pivotal role in inflammation and infection.