key: cord-0011943-1q3qdk10 authors: Page, Martin J.; Pretorius, Etheresia title: A Champion of Host Defense: A Generic Large-Scale Cause for Platelet Dysfunction and Depletion in Infection date: 2020-04-12 journal: Semin Thromb Hemost DOI: 10.1055/s-0040-1708827 sha: 1fed785973e444ad11eeddc14616fbca15cd1de4 doc_id: 11943 cord_uid: 1q3qdk10 Thrombocytopenia is commonly associated with sepsis and infections, which in turn are characterized by a profound immune reaction to the invading pathogen. Platelets are one of the cellular entities that exert considerable immune, antibacterial, and antiviral actions, and are therefore active participants in the host response. Platelets are sensitive to surrounding inflammatory stimuli and contribute to the immune response by multiple mechanisms, including endowing the endothelium with a proinflammatory phenotype, enhancing and amplifying leukocyte recruitment and inflammation, promoting the effector functions of immune cells, and ensuring an optimal adaptive immune response. During infection, pathogens and their products influence the platelet response and can even be toxic. However, platelets are able to sense and engage bacteria and viruses to assist in their removal and destruction. Platelets greatly contribute to host defense by multiple mechanisms, including forming immune complexes and aggregates, shedding their granular content, and internalizing pathogens and subsequently being marked for removal. These processes, and the nature of platelet function in general, cause the platelet to be irreversibly consumed in the execution of its duty. An exaggerated systemic inflammatory response to infection can drive platelet dysfunction, where platelets are inappropriately activated and face immunological destruction. While thrombocytopenia may arise by condition-specific mechanisms that cause an imbalance between platelet production and removal, this review evaluates a generic large-scale mechanism for platelet depletion as a repercussion of its involvement at the nexus of responses to infection. inflammatory responses against bacterial and viral infections. 3, 21, 22 During infection, the platelet is activated, mobilized, and actively participates in the resultant hemostatic and inflammatory responses. These signaling processes involve many feedback loops that self-amplify initial activation, 23 and platelets can manifest dysfunction even in cases where no bacteremia is present. 10 These processes are irreversible and undoubtedly lead to consumption of the platelet. Activation of platelets leads to their consumption into aggregates with other platelets, leukocytes, and the endothelium. 24 Platelets with bound antibody are targets of phagocytes, and platelets with a bacterial or viral load are sequestrated and also cleared from the circulation. Further, pathogenic compounds induce apoptosis and cytotoxic effects in platelets. 25 In this sense, activated platelets and platelets interacting with pathogens have shortened survival spans and experience increased destruction. The outcome for the patient will be a decrease in normal circulating platelets, and if this manifests widely enough it can be measured as thrombocytopenia. 3, 25 Other mechanisms of platelet decline in infection exist and include the formation of autoantibodies against platelet surface proteins, which leads to clearance of immunoglobulin G (IgG)coated platelets by the reticuloendothelial system, 26, 27 as well as by impaired platelet production in the bone marrow, 3,6 among others. 6 However, a general view of platelet destruction is the simple characteristic that their involvement in thrombotic, hemostatic, immune, and host defense responses is irreversible. Even if platelets are positive contributors to the host response against invading pathogens, they can become dysfunctional, especially in the context of an excessive and unbalanced systemic inflammatory response. 16, 28 Indeed, the dysfunctional state of thrombocytopenia is commonly associated with sepsis and infections. 3, [29] [30] [31] The focus of the current review is platelets and their role in infection. We will examine the interaction of platelets, their receptors, and secretory product with bacteria and viruses, and discuss how this may contribute to platelet dysfunction and ultimately lead to thrombocytopenia. ►Fig. 1 provides the rationale of this review and ►Table 1 lists the abbreviations used in this article. A common feature of many infections, both viral and bacterial, is a systemic inflammatory response that involves a dysregulated proinflammatory biomarker presence in the circulation. 3, 5, 32 These biomarkers may include cytokines (e.g., interleukins [ILs] , tumor necrosis factor [TNF]-α, and interferons) but also molecules originating from bacteria and viruses themselves (e.g., proteases, ribonucleic acid [RNA] , and membrane components like lipopolysaccharide [LPS] , lipoteichoic acid [LTA] , and viral glycoproteins). The presence of such circulating biomarkers has profound agonistic effects on platelets. Platelets contribute to the thromboinflammatory response through the plethora of membrane and cytosolic molecules that they express and release, which possess hemostatic, immunomodulatory, and inflammatory activity. [1] [2] [3] [4] Platelets possess receptors that enable pathogen sensing, and which allow platelets to regulate leukocytes and other cells at the site of infection. During platelet activation, degranulation leads to the surface expression of receptors and the release of abundant proinflammatory mediators, which contribute to numerous signaling events. [1] [2] [3] [4] [5] Platelets also adhere and aggregate to other platelets and to endothelial cells, leukocytes, and erythrocytes. 5, 9, 24 This response is also characteristic during bacterial and viral infections, and can be induced by pathogens directly. 33 This section describes the role of platelets in the immune response. See ►Fig. 2 for a general overview of platelet receptors and secretory products. Endothelial activation markers are raised during infection, and are associated with a thrombotic state. 34 During activation, platelets can bind to the endothelium. 24 This especially occurs upon endothelial damage due to trauma or microbial colonization, 35 as well as in viral infections. 36 Platelets become activated during the adhesion process, and the inflammatory and mitogenic substances that are released alter the chemotactic, adhesive, and proteolytic properties of endothelial cells. 37 Platelet adhesion therefore endows the endothelium with a proinflammatory phenotype. 24 Moreover, platelets that are bound to the endothelium can form a bridging connection with circulating leukocytes. 24 Overall, these mechanisms amplify and facilitate leukocyte recruitment and enhance inflammation. ►Fig. 3 provides an overview of the contact between platelets and cells at the vascular wall to emphasize the involvement of platelets in multiple interactions at the vessel wall. Interactions between platelets and leukocytes are important for the regulation of the immune response and for the clearance of infectious agents. By binding and activating leukocytes, platelets promote their effector functions. Coordination of Fig. 1 Layout of the review. During infection, inflammatory stimuli, and the presence of bacteria, viruses and their products mobilize platelets to exert their immune, antibacterial, and antiviral actions. However, these processes can also lead to platelet dysfunction and ultimately depletion. immune cells by platelets ensures a rapid and targeted host defense response. In a dynamic cross-talk, leukocytes can also release factors that modulate platelet function. Platelets adhere to phagocytes and deliver signals that enhance the killing of internalized pathogens. Platelets are able to modulate neutrophil responses where they enhance neutrophil phagocytosis in a process involving toll-like receptor (TLR) 2 and P-selectin/P-selectin glycoprotein ligand (PSGL)-1. 38 This was demonstrated for both Aggregatibacter actinomycetemcomitans and Porphyromonas gingivalis. 38 Platelets can augment the respiratory burst in neutrophils in response to opsonized Escherichia coli and Staphylococcus aureus. 39 Platelet-neutrophil complexes have more activated adhesion molecules, greater phagocytic ability, and greater toxic oxygen metabolites than noncomplexed neutrophils. 40 Activated platelets can also induce superoxide anion release by monocytes and neutrophils through P-selectin. 41 Soluble CD40 ligand (CD40L) further interacts with CD40 and αMβ2 on neutrophils to induce the adhesive functions of neutrophils as well as cause CD40-dependent reactive oxygen species (ROS) generation. 42 Additionally, the triggering receptor expressed on myeloid cells (TREM)-1 ligand is expressed on platelets and has been shown to induce neutrophil activation, and platelets enhance the neutrophil respiratory burst and release of IL-8 in a TREM-1specific manner in the presence of LPS. 43 The TREM-1 receptor is an important receptor in the innate immune response as well as in severe sepsis where it amplifies the immune response to microbial products. 44 TREM-1 has also been shown to contribute to neutrophil activation in viral infections. 45 Furthermore, platelets induce the release of neutrophil extracellular traps (NETs), deoxyribonucleic acid (DNA) covered with various antimicrobial nuclear and granule-derived molecules 46 that ensnare and kill pathogens, in response to bacterial (septic) stimuli. 39, 47, 48 This NETresponse has been documented in E. coli gram-negative sepsis and S. aureus gram-positive sepsis. 47 Platelets have further been shown to interact with neutrophils following viral challenge, leading to the release of NETs. [49] [50] [51] NETs also deliver antiviral factors such as myeloperoxidase 46 and α-defensin, 50 and capture viruses and promote their elimination. 51 ►Fig. 4 provides an overview of the interactions between platelets and immune cells to emphasize the involvement of platelets in the immune response. Further to the innate immune response, platelets are also important for an optimal adaptive immune response. The periodontopathogens A. actinomycetemcomitans and P. gingivalis have been shown to induce expression of CD40L on human platelets via TLR2 and TLR4. 52 Platelets can modulate B and T cell responses to microbial pathogens through CD40L, and are able to induce isotype switching by B cells and augment CD8 þ T cell function. 53,54 CD40L on platelets enable T cell priming and augment CD8 þ T cell responses against bacterial pathogens by enhancing maturation signals to dendritic cells and lowering the threshold for cell activation [55] [56] [57] (compare with reports that platelets can have an inhibitory effect on dendritic cells 58, 59 ). Platelet-mediated modulation of the adaptive immune system has also been shown to enhance protection against viral rechallenge. 53 Platelets expressing integrin β3 and CD40L are essential for lymphocytic choriomeningitis virus (LCMV) clearance by virus-specific cytotoxic T cells, and protect the host from virus-induced interferon-α/β lethal hemorrhage. 18 Activated platelets can also contribute to immunopathology (e.g., liver damage) by accumulating virus-specific cytotoxic T cells at the site of inflammation in models of acute viral hepatitis. 60 Serotonin released from platelets is vasoactive and can further support viral persistence in the liver by reducing microcirculation, which aggravates virus-induced immunopathology in a model of LCMV-induced hepatitis. 61 Platelets can further shuttle blood-borne gram-positive bacteria to splenic CD8α þ dendritic cells after the bacterium becomes associated to platelets via glycoprotein (GP)-Ib and Platelets are activated by various agonists that interact with surface receptors. Platelets are also replete with secretory granules that store bioactive molecules, which are released into the circulation or translocate to the surface upon platelet activation. These characteristics allow platelets to communicate and modulate the functions of other cells, and trigger hemostatic, inflammatory, and host defense responses against infections (created with https://biorender.com/). ADP, adenosine diphosphate; CAR, coxsackievirus and adenovirus receptor; CCR/CXCR, chemokine receptor; CLEC, C-type lectin-like receptor; CR, complement receptor; DC-SIGN, dendritic cell-specific ICAM-grabbing nonintegrin; FcγRIIa, low-affinity immunoglobulin gamma Fc region receptor II-a; gC1Qr, receptor for the globular heads of C1q; JAM, junction adhesion molecule; MCP, monocyte chemoattractant protein; MHC, major histocompatibility complex; MIP, macrophage inflammatory protein; PAFR, platelet-activating factor receptor; PAR, protease-activated receptor; PDGF, platelet-derived growth factor; PF, platelet factor; RANTES, regulated on activation, normal T-cell expressed and secreted; TGF, transforming growth factor; TLR, toll-like receptor; TNSF14, tumor necrosis factor superfamily member 14; TREM, triggering receptor expressed on myeloid cells; vWF, von Willebrand factor. Platelet-Derived Microparticles: Further Driving the Inflammatory Response Activated platelets produce microparticles during bacterial 63, 64 and viral infection 65,66 that contain both soluble (e.g., regulated on activation, normal T cell expressed and secreted [RANTES] ) and surface mediators (e.g., P-selectin, GPIb, and αIIbβ3), which can exit the vasculature and enter tissues where they are able to activate leukocytes to further drive the inflammatory response. 67, 68 For example, platelet microparticles enhance the expression of cell adhesion molecules such as leukocyte αMβ2 for monocyte adhesion, 69 and can mediate leukocyte activation 70 and leukocyte-leukocyte interactions. 71 Microparticles promote platelet interaction with the endothelium by acting as a substrate for further platelet binding. 72 Further, microparticles can deliver platelet-derived CD40L signals 54, 73 and activate dendritic cells. 74 Platelet microparticles also promote endothelial activation by secreting IL-1β, 75 and can deliver RANTES to the endothelium for monocyte recruitment. 76 Lastly, these microparticles can cause complement activation. 77 Platelets are active role players in antimicrobial defense, and exhibit complex interactions with bacteria and viruses due to the variety of platelet receptors involved in pathogen recognition. Platelets are able to recognize, bind, and internalize pathogens to sequester and neutralize the pathogen. This section describes the interactions of platelets with bacteria, which are summarized in ►Fig. 5. It has long been known that bacteria can cause platelet aggregation and degranulation. 78, 79 A diverse range of platelet receptors can mediate interactions with bacteria, including αIIbβ3, low-affinity immunoglobulin gamma Fc region receptor II-a (FcγRIIa), GPIb, complement receptors (CRs), and TLRs, 80,81 either directly or indirectly through bridging molecules. 11, 12, 81 Alternatively, products shed by bacteria 82 may cause a platelet response independently of direct bacterial attachment to the platelet. 10 Ultimately, engagement of receptors by bacteria and their products leads to common and species-specific intracellular signaling events in platelets. 83 ►Table 2 summarizes platelet receptors that mediate binding of bacteria to cause platelet activation and aggregation. A key mechanism for bacterial adhesion to platelets, which is described for various bacteria, involves αIIbβ3 integrin activation, the FcγRIIa receptor, and IgG, 84 where platelet factor (PF)-4 may potentiate further binding of additional bacteria by forming an immunocomplex with bacteria that bind through FcγRIIa. 85 Platelets also express C-C motif and C-X-C motif chemokine receptors such as CCR1, CCR2, CCR4, and CXCR4, 86 which can detect all four classes of chemokines (C, CC, CXC, and CX 3 C). These receptors allow platelets to recognize and prioritize chemotactic signals and result in rapid vectoring of platelets to sites of infection. 9 They are also involved in stimulating platelet adhesion, aggregation, and secretion. 87 Additionally, platelet activation leads to activation of the complement system, 88, 89 and platelets also express various complement receptors after activation such as CR2, CR3, CR4, C3aR, C5aR, cC1qR, and gC1qR. 3 These may therefore serve as potential receptors for bacteria coated with complement factors, and lead to platelet aggregation. 11 Furthermore, an important class of receptors for pathogen sensing are TLRs, and platelets express numerous TLRs to detect the molecular features of microbes. 21 Platelets are able to respond to many bacterial products, and these products modulate platelet function. 25 LPS can stimulate platelet secretion of dense and α-granules through TLR4/ MyD88 and cyclic guanosine monophosphate (cGMP)/cGMPdependent protein kinase (PKG) signaling pathways. 94 This potentiates secretion-dependent integrin activation and platelet aggregation. Further to this, platelets recognize and discriminate between various isoforms of bacterial LPS and secrete differential cytokine profiles against these danger signals. 95, 96 LPS also induces sCD40L release from platelets 97 as well as ROS generation. 98 Some sources of LPS can activate TLR2, [99] [100] [101] and this has also been implicated in LPS-induced cGMP elevation and platelet activation. 94 However, LPS is described as not always generating conventional platelet activation (e.g., typical P-selectin release from α-granules). 25 Bacterial structures from gram-positive bacteria such as lipoproteins, peptidoglycan, and LTA are TLR2 ligands, and also trigger platelet activation. 92,102 TLR activation in platelets induces a thromboinflammatory response, including platelet aggregation, formation of platelet-leukocyte complexes, and ROS generation 103 as well as the elaboration of acutephase reactants like TNF-α. 91 However, studies have shown mixed effects of TLR2 agonists and LTA on platelet aggregation. 104, 105 Platelets can migrate toward the chemotactic signal of bacterial N-formyl peptide by their receptors for this peptide. 106 The gingipain proteases HRgpA and RgpB from P. gingivalis activate platelet protease-activated receptor (PAR)-1 and PAR4, leading to platelet aggregation. 107,108 S. aureus α-toxin also causes platelet activation and leads to enhanced prothrombinase activity on the platelet surface. 109, 110 Staphylococcal superantigen-like (SSL)-5 from S. aureus additionally induces platelet activation via platelet receptors GPVI and GPIb, 111, 112 whereas the Panton-Valentine leukocidin toxin leads to platelet activation via neutrophil secretion products from damaged neutrophils. 113 Another class of exotoxins from S. aureus, extracellular adherence protein (Eap) and extracellular fibrinogen-binding protein (Efb) fibrinogen-binding proteins, also interacts with platelets. On the one hand, Eap enhances αIIbβ3 integrin activation, granule secretion, and aggregation, 114 whereas Efb inhibits platelet activation and aggregation 115, 116 and has powerful antiplatelet actions. 117 Staphylococcus aureus enterotoxin B similarly inhibits platelet aggregation. 118 LTA from S. aureus has also been reported to inhibit platelet activation through platelet-activating factor (PAF) receptor and raised cyclic adenosine monophosphate (cAMP), 119 as well as to inhibit platelet aggregation, [120] [121] [122] but may support platelet adhesion to Staphylococcus epidermidis. 123 Additional products released by S. aureus also have opposing functions on platelet aggregation. While staphylothrombin mediates fibrin formation that supports aggregation, 124 staphylokinase prevents aggregation by degrading fibrinogen. 125 Bacterial toxins can also cause platelet destruction. For example, α-toxin from S. aureus and α-hemolysin from E. coli 126 as well as peptidoglycan from S. aureus 127 can induce platelet apoptosis. Indeed, these pore-toxins stimulate disturbances in the platelet membrane and can be cytotoxic. 3, 128 Escherichia coli Shiga toxin causes downregulation of platelet CD47 expression, which leads to enhanced platelet activation and phagocytosis of platelets by macrophages. 129 Toxins such as pneumolysin from Streptococcus pneumoniae 130 and α-toxin from S. aureus 131 can cause platelet lysis, whereas streptolysin O from Streptococcus pyogenes 132 and phospholipase C from Clostridium perfringens 133 induce the formation of platelet-leukocyte complexes. A further function of platelets in bacterial infection is mediating antimicrobial attack. Platelets mediate some of their antimicrobial actions through the secretion of potent antimicrobial proteins from their α-granules. 8, 35 Moreover, platelets rapidly form clusters around bacteria that have been captured by Kupffer cells in the liver sinusoids (specialized macrophages in the liver), encasing the bacterium and facilitating its destruction. 13 Further, sCD40L causes increased generation and release of reactive oxygen (e.g., superoxide) and nitrogen (e.g., nitric oxide) species by platelets, which assists in pathogen destruction. 134, 135 Platelets are able to bind and endocytose/phagocytose bacteria through engulfing endosome-like vacuoles that are formed by membrane endocytosis and become the site of α-granule release for the granular proteins to access the pathogen. 136, 137 A mechanism of internalizing bacteria via the open canalicular system has also been proposed 138 (compare with Boukour and Cramer 139 ). Nonetheless, the platelet FcγRIIa receptor can bind IgG complexes and allows platelets to clear these complexes from the circulation. 140 Internalization of IgG-coated particles results in platelet activation and the release of RANTES and sCD40L. 141 Platelets opsonized by IgG can be destroyed by Fc-mediated platelet phagocytosis, contributing to the clearance of IgG-containing complexes from the circulation. 142, 143 More broadly, activated platelets expose phosphatidylserine, and neutrophils have been shown to phagocytose activated platelets in a clearance program involving phosphatidylserine and P-selectin. [144] [145] [146] Platelet Interactions with Viruses Viruses have been observed to interact directly with platelets. Various viruses have been identified adsorbed to or inside platelets, including influenza virus, 147, 148 HIV, 136, 149, 150 hepatitis C, [151] [152] [153] and herpes simplex virus 154 as well as others such as vaccinia virus 155 and dengue virus. [156] [157] [158] However, the interactions between viruses and platelets are less well characterized compared with those of gram-positive bacteria. This section describes the interaction of platelets with viruses, which are summarized in ►Fig. 6. Several platelet receptors have been identified to mediate binding to viral particles, 6, 7, 30, 159 and are summarized in ►Table 3. Similarly to bacteria, IgG is important for the adhesion of viral particles to platelets, where IgG-coated particles can interact with the FcγRIIa receptor 151,160-162 to be internalized into the platelet. 140 However, other antibody-dependent mechanisms that enhance viral binding to platelets are also described, 156 and platelets can further bind viruses in a receptor-independent manner. 163 For example, although the coxsackievirus and adenovirus receptor (CAR) is expressed on platelets, coxsackie B virus interaction with platelets has also been described independently of CAR and can result in P-selectin and phosphatidylserine exposure. 163 More broadly, β3 integrins are important platelet-adhesion receptors, and these receptors appear to facilitate viral adhesion to platelets. 18, 65, 164 Even though various receptors that are expressed on platelets have been implicated in viral adhesion and cell entry, the direct effect of this interaction on the platelet has not always been described. Platelets can also detect viruses through TLRs. Platelet TLR2 can bind cytomegalovirus, which triggers platelet activation, degranulation, and the formation of plateletleukocyte aggregates. 165 TLR7 recognizes the classical viral PAMP, single-stranded RNA. 92 Platelets express functional TLR7, and activation via TLR7 leads to expression of CD40L and P-selectin, and P-selectin supports the adhesion of virally activated platelets to neutrophils. 22, 166 Moreover, platelet TLR7 mediates complement C3 release from platelets, which in turn leads to platelet-neutrophil aggregation and NET release by neutrophils. 167 Encephalomyocarditis virus has been shown to interact with platelet TLR7. 166 Platelet TLR9 recognizes unmethylated CpG islands found in bacterial and viral DNA, which also leads to P-selectin surface expression. 92 Viruses secrete various products that modulate platelet function. The secreted HIV Tat protein directly interacts with platelets in a process requiring the platelet receptors CCR3 and β3 integrin as well as calcium influx. This leads to platelet activation and CD40L expression as well as microparticle formation. 65 Indeed, platelet activation persists even in virologically suppressed HIV infection. 169 Viral enzymes such as neuraminidase can cause desialylation of platelet surface receptors, 6 and desialylation might promote platelet clearance in the liver. 170, 171 Platelets Mediate Antiviral Attack The secretory products of platelets can also exert virucidal effects, including the inactivation of adenovirus, poliovirus and vaccinia virus, 172 and HIV suppression. 20 Moreover, platelets exhibit phagocytic behavior toward viruses such as HIV and can form engulfing vacuoles that lead to granular components being secreted on the virus particle, as described for bacteria. 136 Indeed, intact HIV-1 particles enclosed in endocytic vesicles have been found in the open canalicular system. 173, 174 Recently, it has been proposed that platelets may also potentially phagocytose influenza virus. 175, 176 Platelets may then Fig. 6 Platelet interactions with viruses. Various platelet receptors can mediate binding to viral particles; however, the direct effect of this binding on platelets is less well described than for bacteria. Pattern recognition receptors recognize classical viral signals, and viral products also modulate platelet function. Platelets mediate viral attack by secreting virucidal proteins and by engulfing viral particles, as well as by interacting with immune cells and enhancing the immune response. Overall, platelets may be activated and aggregate, but also face apoptosis. Virus-platelet aggregates and platelets with a viral load are targeted by leukocytes, and platelets are ultimately cleared from the circulation (created with https://biorender.com/). CAR, coxsackievirus and adenovirus receptor; CCR/CXCR, chemokine receptor; CLEC, C-type lectin-like receptor; CR, complement receptor; DC-SIGN, dendritic cell-specific ICAMgrabbing nonintegrin; FcγRIIa, low-affinity immunoglobulin gamma Fc region receptor II-a; HIV, human immunodeficiency virus; Ig, immunoglobulin; RNA, ribonucleic acid; ROS, reactive oxygen species; Tat, trans-activator of transcription; TLR, toll-like receptor; vWF, von Willebrand factor. cause disruption of viral integrity. 174 Overall, it has been suggested that internalization of viral particles by platelets may function to clear viruses from the circulation. 177 Viruses can cause the expression of P-selectin and phosphatidylserine exposure on platelets, and these components promote interactions with leukocytes as well as lead to phagocytosis of the platelet. 163, 178 Interaction between platelets and viruses can also lead to sequestration to the reticuloendothelial system of the liver, where virus-platelet aggregates can be taken up by Kupffer cells and degraded. 179 Spleen macrophages also assist in clearing platelets with a viral load. 30 Platelets are among the first cells to accumulate at sites of infection and inflammation, and can be considered as first responders to invading pathogens. Here, platelets have a key role in sensing and effecting the first wave of responses to microbial and viral threat. 8, 9 This is achieved by the inflammatory activity of platelets but also through direct antibacterial and antiviral actions that facilitate the clearance of pathogens from the circulation. Platelets are therefore represented at the interface of hemostasis, inflammation, and antimicrobial host defense. Their position at the crossroads of these processes emphasizes their role as signaling entities in infection and inflammation. Various stimuli that are relevant to infection impinge on platelets, activating and forcing them to exert their effector actions. Recursive stimulation of activation receptors and successive activation of bystander platelets intensify the hostdefense functions of platelets even at threshold stoichiometric ratios of platelets to pathogens. 180 Platelets face inappropriate activation and immunological destruction, and are inevitably consumed by their participation in host defense. An inflammatory milieu can thereby drive platelet dysfunction. In this review, we emphasize that platelet dysfunction can arise as a general consequence of an exaggerated systemic (immune) response to infection. Increased platelet consumption and removal can lead to thrombocytopenia, which is frequently observed during infection. ►Fig. 7 summarizes and links together the various processes we have discussed, to show a general mechanism of platelet depletion during infection. Because of their largely protective role, lower platelet counts are associated with worse prognosis and greater likelihood of infection; however, platelets are also presented as having an ambivalent role in infections by possibly sheltering pathogens in certain cases. 6, 7, 9, 12, 30, 181 Nonetheless, in the context of impairment of the immune system, the functions of platelets become more important. Following the contribution of platelets to diverse immunological processes, dysregulation of platelet-leukocyte interactions, which are important for inflammatory and immune reactions, together with dysregulation of inflammatory mediators, establish an excessive and unbalanced systemic inflammatory response. In this context, platelets can contribute to pathophysiological processes and immunopathology, and become dysfunctional. Achieving a balance between pro-and anti-inflammatory responses during infection is difficult to manipulate effectively in a therapeutic context. Following from the diverse functions of platelets in infections, platelets are also placed at an interface between health and disease. Platelets are acutely affected by the surrounding environment. This, together with other characteristics of platelets such as their fast turnover, might position platelets as relevant signaling entities with clinical potential in disease tracking and targeting to evaluate or manage the course of infections. Although platelets are perhaps a lesser-known participant in the host-defense system, their large-scale depletion may cause significant health issues. Managing a generic depletion of platelets during the presence of infection should possibly be a more actively pursued clinical goal. The key points encapsulating the main ideas of this review are presented in ►Table 4. • Platelets are versatile cells positioned at the interface of hemostasis, inflammation, and antimicrobial host defense, and their immune, antibacterial, and antiviral actions establish them as active participants in infection. • By nature of their normal functioning, platelets are invariably and irreversibly expended in the processes to which they contribute. • During infection, an onslaught of inflammatory and pathogen-derived stimuli can evoke and challenge platelets, leading to inappropriate activation, immunological destruction, and sequestration. • In the context of a dysregulated host response to infection, platelets can experience overwhelming activation and, consequently, consumption, and this represents a generic large-scale mechanism for platelet depletion in infection. Platelets and innate immunity Platelets and the immune continuum Platelets as immune cells in infectious diseases Platelets: signaling cells in the immune continuum Platelets in inflammation and infection Platelets and infection -an emerging role of platelets in viral infection Platelets in immune response to virus and immunopathology of viral infections Platelets in defense against bacterial pathogens Platelets: at the nexus of antimicrobial defence The interaction of bacterial pathogens with platelets Platelets and infections -complex interactions with bacteria Platelets and the innate immune system: mechanisms of bacterial-induced platelet activation Nucleation of platelets with blood-borne pathogens on Kupffer cells precedes other innate immunity and contributes to bacterial clearance Effect of thrombocytopenia on the early course of streptococcal endocarditis Inhibiting platelets aggregation could aggravate the acute infection caused by Staphylococcus aureus Molecular Diagnosis and Risk Stratification of Sepsis Consortium. Thrombocytopenia is associated with a dysregulated host response in critically ill sepsis patients Blood platelets and sepsis pathophysiology: a new therapeutic prospect in critically ill patients? Ann Intensive Care Platelets prevent IFN-α/βinduced lethal hemorrhage promoting CTL-dependent clearance of lymphocytic choriomeningitis virus Platelets support a protective immune response to LCMV by preventing splenic necrosis Platelet activation suppresses HIV-1 infection of T cells Platelet toll-like receptors in thromboinflammation Platelets: crossroads of immunity and hemostasis Signaling during platelet adhesion and activation Molecular and functional interactions among monocytes, platelets, and endothelial cells and their relevance for cardiovascular diseases Platelet interaction with bacterial toxins and secreted products Role of molecular mimicry of hepatitis C virus protein with platelet GPIIIa in hepatitis C-related immunologic thrombocytopenia Antiplatelet antibodies contribute to thrombocytopenia associated with chronic hepatitis C virus infection Platelets: emerging facilitators of cellular crosstalk in rheumatoid arthritis Thrombocytopenia and infections Platelets and viruses: an ambivalent relationship Sepsisassociated thrombocytopenia Innate immune responses to infection Streptococcus sanguinis-induced cytokine release from platelets Biomarkers of endothelial activation/dysfunction in infectious diseases The role of platelets in antimicrobial host defense Influenza virus infection induces platelet-endothelial adhesion which contributes to lung injury Platelets in inflammation and atherogenesis Efficient phagocytosis of periodontopathogens by neutrophils requires plasma factors, platelets and TLR2 Platelets augment respiratory burst in neutrophils activated by selected species of gram-positive or gram-negative bacteria Circulating platelet-neutrophil complexes represent a subpopulation of activated neutrophils primed for adhesion, phagocytosis and intracellular killing Activated platelets induce superoxide anion release by monocytes and neutrophils through P-selectin (CD62) Seminars in Thrombosis & Hemostasis Soluble CD40 ligand stimulates CD40-dependent activation of the β2 integrin Mac-1 and protein kinase C zeda (PKCζ) in neutrophils: implications for neutrophil-platelet interactions and neutrophil oxidative burst TREM-1 ligand expression on platelets enhances neutrophil activation TREM-1 amplifies inflammation and is a crucial mediator of septic shock Activation of triggering receptor expressed on myeloid cells-1 on human neutrophils by marburg and ebola viruses Neutrophil extracellular traps kill bacteria Intravascular neutrophil extracellular traps capture bacteria from the bloodstream during sepsis Platelets, neutrophils, and neutrophil extracellular traps (NETs) in sepsis Neutrophils recruited to sites of infection protect from virus challenge by releasing neutrophil extracellular traps Neutrophil extracellular traps contain calprotectin, a cytosolic protein complex involved in host defense against Candida albicans Neutrophil extracellular traps mediate a host defense response to human immunodeficiency virus-1 Periodontopathogens induce expression of CD40L on human platelets via TLR2 and TLR4 Platelet-mediated modulation of adaptive immunity. A communication link between innate and adaptive immune compartments Platelet influence on T-and B-cell responses Platelet-derived CD154 enables T-cell priming and protection against Listeria monocytogenes challenge CD40 ligand-dependent maturation of human monocyte-derived dendritic cells by activated platelets Platelets deliver costimulatory signals to antigen-presenting cells: a potential bridge between injury and immune activation Human platelets target dendritic cell differentiation and production of proinflammatory cytokines Direct contact of platelets and their released products exert different effects on human dendritic cell maturation Platelets mediate cytotoxic T lymphocyte-induced liver damage Aggravation of viral hepatitis by platelet-derived serotonin A plateletmediated system for shuttling blood-borne bacteria to CD8αþ dendritic cells depends on glycoprotein GPIb and complement C3 Activated platelets enhance microparticle formation and platelet-leukocyte interaction in severe trauma and sepsis Microparticle generation and leucocyte death in Shiga toxin-mediated HUS HIV-1 Tat-induced platelet activation and release of CD154 contribute to HIV-1-associated autoimmune thrombocytopenia Increased platelet and microparticle activation in HIV infection: upregulation of Pselectin and tissue factor expression Platelets amplify inflammation in arthritis via collagen-dependent microparticle production Function and clinical significance of platelet-derived microparticles Modulation of monocyte-endothelial cell interactions by platelet microparticles Involvement of platelet glycoprotein Ib in platelet microparticle mediated neutrophil activation Leukocyte-leukocyte interactions mediated by platelet microparticles under flow Platelet microparticles promote platelet interaction with subendothelial matrix in a glycoprotein IIb/IIIa-dependent mechanism Platelet-mediated modulation of adaptive immunity: unique delivery of CD154 signal by platelet-derived membrane vesicles Platelet-derived microparticles cause CD154-dependent activation of dendritic cells Lipopolysaccharide signaling without a nucleus: kinase cascades stimulate platelet shedding of proinflammatory IL-1β-rich microparticles Platelet microparticles: a transcellular delivery system for RANTES promoting monocyte recruitment on endothelium Expression of complement components and inhibitors on platelet microparticles Platelet interaction with bacteria. I. Reaction phases and effects of inhibitors Platelet interaction with bacteria. IV. Stimulation of the release reaction Platelets: essential components of the immune system The expanding field of platelet-bacterial interconnections Platelet-platelet and platelet-leukocyte interactions induced by outer membrane vesicles from N. meningitidis Bacteria-induced intracellular signalling in platelets Seminars in Thrombosis & Hemostasis Platelet Dysfunction and Depletion in Infection Page Amplification of bacteriainduced platelet activation is triggered by FcγRIIA, integrin αIIbβ3, and platelet factor 4 Bacteria exploit platelets Functional expression of CCR1, CCR3, CCR4, and CXCR4 chemokine receptors on human platelets Platelet chemokines and chemokine receptors: linking hemostasis, inflammation, and host defense Afshar-Kharghan V. Platelet activation leads to activation and propagation of the complement system Platelet mediated complement activation Evidence of Toll-like receptor molecules on human platelets Platelet Toll-like receptor expression modulates lipopolysaccharide-induced thrombocytopenia and tumor necrosis factor-alpha production in vivo Platelet Toll-like receptor expression: the link between "danger" ligands and inflammation Platelets express functional Toll-like receptor-4 Lipopolysaccharide stimulates platelet secretion and potentiates platelet aggregation via TLR4/MyD88 and the cGMP-dependent protein kinase pathway Human platelets can discriminate between various bacterial LPS isoforms via TLR4 signaling and differential cytokine secretion Toll-like receptor 4 ligand can differentially modulate the release of cytokines by human platelets Lipopolysaccharide induces sCD40L release through human platelets TLR4, but not TLR2 and TLR9 The generation of superoxide anion in blood platelets in response to different forms of Proteus mirabilis lipopolysaccharide: effects of staurosporin, wortmannin, and indomethacin Leptospiral lipopolysaccharide activates cells through a TLR2-dependent mechanism Porphyromonas gingivalis lipopolysaccharide contains multiple lipid A species that functionally interact with both toll-like receptors 2 and 4 Lipopolysaccharides of Bacteroides fragilis, Chlamydia trachomatis and Pseudomonas aeruginosa signal via toll-like receptor 2 Invasive Streptococcus pneumoniae trigger platelet activation via Toll-like receptor 2 Stimulation of Toll-like receptor 2 in human platelets induces a thromboinflammatory response through activation of phosphoinositide 3-kinase Toll-like receptor 2 stimulation of platelets is mediated by purinergic P2X1-dependent Ca2þ mobilisation, cyclooxygenase and purinergic P2Y1 and P2Y12 receptor activation Agonists of toll-like receptor (TLR)2 and TLR4 are unable to modulate platelet activation by adenosine diphosphate and platelet activating factor Human platelets exhibit chemotaxis using functional N-formyl peptide receptors Activation of proteaseactivated receptors by gingipains from Porphyromonas gingivalis leads to platelet aggregation: a new trait in microbial pathogenicity Aggregation of human platelets by gingipain-R from Porphyromonas gingivalis cells and membrane vesicles Staphylococcal alpha toxin promotes blood coagulation via attack on human platelets Staphylococcus aureus alpha-toxin attack on human platelets promotes assembly of the prothrombinase complex GPVI and GPIbα mediate staphylococcal superantigen-like protein 5 (SSL5) induced platelet activation and direct toward glycans as potential inhibitors Staphylococcal superantigen-like 5 activates platelets and supports platelet adhesion under flow conditions, which involves glycoprotein Ibα and αIIbβ3 Panton-Valentine leukocidin associated with S. aureus osteomyelitis activates platelets via neutrophil secretion products Staphylococcal extracellular adherence protein induces platelet activation by stimulation of thiol isomerases Extracellular fibrinogen-binding protein, Efb, from Staphylococcus aureus blocks platelet aggregation due to its binding to the alpha-chain Extracellular fibrinogen binding protein, Efb, from Staphylococcus aureus binds to platelets and inhibits platelet aggregation Extracellular fibrinogen binding protein, Efb, from Staphylococcus aureus as an antiplatelet agent in vivo Staphylococcal enterotoxin B initiates protein kinase C translocation and eicosanoid metabolism while inhibiting thrombin-induced aggregation in human platelets Staphylococcus aureus lipoteichoic acid inhibits platelet activation and thrombus formation via the Paf receptor Interaction of lipoteichoic acid of group A streptococci with human platelets Inhibitory effects of lipoteichoic acid from Staphylococcus aureus on platelet function and platelet-monocyte aggregation Seminars in Thrombosis & Hemostasis Mechanisms involved in the antiplatelet activity of Staphylococcus aureus lipoteichoic acid in human platelets Adherence of Staphylococcus epidermidis to fibrin-platelet clots in vitro mediated by lipoteichoic acid Fibrin formation by staphylothrombin facilitates Staphylococcus aureus-induced platelet aggregation Inhibitory effect of staphylokinase on platelet aggregation Bacteria differentially induce degradation of Bcl-xL, a survival protein, by human platelets Stimulation of platelet apoptosis by peptidoglycan from Staphylococcus aureus 113 Alpha-toxin of Staphylococcus aureus Down-regulation of platelet surface CD47 expression in Escherichia coli O157:H7 infection-induced thrombocytopenia Effects of pneumolysin on human polymorphonuclear leukocytes and platelets Hyperproduction of alpha-toxin by Staphylococcus aureus results in paradoxically reduced virulence in experimental endocarditis: a host defense role for platelet microbicidal proteins Vascular dysfunction and ischemic destruction of tissue in Streptococcus pyogenes infection: the role of streptolysin Oinduced platelet/neutrophil complexes Clostridial gas gangrene. II. Phospholipase C-induced activation of platelet gpIIbIIIa mediates vascular occlusion and myonecrosis in Clostridium perfringens gas gangrene Soluble CD40 ligand accumulates in stored blood components, primes neutrophils through CD40, and is a potential cofactor in the development of transfusion-related acute lung injury CD40 ligand influences platelet release of reactive oxygen intermediates Host defense role of platelets: engulfment of HIV and Staphylococcus aureus occurs in a specific subcellular compartment and is enhanced by platelet activation An ultrastructural study of Porphyromonas gingivalis-induced platelet aggregation Platelets are covercytes, not phagocytes: uptake of bacteria involves channels of the open canalicular system Platelet interaction with bacteria Platelet FcgammaRIIA binds and internalizes IgG-containing complexes Internalization of IgGcoated targets results in activation and secretion of soluble CD40 ligand and RANTES by human platelets Plateletbound lipopolysaccharide enhances Fc receptor-mediated phagocytosis of IgG-opsonized platelets Human platelet FcγRIIA and phagocytes in immune-complex clearance Neutrophils phagocytose activated platelets in vivo: a phosphatidylserine, Pselectin, and β2 integrin-dependent cell clearance program Dangerous connections: neutrophils and the phagocytic clearance of activated platelets Platelet clearance by circulating leukocytes: a rare event or a determinant of the "immune continuum Interaction of influenza virus with blood platelets Incorporation of influenza virus in human blood platelets in vitro. Electron microscopical observation Internalization of human immunodeficiency virus type I and other retroviruses by megakaryocytes and platelets Platelets and erythrocytebound platelets bind infectious HIV-1 in plasma of chronically infected patients The dynamics of hepatitis C virus binding to platelets and 2 mononuclear cell lines HCV infective virions can be carried by human platelets Detection of hepatitis C virus in platelets: evaluating its relationship to antiviral therapy outcome Association of herpes simplex virus with platelets of experimentally infected mice Interaction between vaccinia virus and human blood platelets Antibodyenhanced binding of dengue-2 virus to human platelets Imaging the interaction between dengue 2 virus and human blood platelets using atomic force and electron microscopy Detection of dengue virus in platelets isolated from dengue patients. Southeast Asian Platelet interactions with viruses and parasites Influenza virus H1N1 activates platelets through FcγRIIA signaling and thrombin generation Involvement of the Fc gamma receptor IIA cytoplasmic domain in antibody-dependent enhancement of dengue virus infection Immature dengue virus: a veiled pathogen? Seminars in Thrombosis & Hemostasis Platelet Dysfunction and Depletion in Infection Page Platelets interact with coxsackieviruses B and have a critical role in the pathogenesis of virus-induced myocarditis Cellular receptors and hantavirus pathogenesis Human cytomegalovirusplatelet interaction triggers toll-like receptor 2-dependent proinflammatory and proangiogenic responses Platelet-TLR7 mediates host survival and platelet count during viral infection in the absence of platelet-dependent thrombosis The role of platelets in mediating a response to human influenza infection T granules in human platelets function in TLR9 organization and signaling Persistent platelet activation and apoptosis in virologically suppressed HIVinfected individuals Desialylation is a mechanism of Fc-independent platelet clearance and a therapeutic target in immune thrombocytopenia Role of sialic acid for platelet life span: exposure of beta-galactose results in the rapid clearance of platelets from the circulation by asialoglycoprotein receptor-expressing liver macrophages and hepatocytes The virucidal effect of platelet concentrates: preliminary study and first conclusions Human platelets and their capacity of binding viruses: meaning and challenges Lentivirus degradation and DC-SIGN expression by human platelets and megakaryocytes Uptake of influenza virus by platelets occurs via phagocytosis Platelets can phagocytose influenza virus which may contribute to the occurrence of thrombocytopenia during influenza infection Platelets modulate innate immune response against human respiratory syncytial virus in vitro Platelet apoptosis and apoptotic platelet clearance by macrophages in secondary dengue virus infections Adenovirusplatelet interaction in blood causes virus sequestration to the reticuloendothelial system of the liver Platelet antistaphylococcal responses occur through P2X1 and P2Y12 receptor-induced activation and kinocidin release Why human platelets fail to kill bacteria Integrin alpha IIb beta 3 mediates binding of the Lyme disease agent Borrelia burgdorferi to human platelets Chlamydia pneumoniae binds to platelets and triggers P-selectin expression and aggregation: a causal role in cardiovascular disease? Helicobacter pylori binds von Willebrand factor and interacts with GPIb to induce platelet aggregation Porphyromonas gingivalis-induced platelet aggregation in plasma depends on Hgp44 adhesin but not Rgp proteinase FbsA, a fibrinogen-binding protein from Streptococcus agalactiae, mediates platelet aggregation Molecular basis for Staphylococcus aureus-mediated platelet aggregate formation under arterial shear in vitro Roles for fibrinogen, immunoglobulin and complement in platelet activation promoted by Staphylococcus aureus clumping factor A Both complement-and fibrinogen-dependent mechanisms contribute to platelet aggregation mediated by Staphylococcus aureus clumping factor B Fibronectin-binding proteins of Staphylococcus aureus mediate activation of human platelets via fibrinogen and fibronectin bridges to integrin GPIIb/IIIa and IgG binding to the FcgammaRIIa receptor Multiple mechanisms for the activation of human platelet aggregation by Staphylococcus aureus: roles for the clumping factors ClfA and ClfB, the serine-aspartate repeat protein SdrE and protein A Thrombospondin binds to Staphylococcus aureus and promotes staphylococcal adherence to surfaces Adhesion of Staphylococcus aureus to surface-bound platelets: role of fibrinogen/fibrin and platelet integrins Staphylococcus aureus induces platelet aggregation via a fibrinogen-dependent mechanism which is independent of principal platelet glycoprotein IIb/IIIa fibrinogen-binding domains Staphylococci-induced human platelet injury mediated by protein A and immunoglobulin G Fc fragment receptor Staphylococcus aureus protein A recognizes platelet gC1qR/p33: a novel mechanism for staphylococcal interactions with platelets Ironregulated surface determinant B (IsdB) promotes Staphylococcus aureus adherence to and internalization by non-phagocytic human cells Direct interaction of iron-regulated surface determinant IsdB of Staphylococcus aureus with the GPIIb/IIIa receptor on platelets Staphylococcus aureus protein A binding to von Willebrand factor A1 domain is mediated by conserved IgG binding regions Seminars in Thrombosis & Hemostasis Protein A is the von Willebrand factor binding protein on Staphylococcus aureus Induction of platelet thrombi by bacteria and antibodies Elucidating the role of Staphylococcus epidermidis serine-aspartate repeat protein G in platelet activation Human platelets recognize a novel surface protein, PadA, on Streptococcus gordonii through a unique interaction involving fibrinogen receptor GPIIbIIIa Mechanism of outside-in αIIbβ3-mediated activation of human platelets by the colonizing Bacterium, Streptococcus gordonii Binding of the Streptococcus gordonii surface glycoproteins GspB and Hsa to specific carbohydrate structures on platelet membrane glycoprotein Ibalpha Role of Streptococcus gordonii surface proteins SspA/SspB and Hsa in platelet function Characterization of the fibrinogenbinding surface protein Fbl of Staphylococcus lugdunensis Bacteriophage lysin mediates the binding of streptococcus mitis to human platelets through interaction with fibrinogen Streptococcus mitis phage-encoded adhesins mediate attachment to alpha2-8-linked sialic acid residues on platelet membrane gangliosides Glycoprotein Ibα and FcγRIIa play key roles in platelet activation by the colonizing bacterium, Streptococcus oralis Review manuscript: mechanisms of platelet activation by the pneumococcus and the role of platelets in community-acquired pneumonia Hammerschmidt S. Pneumococcal adhesins PavB and PspC are important for the interplay with human thrombospondin-1 Serotype 3 pneumococci sequester platelet-derived human thrombospondin-1 via the adhesin and immune evasion protein Hic Platelet activation by Streptococcus pyogenes leads to entrapment in platelet aggregates, from which bacteria subsequently escape The role of immunoglobulin G and fibrinogen in platelet aggregation by Streptococcus sanguis A serine-rich glycoprotein of Streptococcus sanguis mediates adhesion to platelets via GPIb A role for glycoprotein Ib in Streptococcus sanguis-induced platelet aggregation Adenovirus receptors Adenoviral vectors do not induce, inhibit, or potentiate human platelet aggregation Adenovirus type 3 induces platelet activation in vitro Adenovirus-induced thrombocytopenia: the role of von Willebrand factor and P-selectin in mediating accelerated platelet clearance Dengue virus binding and replication by platelets Dengue induces platelet activation, mitochondrial dysfunction and cell death through mechanisms that involve DC-SIGN and caspases Ctype lectins DC-SIGN and L-SIGN mediate cellular entry by Ebola virus in cis and in trans Integrin alpha(v)beta3 (vitronectin receptor) is a candidate receptor for the virulent echovirus 9 strain Barty Activation of human platelets through gp140, the C3d/EBV receptor (CR2) Pathogenic hantaviruses direct the adherence of quiescent platelets to infected endothelial cells Hepatitis C virus interacts with human platelet glycoprotein VI Megakaryocyte precursors, megakaryocytes and platelets express the HIV co-receptor CXCR4 on their surface: determination of response to stromal-derived factor-1 by megakaryocytes and platelets DC-SIGN and CLEC-2 mediate human immunodeficiency virus type 1 capture by platelets αvβ3-integrin is a major sensor and activator of innate immunity to herpes simplex virus-1 Human parechovirus 1 utilizes integrins alphavbeta3 and alphavbeta1 as receptors Identification of cell surface molecules involved in dystroglycanindependent Lassa virus cell entry Rotavirus contains integrin ligand sequences and a disintegrin-like domain that are implicated in virus entry into cells Determinants of the specificity of rotavirus interactions with the alpha2beta1 integrin Seminars in Thrombosis & Hemostasis We thank the Medical Research Council of South Africa (Self-Initiated Research Program) for supporting our research. The funders had no role in the design and preparation of the manuscript or decision to publish.