Platelets support a protective immune response to LCMV by preventing splenic necrosis

Increased platelet-CD8+ T-cell aggregates displaying high activation, exhaustion, and tendency to death correlate with disease progression in people with HIV-1

Platelets engage in HIV-1 infection by interacting with immune cells, which has been realized broadly. However, the potential interaction between platelets and CD8+ T cells remains unidentified. Here, treatment-naive individuals with HIV-1, complete immunological responders to antiretroviral therapy, and healthy controls were enrolled. First, we found that treatment-naive individuals with HIV-1 had low platelet numbers and high CD8+ T-cell counts when compared with complete immunological responders to antiretroviral therapy and healthy controls, leading to a low platelet/CD8+ T-cell ratio in peripheral blood, which could effectively differentiate the status of HIV-1 infection. Moreover, cytokines that may have been derived from platelets were higher in the plasma of people with HIV-1 despite viral suppression. Furthermore, we demonstrated that platelet-CD8+ T-cell aggregates were elevated in treatment-naive individuals with HIV-1, which positively correlated with HIV-1 viral load but negatively correlated with CD4+ T-cell count and CD4/CD8 ratio. Finally, we revealed that platelet-CD8+ T-cell aggregates correlate with enhanced activation/exhaustion and pyroptosis/apoptosis compared with free CD8+ T cells. Moreover, platelet-induced caspase 1 activation of CD8+ T cells correlated with IL-1β and IL-18 plasma levels. In brief, we reveal the importance of platelets in HIV-1 infection, which might secrete more cytokines and mediate CD8+ T-cell phenotypic characteristics by forming platelet-CD8+ T-cell aggregates, which are related to poor prognosis.

Hemostasis without clot formation: how platelets guard the vasculature in inflammation, infection, and malignancy

Platelets are key vascular effectors in hemostasis, with activation signals leading to fast recruitment, aggregation, and clot formation. The canonical process of hemostasis is well-characterized and shares many similarities with pathological thrombus formation. However, platelets are also crucially involved in the maintenance of vascular integrity under both steady-state and inflammatory conditions by ensuring blood vessel homeostasis and preventing microbleeds. In these settings, platelets use distinct receptors, signaling pathways, and ensuing effector functions to carry out their deeds. Instead of simply forming clots, they mainly act as individual sentinels that swiftly adapt their behavior to the local microenvironment. In this review, we summarize previously recognized and more recent studies that have elucidated how anucleate, small platelets manage to maintain vascular integrity when faced with challenges of infection, sterile inflammation, and even malignancy. We dissect how platelets are recruited to the vascular wall, how they identify sites of injury, and how they prevent hemorrhage as single cells. Furthermore, we discuss mechanisms and consequences of platelets' interaction with leukocytes and endothelial cells, the relevance of adhesion as well as signaling receptors, in particular immunoreceptor tyrosine-based activation motif receptors, and cross talk with the coagulation system. Finally, we outline how recent insights into inflammatory hemostasis and vascular integrity may aid in the development of novel therapeutic strategies to prevent hemorrhagic events and vascular dysfunction in patients who are critically ill.

Platelet-lymphocyte co-culture serves as an ex vivo platform of dynamic heterotypic cross-talk

Platelets are well known for their roles in hemostasis and thrombosis, and are increasingly recognized for their abilities to interact with white blood cells during inflammatory diseases, via secreted soluble factors as well as cell-cell contact. This interaction has been investigated in animal models and patient samples and has shown to be implicated in patient outcomes in several diseases. Platelet-leukocyte co-cultures are widely used to study platelet-leukocyte interactions ex vivo. However, there is a paucity with regard to the systematic characterization of cell activation and functional behaviors of platelets and leukocytes in these co-cultures. Hence we aimed to characterize a model of platelet-leukocyte co-culture ex vivo. Human peripheral blood mononuclear cell (PBMC) and platelets were isolated and co-cultured for 5 days at 37 °C in the presence or absence of anti-CD3/CD28 antibodies or PHA. We evaluated PF-4 secretion and p-selectin expression in platelets as markers of platelet activation. Lymphocyte activation was assessed by cell proliferation and cell population phenotyping, in addition to platelet-lymphocyte aggregation. Platelet secretion and p-selectin expression is maintained throughout the co-culture, indicating that platelets were viable and reactive over the 5 days. Similarly PBMCs were viable and maintained proliferative capacity. Finally, dynamic heterotypic conjugation between platelets and T lymphocytes was also observed throughout co-culture (with a peak at days 3 and 4) upon T lymphocyte activation. In conclusion, this in vitro model can successfully mimic the in vivo interaction between platelets and T lymphocytes, and can be used to confirm and/or support in vivo results.

Arenaviral infection causes bleeding in mice due to reduced serotonin release from platelets

Bleeding correlates with disease severity in viral hemorrhagic fevers. We found that the increase in type I interferon (IFN-I) in mice caused by infection with the Armstrong strain of lymphocytic choriomeningitis virus (LCMV; an arenavirus) reduced the megakaryocytic expression of genes encoding enzymes involved in lipid biosynthesis (cyclooxygenase 1 and thromboxane A synthase 1) and a thrombopoietic transcription factor (Nf-e2). The decreased expression of these genes was associated with reduced numbers of circulating platelets and defects in the arachidonic acid synthetic pathway, thereby suppressing serotonin release from δ-granules in platelets. Bleeding resulted when severe thrombocytopenia and altered platelet function reduced the amount of platelet-derived serotonin below a critical threshold. Bleeding was facilitated by the absence of the activity of the kinase Lyn or the administration of aspirin, an inhibitor of arachidonic acid synthesis. Mouse platelets were not directly affected by IFN-I because they lack the receptor for the cytokine (IFNAR1), suggesting that transfusion of normal platelets into LCMV-infected mice could increase the amount of platelet-released serotonin and help to control hemorrhage.

The Underestimated Role of Platelets in Severe Infection a Narrative Review

Beyond their role in hemostasis, platelets have emerged as key contributors in the immune response; accordingly, the occurrence of thrombocytopenia during sepsis/septic shock is a well-known risk factor of mortality and a marker of disease severity. Recently, some studies elucidated that the response of platelets to infections goes beyond a simple fall in platelets count; indeed, sepsis-induced thrombocytopenia can be associated with—or even anticipated by—several changes, including an altered morphological pattern, receptor expression and aggregation. Of note, alterations in platelet function and morphology can occur even with a normal platelet count and can modify, depending on the nature of the pathogen, the pattern of host response and the severity of the infection. The purpose of this review is to give an overview on the pathophysiological interaction between platelets and pathogens, as well as the clinical consequences of platelet dysregulation. Furthermore, we try to clarify how understanding the nature of platelet dysregulation may help to optimize the therapeutic approach.

Platelets in COVID-19 disease: friend, foe, or both?

Immuno-thrombosis of COVID-19 results in the activation of platelets and coagulopathy. Antiplatelet therapy has been widely used in COVID-19 patients to prevent thrombotic events. However, recent analysis of clinical trials does not support the major effects of antiplatelet therapy on mortality in hospitalized COVID-19 patients, despite the indisputable evidence for an increased risk of thrombotic complications in COVID-19 disease. This apparent paradox calls for an explanation. Platelets have an important role in sensing and orchestrating host response to infection, and several platelet functions related to host defense response not directly related to their well-known hemostatic function are emerging. In this paper, we aim to review the evidence supporting the notion that platelets have protective properties in maintaining endothelial barrier integrity in the course of an inflammatory response, and this role seems to be of particular importance in the lung. It might, thus, well be that the inhibition of platelet function, if affecting the protective aspect of platelet activity, might diminish clinical benefits resulting from the inhibition of the pro-thrombotic phenotype of platelets in immuno-thrombosis of COVID-19. A better understanding of the platelet-dependent mechanisms involved in the preservation of the endothelial barrier is necessary to design the antiplatelet therapeutic strategies that inhibit the pro-thrombotic activity of platelets without effects on the vaso-protective function of platelets safeguarding the pulmonary endothelial barrier during multicellular host defense in pulmonary circulation.

Thrombocytopathy and endotheliopathy: crucial contributors to COVID-19 thromboinflammation

The core pathology of coronavirus disease 2019 (COVID-19) is infection of airway cells by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) that results in excessive inflammation and respiratory disease, with cytokine storm and acute respiratory distress syndrome implicated in the most severe cases. Thrombotic complications are a major cause of morbidity and mortality in patients with COVID-19. Patients with pre-existing cardiovascular disease and/or traditional cardiovascular risk factors, including obesity, diabetes mellitus, hypertension and advanced age, are at the highest risk of death from COVID-19. In this Review, we summarize new lines of evidence that point to both platelet and endothelial dysfunction as essential components of COVID-19 pathology and describe the mechanisms that might account for the contribution of cardiovascular risk factors to the most severe outcomes in COVID-19. We highlight the distinct contributions of coagulopathy, thrombocytopathy and endotheliopathy to the pathogenesis of COVID-19 and discuss potential therapeutic strategies in the management of patients with COVD-19. Harnessing the expertise of the biomedical and clinical communities is imperative to expand the available therapeutics beyond anticoagulants and to target both thrombocytopathy and endotheliopathy. Only with such collaborative efforts can we better prepare for further waves and for future coronavirus-related pandemics.

Thrombocytopenia in Virus Infections

Thrombocytopenia, which signifies a low platelet count usually below 150 × 10⁹/L, is a common finding following or during many viral infections. In clinical medicine, mild thrombocytopenia, combined with lymphopenia in a patient with signs and symptoms of an infectious disease, raises the suspicion of a viral infection. This phenomenon is classically attributed to platelet consumption due to inflammation-induced coagulation, sequestration from the circulation by phagocytosis and hypersplenism, and impaired platelet production due to defective megakaryopoiesis or cytokine-induced myelosuppression. All these mechanisms, while plausible and supported by substantial evidence, regard platelets as passive bystanders during viral infection. However, platelets are increasingly recognized as active players in the (antiviral) immune response and have been shown to interact with cells of the innate and adaptive immune system as well as directly with viruses. These findings can be of interest both for understanding the pathogenesis of viral infectious diseases and predicting outcome. In this review, we will summarize and discuss the literature currently available on various mechanisms within the relationship between thrombocytopenia and virus infections.

Identification of a Locus in Mice that Regulates the Collateral Damage and Lethality of Virus Infection

Arenaviruses can cause severe hemorrhagic disease in humans, which can progress to organ failure and death. The underlying mechanisms causing lethality and person-to-person variation in outcome remain incompletely explained. Herein, we characterize a mouse model that recapitulates many features of pathogenesis observed in humans with arenavirus-induced hemorrhagic disease, including thrombocytopenia, severe vascular leakage, lung edema, and lethality. The susceptibility of congenic B6.PL mice to lymphocytic choriomeningitis virus (LCMV) infection is associated with increased antiviral T cell responses in B6.PL mice compared with C57BL/6 mice and is T cell dependent. Pathogenesis imparted by the causative locus is inherited in a semi-dominant manner in F1 crosses. The locus includes PL-derived sequence variants in both poorly annotated genes and genes known to contribute to immune responses. This model can be used to further interrogate how limited genetic differences in the host can remarkably alter the disease course of viral infection.

Pulmonary and cardiac pathology in African American patients with COVID-19: an autopsy series from New Orleans

Background

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) spread rapidly across the USA, causing extensive morbidity and mortality, particularly in the African American community. Autopsy can considerably contribute to our understanding of many disease processes and could provide crucial information to guide management of patients with coronavirus disease 2019 (COVID-19). We report on the relevant cardiopulmonary findings in, to our knowledge, the first autopsy series of ten African American decedents, with the cause of death attributed to COVID-19.

Methods

Autopsies were performed on ten African American decedents aged 44–78 years with cause of death attributed to COVID-19, reflective of the dominant demographic of deaths following COVID-19 diagnosis in New Orleans. Autopsies were done with consent of the decedents' next of kin. Pulmonary and cardiac features were examined, with relevant immunostains to characterise the inflammatory response, and RNA labelling and electron microscopy on representative sections.

Findings

Important findings include the presence of thrombosis and microangiopathy in the small vessels and capillaries of the lungs, with associated haemorrhage, that significantly contributed to death. Features of diffuse alveolar damage, including hyaline membranes, were present, even in patients who had not been ventilated. Cardiac findings included individual cell necrosis without lymphocytic myocarditis. There was no evidence of secondary pulmonary infection by microorganisms.

Interpretation

We identify key pathological states, including thrombotic and microangiopathic pathology in the lungs, that contributed to death in patients with severe COVID-19 and decompensation in this demographic. Management of these patients should include treatment to target these pathological mechanisms.

Funding

None.

A Champion of Host Defense: A Generic Large-Scale Cause for Platelet Dysfunction and Depletion in Infection

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.

Characterization of acute TLR-7 agonist-induced hemorrhagic myocarditis in mice by multiparametric quantitative cardiac magnetic resonance imaging

Hemorrhagic myocarditis is a potentially fatal complication of excessive levels of systemic inflammation. It has been reported in viral infection, but is also possible in systemic autoimmunity. Epicutaneous treatment of mice with the Toll-like receptor 7 (TLR-7) agonist Resiquimod induces auto-antibodies and systemic tissue damage, including in the heart, and is used as an inducible mouse model of systemic lupus erythematosus (SLE). Here, we show that overactivation of the TLR-7 pathway of viral recognition by Resiquimod treatment of CFN mice induces severe thrombocytopenia and internal bleeding, which manifests most prominently as hemorrhagic myocarditis. We optimized a cardiac magnetic resonance (CMR) tissue mapping approach for the in vivo detection of diffuse infiltration, fibrosis and hemorrhages using a combination of T1, T2 and T2* relaxation times, and compared results with ex vivo histopathology of cardiac sections corresponding to CMR tissue maps. This allowed detailed correlation between in vivo CMR parameters and ex vivo histopathology, and confirmed the need to include T2* measurements to detect tissue iron for accurate interpretation of pathology associated with CMR parameter changes. In summary, we provide detailed histological and in vivo imaging-based characterization of acute hemorrhagic myocarditis as an acute cardiac complication in the mouse model of Resiquimod-induced SLE, and a refined CMR protocol to allow non-invasive longitudinal in vivo studies of heart involvement in acute inflammation. We propose that adding T2* mapping to CMR protocols for myocarditis diagnosis improves diagnostic sensitivity and interpretation of disease mechanisms.This article has an associated First Person interview with the first author of the paper.

Spontaneous bleeding in thrombocytopenia: Is it really spontaneous?

Spontaneous bleeding is a clinical hallmark of thrombocytopenia and can take multiple forms including petechiae, epistaxis, gum bleeding, or, in worst cases, intracranial hemorrhage. Those bleeding events are called " spontaneous " because they occur in the absence of overt trauma. Spontaneous bleeding manifestations have long been considered to be a direct consequence of low platelet counts. Nevertheless, although low platelet counts may lead to ultrastructural endothelial alterations, those alterations and the associated state of vascular fragility are unlikely sufficient to cause spontaneous rupture of the microvessel wall. Indeed, in addition to endothelial injury, factors capable of damaging the basement membrane are required to allow escape of red blood cells in the extravascular space. Therefore, despite their misleading name, spontaneous bleeding events in thrombocytopenia are most likely provoked and involve subclinical biological processes in which platelets normally intervene to ensure hemostasis. In this review, we discuss past and more recent studies on the possible triggers of spontaneous bleeding events in thrombocytopenia, with a particular focus on the role of inflammatory reactions.

Platelets in Immune Response to Virus and Immunopathology of Viral Infections

Platelets are essential effector cells in hemostasis. Aside from their role in coagulation, platelets are now recognized as major inflammatory cells with key roles in the innate and adaptive arms of the immune system. Activated platelets have key thromboinflammatory functions linking coagulation to immune responses in various infections, including in response to virus. Recent studies have revealed that platelets exhibit several pattern recognition receptors (PRR) including those from the toll-like receptor, NOD-like receptor, and C-type lectin receptor family and are first-line sentinels in detecting and responding to pathogens in the vasculature. Here, we review the main mechanisms of platelets interaction with viruses, including their ability to sustain viral infection and replication, their expression of specialized PRR, and activation of thromboinflammatory responses against viruses. Finally, we discuss the role of platelet-derived mediators and platelet interaction with vascular and immune cells in protective and pathophysiologic responses to dengue, influenza, and human immunodeficiency virus 1 infections.

Glycoprotein VI in securing vascular integrity in inflamed vessels

Glycoprotein VI (GPVI), the main platelet receptor for collagen, has been shown to play a central role in various models of thrombosis, and to be a minor actor of hemostasis at sites of trauma. These observations have made of GPVI a novel target for antithrombotic therapy, as its inhibition would ideally combine efficacy with safety. Nevertheless, recent studies have indicated that GPVI could play an important role in preventing bleeding caused by neutrophils in the inflamed skin and lungs. Remarkably, there is evidence that the GPVI-dependent hemostatic function of platelets at the acute phase of inflammation in these organs does not involve aggregation. From a therapeutic perspective, the vasculoprotective action of GPVI in inflammation suggests that blocking of GPVI might bear some risks of bleeding at sites of neutrophil infiltration. In this review, we summarize recent findings on GPVI functions in inflammation and discuss their possible clinical implications and applications.

Platelets and vascular integrity: how platelets prevent bleeding in inflammation

Platelets play a central role in primary hemostasis by forming aggregates that plug holes in injured vessels. Half a century ago, detailed studies of the microvasculature by electron microscopy revealed that under inflammatory conditions that do not induce major disruption to vascular structure, individual platelets are mobilized to the vessel wall, where they interact with leukocytes and appear to seal gaps that arise between endothelial cells. Recent developments in genetic engineering and intravital microscopy have allowed further molecular and temporal characterization of these events. Surprisingly, it turns out that platelets support the recruitment of leukocytes to sites of inflammation. In parallel, however, they exercise their hemostatic function by securing the integrity of inflamed blood vessels to prevent bleeding from sites of leukocyte infiltration. It thus appears that platelets not only serve in concert as building blocks of the hemostatic plug but also act individually as gatekeepers of the vascular wall to help preserve vascular integrity while coordinating host defense. Variants of this recently appreciated hemostatic function of platelets that we refer to as "inflammation-associated hemostasis" are engaged in different contexts in which the endothelium is challenged or dysfunctional. Although the distinguishing characteristics of these variants and the underlying mechanisms of inflammation-associated hemostasis remain to be fully elucidated, they can differ notably from those supporting thrombosis, thus presenting therapeutic opportunities.

Small but mighty: Platelets as central effectors of host defense

Platelets actively participate in inflammatory processes and drive diseases such as atherosclerosis, rheumatoid arthritis and cancer metastasis. However, platelets also have anti-inflammatory and anti-infective properties, which have received less consideration in the past. In this review, we highlight recent findings on the role of platelets in host defense and describe regulatory pathways modulating immuneresponses. Furthermore, we discuss the role of platelets for the resolution of inflammation and tissue repair. These conceptual changes contribute to our understanding of platelet biology in disease.

Platelets and infections in the resource-limited countries with a focus on malaria and viral haemorrhagic fevers

Infections continue to cause a high incidence of mortality and morbidity in resource-poor nations. Although antimicrobial therapy has aided mostly in dealing with the pathogenic micro-organisms themselves, the collateral damage caused by the infections continue to cause many deaths. Intensive care support and manipulation of the hosts' abnormal response to the infection have helped to improve mortality in well-resourced countries. But, in those areas with limited resources, this is not yet the case and simpler methods of diagnosis and interventions are required. Thrombocytopenia is one of the most common manifestations in all these infections and may be used as an easily available prognostic indicator and marker for the severity of the infections. In this review, the relevance of platelets in infections in general, and specifically to tropical infections, malaria, and viral haemorrhagic fevers in the emerging countries is discussed. Better understanding of the pathophysiology and the role of platelets in particular in such conditions is likely to translate into better patient care and thus reduce morbidity and mortality.

Platelets in Pulmonary Immune Responses and Inflammatory Lung Diseases

Platelets are essential for physiological hemostasis and are central in pathological thrombosis. These are their traditional and best known activities in health and disease. In addition, however, platelets have specializations that broaden their functional repertoire considerably. These functional capabilities, some of which are recently discovered, include the ability to sense and respond to infectious and immune signals and to act as inflammatory effector cells. Human platelets and platelets from mice and other experimental animals can link the innate and adaptive limbs of the immune system and act across the immune continuum, often also linking immune and hemostatic functions. Traditional and newly recognized facets of the biology of platelets are relevant to defensive, physiological immune responses of the lungs and to inflammatory lung diseases. The emerging view of platelets as blood cells that are much more diverse and versatile than previously thought further predicts that additional features of the biology of platelets and of megakaryocytes, the precursors of platelets, will be discovered and that some of these will also influence pulmonary immune defenses and inflammatory injury.

Human platelets and their capacity of binding viruses: meaning and challenges?

Blood platelets are first aimed at ensuring primary hemostasis. Beyond this role, they have been acknowledged as having functions in the maintenance of the vascular arborescence and, more recently, as being also innate immune cells, devoted notably to the detection of danger signals, of which infectious ones. Platelets express pathogen recognition receptors that can sense bacterial and viral moieties. Besides, several molecules that bind epithelial or sub-endothelial molecules and, so forth, are involved in hemostasis, happen to be able to ligate viral determinants, making platelets capable of either binding viruses or even to be infected by some of them. Further, as platelets express both Fc-receptors for Ig and complement receptors, they also bind occasionally virus-Ig or virus-Ig-complement immune complexes. Interplays of viruses with platelets are very complex and viral infections often interfere with platelet number and functions. Through a few instances of viral infections, the present review aims at presenting some of the most important interactions from pathophysiological and clinical points of view, which are observed between human viruses and platelets.

Platelet interactions with viruses and parasites

While the interactions between Gram-positive bacteria and platelets have been well characterized, there is a paucity of data on the interaction between other pathogens and platelets. However, thrombocytopenia is a common feature with many infections especially viral hemorrhagic fever. The little available data on these interactions indicate a similarity with bacteria-platelet interactions with receptors such as FcγRIIa and Toll-Like Receptors (TLR) playing key roles with many pathogens. This review summarizes the known interactions between platelets and pathogens such as viruses, fungi and parasites.

Platelets in inflammation and infection

Although platelets are traditionally recognized for their central role in hemostasis, many lines of research clearly demonstrate these rather ubiquitous blood components are potent immune modulators and effectors. Platelets have been shown to directly recognize, sequester and kill pathogens, to activated and recruit leukocytes to sites of infection and inflammation, and to modulate leukocyte behavior, enhancing their ability to phagocytose and kill pathogens and inducing unique effector functions, such as the production of Neutrophil Extracellular Traps (NETs). This multifaceted response to infection and inflammation is due, in part, to the huge array of soluble mediators and cell surface molecules expressed by platelets. From their earliest origins as primordial hemocytes in invertebrates to their current form as megakaryocyte-derived cytoplasts, platelets have evolved to be one of the key regulators of host intravascular immunity and inflammation. In this review, we present the diverse roles platelets play in immunity and inflammation associated with autoimmune diseases and infection. Additionally, we highlight recent advances in our understanding of platelet behavior made possible through the use of advanced imaging techniques that allow us to visualize platelets and their interactions, in real-time, within the intact blood vessels of a living host.

Platelets interact with Coxsackieviruses B and have a critical role in the pathogenesis of virus-induced myocarditis

BACKGROUND

To further understand the role of platelets in the pathogenesis of viral infections we explored platelet interaction with Coxsackieviruses B (CVB) 1 and 3. CVB is a group of viruses that cause the majority of human enterovirus-related viral myocarditis; their receptor (CAR) is expressed on the platelet surface and there is a well-characterized CVB3-induced myocarditis murine model.

METHODS

Human platelets were infected with CVB1 and 3 and viruses were detected in pellets and in supernatants. C57BL/6J mice with or without platelet depletion were inoculated with CVB3 and peripheral blood and heart samples collected at different times post-infection.

RESULTS

CVB1 and 3 RNA and a capsid protein were detected in infected platelets. Despite the fact that titration assays in Vero cells showed increasing infectivity titers over time, supernatants and pellets from infected platelets showed similar levels, suggesting that platelets were not susceptible to a replicative infectivity cycle. CVB binding was CAR-independent and resulted in P-selectin and phosphatidylserine (PS) exposure. CVB3-infected mice showed a rapid thrombocytopenia that correlated with an increase in platelet PS exposure and platelet-leukocyte aggregates without modification of platelet P-selectin expression or von Willebrand factor levels. Mortality, viremia, heart viral titers and myocarditis were significantly higher in platelet-depleted than normal animals. Type I IFN levels were not changed but IgG levels were lower in infected and platelet-depleted mice.

CONCLUSIONS

Our data reveal that platelets play a critical role in host survival and immune response against CVB3 infection.

Platelets: essential components of the immune system

Platelets are anucleate cell fragments known for their central role in coagulation and vascular integrity. However, it is becoming increasingly clear that platelets contribute to diverse immunological processes extending beyond the traditional view of platelets as fragmentary mediators of hemostasis and thrombosis. There is recent evidence that platelets participate in: 1) intervention against microbial threats; 2) recruitment and promotion of innate effector cell functions; 3) modulating antigen presentation; and 4) enhancement of adaptive immune responses. In this way, platelets should be viewed as the underappreciated orchestrator of the immune system. This review will discuss recent and historical evidence regarding how platelets influence both innate and adaptive immune responses.

Platelets and infection - an emerging role of platelets in viral infection

Platelets are anucleate blood cells that play a crucial role in the maintenance of hemostasis. While platelet activation and elevated platelet counts (thrombocytosis) are associated with increased risk of thrombotic complications, low platelet counts (thrombocytopenia) and several platelet function disorders increase the risk of bleeding. Over the last years, more and more evidence has emerged that platelets and their activation state can also modulate innate and adaptive immune responses and low platelet counts have been identified as a surrogate marker for poor prognosis in septic patients. Viral infections often coincide with platelet activation. Host inflammatory responses result in the release of platelet activating mediators and a pro-oxidative and pro-coagulant environment, which favors platelet activation. However, viruses can also directly interact with platelets and megakaryocytes and modulate their function. Furthermore, platelets can be activated by viral antigen-antibody complexes and in response to some viruses B-lymphocytes also generate anti-platelet antibodies. All these processes contributing to platelet activation result in increased platelet consumption and removal and often lead to thrombocytopenia, which is frequently observed during viral infection. However, virus-induced platelet activation does not only modulate platelet count but also shape immune responses. Platelets and their released products have been reported to directly and indirectly suppress infection and to support virus persistence in response to certain viruses, making platelets a double-edged sword during viral infections. This review aims to summarize the current knowledge on platelet interaction with different types of viruses, the viral impact on platelet activation, and platelet-mediated modulations of innate and adaptive immune responses.

Thrombocytopenia impairs host defense in gram-negative pneumonia-derived sepsis in mice

Thrombocytopenia is a common finding in sepsis and associated with a worse outcome. We used a mouse model of pneumonia-derived sepsis caused by the human pathogen Klebsiella pneumoniae to study the role of platelets in host response to sepsis. Platelet counts (PCs) were reduced to less than a median of 5 × 10(9)/L or to 5 to 13 × 10(9)/L by administration of a depleting antibody in mice infected with Klebsiella via the airways. Thrombocytopenia was associated with strongly impaired survival during pneumonia-derived sepsis proportional to the extent of platelet depletion. Thrombocytopenic mice demonstrated PC-dependent enhanced bacterial growth in lungs, blood, and distant organs. Severe thrombocytopenia resulted in hemorrhage at the primary site of infection, but not in distant organs. PCs of 5 to 13 × 10(9)/L were sufficient to largely maintain hemostasis in infected lungs. Thrombocytopenia did not influence lung inflammation or neutrophil recruitment and did not attenuate local or systemic activation of coagulation or the vascular endothelium. PCs <5 × 10(9)/L even resulted in enhanced coagulation and endothelial cell activation, which coincided with increased proinflammatory cytokine levels. In accordance, low PCs in whole blood enhanced Klebsiella-induced cytokine release in vitro. These data suggest that platelets play an important role in host defense to Klebsiella pneumosepsis.

The role of platelets in sepsis

Platelets are small circulating anucleate cells that are of crucial importance in haemostasis. Over the last decade, it has become increasingly clear that platelets play an important role in inflammation and can influence both innate and adaptive immunity. Sepsis is a potentially lethal condition caused by detrimental host response to an invading pathogen. Dysbalanced immune response and activation of the coagulation system during sepsis are fundamental events leading to sepsis complications and organ failure. Platelets, being major effector cells in both haemostasis and inflammation, are involved in sepsis pathogenesis and contribute to sepsis complications. Platelets catalyse the development of hyperinflammation, disseminated intravascular coagulation and microthrombosis, and subsequently contribute to multiple organ failure. Inappropriate accumulation and activity of platelets are key events in the development of sepsis-related complications such as acute lung injury and acute kidney injury. Platelet activation readouts could serve as biomarkers for early sepsis recognition; inhibition of platelets in septic patients seems like an important target for immune-modulating therapy and appears promising based on animal models and retrospective human studies.

Emerging Roles for Platelets in Inflammation and Disease

Platelets and their interaction with cells of the immune system contribute through a variety of molecular mechanisms to support hemostasis and inflammation. These simple yet essential cells exert their effects in lymphocytes, monocytes, and neutrophils, both recruiting and modulating their function after activation. Emerging evidence is starting to define the mechanisms that allow platelets to also play pivotal roles in host defense. For example, platelet cell-surface expression of toll-like receptors allows platelets to direct neutrophil activation toward extracellular trap formation and facilitate the elimination of blood pathogens. In addition to these well-known receptors, two of the most recently discovered platelet receptors, C-type lectin receptor 2 (CLEC-2), and TREM-like transcript-1 (TLT-1), have been shown to modulate hemostatic and inflammation-related roles in platelets. This review will discuss the evolution of our understanding of platelet functions from hemostasis to inflammation, and highlight novel mechanisms that platelets use to mediate hemostasis under inflammatory pressure.

The role of platelets in the pathogenesis of viral hemorrhagic fevers

Viral hemorrhagic fevers (VHF) are acute zoonotic diseases that, early on, seem to cause platelet destruction or dysfunction. Here we present the four major ways viruses affect platelet development and function and new evidence of molecular factors that are preferentially induced by the more pathogenic members of the families Flaviviridae, Bunyaviridae, Arenaviridae, and Filoviridae. A systematic search was performed through the main medical electronic databases using as parameters all current findings concerning platelets in VHF. Additionally, the review contains information from conference proceedings.

Platelets protect from septic shock by inhibiting macrophage-dependent inflammation via the cyclooxygenase 1 signalling pathway

Although it has long been known that patients with sepsis often have thrombocytopenia and that septic patients with severe thrombocytopenia have a poor prognosis and higher mortality, the role of platelets in the pathogenesis of sepsis is poorly understood. Here we report a protective role of platelets in septic shock. We show that experimental thrombocytopenia induced by intraperitoneal injection of an anti-glycoprotein Ibα monoclonal antibody increases mortality and aggravates organ failure, whereas transfusion of platelets reduces mortality in lipopolysaccharide-induced endotoxemia and a bacterial infusion mouse sepsis model. Plasma concentrations of proinflammatory cytokines TNF-α and IL-6 are elevated by thrombocytopenia and decreased by platelet transfusion in septic mice. Furthermore, we identify that platelets protect from septic shock by inhibiting macrophage-dependent inflammation via the COX1/PGE₂/EP4-dependent pathway. Thus, these findings demonstrate a previously unappreciated role for platelets in septic shock and suggest that platelet transfusion may be effective in treating severely septic patients.

Graded defects in cytotoxicity determine severity of hemophagocytic lymphohistiocytosis in humans and mice

Primary hemophagocytic lymphohistiocytosis (HLH) is a life-threatening disease of hyperinflammation resulting from immune dysregulation due to inherited defects in the cytolytic machinery of natural killer and T cells. In humans, mutations in seven genes encoding proteins involved in cytolytic effector functions have so far been identified that predispose to HLH. However, although most affected patients develop HLH eventually, disease onset and severity are highly variable. Due to the genetic heterogeneity and variable time and nature of disease triggers, the immunological basis of these variations in HLH progression is incompletely understood. Several murine models of primary HLH have been established allowing to study HLH pathogenesis under more defined conditions. Here we directly compare the clinical HLH phenotype in six HLH-prone mouse strains with defects in the granule-dependent cytotoxic pathway. A severity gradient of HLH manifestations could be identified that is defined by the genetically determined residual lytic activity of cytotoxic T lymphocytes (CTL) and their ability to control lymphocytic choriomeningitis virus, which was used as a trigger for disease induction. Importantly, analysis of cohorts of HLH patients with severe bi-allelic mutations in the corresponding genes yielded a similar severity gradient in human HLH as reflected by the age at disease onset. Our findings define HLH as a threshold disease determined by subtle differences in the residual lytic activity of CTL.

Pathogenic mechanisms involved in the hematological alterations of arenavirus-induced hemorrhagic fevers

Viral hemorrhagic fevers (VHFs) caused by arenaviruses are acute diseases characterized by fever, headache, general malaise, impaired cellular immunity, eventual neurologic involvement, and hemostatic alterations that may ultimately lead to shock and death. The causes of the bleeding are still poorly understood. However, it is generally accepted that these causes are associated to some degree with impaired hemostasis, endothelial cell dysfunction and low platelet counts or function. In this article, we present the current knowledge about the hematological alterations present in VHF induced by arenaviruses, including new aspects on the underlying pathogenic mechanisms.