Platelets Mediate Host Defense against Staphylococcus aureus through Direct Bactericidal Activity and by Enhancing Macrophage Activities

The distinct effects of aspirin on platelet aggregation induced by infectious bacteria

Bacteria induce platelet aggregation triggered by several mechanisms. The goal of this work was to characterize platelet aggregates induced by different bacterial strains and to quantify the effect of aspirin treatment using aggregation tests, as well as a novel approach based on confocal analysis. Blood samples were obtained from either healthy donors (n = 27) or patients treated with long-term aspirin (n = 15). The bacterial species included were Staphylococcus aureus, Enterococcus faecalis, and Streptococcus sanguinis. The different aggregate's ultrastructures depending on the bacterial strain were analyzed using Scanning electron microscopy. Quantification of the size of the platelet aggregates, their mean number as well as the bacterial impregnation within the aggregates was performed using confocal laser scanning light microscopy. Light Transmission Aggregometry was also performed. Our results reported distinct characteristics of platelet aggregates depending on the bacterial strain. Using confocal analysis, we have shown that aspirin significantly reduced platelet aggregation induced by S. aureus (p = .003) and E. faecalis (p = .006) with no effect in the case of S. sanguinis (p = .529). The results of the aggregometry were concordant with those of the confocal technique in the case of S. aureus and S. sanguinis. Interestingly, aggregation induced by E. faecalis was detected only with confocal analysis. In conclusion, our confocal scanning microscopy allowed a detailed study of the platelet aggregation induced by bacteria. We showed that aspirin acts on bacterial-induced platelet aggregation depending on the species. These results are in favor of the use of aspirin considering the species and the bacterial strain involved.

Aspirin Effect on Staphylococcus aureus-Platelet Interactions During Infectious Endocarditis

Infectious endocarditis (IE) is a rare disease associated with high mortality and morbidity rate. The platelet-bacterial interaction presents the cornerstone of the development of endocardial vegetation. The epidemiology of IE has undergone profound changes between the last and the new decade, with Staphylococcus aureus becoming the main incriminated species. Despite improvements in antibiotic and surgical therapies, embolic disorders remain highly associated with IE that can be fatal. Antiplatelet drugs have been widely proposed to overcome embolic events associated with IE. This proposal has been supported by numerous in vitro, experimental, and clinical studies. However, other studies have yielded conflicting results. In this review, we focus on the effect of aspirin on the genesis of S. aureus endocarditic vegetation, as well as on the management of embolic and hemorrhagic events related to it, starting by its influence on the platelet-bacteria interaction.

Platelets and Intravascular Immunity: Guardians of the Vascular Space During Bloodstream Infections and Sepsis

Despite their humble origins as anuclear fragments of megakaryocytes, platelets have emerged as versatile mediators of thrombosis and immunity. The diverse spectrum of platelet functions are on full display during the host response to severe infection and sepsis, with platelets taking center-stage in the intravascular immune response to blood-borne pathogens. Platelets are endowed with a comprehensive armamentarium of pathogen detection systems that enable them to function as sentinels in the bloodstream for rapid identification of microbial invasion. Through both autonomous anti-microbial effector functions and collaborations with other innate immune cells, platelets orchestrate a complex intravascular immune defense system that protects against bacterial dissemination. As with any powerful immune defense system, dysregulation of platelet-mediated intravascular immunity can lead to profound collateral damage to host cells and tissues, resulting in sepsis-associated organ dysfunction. In this article, the cellular and molecular contributions of platelets to intravascular immune defenses in sepsis will be reviewed, including the roles of platelets in surveillance of the microcirculation and elicitation of protective anti-bacterial responses. Mechanisms of platelet-mediated thromboinflammatory organ dysfunction will be explored, with linkages to clinical biomarkers of platelet homeostasis that aid in the diagnosis and prognostication of human sepsis. Lastly, we discuss novel therapeutic opportunities that take advantage of our evolving understanding of platelets and intravascular immunity in severe infection.

Platelets in Host Defense: Experimental and Clinical Insights

Platelets are central players in thrombosis and hemostasis but are increasingly recognized as key components of the immune system. They shape ensuing immune responses by recruiting leukocytes, and support the development of adaptive immunity. Recent data shed new light on the complex role of platelets in immunity. Here, we summarize experimental and clinical data on the role of platelets in host defense against bacteria. Platelets bind, contain, and kill bacteria directly; however, platelet proinflammatory effector functions and cross-talk with the coagulation system, can also result in damage to the host (e.g., acute lung injury and sepsis). Novel clinical insights support this dichotomy: platelet inhibition/thrombocytopenia can be either harmful or protective, depending on pathophysiological context. Clinical studies are currently addressing this aspect in greater depth.

Characterization of a transgenic mouse model of chronic conditional platelet depletion

BACKGROUND

Platelets are widely recognized for their role in maintaining hemostasis. Recently, platelets have become appreciated for their varying roles in immunity, neuroprotection, and other physiological processes. While there are currently excellent methods to transiently deplete platelets and models of thrombocytopenia, studying the roles of platelets in chronic processes can be challenging.

OBJECTIVE

Phenotypic characterization of the PF4-DTR mouse model of conditional platelet depletion compared to antibody depletion.

METHODS

We describe the ability of the PF4-DTR mouse to maintain chronic platelet depletion, along with examining the bleeding phenotype compared to antibody-mediated platelet depletion.

RESULTS

Systemic administration of diphtheria toxin resulted in >99% platelet depletion that can be maintained for >2 weeks. When compared to an antibody depletion model, PF4-DTR mice showed similar phenotypes when challenged with tail bleed and saphenous vein measurements of hemostasis. Mice depleted with diphtheria toxin were also able to undergo adoptive transfer of platelets. If the frequency and amount of diphtheria toxin is reduced, mice can be maintained at >40% depletion for >28 days, showing that this model is tunable.

CONCLUSIONS

When compared to the gold standard of antibody-mediated depletion, PF4-DTR mice showed similar phenotypes and should be considered an important tool for examining the impact of thrombocytopenia over longer periods of time.

Regulation of Innate Immune Responses by Platelets

The role of platelets has been extensively studied in the context of coagulation and vascular integrity. Their hemostatic imbalance can lead to known conditions as atherosclerotic plaques, thrombosis, and ischemia. Nevertheless, the knowledge regarding the regulation of different cell types by platelets has been growing exponentially in the past years. Among these biological systems, the innate immune response is remarkably affected by the crosstalk with platelets. This interaction can come from the formation of platelet-leukocyte aggregates, signaling by direct contact between membrane surface molecules or by the stimulation of immune cells by soluble factors and active microparticles secreted by platelets. These ubiquitous blood components are able to sense and react to danger signals, guiding leukocytes to an injury site and providing a scaffold for the formation of extracellular traps for efficient microbial killing and clearance. Using several different mechanisms, platelets have an important task as they regulate the release of different cytokines and chemokines upon sterile or infectious damage, the expression of cell markers and regulation of cell death and survival. Therefore, platelets are more than clotting agents, but critical players within the fine inflammatory equilibrium for the host. In this review, we present pointers to a better understanding about how platelets control and modulate innate immune cells, as well as a summary of the outcome of this interaction, providing an important step for therapeutic opportunities and guidance for future research on infectious and autoimmune diseases.

Platelets, On Your Marks, Get Set, Migrate!

The idea that anucleate platelets display autonomous migration has long been viewed with skepticism. Gaertner et al. provide in vivo evidence that platelets undergo active migration at sites of thrombus formation and in inflamed liver sinusoids. Integrin-dependent migration allows platelets to scavenge and bundle fibrin-bound material, including intravascular bacteria.

Dual actions of group B Streptococcus capsular sialic acid provide resistance to platelet-mediated antimicrobial killing

Circulating platelets have important functions in thrombosis and in modulating immune and inflammatory responses. However, the role of platelets in innate immunity to bacterial infection is largely unexplored. While human platelets rapidly kill Staphylococcus aureus, we found the neonatal pathogen group B Streptococcus (GBS) to be remarkably resistant to platelet killing. GBS possesses a capsule polysaccharide (CPS) with terminal α2,3-linked sialic acid (Sia) residues that mimic a common epitope present on the human cell surface glycocalyx. A GBS mutant deficient in CPS Sia was more efficiently killed by human platelets, thrombin-activated platelet releasate, and synthetic platelet-associated antimicrobial peptides. GBS Sia is known to bind inhibitory Sia-recognizing Ig superfamily lectins (Siglecs) to block neutrophil and macrophage activation. We show that human platelets also express high levels of inhibitory Siglec-9 on their surface, and that GBS can engage this receptor in a Sia-dependent manner to suppress platelet activation. In a mouse i.v. infection model, antibody-mediated platelet depletion increased susceptibility to platelet-sensitive S. aureus but did not alter susceptibility to platelet-resistant GBS. Elimination of murine inhibitory Siglec-E partially reversed platelet suppression in response to GBS infection. We conclude that GBS Sia has dual roles in counteracting platelet antimicrobial immunity: conferring intrinsic resistance to platelet-derived antimicrobial components and inhibiting platelet activation through engagement of inhibitory Siglecs. We report a bacterial virulence factor for evasion of platelet-mediated innate immunity.

Platelets kill circulating parasites of all major Plasmodium species in human malaria

Platelets are understood to assist host innate immune responses against infection, although direct evidence of this function in any human disease, including malaria, is unknown. Here we characterized platelet-erythrocyte interactions by microscopy and flow cytometry in patients with malaria naturally infected with Plasmodium falciparum, Plasmodium vivax, Plasmodium malariae, or Plasmodium knowlesi Blood samples from 376 participants were collected from malaria-endemic areas of Papua, Indonesia, and Sabah, Malaysia. Platelets were observed binding directly with and killing intraerythrocytic parasites of each of the Plasmodium species studied, particularly mature stages, and was greatest in P vivax patients. Platelets preferentially bound to the infected more than to the uninfected erythrocytes in the bloodstream. Analysis of intraerythrocytic parasites indicated the frequent occurrence of platelet-associated parasite killing, characterized by the intraerythrocytic accumulation of platelet factor-4 and terminal deoxynucleotidyl transferase deoxyuridine triphosphate nick-end labeling of parasite nuclei (PF4+TUNEL+ parasites). These PF4+TUNEL+ parasites were not associated with measures of systemic platelet activation. Importantly, patient platelet counts, infected erythrocyte-platelet complexes, and platelet-associated parasite killing correlated inversely with patient parasite loads. These relationships, taken together with the frequency of platelet-associated parasite killing observed among the different patients and Plasmodium species, suggest that platelets may control the growth of between 5% and 60% of circulating parasites. Platelet-erythrocyte complexes made up a major proportion of the total platelet pool in patients with malaria and may therefore contribute considerably to malarial thrombocytopenia. Parasite killing was demonstrated to be platelet factor-4-mediated in P knowlesi culture. Collectively, our results indicate that platelets directly contribute to innate control of Plasmodium infection in human malaria.

Platelets Enhance Dendritic Cell Responses against Staphylococcus aureus through CD40-CD40L

Staphylococcus aureus is a major human pathogen that can cause mild to severe life-threatening infections in many tissues and organs. Platelets are known to participate in protection against S. aureus by direct killing and by enhancing the activities of neutrophils and macrophages in clearing S. aureus infection. Platelets have also been shown to induce monocyte differentiation into dendritic cells and to enhance activation of dendritic cells. Therefore, in the present study, we explored the role of platelets in enhancing bone marrow-derived dendritic cell (BMDC) function against S. aureus We observed a significant increase in dendritic cell phagocytosis and intracellular killing of a methicillin-resistant Staphylococcus aureus (MRSA) strain (USA300) by thrombin-activated platelets or their releasates. Enhancement of bacterial uptake and killing by DCs is mediated by platelet-derived CD40L. Coculture of USA300 and BMDCs in the presence of thrombin-activated platelet releasates invokes upregulation of the maturation marker CD80 on DCs and enhanced production of the proinflammatory cytokines tumor necrosis factor alpha (TNF-α), interleukin 12 (IL-12), and IL-6. Overall, these observations support our hypothesis that platelets play a critical role in the host defense against S. aureus infection. Platelets stimulate DCs, leading to direct killing of S. aureus and enhanced DC maturation, potentially leading to adaptive immune responses against S. aureus.

Innate Immunity in the Persistent Inflammation, Immunosuppression, and Catabolism Syndrome and Its Implications for Therapy

Clinical and technological advances promoting early hemorrhage control and physiologic resuscitation as well as early diagnosis and optimal treatment of sepsis have significantly decreased in-hospital mortality for many critically ill patient populations. However, a substantial proportion of severe trauma and sepsis survivors will develop protracted organ dysfunction termed chronic critical illness (CCI), defined as ≥14 days requiring intensive care unit (ICU) resources with ongoing organ dysfunction. A subset of CCI patients will develop the persistent inflammation, immunosuppression, and catabolism syndrome (PICS), and these individuals are predisposed to a poor quality of life and indolent death. We propose that CCI and PICS after trauma or sepsis are the result of an inappropriate bone marrow response characterized by the generation of dysfunctional myeloid populations at the expense of lympho- and erythropoiesis. This review describes similarities among CCI/PICS phenotypes in sepsis, cancer, and aging and reviews the role of aberrant myelopoiesis in the pathophysiology of CCI and PICS. In addition, we characterize pathogen recognition, the interface between innate and adaptive immune systems, and therapeutic approaches including immune modulators, gut microbiota support, and nutritional and exercise therapy. Finally, we discuss the future of diagnostic and prognostic approaches guided by machine and deep-learning models trained and validated on big data to identify patients for whom these approaches will yield the greatest benefits. A deeper understanding of the pathophysiology of CCI and PICS and continued investigation into novel therapies harbor the potential to improve the current dismal long-term outcomes for critically ill post-injury and post-infection patients.

Platelets directly regulate DNA damage and division of Staphylococcus aureus

Platelets (PLTs) are classically used in the clinical setting to maintain hemostasis. Recent evidence supports important roles for PLTs in host inflammatory and immune responses, and PLT-rich plasma has been demonstrated to inhibit the growth of bacteria in vitro and in vivo; however, few studies have examined whether PLTs can inhibit bacterial growth directly, and related mechanisms have not been elucidated further. Accordingly, in this study, we evaluated the effects of PLTs on bacterial growth. We washed and purified PLTs from peripheral blood, then confirmed that PLTs significantly inhibited the growth of Staphylococcus aureus when cocultured in vitro. Moreover, PLTs damaged DNA and blocked cell division in S. aureus. During coculture, PLT-derived TGF-β1 was dramatically down-regulated compared with that in PLT culture alone, and the addition of TGF-β1 to the coculture system promoted the inhibition of PLTs on S. aureus. Analysis of a murine S. aureus infection model demonstrated that the depletion of PLTs exacerbated the severity of infection, whereas the transfusion of PLTs alleviated this infection. Our observations demonstrate that PLTs could directly inhibit the growth of S. aureus by damaging DNA and blockage cell division, and that PLT-derived TGF-β1 may play an important role in this machinery.-Xu, J., Yi, J., Zhang, H., Feng, F., Gu, S., Weng, L., Zhang, J., Chen, Y., An, N., Liu, Z., An, Q., Yin, W., Hu, X. Platelets directly regulate DNA damage and division of Staphylococcus aureus.

Migrating Platelets Are Mechano-scavengers that Collect and Bundle Bacteria

Blood platelets are critical for hemostasis and thrombosis and play diverse roles during immune responses. Despite these versatile tasks in mammalian biology, their skills on a cellular level are deemed limited, mainly consisting in rolling, adhesion, and aggregate formation. Here, we identify an unappreciated asset of platelets and show that adherent platelets use adhesion receptors to mechanically probe the adhesive substrate in their local microenvironment. When actomyosin-dependent traction forces overcome substrate resistance, platelets migrate and pile up the adhesive substrate together with any bound particulate material. They use this ability to act as cellular scavengers, scanning the vascular surface for potential invaders and collecting deposited bacteria. Microbe collection by migrating platelets boosts the activity of professional phagocytes, exacerbating inflammatory tissue injury in sepsis. This assigns platelets a central role in innate immune responses and identifies them as potential targets to dampen inflammatory tissue damage in clinical scenarios of severe systemic infection.