Well established for their central role in hemostasis, platelets have increasingly been appreciated as immune cells in recent years. This emerging role should not come as a surprise as the central ...immune cells of invertebrates, hemocytes, are able to phagocytose, secrete soluble mediators and promote coagulation of hemolymph, blurring the line between immunity and hemostasis. The undeniable evolutionary link between coagulation and immunity becomes even clearer as the role of platelets in inflammation is better understood. Platelets exert a range of immune-related functions, many of which involve an intimate interplay with leukocytes. Platelets promote leukocyte recruitment via endothelial activation and can serve as "landing pads" for leukocytes, facilitating cellular adhesion in vascular beds devoid of classic adhesion molecules. Moreover, platelets enhance leukocyte function both through direct interactions and through release of soluble mediators. Among neutrophil-platelets interactions, the modulation of neutrophil extracellular traps (NETs) is of great interest. Platelets have been shown to induce NET formation; and, in turn, NET components further regulate platelet and neutrophil function. While NETs have been shown to ensnare and kill pathogens, they also initiate coagulation via thrombin activation. In fact, increased NET formation has been associated with hypercoagulability in septic patients as well as in chronic vascular disorders. This review will delve into current knowledge of platelet-neutrophil interactions, with a focus on NET-driven coagulation, in the context of infectious diseases. A better understanding of these mechanisms will shed a light on the therapeutic potential of uncoupling immunity and coagulation through targeting of NETs.
Abstract
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Eradication of pathogens from the bloodstream is critical to prevent disseminated infections and sepsis. Kupffer cells in the liver form an intravascular firewall that captures and clears pathogens ...from the blood. Here, we show that the catching and killing of circulating pathogens by Kupffer cells in vivo are promoted by the gut microbiota through commensal-derived D-lactate that reaches the liver via the portal vein. The integrity of this Kupffer cell-mediated intravascular firewall requires continuous crosstalk with gut commensals, as microbiota depletion with antibiotics leads to a failure of pathogen clearance and overwhelming disseminated infection. Furthermore, administration of purified D-lactate to germ-free mice, or gnotobiotic colonization with D-lactate-producing commensals, restores Kupffer cell-mediated pathogen clearance by the liver firewall. Thus, the gut microbiota programs an intravascular immune firewall that protects against the spread of bacterial infections via the bloodstream.
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•The gut microbiota promotes clearance of pathogens from the bloodstream by the liver•Commensal D-lactate programs Kupffer cells to capture and kill circulating pathogens•Dysbiosis causes impaired Kupffer cell function, leading to disseminated infection
McDonald et al., used gnotobiotic mice and in vivo imaging to show that clearance of circulating pathogens by Kupffer cells in the liver is governed by the gut microbiota and its production of D-lactate, which reaches the liver via the portal vein and programs Kupffer cells to capture and kill pathogens in the bloodstream.
Extra-intestinal pathogenic Escherichia coli (ExPEC) can cause a variety of infections outside of the intestine and are a major causative agent of urinary tract infections. Treatment of these ...infections is increasingly frustrated by antimicrobial resistance (AMR) diminishing the number of effective therapies available to clinicians. Incidence of multidrug resistance (MDR) is not uniform across the phylogenetic spectrum of E. coli. Instead, AMR is concentrated in select lineages, such as ST131, which are MDR pandemic clones that have spread AMR globally. Using a gnotobiotic mouse model, we demonstrate that an MDR E. coli ST131 is capable of out-competing and displacing non-MDR E. coli from the gut in vivo. This is achieved in the absence of antibiotic treatment mediating a selective advantage. In mice colonised with non-MDR E. coli strains, challenge with MDR E. coli either by oral gavage or co-housing with MDR E. coli colonised mice results in displacement and dominant intestinal colonisation by MDR E. coli ST131. To investigate the genetic basis of this superior gut colonisation ability by MDR E. coli, we assayed the metabolic capabilities of our strains using a Biolog phenotypic microarray revealing altered carbon metabolism. Functional pangenomic analysis of 19,571 E. coli genomes revealed that carriage of AMR genes is associated with increased diversity in carbohydrate metabolism genes. The data presented here demonstrate that independent of antibiotic selective pressures, MDR E. coli display a competitive advantage to colonise the mammalian gut and points to a vital role of metabolism in the evolution and success of MDR lineages of E. coli via carriage and spread.
Critically ill patients in intensive care units experience profound alterations of their gut microbiota that have been linked to a high risk of hospital-acquired (nosocomial) infections and adverse ...outcomes through unclear mechanisms. Abundant mouse and limited human data suggest that the gut microbiota can contribute to maintenance of systemic immune homeostasis, and that intestinal dysbiosis may lead to defects in immune defense against infections. Here we use integrated systems-level analyses of fecal microbiota dynamics in rectal swabs and single-cell profiling of systemic immune and inflammatory responses in a prospective longitudinal cohort study of critically ill patients to show that the gut microbiota and systemic immunity function as an integrated metasystem, where intestinal dysbiosis is coupled to impaired host defense and increased frequency of nosocomial infections. Longitudinal microbiota analysis by 16s rRNA gene sequencing of rectal swabs and single-cell profiling of blood using mass cytometry revealed that microbiota and immune dynamics during acute critical illness were highly interconnected and dominated by Enterobacteriaceae enrichment, dysregulated myeloid cell responses and amplified systemic inflammation, with a lesser impact on adaptive mechanisms of host defense. Intestinal Enterobacteriaceae enrichment was coupled with impaired innate antimicrobial effector responses, including hypofunctional and immature neutrophils and was associated with an increased risk of infections by various bacterial and fungal pathogens. Collectively, our findings suggest that dysbiosis of an interconnected metasystem between the gut microbiota and systemic immune response may drive impaired host defense and susceptibility to nosocomial infections in critical illness.
The influenza A virus (IAV) causes a respiratory tract infection with approximately 10% of the population infected by the virus each year. Severe IAV infection is characterized by excessive ...inflammation and tissue pathology in the lungs. Platelet and neutrophil recruitment to the lung are involved in the pathogenesis of IAV, but the specific mechanisms involved have not been clarified. Using confocal intravital microscopy in a mouse model of IAV infection, we observed profound neutrophil recruitment, platelet aggregation, neutrophil extracellular trap (NET) production and thrombin activation within the lung microvasculature
. Importantly, deficiency or antagonism of the protease-activated receptor 4 (PAR4) reduced platelet aggregation, NET production, and neutrophil recruitment. Critically, inhibition of thrombin or PAR4 protected mice from virus-induced lung tissue damage and edema. Together, these data imply thrombin-stimulated platelets play a critical role in the activation/recruitment of neutrophils, NET release and directly contribute to IAV pathogenesis in the lung.
During bloodstream infections, neutrophils home to the liver as part of an intravascular immune response to eradicate blood-borne pathogens, but the mechanisms regulating this crucial response are ...unknown. Using in vivo imaging of neutrophil trafficking in germ-free and gnotobiotic mice, we demonstrate that the intestinal microbiota guides neutrophil homing to the liver in response to infection mediated by the microbial metabolite D-lactate. Commensal-derived D-lactate augments neutrophil adhesion in the liver independent of granulopoiesis in bone marrow or neutrophil maturation and activation in blood. Instead, gut-to-liver D-lactate signaling primes liver endothelial cells to upregulate adhesion molecule expression in response to infection and promote neutrophil adherence. Targeted correction of microbiota D-lactate production in a model of antibiotic-induced dysbiosis restores neutrophil homing to the liver and reduces bacteremia in a model of Staphylococcus aureus infection. These findings reveal long-distance traffic control of neutrophil recruitment to the liver by microbiota-endothelium crosstalk.
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•Gut commensals promote neutrophil recruitment to liver during systemic infections•Impaired neutrophil recruitment in GF mice is restored by microbiota D-lactate•D-lactate primes liver endothelium to upregulate adhesion molecules for neutrophils•Therapeutic correction of D-lactate production improved defense against infection
The gut microbiota promotes neutrophil production and development, but its role in directing neutrophil trafficking is poorly understood. Zucoloto et al. show that the gut microbiota promotes neutrophil recruitment to the liver during systemic infections through priming of microvascular endothelium to upregulate adhesion molecules that mediate neutrophil homing to liver.
Probucol 4,4′- (Isopropylidenedithio)bis(2,6-di-tert-butylphenol) is a synthetic molecule clinically used for prevention and treatment of hypercholesterolemia and atherosclerosis. Recent studies have ...shown that the beneficial effects of probucol mainly derive from its anti-inflammatory and antioxidant properties. Gram-negative bacteria are common infectious agents and their wall components, e.g. lipopolysaccharide (LPS), are important elicitors of inflammation. LPS is sensed by tissue resident cells and it triggers a Toll-like receptor 4/MyD88-dependent signaling cascade resulting in endothelial activation, leukocyte recruitment and nociception. Therefore the present study aimed to investigate the anti-inflammatory and analgesic effects of probucol in models of LPS-induced acute inflammation. Probucol at 0.3–30mg/kg was administrated to male Swiss mice per oral 1h before intraplantar or intraperitoneal lipopolysaccharide stimulus. Probucol at 3mg/kg reduced lipopolysaccharide-induced mechanical and thermal hyperalgesia. These effects were accompanied by reduced leukocyte influx and cytokine production in both paw skin and peritoneum exudate. Unexpectedly, probucol did not alter lipopolysaccharide-induced tissue oxidative stress at anti-inflammatory /analgesic dose. On the other hand, probucol inhibited lipopolysaccharide-induced nuclear factor kappa B (NF-кB) activation in paw tissue as well as NF-кB activity in cultured macrophages in vitro, reinforcing the inhibitory effect of probucol over the NF-кB signaling pathway. In this sense, we propose that probucol acts on resident immune cells, such as macrophages, targeting the NF-кB pathway. As a result, it prevents the amplification and persistence of the inflammatory response by attenuating NF-кB-dependent cytokine production and leukocyte recruitment explaining its analgesic effects as well.
Healthy host-microbial mutualism with our intestinal microbiota relies to a large degree on compartmentalization and careful regulation of adaptive mucosal and systemic anti-microbial immune ...responses. However, commensal intestinal bacteria are never exclusively or permanently restricted to the intestinal lumen and regularly reach the systemic circulation. This results in various degrees of commensal bacteremia that needs to be appropriately dealt with by the systemic immune system. While most intestinal commensal bacteria, except for pathobionts or opportunistic pathogen, have evolved to be non-pathogenic, this does not mean that they are non-immunogenic. Mucosal immune adaptation is carefully controlled and regulated to avoid an inflammatory response, but the systemic immune system usually responds differently and more vigorously to systemic bacteremia. Here we show that germ-free mice have increased systemic immune sensitivity and display anti-commensal hyperreactivity in response to the addition of a single defined T helper cell epitope to the outer membrane porin C (OmpC) of a commensal
strain demonstrated by increased
-specific T cell-dependent IgG responses following systemic priming. This increased systemic immune sensitivity was not observed in mice colonized with a defined microbiota at birth indicating that intestinal commensal colonization also regulates systemic, and not only mucosal, anti-commensal responses. The observed increased immunogenicity of the
strain with the modified OmpC protein was not due to a loss of function and associated metabolic changes as a control
strain without OmpC did not display increased immunogenicity.
Acute respiratory distress syndrome (ARDS) is a life-threatening syndrome, constituted by respiratory failure and diffuse alveolar damage that results from dysregulated local and systemic immune ...activation, causing pulmonary vascular, parenchymal, and alveolar damage. SARS-CoV-2 infection has become the dominant cause of ARDS worldwide, and emerging evidence implicates neutrophils and their cytotoxic arsenal of effector functions as central drivers of immune-mediated lung injury in COVID-19 ARDS. However, key outstanding questions are whether COVID-19 drives a unique program of neutrophil activation or effector functions that contribute to the severe pathogenesis of this pandemic illness and whether this unique neutrophil response can be targeted to attenuate disease. Using a combination of high-dimensional single-cell analysis and ex vivo functional assays of neutrophils from patients with COVID-19 ARDS, compared with those with non-COVID ARDS (caused by bacterial pneumonia), we identified a functionally distinct landscape of neutrophil activation in COVID-19 ARDS that was intrinsically programmed during SARS-CoV-2 infection. Furthermore, neutrophils in COVID-19 ARDS were functionally primed to produce high amounts of neutrophil extracellular traps. Surprisingly, this unique pathological program of neutrophil priming escaped conventional therapy with dexamethasone, thereby revealing a promising target for adjunctive immunotherapy in severe COVID-19.
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