Neutrophil extracellular traps in the host defense against sepsis induced by Burkholderia pseudomallei (melioidosis)

Dysfunctional host cellular immune responses are associated with mortality in melioidosis

Melioidosis is a tropical infection caused by the intracellular pathogen Burkholderia pseudomallei, an underreported and emerging global threat. As melioidosis-associated mortality is frequently high despite antibiotics, novel management strategies are critically needed. Therefore, we sought to determine whether functional changes in the host innate and adaptive immune responses are induced during acute melioidosis and are associated with outcome. Using a unique whole blood stimulation assay developed for use in resource-limited settings, we examined induced cellular functional and phenotypic changes in a cohort of patients with bacteremic melioidosis prospectively enrolled within 24 h of positive blood culture and followed for 28 days. Compared to healthy controls, melioidosis survivors generated an IL-17 response mediated by Th17 cells and terminally-differentiated effector memory CD8+ T cells (P < .05, both), persisting to 28 days after enrolment. Furthermore, melioidosis survivors developed polyfunctional cytokine production in CD8+ T cells (P < .01). Conversely, a reduction in CCR6+ CD4+ T cells was associated with higher mortality, even after adjustments for severity of illness (P = 0.004). Acute melioidosis was also associated with a profound acute impairment in monocyte function as stimulated cytokine responses were reduced in classical, intermediate and non-classical monocytes. Impaired monocyte cytokine function improved by 28-days after enrolment. These data suggest that IL-17 mediated cellular responses may be contributors to host defense during acute melioidosis, and that innate immune function may be impaired. These insights could provide novel targets for the development of therapies and vaccine targets in this frequently lethal disease.

Circulating Nucleosomes as a Novel Biomarker for Sepsis: A Scoping Review

Circulating nucleosome levels are commonly elevated in physiological and pathological conditions. Their potential as biomarkers for diagnosing and prognosticating sepsis remains uncertain due, in part, to technical limitations in existing detection methods. This scoping review explores the possible role of nucleosome concentrations in the diagnosis, prognosis, and therapeutic management of sepsis. A comprehensive literature search of the Cochrane and Medline libraries from 1996 to 1 February 2024 identified 110 potentially eligible studies, of which 19 met the inclusion criteria, encompassing a total of 39 SIRS patients, 893 sepsis patients, 280 septic shock patients, 117 other ICU control patients, and 345 healthy volunteers. The enzyme-linked immunosorbent assay [ELISA] was the primary method of nucleosome measurement. Studies consistently reported significant correlations between nucleosome levels and other NET biomarkers. Nucleosome levels were higher in patients with sepsis than in healthy volunteers and associated with disease severity, as indicated by SOFA and APACHE II scores. Non-survivors had higher nucleosome levels than survivors. Circulating nucleosome levels, therefore, show promise as early markers of NETosis in sepsis, with moderate diagnostic accuracy and strong correlations with disease severity and prognosis. However, the available evidence is drawn mainly from single-center, observational studies with small sample sizes and varied detection methods, warranting further investigation.

Burkholderia pseudomallei and melioidosis

Burkholderia pseudomallei, the causative agent of melioidosis, is found in soil and water of tropical and subtropical regions globally. Modelled estimates of the global burden predict that melioidosis remains vastly under-reported, and a call has been made for it to be recognized as a neglected tropical disease by the World Health Organization. Severe weather events and environmental disturbance are associated with increased case numbers, and it is anticipated that, in some regions, cases will increase in association with climate change. Genomic epidemiological investigations have confirmed B. pseudomallei endemicity in newly recognized regions, including the southern United States. Melioidosis follows environmental exposure to B. pseudomallei and is associated with comorbidities that affect the immune response, such as diabetes, and with socioeconomic disadvantage. Several vaccine candidates are ready for phase I clinical trials. In this Review, we explore the global burden, epidemiology and pathophysiology of B. pseudomallei as well as current diagnostics, treatment recommendations and preventive measures, highlighting research needs and priorities.

Rethinking neutrophil extracellular traps

Neutrophils are a major subset of leukocytes in human circulating blood. In some circumstances, neutrophils release neutrophil extracellular traps (NETs). lnitially, NETs were considered to have a strong antibacterial capacity. However, currently, NETs have been shown to have a pivotal impact on various diseases. Different stimulators induce the production of different types of NETs, and their biological functions and modes of clearance do not appear to be the same. In this review, we will discuss several important issues related to NETs in order to better understand the relationship between NETs and diseases, as well as how to utilize the characteristics of NETs for disease treatment.

Outer membrane vesicles from bacteria: Role and potential value in the pathogenesis of chronic respiratory diseases

Infectious diseases are the leading cause of death in both adults and children, with respiratory infections being the leading cause of death. A growing body of evidence suggests that bacterially released extracellular membrane vesicles play an important role in bacterial pathogenicity by targeting and (de)regulating host cells through the delivery of nucleic acids, proteins, lipids, and carbohydrates. Among the many factors contributing to bacterial pathogenicity are the outer membrane vesicles produced by the bacteria themselves. Bacterial membrane vesicles are being studied in more detail because of their potential role as deleterious mediators in bacterial infections. This review provides an overview of the most current information on the emerging role of bacterial membrane vesicles in the pathophysiology of pneumonia and its complications and their adoption as promising targets for future preventive and therapeutic approaches.

Burkholderia pseudomallei biofilm phenotypes confined but surviving in neutrophil extracellular traps of varying appearance

Melioidosis is a fatal infectious disease caused by Burkholderia pseudomallei. Complications following treatment are usually due to antibiotic resistance and relapse is mainly caused by B. pseudomallei biofilm. Although the release of neutrophil extracellular traps (NETs) is crucial to capture and eliminate bacterial pathogens, to date response of NETs to B. pseudomallei biofilm is poorly understood. Here we compare the NETs produced by neutrophils in response to B. pseudomallei H777 (a biofilm-producing strain containing the bpsl0618 gene), a biofilm-defect strain lacking this gene (B. pseudomallei M10) and a bpsl0618 biofilm-complemented strain, B. pseudomallei C17, in which function of bpsl0618 was restored. Co-cultivation of these strains with healthy human neutrophils at MOI 10 with or without cytochalasin D demonstrated that H777 significantly resisted neutrophil-mediated killing and non-phagocytotic mechanisms compared to M10 (p < 0.0001). Three distinct morphotypes of NETs were seen: “aggregated”, “spiky” and “cloudy”. These were induced in different proportions by the different bacterial strains. All types of NETs were shown to confine all B. pseudomallei strains. Strains H777 and C17 could stimulate production of twice as much extracellular DNA (234.62 ng/mL and 205.43 ng/mL, respectively) as did M10 (111.87 ng/mL). Cells of H777 and C17 were better able to survive in the presence of neutrophil killing mechanisms relative to M10 (p < 0.0001) and NET formation (p < 0.0001 and 0.05). These findings suggest that NET stimulation was insufficient to eradicate B. pseudomallei H777 and C17 despite their possession of bpsl0618, a sugar-transferase gene associated with biofilm formation ability. Our findings demonstrate that B. pseudomallei biofilm phenotype may be a key factor in assisting pathogens to escape killing by neutrophils. This work provides a better understanding of how B. pseudomallei biofilm-associated infections induce and survive NET formation, resulting in bacterial persistence and increased severity of disease.

Role of Extracellular Trap Release During Bacterial and Viral Infection

Neutrophils are innate immune cells that play an essential role during the clearance of pathogens that can release chromatin structures coated by several cytoplasmatic and granular antibacterial proteins, called neutrophil extracellular traps (NETs). These supra-molecular structures are produced to kill or immobilize several types of microorganisms, including bacteria and viruses. The contribution of the NET release process (or NETosis) to acute inflammation or the prevention of pathogen spreading depends on the specific microorganism involved in triggering this response. Furthermore, studies highlight the role of innate cells different from neutrophils in triggering the release of extracellular traps during bacterial infection. This review summarizes the contribution of NETs during bacterial and viral infections, explaining the molecular mechanisms involved in their formation and the relationship with different components of such pathogens.

Zingerone Inhibits the Neutrophil Extracellular Trap Formation and Protects against Sepsis via Nrf2-Mediated ROS Inhibition

Neutrophils release chromatin and antimicrobial proteins to trap and kill microbes, which is termed as neutrophil extracellular trap (NET) formation. NETs play a pivotal role in host defense against infection. However, emerging evidence indicated that NETs also contribute to an exaggerated inflammatory response and organic injuries in sepsis. Zingerone, a natural compound extracted from Zingiber officinale, exerts antioxidant, anti-inflammatory, and antioncogenic properties. In this study, we found that treatment with zingerone reduced organ injury and improved the outcome in a cecal ligation puncture- (CLP-) induced polymicrobial sepsis model. Administration of zingerone also alleviates reactive oxygen species (ROS) accumulation and systematic inflammation in septic mice and inhibits neutrophil extracellular traps (NETs) formation in vivo and in vitro. Furthermore, inhibition of nuclear factor erythroid 2-related factor 2 (Nrf2) with its specific antagonist significantly counteracted the suppressive effects of zingerone on ROS and NETs and retarded the protective role of zingerone against sepsis-associated organ injury. In addition, exposure to zingerone does not affect phagocytic activity of neutrophils in vitro and bacterial dissemination in vivo. Above all, our results indicate that zingerone treatment obviously attenuates NET formation and inflammatory response via Nrf2-mediated ROS inhibition, thus providing a novel therapeutic strategy against sepsis-induced injury.

Peptidylarginine Deiminase 2 in Host Immunity: Current Insights and Perspectives

Peptidylarginine deiminases (PADs) are a group of enzymes that catalyze post-translational modifications of proteins by converting arginine residues into citrullines. Among the five members of the PAD family, PAD2 and PAD4 are the most frequently studied because of their abundant expression in immune cells. An increasing number of studies have identified PAD2 as an essential factor in the pathogenesis of many diseases. The successes of preclinical research targeting PAD2 highlights the therapeutic potential of PAD2 inhibition, particularly in sepsis and autoimmune diseases. However, the underlying mechanisms by which PAD2 mediates host immunity remain largely unknown. In this review, we will discuss the role of PAD2 in different types of cell death signaling pathways and the related immune disorders contrasted with functions of PAD4, providing novel therapeutic strategies for PAD2-associated pathology.

Neutrophil extracellular traps and organ dysfunction in sepsis

Sepsis is a clinical syndrome resulting from infection followed by inflammation and is one of the significant causes of mortality worldwide. The underlying reason is the host's uncontrolled inflammatory response due to an infection led to multiple organ dysfunction/failure. Neutrophils, an innate immune cell, are forerunners to reach the site of infection/inflammation for clearing the infection and resolute the inflammation during sepsis. A relatively new neutrophil effector function, neutrophil extracellular traps (NETs), have been demonstrated to kill the pathogens by releasing DNA decorated with histone and granular proteins. A growing number of pieces of shreds of evidence suggest that unregulated incidence of NETs have a significant influence on the pathogenesis of sepsis-induced multiple organ damage, including arterial hypotension, hypoxemia, coagulopathy, renal, neurological, and hepatic dysfunction. Thus, excessive production and improper resolution of NETs are of significant therapeutic value in combating sepsis-induced multiple organ failure. The purpose of this review is intended to highlight the role of NETs in sepsis-induced organ failure. Furthermore, the current status of therapeutic strategies to intersect the harmful effects of NETs to restore organ functions is discussed.

Hijacking of the Host's Immune Surveillance Radars by Burkholderia pseudomallei

Burkholderia pseudomallei (B. pseudomallei) causes melioidosis, a potentially fatal disease for which no licensed vaccine is available thus far. The host-pathogen interactions in B. pseudomallei infection largely remain the tip of the iceberg. The pathological manifestations are protean ranging from acute to chronic involving one or more visceral organs leading to septic shock, especially in individuals with underlying conditions similar to COVID-19. Pathogenesis is attributed to the intracellular ability of the bacterium to ‘step into’ the host cell’s cytoplasm from the endocytotic vacuole, where it appears to polymerize actin filaments to spread across cells in the closer vicinity. B. pseudomallei effectively evades the host’s surveillance armory to remain latent for prolonged duration also causing relapses despite antimicrobial therapy. Therefore, eradication of intracellular B. pseudomallei is highly dependent on robust cellular immune responses. However, it remains ambiguous why certain individuals in endemic areas experience asymptomatic seroconversion, whereas others succumb to sepsis-associated sequelae. Here, we propose key insights on how the host’s surveillance radars get commandeered by B. pseudomallei.

Peptidylarginine deiminase 2 has potential as both a biomarker and therapeutic target of sepsis

Peptidylarginine deiminases (PADs) are a family of calcium-dependent enzymes that are involved in a variety of human disorders, including cancer and autoimmune diseases. Although targeting PAD4 has shown no benefit in sepsis, the role of PAD2 remains unknown. Here, we report that PAD2 is engaged in sepsis and sepsis-induced acute lung injury in both human patients and mice. Pad2^–/– or selective inhibition of PAD2 by a small molecule inhibitor increased survival and improved overall outcomes in mouse models of sepsis. Pad2 deficiency decreased neutrophil extracellular trap (NET) formation. Importantly, Pad2 deficiency inhibited Caspase-11–dependent pyroptosis in vivo and in vitro. Suppression of PAD2 expression reduced inflammation and increased macrophage bactericidal activity. In contrast to Pad2^–/–, Pad4 deficiency enhanced activation of Caspase-11–dependent pyroptosis in BM-derived macrophages and displayed no survival improvement in a mouse sepsis model. Collectively, our findings highlight the potential of PAD2 as an indicative marker and therapeutic target for sepsis.

Peptidylarginine deiminases 2 (PAD2) regulates neutrophil extracellular trap (NET) formation in sepsis and sepsis-induced acute lung injury.

Neutrophil Elastase and Neutrophil Extracellular Traps in the Tumor Microenvironment

Tumor-associated neutrophils (TANs) play a major role during cancer development and progression in the tumor microenvironment. Neutrophil elastase (NE) is a serine protease normally expressed in neutrophil primary granules. Formation of neutrophil extracellular traps (NETs), a mechanism used by neutrophils, has been traditionally associated with the capture and killing of bacteria. However, there are recent discoveries suggesting that NE secretion and NETs formation are also involved in the tumor microenvironment. Here, we focus on how NE and NETs play a key regulatory function in the tumor microenvironment, such as tumor proliferation, distant metastasis, tumor-associated thrombosis, and antitumor activity. Additionally, the potential use of NETs, NE, or associated molecules as potential disease activity biomarkers or therapeutic targets will be introduced.

Thrombocytopenia Impairs Host Defense Against Burkholderia pseudomallei (Melioidosis)

Background

Infection with the gram-negative bacillus Burkholderia pseudomallei (melioidosis) is an important cause of pneumosepsis in Southeast Asia and has a mortality of up to 40%. We aimed to assess the role of platelets in the host response against B. pseudomallei infection.

Methods

Association between platelet counts and mortality was determined in 1160 patients with culture-proven melioidosis. Mice treated with (low- or high-dose) platelet-depleting antibody were inoculated intranasally with B. pseudomallei and killed. Additional studies using functional glycoprotein Ibα-deficient mice were conducted.

Results

Thrombocytopenia was present in 31% of patients at admission and predicted mortality in melioidosis patients even after adjustment for confounders. In our murine-melioidosis model, platelet counts decreased, and mice treated with a platelet-depleting antibody showed enhanced mortality and higher bacterial loads compared to mice with normal platelet counts. Low platelet counts had a modest impact on early-pulmonary neutrophil influx. Reminiscent of their role in hemostasis, platelet depletion impaired vascular integrity, resulting in early lung bleeding. Glycoprotein Ibα-deficient mice had reduced platelet counts during B. pseudomallei infection together with an impaired local host defense in the lung.

Conclusions

Thrombocytopenia predicts mortality in melioidosis patients and, during experimental melioidosis, platelets play a protective role in both innate immunity and vascular integrity.

Quantitative Proteomics Reveals Differences in the Response of Neutrophils Isolated from Healthy or Diabetic Subjects to Infection with Capsule-Variant Burkholderia thailandensis

In Thailand, diabetes mellitus is the most significant risk factor for melioidosis, a severe disease caused by Burkholderia pseudomallei. In this study, neutrophils isolated from healthy or diabetic subjects were infected with B. thailandensis E555, a variant strain with a B. pseudomallei-like capsular polysaccharide used here as a surrogate micro-organism for B. pseudomallei. At 2 h post-infection, neutrophil proteins were subjected to 4-plex iTRAQ-based comparative proteomic analysis. A total of 341 proteins were identified in two or more samples, of which several proteins involved in oxidative stress and inflammation were enriched in infected diabetic neutrophils. We validated this finding by demonstrating that infected diabetic neutrophils generated significantly elevated levels of pro-inflammatory cytokines TNFα, IL-6, IL-1β, and IL-17 compared to healthy neutrophils. Our data also revealed that infected neutrophils from healthy or diabetic individuals undergo apoptotic cell death at distinctly different rates, with infected diabetic neutrophils showing a diminished ability to delay apoptosis and an increased likelihood of undergoing a lytic form of cell death, compared to infected neutrophils from healthy individuals. Increased expression of inflammatory proteins by infected neutrophils could contribute to the increased susceptibility to infection and inflammation in diabetic patients in melioidosis-endemic areas.

Root extracellular traps versus neutrophil extracellular traps in host defence, a case of functional convergence?

The root cap releases cells that produce massive amounts of mucilage containing polysaccharides, proteoglycans, extracellular DNA (exDNA) and a variety of antimicrobial compounds. The released cells - known as border cells or border-like cells - and mucilage secretions form networks that are defined as root extracellular traps (RETs). RETs are important players in root immunity. In animals, phagocytes are some of the most abundant white blood cells in circulation and are very important for immunity. These cells combat pathogens through multiple defence mechanisms, including the release of exDNA-containing extracellular traps (ETs). Traps of neutrophil origin are abbreviated herein as NETs. Similar to phagocytes, plant root cap-originating cells actively contribute to frontline defence against pathogens. RETs and NETs are thus components of the plant and animal immune systems, respectively, that exhibit similar compositional and functional properties. Herein, we describe and discuss the formation, molecular composition and functional similarities of these similar but different extracellular traps.

Mechanisms and disease relevance of neutrophil extracellular trap formation

While the microscopic appearance of neutrophil extracellular traps (NETs) has fascinated basic researchers since its discovery, the (patho)physiological mechanisms triggering NET release, the disease relevance and clinical translatability of this unconventional cellular mechanism remained poorly understood. Here, we summarize and discuss current concepts of the mechanisms and disease relevance of NET formation.

Living dangerously: Burkholderia pseudomallei modulates phagocyte cell death to survive

Melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, is a major cause of sepsis and mortality in endemic regions of Southeast Asia and Northern Australia. As a facultative intracellular pathogen, B. pseudomallei produces virulence factors to evade innate host response and survive within host cells. Neutrophils and macrophages are phagocytes that play critical roles in host defense against pathogens by their ability to detect and eliminate microbes. Host defense processes against B. pseudomallei including phagocytosis, oxidative burst, autophagy, apoptosis, and proinflammatory cytokine release are all initiated by these two phagocytes in the fight against this bacterium. In vitro studies with mouse macrophage cell lines revealed multiple evasion strategies used by B. pseudomallei to counteract these innate processes. B. pseudomallei invades and replicates in neutrophils but little is known regarding its evasion mechanisms. The bidirectional interaction of neutrophils and macrophages in controlling B. pseudomallei infection has also been overlooked. Here the hypothesis that B. pseudomallei hijacks neutrophils and uses them to transport and infect new phagocytes is proposed as an evasion strategy to survive and persist in host phagocytes. This two-pronged approach by B. pseudomallei to replicate in two different types of phagocytes and to modulate their cell death modes is effective in promoting persistence and survival of the bacterium.

The diverse origins of circulating cell-free DNA in the human body: a critical re-evaluation of the literature

Since the detection of cell-free DNA (cfDNA) in human plasma in 1948, it has been investigated as a non-invasive screening tool for many diseases, especially solid tumours and foetal genetic abnormalities. However, to date our lack of knowledge regarding the origin and purpose of cfDNA in a physiological environment has limited its use to more obvious diagnostics, neglecting, for example, its potential utility in the identification of predisposition to disease, earlier detection of cancers, and lifestyle-induced epigenetic changes. Moreover, the concept or mechanism of cfDNA could also have potential therapeutic uses such as in immuno- or gene therapy. This review presents an extensive compilation of the putative origins of cfDNA and then contrasts the contributions of cellular breakdown processes with active mechanisms for the release of cfDNA into the extracellular environment. The involvement of cfDNA derived from both cellular breakdown and active release in lateral information transfer is also discussed. We hope to encourage researchers to adopt a more holistic view of cfDNA research, taking into account all the biological pathways in which cfDNA is involved, and to give serious consideration to the integration of in vitro and in vivo research. We also wish to encourage researchers not to limit their focus to the apoptotic or necrotic fraction of cfDNA, but to investigate the intercellular messaging capabilities of the actively released fraction of cfDNA and to study the role of cfDNA in pathogenesis.

Consequences of extracellular trap formation in sepsis

PURPOSE OF REVIEW

This review will focus on in-vivo findings derived from animal models of sepsis regarding the trapping role of neutrophil extracellular traps (NETs) which is difficult to assess ex vivo. The NETotic response of neutrophils at sites of sterile injury or autoimmune disease is destructive as no antimicrobial advantage to the host is realized and dampening NETosis is largely beneficial. In early stages of local infection or in sepsis, the trapping function of NETs may help abscess formation and limit microbial dissemination.

RECENT FINDINGS

The trapping function of NETs limits bacterial dissemination keeping an abscess from becoming bacteremic or confining tissue infection to local sites. Once containment is lost and disease has progressed, the best therapeutic approach suggested by animal studies to date is to inhibit protein arginine deiminase 4 and prevent NETosis rather than attempting to neutralize caustic NET components. Prognostic value may best be realized by taking cell free DNA, citrulllinated histones, neutrophil function and counts of immature granulocytes into consideration rather than rely on any one measure alone.

SUMMARY

The trapping function of NETs may supercede the value of antimicrobial function in the early phases of sepsis such that degradation of the DNA backbone is contraindicated.

Neutrophil extracellular traps in immunity and disease

Neutrophils are innate immune phagocytes that have a central role in immune defence. Our understanding of the role of neutrophils in pathogen clearance, immune regulation and disease pathology has advanced dramatically in recent years. Web-like chromatin structures known as neutrophil extracellular traps (NETs) have been at the forefront of this renewed interest in neutrophil biology. The identification of molecules that modulate the release of NETs has helped to refine our view of the role of NETs in immune protection, inflammatory and autoimmune diseases and cancer. Here, I discuss the key findings and concepts that have thus far shaped the field of NET biology.

The structural and functional changes of blood cells and molecular components in diabetes mellitus

It is known fact that diabetes mellitus (DM) affects blood cells. Changes in the erythrocyte membrane, disorder in hemoglobin oxygen-binding and modification in mechanical characteristics, are effects of hyperglycemia on red blood cells. Altered susceptibility infection of patients with diabetes has been ascribed to a depression in the function of polymorphonuclear leukocytes. Neutrophil function in patients with diabetes with good glucose control is slightly different than in healthy ones. DM causes significant changes in lymphocytes metabolism and their functions. Patients with diabetes, presenting with acute coronary syndrome, are at higher risk of cardiovascular complications and recurrent ischemic events in comparison to non-diabetic counterparts. Various mechanisms, including endothelial dysfunction, platelet hyperactivity, and abnormalities in coagulation and fibrynolysis have been implicated for this increased atherothrombotic risk. There are many other alterations of blood cells due to DM. In the present review we focused on modifications of blood cells due to DM. Then, as a second point, we explored how the changes affect functions of red blood cells, white blood cells and platelets.

Increased Von Willebrand factor, decreased ADAMTS13 and thrombocytopenia in melioidosis

BACKGROUND

Melioidosis, caused by bioterror treat agent Burkholderia pseudomallei, is an important cause of community-acquired Gram-negative sepsis in Southeast Asia and Northern Australia. New insights into the pathogenesis of melioidosis may help improve treatment and decrease mortality rates from this dreadful disease. We hypothesized that changes in Von Willebrand factor (VWF) function should occur in melioidosis, based on the presence of endothelial stimulation by endotoxin, pro-inflammatory cytokines and thrombin in melioidosis, and investigated whether this impacted on outcome.

METHODS/PRINCIPAL FINDINGS

We recruited 52 controls and 34 culture-confirmed melioidosis patients at Sappasithiprasong Hospital in Ubon Ratchathani, Thailand. All subjects were diabetic. Platelet counts in melioidosis patients were lower compared to controls (p = 0.0001) and correlated with mortality (p = 0.02). VWF antigen levels were higher in patients (geometric mean, 478 U/dl) compared to controls (166 U/dL, p<0.0001). The high levels of VWF in melioidosis appeared to be due to increased endothelial stimulation (VWF propeptide levels were elevated, p<0.0001) and reduced clearance (ADAMTS13 reduction, p<0.0001). However, VWF antigen levels did not correlate with platelet counts implying that thrombocytopenia in acute melioidosis has an alternative cause.

CONCLUSIONS/SIGNIFICANCE

Thrombocytopenia is a key feature of melioidosis and is correlated with mortality. Additionally, excess VWF and ADAMTS13 deficiency are features of acute melioidosis, but are not the primary drivers of thrombocytopenia in melioidosis. Further studies on the role of thrombocytopenia in B. pseudomallei infection are needed.

Boosting of post-exposure human T-cell and B-cell recall responses in vivo by Burkholderia pseudomallei-related proteins

Burkholderia pseudomallei is the causative agent of melioidosis, an infectious disease with high incidence and mortality in South East Asia and northern Australia. To date there is no protective vaccine and antibiotic treatment is prolonged and not always effective. Most people living in endemic areas have been exposed to the bacteria and have developed some immunity, which may have helped to prevent disease. Here, we used a humanized mouse model (hu-PBL-SCID), reconstituted with human peripheral blood mononuclear cells from seropositive donors, to illustrate the potential of three known antigens (FliC, OmpA and N-PilO2) for boosting both T-cell and B-cell immune responses. All three antigens boosted the production of specific antibodies in vivo, and increased the number of antibody and interferon-γ-secreting cells, and induced antibody affinity maturation. Moreover, antigen-specific antibodies isolated from either seropositive individuals or boosted mice, were found to enhance phagocytosis and oxidative burst activities from human polymorphonuclear cells. Our study demonstrates that FliC, OmpA and N-PilO2 can stimulate human memory T and B cells and highlight the potential of the hu-PBL-SCID system for screening and evaluation of novel protein antigens for inclusion in future vaccine trials against melioidosis.

Extracellular histones, cell-free DNA, or nucleosomes: differences in immunostimulation

In inflammation, extensive cell death may occur, which results in the release of chromatin components into the extracellular environment. Individually, the purified chromatin components double stranded (ds)DNA and histones have been demonstrated, both in vitro and in vivo, to display various immunostimulatory effects, for example, histones induce cytotoxicity and proinflammatory signaling through toll-like receptor (TLR)2 and 4, while DNA induces signaling through TLR9 and intracellular nucleic acid sensing mechanisms. However, DNA and histones are organized in nucleosomes in the nucleus, and evidence suggests that nucleosomes are released as such in inflammation. The cytotoxicity and proinflammatory signaling induced by nucleosomes have not been studied as extensively as the separate effects brought about by histones and dsDNA, and there appear to be some marked differences. Remarkably, little distinction between the different forms in which histones circulate has been made throughout literature. This is partly due to the limitations of existing techniques to differentiate between histones in their free or DNA-bound form. Here we review the current understanding of immunostimulation induced by extracellular histones, dsDNA and nucleosomes, and discuss the importance of techniques that in their detection differentiate between these different chromatin components.

Diabetes-independent increase of factor VII-activating protease activation in patients with Gram-negative sepsis (melioidosis)

BACKGROUND

The plasma protease factor VII-activating protease (FSAP) can release nucleosomes from late apoptotic cells. Nucleosomes are markers of cell death, and extracellular cell-free DNA has been suggested to play an important role in inflammation and has been demonstrated to correlate with severity and outcome in sepsis patients.

OBJECTIVE

To investigate FSAP activation in patients suffering from Burkholderia pseudomallei infection (melioidosis), an important cause of Gram-negative sepsis in Southeast Asia. As diabetes mellitus (DM) is the most important risk factor for both melioidosis and sepsis, we were also able to examine the role of DM in FSAP activation in this cohort of patients.

METHODS

In a prospective observational study, complexes of FSAP with α2 -antiplasmin (AP) were assayed in 44 patients with melioidosis, 34 of whom were classified as diabetic. Eighty-two healthy subjects served as controls (52 with DM and 30 without).

RESULTS

FSAP-AP complex levels were markedly elevated in patients as compared with controls. The FSAP level increased by 16.82 AU mL(-1) in patients with melioidosis after adjustment for the effect of DM in the regression model. As expected, FSAP activation was correlated with nucleosome release (slope = 0.74). No difference in FSAP activation on admission was seen between survivors and non-survivors, but the extent of FSAP activation correlated with stage of the disease; repeated testing during convalescence showed a return towards normal values (day 0 vs. day 28, 4.16 AU mL(-1) , 95% confidence interval [CI] 1.42-12.22).

CONCLUSION

Patients with Gram-negative sepsis caused by B. pseudomallei have abundant FSAP activation, which significantly correlates with stage of disease. The presence of DM, however, does not influence the extent of FSAP activation.