The role of NOD2 in murine and human melioidosis

Lack of Association of TLR1 and TLR5 Coding Variants with Mortality in a Large Multicenter Cohort of Melioidosis Patients

Melioidosis, infection caused by Burkholderia pseudomallei, is characterized by robust innate immune responses. We have previously reported associations of TLR1 single nucleotide missense variant rs76600635 with mortality and of TLR5 nonsense variant rs5744168 with both bacteremia and mortality in single-center studies of patients with melioidosis in northeastern Thailand. The objective of this study was to externally validate the associations of rs76600635 and rs5744168 with bacteremia and mortality in a large multicenter cohort of melioidosis patients. We genotyped rs76600635 and rs5744168 in 1,338 melioidosis patients enrolled in a prospective parent cohort study conducted at nine hospitals in northeastern Thailand. The genotype frequencies of rs76600635 did not differ by bacteremia status (P = 0.27) or 28-day mortality (P = 0.84). The genotype frequencies of rs5744168 did not differ by either bacteremia status (P = 0.46) or 28-day mortality (P = 0.10). Assuming a dominant genetic model, there was no association of the rs76600635 variant with bacteremia (adjusted odds ratio [OR], 0.75; 95% CI, 0.54-1.04, P = 0.08) or 28-day mortality (adjusted OR, 0.96; 95% CI, 0.71-1.28, P = 0.77). There was no association of the rs5744168 variant with bacteremia (adjusted OR, 1.24; 95% CI, 0.76-2.03, P = 0.39) or 28-day mortality (adjusted OR, 1.22; 95% CI, 0.83-1.79, P = 0.21). There was also no association of either variant with 1-year mortality. We conclude that in a large multicenter cohort of patients hospitalized with melioidosis in northeastern Thailand, neither TLR1 missense variant rs76600635 nor TLR5 nonsense variant rs5744168 is associated with bacteremia or mortality.

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.

Comparative virulence of three different strains of Burkholderia pseudomallei in an aerosol non-human primate model

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. B. pseudomallei is a potential bioterrorism agent due to its high infectivity, especially via inhalation, and its inherent resistance to antimicrobials. There is currently no vaccine for melioidosis and antibiotic treatment can fail due to innate drug resistance, delayed diagnosis and treatment, or insufficient duration of treatment. A well-characterized animal model that mimics human melioidosis is needed for the development of new medical countermeasures. This study first characterized the disease progression of melioidosis in the African green monkey (AGM) and rhesus macaque (RM) for non-human primate model down-selection. All AGMs developed acute lethal disease similar to that described in human acute infection following exposure to aerosolized B. pseudomallei strain HBPUB10134a. Only 20% of RMs succumbed to acute disease. Disease progression, immune response and pathology of two other strains of B. pseudomallei, K96243 and MSHR5855, were also compared using AGMs. These three B. pseudomallei strains represent a highly virulent strain from Thailand (HBPUB101034a), a highly virulent strains from Australia (MSHR5855), and a commonly used laboratory strains originating from Thailand (K96243). Animals were observed for clinical signs of infection and blood samples were analyzed for cytokine responses, blood chemistry and leukocyte changes in order to characterize bacterial infection. AGMs experienced fever after exposure to aerosolized B. pseudomallei at the onset of acute disease. Inflammation, abscesses and/or pyogranulomas were observed in lung with all three strains of B. pseudomallei. Inflammation, abscesses and/or pyogranulomas were observed in lymph nodes, spleen, liver and/or kidney with B. pseudomallei, HBPUB10134a and K96243. Additionally, the Australian strain MSHR5855 induced brain lesions in one AGM similar to clinical cases of melioidosis seen in Australia. Elevated serum levels of IL-1β, IL-1 receptor antagonist, IL-6, MCP-1, G-CSF, HGF, IFNγ, MIG, I-TAC, and MIP-1β at terminal end points can be significantly correlated with non-survivors with B. pseudomallei infection in AGM. The AGM model represents an acute model of B. pseudomallei infection for all three strains from two geographical locations and will be useful for efficacy testing of vaccines and therapeutics against melioidosis. In summary, a dysregulated immune response leading to excessive persistent inflammation and inflammatory cell death is the key driver of acute melioidosis. Early intervention in these pathways will be necessary to counter B. pseudomallei and mitigate the pathological consequences of melioidosis.

Melioidosis, caused by the Gram-negative bacterium Burkholderia pseudomallei, is endemic in tropical regions globally and is an emerging threat in non-tropical areas worldwide. Its mortality rate is high in endemic areas due to its high infectivity, antimicrobial resistance, lack of available vaccines and limited treatment options. Animal model development and pathogenicity studies of various isolates are critical for the development of countermeasures against this pathogen. In this study, we compared the virulence of three different isolates of B. pseudomallei from two geographical locations in an aerosol non-human primate model. We found that early elevations of both pro-inflammatory and anti-inflammatory mediators, as well as the persistence of these mediators in the terminal phase of bacterial infection correlate with mortality. Histopathological analysis showed that the severity of lesions in various organs also correlates with the virulence of the B. pseudomallei strains, HBPUB10134a, MSHR5855 and K96243. Thus, a dysregulated immune response leading to excessive IL-1β and IL-6 at terminal end points and necrosis are key drivers of acute melioidosis. Development of drugs targeting these host response processes will be necessary to counter B. pseudomallei and mitigate the pathological consequences of melioidosis. This non-human primate model will facilitate the screening of vaccines and novel therapeutics.

An Evolutionary Arms Race Between Burkholderia pseudomallei and Host Immune System: What Do We Know?

A better understanding of co-evolution between pathogens and hosts holds promise for better prevention and control strategies. This review will explore the interactions between Burkholderia pseudomallei, an environmental and opportunistic pathogen, and the human host immune system. B. pseudomallei causes "Melioidosis," a rapidly fatal tropical infectious disease predicted to affect 165,000 cases annually worldwide, of which 89,000 are fatal. Genetic heterogeneities were reported in both B. pseudomallei and human host population, some of which may, at least in part, contribute to inter-individual differences in disease susceptibility. Here, we review (i) a multi-host-pathogen characteristic of the interaction; (ii) selection pressures acting on B. pseudomallei and human genomes with the former being driven by bacterial adaptation across ranges of ecological niches while the latter are driven by human encounter of broad ranges of pathogens; (iii) the mechanisms that generate genetic diversity in bacterial and host population particularly in sequences encoding proteins functioning in host-pathogen interaction; (iv) reported genetic and structural variations of proteins or molecules observed in B. pseudomallei-human host interactions and their implications in infection outcomes. Together, these predict bacterial and host evolutionary trajectory which continues to generate genetic diversity in bacterium and operates host immune selection at the molecular level.

Human Melioidosis

The causative agent of melioidosis, Burkholderia pseudomallei, a tier 1 select agent, is endemic in Southeast Asia and northern Australia, with increased incidence associated with high levels of rainfall. Increasing reports of this condition have occurred worldwide, with estimates of up to 165,000 cases and 89,000 deaths per year. The ecological niche of the organism has yet to be clearly defined, although the organism is associated with soil and water. The culture of appropriate clinical material remains the mainstay of laboratory diagnosis. Identification is best done by phenotypic methods, although mass spectrometric methods have been described. Serology has a limited diagnostic role. Direct molecular and antigen detection methods have limited availability and sensitivity. Clinical presentations of melioidosis range from acute bacteremic pneumonia to disseminated visceral abscesses and localized infections. Transmission is by direct inoculation, inhalation, or ingestion. Risk factors for melioidosis include male sex, diabetes mellitus, alcohol abuse, and immunosuppression. The organism is well adapted to intracellular survival, with numerous virulence mechanisms. Immunity likely requires innate and adaptive responses. The principles of management of this condition are drainage and debridement of infected material and appropriate antimicrobial therapy. Global mortality rates vary between 9% and 70%. Research into vaccine development is ongoing.

Longitudinal profiling of plasma cytokines in melioidosis and their association with mortality: a prospective cohort study

OBJECTIVES

To characterize plasma cytokine responses in melioidosis and analyse their association with mortality.

METHODS

A prospective longitudinal study was conducted in two hospitals in Northeast Thailand to enrol 161 individuals with melioidosis, plus 13 uninfected healthy individuals and 11 uninfected individuals with diabetes to act as controls. Blood was obtained from all individuals at enrolment (day 0), and at days 5, 12 and 28 from surviving melioidosis patients. Interferon-γ (IFN-γ), interleukin-1β (IL-1β), IL-2, IL-4, IL-6, IL-8, IL-10, IL-12p70, IL-13, IL-17A, IL-23, and tumour necrosis factor-α (TNF-α) were assayed in plasma. The association of each cytokine and its dynamics with 28-day mortality was determined.

RESULTS

Of the individuals with melioidosis, 131/161 (81%) were bacteraemic, and 68/161 (42%) died. On enrolment, median levels of IFN-γ, IL-6, IL-8, IL-10, IL-23 and TNF-α were higher in individuals with melioidosis compared with uninfected healthy individuals and all but IFN-γ were positively associated with 28-day mortality. Interleukin-8 provided the best discrimination of mortality (area under the receiver operating characteristic curve 0.78, 95% CI 0.71-0.85). Over time, non-survivors had increasing IL-6, IL-8 and IL-17A levels, in contrast to survivors. In joint modelling, temporal trajectories of IFN-γ, IL-6, IL-8, IL-10 and TNF-α predicted survival.

CONCLUSIONS

In a severely ill cohort of individuals with melioidosis, specific pro- and anti-inflammatory and T helper type 17 cytokines were associated with survival from melioidosis, at enrolment and over time. Persistent inflammation preceded death. These findings support further evaluation of these mediators as prognostic biomarkers and to guide targeted immunotherapeutic development for severe melioidosis.

Melioidosis

Burkholderia pseudomallei is a Gram-negative environmental bacterium and the aetiological agent of melioidosis, a life-threatening infection that is estimated to account for ∼89,000 deaths per year worldwide. Diabetes mellitus is a major risk factor for melioidosis, and the global diabetes pandemic could increase the number of fatalities caused by melioidosis. Melioidosis is endemic across tropical areas, especially in southeast Asia and northern Australia. Disease manifestations can range from acute septicaemia to chronic infection, as the facultative intracellular lifestyle and virulence factors of B. pseudomallei promote survival and persistence of the pathogen within a broad range of cells, and the bacteria can manipulate the host's immune responses and signalling pathways to escape surveillance. The majority of patients present with sepsis, but specific clinical presentations and their severity vary depending on the route of bacterial entry (skin penetration, inhalation or ingestion), host immune function and bacterial strain and load. Diagnosis is based on clinical and epidemiological features as well as bacterial culture. Treatment requires long-term intravenous and oral antibiotic courses. Delays in treatment due to difficulties in clinical recognition and laboratory diagnosis often lead to poor outcomes and mortality can exceed 40% in some regions. Research into B. pseudomallei is increasing, owing to the biothreat potential of this pathogen and increasing awareness of the disease and its burden; however, better diagnostic tests are needed to improve early confirmation of diagnosis, which would enable better therapeutic efficacy and survival.

T-Cell Responses Are Associated with Survival in Acute Melioidosis Patients

Background

Melioidosis is an increasingly recognised cause of sepsis and death across South East Asia and Northern Australia, caused by the bacterium Burkholderia pseudomallei. Risk factors include diabetes, alcoholism and renal disease, and a vaccine targeting at-risk populations is urgently required. A better understanding of the protective immune response in naturally infected patients is essential for vaccine design.

Methods

We conducted a longitudinal clinical and immunological study of 200 patients with melioidosis on admission, 12 weeks (n = 113) and 52 weeks (n = 65) later. Responses to whole killed B. pseudomallei were measured in peripheral blood mononuclear cells (PBMC) by interferon-gamma (IFN-γ) ELIspot assay and flow cytometry and compared to those of control subjects in the region with diabetes (n = 45) and without diabetes (n = 43).

Results

We demonstrated strong CD4+ and CD8+ responses to B. pseudomallei during acute disease, 12 weeks and 52 weeks later. 28-day mortality was 26% for melioidosis patients, and B. pseudomallei-specific cellular responses in fatal cases (mean 98 IFN-γ cells per million PBMC) were significantly lower than those in the survivors (mean 142 IFN-γ cells per million PBMC) in a multivariable logistic regression model (P = 0.01). A J-shaped curve association between circulating neutrophil count and mortality was seen with an optimal count of 4000 to 8000 neutrophils/μl.

Melioidosis patients with known diabetes had poor diabetic control (median glycated haemoglobin HbA_1c 10.2%, interquartile range 9.2–13.1) and showed a stunted B. pseudomallei-specific cellular response during acute illness compared to those without diabetes.

Conclusions

The results demonstrate the role of both CD4+ and CD8+ T-cells in protection against melioidosis, and an interaction between diabetes and cellular responses. This supports development of vaccine strategies that induce strong T-cell responses for the control of intracellular pathogens such as B. pseudomallei.

Melioidosis is a key cause of death in South East Asia and Northern Australia. It is caused by the soil-dwelling bacteria Burkholderia pseudomallei, and presents as a range of clinical illnesses including pneumonia and bloodstream infections. About two-thirds of patients with melioidosis in Thailand have diabetes, but the immune responses associated with death and diabetes are unknown. This study examined the relationship between immune responses to the bacteria and death by studying the bacteria-specific lymphocyte responses in 200 patients admitted to hospital with acute melioidosis and following the patients for up to one year where possible. 26% of patients died within 28 days despite receiving antibiotics and supportive care. We showed people with melioidosis make bacteria-specific lymphocyte responses, with lower levels seen in fatal cases compared to survivors. People with diabetes make lower responses than people without diabetes, and higher levels of circulating neutrophils on admission to hospital were associated with lower lymphocyte responses both during illness and three months later in survivors. This highlighting of lymphocyte responses to melioidosis is important for the design of vaccines to target at risk groups.

Analysis of the immunomodulatory properties of two heat-killed mycobacterial preparations in a human whole blood model

The significant role played by mycobacteria in modulating immune responses through enhancing the crosstalk between innate and adaptive immunity has been highlighted in several studies. Owing to their unique antigenic profile, heat killed (HK) preparations of rapid-growing mycobacteria, currently undergoing clinical development, have been assessed as adjuvant therapy in various diseases. The purpose of this study is to investigate the regulation of leukocyte surface receptors, in whole blood from healthy donors, following in vitro stimulation with HK Mycobacterium vaccae (M. vaccae) or M. obuense. We have demonstrated the ability of both mycobacterial preparations to target monocytes and neutrophils and to regulate the surface expression of selected adhesion receptors, antigen-presenting and costimulatory receptors, pattern recognition receptors, complement and Fc receptors, as well as cytokine/chemokine receptors. Toll-like receptors (TLRs) 1 and 2 were also shown to be involved in mediating the M. obuense-induced upregulation of selected surface receptors on monocytes. Whole blood stimulation with M. vaccae or M. obuense resulted in a significant increase in the secretion of a specific set of cytokines and chemokines. Both mycobacterial preparations induced strong antigen-specific proliferative responses in peripheral blood mononuclear cells. Collectively, our data shows that M. vaccae and M. obuense have the potential to act as potent immunomodulators. Future research based on these findings may reveal novel immune pathways induced by these preparations with potential implication for their use in diverse immunotherapeutic approaches.

Delineating the importance of serum opsonins and the bacterial capsule in affecting the uptake and killing of Burkholderia pseudomallei by murine neutrophils and macrophages

Infection of susceptible hosts by the encapsulated Gram-negative bacterium Burkholderia pseudomallei (Bp) causes melioidosis, with septic patients attaining mortality rates ≥ 40%. Due to its high infectivity through inhalation and limited effective therapies, Bp is considered a potential bioweapon. Thus, there is great interest in identifying immune effectors that effectively kill Bp. Our goal is to compare the relative abilities of murine macrophages and neutrophils to clear Bp, as well as determine the importance of serum opsonins and bacterial capsule. Our findings indicate that murine macrophages and neutrophils are inherently unable to clear either unopsonized Bp or the relatively-avirulent acapsular bacterium B. thailandensis (Bt). Opsonization of Bp and Bt with complement or pathogen-specific antibodies increases macrophage-uptake, but does not promote clearance, although antibody-binding enhances complement deposition. In contrast, complement opsonization of Bp and Bt causes enhanced uptake and killing by neutrophils, which is linked with rapid ROS induction against bacteria exhibiting a threshold level of complement deposition. Addition of bacteria-specific antibodies enhances complement deposition, but antibody-binding alone cannot elicit neutrophil clearance. Bp capsule provides some resistance to complement deposition, but is not anti-phagocytic or protective against reactive oxygen species (ROS)-killing. Macrophages were observed to efficiently clear Bp only after pre-activation with IFNγ, which is independent of serum- and/or antibody-opsonization. These studies indicate that antibody-enhanced complement activation is sufficient for neutrophil-clearance of Bp, whereas macrophages are ineffective at clearing serum-opsonized Bp unless pre-activated with IFNγ. This suggests that effective immune therapies would need to elicit both antibodies and Th1-adaptive responses for successful prevention/eradication of melioidosis.

Type three secretion system-mediated escape of Burkholderia pseudomallei into the host cytosol is critical for the activation of NFκB

Background

Burkholderia pseudomallei is the causative agent of melioidosis, a potentially fatal disease endemic in Southeast Asia and Northern Australia. This Gram-negative pathogen possesses numerous virulence factors including three “injection type” type three secretion systems (T3SSs). B. pseudomallei has been shown to activate NFκB in HEK293T cells in a Toll-like receptor and MyD88 independent manner that requires T3SS gene cluster 3 (T3SS3 or T3SS_Bsa). However, the mechanism of how T3SS3 contributes to NFκB activation is unknown.

Results

Known T3SS3 effectors are not responsible for NFκB activation. Furthermore, T3SS3-null mutants are able to activate NFκB almost to the same extent as wildtype bacteria at late time points of infection, corresponding to delayed escape into the cytosol. NFκB activation also occurs when bacteria are delivered directly into the cytosol by photothermal nanoblade injection.

Conclusions

T3SS3 does not directly activate NFκB but facilitates bacterial escape into the cytosol where the host is able to sense the presence of the pathogen through cytosolic sensors leading to NFκB activation.

A thrombomodulin mutation that impairs active protein C generation is detrimental in severe pneumonia-derived gram-negative sepsis (melioidosis)

Background

During severe (pneumo)sepsis inflammatory and coagulation pathways become activated as part of the host immune response. Thrombomodulin (TM) is involved in a range of host defense mechanisms during infection and plays a pivotal role in activation of protein C (PC) into active protein C (APC). APC has both anticoagulant and anti-inflammatory properties. In this study we investigated the effects of impaired TM-mediated APC generation during melioidosis, a common form of community-acquired Gram-negative (pneumo)sepsis in South-East Asia caused by Burkholderia (B.) pseudomallei.

Methodology/Principal Findings

(WT) mice and mice with an impaired capacity to activate protein C due to a point mutation in their Thbd gene (TM^pro/pro mice) were intranasally infected with B. pseudomallei and sacrificed after 24, 48 or 72 hours for analyses. Additionally, survival studies were performed. When compared to WT mice, TM^pro/pro mice displayed a worse survival upon infection with B. pseudomallei, accompanied by increased coagulation activation, enhanced lung neutrophil influx and bronchoalveolar inflammation at late time points, together with increased hepatocellular injury. The TM^pro/pro mutation had limited if any impact on bacterial growth and dissemination.

Conclusion/Significance

TM-mediated protein C activation contributes to protective immunity after infection with B. pseudomallei. These results add to a better understanding of the regulation of the inflammatory and procoagulant response during severe Gram-negative (pneumo)sepsis.

Pneumonia and sepsis are conditions in which a procoagulant state is observed, with activation of coagulation and downregulation of anticoagulant pathways, both closely interrelated with inflammation. The protein C (PC) system is an important anticoagulant pathway implicated in the pathogenesis of sepsis. After binding to thrombomodulin (TM), PC is converted into active protein C (APC), mediated via high-affinity binding of thrombin to thrombomodulin (TM) and further augmented via association of the endothelial protein C receptor (EPCR) to the TM-thrombin complex. We studied the role of TM-associated PC-activation during the host response during pneumonia-derived sepsis caused by Burkholderia (B.) pseudomallei, the causative agent of melioidosis, a common form of community-acquired Gram-negative (pneumo)sepsis in South-East Asia and a serious potential bioterrorism threat agent. Mice with an impaired capacity to activate protein C displayed a worse survival upon infection with B. pseudomallei, accompanied by increased coagulation activation, enhanced lung neutrophil influx and bronchoalveolar inflammation at late time points, together with increased hepatocellular injury. These data further expand the knowledge about the role of the protein C system during melioidosis and may be of value in the development of therapeutic strategies against this dangerous pathogen.