Nicolle D. Myers

Pathogenicity of High-Dose Enteral Inoculation of Burkholderia pseudomallei to Mice

Melioidosis is a frequently lethal tropical infection caused by the environmental saprophyte Burkholderia pseudomallei. Although transcutaneous inoculation and inhalation are considered the primary routes of infection, suggestive clinical evidence implicates ingestion as a possible alternative route. We show that in BALB/c and C57BL/6 mice, direct gastric inoculation of high doses of B. pseudomallei causes systemic infection that may be lethal or cause chronic disseminated infection. Mice may shed bacteria in the stool for weeks after infection, and high titers of B. pseudomallei-specific IgG are detectable. This report of enteric murine melioidosis supports further consideration of this route of infection.

Defensins Potentiate a Neutralizing Antibody Response to Enteric Viral Infection

α-defensins are abundant antimicrobial peptides with broad, potent antibacterial, antifungal, and antiviral activities in vitro. Although their contribution to host defense against bacteria in vivo has been demonstrated, comparable studies of their antiviral activity in vivo are lacking. Using a mouse model deficient in activated α-defensins in the small intestine, we show that Paneth cell α-defensins protect mice from oral infection by a pathogenic virus, mouse adenovirus 1 (MAdV-1). Survival differences between mouse genotypes are lost upon parenteral MAdV-1 infection, strongly implicating a role for intestinal defenses in attenuating pathogenesis. Although differences in α-defensin expression impact the composition of the ileal commensal bacterial population, depletion studies using broad-spectrum antibiotics revealed no effect of the microbiota on α-defensin-dependent viral pathogenesis. Moreover, despite the sensitivity of MAdV-1 infection to α-defensin neutralization in cell culture, we observed no barrier effect due to Paneth cell α-defensin activation on the kinetics and magnitude of MAdV-1 dissemination to the brain. Rather, a protective neutralizing antibody response was delayed in the absence of α-defensins. This effect was specific to oral viral infection, because antibody responses to parenteral or mucosal ovalbumin exposure were not affected by α-defensin deficiency. Thus, α-defensins play an important role as adjuvants in antiviral immunity in vivo that is distinct from their direct antiviral activity observed in cell culture.

Survey of Innate Immune Responses to Burkholderia pseudomallei in Human Blood Identifies a Central Role for Lipopolysaccharide

B. pseudomallei is a gram-negative bacterium that causes the tropical infection melioidosis. In northeast Thailand, mortality from melioidosis approaches 40%. As exemplified by the lipopolysaccharide-Toll-like receptor 4 interaction, innate immune responses to invading bacteria are precipitated by activation of host pathogen recognition receptors by pathogen associated molecular patterns. Human melioidosis is characterized by up-regulation of pathogen recognition receptors and pro-inflammatory cytokine release. In contrast to many gram-negative pathogens, however, the lipopolysaccharide of B. pseudomallei is considered only weakly inflammatory. We conducted a study in 300 healthy Thai subjects to investigate the ex vivo human blood response to various bacterial pathogen associated molecular patterns, including lipopolysaccharide from several bacteria, and to two heat-killed B. pseudomallei isolates. We measured cytokine levels after stimulation of fresh whole blood with a panel of stimuli. We found that age, sex, and white blood cell count modulate the innate immune response to B. pseudomallei. We further observed that, in comparison to other stimuli, the innate immune response to B. pseudomallei is most highly correlated with the response to lipopolysaccharide. The magnitude of cytokine responses induced by B. pseudomallei lipopolysaccharide was significantly greater than those induced by lipopolysaccharide from Escherichia coli and comparable to many responses induced by lipopolysaccharide from Salmonella minnesota despite lower amounts of lipid A in the B. pseudomallei lipopolysaccharide preparation. In human monocytes stimulated with B. pseudomallei, addition of polymyxin B or a TLR4/MD-2 neutralizing antibody inhibited the majority of TNF-α production. Challenging existing views, our data indicate that the innate immune response to B. pseudomallei in human blood is largely driven by lipopolysaccharide, and that the response to B. pseudomallei lipopolysaccharide in blood is greater than the response to other lipopolysaccharide expressing isolates. Our findings suggest that B. pseudomallei lipopolysaccharide may play a central role in stimulating the host response in melioidosis.

Impaired TLR5 Functionality Is Associated with Survival in Melioidosis

Melioidosis is infection caused by the flagellated saprophyte Burkholderia pseudomallei. TLR5 is a pathogen recognition receptor activated by bacterial flagellin. We studied a genetic variant that encodes a defective TLR5 protein, TLR51174C>T, to elucidate the role of TLR5 in melioidosis. We measured NF-κB activation induced by B. pseudomallei in human embryonic kidney–293 cells transfected with TLR5 and found that B. pseudomallei induced TLR51174C- but not TLR51174T-dependent activation of NF-κB. We tested the association of TLR51174C>T with outcome in 600 Thai subjects with melioidosis. In a dominant model, TLR51174C>T was associated with protection against in-hospital death (adjusted odds ratio: 0.20; 95% confidence interval: 0.08–0.50; p = 0.001) and organ failure (adjusted odds ratio: 0.37; 95% confidence interval: 0.19–0.71; p = 0.003). We analyzed blood cytokine production induced by flagellin or heat-killed B. pseudomallei by TLR51174C>T genotype in healthy subjects. Flagellin induced lower monocyte-normalized levels of IL-6, IL-8, TNF-α, IL-10, MCP-1, IL-1ra, G-CSF, and IL-1β in carriers of TLR51174T compared with carriers of TLR51174C. B. pseudomallei induced lower monocyte-normalized levels of IL-10 in carriers of TLR51174T. We conclude that the hypofunctional genetic variant TLR51174C>T is associated with reduced organ failure and improved survival in melioidosis. This conclusion suggests a deleterious immunoregulatory effect of TLR5 that may be mediated by IL-10 and identifies this receptor as a potential therapeutic target in melioidosis.

Murine pulmonary infection and inflammation induced by inhalation of Burkholderia pseudomallei

Melioidosis is a tropical disease caused by ingestion, percutaneous inoculation or inhalation of the Gram‐negative soil saprophyte Burkholderia pseudomallei. We developed a reproducible experimental murine model of pneumonic melioidosis induced by inhalation of aerosolized B. pseudomallei 1026b. In a series of experiments performed to bracket the lethal dose, we found that C57BL/6 mice were modestly more resistant than BALB/c mice (median lethal dose 334 CFU/lung vs 204 CFU/lung). We further characterized infection and pulmonary inflammation in C57BL/6 mice infected with a sublethal dose. We observed pulmonary replication and dissemination of bacteria to distant organs in the first days after infection, followed by bacterial containment by day 4 and no evidence of recrudescent infection for up to 2 months. We measured a robust host inflammatory response notable for a neutrophilic bronchoalveolar lavage fluid profile, elevated cytokines and chemokines in the lung and serum and scattered foci of neutrophilic infiltrates in the alveoli and in a perivascular distribution on histological analysis. We previously noted a similar pattern of inflammation in mice infected with aerosolized B. thailandensis. This report builds on the limited literature describing experimental murine pneumonic melioidosis induced by aerosol and characterizes pulmonary infection and resultant inflammation in C57BL/6 mice infected with aerosolized B. pseudomallei. This model has utility for the study of bacterial and host factors that contribute to the virulence of melioidosis.

NLRC4 and TLR5 Each Contribute to Host Defense in Respiratory Melioidosis

Burkholderia pseudomallei causes the tropical infection melioidosis. Pneumonia is a common manifestation of melioidosis and is associated with high mortality. Understanding the key elements of host defense is essential to developing new therapeutics for melioidosis. As a flagellated bacterium encoding type III secretion systems, B. pseudomallei may trigger numerous host pathogen recognition receptors. TLR5 is a flagellin sensor located on the plasma membrane. NLRC4, along with NAIP proteins, assembles a canonical caspase-1-dependent inflammasome in the cytoplasm that responds to flagellin (in mice) and type III secretion system components (in mice and humans). In a murine model of respiratory melioidosis, Tlr5 and Nlrc4 each contributed to survival. Mice deficient in both Tlr5 and Nlrc4 were not more susceptible than single knockout animals. Deficiency of Casp1/Casp11 resulted in impaired bacterial control in the lung and spleen; in the lung much of this effect was attributable to Nlrc4, despite relative preservation of pulmonary IL-1β production in Nlrc4−/− mice. Histologically, deficiency of Casp1/Casp11 imparted more severe pulmonary inflammation than deficiency of Nlrc4. The human NLRC4 region polymorphism rs6757121 was associated with survival in melioidosis patients with pulmonary involvement. Co-inheritance of rs6757121 and a functional TLR5 polymorphism had an additive effect on survival. Our results show that NLRC4 and TLR5, key components of two flagellin sensing pathways, each contribute to host defense in respiratory melioidosis.

The Role of NOD2 in Murine and Human Melioidosis

Nucleotide-binding oligomerization domain 2 (NOD2) is a cytosolic pathogen recognition receptor that regulates susceptibility to a variety of infections and chronic diseases. Burkholderia pseudomallei, a facultative intracellular bacterium, causes the tropical infection melioidosis. We hypothesized that NOD2 may participate in host defense in melioidosis. We performed a series of in vitro assays and in vivo experiments and analyzed the association of human genetic variation with infection to delineate the contribution of NOD2 to the host response to B. pseudomallei. We found that transfection with NOD2 mediated NF-κB activation induced by B. pseudomallei stimulation of HEK293 cells. After low-dose inoculation with aerosolized B. pseudomallei, Nod2-deficient mice showed impaired clinical responses and permitted greater bacterial replication in the lung and dissemination to the spleen compared with wild-type mice. IL-6 and KC levels were higher in the lungs of Nod2-deficient mice. In a cohort of 1562 Thai subjects, a common genetic polymorphism in the NOD2 region, rs7194886, was associated with melioidosis, and this effect was most pronounced in women. rs7194886 was not associated with differences in cytokine production induced by whole-blood stimulation with the NOD2 ligand, muramyl dipeptide, or B. pseudomallei. To our knowledge, these findings are the first to characterize the role of NOD2 in host defense in mammalian melioidosis.

Screen of whole blood responses to flagellin identifies TLR5 variation associated with outcome in melioidosis.

Melioidosis is a severe infection caused by the flagellated bacterium Burkholderia pseudomallei. The nonsense polymorphism TLR51174C>T is associated with improved outcome in Thais with melioidosis. We hypothesized that other TLR5 variants may modulate the host response and determine outcome in melioidosis. We genotyped 12 TLR5 variants selected de novo from the HapMap database and examined the association of each with cytokines induced by flagellin stimulation of whole blood from healthy Thai subjects. We found a blunted cytokine response for three related markers that were in linkage disequilibrium (LD) with a non-synonymous variant, TLR51846T>C. Carriers of TLR51846T>C had broadly impaired cytokine responses induced by flagellin. TLR51846T>C was associated with protection against death in melioidosis patients (odds ratio: 0.62, 95% confidence interval: 0.42–0.93, P=0.021). We observed no impairment in TLR51846C-dependent nuclear factor κB activation, however, suggesting an alternative mechanism for the effect. We found that TLR51846T>C was in strong LD with TLR51174C>T. Many of the blunted cytokine responses observed and the association of TLR51846T>C with survival in melioidosis patients may be attributable to TLR51174C>T, but we could not exclude an independent effect of TLR51846T>C. These data identify novel associations for TLR51846T>C, enhance our understanding of TLR5 genetic architecture in Thais and highlight the role of TLR5 in melioidosis.

Directed Evolution of Mutator Adenoviruses Resistant to Antibody Neutralization

ABSTRACT We incorporated a previously identified mutation that reduces the fidelity of the DNA polymerase into a human adenovirus vector. Using this mutator vector, we demonstrate rapid selection of resistance to a neutralizing anti-hexon monoclonal antibody due to a G434D mutation in hexon that precludes antibody binding. Since mutator adenoviruses can accumulate compound mutations that are unattainable using traditional random mutagenesis techniques, this approach will be valuable to the study of antivirals and host factor interactions.

Common TLR1 Genetic Variation Is Not Associated with Death from Melioidosis, a Common Cause of Sepsis in Rural Thailand

Melioidosis, infection caused by the Gram-negative bacterium Burkholderia pseudomallei, is a common cause of sepsis in northeast Thailand. In white North Americans, common functional genetic variation in TLR1 is associated with organ failure and death from sepsis. We hypothesized that TLR1 variants would be associated with outcomes in Thais with melioidosis. We collated the global frequencies of three TLR1 variants that are common in white North American populations: rs5743551 (-7202A/G), rs4833095 (742A/G), and rs5743618 (1804G/T). We noted a reversal of the minor allele from white North American subjects to Asian populations that was particularly pronounced for rs5743618. In the Utah residents of European ancestry, the frequency of the rs5743618 T allele was 17% whereas in Vietnamese subjects the frequency was >99%. We conducted a genetic association study in 427 patients with melioidosis to determine the association of TLR1 variation with organ failure or death. We genotyped rs5743551 and rs4833095. The variants were in high linkage disequilibrium but neither variant was associated with organ failure or in-hospital death. In 300 healthy Thai individuals we further tested the association of TLR1 variation with ex vivo blood responses to Pam3CSK4, a TLR1 agonist. Neither variant was robustly associated with blood cytokine responses induced by Pam3CSK4. We identified additional common variation in TLR1 by searching public databases and the published literature and screened three additional TLR1 variants for associations with Pam3CSK4-induced responses but found none. We conclude that the genetic architecture of TLR1 variation differs substantially in southeast Asians compared to other populations and common variation in TLR1 in Thais is not associated with outcome from melioidosis or with altered blood responses to Pam3CSK4. Our findings highlight the need for additional studies of TLR1 and other innate immune genetic modulators of the inflammatory host response and determinants of sepsis in southeast Asian populations.