Development of an acute model of inhalational melioidosis in the common marmoset (Callithrix jacchus)

Marmosets as models of infectious diseases

Animal models of infectious disease often serve a crucial purpose in obtaining licensure of therapeutics and medical countermeasures, particularly in situations where human trials are not feasible, i.e., for those diseases that occur infrequently in the human population. The common marmoset (Callithrix jacchus), a Neotropical new-world (platyrrhines) non-human primate, has gained increasing attention as an animal model for a number of diseases given its small size, availability and evolutionary proximity to humans. This review aims to (i) discuss the pros and cons of the common marmoset as an animal model by providing a brief snapshot of how marmosets are currently utilized in biomedical research, (ii) summarize and evaluate relevant aspects of the marmoset immune system to the study of infectious diseases, (iii) provide a historical backdrop, outlining the significance of infectious diseases and the importance of developing reliable animal models to test novel therapeutics, and (iv) provide a summary of infectious diseases for which a marmoset model exists, followed by an in-depth discussion of the marmoset models of two studied bacterial infectious diseases (tularemia and melioidosis) and one viral infectious disease (viral hepatitis C).

Melioidosis in Cynomolgus Macaques ( Macaca Fascicularis ) Imported to the United States from Cambodia

Melioidosis, a potentially fatal infectious disease of humans and animals, including nonhuman primates (NHPs), is caused by the high-consequence pathogen Burkholderia pseudomallei. This environmental bacterium is found in the soil and water of tropical regions, such as Southeast Asia, where melioidosis is endemic. The global movement of humans and animals can introduce B. pseudomallei into nonendemic regions of the United States, where environmental conditions could allow establishment of the organism. Approximately 60% of NHPs imported into the United States originate in countries considered endemic for melioidosis. To prevent the introduction of infectious agents to the United States, the Centers for Disease Control and Prevention (CDC) requires newly imported NHPs to be quarantined for at least 31 d, during which time their health is closely monitored. Most diseases of public health concern that are transmissible from imported NHPs have relatively short incubation periods that fall within the 31-d quarantine period. However, animals infected with B. pseudomallei may appear healthy for months to years before showing signs of illness, during which time they can shed the organism into the environment. Melioidosis presents diagnostic challenges because it causes nonspecific clinical signs, serologic screening can produce unreliable results, and culture isolates are often misidentified on rapid commercial testing systems. Here, we present a case of melioidosis in a cynomolgus macaque (Macaca fascicularis) that developed a subcutaneous abscess after importation from Cambodia to the United States. The bacterial isolate from the abscess was initially misidentified on a commercial test. This case emphasizes the possibility of melioidosis in NHPs imported from endemic countries and its associated diagnostic challenges. If melioidosis is suspected, diagnostic samples and culture isolates should be submitted to a laboratory in the CDC Laboratory Response Network for conclusive identification and characterization of the pathogen.

Efficacy of Co-Trimoxazole against Experimental Melioidosis Acquired by Different Routes of Infection

Burkholderia pseudomallei is the causative agent of melioidosis and presents with diverse clinical manifestations. Naturally occurring infection occurs following contamination of cuts or skin abrasions, or ingestion of contaminated water, and occasionally through inhalational of infected soil or water particles. The influence of the route of disease acquisition on the efficacy of medical countermeasures has not been explored in humans or in appropriate animal models. The efficacy of co-trimoxazole against melioidosis acquired by different routes of exposure was assessed in postexposure prophylaxis (PEP) and treatment studies in marmoset models of melioidosis. Following challenge with B. pseudomallei by the inhalational, subcutaneous, or ingestion routes of administration, animals were given co-trimoxazole at 12 hourly intervals for 14 days, starting either 6 h postchallenge or at the onset of fever. Animals were then observed for 28 days. All animals that received antibiotic 6 h postchallenge survived the duration of dosing. All animals that received antibiotics at the onset of fever completed the treatment, but 10%, 57%, and 60% of those with ingestion, subcutaneous, and inhalation challenge relapsed, respectively. Bacteriological and histological differences were observed between placebo-control animals and those that relapsed. Immunological profiles indicate difference between animals given placebo and those that relapsed or survived the duration of the study. A broad T-cell activation was observed in animals that survived. Overall, these data suggest the efficacy of co-trimoxazole, as measured in the incidence of relapse, differs depending on the disease-acquisition route. Therefore, there are implications in treating this disease in regions of endemicity.

Histopathological and Immunological Findings in the Common Marmoset Following Exposure to Aerosolized SARS-CoV-2

There is an enduring requirement to develop animal models of COVID-19 to assess the efficacy of vaccines and therapeutics that can be used to treat the disease in humans. In this study, six marmosets were exposed to a small particle aerosol (1–3 µm) of SARS-CoV-2 VIC01 that delivered the virus directly to the lower respiratory tract. Following the challenge, marmosets did not develop clinical signs, although a disruption to the normal diurnal temperature rhythm was observed in three out of six animals. Early weight loss and changes to respiratory pattern and activity were also observed, yet there was limited evidence of viral replication or lung pathology associated with infection. There was a robust innate immunological response to infection, which included an early increase in circulating neutrophils and monocytes and a reduction in the proportion of circulating T-cells. Expression of the ACE2 receptor in respiratory tissues was almost absent, but there was ubiquitous expression of TMPRSS2. The results of this study indicate that exposure of marmosets to high concentrations of aerosolised SARS-CoV-2 did not result in the development of clear, reproducible signs of COVID-19.

The innate immune response in the marmoset during the acute pneumonic disease caused by Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, a severe human infection that is difficult to treat with antibiotics and for which there is no effective vaccine. Development of novel treatments rely upon appropriately characterized animal models. The common marmoset (Callithrix jacchus) has been established at Defense Science and Technology laboratories (DSTL) as a model of melioidosis. Further analysis was performed on samples generated in these studies to provide a description of the innate immune response. Many of the immunological features described, (migration/activation of neutrophils and macrophages, activation of T cells, elevation of key cytokines IFNγ, TNF-α, IL-6, and IL-1β) have been observed in acute melioidosis human cases and correlated with prognosis. Expression of the MHCII marker (HLA-DR) on neutrophils showed potential as a diagnostic with 80% accuracy when comparing pre- and postchallenge levels in paired blood samples. Discriminant analysis of cell surface, activation markers on neutrophils combined with levels of key cytokines, differentiated between disease states from single blood samples with 78% accuracy. These key markers have utility as a prototype postexposure, presymptomatic diagnostic. Ultimately, these data further validate the use of the marmoset as a suitable model for determining efficacy of medical countermeasures against B. pseudomallei.

Laboratory Safety: Handling Burkholderia pseudomallei Isolates without a Biosafety Cabinet

Burkholderia pseudomallei is a tier 1 select agent that is associated with laboratory-acquired melioidosis, with international guidelines recommending isolate handling within a class II biosafety cabinet (BSC) in a biosafety level 3 (BSL3) facility. In low-resource settings, this may not be practical; therefore, we aimed to assess the risk of laboratory-acquired melioidosis during routine work. Prior exposure to the organism was determined with a questionnaire and concomitant serology. Of 30 laboratory scientists handling B. pseudomallei on 1,267 occasions outside a biosafety cabinet, no infections were documented and all participants remained seronegative. Additionally, we performed controlled environmental air sampling during 78 laboratory handling events, including plate opening, oxidase testing, and McFarland suspension creation. None of the experiments demonstrated aerosolization of the organism. This study suggests the risk of laboratory-acquired melioidosis is low. However, individual laboratories will need to undertake a risk assessment, including melioidosis endemicity, availability of resources for containment, the nature of routine handling to be undertaken, and the presence of predisposing risk factors for infection in the staff concerned. Additionally, laboratories should take region-specific guidelines into consideration. Further research is required to better inform on the overall risk of infection in the microbiology laboratory.

Comparison of three non-human primate aerosol models for glanders, caused by Burkholderia mallei

Burkholderia mallei is a gram-negative obligate animal pathogen that causes glanders, a highly contagious and potentially fatal disease of solipeds including horses, mules, and donkeys. Humans are also susceptible, and exposure can result in a wide range of clinical forms, i.e., subclinical infection, chronic forms with remission and exacerbation, or acute and potentially lethal septicemia and/or pneumonia. Due to intrinsic antibiotic resistance and the ability of the organisms to survive intracellularly, current treatment regimens are protracted and complicated; and no vaccine is available. As a consequence of these issues, and since B. mallei is infectious by the aerosol route, B. mallei is regarded as a major potential biothreat agent. To develop optimal medical countermeasures and diagnostic tests, well characterized animal models of human glanders are needed. The goal of this study was to perform a head-to-head comparison of models employing three commonly used nonhuman primate (NHP) species, the African green monkey (AGM), Rhesus macaque, and the Cynomolgus macaque. The natural history of infection and in vitro clinical, histopathological, immunochemical, and bacteriological parameters were examined. The AGMs were the most susceptible NHP to B. mallei; five of six expired within 14 days. Although none of the Rhesus or Cynomolgus macaques succumbed, the Rhesus monkeys exhibited abnormal signs and clinical findings associated with B. mallei infection; and the latter may be useful for modeling chronic B. mallei infection. Based on the disease progression observations, gross and histochemical pathology, and humoral and cellular immune response findings, the AGM appears to be the optimal model of acute, lethal glanders infection. AGM models of infection by B. pseudomallei, the etiologic agent of melioidosis, have been characterized recently. Thus, the selection of the AGM species provides the research community with a single NHP model for investigations on acute, severe, inhalational melioidosis and glanders.

The lymphatic system as a potential mechanism of spread of melioidosis following ingestion of Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis, which is a Gram negative, facultative intracellular bacterium. Disease is prevalent in SE Asia and in northern Australia, as well as in other tropical and subtropical regions. Recently, there is an increasing awareness of the importance of bacterial ingestion as a potential route of infection, particularly in cases of unexplained origin of the disease. The marmoset is a New World Monkey (NWM) species that is being developed as an alternative NHP model to complement the more traditionally used Old World Monkeys (OWM). Models have been developed for the traditional routes of disease acquisition, subcutaneous and inhalational. This manuscript details the development and characterisation of an ingestion model of melioidosis. Dose-ranging study assessed the lethality of B. pseudomallei and disease progression was assessed by euthanizing animals at predetermined time points, 12, 36, 48 and 54 hours post-challenge. Challenge doses of greater than 6.2 x 106 cfu resulted in an acute, lethal, febrile disease. Following challenge the lung was the first organ, outside of the gastrointestinal tract, to become colonised. Enteritis (duodenitis, ileitis and/or jejunitis) was observed in sections of the small intestine from animals that succumbed to disease. However, the most severe pathological features were observed in the mesenteric lymph nodes from these animals. These findings are consistent with lymphatic draining as route of dissemination.

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.

Dose-dependant acute or subacute disease caused by Burkholderia pseudomallei strain NCTC 13392 in a BALB/c aerosol model of infection

AIMS

The goal of this study was to examine, for the first time, the virulence and pathogenicity of aerosolized Burkholderia pseudomallei, strain NCTC 13392, in BALB/c mice in order to develop an animal model for testing novel medical countermeasures (MCMs) for the treatment of human acute and subacute (a disease state between acute and chronic) melioidosis.

METHODS AND RESULTS

BALB/c mice were exposed to varying doses of aerosolized bacteria. Acute disease was seen in animals exposed to a very-high dose (≥10³  CFU per animal) and death occurred 3-4 days postchallenge (pc). Bacteria were detected in the lungs, liver, kidney and spleen. In contrast, animals exposed to a low dose (<10 CFU per animal) survived to the end of the study (day 30 pc) but developed weight loss, a bacterial tissue burden and increasing clinical signs of infection from day 20 pc onwards, mimicking a subacute form of the disease. Pathological changes in the tissues mirrored these findings.

CONCLUSIONS

This proof of concept study has shown that B. pseudomallei strain NCTC 13392 is virulent and pathogenic in BALB/c mice, when delivered by aerosol. By varying the doses of aerosolized bacteria it was possible to mimic characteristics of both human acute and subacute melioidosis, at the same time, within the same study.

SIGNIFICANCE AND IMPACT OF THE STUDY

Burkholderia pseudomallei, the aetiological agent of melioidosis, causes a serious and often fatal disease in humans and animals. Novel MCMs are urgently needed for both public health and biodefense purposes. The present model provides a useful tool for the assessment and evaluation of new MCMs (e.g. therapeutics and vaccines) and offers the potential for testing new treatments for both subacute to chronic and acute melioidosis prior to human clinical trials.

Novel multi-component vaccine approaches for Burkholderia pseudomallei

Burkholderia pseudomallei is the causative agent of melioidosis. Historically believed to be a relatively rare human disease in tropical countries, a recent study estimated that, worldwide, there are approximately 165 000 human melioidosis cases per year, more than half of whom die. The bacterium is inherently resistant to many antibiotics and treatment of the disease is often protracted and ineffective. There is no licensed vaccine against melioidosis, but a vaccine is predicted to be of value if used in high-risk populations. There has been progress over the last decade in the pursuit of an effective vaccine against melioidosis. Animal models of disease including mouse and non-human primates have been developed, and these models show that antibody responses play a key role in protection against melioidosis. Surprisingly, although B. pseudomallei is an intracellular pathogen there is limited evidence that CD8⁺  T cells play a role in protection. It is evident that a multi-component vaccine, incorporating one or more protective antigens, will probably be essential for protection because of the pathogen's sophisticated virulence mechanisms as well as strain heterogeneity. Multi-component vaccines in development include glycoconjugates, multivalent subunit preparations, outer membrane vesicles and other nano/microparticle platforms and live-attenuated or inactivated bacteria. A consistent finding with vaccine candidates tested in mice is the ability to induce sterilizing immunity at low challenge doses and extended time to death at higher challenge doses. Further research to identify ways of eliciting more potent immune responses might provide a path for licensing an effective vaccine.

Characterization of lesion formation in marmosets following inhalational challenge with different strains of Burkholderia pseudomallei

The marmoset model of melioidosis was used to explore whether there was any difference in the disease presentation and/or the lesion formation following inhalational challenge with one of four strains of Burkholderia pseudomallei (K96243, 1026b, HBPUB10303a and HBPUB10134a). Marmosets were challenged with a range of bacterial doses and bacterial load, histological and physiological features were determined temporally following lethal disease. Melioidosis presented as an acute, febrile disease with bacteraemia, bacterial dissemination, necrotizing hepatitis, splenitis and pneumonia which was independent of the challenge strain. Generally, there were no major differences in the manifestation of melioidosis following challenge by the different strains of B. pseudomallei; however, there were some differences in the time to death and the severity of the pathological features. The pathological features observed in the liver and spleen of animals challenged with B. pseudomallei strain 1026b were statistically less severe (P < 0.05) and less frequent. However, more severe foci of disease were evident in the lungs of animals challenged with strain 1026b. In all cases, the lesions developed from small areas of bacteria-infected macrophages surrounded by non-infected neutrophils into large lesions with both immune cell types infected. The marmoset model was a useful tool enabling the distinction of subtle difference in the pathological response to B. pseudomallei.

Use of the common marmoset to study Burkholderia mallei infection

Burkholderia mallei is a host-adapted bacterium that does not persist outside of its equine reservoir. The organism causes the zoonosis glanders, which is endemic in Asia, Africa, the Middle East and South America. Infection by B. mallei typically occurs via the respiratory or percutaneous route, and the most common manifestations are life-threatening pneumonia and bacteremia. Glanders is difficult to diagnose and requires prolonged antibiotic therapy with low success rates. There is no vaccine to protect against B. mallei and there is concern regarding its use as a biothreat agent. Thus, experiments were performed to establish a non-human primate model of intranasal infection to study the organism and develop countermeasures. Groups of marmosets (Callithrix jacchus) were inoculated intranasally with B. mallei strain ATCC 23344 and monitored for clinical signs of illness for up to 13 days. We discovered that 83% of marmosets inoculated with doses of 2.5 X 10(4) to 2.5 X 10(5) bacteria developed acute lethal infection within 3-4 days. Signs of disease were severe and included lethargy, inappetence, conjunctivitis, mucopurulent and hemorrhagic nasal discharges, and increased respiratory effort with abdominal lifts. Burkholderia mallei was cultured from the lungs, spleen and liver of these animals, and pathologic examination of tissues revealed lesions characteristic of glanders. Challenge experiments also revealed that 91% of animals infected with doses ranging from 25 to 2.5 X 10(3) bacteria exhibited mild non-specific signs of illness and were culture negative. One marmoset inoculated with 2.5 X 10(3) organisms developed moderate signs of disease and reached humane end-points 8 days post-infection. The liver and spleen of this animal were colonized with the agent and pathological analysis of tissues showed nasal, splenic and hepatic lesions. Taken together, these data indicate that the marmoset is a suitable model to study respiratory infection by B. mallei.

Comparative experimental subcutaneous glanders and melioidosis in the common marmoset (Callithrix jacchus)

Glanders and melioidosis are caused by two distinct Burkholderia species and have generally been considered to have similar disease progression. While both of these pathogens are HHS/CDC Tier 1 agents, natural infection with both these pathogens is primarily through skin inoculation. The common marmoset (Callithrix jacchus) was used to compare disease following experimental subcutaneous challenge. Acute, lethal disease was observed in marmosets following challenge with between 26 and 1.2 × 10(8) cfu Burkholderia pseudomallei within 22-85 h. The reproducibility and progression of the disease were assessed following a challenge of 1 × 10(2) cfu of B. pseudomallei. Melioidosis was characterised by high levels of bacteraemia, focal microgranuloma progressing to non-necrotic multifocal solid lesions in the livers and spleens and multi-organ failure. Lethal disease was observed in 93% of animals challenged with Burkholderia mallei, occurring between 5 and 10.6 days. Following challenge with 1 × 10(2) cfu of B. mallei, glanders was characterised with lymphatic spread of the bacteria and non-necrotic, multifocal solid lesions progressing to a multifocal lesion with severe necrosis and pneumonia. The experimental results confirmed that the disease pathology and presentation is strikingly different between the two pathogens. The marmoset provides a model of the human syndrome for both diseases facilitating the development of medical countermeasures.

Characterization of major histocompatibility complex class I allele polymorphisms in common marmosets

Currently, little information is available for major histocompatibility complex (MHC)-I that conditions the T-cell response of marmosets. In this study, 471 clones of MHC-I cDNA sequences were isolated from 12 marmosets. Twenty full-length sequences of class I G (Caja-G) alleles were obtained from these marmosets, 15 of them were novel. Among these 20 Caja-G alleles, 10 were found in individual animals while the rests were in two to four marmosets, but none was common to all animals. Ten marmosets possessed one to three Caja-G alleles, and two marmosets carried five or six alleles, which suggested that the Caja-G locus was duplicated in marmoset's genome. The high polymorphisms of Caja-G sequences provided important information helpful for understanding the cellular immune response in virus-infected marmosets.

Exploring the innate immunological response of an alternative nonhuman primate model of infectious disease; the common marmoset

The common marmoset (Callithrix jacchus) is increasingly being utilised as a nonhuman primate model for human disease, ranging from autoimmune to infectious disease. In order to fully exploit these models, meaningful comparison to the human host response is necessary. Commercially available reagents, primarily targeted to human cells, were utilised to assess the phenotype and activation status of key immune cell types and cytokines in naive and infected animals. Single cell suspensions of blood, spleen, and lung were examined. Generally, the phenotype of cells was comparable between humans and marmosets, with approximately 63% of all lymphocytes in the blood of marmosets being T cells, 25% B-cells, and 12% NK cells. The percentage of neutrophils in marmoset blood were more similar to human values than mouse values. Comparison of the activation status of cells following experimental systemic or inhalational infection exhibited different trends in different tissues, most obvious in cell types active in the innate immune response. This work significantly enhances the ability to understand the immune response in these animals and fortifies their use as models of infectious disease.

Protection against experimental melioidosis following immunisation with a lipopolysaccharide-protein conjugate

Melioidosis is a severe infectious disease caused by Burkholderia pseudomallei. It is refractory to antibiotic treatment and there is currently no licensed vaccine. In this report we detail the construction and protective efficacy of a polysaccharide-protein conjugate composed of B. pseudomallei lipopolysaccharide and the Hc fragment of tetanus toxin. Immunisation of mice with the lipopolysaccharide-conjugate led to significantly reduced bacterial burdens in the spleen 48 hours after challenge and afforded significant protection against a lethal challenge with B. pseudomallei. The conjugate generated significantly higher levels of antigen-specific IgG1 and IgG2a than in lipopolysaccharide-immunised mice. Immunisation with the conjugate also demonstrated a bias towards Th1 type responses, evidenced by high levels of IgG2a. In contrast, immunisation with unconjugated lipopolysaccharide evoked almost no IgG2a demonstrating a bias towards Th2 type responses. This study demonstrates the effectiveness of this approach in the development of an efficacious and protective vaccine against melioidosis.

Pathogenesis of percutaneous infection of goats with Burkholderia pseudomallei: clinical, pathologic, and immunological responses in chronic melioidosis

Melioidosis is a severe suppurative to granulomatous infection caused by Burkholderia pseudomallei. The disease is endemic to South-East Asia and Northern Australasia and is also of interest as a potential biological weapon. Natural infection can occur by percutaneous inoculation, inhalation or ingestion, but the relative importance of each route is unknown. Experimental infection models using mice have shown inhalation to be the most lethal route of exposure, but few studies have examined the pathogenesis of percutaneous infection despite its presumptive importance in natural disease. Caprine models are useful in the study of melioidosis because goats are susceptible to natural infection by B. pseudomallei, display similar epizootiology/epidemiology to that of humans within the endemic range and develop similar pathologic lesions. Percutaneous inoculation with 10(4)  CFU of B. pseudomallei produced disease in all experimental animals with rapid dissemination to the lungs, spleen and kidneys. Initial fever was brief, but temperatures did not return to pre-infection levels until day 18, concurrent with a dramatic lymphocytosis and the transition to chronic disease. Distribution and appearance of gross pathologic and radiographic lesions in goats were similar to caprine aerosol infection and to reported human disease. The similarities seen despite different routes of infection suggest that host or bacterial factors may be more important than the route of infection in disease pathogenesis. The nature of melioidosis in goats makes it amenable for modelling additional risk factors to produce acute clinical disease, which is important to the study of human melioidosis.

Pathological findings and diagnostic implications of a rhesus macaque (Macaca mulatta) model of aerosol-exposure melioidosis (Burkholderia pseudomallei)

Aerosolized Burkholderia pseudomallei, the causative agent of melioidosis, can infect many species of mammals (including humans), causing rapid, severe pneumonia with high mortality. Diagnosis in humans is challenging, as few organisms can be detected in blood or other non-invasive samples. Although it cannot be said that the model is established, studies to date indicate that rhesus macaques may represent a good model of human melioidosis. This is supported by the results of this study. The early progression of meliodosis in the rhesus macaque was studied in an effort to better understand the disease and the application of rapid diagnostic methods. Results indicate that a PCR analysis of key diagnostic samples such as nasal swabs, throat swabs, tracheo bronchial lymph node aspirates and broncho-alveolar lavage may be a useful component of a rapid diagnostic algorithm in case of aerosol exposure.

In vivo manipulation of γ9(+) T cells in the common marmoset (Callithrix Jacchus) with phosphoantigen and effect on the progression of respiratory melioidosis

Burkholderia pseudomallei is a dangerous human pathogen. Phosphoantigens specifically the target primate specific γ9(+)δ2(+) T cells subset and some have been developed as potential immunotherapeutics. Previously, we demonstrated that, when stimulated with the phosphoantigen CHDMAPP, γ9(+)δ2(+) T cells aid in the killing of intracellular B. pseudomallei bacteria. Moreover, we found that common marmoset (Callithrix Jacchus) γ9(+) T cells increase in frequency and respond to the phosphoantigen CHDMAPP and/or B. pseudomallei, in combination with IL-2, in a similar manner to human γ9(+)δ2(+) T cells. Here we evaluate the efficacy of the phosphoantigen CHDMAPP, in combination with IL-2, as a therapy against B. pseudomallei infection, in vivo. We found that the previous studies predicted the in vivo responsiveness of γ9(+) T cells to the CHDMAPP+IL-2 treatment and significant expansion of the numbers of peripheral and splenic γ9(+) T cells were observed. This effect was similar to those reported in other primate species treated with phosphoantigen. Furthermore, splenocytes were retrieved 7 days post onset of treatment, restimulated with CHDMAPP or heat-killed B. pseudomallei and the cultured γ9(+) T cells demonstrated no reduction in IFN-γ response when CHDMAPP+IL-2 animals were compared to IL-2 only treated animals. Using an established model of B. pseudomallei infection in the marmoset, we assessed the potential for using phosphoantigen as a novel immunotherapy. The CHDMAPP treatment regime had no effect on the progression of respiratory melioidosis and this was despite the presence of elevated numbers of γ9(+) T cells in the spleen, liver and lung and an increased proportion of IFN-γ(+) cells in response to infection. We therefore report that the common marmoset has proven a good model for studying the effect in vivo of γ9(+) T cell stimulation; however, γ9(+) T cells have little or no effect on the progression of lethal, respiratory B. pseudomallei infection.

Genome Sequence of Burkholderia pseudomallei NCTC 13392

Here, we describe the draft genome sequence of Burkholderia pseudomallei NCTC 13392. This isolate has been distributed as K96243, but distinct genomic differences have been identified. The genomic sequence of this isolate will provide the genomic context for previously conducted functional studies.

Assessment of antimicrobial peptide LL-37 as a post-exposure therapy to protect against respiratory tularemia in mice

Early activation of the innate immune response is important for protection against infection with Francisella tularensis live vaccine strain (LVS) in mice. The human cathelicidin antimicrobial peptide LL-37 is known to have immunomodulatory properties, and therefore exogenously administered LL-37 may be suitable as an early post-exposure therapy to protect against LVS infection. LL-37 has been evaluated for immunostimulatory activity in uninfected mice and for activity against LVS in macrophage assays and protective efficacy when administered post-challenge in a mouse model of respiratory tularemia. Increased levels of pro-inflammatory cytokine IL-6, chemokines monocyte chemoattractant protein 1 (MCP-1) and CXCL1 with increased neutrophil influx into the lungs were observed in uninfected mice after intranasal administration of LL-37. Following LVS challenge, LL-37 administration resulted in increased IL-6, IL-12 p70, IFNγ and MCP-1 production, a slowing of LVS growth in the lung, and a significant extension of mean time to death compared to control mice. However, protection was transient, with the LL-37 treated mice eventually succumbing to infection. As this short course of nasally delivered LL-37 was moderately effective at overcoming the immunosuppressive effects of LVS infection this suggests that a more sustained treatment regimen may be an effective therapy against this pathogen.

Clinical definitions of melioidosis

Clinical definitions of melioidosis and inhalation-acquired melioidosis (Burkholderia pseudomallei infection) are described together with the evidence used to develop these definitions. Such definitions support accurate public health reporting, preparedness planning for deliberate B. pseudomallei release, design of experimental models, and categorization of naturally acquired melioidosis.

Animal models for Francisella tularensis and Burkholderia species: scientific and regulatory gaps toward approval of antibiotics under the FDA Animal Rule

The development and regulatory approval of medical countermeasures (MCMs) for the treatment and prevention of bacterial threat agent infections will require the evaluation of products in animal models. To obtain regulatory approval, these models must accurately recapitulate aspects of human disease, including, but not necessarily limited to, route of exposure, time to disease onset, pathology, immune response, and mortality. This article focuses on the state of animal model development for 3 agents for which models are largely immature: Francisella tularensis, Burkholderia mallei, and Burkholderia pseudomallei. An overview of available models and a description of scientific and regulatory gaps are provided.

Experimental respiratory Marburg virus haemorrhagic fever infection in the common marmoset (Callithrix jacchus)

Marburg virus causes a highly infectious and lethal haemorrhagic fever in primates and may be exploited as a potential biothreat pathogen. To combat the infection and threat of Marburg haemorrhagic fever, there is a need to develop and license appropriate medical countermeasures. To determine whether the common marmoset (Callithrix jacchus) would be an appropriate model to assess therapies against Marburg haemorrhagic fever, initial susceptibility, lethality and pathogenesis studies were performed. Low doses of virus, between 4 and 28 TCID50 , were sufficient to cause a lethal, reproducible infection. Animals became febrile between days 5 and 6, maintaining a high fever before succumbing to disease between 8 and 11 days postchallenge. Typical signs of Marburg virus infection were observed including haemorrhaging and a transient rash. In pathogenesis studies, virus was isolated from the animals' lungs from day 3 postchallenge and from the liver, spleen and blood from day 5 postchallenge. Early signs of histopathology were apparent in the kidney and liver from day 3. The most striking features were observed in animals exhibiting severe clinical signs, which included high viral titres in all organs, with the highest levels in the blood, increased levels in liver function enzymes and blood clotting times, decreased levels in platelets, multifocal moderate-to-severe hepatitis and perivascular oedema.

An assessment of common marmoset (Callithrix jacchus) γ9(+) T cells and their response to phosphoantigen in vitro

γ9δ2 T cells are a primate-specific γδ T cell subtype that expand and become activated during infection, responding directly to phosphoantigens which are by-products of essential metabolic pathways in both bacteria and mammals. Analogues of natural phosphoantigens have been developed as potential immunotherapeutics for treatment of tumours and infectious diseases. Several non-human primate models have been used in preclinical studies, however, little is known about marmoset γ9δ2 T cell responses. We identified γ9(+) T cells in various tissues in the marmoset and determined that these cells respond to phosphoantigen in a similar manner to human γ9δ2 T cells in vitro. Both human γ9δ2 T cells and marmoset γ9(+) T cells were able to reduce growth of the intracellular bacterium Burkholderia pseudomallei in vitro following expansion with phosphoantigen. This suggests that the marmoset is an appropriate model for examining the immunotherapeutic potential of compounds which target γ9δ2 T cells.

An objective approach for Burkholderia pseudomallei strain selection as challenge material for medical countermeasures efficacy testing

Burkholderia pseudomallei is the causative agent of melioidosis, a rare disease of biodefense concern with high mortality and extreme difficulty in treatment. No human vaccines are available that protect against B. pseudomallei infection, and with the current limitations of antibiotic treatment, the development of new preventative and therapeutic interventions is crucial. Although clinical trials could be used to test the efficacy of new medical countermeasures (MCMs), the high mortality rates associated with melioidosis raises significant ethical issues concerning treating individuals with new compounds with unknown efficacies. The US Food and Drug Administration (FDA) has formulated a set of guidelines for the licensure of new MCMs to treat diseases in which it would be unethical to test the efficacy of these drugs in humans. The FDA “Animal Rule” 21 CFR 314 calls for consistent, well-characterized B. pseudomallei strains to be used as challenge material in animal models. In order to facilitate the efficacy testing of new MCMs for melioidosis using animal models, we intend to develop a well-characterized panel of strains for use. This panel will comprise of strains that were isolated from human cases, have a low passage history, are virulent in animal models, and are well-characterized phenotypically and genotypically. We have reviewed published and unpublished data on various B. pseudomallei strains to establish an objective method for selecting the strains to be included in the panel of B. pseudomallei strains with attention to five categories: animal infection models, genetic characterization, clinical and passage history, and availability of the strain to the research community. We identified 109 strains with data in at least one of the five categories, scored each strain based on the gathered data and identified six strains as candidate for a B. pseudomallei strain panel.

Particle-size dependent effects in the Balb/c murine model of inhalational melioidosis

Deposition of Burkholderia pseudomallei within either the lungs or nasal passages of the Balb/c murine model resulted in different infection kinetics. The infection resulting from the inhalation of B. pseudomallei within a 12 μm particle aerosol was prolonged compared to a 1 μm particle aerosol with a mean time-to-death (MTD) of 174.7 ± 14.9 h and 73.8 ± 11.3 h, respectively. Inhalation of B. pseudomallei within 1 μm or 12 μm particle aerosols resulted in a median lethal dose (MLD) of 4 and 12 cfu, respectively. The 12 μm particle inhalational infection was characterized by a marked involvement of the nasal mucosa and extension of bacterial colonization and inflammatory lesions from the olfactory epithelium through the olfactory nerves (or tracts) to the olfactory bulb (100%), culminating in abscessation of the brain (33%). Initial involvement of the upper respiratory tract lymphoid tissues (nasal-associated lymphoid tissue (NALT) and cervical lymph nodes) was observed in both the 1 and 12 μm particle inhalational infections (80-85%). Necrotising alveolitis and bronchiolitis were evident in both inhalational infections, however, lung pathology was greater after inhalation of the 1 μm particle aerosol with pronounced involvement of the mediastinal lymph node (50%). Terminal disease was characterized by bacteraemia in both inhalational infections with dissemination to the spleen, liver, kidneys, and thymus. Treatment with co-trimoxazole was more effective than treatment with doxycycline irrespective of the size of the particles inhaled. Doxycycline was more effective against the 12 μm particle inhalational infection as evidenced by increased time to death. However, both treatment regimes exhibited significant relapse when therapy was discontinued with massive enlargement and abscessation of the lungs, spleen, and cervical lymph nodes observed.

Natural history of inhalation melioidosis in rhesus macaques (Macaca mulatta) and African green monkeys (Chlorocebus aethiops)

Burkholderia pseudomallei, the causative agent of melioidosis, is recognized as a serious health threat due to its involvement in septic and pulmonary infections in areas of endemicity and is recognized by the Centers for Disease Control and Prevention as a category B biothreat agent. An animal model is desirable to evaluate the pathogenesis of melioidosis and medical countermeasures. A model system that represents human melioidosis infections is essential in this process. A group of 10 rhesus macaques (RMs) and 10 African green monkeys (AGMs) was exposed to aerosolized B. pseudomallei 1026b. The first clinical signs were fever developing 24 to 40 h postexposure followed by leukocytosis resulting from a high percentage of neutrophils. Dyspnea manifested 2 to 4 days postexposure. In the AGMs, an increase in interleukin 1β (IL-1β), IL-6, IL-8, gamma interferon (IFN-γ), and tumor necrosis factor alpha (TNF-α) was observed. In the RMs, IL-1β, IL-6, and TNF-α increased. All the RMs and AGMs had various degrees of bronchopneumonia, with inflammation consisting of numerous neutrophils and a moderate number of macrophages. Both the RMs and the AGMs appear to develop a melioidosis infection that closely resembles that seen in acute human melioidosis. However, for an evaluation of medical countermeasures, AGMs appear to be a more appropriate model.