Establishment of lethal inhalational infection with Francisella tularensis (tularaemia) 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).

The rLVS ΔcapB/iglABC vaccine provides potent protection in Fischer rats against inhalational tularemia caused by various virulent Francisella tularensis strains

Francisella tularensis is one of the several biothreat agents for which a licensed vaccine is needed. To ensure vaccine protection is achieved across a range of virulent F. tularensis strains, we assembled and characterized a panel of F. tularensis isolates to be utilized as challenge strains. A promising tularemia vaccine candidate is rLVS ΔcapB/iglABC (rLVS), in which the vector is the LVS strain with a deletion in the capB gene and which additionally expresses a fusion protein comprising immunodominant epitopes of proteins IglA, IglB, and IglC. Fischer rats were immunized subcutaneously 1-3 times at 3-week intervals with rLVS at various doses. The rats were exposed to a high dose of aerosolized Type A strain Schu S4 (FRAN244), a Type B strain (FRAN255), or a tick derived Type A strain (FRAN254) and monitored for survival. All rLVS vaccination regimens including a single dose of 107 CFU rLVS provided 100% protection against both Type A strains. Against the Type B strain, two doses of 107 CFU rLVS provided 100% protection, and a single dose of 107 CFU provided 87.5% protection. In contrast, all unvaccinated rats succumbed to aerosol challenge with all of the F. tularensis strains. A robust Th1-biased antibody response was induced in all vaccinated rats against all F. tularensis strains. These results demonstrate that rLVS ΔcapB/iglABC provides potent protection against inhalational challenge with either Type A or Type B F. tularensis strains and should be considered for further analysis as a future tularemia vaccine.

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.

Comparison of experimental MERS-CoV infection acquired by three individual routes of infection in the common marmoset

Two strains of Middle East respiratory syndrome coronavirus (MERS-CoV), England 1 and Erasmus Medical Centre/2012 (EMC/2012), were used to challenge common marmosets (Callithrix jacchus) by three routes of infection: aerosol, oral, and intranasal. Animals challenged by the intranasal and aerosol routes presented with mild, transient disease, while those challenged by the oral route presented with a subclinical immunological response. Animals challenged with MERS-CoV strain EMC/2012 by the aerosol route responded with primary and/or secondary pyrexia. Marmosets had minimal to mild multifocal interstitial pneumonia, with the greatest relative severity being observed in animals challenged by the aerosol route. Viable virus was isolated from the host in throat swabs and lung tissue. The transient disease described is consistent with a successful host response and was characterized by the upregulation of macrophage and neutrophil function observed in all animals at the time of euthanasia.

IMPORTANCE Middle East respiratory syndrome is caused by a human coronavirus, MERS-CoV, similar to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Humans typically exhibit fever, cough, shortness of breath, gastrointestinal issues, and breathing difficulties, which can lead to pneumonia and/or renal complications. This emerging disease resulted in the first human lethal cases in 2012 and has a case fatality rate of approximately 36%. Consequently, there is a need for medical countermeasures and appropriate animal models for their assessment. This work has demonstrated the requirement for higher concentrations of virus to cause overt disease. Challenge by the aerosol, intranasal, and oral routes resulted in no or mild disease, but all animals had an immunological response. This shows that an appropriate early immunological response is able to control the disease.

Differential Immune Response Following Intranasal and Intradermal Infection with Francisella tularensis: Implications for Vaccine Development

Francisella tularensis (Ft) is a Gram-negative, facultative intracellular coccobacillus that is the etiological agent of tularemia. Interestingly, the disease tularemia has variable clinical presentations that are dependent upon the route of infection with Ft. Two of the most likely routes of Ft infection include intranasal and intradermal, which result in pneumonic and ulceroglandular tularemia, respectively. While there are several differences between these two forms of tularemia, the most notable disparity is between mortality rates: the mortality rate following pneumonic tularemia is over ten times that of the ulceroglandular disease. Understanding the differences between intradermal and intranasal Ft infections is important not only for clinical diagnoses and treatment but also for the development of a safe and effective vaccine. However, the immune correlates of protection against Ft, especially within the context of infection by disparate routes, are not yet fully understood. Recent advances in different animal models have revealed new insights in the complex interplay of innate and adaptive immune responses, indicating dissimilar patterns in both responses following infection with Ft via different routes. Further investigation of these differences will be crucial to predicting disease outcomes and inducing protective immunity via vaccination or natural infection.

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.

A Novel Marmoset (Callithrix jacchus) Model of Human Inhalational Q Fever

Common marmosets (Callithrix jacchus) were shown to be susceptible to inhalational infection with Coxiella burnetii, in a dose-dependent manner, producing a disease similar to human Q fever, characterized by a resolving febrile response. Illness was also associated with weight loss, liver enzyme dysfunction, characteristic cellular activation, circulating INF-γ and bacteraemia. Viable C. burnetii was recovered from various tissues during disease and from 75% of the animal's lungs on 28 days post challenge, when there were no overt clinical features of disease but there was histological evidence of macrophage and lymphocyte infiltration into the lung resulting in granulomatous alveolitis. Taken together, these features of disease progression, physiology and bacterial spread appear to be consistent with human disease and therefore the common marmoset can be considered as a suitable model for studies on the pathogenesis or the development of medical counter measures of inhalational Q fever.

Adaptive Immunity to Francisella tularensis and Considerations for Vaccine Development

Francisella tularensis is an intracellular bacterium that causes the disease tularemia. There are several subspecies of F. tularensis whose ability to cause disease varies in humans. The most virulent subspecies, tularensis, is a Tier One Select Agent and a potential bioweapon. Although considerable effort has made to generate efficacious tularemia vaccines, to date none have been licensed for use in the United States. Despite the lack of a tularemia vaccine, we have learned a great deal about the adaptive immune response the underlies protective immunity. Herein, we detail the animal models commonly used to study tularemia and their recapitulation of human disease, the field's current understanding of vaccine-mediated protection, and discuss the challenges associated with new vaccine development.

The Natural History of Pneumonic Tularemia in Female Fischer 344 Rats after Inhalational Exposure to Aerosolized Francisella tularensis Subspecies tularensis Strain SCHU S4

The inbred Fischer 344 rat is being evaluated for testing novel vaccines and therapeutics against pneumonic tularemia. Although primary pneumonic tularemia in humans typically occurs by inhalation of aerosolized bacteria, the rat model has relied on intratracheal inoculation of organisms because of safety and equipment issues. We now report the natural history of pneumonic tularemia in female Fischer 344 rats after nose-only inhalational exposure to lethal doses of aerosolized Francisella tularensis subspecies tularensis, strain SCHU S4. Our results are consistent with initial uptake of aerosolized SCHU S4 from the nasal cavity, lungs, and possibly the gastrointestinal tract. Bacteremia with hematogenous dissemination was first detected 2 days after exposure. Shortly thereafter, the infected rats exhibited fever, tachypnea, and hypertension that persisted for 24 to 36 hours and then rapidly decreased as animals succumbed to infection between days 5 and 8 after exposure. Tachycardia was observed briefly, but only after the core body temperature and blood pressure began to decrease as the animals were near death. Initial neutrophilic and histiocytic inflammation in affected tissues became progressively more fibrinous and necrotizing over time. At death, as many as 10¹⁰ colony-forming units were found in the lungs, spleen, and liver. Death was attributed to sepsis and disseminated intravascular coagulation. Overall, the pathogenesis of pneumonic tularemia in the female F344 rat model appears to replicate the disease in humans.

Tularemia vaccine development: paralysis or progress?

Francisella tularensis (Ft) is a gram-negative intercellular pathogen and category A biothreat agent. However, despite 15 years of strong government investment and intense research focused on the development of a US Food and Drug Administration-approved vaccine against Ft, the primary goal remains elusive. This article reviews research efforts focused on developing an Ft vaccine, as well as a number of important factors, some only recently recognized as such, which can significantly impact the development and evaluation of Ft vaccine efficacy. Finally, an assessment is provided as to whether a US Food and Drug Administration-approved Ft vaccine is likely to be forthcoming and the potential means by which this might be achieved.

Progress, challenges, and opportunities in Francisella vaccine development

Renewed interest in Francisella tularensis has resulted in substantial new information about its pathogenesis and immunology, along with development of useful animal models. While understanding of protective immunity against Francisella remains incomplete, data in both animals and humans suggest that inducing T cell-mediated immunity is crucial for successful vaccination with current candidates such as the Live Vaccine Strain (LVS), with specific antibodies and immune B cells playing supporting roles. Consistent with this idea, recent results indicate that measurements of T cell functions and relative gene expression by immune T cells predict vaccine-induced protection in animal models. Because field trials of new vaccines will be difficult to design, using such measurements to derive potential correlates of protection may be important to bridge between animal efficacy studies and people.

Francisella Tularensis Clades B.FTN002-00 and B.13 Are Associated With Distinct Pathology in the European Brown Hare (Lepus europaeus)

Tularemia is a severe disease caused by Francisella tularensis This bacterium has a major pathogenic potential in countless animal species as well as in humans. Despite the relatively significant body of literature available on this microorganism, many questions are still open concerning its biological cycle in the environment, the pathology and pathogenesis of the disease, the possible routes of infection in animals, and the pathologic and ecological relevance of the distinct phylogenetic clusters of F. tularensis In order to address these questions, we have thoroughly characterized the pathology and microbiology of terminally ill European brown hares (Lepus europaeus) infected with F. tularensis subsp. holarctica, collected in Switzerland from 2012 to 2014. F tularensis isolates were typed by defining their phylogenetic clusters. We showed that the pathology associated with F. tularensis subsp. holarctica belonging to the clade B.FTNF002-00 is different from that previously reported to be associated with the clade B.13. In particular, strains of the clade B.FTNF002-00 were almost invariably associated with splenitis and hepatitis and not with the polyserositis affecting pleura, pericardium, and kidney reported in the literature for infections caused by the clade B.13. We describe findings suggesting that the ports of entry for the bacteria might be the respiratory and digestive routes.

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.

Experimental Respiratory Infection of Marmosets (Callithrix jacchus) With Ebola Virus Kikwit

Ebola virus (EBOV) causes a highly infectious and lethal hemorrhagic fever in primates with high fatality rates during outbreaks and EBOV may be exploited as a potential biothreat pathogen. There is therefore a need to develop and license appropriate medical countermeasures against this virus. To determine whether the common marmoset (Callithrix jacchus) would be an appropriate model to assess vaccines or therapies against EBOV disease (EVD), initial susceptibility, lethality and pathogenesis studies were performed. Low doses of EBOV-Kikwit, between 4 and 27 times the 50% tissue culture infectious dose, were sufficient to cause a lethal, reproducible infection. Animals became febrile between days 5 and 6, maintaining a high fever before succumbing to EVD between 6 and 8 days after challenge. Typical signs of EVD were observed. Pathogenesis studies revealed that virus was isolated from the lungs of animals beginning on day 3 after challenge and from the liver, spleen and blood beginning on day 5. The most striking features were observed in animals that succumbed to infection, including high viral titers in all organs, increased levels of liver function enzymes and blood clotting times, decreased levels of platelets, multifocal moderate to severe hepatitis, and perivascular edema.

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.

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.

Evaluation of the use of primate undershirts as a refinement practice for jacketed rhesus macaques (Macaca mulatta)

Jacketing of nonhuman primates (NHP) is a commonly used practice in the laboratory animal setting to support data collection with reduced direct human-to-animal interaction. NHP often wear jackets for several weeks, potentially leading to the formation of dermal lesions ranging from mild alopecia to severe full-thickness ulceration. We sought to evaluate the addition of a commercially available undershirt for primates as a possible refinement practice for our jacketed rhesus macaques. In this study, we compared the lesion count, location, and severity and differences in rectal body temperature between jacketed NHP with undershirts with those wearing the jackets alone. In both groups, most lesions (75%) were located at either the underarm or shoulder. The percentages of total lesions in the back and neck were lower in jacketed NHP that wore undershirts than in those that did not. In addition, the estimated odds of increased severity scores in jacketed NHP without undershirts was 1.80 times that for NHP that wore both jackets and undershirts. Both groups of NHP showed a significant decrease in dermal scores with time, indicating adaptation to the jackets with or without undershirts. However, there was no statistically significant decrease in lesion count, severity, or location in jacketed NHP that wore undershirts compared with those that did not.

Differences in aerosolization of Rift Valley fever virus resulting from choice of inhalation exposure chamber: implications for animal challenge studies

The aerosol characteristics of Rift Valley fever virus (RVFV) were evaluated to achieve reproducible infection of experimental animals with aerosolized RVFV suitable for animal efficacy studies. Spray factor (SF), the ratio between the concentrations of the aerosolized agent to the agent in the aerosol generator, is used to compare performance differences between aerosol exposures. SF indicates the efficiency of the aerosolization process; a higher SF means a lower nebulizer concentration is needed to achieve a desired inhaled dose. Relative humidity levels as well as the duration of the exposure and choice of exposure chamber all impacted RVFV SF. Differences were also noted between actual and predicted minute volumes for different species of nonhuman primates. While NHP from Old World species (Macaca fascicularis, M. mulatta, Chlorocebus aethiops) generally had a lower actual minute volume than predicted, the actual minute volume for marmosets (Callithrix jacchus) was higher than predicted (150% for marmosets compared with an average of 35% for all other species examined). All of these factors (relative humidity, chamber, duration, and minute volume) impact the ability to reliably and reproducibly deliver a specific dose of aerosolized RVFV. The implications of these findings for future pivotal efficacy studies are discussed.

A ΔclpB mutant of Francisella tularensis subspecies holarctica strain, FSC200, is a more effective live vaccine than F. tularensis LVS in a mouse respiratory challenge model of tularemia

Francisella tularensis subsp. tularensis is a highly virulent pathogen for humans especially if inhaled. Consequently, it is considered to be a potential biothreat agent. An experimental vaccine, F. tularensis live vaccine strain, derived from the less virulent subsp. holarctica, was developed more than 50 years ago, but remains unlicensed. Previously, we developed a novel live vaccine strain, by deleting the chaperonin clpB gene from F. tularensis subsp. tularensis strain, SCHU S4. SCHU S4ΔclpB was less virulent for mice than LVS and a more effective vaccine against respiratory challenge with wild type SCHU S4. In the current study, we were interested to determine whether a similar mutant on the less virulent subsp. holarctica background would also outperform LVS in terms of safety and efficacy. To this end, clpB was deleted from clinical holarctica strain, FSC200. FSC200ΔclpB had a significantly higher intranasal LD₅₀ than LVS for BALB/c mice, but replicated to higher numbers at foci of infection after dermal inoculation. Moreover, FSC200ΔclpB killed SCID mice more rapidly than LVS. However, dermal vaccination of BALB/c mice with the former versus the latter induced greater protection against respiratory challenge with SCHU S4. This increased efficacy was associated with enhanced production of pulmonary IL-17 after SCHU S4 challenge.

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.

Live attenuated tularemia vaccines: recent developments and future goals

In the aftermath of the 2001 anthrax attacks in the U.S., numerous efforts were made to increase the level of preparedness against a biological attack both in the US and worldwide. As a result, there has been an increase in research interest in the development of vaccines and other countermeasures against a number of agents with the potential to be used as biological weapons. One such agent, Francisella tularensis, has been the subject of a surge in the level of research being performed, leading to a substantial increase in knowledge of the pathogenic mechanisms of the organism and the induced immune responses. This information has facilitated the development of multiple new Francisella vaccine candidates. Herein we review the latest live attenuated F. tularensis vaccine efforts. Historically, live attenuated vaccines have demonstrated the greatest degree of success in protection against tularemia and the greatest promise in recent efforts to develop of a fully protective vaccine. This review summarizes recent live attenuated Francisella vaccine candidates and the lessons learned from those studies, with the goal of collating known characteristics associated with successful attenuation, immunogenicity, and protection.

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.

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

Studies of inhalational melioidosis were undertaken in the common marmoset (Callithrix jacchus). Following exposure to an inhaled challenge with aerosolized Burkholderia pseudomallei, lethal infection was observed in marmosets challenged with doses below 10 cfu; a precise LD(50) determination was not possible. The model was further characterized using a target challenge dose of approximately 10(2) cfu. A separate pathogenesis time-course experiment was also conducted. All animals succumbed, between 27 and 78 h postchallenge. The challenge dose received and the time to the humane endpoint (1 °C below normal body temperature postfever) were correlated. The first indicator of disease was an increased core body temperature (T(c) ), at 22 h postchallenge. This coincided with bacteraemia and bacterial dissemination. Overt clinical signs were first observed 3-5 h later. A sharp decrease (typically within 3-6 h) in the T(c) was observed prior to humanely culling the animals in the lethality study. Pathology was noted in the lung, liver and spleen. Disease progression in the common marmoset appears to be consistent with human infection in terms of bacterial spread, pathology and physiology. The common marmoset can therefore be considered a suitable animal model for further studies of inhalational melioidosis.

Tularemia vaccines: recent developments and remaining hurdles

Francisella tularensis subsp. tularensis is a facultative intracellular bacterial pathogen of humans and other mammals. Its inhaled infectious dose is very low and can result in very high mortality. Historically, subsp. tularensis was developed as a biological weapon and there are now concerns about its abuse as such by terrorists. A live attenuated vaccine developed pragmatically more than half a century ago from the less virulent holarctica subsp. is the sole prophylactic available, but it remains unlicensed. In recent years several other potential live, killed and subunit vaccine candidates have been developed and tested in mice for their efficacy against respiratory challenge with subsp. tularensis. This article will review these vaccine candidates and the development hurdles they face.

Comparative genome analysis of the major histocompatibility complex (MHC) class I B/C segments in primates elucidated by genomic sequencing in common marmoset (Callithrix jacchus)

Common marmoset monkeys (Callithrix jacchus) have emerged as important animal models for biomedical research, necessitating a more extensive characterization of their major histocompatibility complex (MHC) region. However, the genomic information of the marmoset MHC (Caja) is still lacking. The MHC-B/C segment represents the most diverse MHC region among primates. Therefore, in this paper, to elucidate the detailed gene organization and evolutionary processes of the Caja class I B (Caja-B) segment, we determined two parts of the Caja-B sequences with 1,079 kb in total, ranging from H6orf15 to BAT1 and compared the structure and phylogeny with that of other primates. This segment contains 54 genes in total, nine Caja-B genes (Caja-B1 to Caja-B9), two MIC genes (MIC1 and MIC2), eight non-MHC genes, two non-coding genes, and 33 non-MHC pseudogenes that have not been observed in other primate MHC-B/C segments. Caja-B3, Caja-B4, and Caja-B7 encode proper MHC class I proteins according to amino acid structural characteristics. Phylogenetic relationships based on 48 MHC-I nucleotide sequences in primates suggested (1) species-specific divergence for Caja, Mamu, and HLA/Patr/Gogo lineages, (2) independent generation of the "seven coding exon" type MHC-B genes in Mamu and HLA/Patr/Gogo lineages from an ancestral "eight coding exon" type MHC-I gene, (3) parallel correlation with the long and short segmental duplication unit length in Caja and Mamu lineages. These findings indicate that the MHC-B/C segment has been under permanent selective pressure in the evolution of primates.

Immunity to Francisella

In recent years, studies on the intracellular pathogen Francisella tularensis have greatly intensified, generating a wealth of new information on the interaction of this organism with the immune system. Here we review the basic elements of the innate and adaptive immune responses that contribute to protective immunity against Francisella species, with special emphasis on new data that has emerged in the last 5 years. Most studies have utilized the mouse model of infection, although there has been an expansion of work on human cells and other new animal models. In mice, basic immune parameters that operate in defense against other intracellular pathogen infections, such as interferon gamma, TNF-α, and reactive nitrogen intermediates, are central for control of Francisella infection. However, new important immune mediators have been revealed, including IL-17A, Toll-like receptor 2, and the inflammasome. Further, a variety of cell types in addition to macrophages are now recognized to support Francisella growth, including epithelial cells and dendritic cells. CD4⁺ and CD8⁺ T cells are clearly important for control of primary infection and vaccine-induced protection, but new T cell subpopulations and the mechanisms employed by T cells are only beginning to be defined. A significant role for B cells and specific antibodies has been established, although their contribution varies greatly between bacterial strains of lower and higher virulence. Overall, recent data profile a pathogen that is adept at subverting host immune responses, but susceptible to many elements of the immune system's antimicrobial arsenal.

Antibodies contribute to effective vaccination against respiratory infection by type A Francisella tularensis strains

Pneumonic tularemia is a life-threatening disease caused by inhalation of the highly infectious intracellular bacterium Francisella tularensis. The most serious form of the disease associated with the type A strains can be prevented in experimental animals through vaccination with the attenuated live vaccine strain (LVS). The protection is largely cell mediated, but the contribution of antibodies remains controversial. We addressed this issue in a series of passive immunization studies in Fischer 344 (F344) rats. Subcutaneous LVS vaccination induced a robust serum antibody response dominated by IgM, IgG2a, and IgG2b antibodies. Prophylactic administration of LVS immune serum or purified immune IgG reduced the severity and duration of disease in naïve rats challenged intratracheally with a lethal dose of the virulent type A strain SCHU S4. The level of resistance increased with the volume of immune serum given, but the maximum survivable SCHU S4 challenge dose was at least 100-fold lower than that shown for LVS-vaccinated rats. Protection correlated with reduced systemic bacterial growth, less severe histopathology in the liver and spleen during the early phase of infection, and bacterial clearance by a T cell-dependent mechanism. Our results suggest that treatment with immune serum limited the sequelae associated with infection, thereby enabling a sterilizing T cell response to develop and resolve the infection. Thus, antibodies induced by LVS vaccination may contribute to the defense of F344 rats against respiratory infection by type A strains of F. tularensis.

Bioavailability and efficacy of levofloxacin against Francisella tularensis in the common marmoset (Callithrix jacchus)

Pharmacokinetic and efficacy studies with levofloxacin were performed in the common marmoset (Callithrix jacchus) model of inhalational tularemia. Plasma levofloxacin pharmacokinetics were determined in six animals in separate single-dose and multidose studies. Plasma drug concentrations were analyzed using liquid chromatography-tandem mass spectrometry-electrospray ionization. On day 7 of a twice-daily dosing regimen of 40 mg/kg, the levofloxacin half-life, maximum concentration, and area under the curve in marmoset plasma were 2.3 h, 20.9 microg/ml, and 81.4 microg/liter/h, respectively. An efficacy study was undertaken using eight treated and two untreated control animals. Marmosets were challenged with a mean of 1.5 x 10(2) CFU of Francisella tularensis by the airborne route. Treated animals were administered 16.5 mg/kg levofloxacin by mouth twice daily, based on the pharmacokinetic parameters, beginning 24 h after challenge. Control animals had a raised core body temperature by 57 h postchallenge and died from infection by day 5. All of the other animals survived, remained afebrile, and lacked overt clinical signs. No bacteria were recovered from the organs of these animals at postmortem after culling at day 24 postchallenge. In conclusion, postexposure prophylaxis with orally administered levofloxacin was efficacious against acute inhalational tularemia in the common marmoset. The marmoset appears to be an appropriate animal model for the evaluation of postexposure therapies.

Characterization of lethal inhalational infection with Francisella tularensis in the common marmoset (Callithrix jacchus)

The intracellular Gram-negative pathogen Francisella tularensis is the causative agent of tularaemia and is prevalent in many countries in the northern hemisphere. To determine whether the common marmoset (Callithrix jacchus) would be a suitable non-human primate model of inhalational tularaemia, a pathophysiology study was undertaken. Ten animals were challenged with approximately 10(2) c.f.u. F. tularensis strain SCHU S4 (F. tularensis subsp. tularensis). To look for trends in the infection, pairs of animals were sacrificed at 24 h intervals between 0 and 96 h post-challenge and blood and organs were assessed for bacteriology, pathology and haematological and immunological parameters. The first indication of infection was a raised core temperature at 3 days post-challenge. This coincided with a number of other factors: a rapid increase in the number of bacteria isolated from all organs, more pronounced gross pathology and histopathology, and an increase in the immunological response. As the disease progressed, higher bacterial and cytokine levels were detected. More extensive pathology was observed, with multifocal lesions seen in the lungs, liver and spleen. Disease progression in the common marmoset appears to be consistent with human clinical and pathological features of tularaemia, indicating that this may be a suitable animal model for the investigation of novel medical interventions such as vaccines or therapeutics.

Francisella virulence: significant advances, ongoing challenges and unmet needs

Francisella tularensis, the causative agent of tularemia, is an organism of concern as a potential biowarfare agent. Progress towards understanding the molecular basis of pathogenicity has been hampered by a lack of tools with which to manipulate the pathogen. However, the availability of genome sequence data for a range of strains and the development of a range of plasmids and mutagenesis protocols for use in Francisella has resulted in a huge advance in understanding. No licensed vaccine is yet available. Various approaches towards a new vaccine are being evaluated, but novel adjuvants and delivery systems are needed to induce the complex response required for immunity. Better animal models to more accurately represent human responses to infection are also required.

Francisella tularensis vaccines

Francisella tularensis has attracted attention historically as a biological weapon, due to its high infectivity in aerosols, and the severity of disease in humans. There is no licensed vaccine currently available, although an attenuated live vaccine strain (LVS) was identified in the middle of the last century and has been successfully used to protect humans. Efforts are underway to determine the basis of attenuation of LVS, and to understand the immunity required for protection. Alternative approaches to produce subunit vaccines and defined attenuated strains are also in progress. However, the limitations of animal models may make licensing a candidate vaccine challenging.