Attenuated Mycobacterium marinum protects zebrafish against mycobacteriosis

Antigen identification strategies and preclinical evaluation models for advancing tuberculosis vaccine development

In its myriad devastating forms, Tuberculosis (TB) has existed for centuries, and humanity is still affected by it. Mycobacterium tuberculosis (M. tuberculosis), the causative agent of TB, was the foremost killer among infectious agents until the COVID-19 pandemic. One of the key healthcare strategies available to reduce the risk of TB is immunization with bacilli Calmette-Guerin (BCG). Although BCG has been widely used to protect against TB, reports show that BCG confers highly variable efficacy (0-80%) against adult pulmonary TB. Unwavering efforts have been made over the past 20 years to develop and evaluate new TB vaccine candidates. The failure of conventional preclinical animal models to fully recapitulate human response to TB, as also seen for the failure of MVA85A in clinical trials, signifies the need to develop better preclinical models for TB vaccine evaluation. In the present review article, we outline various approaches used to identify protective mycobacterial antigens and recent advancements in preclinical models for assessing the efficacy of candidate TB vaccines.

Protective Efficacy of BCG Vaccine against Mycobacterium leprae and Non-Tuberculous Mycobacterial Infections

The genus mycobacterium includes several species that are known to cause infections in humans. The microorganisms are classified into tuberculous and non-tuberculous based on their morphological characteristics, defined by the dynamic relationship between the host defenses and the infectious agent. Non-tuberculous mycobacteria (NTM) include all the species of mycobacterium other than the ones that cause tuberculosis (TB). The group of NTM contains almost 200 different species and they are found in soil, water, animals—both domestic and wild—milk and food products, and from plumbed water resources such as sewers and showerhead sprays. A systematic review of Medline between 1946 and 2014 showed an 81% decline in TB incidence rates with a simultaneous 94% increase in infections caused by NTM. Prevalence of infections due to NTM has increased relative to infections caused by TB owing to the stringent prevention and control programs in Western countries such as the USA and Canada. While the spread of typical mycobacterial infections such as TB and leprosy involves human contact, NTM seem to spread easily from the environment without the risk of acquiring from a human contact except in the case of M. abscessus in patients with cystic fibrosis, where human transmission as well as transmission through fomites and aerosols has been recorded. NTM are opportunistic in their infectious processes, making immunocompromised individuals such as those with other systemic infections such as HIV, immunodeficiencies, pulmonary disease, or usage of medications such as long-term corticosteroids/TNF-α inhibitors more susceptible. This review provides insight on pathogenesis, treatment, and BCG vaccine efficacy against M. leprae and some important NTM infections.

Mycobacteriosis and Infections with Non-tuberculous Mycobacteria in Aquatic Organisms: A Review

The Mycobacteriaceae constitute a family of varied Gram-positive organisms that include a large number of pathogenic bacteria. Among these, non-tuberculous mycobacteria are endemic worldwide and have been associated with infections in a large number of organisms, including humans and other mammals and reptiles, as well as fish. In this review, we summarize the most recent findings regarding this group of pathogens in fish. There, four species are most commonly associated with disease outbreaks: Mycobacterium marinum, the most common of these fish mycobacterial pathogens, Mycobacterium fortuitum, Mycobacterium gordonae, and Mycobacterium chelonae. These bacteria have a broad host range: they are zoonotic, and infections have been reported in a large number of fish species. The main route of entry of the bacterium into the fish is through the gastrointestinal route, and the disease is associated with ulcerative dermatitis as well as organomegaly and the development of granulomatous lesions in the internal organs. Mycobacteriaceae are slow-growing and fastidious and isolation is difficult and time consuming and diagnostic is mostly performed using serological and molecular tools. Control of the disease is also difficult: there is currently no effective vaccine and infections react poorly to antibiotherapy. For this reason, more research is needed on the subject of these vexing pathogens.

Integrating fish models in tuberculosis vaccine development

Tuberculosis is a chronic infection by Mycobacterium tuberculosis that results in over 1.5 million deaths worldwide each year. Currently, there is only one vaccine against tuberculosis, the Bacillus Calmette–Guérin (BCG) vaccine. Despite widespread vaccination programmes, over 10 million new M. tuberculosis infections are diagnosed yearly, with almost half a million cases caused by antibiotic-resistant strains. Novel vaccination strategies concentrate mainly on replacing BCG or boosting its efficacy and depend on animal models that accurately recapitulate the human disease. However, efforts to produce new vaccines against an M. tuberculosis infection have encountered several challenges, including the complexity of M. tuberculosis pathogenesis and limited knowledge of the protective immune responses. The preclinical evaluation of novel tuberculosis vaccine candidates is also hampered by the lack of an appropriate animal model that could accurately predict the protective effect of vaccines in humans. Here, we review the role of zebrafish (Danio rerio) and other fish models in the development of novel vaccines against tuberculosis and discuss how these models complement the more traditional mammalian models of tuberculosis.

Summary: In this Review, we discuss how zebrafish (Danio rerio) and other fish models can complement the more traditional mammalian models in the development of novel vaccines against tuberculosis.

Zebrafish as a Model for Fish Diseases in Aquaculture

The use of zebrafish as a model for human conditions is widely recognized. Within the last couple of decades, the zebrafish has furthermore increasingly been utilized as a model for diseases in aquacultured fish species. The unique tools available in zebrafish present advantages compared to other animal models and unprecedented in vivo imaging and the use of transgenic zebrafish lines have contributed with novel knowledge to this field. In this review, investigations conducted in zebrafish on economically important diseases in aquacultured fish species are included. Studies are summarized on bacterial, viral and parasitic diseases and described in relation to prophylactic approaches, immunology and infection biology. Considerable attention has been assigned to innate and adaptive immunological responses. Finally, advantages and drawbacks of using the zebrafish as a model for aquacultured fish species are discussed.

Teleost contributions to the understanding of mycobacterial diseases

Few pathogens have shaped human medicine as the mycobacteria. From understanding biological phenomena driving disease spread, to mechanisms of host-pathogen interactions and antibiotic resistance, the Mycobacterium genus continues to challenge and offer insights into the basis of health and disease. Teleost fish models of mycobacterial infections have progressed significantly over the past three decades, now supplying a range of unique tools and new opportunities to define the strategies employed by these Gram-positive bacteria to overcome host defenses, as well as those host antimicrobial pathways that can be used to limit its growth and spread. Herein, we take a comparative perspective and provide an update on the contributions of teleost models to our understanding of mycobacterial diseases.

Vaccination of European sea bass Dicentrarchus labrax with avirulent Mycobacterium marinum (iipA::kan mutant)

Mycobacteriosis is a chronic progressive disease affecting teleost fishes all over the world. No vaccine is commercially available against its main etiological agent, Mycobacterium marinum. The mycobacterial gene responsible for invasion and intracellular persistence, iipA, is known to modulate M. marinum pathology. The innate and adaptive immune responses in sea bass (Dicentrarchus labrax) vaccinated with M. marinum iipA::kan mutant with (and without) the use of adjuvant, with (and without) a booster vaccination were monitored. The adjuvanted vaccine induced enhanced immune responses. TNF-α transcription levels were extremely high in spleen of the fish vaccinated with the addition of adjuvant in both fish vaccinated once and twice, followed by an IgM response highly specific for M. marinum. Also, histologically, granulomas started appearing in spleen and head-kidney tissues (but with no visible bacteria) within a month after vaccination, mainly with the adjuvanted vaccine. This was followed by reduction in pathology, as demonstrated by the lower number of granulomas (with visible bacteria), indicating that even heat-killed bacteria were able to elicit granulomatous formations. Adhesion of the internal organs and moderate pigmentation were observed in the perivisceral adipose tissue of nearly all vaccinated fish. Although the adjuvanted heat-killed avirulent iipA::kan mutant clearly induced a strong humoral and adaptive immune response, the booster treatment did not seem to have produced a significantly higher degree of protection from the disease compared to fish that received a single vaccination.

The zebrafish model of tuberculosis - no lungs needed

Tuberculosis is still a global health burden. It is caused by Mycobacterium tuberculosis which afflicts around one third of the world's population and costs around 1.3 million people their lives every year. Bacillus Calmette-Guerin vaccine is inefficient to prevent overt infection. Additionally, the lengthy inconvenient course of treatment, along with the raising issue of antimicrobial resistance, result in incomplete eradication of this infectious disease. The lack of proper animal models that replicate the latent and active courses of human tuberculosis infection remains one of the main reasons behind the poor advancement in tuberculosis research. Danio rerio, commonly known as zebrafish, is catching more attention as an animal model in tuberculosis research field. This shift is based on the histological and pathological similarities between Mycobacterium marinum infection in zebrafish and Mycobacterium tuberculosis infection in humans. Being small, cheap, transparent, and easy to handle have added further advantages to the use of zebrafish model. Besides better understanding of the pathogenesis of tuberculosis, Mycobacterium marinum infected zebrafish model is useful for evaluating novel vaccines against human tuberculosis, high throughput small molecule screening, repurposing established drugs with possible antitubercular activity, and assessing novel antituberculars for hepatotoxicity.

Humoral and Cellular Immune Response of European Seabass Dicentrarchus labrax Vaccinated with Heat-Killed Mycobacterium marinum (iipA::kan Mutant)

No vaccine is yet commercially available against Mycobacterium marinum, the etiological agent of fish mycobacteriosis (also known as "fish tuberculosis"). The mycobacterial gene responsible for invasion and intracellular persistence, iipA, is known to moderate M. marinum pathology in Zebrafish Danio rerio. Two doses of heat-killed, wild-type, virulent M. marinum and two doses of a heat-killed, avirulent M. marinum iipA::kan mutant strain were used in parallel to vaccinate European Seabass Dicentrarchus labrax. The fish were then challenged with live, virulent M. marinum, and the pathogenesis of the infection was monitored. High specific immunoglobulin M (IgM) response and an increase in cytokine tumor necrosis factor alpha (TNF-α) messenger RNA expression levels were observed in all vaccinated fish. At 1 month postchallenge, TNF-α expression levels increased in spleen tissues of fish vaccinated with the virulent type and in those of unvaccinated fish, whereas in the head kidney, expression was up-regulated only in unvaccinated fish. The expression then decreased, and at 2 months postchallenge, expression appeared similar in all vaccination types. The highest survival rate (75%) was recorded in the group of fish that were vaccinated with a high dose of avirulent iipA::kan mutant. The iipA::kan mutant induced a strong immune response accompanied by only modest tissue disruption. Coupled with an effective program of booster treatments, the iipA::kan mutant vaccine may be developed into a powerful preventive measure against fish mycobacteriosis.

Heat-inactivated Mycobacterium bovis protects zebrafish against mycobacteriosis

Control of mycobacterial infection constitutes a priority for human and animal health worldwide. However, effective vaccines are needed for the control of human and animal tuberculosis (TB). Adult zebrafish have become a useful model for studying the pathophysiology of mycobacterial infection and for the development of novel interventions for TB control and prevention. Recently, parenteral and oral immunization with the heat-inactivated Mycobacterium bovis vaccine (M. bovis IV) protected wild boar against TB. The objectives of this study were to provide additional support for the role of M. bovis IV in TB control using the zebrafish model and to conduct the first trial with this vaccine for the control of fish mycobacteriosis. The results showed that M. bovis IV protected zebrafish against mycobacteriosis caused by low and high infection doses of Mycobacterium marinum and provided evidence suggesting that the protective mechanism elicited by M. bovis IV in zebrafish as in other species is based on the activation of the innate immune response through the C3 pathway, with a role for the regulatory protein Akr2 in this process. These results encourage the use of M. bovis IV for TB control in different species.

Transcriptome profiling and expression analysis of immune responsive genes in the liver of Golden mahseer (Tor putitora) challenged with Aeromonas hydrophila

Transcriptome profiling has been used to decipher the novel mechanisms behind immune responses of the fishes. However, the molecular mechanism underlining immune response in mahseer is not studied so far. Fishes are greatly affected by bacterial pathogens such as Aeromonas hydrophila. In this study, transcriptome response of golden mahseer (Tor putitora) infected with A. hydrophila was examined using paired end Illumina sequencing of liver tissue to understand the immune response of the fish. The de novo assembly generated 61,042 unigenes ranging from 200 to 9322 bp in length and an average length of 463 bp. The gene ontology annotations resulted a total of 131,826 term assignments to the annotated transcriptome including 60,846 (46.16%) allocations from the biological process; 21,603 (16.39%) from molecular function and 49,377 (37.46%) from cellular components. Differential gene expression analysis of the transcriptome data from challenged and control group revealed 1104 upregulated and 1304 down-regulated unigenes. The differentially expressed genes were mainly involved in the pathways including cell surface receptor signaling, TH1 and TH2 cell differentiation, pathogen recognition, and immune system process/defense response especially complement cascade. Twelve unigenes including ankyrin, serum amyloid, hsp4b, STAT3, complement factor c3 and c7 were validated using qPCR and found differentially expressed in accordance with in silico expression analysis. The results obtained in this study will provide the first and crucial information on the molecular mechanism of mahseer fishes against bacterial infection.

The Zebrafish Breathes New Life into the Study of Tuberculosis

Tuberculosis (TB) is a global health emergency. Up to one-third of the world’s population is infected with Mycobacterium tuberculosis, and the pathogen continues to kill 1.5 million people annually. Currently, the means for preventing, diagnosing, and treating TB are unsatisfactory. One of the main reasons for the poor progress in TB research has been a lack of good animal models to study the latency, dormancy, and reactivation of the disease. Although sophisticated in vitro and in silico methods suitable for TB research are constantly being developed, they cannot reproduce the complete vertebrate immune system and its interplay with pathogens and vaccines. However, the zebrafish has recently emerged as a useful alternative to more traditional models, such as mice, rabbits, guinea pigs, and non-human primates, for studying the complex pathophysiology of a mycobacterial infection. The model is based on the similarity between Mycobacterium marinum – a natural fish pathogen – and M. tuberculosis. In both zebrafish larvae and adult fish, an infection with M. marinum leads to the formation of macrophage aggregates and granulomas, which resemble the M. tuberculosis infections in humans. In this review, we will summarize the current status of the zebrafish model in TB research and highlight the advantages of using zebrafish to dissect mycobacterial virulence strategies as well as the host immune responses elicited against them. In addition, we will discuss the possibilities of using the adult zebrafish model for studying latency, dormancy, and reactivation in a mycobacterial infection.

Controlling Endemic Pathogens-Challenges and Opportunities

By most measures, the University of Utah Centralized Zebrafish Animal Resource is a successful zebrafish core facility: we house ∼4000-5000 tanks for over 16 research groups; provide services and equipment for ∼150 users; are currently undergoing an expansion by 3000 tanks; and have been praised by institutional and national regulatory agencies for the cleanliness and efficiency of our facility. In recent years, we have implemented new programs to improve the overall health of our colony and believe we have seen a reduction in apparently sick fish. However, there are still deficiencies in our monitoring and pathogen control programs. Our histopathology sample sizes have been insufficient to estimate prevalence, but our sentinel tank program reveals the presence of Pseudoloma neurophilia and myxozoan, presumably Myxidium streisinger, in our facility. As we develop protocols to further reduce the burden of disease, we are focused on defining our baseline, establishing goals, and implementing methods to monitor our progress. The data generated by this approach will allow us to evaluate and implement the most cost-effective protocols to improve fish health.

Zebrafish and Streptococcal Infections

Streptococcal bacteria are a versatile group of gram-positive bacteria capable of infecting several host organisms, including humans and fish. Streptococcal species are common colonizers of the human respiratory and gastrointestinal tract, but they also cause some of the most common life-threatening, invasive infections in humans and aquaculture. With its unique characteristics and efficient tools for genetic and imaging applications, the zebrafish (Danio rerio) has emerged as a powerful vertebrate model for infectious diseases. Several zebrafish models introduced so far have shown that zebrafish are suitable models for both zoonotic and human-specific infections. Recently, several zebrafish models mimicking human streptococcal infections have also been developed. These models show great potential in providing novel information about the pathogenic mechanisms and host responses associated with human streptococcal infections. Here, we review the zebrafish infection models for the most relevant streptococcal species: the human-specific Streptococcus pneumoniae and Streptococcus pyogenes, and the zoonotic Streptococcus iniae and Streptococcus agalactiae. The recent success and the future potential of these models for the study of host-pathogen interactions in streptococcal infections are also discussed.

Zebrafish as a model for zoonotic aquatic pathogens

Aquatic habitats harbor a multitude of bacterial species. Many of these bacteria can act as pathogens to aquatic species and/or non-aquatic organisms, including humans, that come into contact with contaminated water sources or colonized aquatic organisms. In many instances, the bacteria are not pathogenic to the aquatic species they colonize and are only considered pathogens when they come into contact with humans. There is a general lack of knowledge about how the environmental lifestyle of these pathogens allows them to persist, replicate and produce the necessary pathogenic mechanisms to successfully transmit to the human host and cause disease. Recently, the zebrafish infectious disease model has emerged as an ideal system for examining aquatic pathogens, both in the aquatic environment and during infection of the human host. This review will focus on how the zebrafish has been used successfully to analyze the pathogenesis of aquatic bacterial pathogens.

Adult zebrafish model for pneumococcal pathogenesis

Streptococcus pneumoniae (pneumococcus) is a leading cause of community acquired pneumonia, septicemia, and meningitis. Due to incomplete understanding of the host and bacterial factors contributing to these diseases optimal treatment and prevention methods are lacking. In the present study we examined whether the adult zebrafish (Danio rerio) can be used to investigate the pathophysiology of pneumococcal diseases. Here we show that both intraperitoneal and intramuscular injections of the pneumococcal strain TIGR4 cause a fulminant, dose-dependent infection in adult zebrafish, while isogenic mutant bacteria lacking the polysaccharide capsule, autolysin, or pneumolysin are attenuated in the model. Infection through the intraperitoneal route is characterized by rapid expansion of pneumococci in the bloodstream, followed by penetration of the blood-brain barrier and progression to meningitis. Using Rag1 mutant zebrafish, which are devoid of somatic recombination and thus lack adaptive immune responses, we show that clearance of pneumococci in adult zebrafish depends mainly on innate immune responses. In conclusion, this study provides evidence that the adult zebrafish can be used as a model for a pneumococcal infection, and that it can be used to study both host and bacterial factors involved in the pathogenesis. However, our results do not support the use of the zebrafish in studies on the role of adaptive immunity in pneumococcal disease or in the development of new pneumococcal vaccines.

Mycobacteriosis in zebrafish colonies

Mycobacteriosis, a chronic bacterial infection, has been associated with severe losses in some zebrafish facilities and low-level mortalities and unknown impacts in others. The occurrence of at least six different described species (Mycobacterium abscessus, M. chelonae, M. fortuitum, M. haemophilum, M. marinum, M. peregrinum) from zebrafish complicates diagnosis and control because each species is unique. As a generalization, mycobacteria are often considered opportunists, but M. haemophilum and M. marinum appear to be more virulent. Background genetics of zebrafish and environmental conditions influence the susceptibility of fish and progression of disease, emphasizing the importance of regular monitoring and good husbandry practices. A combined approach to diagnostics is ultimately the most informative, with histology as a first-level screen, polymerase chain reaction for rapid detection and species identification, and culture for strain differentiation. Occurrence of identical strains of Mycobacterium in both fish and biofilms in zebrafish systems suggests transmission can occur when fish feed on infected tissues or tank detritus containing mycobacteria. Within a facility, good husbandry practices and sentinel programs are essential for minimizing the impacts of mycobacteria. In addition, quarantine and screening of animals coming into a facility is important for eliminating the introduction of the more severe pathogens. Elimination of mycobacteria from an aquatic system is likely not feasible because these species readily establish biofilms on surfaces even in extremely low nutrient conditions. Risks associated with each commonly encountered species need to be identified and informed management plans developed. Basic research on the growth characteristics, disinfection, and pathogenesis of zebrafish mycobacteria is critical moving forward.

RNA-seq liver transcriptome analysis reveals an activated MHC-I pathway and an inhibited MHC-II pathway at the early stage of vaccine immunization in zebrafish

Background

Zebrafish (Danio rerio) is a prominent vertebrate model of human development and pathogenic disease and has recently been utilized to study teleost immune responses to infectious agents threatening the aquaculture industry. In this work, to clarify the host immune mechanisms underlying the protective effects of a putative vaccine and improve its immunogenicity in the future efforts, high-throughput RNA sequencing technology was used to investigate the immunization-related gene expression patterns of zebrafish immunized with Edwardsiella tarda live attenuated vaccine.

Results

Average reads of 18.13 million and 14.27 million were obtained from livers of zebrafish immunized with phosphate buffered saline (mock) and E. tarda vaccine (WED), respectively. The reads were annotated with the Ensembl zebrafish database before differential expressed genes sequencing (DESeq) comparative analysis, which identified 4565 significantly differentially expressed genes (2186 up-regulated and 2379 down-regulated in WED; p<0.05). Among those, functional classifications were found in the Gene Ontology database for 3891 and in the Kyoto Encyclopedia of Genes and Genomes database for 3467. Several pathways involved in acute phase response, complement activation, immune/defense response, and antigen processing and presentation were remarkably affected at the early stage of WED immunization. Further qPCR analysis confirmed that the genes encoding the factors involved in major histocompatibility complex (MHC)-I processing pathway were up-regulated, while those involved in MHC-II pathway were down-regulated.

Conclusion

These data provided insights into the molecular mechanisms underlying zebrafish immune response to WED immunization and might aid future studies to develop a highly immunogenic vaccine against gram-negative bacteria in teleosts.

Immune responses of zebrafish (Danio rerio) induced by bath-vaccination with a live attenuated Vibrio anguillarum vaccine candidate

A fish vaccine candidate, live attenuated Vibrio anguillarum, which can protect fish from vibriosis, was established in our laboratory. In this study, the protective immunological mechanism of live attenuated V. anguillarum was investigated in zebrafish as a model animal. After bath-vaccinated with the live attenuated strain, zebrafish were challenged with wild pathogenic strain to test the immunoprotection of the live attenuated strain. As the results, specific antibody response of fish against V. anguillarum was found to gradually increase during 28 days post-vaccination, and remarkable protection was showed with a high relative protection survival (RPS) of about 90%. Moreover, the vaccination changed the expressions of several immune-related genes in the spleens and livers of zebrafish. Among them, the expressions of pro-inflammatory factors such as IL-1 and IL-8 were tenderly up-regulated with about 3-4 fold in 1-7 days post-vaccination, while MHC II rose to a peak level of 4-fold in 7th day post-vaccination. These results gave some important messages about the mechanism of specific protection induced by live attenuated V. anguillarum and showed the availability of zebrafish model in the evaluation of the vaccine candidate.

The challenges of implementing pathogen control strategies for fishes used in biomedical research

Over the past several decades, a number of fish species, including the zebrafish, medaka, and platyfish/swordtail, have become important models for human health and disease. Despite the increasing prevalence of these and other fish species in research, methods for health maintenance and the management of diseases in laboratory populations of these animals are underdeveloped. There is a growing realization that this trend must change, especially as the use of these species expands beyond developmental biology and more towards experimental applications where the presence of underlying disease may affect the physiology animals used in experiments and potentially compromise research results. Therefore, there is a critical need to develop, improve, and implement strategies for managing health and disease in aquatic research facilities. The purpose of this review is to report the proceedings of a workshop entitled "Animal Health and Disease Management in Research Animals" that was recently held at the 5th Aquatic Animal Models for Human Disease in September 2010 at Corvallis, Oregon to discuss the challenges involved with moving the field forward on this front.

The effect of stocking densities on reproductive performance in laboratory zebrafish (Danio rerio)

Despite the growing popularity of the zebrafish model system, the optimal husbandry conditions for this animal are not well defined. The aim of this study was to examine the effect of stocking density on reproductive performance in zebrafish. In this study, undertaken by eight different zebrafish facilities, clutches of at least 200 wild-type zebrafish embryos from a single pairwise mating were produced at each participating institution and subsequently reared according to "in-house protocols" until they were 14 weeks old. Fish were then randomly assigned into treatment groups with balanced sex ratios and densities of 3, 6, or 12 fish/L. After a 1-month acclimation period, fish were spawned in pair crosses every 2 weeks for 3 months, for a total of six spawning dates. The number of viable and nonviable embryos produced in each clutch were counted at 1 day postfertilization. Although there was a great deal of variability in clutch size and percent spawning success among laboratories, there were no significant differences in average clutch size, spawning success, or percent viable among the treatment densities. These data suggest that using stocking densities as high as 12 fish/L does not have a negative impact on performance, when measured by reproductive performance.