A Prototype Recombinant-Protein Based Chlamydia pecorum Vaccine Results in Reduced Chlamydial Burden and Less Clinical Disease in Free-Ranging Koalas (Phascolarctos cinereus)

Immunohistochemical characterization of the immune cell response during chlamydial infection in the male and female koala (Phascolarctos cinereus) reproductive tract

Chlamydiosis is one of the main causes of the progressive decline of koala populations in eastern Australia. While histologic, immunologic, and molecular studies have provided insights into the basic function of the koala immune system, the in situ immune cell signatures during chlamydial infection of the reproductive tract in koalas have not been investigated. Thirty-two female koalas and 47 males presented to wildlife hospitals with clinical signs suggestive of Chlamydia infection were euthanized with the entire reproductive tract collected for histology; immunohistochemistry (IHC) for T-cell (CD3ε, CD4, and CD8α), B-cell (CD79b), and human leukocyte antigen (HLA)-DR markers; and quantitative real-time polymerase chain reaction (rtPCR) for Chlamydia pecorum. T-cells, B-cells, and HLA-DR-positive cells were observed in both the lower and upper reproductive tracts of male and female koalas with a statistically significant associations between the degree of the inflammatory reaction; the number of CD3, CD4, CD79b, and HLA-DR positive cells; and the PCR load. CD4-positive cells were negatively associated with the severity of the gross lesions. The distribution of immune cells was also variable according to the location within the genital tract in both male and female koalas. These preliminary results represent a step forward towards further exploring mechanisms behind chlamydial infection immunopathogenesis, thus providing valuable information about the immune response and infectious diseases in free-ranging koalas.

Efficacy of a synthetic peptide Chlamydia pecorum major outer membrane protein vaccine in a wild koala (Phascolarctos cinereus) population

Chlamydiosis is a significant disease affecting Eastern Australian koala (Phascolarctos cinereus) populations, impacting individual animal welfare and fecundity and therefore influencing population dynamics. The aim of this study was to investigate the effect of a synthetic peptide vaccine based on 4 components of the Chlamydia pecorum major outer membrane protein (MOMP), over an 18-month period in a koala population severely impacted by chlamydiosis. Wild koalas were recruited into a vaccination or a placebo treatment group on a random allocation, then followed through a period of 18 months, with recapture at 6 monthly intervals. Vaccination did not alter clinical disease expression or chlamydial shedding from the ocular or urogenital sites. Vaccination did not stimulate a significant plasma anti-MOMP IgG response, when compared to the placebo group. There was no significant effect of vaccination on IFN-γ and IL-17A mRNA expression of peripheral blood lymphocytes when stimulated with rMOMP. We have demonstrated that a synthetic peptide vaccination against chlamydiosis is not an effective management tool in a koala population with a high prevalence of C. pecorum infection and related disease. The lack of antigenic response found in this study suggests that further research utilising a larger, full-length antigen is an avenue worth investigation if we are to consider vaccination as a part of a management strategy in diseased koala populations.

Reduction of Chlamydia pecorum and Koala Retrovirus subtype B expression in wild koalas vaccinated with novel peptide and peptide/recombinant protein formulations

Koalas are an endangered species under threat of extinction from several factors, including infections agents. Chlamydia pecorum infection results in morbidity and mortality from ocular and urogenital diseases while Koala Retrovirus (KoRV) infection has been linked to increased rates of cancer and chlamydiosis. Both C. pecorum and KoRV are endemic in many wild Australian koala populations, with limited treatment options available. Fortunately, vaccines for these pathogens are under development and have generated effective immune responses in multiple trials. The current study aimed to improve vaccine formulations by testing a novel peptide version of the Chlamydia vaccine and a combination Chlamydia - KoRV vaccine. Utilising a monitored wild population in Southeast Queensland, this trial followed koalas given either a 'Chlamydia only' vaccine (utilising four peptides from the chlamydial Major Outer Membrane Protein, MOMP), a combination 'Chlamydia and KoRV' vaccine (comprised of the chlamydial peptides plus a KoRV recombinant envelope protein (rEnv)) or no treatment. Clinical observations, C. pecorum and KoRV gene expression, serum IgG, and mucosal immune gene expression were assessed over a 17-month period. Overall, both vaccine formulations resulted in a decrease in chlamydiosis mortality, with decreases in C. pecorum, CD4, CD8β and IL-17A gene expression observed. In addition, the combination vaccine group also showed an increase in anti-KoRV IgG production that corresponded to a decrease in detected KoRV-B expression. While these results are favourable, the chlamydial peptide vaccine did not appear to outperform the established recombinant chlamydial vaccine and suggests that a combination vaccine formulated with recombinant MOMP plus KoRV rEnv could capitalize on the demonstrated benefits of both for the betterment of koalas into the future.

Susceptibility to a sexually transmitted disease in a wild koala population shows heritable genetic variance but no inbreeding depression

The koala, one of the most iconic Australian wildlife species, is facing several concomitant threats that are driving population declines. Some threats are well known and have clear methods of prevention (e.g., habitat loss can be reduced with stronger land-clearing control), whereas others are less easily addressed. One of the major current threats to koalas is chlamydial disease, which can have major impacts on individual survival and reproduction rates and can translate into population declines. Effective management strategies for the disease in the wild are currently lacking, and, to date, we know little about the determinants of individual susceptibility to disease. Here, we investigated the genetic basis of variation in susceptibility to chlamydia using one of the most intensively studied wild koala populations. We combined data from veterinary examinations, chlamydia testing, genetic sampling and movement monitoring. Out of our sample of 342 wild koalas, 60 were found to have chlamydia. Using genotype information on 5007 SNPs to investigate the role of genetic variation in determining disease status, we found no evidence of inbreeding depression, but a heritability of 0.11 (95% CI: 0.06-0.23) for the probability that koalas had chlamydia. Heritability of susceptibility to chlamydia could be relevant for future disease management, as it suggests adaptive potential for the population.

The Koala Immune Response to Chlamydial Infection and Vaccine Development-Advancing Our Immunological Understanding

Simple Summary

Chlamydia is a major pathogen of the Australian marsupial, the koala (Phascolarctos cinereus). One approach to improving this situation is to develop a vaccine. Human Chlamydia research suggests that an effective anti-chlamydial response will involve a balance between a cell-mediated Th1 response and a humoral Th2 responses, involving systemic IgG and mucosal IgA. Characterization of koalas with chlamydial disease suggests that increased expression for similar immunological pathways and monitoring of koalas’ post-vaccination can be successful and subsequently lead to improved vaccines. These findings offer optimism that a chlamydial vaccine for wider distribution to koalas is not far off.

Abstract

Chlamydia is a significant pathogen for many species, including the much-loved Australian marsupial, the koala (Phascolarctos cinereus). To combat this situation, focused research has gone into the development and refinement of a chlamydial vaccine for koalas. The foundation of this process has involved characterising the immune response of koalas to both natural chlamydial infection as well as vaccination. From parallels in human and mouse research, it is well-established that an effective anti-chlamydial response will involve a balance of cell-mediated Th1 responses involving interferon-gamma (IFN-γ), humoral Th2 responses involving systemic IgG and mucosal IgA, and inflammatory Th17 responses involving interleukin 17 (IL-17) and neutrophils. Characterisation of koalas with chlamydial disease has shown increased expression within all three of these major immunological pathways and monitoring of koalas’ post-vaccination has detected further enhancements to these key pathways. These findings offer optimism that a chlamydial vaccine for wider distribution to koalas is not far off. Recent advances in marsupial genetic knowledge and general nucleic acid assay technology have moved koala immunological research a step closer to other mammalian research systems. However, koala-specific reagents to directly assay cytokine levels and cell-surface markers are still needed to progress our understanding of koala immunology.

Koala retrovirus epidemiology, transmission mode, pathogenesis, and host immune response in koalas (Phascolarctos cinereus): a review

Koala retrovirus (KoRV) is a major threat to koala health and conservation. It also represents a series of challenges across the fields of virology, immunology, and epidemiology that are of great potential interest to any researcher in the field of retroviral diseases. KoRV is a gammaretrovirus that is present in both endogenous and exogenous forms in koala populations, with a still-active endogenization process. KoRV may induce immunosuppression and neoplastic conditions such as lymphoma and leukemia and play a role in chlamydiosis and other diseases in koalas. KoRV transmission modes, pathogenesis, and host immune response still remain unclear, and a clear understanding of these areas is critical for devising effective preventative and therapeutic strategies. Research on KoRV is clearly critical for koala conservation. In this review, we provide an overview of the current understanding and future challenges related to KoRV epidemiology, transmission mode, pathogenesis, and host immune response and discuss prospects for therapeutic and preventive vaccines.

Isolation and sequence-based characterization of a koala symbiont: Lonepinella koalarum

Koalas (Phascolarctos cinereus) are highly specialized herbivorous marsupials that feed almost exclusively on Eucalyptus leaves, which are known to contain varying concentrations of many different toxic chemical compounds. The literature suggests that Lonepinella koalarum, a bacterium in the Pasteurellaceae family, can break down some of these toxic chemical compounds. Furthermore, in a previous study, we identified L. koalarum as the most predictive taxon of koala survival during antibiotic treatment. Therefore, we believe that this bacterium may be important for koala health. Here, we isolated a strain of L. koalarum from a healthy koala female and sequenced its genome using a combination of short-read and long-read sequencing. We placed the genome assembly into a phylogenetic tree based on 120 genome markers using the Genome Taxonomy Database (GTDB), which currently does not include any L. koalarum assemblies. Our genome assembly fell in the middle of a group of Haemophilus, Pasteurella and Basfia species. According to average nucleotide identity and a 16S rRNA gene tree, the closest relative of our isolate is L. koalarum strain Y17189. Then, we annotated the gene sequences and compared them to 55 closely related, publicly available genomes. Several genes that are known to be involved in carbohydrate metabolism could exclusively be found in L. koalarum relative to the other taxa in the pangenome, including glycoside hydrolase families GH2, GH31, GH32, GH43 and GH77. Among the predicted genes of L. koalarum were 79 candidates putatively involved in the degradation of plant secondary metabolites. Additionally, several genes coding for amino acid variants were found that had been shown to confer antibiotic resistance in other bacterial species against pulvomycin, beta-lactam antibiotics and the antibiotic efflux pump KpnH. In summary, this genetic characterization allows us to build hypotheses to explore the potentially beneficial role that L. koalarum might play in the koala intestinal microbiome. Characterizing and understanding beneficial symbionts at the whole genome level is important for the development of anti- and probiotic treatments for koalas, a highly threatened species due to habitat loss, wildfires, and high prevalence of Chlamydia infections.

Capturing Complex Vaccine-Immune-Disease Relationships for Free-Ranging Koalas: Higher Chlamydial Loads Are Associated With Less IL17 Expression and More Chlamydial Disease

Background: Chlamydial disease is a major factor negatively affecting koala populations. Vaccination is a promising management option that would result in immune-mediated protection against disease. Measuring and assessing vaccine efficacy can be challenging owing to both direct and indirect interactions caused by vaccination. In this study, we investigate vaccine-immune-chlamydial load-disease relationships from MOMP (major outer membrane protein) vaccine trials to protect healthy free-ranging koalas against Chlamydia-related diseases.

Methods: We created a priori hypotheses based on data sources and perceived direct and indirect interactions from koalas vaccinated 6 months prior. Each hypothesis was tested as a structural equation model separately for either the urogenital or the ocular site to evaluate possible causality among measured variables. Model averaging was used as multiple models fit the data, and the strength of relationships was examined through averaged coefficients and the raw data.

Results: We found more relationships in urogenital models as compared to ocular models, particularly those with interleukin 17 (IL17) mRNA expression compared to models with interferon gamma (IFNγ) expression. In the averaged model with IL17, urogenital chlamydial load was positively associated with disease and negatively associated with IL17 expression. MOMP vaccination had a trending effect for reducing urogenital chlamydial load and also had a strong effect on increasing IL17 expression. Not surprisingly, urogenital chlamydial load was a positive predictor for the development of urogenital disease at 6 months post-vaccination.

Conclusions: Despite multiple potential sources of variation owing to the koalas in this study being free-ranging, our analyses provide unique insights into the effects of vaccinating against Chlamydia. Using structural equation modeling, this study has helped illuminate that the expression of the immune cytokine IL17 is linked to MOMP vaccination, and animals with a high urogenital chlamydial load expressed less IL17 and were more likely to develop disease, enhancing previous investigations. Going beyond univariate statistics, the methods used in this study can be applied to other preclinical vaccination experiments to identify important direct and indirect factors underpinning the effects of a vaccine.

Ecological and Evolutionary Challenges for Wildlife Vaccination

Wildlife vaccination is of urgent interest to reduce disease-induced extinction and zoonotic spillover events. However, several challenges complicate its application to wildlife. For example, vaccines rarely provide perfect immunity. While some protection may seem better than none, imperfect vaccination can present epidemiological, ecological, and evolutionary challenges. While anti-infection and antitransmission vaccines reduce parasite transmission, antidisease vaccines may undermine herd immunity, select for increased virulence, or promote spillover. These imperfections interact with ecological and logistical constraints that are magnified in wildlife, such as poor control and substantial trait variation within and among species. Ultimately, we recommend approaches such as trait-based vaccination, modeling tools, and methods to assess community- and ecosystem-level vaccine safety to address these concerns and bolster wildlife vaccination campaigns.

Helping koalas battle disease - Recent advances in Chlamydia and koala retrovirus (KoRV) disease understanding and treatment in koalas

The iconic Australian marsupial, the koala (Phascolarctos cinereus), has suffered dramatic population declines as a result of habitat loss and fragmentation, disease, vehicle collision mortality, dog attacks, bushfires and climate change. In 2012, koalas were officially declared vulnerable by the Australian government and listed as a threatened species. In response, research into diseases affecting koalas has expanded rapidly. The two major pathogens affecting koalas are Chlamydia pecorum, leading to chlamydial disease and koala retrovirus (KoRV). In the last eight years, these pathogens and their diseases have received focused study regarding their sources, genetics, prevalence, disease presentation and transmission. This has led to vast improvements in pathogen detection and treatment, including the ongoing development of vaccines for each as a management and control strategy. This review will summarize and highlight the important advances made in understanding and combating C. pecorum and KoRV in koalas, since they were declared a threatened species. With complementary advances having also been made from the koala genome sequence and in our understanding of the koala immune system, we are primed to make a significant positive impact on koala health into the future.

New technologies in the mix: Assessing N-mixture models for abundance estimation using automated detection data from drone surveys

Reliable estimates of abundance are critical in effectively managing threatened species, but the feasibility of integrating data from wildlife surveys completed using advanced technologies such as remotely piloted aircraft systems (RPAS) and machine learning into abundance estimation methods such as N-mixture modeling is largely unknown due to the unique sources of detection errors associated with these technologies.We evaluated two modeling approaches for estimating the abundance of koalas detected automatically in RPAS imagery: (a) a generalized N-mixture model and (b) a modified Horvitz-Thompson (H-T) estimator method combining generalized linear models and generalized additive models for overall probability of detection, false detection, and duplicate detection. The final estimates from each model were compared to the true number of koalas present as determined by telemetry-assisted ground surveys.The modified H-T estimator approach performed best, with the true count of koalas captured within the 95% confidence intervals around the abundance estimates in all 4 surveys in the testing dataset (n = 138 detected objects), a particularly strong result given the difficulty in attaining accuracy found with previous methods.The results suggested that N-mixture models in their current form may not be the most appropriate approach to estimating the abundance of wildlife detected in RPAS surveys with automated detection, and accurate estimates could be made with approaches that account for spurious detections.

Chlamydiae from Down Under: The Curious Cases of Chlamydial Infections in Australia

In Australia, the most researched and perhaps the most successful chlamydial species are the human pathogen Chlamydia trachomatis, animal pathogens Chlamydia pecorum and Chlamydia psittaci. C. trachomatis remains the leading cause of sexually transmitted infections in Australians and trachoma in Australian Indigenous populations. C. pecorum is globally recognised as the infamous koala and widespread livestock pathogen, whilst the avian C. psittaci is emerging as a horse pathogen posing zoonotic risks to humans. Certainly not innocuous, the human infections with Chlamydia pneumoniae seem to be less prevalent that other human chlamydial pathogens (namely C. trachomatis). Interestingly, the complete host range for C. pecorum and C. psittaci remains unknown, and infections by other chlamydial organisms in Australian domesticated and wildlife animals are understudied. Considering that chlamydial organisms can be encountered by either host at the human/animal interface, I review the most recent findings of chlamydial organisms infecting Australians, domesticated animals and native wildlife. Furthermore, I also provide commentary from leading Australian Chlamydia experts on challenges and future directions in the Chlamydia research field.

Antibiotic treatment of Chlamydia-induced cystitis in the koala is linked to expression of key inflammatory genes in reactive oxygen pathways

Chlamydial-induced cystitis in the koala (Phascolarctos cinereus) is currently treated by antibiotics. However, while reducing the chlamydial load, this treatment can also lead to gastrointestinal complications and death. Development of alternative treatments, such as a therapeutic chlamydial vaccine, are hindered by the lack of detailed understanding of the innate immune response to chlamydial clearance and disease regression during antibiotic treatment. Through clinical, microbiological and transcriptomic approaches, disease regression, bacterial clearance and innate immune responses were mapped in koalas with signs of chlamydial-induced cystitis while receiving anti-chlamydial antibiotics. Significant reduction in the signs of cystitis were observed during and post antibiotic treatment. This was observed as a thinning of the bladder wall and complete reversal of urinary incontinence. Transcriptomic analysis before treatment, at the end of treatment and prior to release identified significant down-regulation of specific genes involved in 21 biological pathways. Of these, the chemokine receptor signalling and NOD-like receptor signalling pathways where identified as important markers of inflammation. Specific genes within these pathways (NCF1 and NOX2) were significantly down-regulated, suggesting a decrease in reactive oxygen species production. Through the monitoring of specific clinical and transcriptomic markers, these findings allow detailed profiling of the clinical response to therapeutic vaccination in koalas with current signs of disease. This also adds to our understanding of innate immune responses to chlamydial infections and indicates that chlamydial-induced cystitis in the koala is linked to the regulation of reactive oxygen pathways.

Automated detection of koalas using low-level aerial surveillance and machine learning

Effective wildlife management relies on the accurate and precise detection of individual animals. These can be challenging data to collect for many cryptic species, particularly those that live in complex structural environments. This study introduces a new automated method for detection using published object detection algorithms to detect their heat signatures in RPAS-derived thermal imaging. As an initial case study we used this new approach to detect koalas (Phascolarctus cinereus), and validated the approach using ground surveys of tracked radio-collared koalas in Petrie, Queensland. The automated method yielded a higher probability of detection (68–100%), higher precision (43–71%), lower root mean square error (RMSE), and lower mean absolute error (MAE) than manual assessment of the RPAS-derived thermal imagery in a comparable amount of time. This new approach allows for more reliable, less invasive detection of koalas in their natural habitat. This new detection methodology has great potential to inform and improve management decisions for threatened species, and other difficult to survey species.

Seventy Years of Chlamydia Vaccine Research - Limitations of the Past and Directions for the Future

Chlamydia is a major bacterial pathogen that infects humans, as well as a wide range of animals, including marsupials, birds, cats, pigs, cattle, and sheep. Antibiotics are the only treatment currently available, however, with high rates of re-infection, there is mounting pressure to develop Chlamydia vaccines. In this review, we analyzed how Chlamydia vaccine trials have developed over the past 70 years and identified where future trials need to be focused. There has been a strong bias toward studies targeting C. muridarum and C. trachomatis within mice and a lack of studies matching chlamydial species to their end target host. Even though a large number of specific antigenic targets have been studied, the results from whole-cell vaccine targets show slightly more promising results overall. There has also been a strong bias toward systemic vaccine delivery systems, despite the finding that mucosal delivery systems have shown more promising outcomes. However, the only successful vaccines with matched chlamydial species/infecting host are based on systemic vaccine delivery methods. We highlight the extensive work done with mouse model trials and indicate that whole cell antigenic targets are capable of inducing an effective response, protecting from disease and reducing shedding rates. However, replication of these results using antigen preparations more conducive to commercial vaccine production has proven difficult. To date, the Major Outer Membrane Protein (MOMP) has emerged as the most suitable substitute for whole cell targets and its delivery as a combined systemic and mucosal vaccine is most effective. Finally, although mouse model trials are useful, differences between hosts and infecting chlamydial strains are preventing vaccine formulations from mouse models to be translated into larger animals or intended hosts.

Therapeutic effect of a Chlamydia pecorum recombinant major outer membrane protein vaccine on ocular disease in koalas (Phascolarctos cinereus)

Chlamydia pecorum is responsible for causing ocular infection and disease which can lead to blindness in koalas (Phascolarctos cinereus). Antibiotics are the current treatment for chlamydial infection and disease in koalas, however, they can be detrimental for the koala’s gastrointestinal tract microbiota and in severe cases, can lead to dysbiosis and death. In this study, we evaluated the therapeutic effects provided by a recombinant chlamydial major outer membrane protein (MOMP) vaccine on ocular disease in koalas. Koalas with ocular disease (unilateral or bilateral) were vaccinated and assessed for six weeks, evaluating any changes to the conjunctival tissue and discharge. Samples were collected pre- and post-vaccination to evaluate both humoral and cell-mediated immune responses. We further assessed the infecting C. pecorum genotype, host MHC class II alleles and presence of koala retrovirus type (KoRV-B). Our results clearly showed an improvement in the clinical ocular disease state of all seven koalas, post-vaccination. We observed increases in ocular mucosal IgA antibodies to whole C. pecorum elementary bodies, post-vaccination. We found that systemic cell-mediated immune responses to interferon-γ, interleukin-6 and interleukin-17A were not significantly predictive of ocular disease in koalas. Interestingly, one koala did not have as positive a clinical response (in one eye primarily) and this koala was infected with a C. pecorum genotype (E’) that was not used as part of the vaccine formula (MOMP genotypes A, F and G). The predominant MHC class II alleles identified were DAb*19, DAb*21 and DBb*05, with no two koalas identified with the same genetic sequence. Additionally, KoRV-B, which is associated with chlamydial disease outcome, was identified in two (29%) ocular diseased koalas, which still produced vaccine-induced immune responses and clinical ocular improvements post-vaccination. Our findings show promise for the use of a recombinant chlamydial MOMP vaccine for the therapeutic treatment of ocular disease in koalas.

Optimising the short and long-term clinical outcomes for koalas (Phascolarctos cinereus) during treatment for chlamydial infection and disease

Koalas (Phascolarctos cinereus) have suffered severe declines in the northern extent of their range due to a variety of threats, including habitat destruction, trauma from cars and dogs, climate change and importantly, disease. The most significant pathogen in koalas is Chlamydia pecorum, which causes inflammation and fibrosis at mucosal sites, resulting in blindness, infertility and death in severe cases. Chlamydia treatment can be problematic in koalas as the response to treatment is often poor in chronic cases and antimicrobial choice is limited. Thus, chlamydial disease is a severely threatening process for koala conservation. We investigated the short and long-term clinical outcomes for 167 koalas with Chlamydia that underwent capture, telemetric monitoring and intensive veterinary management as part of a large-scale population management program in South East Queensland. Chlamydia treatments included the standard regimen of daily subcutaneous chloramphenicol injections (60mg/kg) for 14 to 28-days, and a variety of non-standard regimens such as topical antimicrobials only (for ocular disease), surgical treatment only (for bilateral reproductive tract disease), and other antimicrobials/treatment lengths. To assess these regimens we analysed clinical records, field monitoring data and swab samples collected from the urogenital tract and ocular conjunctiva. Overall, in contrast to other studies, treatment was generally successful with 86.3% of treated koalas released back into the wild. The success of treatment rose to 94.8% however, when the standard treatment regimen was employed. Further, 100% of koalas that were also treated with surgical ovariohysterectomy (n = 12) remained healthy for a median of 466 days of post-treatment monitoring, demonstrating the benefits of surgical treatment. Previous studies reported 45-day chloramphenicol regimens, but the shorter standard regimen still achieved microbiological cure and reduces the risk of negative sequelae associated with treatment and/or captivity and treatment costs. Despite these positive clinical outcomes, alternatives to chloramphenicol are warranted due to its decreasing availability.

Induction of neutralizing antibody response against koala retrovirus (KoRV) and reduction in viral load in koalas following vaccination with recombinant KoRV envelope protein

Koala retrovirus (KoRV) infects the majority of Australia’s koalas (Phascolarctos cinereus) and has been linked to several life-threatening diseases such as lymphoma and leukemia, as well as Chlamydia and thus poses a threat to the continued survival of this species. While quarantine and antiretroviral drug treatment are possible control measures, they are impractical, leaving vaccination as the only realistic option. In this study, we examined the effect of a recombinant envelope protein-based anti-KoRV vaccine in two groups of South Australian koalas: KoRV infected or KoRV free. We report a successful vaccination response in the koalas with no vaccine-associated side effects. The vaccine induced a significant humoral immune response as well as the production of neutralizing antibodies in both groups of koalas. We also identified B-cell epitopes that were differentially recognized in KoRV-infected versus KoRV-free koalas following vaccination. Importantly, we also showed that vaccination had a therapeutic effect on koalas infected exogenously with KoRV by reducing their circulating viral load. Together, this study highlights the possibility of successfully developing a vaccine against KoRV infection in koalas.

A vaccine candidate for Koala retrovirus elicits a protective antibody response and reduces the viral load in already-infected koalas. Koala retrovirus (KoRV), first identified in the last 20 years, is a life-threatening, endemic pathogen affecting Australian koalas. In pursuit of an effective KoRV vaccine, the University of the Sunshine Coast’s Peter Timms led a group of Australian scientists to develop a candidate based on the transmembrane section of the virus’ envelope protein. The six koalas vaccinated in the study all generated a strong antibody response to the envelope protein, and a strong neutralizing antibody response was reported during in vitro tests. Vaccinated koalas with pre-existing KoRV infection benefited from an average 79% reduction in viral load when measured 12 weeks after vaccination. Further research should be prioritized to provide much-needed protection to Australia’s koalas.

Vaccination of koalas (Phascolarctos cinereus) against Chlamydia pecorum using synthetic peptides derived from the major outer membrane protein

Chlamydia pecorum is a mucosal infection, which causes debilitating disease of the urinary tract, reproductive tract and ocular sites of koalas (Phascolarctos cinereus). While antibiotics are available for treatment, they are detrimental to the koalas' gastrointestinal tract microflora leaving the implementation of a vaccine as an ideal option for the long-term management of koala populations. We have previously reported on the successes of an anti-chlamydial recombinant major outer membrane protein (rMOMP) vaccine however, recombinant protein based vaccines are not ideal candidates for scale up from the research level to small-medium production level for wider usage. Peptide based vaccines are a promising area for vaccine development, because peptides are stable, cost effective and easily produced. In this current study, we assessed, for the first time, the immune responses to a synthetic peptide based anti-chlamydial vaccine in koalas. Five healthy male koalas were vaccinated with two synthetic peptides derived from C. pecorum MOMP and another five healthy male koalas were vaccinated with full length recombinant C. pecorum MOMP (genotype G). Systemic (IgG) and mucosal (IgA) antibodies were quantified and pre-vaccination levels compared to post-vaccination levels (12 and 26 weeks). MOMP-peptide vaccinated koalas produced Chlamydia-specific IgG and IgA antibodies, which were able to recognise not only the genotype used in the vaccination, but also MOMPs from several other koala C. pecorum genotypes. In addition, IgA antibodies induced at the ocular site not only recognised recombinant MOMP protein but also, whole native chlamydial elementary bodies. Interestingly, some MOMP-peptide vaccinated koalas showed a stronger and more sustained vaccine-induced mucosal IgA antibody response than observed in MOMP-protein vaccinated koalas. These results demonstrate that a synthetic MOMP peptide based vaccine is capable of inducing a Chlamydia-specific antibody response in koalas and is a promising candidate for future vaccine development.

Prevalence of Chlamydia pecorum in Juvenile Koalas ( Phascolarctos cinereus) and Evidence for Protection from Infection via Maternal Immunization

Chlamydia pecorum in koalas ( Phascolarctos cinereus) is considered a sexually transmitted infection. Analysis of samples from koala joeys (<1 yr) suggested that mother-to-young direct transmission was also occurring. Further, evidence suggested that joeys from vaccinated mothers were less likely to contract infections than joeys with unvaccinated mothers.

Koala immunology and infectious diseases: How much can the koala bear?

Infectious diseases are contributing to the decline of the iconic Australian marsupial, the koala (Phascolarctos cinereus). Infections with the obligate intracellular bacteria, Chlamydia pecorum, cause debilitating ocular and urogenital-tract disease while the koala-retrovirus (KoRV) has been implicated in host immunosuppression and exacerbation of chlamydial pathogenesis. Although histological studies have provided insight into the basic architecture of koala immune tissues, our understanding of the koala immune response to infectious disease has been limited, until recently, by a lack of species-specific immune reagents. Recent advances in the characterisation of key immune genes have focused on advancing our understanding of the immune response to Chlamydia infection, revealing commonalities in disease pathologies and immunity between koalas and other hosts and paving the way for the development of a koala Chlamydia vaccine. This review summarises these recent findings and highlights key aspects of the koala immune system requiring further attention with particular regard to their most prominent infectious diseases.

Immunization Strategies Producing a Humoral IgG Immune Response against Devil Facial Tumor Disease in the Majority of Tasmanian Devils Destined for Wild Release

Devil facial tumor disease (DFTD) is renowned for its successful evasion of the host immune system. Down regulation of the major histocompatabilty complex class I molecule (MHC-I) on the DFTD cells is a primary mechanism of immune escape. Immunization trials on captive Tasmanian devils have previously demonstrated that an immune response against DFTD can be induced, and that immune-mediated tumor regression can occur. However, these trials were limited by their small sample sizes. Here, we describe the results of two DFTD immunization trials on cohorts of devils prior to their wild release as part of the Tasmanian Government’s Wild Devil Recovery project. 95% of the devils developed anti-DFTD antibody responses. Given the relatively large sample sizes of the trials (N = 19 and N = 33), these responses are likely to reflect those of the general devil population. DFTD cells manipulated to express MHC-I were used as the antigenic basis of the immunizations in both trials. Although the adjuvant composition and number of immunizations differed between trials, similar anti-DFTD antibody levels were obtained. The first trial comprised DFTD cells and the adjuvant combination of ISCOMATRIX™, polyIC, and CpG with up to four immunizations given at monthly intervals. This compared to the second trial whereby two immunizations comprising DFTD cells and the adjuvant combination ISCOMATRIX™, polyICLC (Hiltonol^®) and imiquimod were given a month apart, providing a shorter and, therefore, more practical protocol. Both trials incorporated a booster immunization given up to 5 months after the primary course. A key finding was that devils in the second trial responded more quickly and maintained their antibody levels for longer compared to devils in the first trial. The different adjuvant combination incorporating the RNAase resistant polyICLC and imiquimod used in the second trial is likely to be responsible. The seroconversion in the majority of devils in these anti-DFTD immunization trials was remarkable, especially as DFTD is hallmarked by its immune evasion mechanisms. Microsatellite analyzes of MHC revealed that some MHC-I microsatellites correlated to stronger immune responses. These trials signify the first step in the long-term objective of releasing devils with immunity to DFTD into the wild.

Humoral immune response against two surface antigens of Chlamydia pecorum in vaccinated and naturally infected sheep

Chlamydia pecorum is a globally recognised livestock pathogen due to the significant clinical and economic impact it poses to livestock producers. Routine serological diagnosis is through a complement fixation test (CFT), which is often criticised for cross-reactivity, poor sensitivity and specificity. Although serology remains the preferred method in veterinary diagnostic laboratories, serological assays based on surface antigens of C. pecorum have not been established until now. In this study, we evaluated the use of two chlamydial recombinant protein antigens (PmpG and MOMP-G) by a direct IgG ELISA method for detection of ovine anti-chlamydial antibodies. Using the Pepscan method we then identified B cell epitopes across PmpG and MOMP-G proteins, in lambs with (a) naturally occurring asymptomatic C. pecorum infections (b) C. pecorum-associated polyarthritis and (c) recombinant PmpG and MOMP-G vaccine. Plasma IgG antibodies to PmpG in natural infection of lambs were detected earlier in infection than CFT and served as an acute phase marker. Antibodies to MOMP-G IgG were significantly heightened in lambs with C. pecorum-associated polyarthritis. PmpG and MOMP-G specific B-cell epitope mapping revealed epitope responses in immunised lambs cluster with some of the epitope responses in naturally infected lambs. B-cell epitope mapping further revealed that lambs with polyarthritis recognised several unique PmpG (50% frequency, peptide 8, 25, 40, 41 and 50) and MOMP (50% frequency, peptide 50) epitopes in comparison to asymptomatic infections. The findings of this study will have implications towards improved serodiagnosis of C. pecorum infections in livestock and inform the downstream development of alternative peptide-based antigens for future C. pecorum vaccine studies.

Dendrimer-conjugated peptide vaccine enhances clearance of Chlamydia trachomatis genital infection

Peptide-based vaccines have emerged in recent years as promising candidates in the prevention of infectious diseases. However, there are many challenges to maintaining in vivo peptide stability and enhancement of peptide immunogenicity to generate protective immunity which enhances clearance of infections. Here, a dendrimer-based carrier system is proposed for peptide-based vaccine delivery, and shows its anti-microbial feasibility in a mouse model of Chlamydia trachomatis. Chlamydiae are the most prevalent sexually transmitted bacteria worldwide, and also the causal agent of trachoma, the leading cause of preventable infectious blindness. In spite of the prevalence of this infectious agent and the many previous vaccine-related studies, there is no vaccine commercially available. The carrier system proposed consists of generation 4, hydroxyl-terminated, polyamidoamine (PAMAM) dendrimers (G4OH), to which a peptide mimic of a chlamydial glycolipid antigen-Peptide 4 (Pep4, AFPQFRSATLLL) was conjugated through an ester bond. The ester bond between G4OH and Pep4 is expected to break down mainly in the intracellular environment for antigen presentation. Pep4 conjugated to dendrimer induced Chlamydia-specific serum antibodies after subcutaneous immunizations. Further, this new vaccine formulation significantly protected immunized animals from vaginal challenge with infectious Chlamydia trachomatis, and it reduced infectious loads and tissue (genital tract) damage. Pep4 conjugated to G4OH or only mixed with peptide provided enhanced protection compared to Pep4 and adjuvant (i.e. alum), suggesting a potential adjuvant effect of the PAMAM dendrimer. Combined, these results demonstrate that hydroxyl-terminated PAMAM dendrimer is a promising polymeric nanocarrier platform for the delivery of peptide vaccines and this approach has potential to be expanded to other infectious intracellular bacteria and viruses of public health significance.

Microbiota composition of the koala (Phascolarctos cinereus) ocular and urogenital sites, and their association with Chlamydia infection and disease

Disease caused by Chlamydia pecorum is characterised by ocular and urogenital infections that can lead to blindness and infertility in koalas. However, koalas that are infected with C. pecorum do not always progress to disease. In other host systems, the influence of the microbiota has been implicated in either accelerating or preventing infections progressing to disease. This study investigates the contribution of koala urogenital and ocular microbiota to Chlamydia infection and disease in a free ranging koala population. Using univariate and multivariate analysis, it was found that reproductive status in females and sexual maturation in males, were defining features in the koala urogenital microbiota. Changes in the urogenital microbiota of koalas is correlated with infection by the common pathogen, C. pecorum. The correlation of microbiota composition and C. pecorum infection is suggestive of members of the microbiota being involved in the acceleration or prevention of infections progressing to disease. The analysis also suggests that multiple microbes are likely to be associated with this process of disease progression, rather than a single organism. While other Chlamydia-like organisms were also detected, they are unlikely to contribute to chlamydial disease as they are rare members of the urogenital and ocular microbiota communities.

Infection with koala retrovirus subgroup B (KoRV-B), but not KoRV-A, is associated with chlamydial disease in free-ranging koalas (Phascolarctos cinereus)

The virulence of chlamydial infection in wild koalas is highly variable between individuals. Some koalas can be infected (PCR positive) with Chlamydia for long periods but remain asymptomatic, whereas others develop clinical disease. Chlamydia in the koala has traditionally been studied without regard to coinfection with other pathogens, although koalas are usually subject to infection with koala retrovirus (KoRV). Retroviruses can be immunosuppressive, and there is evidence of an immunosuppressive effect of KoRV in vitro. Originally thought to be a single endogenous strain, a new, potentially more virulent exogenous variant (KoRV-B) was recently reported. We hypothesized that KoRV-B might significantly alter chlamydial disease outcomes in koalas, presumably via immunosuppression. By studying sub-groups of Chlamydia and KoRV infected koalas in the wild, we found that neither total KoRV load (either viraemia or proviral copies per genome), nor chlamydial infection level or strain type, was significantly associated with chlamydial disease risk. However, PCR positivity with KoRV-B was significantly associated with chlamydial disease in koalas (p = 0.02961). This represents an example of a recently evolved virus variant that may be predisposing its host (the koala) to overt clinical disease when co-infected with an otherwise asymptomatic bacterial pathogen (Chlamydia).

Chlamydial infections in wildlife-conservation threats and/or reservoirs of 'spill-over' infections?

Members of the order Chlamydiales are biphasic intracellular pathogens known to cause disease in both humans and animals. As we learn more about the genetic diversity of this group of pathogens, evidence is growing that these bacteria infect a broader range of animal hosts than previously thought. Over 400 host species are now documented globally with the majority of these being wild animals. Given the impact of chlamydial infections on humans and domesticated animals, the identification of members of the order Chlamydiales in wildlife raises significant questions over a) their impact on animal health and b) the relationships to those strains also found in humans and domestic animals. In some species such as the iconic marsupial, the koala, the conservation impact is known with chlamydial infections associated with debilitating disease, however, in general, little is known about the pathogenic potential of Chlamydiae infecting most wildlife hosts. Accumulating evidence suggests contact with wild animals is a risk factor for infections in domestic animals and/or humans. Beyond the well-recognised zoonotic pathogen, Chlamydia psittaci, a range of studies have now reported traditional pathogens in the family Chlamydiaceae such as Chlamydia pecorum, Chlamydia suis, Chlamydia pneumoniae and Chlamydia abortus in wild animals. The spectre of cross-host transmission 'spill-over' and 'spill-back' in the epidemiology of infections is of potential concern, however, comprehensive epidemiological studies are lacking for most of these. Accurate evaluation of the significance of chlamydial infections in wildlife is otherwise hampered by i) the cross-sectional nature of most impact studies, ii) a lack of standardised diagnostic approaches, iii) limited study sizes, and iv) biases associated with opportunistic sampling.

Treatment of Chlamydia-associated ocular disease via a recombinant protein based vaccine in the koala (Phascolarctos cinereus)

Koalas (Phascolarctos cinereus) are affected by debilitating chlamydial disease that can lead to blindness, infertility, and death. The causative agent is the intracellular bacterium Chlamydia pecorum. While antibiotics can be used to treat koala chlamydial infection, they are often ineffective or cause severe dysbiosis to the animal's unique gut flora. Recent work has progressed on the development of a protective vaccine for Chlamydia in the koala. This study demonstrates that the use of a vaccine can have a positive effect in koalas already with clinical signs of ocular disease, suggesting a possible therapeutic effect and an alternative to antibiotic therapy.

Antibody and Cytokine Responses of Koalas (Phascolarctos cinereus) Vaccinated with Recombinant Chlamydial Major Outer Membrane Protein (MOMP) with Two Different Adjuvants

Developing a vaccine against Chlamydia is key to combating widespread mortalities and morbidities associated with this infection in koalas (Phascolarctos cinereus). In previous studies, we have shown that two or three doses of a Recombinant Major Outer Membrane Protein (rMOMP) antigen-based vaccine, combined with immune stimulating complex (ISC) adjuvant, results in strong cellular and humoral immune responses in koalas. We have also separately evaluated a single dose vaccine, utilising a tri-adjuvant formula that comprises polyphosphazine based poly I: C and host defense peptides, with the same antigen. This formulation also produced strong cellular and humoral immune responses in captive koalas. In this current study, we directly compared the host immune responses of two sub-groups of wild Chlamydia negative koalas in one population vaccinated with the rMOMP protein antigen and adjuvanted with either the ISC or tri-adjuvant formula. Overall, both adjuvants produced strong Chlamydia-specific cellular (IFN-γ and IL-17A) responses in circulating PBMCs as well as MOMP-specific and functional, in vitro neutralising antibodies. While the immune responses were similar, there were adjuvant-specific immune differences between the two adjuvants, particularly in relation to the specificity of the MOMP epitope antibody responses.