Koala retrovirus load and non-A subtypes are associated with secondary disease among wild northern koalas

A high frequency of detection of koala retrovirus fragments in Victorian koalas suggests historic integration of KoRV

Recombinant koala retrovirus (recKoRV) is a recently discovered variant of koala retrovirus (KoRV), which likely emerged due to recombination with another retrovirus (such as Phascolarctos endogenous retrovirus). KoRV spread and endogenization in Australia were thought to be ongoing in a north to south direction given the low prevalence of the virus in southern koala populations, based on molecular detection of the pol gene. However, recKoRV has highlighted that fragments of KoRV with the pol region missing are present within southern koalas. In this study, a new 5'-region-based KoRV PCR assay was developed, capable of detecting both intact KoRV and all known variants of recKoRV. Using this assay, 319 archived DNA samples from 287 Victorian koalas were retested to investigate KoRV endogenization. We found 98.3% (282/287) of these samples were positive for the KoRV-5' fragment, the majority of which were KoRV-pol negative (222/287) on prior testing. Our findings demonstrate extensive KoRV integration into the Victorian koala populations, suggestive of a historic presence of KoRV in Victorian koalas. This finding makes biological sense relative to the translocation history of Victorian koalas, compared to the prior paradigm of low virus prevalence, and provides new epidemiological and practical management implications.

Correlation Between Cryptococcus Infection and the Nasal Mycobiota in a Population of Free-Ranging Koalas (Phascolarctos cinereus) in New South Wales, Australia

Cryptococcosis is a fungal disease in humans and animals, caused by the Cryptococcus neoformans and Cryptococcus gattii species complexes. Clinical cryptococcosis primarily manifests as upper respiratory tract disease; however, dissemination to other organs, particularly the brain, can occur. Nasal colonisation and subclinical cryptococcosis are common in koalas (Phascolarctos cinereus) due to their shared environmental niche with Cryptococcus: Eucalyptus trees. However, for reasons that remain unclear, the prevalence of clinical disease is low in koalas. Interactions between respiratory pathogens and the nasal mycobiome are thought to play a role in the development and progression of numerous respiratory diseases. As such, this study aimed to characterise the mycobiome of the nasal vestibule in koalas with and without evidence of cryptococcal colonisation and subclinical disease via the next-generation sequencing (NGS) of the ITS1 region of the fungal internal transcribed spacer (ITS) gene. Samples were collected from 47 koalas from a population of free-ranging koalas in the Liverpool Plains, NSW, Australia, with a known history of Cryptococcus exposure and nasal colonisation. Of the 47 animals tested, 6.4% were culture-positive only, 4.3% were seropositive only, and 2.1% were culture- and seropositive. C. gattii was detected in four samples via NGS. C. neoformans was not detected via NGS. There were no significant differences in the nasal mycobiomes of Cryptococcus-positive and -negative animals; thus, we could not establish a definitive association between the mycobiome and infection outcomes. We identified a number of fungal genera that were significantly more abundant in samples from Cryptococcus-positive animals, but there was no apparent relationship between these genera and the development of cryptococcosis. This study represents the first investigation of the nasal mycobiota of wild koalas. Further studies involving koalas with clinical disease are necessary to determine the role of the nasal mycobiota in the development of cryptococcosis.

A preliminary study of gene expression changes in Koalas Infected with Koala Retrovirus (KoRV) and identification of potential biomarkers for KoRV pathogenesis

BACKGROUND

Koala retrovirus (KoRV), a major pathogen of koalas, exists in both endogenous (KoRV-A) and exogenous forms (KoRV-A to I and K to M) and causes multiple disease phenotypes, including carcinomas and immunosuppression. However, the direct association between the different KoRV subtypes and carcinogenesis remains unknown. Differentially expressed gene (DEG) analysis of peripheral blood mononuclear cells (PBMCs) of koalas carrying both endogenous (KoRV-A) and exogenous (KoRV-A, B, and C) subtypes was performed using a high-throughput RNA-seq approach. PBMCs were obtained from three healthy koalas: one infected with endogenous (KoRV-A; Group I) and two infected with exogenous (KoRV-B and/or KoRV-C; Group II) subtypes. Additionally, spleen samples (n = 6) from six KoRV-infected deceased koalas (K1- K6) and blood samples (n = 1) from a live koala (K7) were collected and examined to validate the findings.

RESULTS

All koalas were positive for the endogenous KoRV-A subtype, and eight koalas were positive for KoRV-B and/or KoRV-C. Transcription of KoRV gag, pol, and env genes was detected in all koalas. Upregulation of cytokine and immunosuppressive genes was observed in koalas infected with KoRV-B or KoRV-B and -C subtypes, compared to koalas infected with only KoRV-A. We found 550 DEG signatures with significant (absolute p < 0.05, and absolute log2 Fold Change (FC) > 1.5) dysregulation, out of which 77.6% and 22.4% DEGs were upregulated (log2FC > 1.5) and downregulated (log2FC <  - 1.5), and downregulated (log2 FC <  - 1), respectively. We identified 17 unique hub genes (82.3% upregulated and 17.7% down-regulated), with KIF23, CCNB2, POLR3F, and RSL24D1 detected as the potential hub genes modified with KoRV infection. Real-time RT-qPCR was performed on seven koalas to ascertain the expression levels of four potential hub genes, which were subsequently normalized to actin copies. Notably, all seven koalas exhibited distinct expression signatures for the hub genes, especially, KIF23 and CCNB2 show the highest expression in healthy koala PBMC, and POLR3F shows the highest expression in koala with lymphoma (K1).

CONCLUSION

Thus, it can be concluded that multiple KoRV subtypes affect disease progression in koalas and that the predicted hub genes could be promising prognostic biomarkers for pathogenesis.

Koala retrovirus and neoplasia: correlation and underlying mechanisms

The koala retrovirus, KoRV, is one of the few models for understanding the health consequences of retroviral colonization of the germline. Such colonization events transition exogenous infectious retroviruses to Mendelian traits or endogenous retroviruses (ERVs). KoRV is currently in a transitional state from exogenous retrovirus to ERV, which in koalas (Phascolarctos cinereus) has been associated with strongly elevated levels of neoplasia. In this review, we describe what is currently known about the associations and underlying mechanisms of KoRV-induced neoplasia.

GALV-KoRV-related retroviruses in diverse Australian and African rodent species

The enigmatic origins and transmission events of the gibbon ape leukemia virus (GALV) and its close relative the koala retrovirus (KoRV) have been a source of enduring debate. Bats and rodents are each proposed as major reservoirs of interspecies transmission, with ongoing efforts to identify additional animal hosts of GALV-KoRV-related retroviruses. In this study, we identified nine rodent species as novel hosts of GALV-KoRV-related retroviruses. Included among these hosts are two African rodents, revealing the first appearance of this clade beyond the Australian and Southeast Asian region. One of these African rodents, Mastomys natalensis, carries an endogenous GALV-KoRV-related retrovirus that is fully intact and potentially still infectious. Our findings support the hypothesis that rodents are the major carriers of GALV-KoRV-related retroviruses.

Wildlife endogenous retroviruses: colonization, consequences, and cooption

Endogenous retroviruses (ERVs) are inherited genomic remains of past germline retroviral infections. Research on human ERVs has focused on medical implications of their dysregulation on various diseases. However, recent studies incorporating wildlife are yielding remarkable perspectives on long-term retrovirus-host interactions. These initial forays into broader taxonomic analysis, including sequencing of multiple individuals per species, show the incredible plasticity and variation of ERVs within and among wildlife species. This demonstrates that stochastic processes govern much of the vertebrate genome. In this review, we elaborate on discoveries pertaining to wildlife ERV origins and evolution, genome colonization, and consequences for host biology.

Diversity and transmission of koala retrovirus: a case study in three captive koala populations

Koala retrovirus is a recently endogenized retrovirus associated with the onset of neoplasia and infectious disease in koalas. There are currently twelve described KoRV subtypes (KoRV-A to I, K–M), most of which were identified through recently implemented deep sequencing methods which reveal an animals’ overall KoRV profile. This approach has primarily been carried out on wild koala populations around Australia, with few investigations into the whole-population KoRV profile of captive koala colonies to date. This study conducted deep sequencing on 64 captive koalas of known pedigree, housed in three institutions from New South Wales and South-East Queensland, to provide a detailed analysis of KoRV genetic diversity and transmission. The final dataset included 93 unique KoRV sequences and the first detection of KoRV-E within Australian koala populations. Our analysis suggests that exogenous transmission of KoRV-A, B, D, I and K primarily occurs between dam and joey. Detection of KoRV-D in a neonate sample raises the possibility of this transmission occurring in utero. Overall, the prevalence and abundance of KoRV subtypes was found to vary considerably between captive populations, likely due to their different histories of animal acquisition. Together these findings highlight the importance of KoRV profiling for captive koalas, in particular females, who play a primary role in KoRV exogenous transmission.

Molecular Insights into Innate Immune Response in Captive Koala Peripheral Blood Mononuclear Cells Co-Infected with Multiple Koala Retrovirus Subtypes

Koala retrovirus (KoRV) exists in both endogenous and exogenous forms and has appeared as a major threat to koala health and conservation. Currently, there are twelve identified KoRV subtypes: an endogenous subtype (KoRV-A) and eleven exogenous subtypes (KoRV-B to -I, KoRV-K, -L, and -M). However, information about subtype-related immune responses in koalas against multiple KoRV infections is limited. In this study, we investigated KoRV-subtype (A, B, C, D, and F)-related immunophenotypic changes, including CD4, CD8b, IFN-γ, IL-6, and IL-10 mRNA expression, in peripheral blood mononuclear cells (PBMCs) obtained from captive koalas (n = 37) infected with multiple KoRV subtypes (KoRV-A to F) reared in seven Japanese zoos. Based on KoRV subtype infection profiles, no significant difference in CD4 and CD8b mRNA expression was observed in the study populations. Based on the different KoRV subtype infections, we found that the IFN-γ mRNA expression in koala PMBCs differs insignificantly (p = 0.0534). In addition, IL-6 and IL-10 mRNA expression also did not vary significantly in koala PBMCs based on KoRV subtype differences. We also investigated the Toll-like receptors (TLRs) response, including TLR2–10, and TLR13 mRNA in koala PBMCs infected with multiple KoRV subtypes. Significant differential expression of TLR5, 7, 9, 10, and 13 mRNA was observed in the PBMCs from koalas infected with different KoRV subtypes. Therefore, based on the findings of this study, it is assumed that co-infection of multiple KoRV subtypes might modify the host innate immune response, including IFN-γ and TLRs responses. However, to have a more clear understanding regarding the effect of multiple KoRV subtypes on host cytokines and TLR response and pathogenesis, further large-scale studies including the koalas negative for KoRV and koalas infected with other KoRV subtypes (KoRV-A to -I, KoRV-K, -L and -M) are required.