Retroviral invasion of the koala genome

Contrasting segregation patterns among endogenous retroviruses across the koala population

Koalas (Phascolarctos cinereus) have experienced a history of retroviral epidemics leaving their trace as heritable endogenous retroviruses (ERVs) in their genomes. A recently identified ERV lineage, named phaCin-β, shows a pattern of recent, possibly current, activity with high insertional polymorphism in the population. Here, we investigate geographic patterns of three focal ERV lineages of increasing estimated ages, from the koala retrovirus (KoRV) to phaCin-β and to phaCin-β-like, using the whole-genome sequencing of 430 koalas from the Koala Genome Survey. Thousands of ERV loci were found across the population, with contrasting patterns of polymorphism. Northern individuals had thousands of KoRV integrations and hundreds of phaCin-β ERVs. In contrast, southern individuals had higher phaCin-β frequencies, possibly reflecting more recent activity and a founder effect. Overall, our findings suggest high ERV burden in koalas, reflecting historic retrovirus-host interactions. Importantly, the ERV catalogue supplies improved markers for conservation genetics in this endangered species.

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.

Ribosomal profiling of human endogenous retroviruses in healthy tissues

Human endogenous retroviruses (HERVs) are the germline embedded proviral fragments of ancient retroviral infections that make up roughly 8% of the human genome. Our understanding of HERVs in physiology primarily surrounds their non-coding functions, while their protein coding capacity remains virtually uncharacterized. Therefore, we applied the bioinformatic pipeline "hervQuant" to high-resolution ribosomal profiling of healthy tissues to provide a comprehensive overview of translationally active HERVs. We find that HERVs account for 0.1-0.4% of all translation in distinct tissue-specific profiles. Collectively, our study further supports claims that HERVs are actively translated throughout healthy tissues to provide sequences of retroviral origin to the human proteome.

Investigation of koala retrovirus in captive koalas with pneumonia and comparative analysis of subtype distribution

This study focused on the involvement of koala retrovirus (KoRV) in pneumonia in koalas. Three deceased pneumonic koalas from a Japanese zoo were examined in this study. Hematological and histopathological findings were assessed, and KoRV proviral DNA loads in the blood and tissues were compared with those of eight other KoRV-infected koalas from different zoos. Demographic data and routine blood profiles were collected, and blood and tissue samples were analyzed to rule out concurrent infections in pneumonic koalas. KoRV subtyping and measurement of the KoRV proviral DNA load were performed by polymerase chain reaction (PCR) using specific primers targeting the pol and env genes. The results showed that the koalas had histopathologically suppurative and fibrinous pneumonia. Chlamydiosis was not detected in any of the animals. PCR analysis revealed KoRV-A, -B, and -C infections in all koalas, except for animals K10-11, which lacked KoRV-B. Significant variations in the proviral DNA loads of these KoRV subtypes were observed in all tissues and disease groups. Most tissues showed reduced KoRV loads in koalas with pneumonia, except in the spleen, which had significantly higher loads of total KoRV (2.54 × 107/µg DNA) and KoRV-A (4.74 × 107/µg DNA), suggesting potential immunosuppression. This study revealed the intricate dynamics of KoRV in various tissues, indicating its potential role in koala pneumonia via immunosuppression and opportunistic infections. Analysis of the levels of KoRV proviral DNA in different tissues will shed light on viral replication and the resulting pathogenesis in future studies.

Themes and variations on piRNA-guided transposon control

PIWI-interacting RNAs (piRNAs) are responsible for preventing the movement of transposable elements in germ cells and protect the integrity of germline genomes. In this review, we examine the common elements of piRNA-guided silencing as well as the differences observed between species. We have categorized the mechanisms of piRNA biogenesis and function into modules. Individual PIWI proteins combine these modules in various ways to produce unique PIWI-piRNA pathways, which nevertheless possess the ability to perform conserved functions. This modular model incorporates conserved core mechanisms and accommodates variable co-factors. Adaptability is a hallmark of this RNA-based immune system. We believe that considering the differences in germ cell biology and resident transposons in different organisms is essential for placing the variations observed in piRNA biology into context, while still highlighting the conserved themes that underpin this process.

Identification of a syncytin gene in a non-rodent laboratory mammal, Suncus murinus

An endogenous retrovirus-derived membrane protein, syncytin (SYN), contributes to placental function via trophoblast fusion. Multinuclear trophoblasts (syncytiotrophoblasts) physically and functionally mediate the interaction between fetal and maternal vessels in various ways. Suncus murinus (suncus) is a small mammalian species with a pregnancy duration of approximately 30 days, 1.5 times longer than mice. However, the molecular basis for the longer pregnancy duration is unknown. In this study, we first isolated two genes that encoded putative SYN proteins expressed in the suncus placenta, which were named syncytin-1-like proteins 1 and 2 (SYN1L1 and SYN1L2). When their expression vectors were introduced into cultured cells, suncus SYN1L2 was found to be active in cell fusion. Moreover, the SYN1L2 protein was homologous to a SYN1-like protein identified in greater mouse-eared bats (bat SYN1L) and was structurally compared with bat SYN1L and other SYN proteins, implying the presence of structural features of the SYN1L2 protein.

Genome mining shows that retroviruses are pervasively invading vertebrate genomes

Endogenous retroviruses (ERVs) record past retroviral infections, providing molecular archives for interrogating the evolution of retroviruses and retrovirus-host interaction. However, the vast majority of ERVs are not active anymore due to various disruptive mutations, and ongoing retroviral invasion of vertebrate genomes has been rarely documented. Here we analyze genomics data from 2004 vertebrates for mining invading ERVs (ERVi). We find that at least 412 ERVi elements representing 217 viral operational taxonomic units are invading the genomes of 123 vertebrates, 18 of which have been assessed to be threatened species. Our results reveal an unexpected prevalence of ongoing retroviral invasion in vertebrates and expand the diversity of retroviruses recently circulating in the wild. We characterize the pattern and nature of ERVi in the historical and biogeographical context of their hosts, for instance, the generation of model organisms, sympatric speciation, and domestication. We suspect that these ERVi are relevant to conservation of threatened species, zoonoses in the wild, and emerging infectious diseases in humans.

Transposable elements drive the evolution of metazoan zinc finger genes

Cys2-His2 zinc finger genes (ZNFs) form the largest family of transcription factors in metazoans. ZNF evolution is highly dynamic and characterized by the rapid expansion and contraction of numerous subfamilies across the animal phylogeny. The forces and mechanisms underlying rapid ZNF evolution remain poorly understood, but there is growing evidence that, in tetrapods, the targeting and repression of lineage-specific transposable elements (TEs) plays a critical role in the evolution of the Krüppel-associated box ZNF (KZNF) subfamily. Currently, it is unknown whether this function and coevolutionary relationship is unique to KZNFs or is a broader feature of metazoan ZNFs. Here, we present evidence that genomic conflict with TEs has been a central driver of the diversification of ZNFs in animals. Sampling from 3221 genome assemblies, we show that the copy number of retroelements correlates with that of ZNFs across at least 750 million years of metazoan evolution. Using computational predictions, we show that ZNFs preferentially bind TEs in diverse animal species. We further investigate the largest ZNF subfamily found in cyprinid fish, which is characterized by a conserved sequence we dubbed the fish N-terminal zinc finger-associated (FiNZ) domain. Zebrafish possess approximately 700 FiNZ-ZNFs, many of which are evolving adaptively under positive selection. Like mammalian KZNFs, most zebrafish FiNZ-ZNFs are expressed at the onset of zygotic genome activation, and blocking their translation using morpholinos during early embryogenesis results in derepression of transcriptionally active TEs. Together, these data suggest that ZNF diversification has been intimately connected to TE expansion throughout animal evolution.

'Cannibalism' of exogenous DNA sequences: The ancestral form of adaptive immunity which entails recognition of danger

Adaptive immunity is a sophisticated form of immune response capable of retaining the molecular memory of a very great diversity of target antigens (epitopes) as non-self. It is capable of reactivating itself upon a second encounter with an immunoglobulin or T-cell receptor antigen-binding site with a known epitope that had previously primed the host immune system. It has long been considered that adaptive immunity is a highly evolved form of non-self recognition that appeared quite late in speciation and complemented a more generalist response called innate immunity. Innate immunity offers a relatively non-specific defense (although mediated by sensors that could specifically recognize virus or bacteria compounds) and which does not retain a memory of the danger. But this notion of recent acquisition of adaptive immunity is challenged by the fact that another form of specific recognition mechanisms already existed in prokaryotes that may be able to specifically auto-protect against external danger. This recognition mechanism can be considered a primitive form of specific (adaptive) non-self recognition. It is based on the fact that many archaea and bacteria use a genome editing system that confers the ability to appropriate viral DNA sequences allowing prokaryotes to prevent host damage through a mechanism very similar to adaptive immunity. This is indistinctly called, 'endogenization of foreign DNA' or 'viral DNA predation' or, more pictorially 'DNA cannibalism'. For several years evidence has been accumulating, highlighting the crucial role of endogenization of foreign DNA in the fundamental processes related to adaptive immunity and leading to a change in the dogma that adaptive immunity appeared late in speciation.

Reflection About the Ancient Emergence of HTLV-2 Infection

During millions of years, viruses have emerged and reemerged, with imbalance of photogenicity and transmissivity overtime. This letter describes that sometimes the nomenclature is uncertain what may actually happen during retrovirus evolution nowadays. This article discusses a possibility that human T-lymphotropic virus type 2 (HTLV-2) has been processed to incorporate the human genome in the last millions of years. Persistent viruses such as human immunodeficiency virus type 1 (HIV-1), HIV-2, and human T cell lymphotropic type 2 may also have potential of endogenization instead of a cytolytic process in a long time.

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.

Geographic patterns of koala retrovirus genetic diversity, endogenization, and subtype distributions

Koala retrovirus (KoRV) subtype A (KoRV-A) is currently in transition from exogenous virus to endogenous viral element, providing an ideal system to elucidate retroviral-host coevolution. We characterized KoRV geography using fecal DNA from 192 samples across 20 populations throughout the koala's range. We reveal an abrupt change in KoRV genetics and incidence at the Victoria/New South Wales state border. In northern koalas, pol gene copies were ubiquitously present at above five per cell, consistent with endogenous KoRV. In southern koalas, pol copies were detected in only 25.8% of koalas and always at copy numbers below one, while the env gene was detected in all animals and in a majority at copy numbers above one per cell. These results suggest that southern koalas carry partial endogenous KoRV-like sequences. Deep sequencing of the env hypervariable region revealed three putatively endogenous KoRV-A sequences in northern koalas and a single, distinct sequence present in all southern koalas. Among northern populations, env sequence diversity decreased with distance from the equator, suggesting infectious KoRV-A invaded the koala genome in northern Australia and then spread south. The exogenous KoRV subtypes (B to K), two novel subtypes, and intermediate subtypes were detected in all northern koala populations but were strikingly absent from all southern animals tested. Apart from KoRV subtype D, these exogenous subtypes were generally locally prevalent but geographically restricted, producing KoRV genetic differentiation among northern populations. This suggests that sporadic evolution and local transmission of the exogenous subtypes have occurred within northern Australia, but this has not extended into animals within southern Australia.

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.

Subtype distribution and expression of the koala retrovirus in the Japanese zoo koala population

We investigated the proviral copies and RNA expression in koala retrovirus (KoRV)-infected koalas. To ascertain any variation in viral load by institution, age, sex, or body condition score, we quantified KoRV proviral DNA and RNA loads in captive koalas (n = 37) reared in Japanese zoos. All koalas were positive for KoRV genes (pol, LTRs, and env of KoRV-A) in genomic DNA (gDNA), and 91.89% were positive for the pol gene in RNA. In contrast, the distribution rates of KoRV-B, KoRV-C, KoRV-D, and KoRV-F env genes in gDNA were 94.59%, 27.03%, 67.57%, and 54.05%, respectively. A wide inter-individual variation and/or a significant inter-institutional difference in proviral DNA (p < 0.0001) and RNA (p < 0.001) amounts (copies/10³ koala β-actin copies) were observed in Awaji Farm England Hill Zoo koalas, which were obtained from southern koala populations, suggesting exogenous incorporation of KoRV in these koalas. Significant (p < 0.05) age differences were noted in KoRV RNA load (p < 0.05) and median total RNA load (p < 0.001), with loads higher in younger koalas (joeys and juveniles). Thus, the current study provides the distribution of KoRV subtypes in Japanese zoo koala populations and identifies several additional risk factors (sex, age, and body condition) associated with KoRV expression.

Ongoing endeavors to detect mobilization of transposable elements

Transposable elements (TEs) are DNA sequences capable of mobilization from one location to another in the genome. Since the discovery of ‘Dissociation (Dc) locus’ by Barbara McClintock in maize (1), mounting evidence in the era of genomics indicates that a significant fraction of most eukaryotic genomes is composed of TE sequences, involving in various aspects of biological processes such as development, physiology, diseases and evolution. Although technical advances in genomics have discovered numerous functional impacts of TE across species, our understanding of TEs is still ongoing process due to challenges resulted from complexity and abundance of TEs in the genome. In this mini-review, we briefly summarize biology of TEs and their impacts on the host genome, emphasizing importance of understanding TE landscape in the genome. Then, we introduce recent endeavors especially in vivo retrotransposition assays and long read sequencing technology for identifying de novo insertions/TE polymorphism, which will broaden our knowledge of extraordinary relationship between genomic cohabitants and their host.

Convergent evolution of antiviral machinery derived from endogenous retrovirus truncated envelope genes in multiple species

Host genetic resistance to viral infection controls the pathogenicity and epidemic dynamics of infectious diseases. Refrex-1 is a restriction factor against feline leukemia virus subgroup D (FeLV-D) and an endogenous retrovirus (ERV) in domestic cats (ERV-DC). Refrex-1 is encoded by a subset of ERV-DC loci with truncated envelope genes and secreted from cells as a soluble protein. Here, we identified the copper transporter CTR1 as the entry receptor for FeLV-D and genotype I ERV-DCs. We also identified CTR1 as a receptor for primate ERVs from crab-eating macaques and rhesus macaques, which were found in a search of intact envelope genes capable of forming infectious viruses. Refrex-1 counteracted infection by FeLV-D and ERV-DCs via competition for the entry receptor CTR1; the antiviral effects extended to primate ERVs with CTR1-dependent entry. Furthermore, truncated ERV envelope genes found in chimpanzee, bonobo, gorilla, crab-eating macaque, and rhesus macaque genomes could also block infection by feline and primate retroviruses. Genetic analyses showed that these ERV envelope genes were acquired in a species- or genus-specific manner during host evolution. These results indicated that soluble envelope proteins could suppress retroviral infection across species boundaries, suggesting that they function to control retroviral spread. Our findings revealed that several mammalian species acquired antiviral machinery from various ancient retroviruses, leading to convergent evolution for host defense.

Expansion of a retrovirus lineage in the koala genome

Retroviruses have left their legacy in host genomes over millions of years as endogenous retroviruses (ERVs), and their structure, diversity, and prevalence provide insights into the historical dynamics of retrovirus-host interactions. In bioinformatic analyses of koala (Phascolarctos cinereus) whole-genome sequences, we identify a recently expanded ERV lineage (phaCin-β) that is related to the New World squirrel monkey retrovirus. This ERV expansion shares many parallels with the ongoing koala retrovirus (KoRV) invasion of the koala genome, including highly similar and mostly intact sequences, and polymorphic ERV loci in the sampled koala population. The recent phaCin-β ERV colonization of the koala genome appears to predate the current KoRV invasion, but polymorphic ERVs and divergence comparisons between these two lineages predict a currently uncharacterized, possibly still extant, phaCin-β retrovirus. The genomics approach to ERV-guided discovery of novel retroviruses in host species provides a strong incentive to search for phaCin-β retroviruses in the Australasian fauna.

Revisiting the Tigger Transposon Evolution Revealing Extensive Involvement in the Shaping of Mammal Genomes

Simple Summary

Despite the discovery of the Tigger family of pogo transposons in the mammalian genome, the evolution profile of this family is still incomplete. Here, we conducted a systematic evolution analysis for Tigger in nature. The data revealed that Tigger was found in a broad variety of animals, and extensive invasion of Tigger was observed in mammal genomes. Common horizontal transfer events of Tigger elements were observed across different lineages of animals, including mammals, that may have led to their widespread distribution, while parasites and invasive species may have promoted Tigger HT events. Our results also indicate that the activity of Tigger transposons tends to be low in vertebrates; only one mammalian genome and fish genome may harbor active Tigger.

Abstract

The data of this study revealed that Tigger was found in a wide variety of animal genomes, including 180 species from 36 orders of invertebrates and 145 species from 29 orders of vertebrates. An extensive invasion of Tigger was observed in mammals, with a high copy number. Almost 61% of those species contain more than 50 copies of Tigger; however, 46% harbor intact Tigger elements, although the number of these intact elements is very low. Common HT events of Tigger elements were discovered across different lineages of animals, including mammals, that may have led to their widespread distribution, whereas Helogale parvula and arthropods may have aided Tigger HT incidences. The activity of Tigger seems to be low in the kingdom of animals, most copies were truncated in the mammal genomes and lost their transposition activity, and Tigger transposons only display signs of recent and current activities in a few species of animals. The findings suggest that the Tigger family is important in structuring mammal genomes.

Conserved Viral Transcription Plays a Key Role in Virus-Like Particle Production of the Parasitoid Wasp Venturia canescens

Nudiviruses are large double-stranded DNA viruses related to baculoviruses known to be endogenized in the genomes of certain parasitic wasp species. These wasp-virus associations allow the production of viral particles or virus-like particles that ensure wasp parasitism success within lepidopteran hosts. Venturia canescens is an ichneumonid wasp belonging to the Campopleginae subfamily that has endogenized nudivirus genes belonging to the Alphanudivirus genus to produce "virus-like particles" (Venturia canescens virus-like particles [VcVLPs]), which package proteic virulence factors. The main aim of this study was to determine whether alphanudivirus gene functions have been conserved following endogenization. The expression dynamics of alphanudivirus genes was monitored by a high throughput transcriptional approach, and the functional role of lef-4 and lef-8 genes predicted to encode viral RNA polymerase components was investigated by RNA interference. As described for baculovirus infections and for endogenized nudivirus genes in braconid wasp species producing bracoviruses, a transcriptional cascade involving early and late expressed alphanudivirus genes could be observed. The expression of lef-4 and lef-8 was also shown to be required for the expression of alphanudivirus late genes allowing correct particle formation. Together with previous literature, the results show that endogenization of nudiviruses in parasitoid wasps has repeatedly led to the conservation of the viral RNA polymerase function, allowing the production of viruses or viral-like particles that differ in composition but enable wasp parasitic success. IMPORTANCE This study shows that endogenization of a nudivirus genome in a Campopleginae parasitoid wasp has led to the conservation, as for endogenized nudiviruses in braconid parasitoid wasps, of the viral RNA polymerase function, required for the transcription of genes encoding viral particles involved in wasp parasitism success. We also showed for the first time that RNA interference (RNAi) can be successfully used to downregulate gene expression in this species, a model in behavioral ecology. This opens the opportunity to investigate the function of genes involved in other traits important for parasitism success, such as reproductive strategies and host choice. Fundamental data acquired on gene function in Venturia canescens are likely to be transferable to other parasitoid wasp species used in biological control programs. This study also renders possible the investigation of other nudivirus gene functions, for which little data are available.

Differential and defective transcription of koala retrovirus indicates the complexity of host and virus evolution

Koala retrovirus (KoRV) is unique amongst endogenous (inherited) retroviruses in that its incorporation to the host genome is still active, providing an opportunity to study what drives this fundamental process in vertebrate genome evolution. Animals in the southern part of the natural range of koalas were previously thought to be either virus-free or to have only exogenous variants of KoRV with low rates of KoRV-induced disease. In contrast, animals in the northern part of their range universally have both endogenous and exogenous KoRV with very high rates of KoRV-induced disease such as lymphoma. In this study we use a combination of sequencing technologies, Illumina RNA sequencing of 'southern' (south Australian) and 'northern' (SE QLD) koalas and CRISPR enrichment and nanopore sequencing of DNA of 'southern' (South Australian and Victorian animals) to retrieve full-length loci and intregration sites of KoRV variants. We demonstrate that koalas that tested negative to the KoRV pol gene qPCR, used to detect replication-competent KoRV, are not in fact KoRV-free but harbour defective, presumably endogenous, 'RecKoRV' variants that are not fixed between animals. This indicates that these populations have historically been exposed to KoRV and raises questions as to whether these variants have arisen by chance or whether they provide a protective effect from the infectious forms of KoRV. This latter explanation would offer the intriguing prospect of being able to monitor and selectively breed for disease resistance to protect the wild koala population from KoRV-induced disease.

Non-canonical nematode endogenous retroviruses resulting from RNA virus glycoprotein gene capture by a metavirus

Reverse-transcribing retroviruses exist as horizontally transmitted infectious agents or vertically transmitted endogenous retroviruses (ERVs) resident in eukaryotic genomes, and they are phylogenetically related to the long terminal repeat (LTR) class of retrotransposons. ERVs and retrotransposons are often distinguished only by the presence or absence of a gene encoding the envelope glycoprotein (env). Endogenous elements of the virus family Metaviridae include the insect-restricted Errantivirus genus of ERVs, for which some members possess env, and the pan-eukaryotic Metavirus genus that lacks an envelope glycoprotein gene. Here we report a novel Nematoda endogenous retrovirus (NERV) clade with core retroviral genes arranged uniquely as a continuous gag-env-pro-pol ORF. Reverse transcriptase sequences were phylogenetically related to metaviruses, but envelope glycoprotein sequences resembled those of the Nyamiviridae and Chrysoviridae RNA virus families, suggesting env gene capture during host cell infection by an RNA virus. NERVs were monophyletic, restricted to the nematode subclass Chromadoria, and included additional ORFs for a small hypothetical protein or a large Upf1-like RNA-dependent AAA-ATPase/helicase indicative of viral transduction of a host gene. Provirus LTR identity, low copy number, ORF integrity and segregation of three loci in Meloidogyne incognita, taken together with detection of NERV transcriptional activity, support potential infectivity of NERVs, along with their recent emergence and integration. Altogether, NERVs constitute a new and distinct Metaviridae lineage demonstrating retroviral evolution through sequential heterologous gene capture events.

The Intricate Evolutionary Balance between Transposable Elements and Their Host: Who Will Kick at Goal and Convert the Next Try?

Transposable elements (TEs) are mobile DNA sequences that can jump from one genomic locus to another and that have colonized the genomes of all living organisms. TE mobilization and accumulation are an important source of genomic innovations that greatly contribute to the host species evolution. To ensure their maintenance and amplification, TE transposition must occur in the germ cell genome. As TE transposition is also a major threat to genome integrity, the outcome of TE mobility in germ cell genomes could be highly dangerous because such mutations are inheritable. Thus, organisms have developed specialized strategies to protect the genome integrity from TE transposition, particularly in germ cells. Such effective TE silencing, together with ongoing mutations and negative selection, should result in the complete elimination of functional TEs from genomes. However, TEs have developed efficient strategies for their maintenance and spreading in populations, particularly by using horizontal transfer to invade the genome of novel species. Here, we discuss how TEs manage to bypass the host's silencing machineries to propagate in its genome and how hosts engage in a fightback against TE invasion and propagation. This shows how TEs and their hosts have been evolving together to achieve a fine balance between transposition and repression.

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

Koala Retrovirus (KoRV) has been associated with neoplasia in the vulnerable koala (Phascolarctos cinereus). However, there are conflicting findings regarding its association with secondary disease. We undertook a large-scale assessment of how the different KoRV subtypes and viral load are associated with Chlamydia pecorum infection and a range of disease pathologies in 151 wild koalas admitted for care to Currumbin Wildlife Hospital, Australia. Viral load (KoRV pol copies per ml of plasma) was the best predictor of more disease pathologies than any other KoRV variable. The predicted probability of a koala having disease symptoms increased from 25% to over 85% across the observed range of KoRV load, while the predicted probability of C. pecorum infection increased from 40% to over 80%. We found a negative correlation between the proportion of env deep sequencing reads that were endogenous KoRV-A and total KoRV load. This is consistent with suppression of endogenous KoRV-A, while the exogenous KoRV subtypes obtain high infection levels. Additionally, we reveal evidence that the exogenous subtypes are directly associated with secondary disease, with the proportion of reads that were the endogenous KoRV-A sequence a negative predictor of overall disease probability after the effect of KoRV load was accounted for. Further, koalas that were positive for KoRV-D or KoRV-D/F were more likely to have urogenital C. pecorum infection or low body condition score, respectively, irrespective of KoRV load. By contrast, our findings do not support previous findings that KoRV-B in particular is associated with Chlamydial disease. Based on these findings we suggest that koala research and conservation programs should target understanding what drives individual differences in KoRV load and limiting exogenous subtype diversity within populations, rather than seeking to eliminate any particular subtype.

The diversity and evolution of retroviruses: perspectives from viral “fossils”

Retroviruses exclusively infect vertebrates, causing a variety of diseases. The replication of retroviruses requires reverse transcription and integration into host genomes. When infecting germline cells, retroviruses become inherited vertically, forming endogenous retroviruses (ERVs). ERVs document past viral infections, providing molecular fossils for studying the evolutionary history of retroviruses. In this review, we summarize the recent advances in understanding the diversity and evolution of retroviruses from the perspectives of viral fossils, and discuss the effects of ERVs on the evolution of host biology.

•

Recent advances in understanding the diversity and evolution of retroviruses.

•

Methods to analyze ERVs.

•

The effects of ERVs on the evolution of host biology.

How Well Does Evolution Explain Endogenous Retroviruses?-A Lakatosian Assessment

One of the most sophisticated philosophies of science is the methodology of scientific research programmes (MSRP), developed by Imre Lakatos. According to MSRP, scientists are working within so-called research programmes, consisting of a hard core of fixed convictions and a flexible protective belt of auxiliary hypotheses. Anomalies are accommodated by changes to the protective belt that do not affect the hard core. Under MSRP, research programmes are appraised as 'progressive' if they successfully predict novel facts but are judged as 'degenerative' if they merely offer ad hoc solutions to anomalies. This paper applies these criteria to the evolutionary research programme as it has performed during half a century of ERV research. It describes the early history of the field and the emergence of the endogenization-amplification theory on the origins of retroviral-like sequences. It then discusses various predictions and postdictions that were generated by the programme, regarding orthologous ERVs in different species, the presence of target site duplications and the divergence of long terminal repeats, and appraises how the programme has dealt with data that did not conform to initial expectations. It is concluded that the evolutionary research programme has been progressive with regard to the issues here examined.

Vaccination against the Koala Retrovirus (KoRV): Problems and Strategies

The koala retrovirus (KoRV) is spreading in the koala population from the north to the south of Australia and is also in the process of endogenization into the koala genome. Virus infection is associated with tumorigenesis and immunodeficiency and is contributing to the decline of the animal population. Antibody production is an excellent marker of retrovirus infection; however, animals carrying endogenous KoRV are tolerant. Therefore, the therapeutic immunization of animals carrying endogenous KoRV seems to be ineffective. Using the recombinant transmembrane (TM) envelope protein of the KoRV, we immunized goats, rats and mice, obtaining in all cases neutralizing antibodies which recognize epitopes in the fusion peptide proximal region (FPPR), and in the membrane-proximal external region (MPER). Immunizing several animal species with the corresponding TM envelope protein of the closely related porcine endogenous retrovirus (PERV), as well as the feline leukemia virus (FeLV), we also induced neutralizing antibodies with similar epitopes. Immunizing with the TM envelope protein in addition to the surface envelope proteins of all three viruses resulted in higher titers of neutralizing antibodies. Immunizing KoRV-negative koalas with our vaccine (which is composed of both envelope proteins) may protect these animals from infection, and these may be the starting points of a virus-free population.

Transposable elements shape the evolution of mammalian development

Transposable elements (TEs) promote genetic innovation but also threaten genome stability. Despite multiple layers of host defence, TEs actively shape mammalian-specific developmental processes, particularly during pre-implantation and extra-embryonic development and at the maternal-fetal interface. Here, we review how TEs influence mammalian genomes both directly by providing the raw material for genetic change and indirectly via co-evolving TE-binding Krüppel-associated box zinc finger proteins (KRAB-ZFPs). Throughout mammalian evolution, individual activities of ancient TEs were co-opted to enable invasive placentation that characterizes live-born mammals. By contrast, the widespread activity of evolutionarily young TEs may reflect an ongoing co-evolution that continues to impact mammalian development.

Retroviruses of Bats: a Threat Waiting in the Wings?

Bats are infamous reservoirs of deadly human viruses. While retroviruses, such as the human immunodeficiency virus (HIV), are among the most significant of virus families that have jumped from animals into humans, whether bat retroviruses have the potential to infect and cause disease in humans remains unknown. Recent reports of retroviruses circulating in bat populations builds on two decades of research describing the fossil records of retroviral sequences in bat genomes and of viral metagenomes extracted from bat samples. The impact of the global COVID-19 pandemic demands that we pay closer attention to viruses hosted by bats and their potential as a zoonotic threat. Here we review current knowledge of bat retroviruses and explore the question of whether they represent a threat to humans.

Koala retrovirus genetic diversity and transmission dynamics within captive koala populations

Koala populations are currently in rapid decline across Australia, with infectious diseases being a contributing cause. The koala retrovirus (KoRV) is a gammaretrovirus present in both captive and wild koala colonies that presents an additional challenge for koala conservation in addition to habitat loss, climate change, and other factors. Currently, nine different subtypes (A to I) have been identified; however, KoRV genetic diversity analyses have been limited. KoRV is thought to be exogenously transmitted between individuals, with KoRV-A also being endogenous and transmitted through the germline. The mechanisms of exogenous KoRV transmission are yet to be extensively investigated. Here, deep sequencing was employed on 109 captive koalas of known pedigree, housed in two institutions from Southeast Queensland, to provide a detailed analysis of KoRV transmission dynamics and genetic diversity. The final dataset included 421 unique KoRV sequences, along with the finding of an additional subtype (KoRV-K). Our analysis suggests that exogenous transmission of KoRV occurs primarily between dam and joey, with evidence provided for multiple subtypes, including nonendogenized KoRV-A. No evidence of sexual transmission was observed, with mating partners found to share a similar number of sequences as unrelated koala pairs. Importantly, both distinct captive colonies showed similar trends. These findings indicate that breeding strategies or antiretroviral treatment of females could be employed as effective management approaches in combating KoRV transmission.

Multiple Infiltration and Cross-Species Transmission of Foamy Viruses across the Paleozoic to the Cenozoic Era

Foamy viruses (FVs) are complex retroviruses that can infect humans and other animals. In this study, by integrating transcriptomic and genomic data, we discovered 412 FVs from 6 lineages in amphibians, which significantly increased the known set of FVs in amphibians. Among these lineages, salamander FVs maintained a coevolutionary pattern with their hosts that could be dated back to the Paleozoic era, while in contrast, frog FVs were much more likely acquired from cross-species (class-level) transmission in the Cenozoic era. In addition, we found that three distinct FV lineages had integrated into the genome of a salamander. Unexpectedly, we identified a lineage of endogenous FVs in caecilians that expressed all complete major genes, demonstrating the potential existence of an exogenous form of FV outside of mammals. Our discovery of rare phenomena in amphibian FVs has significantly increased our understanding of the macroevolution of the complex retrovirus. IMPORTANCE Foamy viruses (FVs) represent, more so than other viruses, the best model of coevolution between a virus and a host. This study represents the largest investigation so far of amphibian FVs and reveals 412 FVs of 6 distinct lineages from three major orders of amphibians. Besides a coevolutionary pattern, cross-species and repeated infections were also observed during the evolution of amphibian FVs. Remarkably, expressed FVs including a potential exogenous form were discovered, suggesting that active FVs might be underestimated in nature. These findings revealed that the multiple origins and complex evolution of amphibian FVs started from the Paleozoic era.

Evolutionary History of Endogenous Human Herpesvirus 6 Reflects Human Migration out of Africa

Human herpesvirus 6A and 6B (HHV-6) can integrate into the germline, and as a result, ∼70 million people harbor the genome of one of these viruses in every cell of their body. Until now, it has been largely unknown if 1) these integrations are ancient, 2) if they still occur, and 3) whether circulating virus strains differ from integrated ones. Here, we used next-generation sequencing and mining of public human genome data sets to generate the largest and most diverse collection of circulating and integrated HHV-6 genomes studied to date. In genomes of geographically dispersed, only distantly related people, we identified clades of integrated viruses that originated from a single ancestral event, confirming this with fluorescent in situ hybridization to directly observe the integration locus. In contrast to HHV-6B, circulating and integrated HHV-6A sequences form distinct clades, arguing against ongoing integration of circulating HHV-6A or “reactivation” of integrated HHV-6A. Taken together, our study provides the first comprehensive picture of the evolution of HHV-6, and reveals that integration of heritable HHV-6 has occurred since the time of, if not before, human migrations out of Africa.

Endogenous retroviruses in the origins and treatment of cancer

Endogenous retroviruses (ERVs) are emerging as promising therapeutic targets in cancer. As remnants of ancient retroviral infections, ERV-derived regulatory elements coordinate expression from gene networks, including those underpinning embryogenesis and immune cell function. ERV activation can promote an interferon response, a phenomenon termed viral mimicry. Although ERV expression is associated with cancer, and provisionally with autoimmune and neurodegenerative diseases, ERV-mediated inflammation is being explored as a way to sensitize tumors to immunotherapy. Here we review ERV co-option in development and innate immunity, the aberrant contribution of ERVs to tumorigenesis, and the wider biomedical potential of therapies directed at ERVs.

Toll-Like Receptor and Cytokine Responses to Infection with Endogenous and Exogenous Koala Retrovirus, and Vaccination as a Control Strategy

Koala populations are currently declining and under threat from koala retrovirus (KoRV) infection both in the wild and in captivity. KoRV is assumed to cause immunosuppression and neoplastic diseases, favoring chlamydiosis in koalas. Currently, 10 KoRV subtypes have been identified, including an endogenous subtype (KoRV-A) and nine exogenous subtypes (KoRV-B to KoRV-J). The host’s immune response acts as a safeguard against pathogens. Therefore, a proper understanding of the immune response mechanisms against infection is of great importance for the host’s survival, as well as for the development of therapeutic and prophylactic interventions. A vaccine is an important protective as well as being a therapeutic tool against infectious disease, and several studies have shown promise for the development of an effective vaccine against KoRV. Moreover, CRISPR/Cas9-based genome editing has opened a new window for gene therapy, and it appears to be a potential therapeutic tool in many viral infections, which could also be investigated for the treatment of KoRV infection. Here, we discuss the recent advances made in the understanding of the immune response in KoRV infection, as well as the progress towards vaccine development against KoRV infection in koalas.

Koala retrovirus (KoRV) subtypes and their impact on captive koala (Phascolarctos cinereus) health

Koala retrovirus (KoRV), a major pathogen of koalas, exists in both endogenous (KoRV-A) and exogenous forms (KoRV-B to J). However, the impact of infection with multiple subtypes is not well understood. Accordingly, in this study, we surveyed a representative sample from a Japanese zoo population to determine the infection status for three KoRV subtypes (KoRV-A, B, and C) and to investigate the proviral and RNA load profiles in animals with single- and multiple-subtype infections, using peripheral blood mononuclear cells (PBMCs) and plasma. Six koalas were evaluated in the study; all were infected with KoRV-A, and two koalas were coinfected with non-A subtypes (KoRV-B and/or KoRV-C). The highest KoRV total RNA and viral loads in PBMCs and plasma were found in a koala infected with multiple subtypes (KoRV-A, -B and -C). The other koala infected with multiple subtypes (KoRV-A and B) showed the highest proviral PBMC load but the lowest RNA copy number in PBMC and plasma. PBMCs from this animal were cultured for further investigation, and KoRV RNA was detected in the cells and culture supernatant after 7 and/or 14 days. The koalas harboring multiple subtypes had a higher white blood cell count than those harboring only KoRV-A and were judged to be leukemic, and they subsequently died due to lymphoma. Accordingly, we conclude that coinfection with multiple KoRV subtypes may be linked to more-severe disease. In a sequence alignment, the detected KoRV-A env gene showed 100% sequence identity to the reference gene, whereas the KoRV-B and -C env genes varied from their reference sequences.

Toll-Like Receptor Expression Profiles in Koala (Phascolarctos cinereus) Peripheral Blood Mononuclear Cells Infected with Multiple KoRV Subtypes

Simple Summary

Koala retrovirus (KoRV) is a major pathogen of koala. Toll-like receptors (TLRs) are important innate immune component that are evolutionary conserved and play a crucial role in the early defense against invading pathogens. The expression profile of TLRs in KoRV infection in koalas is not characterized yet. Therefore, in this study, we characterized TLR expression patterns in koalas infected with KoRV-A only vs. KoRV-A with KoRV-B and/or -C. Using qRT-PCR, we measured TLR2–10 and TLR13 mRNA expression in peripheral blood mononuclear cells (PBMCs) and/or tissues from captive koalas in Japanese zoos. We observed variations in TLR expression in koalas with a range of subtype infection profiles (KoRV-A only vs. KoRV-A with KoRV-B and/or -C). The findings of this study might improve our current understanding of koala’s immune response to KoRV infection.

Abstract

Toll-like receptors (TLRs), evolutionarily conserved pattern recognition receptors, play an important role in innate immunity by recognizing microbial pathogen-associated molecular patterns. Koala retrovirus (KoRV), a major koala pathogen, exists in both endogenous (KoRV-A) and exogenous forms (KoRV-B to J). However, the expression profile of TLRs in koalas infected with KoRV-A and other subtypes is yet to characterize. Here, we investigated TLR expression profiles in koalas with a range of subtype infection profiles (KoRV-A only vs. KoRV-A with KoRV-B and/or -C). To this end, we cloned partial sequences for TLRs (TLR2–10 and TLR13), developed real-time PCR assays, and determined TLRs mRNA expression patterns in koala PBMCs and/or tissues. All the reported TLRs for koala were expressed in PBMCs, and variations in TLR expression were observed in koalas infected with exogenous subtypes (KoRV-B and KoRV-C) compared to the endogenous subtype (KoRV-A) only, which indicates the implications of TLRs in KoRV infection. TLRs were also found to be differentially expressed in koala tissues. This is the first report of TLR expression profiles in koala, which provides insights into koala’s immune response to KoRV infection that could be utilized for the future exploitation of TLR modulators in the maintenance of koala health.

Koala Retrovirus in Northern Australia Shows a Mixture of Stable Endogenization and Exogenous Lineage Diversification within Fragmented Koala Populations

The koala population in northern Australia has become increasingly fragmented due to natural and man-made barriers and interventions. This situation has created a unique opportunity to study both endogenous and exogenous koala retrovirus (KoRV). To determine the impact that population isolation has had on KoRV diversity in Queensland, 272 koalas from six fragmented koala populations were profiled for their KoRV provirus across two natural biogeographical barriers (the St Lawrence Gap and the Brisbane Valley Barrier), one man-made geographical barrier (the city of Brisbane) and two translocation events (the single movement of koalas to an island and the repeated movement of koalas into a koala sanctuary). Analysis revealed that all koalas tested were KoRV-A positive, with 90 - 96% of the detected KoRV provirus from each koala representing a single, likely endogenous, KoRV-A strain. The next most abundant proviral sequence was a defective variant of the dominant KoRV-A strain, accounting for 3 - 10% of detected provirus. The remaining KoRV provirus represented expected exogenous strains of KoRV and included geographically localized patterns of KoRV-B, -C, -D, -F, -G, and -I. These results indicate that lineage diversification of exogenous KoRV is actively ongoing. In addition, comparison of KoRV provirus within known dam-sire-joey family groups from the koala sanctuary revealed that joeys consistently had KoRV proviral patterns more similar to their dams than their sires in KoRV-B, -C and -D provirus composition. Collectively, this study highlights both the consistency of endogenous KoRV and the diversity of exogenous KoRV across the fragmented koala populations in northern Australia.IMPORTANCE KoRV infection has become a permanent part of koalas in northern Australia. With KoRV presence and abundance linked to more severe chlamydial disease and neoplasia in these koalas, understanding how KoRV exists throughout an increasingly fragmented koala population is a key first step in designing conservation and management strategies. This survey of KoRV provirus in Queensland koalas indicates that endogenous KoRV provirus is ubiquitous and consistent throughout the state while exogenous KoRV provirus is diverse and distinct in fragmented koala populations. Understanding the prevalence and impact of both endogenous and exogenous KoRV will be needed to ensure a future for all koala populations.

Retroviral integrations contribute to elevated host cancer rates during germline invasion

Repeated retroviral infections of vertebrate germlines have made endogenous retroviruses ubiquitous features of mammalian genomes. However, millions of years of evolution obscure many of the immediate repercussions of retroviral endogenisation on host health. Here we examine retroviral endogenisation during its earliest stages in the koala (Phascolarctos cinereus), a species undergoing germline invasion by koala retrovirus (KoRV) and affected by high cancer prevalence. We characterise KoRV integration sites (IS) in tumour and healthy tissues from 10 koalas, detecting 1002 unique IS, with hotspots of integration occurring in the vicinity of known cancer genes. We find that tumours accumulate novel IS, with proximate genes over-represented for cancer associations. We detect dysregulation of genes containing IS and identify a highly-expressed transduced oncogene. Our data provide insights into the tremendous mutational load suffered by the host during active retroviral germline invasion, a process repeatedly experienced and overcome during the evolution of vertebrate lineages.

Phylogenetic and geographical analysis of a retrovirus during the early stages of endogenous adaptation and exogenous spread in a new host

Most retroviral endogenization and host adaptation happened in the distant past, with the opportunity to study these processes as they occurred lost to time. An exception exists with the discovery that koala retrovirus (KoRV) has recently begun its endogenization into the koala (Phascolarctos cinereus) genome. What makes this opportunity remarkable is the fact that Northern Australian koalas appear to be undergoing endogenization with one KoRV subtype (KoRV‐A), while all subtypes (KoRV‐A‐I) coexist exogenously, and Southern Australian koalas appear to carry all KoRV subtypes as an exogenous virus. To understand the distribution and relationship of all KoRV variants in koalas, the proviral KoRV envelope gene receptor binding domain was assessed across the koala's natural range. Examination of KoRV subtype‐specific proviral copy numbers per cell found that KoRV‐A proviral integration levels were consistent with endogenous incorporation in Northern Australia (southeast Queensland and northeast New South Wales) while revealing lower levels of KoRV‐A proviral integration (suggestive of exogenous incorporation) in southern regions (southeast New South Wales and Victoria). Phylogeographical analysis indicated that several major KoRV‐A variants were distributed uniformly across the country, while non‐KoRV‐A variants appeared to have undergone lineage diversification in geographically distinct regions. Further analysis of the major KoRV‐A variants revealed a distinct shift in variant proportions in southeast New South Wales, suggesting this as the geographical region where KoRV‐A transitions from being predominantly endogenous to exogenous in Australian koalas. Collectively, these findings advance both our understanding of KoRV in koalas and of retroviral endogenization and diversification in general.

see also the Perspective by Elliott S. Chiu and Roderick B. Gagne.

Is the human T-cell lymphotropic virus type 2 in the process of endogenization into the human genome?

Human T-cell lymphotropic virus type 2 (HTLV-2) infection has been shown to be endemic among intravenous drug users in parts of North America, Europe and Southeast Asia and in a number of Amerindian populations. Despite a 65% genetic similarity and common host humoral response, the human T-cell lymphotropic viruses type 1 (HTLV-1) and 2 display different mechanisms of host interaction and capacity for disease development. While HTLV-1 pathogenicity is well documented, HTLV-2 etiology in human disease is not clearly established. From an evolutionary point of view, its introduction and integration into the germ cell chromosomes of host species could be considered as the final stage of parasitism and evasion from host immunity. The extraordinary abundance of endogenous viral sequences in all vertebrate species genomes, including the hominid family, provides evidence of this invasion. Some of these gene sequences still retain viral characteristics and the ability to replicate and hence are potentially able to elicit responses from the innate and adaptive host immunity, which could result in beneficial or pathogenic effects. Taken together, this data may indicate that HTLV-2 is more likely to progress towards endogenization as has happened to the human endogenous retroviruses millions of years ago. Thus, this intimate association (HTLV-2/human genome) may provide protection from the immune system with better adaptation and low pathogenicity.

CD4, CD8b, and Cytokines Expression Profiles in Peripheral Blood Mononuclear Cells Infected with Different Subtypes of KoRV from Koalas (Phascolarctos cinereus) in a Japanese Zoo

Koala retrovirus (KoRV) poses a major threat to koala health and conservation, and currently has 10 identified subtypes: an endogenous subtype (KoRV-A) and nine exogenous subtypes (KoRV-B to KoRV-J). However, subtype-related variations in koala immune response to KoRV are uncharacterized. In this study, we investigated KoRV-related immunophenotypic changes in a captive koala population (Hirakawa zoo, Japan) with a range of subtype infection profiles (KoRV-A only vs. KoRV-A with KoRV-B and/or -C), based on qPCR measurements of CD4, CD8b, IL-6, IL-10 and IL-17A mRNA expression in unstimulated and concanavalin (Con)-A-stimulated peripheral blood mononuclear cells (PBMCs). Although CD4, CD8b, and IL-17A expression did not differ between KoRV subtype infection profiles, IL-6 expression was higher in koalas with exogenous infections (both KoRV-B and KoRV-C) than those with the endogenous subtype only. IL-10 expression did not significantly differ between subtype infection profiles but did show a marked increase—accompanying decreased CD4:CD8b ratio—in a koala with lymphoma and co-infected with KoRV-A and -B, thus suggesting immunosuppression. Taken together, the findings of this study provide insights into koala immune response to multiple KoRV subtypes, which can be exploited for the development of prophylactic and therapeutic interventions for this iconic marsupial species.

Endogenous Retroviruses Drive Resistance and Promotion of Exogenous Retroviral Homologs

Endogenous retroviruses (ERVs) serve as markers of ancient viral infections and provide invaluable insight into host and viral evolution. ERVs have been exapted to assist in performing basic biological functions, including placentation, immune modulation, and oncogenesis. A subset of ERVs share high nucleotide similarity to circulating horizontally transmitted exogenous retrovirus (XRV) progenitors. In these cases, ERV-XRV interactions have been documented and include (a) recombination to result in ERV-XRV chimeras, (b) ERV induction of immune self-tolerance to XRV antigens, (c) ERV antigen interference with XRV receptor binding, and (d) interactions resulting in both enhancement and restriction of XRV infections. Whereas the mechanisms governing recombination and immune self-tolerance have been partially determined, enhancement and restriction of XRV infection are virus specific and only partially understood. This review summarizes interactions between six unique ERV-XRV pairs, highlighting important ERV biological functions and potential evolutionary histories in vertebrate hosts.

Potential for Virus Endogenization in Humans through Testicular Germ Cell Infection: the Case of HIV

Viruses have colonized the germ line of our ancestors on several occasions during evolution, leading to the integration in the human genome of viral sequences from over 30 retroviral groups and a few nonretroviruses. Among the recently emerged viruses infecting humans, several target the testis (e.g., human immunodeficiency virus [HIV], Zika virus, and Ebola virus). Here, we aimed to investigate whether human testicular germ cells (TGCs) can support integration by HIV, a contemporary retrovirus that started to spread in the human population during the last century. We report that albeit alternative receptors enabled HIV-1 binding to TGCs, HIV virions failed to infect TGCs in vitro Nevertheless, exposure of TGCs to infected lymphocytes, naturally present in the testis from HIV+ men, led to HIV-1 entry, integration, and early protein expression. Similarly, cell-associated infection or bypassing viral entry led to HIV-1 integration in a spermatogonial cell line. Using DNAscope, HIV-1 and simian immunodeficiency virus (SIV) DNA were detected within a few TGCs in the testis from one infected patient, one rhesus macaque, and one African green monkey in vivo Molecular landscape analysis revealed that early TGCs were enriched in HIV early cofactors up to integration and had overall low antiviral defenses compared with testicular macrophages and Sertoli cells. In conclusion, our study reveals that TGCs can support the entry and integration of HIV upon cell-associated infection. This could represent a way for this contemporary virus to integrate into our germ line and become endogenous in the future, as happened during human evolution for a number of viruses.IMPORTANCE Viruses have colonized the host germ line on many occasions during evolution to eventually become endogenous. Here, we aimed at investigating whether human testicular germ cells (TGCs) can support such viral invasion by studying HIV interactions with TGCs in vitro Our results indicate that isolated primary TGCs express alternative HIV-1 receptors, allowing virion binding but not entry. However, HIV-1 entered and integrated into TGCs upon cell-associated infection and produced low levels of viral proteins. In vivo, HIV-1 and SIV DNA was detected in a few TGCs. Molecular landscape analysis showed that TGCs have overall weak antiviral defenses. Altogether, our results indicate that human TGCs can support HIV-1 early replication, including integration, suggesting potential for endogenization in future generations.

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.

Koalas vaccinated against Koala retrovirus respond by producing increased levels of interferon-gamma

Koala retrovirus (KoRV) is believed to be in an active state of endogenization into the koala genome. KoRV is present as both an endogenous and exogenous infection in all koalas in northern Australia. KoRV has been linked to koala pathologies including neoplasia and increased susceptibility to Chlamydia. A KoRV vaccine recently trialled in 10 northern koalas improved antibody response and reduced viral load. This communication reports the expression of key immune genes underlining the innate and adaptive immune response to vaccination in these northern koalas. The results showed that prior to vaccination, IL-8 was expressed at the highest levels, with at least 200-fold greater expression compared to other cytokines, while CD8 mRNA expression was significantly higher than CD4 mRNA expression level. Interferon-γ was up-regulated at both 4- and 8-weeks post-vaccination while IL-8 was down-regulated at 8-weeks post-vaccination.

Koala retrovirus diversity, transmissibility, and disease associations

Background

Koalas are infected with the koala retrovirus (KoRV) that exists as exogenous or endogenous viruses. KoRV is genetically diverse with co-infection with up to ten envelope subtypes (A-J) possible; KoRV-A is the prototype endogenous form. KoRV-B, first found in a small number of koalas with an increased leukemia prevalence at one US zoo, has been associated with other cancers and increased chlamydial disease. To better understand the molecular epidemiology of KoRV variants and the effect of increased viral loads (VLs) on transmissibility and pathogenicity we developed subtype-specific quantitative PCR (qPCR) assays and tested blood and tissue samples from koalas at US zoos (n = 78), two Australian zoos (n = 27) and wild-caught (n = 21) in Australia. We analyzed PCR results with available clinical, demographic, and pedigree data.

Results

All koalas were KoRV-A-infected. A small number of koalas (10.3%) at one US zoo were also infected with non-A subtypes, while a higher non-A subtype prevalence (59.3%) was found in koalas at Australian zoos. Wild koalas from one location were only infected with KoRV-A. We observed a significant association of infection and plasma VLs of non-A subtypes in koalas that died of leukemia/lymphoma and other neoplasias and report cancer diagnoses in KoRV-A-positive animals. Infection and VLs of non-A subtypes was not associated with age or sex. Transmission of non-A subtypes occurred from dam-to-offspring and likely following adult-to-adult contact, but associations with contact type were not evaluated. Brief antiretroviral treatment of one leukemic koala infected with high plasma levels of KoRV-A, -B, and -F did not affect VL or disease progression.

Conclusions

Our results show a significant association of non-A KoRV infection and plasma VLs with leukemia and other cancers. Although we confirm dam-to-offspring transmission of these variants, we also show other routes are possible. Our validated qPCR assays will be useful to further understand KoRV epidemiology and its zoonotic transmission potential for humans exposed to koalas because KoRV can infect human cells.

North American Big Brown Bats (Eptesicus fuscus) Harbor an Exogenous Deltaretrovirus

Bats are the reservoir for a large number of zoonotic viruses, including members of Coronaviridae (severe acute respiratory syndrome coronavirus [SARS-CoV] and SARS-CoV-2), Paramyxoviridae (Hendra and Nipah viruses), Rhabdoviridae (rabies virus), and Filoviridae (Ebola virus) as exemplars. Many retroviruses, such as human immunodeficiency virus, are similarly zoonotic; however, only infectious exogenous gammaretroviruses have recently been identified in bats. Here, viral metagenomic sequencing of samples from bats submitted for rabies virus testing, largely due to human exposure, identified a novel, highly divergent exogenous Deltaretrovirus from a big brown bat (Eptesicus fuscus) in South Dakota. The virus sequence, corresponding to Eptesicus fuscus deltaretrovirus (EfDRV), comprised a nearly complete coding region comprised of canonical 5'-gag-pro-pol-env-3' genes with 37% to 51% identity to human T-lymphotropic virus (HTLV), an infectious retrovirus that causes T-cell lymphoma. A putative tax gene with 27% identity to HTLV was located downstream of the pol gene along with a gene harbored in an alternative reading frame which possessed a conserved domain for an Epstein-Barr virus nuclear antigen involved in gene transactivation, suggesting a regulatory function similar to that of the deltaretrovirus rex gene. A TaqMan reverse transcriptase PCR (RT-PCR) targeting the pol gene identified 4/60 (6.7%) bats as positive for EfDRV, which, combined with a search of the E. fuscus genome that failed to identify sequences with homology to EfDRV, suggests that EfDRV is an infectious exogenous virus. As all known members of Deltaretrovirus can cause malignancies and E. fuscus is widely distributed in the Americas, often with a colonial roosting behavior in human dwellings, further studies are needed to investigate potential zoonosis.IMPORTANCE Bats host a large numbers of viruses, many of which are zoonotic. In the United States, the big brown bat (Eptesicus fuscus) is widely distributed and lives in small colonies that roost in cavities, often in human dwellings, leading to frequent human interaction. Viral metagenomic sequencing of samples from an E. fuscus bat submitted for rabies testing identified the first exogenous bat Deltaretrovirus The E. fuscus deltaretrovirus (EfDRV) genome consists of the typical deltaretrovial 5'-gag-pro-pol-env-3' genes along with genes encoding two putative transcriptional transactivator proteins distantly related to the Tax protein of human T-cell lymphotrophic virus and nuclear antigen 3B of Epstein-Barr virus. Searches of the E. fuscus genome sequence failed to identify endogenous EfDRV. RT-PCR targeting the EfDRV pol gene identified 4/60 (6.7%) bats with positive results. Together, these results suggest that EfDRV is exogenous. As all members of Deltaretrovirus are associated with T- and B-cell malignancies or neurologic disease, further studies on possible zoonosis are warranted.

Koala immunogenetics and chlamydial strain type are more directly involved in chlamydial disease progression in koalas from two south east Queensland koala populations than koala retrovirus subtypes

Chlamydial disease control is increasingly utilised as a management tool to stabilise declining koala populations, and yet we have a limited understanding of the factors that contribute to disease progression. To examine the impact of host and pathogen genetics, we selected two geographically separated south east Queensland koala populations, differentially affected by chlamydial disease, and analysed koala major histocompatibility complex (MHC) genes, circulating strains of Chlamydia pecorum and koala retrovirus (KoRV) subtypes in longitudinally sampled, well-defined clinical groups. We found that koala immunogenetics and chlamydial genotypes differed between the populations. Disease progression was associated with specific MHC alleles, and we identified two putative susceptibility (DCb 03, DBb 04) and protective (DAb 10, UC 01:01) variants. Chlamydial genotypes belonging to both Multi-Locus Sequence Typing sequence type (ST) 69 and ompA genotype F were associated with disease progression, whereas ST 281 was associated with the absence of disease. We also detected different ompA genotypes, but not different STs, when long-term infections were monitored over time. By comparison, KoRV profiles were not significantly associated with disease progression. These findings suggest that chlamydial genotypes vary in pathogenicity and that koala immunogenetics and chlamydial strains are more directly involved in disease progression than KoRV subtypes.

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.