Unique Evolution of Antiviral Tetherin in Bats

RNA-Seq analysis reveals the long noncoding RNAs associated with immunity in wild Myotis myotis bats

BACKGROUND

Bats possess a uniquely adapted immune system that enables them to live with viral infections without the expected maladies. The molecular basis and regulation of bats' immune response is still not fully understood. Long non-coding RNAs (lncRNAs) represent an emerging class of molecules with critical regulatory roles in multiple biological processes, including immunity. We hypothesise that lncRNA-based regulation in bats may enable them to limit disease and live with viral pathogens.

RESULTS

We developed a lncRNA prediction pipeline to annotate the long non-coding transcriptome across multiple bat tissues and at the population level. Characterisation of our lncRNA dataset based on 100 blood transcriptomes from wild Myotis myotis bats revealed lower and more tissue-specific expression compared with coding genes, reduced GC content and shorter length distributions, consistent with lncRNA profiles observed in other species. Using WGCNA network analyses and gene ontology, we identified two mRNA-lncRNA co-expression modules in Myotis myotis associated with distinct immune response: one linked to T-cell activation and vial processes, and the other to inflammation. From these immune-related lncRNAs, we selected four candidates with high translational potential for regulating viral infections and inflammation. These include a newly identified lncRNA, BatLnc1, with potential antiviral functions; the M. myotis ortholog of TUG1, implicated in viral-host interactions; and well-known lncRNAs MALAT1 and NEAT1, recognised for their roles in inflammatory regulation.

CONCLUSIONS

We conducted the first ab initio prediction of lncRNAs in a non-model bat species, the wild-caught M. myotis. Our network analysis revealed significant variation in immune status among a subset of individuals, potentially due to pathogenic conditions. From these variations, we identified lncRNAs most likely associated with immune response in bats. This initial exploration lays the groundwork for future experimental validations of lncRNA functions, offering promising insights into their role in bat immunity.

[The diversification of the protein kinase R contributes to the specificity of bat-virus interactions]

Several bat species asymptomatically harbor certain viruses that are highly pathogenic in other mammals. The underlying mechanisms involve an evolutionary balance between tolerance and immune resistance to viral infections. However, how bats innate immunity has evolved in response to viruses remains to be elucidated. Here, we review the evolution of the protein kinase R (PKR) in bats, a major antiviral protein of vertebrate innate antiviral defense. Our recent results indicate that PKR has evolved under selective pressure and has undergone genomic duplications in bats, in contrast to all mammals studied, which possess only a single copy of the gene. The genetic changes in bat PKR are probably partly the result of genetic conflicts with ancient pathogenic poxviruses, shaping a bat-specific host–virus interface. Furthermore, the duplicated PKRs in Myotis species enable them to collectively escape viruses and enhance their viral control. These results suggest that viral adaptations of PKR contribute to the specificity of modern interactions between viruses and bats, and may explain unique antiviral mechanisms in bats.

Single-cell and Spatial Transcriptomics Illuminate Bat Immunity and Barrier Tissue Evolution

Bats have adapted to pathogens through diverse mechanisms, including increased resistance-rapid pathogen elimination, and tolerance-limiting tissue damage following infection. In the Egyptian fruit bat (an important model in comparative immunology), several mechanisms conferring disease tolerance were discovered, but mechanisms underpinning resistance remain poorly understood. Previous studies on other species suggested that the elevated basal expression of innate immune genes may lead to increased resistance to infection. Here, we test whether such transcriptional patterns occur in Egyptian fruit bat tissues through single-cell and spatial transcriptomics of gut, lung, and blood cells, comparing gene expression between bat, mouse, and human. Despite numerous recent loss and expansion events of interferons in the bat genome, interferon expression and induction are remarkably similar to that of mouse. In contrast, central complement system genes are highly and uniquely expressed in key regions in bat lung and gut epithelium, unlike in human and mouse. Interestingly, the unique expression of these genes in the bat gut is strongest in the crypt, where developmental expression programs are highly conserved. The complement system genes also evolve rapidly in their coding sequences across the bat lineage. Finally, the bat complement system displays strong hemolytic activity. Together, these results indicate a distinctive transcriptional divergence of the complement system, which may be linked to bat resistance, and highlight the intricate evolutionary landscape of bat immunity.

Expression of an Efficient Selection Marker Out of a Duplicated Site in the ITRs of a Modified Vaccinia Virus Ankara (MVA)

Background/Objectives: Poxviruses are large DNA viruses that replicate in the host cytoplasm without a nuclear phase. As vaccine vectors, they can package and express large recombinant cassettes from different positions of their genomic core region. We present a comparison between wildtype modified vaccinia Ankara (MVA) and isolate CR19, which has significantly expanded inverted terminal repeats (ITRs). With this expansion, a site in wildtype MVA, called deletion site (DS) IV, has been duplicated at both ends of the genome and now occupies an almost central position in the newly formed ITRs. Methods: We inserted various reporter genes into this site and found that the ITRs can be used for transgene expression. However, ITRs are genomic structures that can rapidly adapt to selective pressure through transient duplication and contraction. To test the potential utility of insertions into viral telomers, we inserted a factor from the cellular innate immune system that interferes with viral replication as an example of a difficult transgene. Results: A site almost in the centre of the ITRs can be used for transgene expression, and both sides are mirrored into identical copies. The example of a challenging transgene, tetherin, proved to be surprisingly efficient in selecting candidate vectors against the large background of parental viruses. Conclusions: Insertion of transgenes into ITRs automatically doubles the gene doses. The functionalisation of viruses with tetherin may accelerate the identification and generation of recombinant vectors for personalised medicine and pandemic preparedness.

Extensive longevity and DNA virus-driven adaptation in nearctic Myotis bats

The genus Myotis is one of the largest clades of bats, and exhibits some of the most extreme variation in lifespans among mammals alongside unique adaptations to viral tolerance and immune defense. To study the evolution of longevity-associated traits and infectious disease, we generated near-complete genome assemblies and cell lines for 8 closely related species of Myotis. Using genome-wide screens of positive selection, analyses of structural variation, and functional experiments in primary cell lines, we identify new patterns of adaptation contributing to longevity, cancer resistance, and viral interactions in bats. We find that Myotis bats have some of the most significant variation in cancer risk across mammals and demonstrate a unique DNA damage response in primary cells of the long-lived M. lucifugus. We also find evidence of abundant adaptation in response to DNA viruses - but not RNA viruses - in Myotis and other bats in sharp contrast with other mammals, potentially contributing to the role of bats as reservoirs of zoonoses. Together, our results demonstrate how genomics and primary cells derived from diverse taxa uncover the molecular bases of extreme adaptations in non-model organisms.

Chromosome-level genome and population genomics of the intermediate horseshoe bat ( Rhinolophus affinis) reveal the molecular basis of virus tolerance in Rhinolophus and echolocation call frequency variation

Horseshoe bats (genus Rhinolophus, family Rhinolophidae) represent an important group within chiropteran phylogeny due to their distinctive traits, including constant high-frequency echolocation, rapid karyotype evolution, and unique immune system. Advances in evolutionary biology, supported by high-quality reference genomes and comprehensive whole-genome data, have significantly enhanced our understanding of species origins, speciation mechanisms, adaptive evolutionary processes, and phenotypic diversity. However, genomic research and understanding of the evolutionary patterns of Rhinolophus are severely constrained by limited data, with only a single published genome of R. ferrumequinum currently available. In this study, we constructed a high-quality chromosome-level reference genome for the intermediate horseshoe bat ( R. affinis). Comparative genomic analyses revealed potential genetic characteristics associated with virus tolerance in Rhinolophidae. Notably, we observed expansions in several immune-related gene families and identified various genes functionally associated with the SARS-CoV-2 signaling pathway, DNA repair, and apoptosis, which displayed signs of rapid evolution. In addition, we observed an expansion of the major histocompatibility complex class II (MHC-II) region and a higher copy number of the HLA- DQB2 gene in horseshoe bats compared to other chiropteran species. Based on whole-genome resequencing and population genomic analyses, we identified multiple candidate loci (e.g., GLI3) associated with variations in echolocation call frequency across R. affinis subspecies. This research not only expands our understanding of the genetic characteristics of the Rhinolophus genus but also establishes a valuable foundation for future research.

Different species of Chiroptera: Immune cells and molecules

The distinct composition and immune response characteristics of bats' innate and adaptive immune systems, which enable them to serve as host of numerous serious zoonotic viruses without falling ill, differ substantially from those of other mammals, it have garnered significant attention. In this article, we offer a systematic review of the names, attributes, and functions of innate and adaptive immune cells & molecules across different bat species. This includes descriptions of the differences shown by research between 71 bat species in 10 families, as well as comparisons between bats and other mammals. Studies of the immune cells & molecules of different bat species are necessary to understand the unique antiviral immunity of bats. By providing comprehensive information on these unique immune responses, it is hoped that new insights will be provided for the study of co-evolutionary dynamics between viruses and the bat immune system, as well as human antiviral immunity.

A tale of endurance: bats, viruses and immune dynamics

The emergence of highly zoonotic viral infections has propelled bat research forward. The viral outbreaks including Hendra virus, Nipah virus, Marburg virus, Ebola virus, Rabies virus, Middle East respiratory syndrome coronavirus, SARS-CoV and the latest SARS-CoV-2 have been epidemiologically linked to various bat species. Bats possess unique immunological characteristics that allow them to serve as a potential viral reservoir. Bats are also known to protect themselves against viruses and maintain their immunity. Therefore, there is a need for in-depth understanding into bat-virus biology to unravel the major factors contributing to the coexistence and spread of viruses.

Screening for Viruses in Indigenous Greek Black Pigs

The successful advancement of xenotransplantation has led to the development of highly sensitive detection systems for the screening of potentially zoonotic viruses in donor pigs and preventing their transmission to the recipient. To validate these methods, genetically modified pigs generated for xenotransplantation, numerous minipigs and other pig breeds have been tested, thereby increasing our knowledge concerning the pig virome and the distribution of pig viruses. Of particular importance are the porcine cytomegalovirus, a porcine roseolovirus (PCMV/PRV) and the hepatitis E virus genotype 3 (HEV3). PCMV/PRV has been shown to reduce the survival time of pig transplants in non-human primates and was also transmitted in the first pig heart transplantation to a human patient. The main aim of this study was to determine the sensitivities of our methods to detect PCMV/PRV, HEV3, porcine lymphotropic herpesvirus-1 (PLHV-1), PLHV-2, PLHV-3, porcine circovirus 2 (PCV2), PCV3, PCV4 and porcine parvovirus 1 (PPV1) and to apply the methods to screen indigenous Greek black pigs. The high number of viruses found in these animals allowed for the evaluation of numerous detection methods. Since porcine endogenous retroviruses (PERVs) type A and B are integrated in the genome of all pigs, but PERV-C is not, the animals were screened for PERV-C and PERV-A/C. Our detection methods were sensitive and detected PCMV/PRV, PLHV-1, PLHV-1, PLHV-3, PVC3 and PERV-C in most animals. PPV1, HEV3, PCV4 and PERV-A/C were not detected. These data are of great interest since the animals are healthy and resistant to diseases.

Independent loss events of a functional tetherin gene in galliform birds

IMPORTANCE

Birds represent important hosts for numerous viruses, including zoonotic viruses and pathogens with the potential to cause major economic losses to the poultry industry. Viral replication and transmission can be inhibited or blocked by the action of antiviral restriction factors (RFs) encoded by the host. One well-characterized RF is tetherin, a protein that directly blocks the release of newly formed viral particles from infected cells. Here, we describe the evolutionary loss of a functional tetherin gene in two galliform birds, turkey (Meleagris gallopavo) and Mikado pheasant (Syrmaticus mikado). Moreover, we demonstrate that the structurally related protein TMCC(aT) exerts antiviral activity in several birds, albeit by a mechanism different from that of tetherin. The evolutionary scenario described here represents the first documented loss-of-tetherin cases in vertebrates.

Functional comparisons of the virus sensor RIG-I from humans, the microbat Myotis daubentonii, and the megabat Rousettus aegyptiacus, and their response to SARS-CoV-2 infection

IMPORTANCE

A common hypothesis holds that bats (order Chiroptera) are outstanding reservoirs for zoonotic viruses because of a special antiviral interferon (IFN) system. However, functional studies about key components of the bat IFN system are rare. RIG-I is a cellular sensor for viral RNA signatures that activates the antiviral signaling chain to induce IFN. We cloned and functionally characterized RIG-I genes from two species of the suborders Yangochiroptera and Yinpterochiroptera. The bat RIG-Is were conserved in their sequence and domain organization, and similar to human RIG-I in (i) mediating virus- and IFN-activated gene expression, (ii) antiviral signaling, (iii) temperature dependence, and (iv) recognition of RNA ligands. Moreover, RIG-I of Rousettus aegyptiacus (suborder Yinpterochiroptera) and of humans were found to recognize SARS-CoV-2 infection. Thus, members of both bat suborders encode RIG-Is that are comparable to their human counterpart. The ability of bats to harbor zoonotic viruses therefore seems due to other features.

The more the merrier? Gene duplications in the coevolution of primate lentiviruses with their hosts

Gene duplications are a major source of genetic diversity and evolutionary innovation. Newly formed, duplicated genes can provide a selection advantage in constantly changing environments. One such example is the arms race of HIV and related lentiviruses with innate immune responses of their hosts. In recent years, it has become clear that both sides have benefited from multiple gene duplications. For example, amplifications of antiretroviral factors such as apolipoprotein-B mRNA-editing enzyme catalytic polypeptide-3 (APOBEC3), interferon-induced transmembrane protein (IFITM), and tripartite motif-containing (TRIM) proteins have expanded the repertoire of cell-intrinsic defense mechanisms and increased the barriers to retroviral replication and cross-species transmission. Conversely, recent studies have also shed light on how duplications of accessory lentiviral genes and Long terminal repeat (LTR) elements can provide a selection advantage in the coevolution with antiviral host proteins.

Regulatory T cell-like response to SARS-CoV-2 in Jamaican fruit bats (Artibeus jamaicensis) transduced with human ACE2

Insectivorous Old World horseshoe bats (Rhinolophus spp.) are the likely source of the ancestral SARS-CoV-2 prior to its spillover into humans and causing the COVID-19 pandemic. Natural coronavirus infections of bats appear to be principally confined to the intestines, suggesting fecal-oral transmission; however, little is known about the biology of SARS-related coronaviruses in bats. Previous experimental challenges of Egyptian fruit bats (Rousettus aegyptiacus) resulted in limited infection restricted to the respiratory tract, whereas insectivorous North American big brown bats (Eptesicus fuscus) showed no evidence of infection. In the present study, we challenged Jamaican fruit bats (Artibeus jamaicensis) with SARS-CoV-2 to determine their susceptibility. Infection was confined to the intestine for only a few days with prominent viral nucleocapsid antigen in epithelial cells, and mononuclear cells of the lamina propria and Peyer's patches, but with no evidence of infection of other tissues; none of the bats showed visible signs of disease or seroconverted. Expression levels of ACE2 were low in the lungs, which may account for the lack of pulmonary infection. Bats were then intranasally inoculated with a replication-defective adenovirus encoding human ACE2 and 5 days later challenged with SARS-CoV-2. Viral antigen was prominent in lungs for up to 14 days, with loss of pulmonary cellularity during this time; however, the bats did not exhibit weight loss or visible signs of disease. From day 7, bats had low to moderate IgG antibody titers to spike protein by ELISA, and one bat on day 10 had low-titer neutralizing antibodies. CD4+ helper T cells became activated upon ex vivo recall stimulation with SARS-CoV-2 nucleocapsid peptide library and exhibited elevated mRNA expression of the regulatory T cell cytokines interleukin-10 and transforming growth factor-β, which may have limited inflammatory pathology. Collectively, these data show that Jamaican fruit bats are poorly susceptible to SARS-CoV-2 but that expression of human ACE2 in their lungs leads to robust infection and an adaptive immune response with low-titer antibodies and a regulatory T cell-like response that may explain the lack of prominent inflammation in the lungs. This model will allow for insight of how SARS-CoV-2 infects bats and how bat innate and adaptive immune responses engage the virus without overt clinical disease.

Genes, inflammatory response, tolerance, and resistance to virus infections in migratory birds, bats, and rodents

Normally, the host immunological response to viral infection is coordinated to restore homeostasis and protect the individual from possible tissue damage. The two major approaches are adopted by the host to deal with the pathogen: resistance or tolerance. The nature of the responses often differs between species and between individuals of the same species. Resistance includes innate and adaptive immune responses to control virus replication. Disease tolerance relies on the immune response allowing the coexistence of infections in the host with minimal or no clinical signs, while maintaining sufficient viral replication for transmission. Here, we compared the virome of bats, rodents and migratory birds and the molecular mechanisms underlying symptomatic and asymptomatic disease progression. We also explore the influence of the host physiology and environmental influences on RNA virus expression and how it impacts on the whole brain transcriptome of seemingly healthy semipalmated sandpiper (Calidris pusilla) and spotted sandpiper (Actitis macularius). Three time points throughout the year were selected to understand the importance of longitudinal surveys in the characterization of the virome. We finally revisited evidence that upstream and downstream regulation of the inflammatory response is, respectively, associated with resistance and tolerance to viral infections.

Antiviral effects of interferon-stimulated genes in bats

The interferon pathway is the first line of defense in viral infection in all mammals, and its induction stimulates broad expression of interferon-stimulated genes (ISGs). In mice and also humans, the antiviral function of ISGs has been extensively studied. As an important viral reservoir in nature, bats can coexist with a variety of pathogenic viruses without overt signs of disease, yet only limited data are available for the role of ISGs in bats. There are multiple species of bats and work has begun deciphering the differences and similarities between ISG function of human/mouse and different bat species. This review summarizes the current knowledge of conserved and bat-specific-ISGs and their known antiviral effector functions.

Comparison of antiviral responses in two bat species reveals conserved and divergent innate immune pathways

Bats host a range of disease-causing viruses without displaying clinical symptoms. The mechanisms behind this are a continuous source of interest. Here, we studied the antiviral response in the Egyptian fruit bat and Kuhl's pipistrelle, representing two subordinal clades. We profiled the antiviral response in fibroblasts using RNA sequencing and compared bat with primate and rodent responses. Both bats upregulate similar genes; however, a subset of these genes is transcriptionally divergent between them. These divergent genes also evolve rapidly in sequence, have specific promoter architectures, and are associated with programs underlying tolerance and resistance. Finally, we characterized antiviral genes that expanded in bats, with duplicates diverging in sequence and expression. Our study reveals a largely conserved antiviral program across bats and points to a set of genes that rapidly evolve through multiple mechanisms. These can contribute to bat adaptation to viral infection and provide directions to understanding the mechanisms behind it.

Characterization of Megabat-Favored, CA-Dependent Susceptibility to Retrovirus Infection

The isolation of the Koala retrovirus-like virus from Australian megabats and the identification of endogenous retroviruses in the bat genome have raised questions on bat susceptibility to retroviruses in general. To answer this, we studied the susceptibility of 12 cell lines from 11 bat species to four well-studied retroviruses (human and simian immunodeficiency viruses [HIV and SIV] and murine leukemia viruses [B- and N-MLV]). Systematic comparison of retroviral susceptibility among bats revealed that megabat cell lines were overall less susceptible to the four retroviruses than microbat cell lines, particularly to HIV-1 infection, whereas lineage-specific differences were observed for MLV susceptibility. Quantitative PCR of reverse transcription (RT) products, infection in heterokaryon cells, and point mutation analysis of the capsid (CA) revealed that (i) HIV-1 and MLV replication were blocked at the nuclear transport of the pre-integration complexes and before and/or during RT, respectively, and (ii) the observed lineage-specific restriction can be attributed to a dominant cellular factor constrained by specific positions in CA. Investigation of bat homologs of the three previously reported post-entry restriction factors constrained by the same residues in CA, tripartite motif-protein 5α (TRIM5α), myxovirus resistance 2/B (Mx2/MxB), and carboxy terminus-truncated cleavage and polyadenylation factor 6 (CPSF6-358), demonstrated poor anti-HIV-1 activity in megabat cells, whereas megabat TRIM5α restricted MLV infection, suggesting that the major known CA-dependent restriction factors were not dominant in the observed lineage-specific susceptibility to HIV-1 in bat cells. Therefore, HIV-1 susceptibility of megabat cells may be determined in a manner distinct from that of primate cells. IMPORTANCE Recent studies have demonstrated the circulation of gammaretroviruses among megabats in Australia and the bats' resistance to HIV-1 infection; however, the origins of these viruses in megabats and the contribution of bats to retrovirus spread to other mammalian species remains unclear. To determine the intrinsic susceptibility of bat cells to HIV-1 infection, we investigated 12 cell lines isolated from 11 bat species. We report that lineage-specific retrovirus restriction in the bat cell lines can be attributed to CA-dependent factors. However, in the megabat cell lines examined, factors known to bind capsid and block infection in primate cell culture, including homologs of TRIM5α, Mx2/MxB, and CPSF6, failed to exhibit significant anti-HIV-1 activities. These results suggested that the HIV-1 susceptibility of megabat cells occurs in a manner distinct from that of primate cells, where cellular factors, other than major known CA-dependent restriction factors, with lineage-specific functions could recognize retroviral proteins in megabats.

Regulatory T Cell-like Response to SARS-CoV-2 in Jamaican Fruit Bats ( Artibeus jamaicensis ) Transduced with Human ACE2

Insectivorous Old World horseshoe bats ( Rhinolophus spp.) are the likely source of the ancestral SARS-CoV-2 prior to its spillover into humans and causing the COVID-19 pandemic. Natural coronavirus infections of bats appear to be principally confined to the intestines, suggesting fecal-oral transmission; however, little is known about the biology of SARS-related coronaviruses in bats. Previous experimental challenges of Egyptian fruit bats ( Rousettus aegyptiacus ) resulted in limited infection restricted to the respiratory tract, whereas insectivorous North American big brown bats ( Eptesicus fuscus ) showed no evidence of infection. In the present study, we challenged Jamaican fruit bats ( Artibeus jamaicensis ) with SARS-CoV-2 to determine their susceptibility. Infection was confined to the intestine for only a few days with prominent viral nucleocapsid antigen in epithelial cells, and mononuclear cells of the lamina propria and Peyer's patches, but with no evidence of infection of other tissues; none of the bats showed visible signs of disease or seroconverted. Expression levels of ACE2 were low in the lungs, which may account for the lack of pulmonary infection. Bats were then intranasally inoculated with a replication-defective adenovirus encoding human ACE2 and 5 days later challenged with SARS-CoV-2. Viral antigen was prominent in lungs for up to 14 days, with loss of pulmonary cellularity during this time; however, the bats did not exhibit weight loss or visible signs of disease. From day 7, bats had low to moderate IgG antibody titers to spike protein by ELISA, and one bat on day 10 had low-titer neutralizing antibodies. CD4 + helper T cells became activated upon ex vivo recall stimulation with SARS-CoV-2 nucleocapsid peptide library and exhibited elevated mRNA expression of the regulatory T cell cytokines interleukin-10 and transforming growth factor-β, which may have limited inflammatory pathology. Collectively, these data show that Jamaican fruit bats are poorly susceptibility to SARS-CoV-2 but that expression of human ACE2 in their lungs leads to robust infection and an adaptive immune response with low-titer antibodies and a regulatory T cell-like response that may explain the lack of prominent inflammation in the lungs. This model will allow for insight of how SARS-CoV-2 infects bats and how bat innate and adaptive immune responses engage the virus without overt clinical disease.

Author Summary

Bats are reservoir hosts of many viruses that infect humans, yet little is known about how they host these viruses, principally because of a lack of relevant and susceptible bat experimental infection models. Although SARS-CoV-2 originated in bats, no robust infection models of bats have been established. We determined that Jamaican fruit bats are poorly susceptible to SARS-CoV-2; however, their lungs can be transduced with human ACE2, which renders them susceptible to SARS-CoV-2. Despite robust infection of the lungs and diminishment of pulmonary cellularity, the bats showed no overt signs of disease and cleared the infection after two weeks. Despite clearance of infection, only low-titer antibody responses occurred and only a single bat made neutralizing antibody. Assessment of the CD4 + helper T cell response showed that activated cells expressed the regulatory T cell cytokines IL-10 and TGFβ that may have tempered pulmonary inflammation.

Identification of Two Isoforms of Canine Tetherin in Domestic Dogs and Characterization of Their Antiviral Activity against Canine Influenza Virus

Canine influenza virus (CIV) significantly threatens the canine population and public health. Tetherin, an innate immune factor, plays an important role in the defense against pathogen invasion and has been discovered to restrict the release of various enveloped viruses. Two isoforms of canine tetherin (tetherin-X1 and tetherin-X2) were identified in peripheral blood leukocytes of mixed-breed dogs using reverse transcription polymerase chain reaction (RT-PCR). Amino acid alignment revealed that relative to full-length tetherin (tetherin-X1) and truncated canine tetherin (tetherin-X2) exhibited deletion of 34 amino acids. The deletion occurred at the C-terminus of the coiled-coiled ectodomain and the N-terminus of the glycosylphosphatidylinositol (GPI)-anchor domain. Tetherin-X2 was localized subcellularly at the cell membrane, which was consistent with the localization of tetherin-X1. In addition, canine tetherin-X1 and tetherin-X2 restricted the release of H3N2 CIV. However, canine tetherin-X1 had higher antiviral activity than canine tetherin-X2, indicating that the C-terminus of the coiled-coiled ectodomain and the N-terminus of the GPI-anchor domain of canine tetherin (containing the amino acids deleted in tetherin-X2) are critical for its ability to restrict H3N2 CIV release. This study provides insights for understanding the key functional domains of tetherin that restrict CIV release.