Characterization of the Antigen Processing Machinery and Endogenous Peptide Presentation of a Bat MHC Class I Molecule

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.

Amino acid insertion in Bat MHC-I enhances complex stability and augments peptide presentation

Bats serve as reservoirs for numerous zoonotic viruses, yet they typically remain asymptomatic owing to their unique immune system. Of particular significance is the MHC-I in bats, which plays crucial role in anti-viral response and exhibits polymorphic amino acid (AA) insertions. This study demonstrated that both 5AA and 3AA insertions enhance the thermal stability of the bat MHC-I complex and enrich the diversity of bound peptides in terms of quantity and length distribution, by stabilizing the 310 helix, a region prone to conformational changes during peptide loading. However, the mismatched insertion could diminish the stability of bat pMHC-I. We proposed that a suitable insertion may help bat MHC-I adapt to high body temperatures during flight while enhancing antiviral responses. Moreover, this site-specific insertions may represent a strategy of evolutionary adaptation of MHC-I molecules to fluctuations in body temperature, as similar insertions have been found in other lower vertebrates.

The gut microbiota of bats confers tolerance to influenza virus (H1N1) infection in mice

Pathogens from wild animals cause approximately 60% of emerging infectious diseases (EIDs). Studies on the immune systems of natural hosts are helpful for preventing the spread of EIDs. Bats are natural hosts for many emerging infectious pathogens and have a unique immune system that often coexists with pathogens without infection. Previous studies have shown that some genes and proteins may help bats fight virus infection, but little is known about the function of the bat gut microbiome on immunity. Here, we transplanted gut microbiota from wild bats (Great Himalayan Leaf-nosed bats, Hipposideros armiger) into antibiotic-treated mice, and found that the gut microbiota from bats regulated the immune system faster than mice after antibiotic treatment. Moreover, we infected mice with H1N1, and found that the gut microbiota of bats could effectively protect mice, leading to decreased inflammatory response and increased survival rate. Finally, metabolomics analysis showed that the gut microbiota of bats produced more flavonoid and isoflavones. Our results demonstrate that the quick-start innate immune response endowed by bat gut microbiota and the regulatory and antiviral effects of gut microbiota metabolites successfully ensured mouse survival after viral challenge. To our knowledge, our study was the first to use fecal microbiota transplantation (FMT) to transplant the gut microbiota of bats into mice, and the first to provide evidence that the gut microbiota of bats confers tolerance to viral infections.

Molecular, ecological, and behavioral drivers of the bat-virus relationship

Bats perform important ecological roles in our ecosystem. However, recent studies have demonstrated that bats are reservoirs of emerging viruses that have spilled over into humans and agricultural animals to cause severe diseases. These viruses include Hendra and Nipah paramyxoviruses, Ebola and Marburg filoviruses, and coronaviruses that are closely related to severe acute respiratory syndrome coronavirus (SARS-CoV), Middle East respiratory syndrome coronavirus (MERS-CoV), and the recently emerged SARS-CoV-2. Intriguingly, bats that are naturally or experimentally infected with these viruses do not show clinical signs of disease. Here we have reviewed ecological, behavioral, and molecular factors that may influence the ability of bats to harbor viruses. We have summarized known zoonotic potential of bat-borne viruses and stress on the need for further studies to better understand the evolutionary relationship between bats and their viruses, along with discovering the intrinsic and external factors that facilitate the successful spillover of viruses from bats.

Transcriptomic analysis elucidates evolution of the major histocompatibility complex class I in neotropical bats

The Order Chiroptera comprises more than 1,400 species, each with its evolutionary history and under unique selective pressures, among which are the host–pathogen interactions. Bats have coped with complex interactions with a broad spectrum of microbes throughout their evolutionary history, prompting the development of unique adaptations that allow them to co-exist with microbes with pathogenic potential more efficiently than other nonadapted species. In this sense, an extraordinary immune system with unique adaptations has been hypothesized in bats. To explore this, we focused on the major histocompatibility complex (MHC), which plays a crucial role in pathogen recognition and presentation to T cells to trigger the adaptive immune response. We analyzed MHC class I transcripts in five species, each from different families of New World bats. From RNA-seq data, we assembled a partial region of the MHC-I comprising the α1 and α2 domains, which are responsible for peptide binding and recognition. We described five putative functional variants, two of which have two independent insertions at the α2 domain. Our results suggest that this insertion appeared after the divergence of the order Chiroptera and may have an adaptive function in the defense against intracellular pathogens, providing evidence of positive selection and trans-specific polymorphism on the peptide-binding sites.

Peptidomes and Structures Illustrate How SLA-I Micropolymorphism Influences the Preference of Binding Peptide Length

Polymorphisms can affect MHC-I binding peptide length preferences, but the mechanism remains unclear. Using a random peptide library combined with LC-MS/MS and de novo sequencing (RPLD-MS) technique, we found that two swine MHC-I molecules with high sequence homology, SLA-1*04:01 and SLA-1*13:01, had significant differences in length preference of the binding peptides. Compared with SLA-1*04:01, SLA-1*13:01 binds fewer short peptides with 8-10 amino acids, but more long peptides. A dodecapeptide peptide (RW12) can bind to both SLA-1*04:01 and SLA-1*13:01, but their crystal structures indicate that the binding modes are significantly different: the entirety of RW12 is embedded in the peptide binding groove of SLA-1*04:01, but it obviously protrudes from the peptide binding groove of SLA-1*13:01. The structural comparative analysis showed that only five differential amino acids of SLA-1*13:01 and SLA-1*04:01 were involved in the binding of RW12, and they determine the different ways of long peptides binding, which makes SLA-1*04:01 more restrictive on long peptides than SLA-1*13:01, and thus binds fewer long peptides. In addition, we found that the N terminus of RW12 extends from the groove of SLA-1*13:01, which is similar to the case previously found in SLA-1*04:01. However, this unusual peptide binding does not affect their preferences of binding peptide length. Our study will be helpful to understand the effect of polymorphisms on the length distribution of MHC-I binding peptides, and to screen SLA-I-restricted epitopes of different lengths and to design effective epitope vaccines.

The immunopeptidomes of two transmissible cancers and their host have a common, dominant peptide motif

Transmissible cancers are malignant cells that can spread between individuals of a population, akin to both a parasite and a mobile graft. The survival of the Tasmanian devil, the largest remaining marsupial carnivore, is threatened by the remarkable emergence of two independent lineages of transmissible cancer, devil facial tumour (DFT) 1 and devil facial tumour 2 (DFT2). To aid the development of a vaccine and to interrogate how histocompatibility barriers can be overcome, we analysed the peptides bound to major histocompatibility complex class I (MHC‐I) molecules from Tasmanian devil cells and representative cell lines of each transmissible cancer. Here, we show that DFT1 + IFN‐γ and DFT2 cell lines express a restricted repertoire of MHC‐I allotypes compared with fibroblast cells, potentially reducing the breadth of peptide presentation. Comparison of the peptidomes from DFT1 + IFNγ, DFT2 and host fibroblast cells demonstrates a dominant motif, despite differences in MHC‐I allotypes between the cell lines, with preference for a hydrophobic leucine residue at position 3 and position Ω of peptides. DFT1 and DFT2 both present peptides derived from neural proteins, which reflects a shared cellular origin that could be exploited for vaccine design. These results suggest that polymorphisms in MHC‐I molecules between tumours and host can be ‘hidden’ by a common peptide motif, providing the potential for permissive passage of infectious cells and demonstrating complexity in mammalian histocompatibility barriers.

Lessons from the host defences of bats, a unique viral reservoir

There have been several major outbreaks of emerging viral diseases, including Hendra, Nipah, Marburg and Ebola virus diseases, severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS)-as well as the current pandemic of coronavirus disease 2019 (COVID-19). Notably, all of these outbreaks have been linked to suspected zoonotic transmission of bat-borne viruses. Bats-the only flying mammal-display several additional features that are unique among mammals, such as a long lifespan relative to body size, a low rate of tumorigenesis and an exceptional ability to host viruses without presenting clinical disease. Here we discuss the mechanisms that underpin the host defence system and immune tolerance of bats, and their ramifications for human health and disease. Recent studies suggest that 64 million years of adaptive evolution have shaped the host defence system of bats to balance defence and tolerance, which has resulted in a unique ability to act as an ideal reservoir host for viruses. Lessons from the effective host defence of bats would help us to better understand viral evolution and to better predict, prevent and control future viral spillovers. Studying the mechanisms of immune tolerance in bats could lead to new approaches to improving human health. We strongly believe that it is time to focus on bats in research for the benefit of both bats and humankind.

Antigenic variation of SARS-CoV-2 in response to immune pressure

Analysis of the bat viruses most closely related to SARS-CoV-2 indicated that the virus probably required limited adaptation to spread in humans. Nonetheless, since its introduction in human populations, SARS-CoV-2 must have been subject to the selective pressure imposed by the human immune system. We exploited the availability of a large number of high-quality SARS-CoV-2 genomes, as well as of validated epitope predictions, to show that B cell epitopes in the spike glycoprotein (S) and in the nucleocapsid protein (N) have higher diversity than nonepitope positions. Similar results were obtained for other human coronaviruses and for sarbecoviruses sampled in bats. Conversely, in the SARS-CoV-2 population, epitopes for CD4+ and CD8+ T cells were not more variable than nonepitope positions. A significant reduction in epitope variability was instead observed for some of the most immunogenic proteins (S, N, ORF8 and ORF3a). Analysis over longer evolutionary time frames indicated that this effect is not due to differential constraints. These data indicate that SARS-CoV-2 evolves to elude the host humoral immune response, whereas recognition by T cells is not actively avoided by the virus. However, we also found a trend of lower diversity of T cell epitopes for common cold coronaviruses, indicating that epitope conservation per se is not directly linked to disease severity. We suggest that conservation serves to maintain epitopes that elicit tolerizing T cell responses or induce T cells with regulatory activity.

Acute experimental infection of bats and ferrets with Hendra virus: Insights into the early host response of the reservoir host and susceptible model species

Bats are the natural reservoir host for a number of zoonotic viruses, including Hendra virus (HeV) which causes severe clinical disease in humans and other susceptible hosts. Our understanding of the ability of bats to avoid clinical disease following infection with viruses such as HeV has come predominantly from in vitro studies focusing on innate immunity. Information on the early host response to infection in vivo is lacking and there is no comparative data on responses in bats compared with animals that succumb to disease. In this study, we examined the sites of HeV replication and the immune response of infected Australian black flying foxes and ferrets at 12, 36 and 60 hours post exposure (hpe). Viral antigen was detected at 60 hpe in bats and was confined to the lungs whereas in ferrets there was evidence of widespread viral RNA and antigen by 60 hpe. The mRNA expression of IFNs revealed antagonism of type I and III IFNs and a significant increase in the chemokine, CXCL10, in bat lung and spleen following infection. In ferrets, there was an increase in the transcription of IFN in the spleen following infection. Liquid chromatography tandem mass spectrometry (LC-MS/MS) on lung tissue from bats and ferrets was performed at 0 and 60 hpe to obtain a global overview of viral and host protein expression. Gene Ontology (GO) enrichment analysis of immune pathways revealed that six pathways, including a number involved in cell mediated immunity were more likely to be upregulated in bat lung compared to ferrets. GO analysis also revealed enrichment of the type I IFN signaling pathway in bats and ferrets. This study contributes important comparative data on differences in the dissemination of HeV and the first to provide comparative data on the activation of immune pathways in bats and ferrets in vivo following infection.

Bats are natural reservoirs for a number of viruses, including HeV that cause severe disease in humans and other susceptible hosts. We examined acute HeV infection in pteropid bats, compared to ferrets, a species that develops fulminating disease following exposure to HeV, similar to humans. Analysis of HeV replication and transcription of innate immune genes was performed at 12, 36 and 60 hpe and global proteomics was performed on tissues at 60 hpe to obtain insight into the mechanisms responsible for innocuous (bats) compared to fatal (ferrets) HeV infection. We confirmed that both animal species had become infected on the basis of detection of viral RNA in bat lung (60 hpe) and ferret lung, lymph node, spleen, heart and intestine (36 and/or 60 hpe). Analysis of the transcription of IFNs and CXCL10, combined with global proteomics analysis revealed differences in the activation of the immune response between bats and ferrets, consistent with the difference in the control of viral replication and the development of pathology associated with disease between the two species. This study represents the first in vivo comparison between bats and a susceptible host and contributes important information on the kinetics and control of HeV in these two model species.

Novel Insights Into Immune Systems of Bats

In recent years, viruses similar to those that cause serious disease in humans and other mammals have been detected in apparently healthy bats. These include filoviruses, paramyxoviruses, and coronaviruses that cause severe diseases such as Ebola virus disease, Marburg haemorrhagic fever and severe acute respiratory syndrome (SARS) in humans. The evolution of flight in bats seem to have selected for a unique set of antiviral immune responses that control virus propagation, while limiting self-damaging inflammatory responses. Here, we summarize our current understanding of antiviral immune responses in bats and discuss their ability to co-exist with emerging viruses that cause serious disease in other mammals. We highlight how this knowledge may help us to predict viral spillovers into new hosts and discuss future directions for the field.

Peptide presentation by bat MHC class I provides new insight into the antiviral immunity of bats

Bats harbor many zoonotic viruses, including highly pathogenic viruses of humans and other mammals, but they are typically asymptomatic in bats. To further understand the antiviral immunity of bats, we screened and identified a series of bat major histocompatibility complex (MHC) I Ptal-N*01:01-binding peptides derived from four different bat-borne viruses, i.e., Hendra virus (HeV), Ebola virus (EBOV), Middle East respiratory syndrome coronavirus (MERS-CoV), and H17N10 influenza-like virus. The structures of Ptal-N*01:01 display unusual peptide presentation features in that the bat-specific 3-amino acid (aa) insertion enables the tight "surface anchoring" of the P1-Asp in pocket A of bat MHC I. As the classical primary anchoring positions, the B and F pockets of Ptal-N*01:01 also show unconventional conformations, which contribute to unusual peptide motifs and distinct peptide presentation. Notably, the features of bat MHC I may be shared by MHC I from various marsupials. Our study sheds light on bat adaptive immunity and may benefit future vaccine development against bat-borne viruses of high impact on humans.

Can extreme MHC class I diversity be a feature of a wide geographic range? The example of Seba's short-tailed bat (Carollia perspicillata)

The major histocompatibility complex (MHC) is one of the most diverse genetic regions under pathogen-driven selection because of its central role in antigen binding and immunity. The highest MHC variability, both in terms of the number of individual alleles and gene copies, has so far been found in passerine birds; this is probably attributable to passerine adaptation to both a wide geographic range and a diverse array of habitats. If extraordinary high MHC variation and duplication rates are adaptive features under selection during the evolution of ecologically and taxonomically diverse species, then similarly diverse MHC architectures should be found in bats. Bats are an extremely species-rich mammalian group that is globally widely distributed. Many bat species roost in multitudinous groups and have high contact rates with pathogens, conspecifics, and allospecifics. We have characterized the MHC class I diversity in 116 Panamanian Seba's short-tailed bats (Carollia perspicillata), a widely distributed, generalist, neotropical species. We have detected a remarkable individual and population-level diversity of MHC class I genes, with between seven and 22 alleles and a unique genotype in each individual. This diversity is comparable with that reported in passerine birds and, in both taxonomic groups, further variability has evolved through length polymorphisms. Our findings support the hypothesis that, for species with a geographically broader range, high MHC class I variability is particularly adaptive. Investigation of the details of the underlying adaptive processes and the role of the high MHC diversity in pathogen resistance are important next steps for a better understanding of the role of bats in viral evolution and as carriers of several deadly zoonotic viruses.

Origin and evolution of the major histocompatibility complex class I region in eutherian mammals

Major histocompatibility complex (MHC) genes in vertebrates are vital in defending against pathogenic infections. To gain new insights into the evolution of MHC Class I (MHCI) genes and test competing hypotheses on the origin of the MHCI region in eutherian mammals, we studied available genome assemblies of nine species in Afrotheria, Xenarthra, and Laurasiatheria, and successfully characterized the MHCI region in six species. The following numbers of putatively functional genes were detected: in the elephant, four, one, and eight in the extended class I region, and κ and β duplication blocks, respectively; in the tenrec, one in the κ duplication block; and in the four bat species, one or two in the β duplication block. Our results indicate that MHCI genes in the κ and β duplication blocks may have originated in the common ancestor of eutherian mammals. In the elephant, tenrec, and all four bats, some MHCI genes occurred outside the MHCI region, suggesting that eutherians may have a more complex MHCI genomic organization than previously thought. Bat-specific three- or five-amino-acid insertions were detected in the MHCI α1 domain in all four bats studied, suggesting that pathogen defense in bats relies on MHCIs having a wider peptide-binding groove, as previously assayed by a bat MHCI gene with a three-amino-acid insertion showing a larger peptide repertoire than in other mammals. Our study adds to knowledge on the diversity of eutherian MHCI genes, which may have been shaped in a taxon-specific manner.

Devil Facial Tumours: Towards a Vaccine

The Tasmanian devil is the only mammalian species to harbour two independent lineages of contagious cancer. Devil facial tumour 1 (DFT1) emerged in the 1990s and has caused significant population declines. Devil facial tumour 2 (DFT2) was identified in 2014, and evidence indicates that this new tumour has emerged independently of DFT1. While DFT1 is widespread across Tasmania, DFT2 is currently found only on the Channel Peninsula in south east Tasmania. Allograft transmission of cancer cells should be prevented by major histocompatibility complex (MHC) molecules. DFT1 avoids immune detection by downregulating MHC class I expression, which can be reversed by treatment with interferon-gamma (IFNγ), while DFT2 currently circulates in hosts with a similar MHC class I genotype to the tumour. Wild Tasmanian devil numbers have not recovered from the emergence of DFT1, and it is feared that widespread transmission of DFT2 will be devastating to the remaining wild population. A preventative solution for the management of the disease is needed. Here, we review the current research on immune responses to devil facial tumours and vaccine strategies against DFT1 and outline our plans moving forward to develop a specific, effective vaccine to support the wild Tasmanian devil population against the threat of these two transmissible tumours.

Nano-based approach to combat emerging viral (NIPAH virus) infection

Emergence of new virus and their heterogeneity are growing at an alarming rate. Sudden outburst of Nipah virus (NiV) has raised serious question about their instant management using conventional medication and diagnostic measures. A coherent strategy with versatility and comprehensive perspective to confront the rising distress could perhaps be effectuated by implementation of nanotechnology. But in concurrent to resourceful and precise execution of nano-based medication, there is an ultimate need of concrete understanding of the NIV pathogenesis. Moreover, to amplify the effectiveness of nano-based approach in a conquest against NiV, a list of developed nanosystem with antiviral activity is also a prerequisite. Therefore the present review provides a meticulous cognizance of cellular and molecular pathogenesis of NiV. Conventional as well several nano-based diagnosis experimentations against viruses have been discussed. Lastly, potential efficacy of different forms of nano-based systems as convenient means to shield mankind against NiV has also been introduced.

Mass spectrometry-based identification of MHC-bound peptides for immunopeptidomics

Peptide antigens bound to molecules encoded by the major histocompatibility complex (MHC) and presented on the cell surface form the targets of T lymphocytes. This critical arm of the adaptive immune system facilitates the eradication of pathogen-infected and cancerous cells, as well as the production of antibodies. Methods to identify these peptide antigens are critical to the development of new vaccines, for which the goal is the generation of effective adaptive immune responses and long-lasting immune memory. Here, we describe a robust protocol for the identification of MHC-bound peptides from cell lines and tissues, using nano-ultra-performance liquid chromatography coupled to high-resolution mass spectrometry (nUPLC-MS/MS) and recent improvements in methods for isolation and characterization of these peptides. The protocol starts with the immunoaffinity capture of naturally processed MHC-peptide complexes. The peptides dissociate from the class I human leukocyte antigens (HLAs) upon acid denaturation. This peptide cargo is then extracted and separated into fractions by HPLC, and the peptides in these fractions are identified using nUPLC-MS/MS. With this protocol, several thousand peptides can be identified from a wide variety of cell types, including cancerous and infected cells and those from tissues, with a turnaround time of 2-3 d.

Structure and Peptidome of the Bat MHC Class I Molecule Reveal a Novel Mechanism Leading to High-Affinity Peptide Binding

Bats are natural reservoir hosts, harboring more than 100 viruses, some of which are lethal to humans. The asymptomatic coexistence with viruses is thought to be connected to the unique immune system of bats. MHC class I (MHC I) presentation is closely related to cytotoxic lymphocyte immunity, which plays an important role in viral resistance. To investigate the characteristics of MHC I presentation in bats, the crystal structures of peptide-MHC I complexes of Pteropus alecto, Ptal-N*01:01/HEV-1 (DFANTFLP) and Ptal-N*01:01/HEV-2 (DYINTNLVP), and two related mutants, Ptal-N*01:01/HEV-1_PΩL (DFANTFLL) and Ptal-N*01:01_ΔMDL/HEV-1, were determined. Through structural analysis, we found that Ptal-N*01:01 had a multi-Ala-assembled pocket B and a flexible hydrophobic pocket F, which could accommodate variable anchor residues and allow Ptal-N*01:01 to bind numerous peptides. Three sequential amino acids, Met, Asp, and Leu, absent from the α1 domain of the H chain in other mammals, were present in this domain in the bat. Upon deleting these amino acids and determining the structure in p/Ptal-N*01:01_ΔMDL/HEV-1, we found they helped form an extra salt-bridge chain between the H chain and the N-terminal aspartic acid of the peptide. By introducing an MHC I random peptide library for de novo liquid chromatography-tandem mass spectrometry analysis, we found that this insertion module, present in all types of bats, can promote MHC I presentation of peptides with high affinity during the peptide exchange process. This study will help us better understand how bat MHC I presents high-affinity peptides from an extensive binding peptidome and provides a foundation to understand the cellular immunity of bats.

The use of proteomics to understand antiviral immunity

Viruses are intracellular pathogens that cause a vast array of diseases, which are often severe and typified by high morbidity and mortality rates. Viral infections continue to be a global health burden and effective vaccines and therapeutics are constantly sought to prevent and treat these infections. The development of such treatments generally relies on understanding the mechanisms that underpin efficient host antiviral immune responses. This review summarises recent developments in our understanding of antiviral adaptive immunity and in particular, highlights the use of mass spectrometry to elucidate viral antigens and their processing and presentation to T cells and other immune effectors. These processed peptides serve as potential vaccine candidates or may facilitate clinical monitoring, diagnosis and immunotherapy of infectious diseases.

Employing proteomics in the study of antigen presentation: an update

INTRODUCTION

Our immune system discriminates self from non-self by examining the peptide cargo of human leukocyte antigen (HLA) molecules displayed on the cell surface. Successful recognition of HLA-bound non-self peptides can induce T cell responses leading to, for example, the destruction of infected cells. Today, largely due to advances in technology, we have an unprecedented capability to identify the nature of these presented peptides and unravel the true complexity of antigen presentation. Areas covered: In addition to conventional linear peptides, HLA molecules also present post-translationally modified sequences comprising a wealth of chemical and structural modifications, including a novel class of noncontiguous spliced peptides. This review focuses on these emerging themes in antigen presentation and how mass spectrometry in particular has contributed to a new view of the antigenic landscape that is presented to the immune system. Expert Commentary: Advances in the sensitivity of mass spectrometers and use of hybrid fragmentation technologies will provide more information-rich spectra of HLA bound peptides leading to more definitive identification of T cell epitopes. Coupled with improvements in sample preparation and new informatics workflows, studies will access novel classes of peptide antigen and allow interrogation of rare and clinically relevant samples.

Tadpole-like Conformations of Huntingtin Exon 1 Are Characterized by Conformational Heterogeneity that Persists regardless of Polyglutamine Length

Soluble huntingtin exon 1 (Httex1) with expanded polyglutamine (polyQ) engenders neurotoxicity in Huntington's disease. To uncover the physical basis of this toxicity, we performed structural studies of soluble Httex1 for wild-type and mutant polyQ lengths. Nuclear magnetic resonance experiments show evidence for conformational rigidity across the polyQ region. In contrast, hydrogen-deuterium exchange shows absence of backbone amide protection, suggesting negligible persistence of hydrogen bonds. The seemingly conflicting results are explained by all-atom simulations, which show that Httex1 adopts tadpole-like structures with a globular head encompassing the N-terminal amphipathic and polyQ regions and the tail encompassing the C-terminal proline-rich region. The surface area of the globular domain increases monotonically with polyQ length. This stimulates sharp increases in gain-of-function interactions in cells for expanded polyQ, and one of these interactions is with the stress-granule protein Fus. Our results highlight plausible connections between Httex1 structure and routes to neurotoxicity.

Mammalia: Chiroptera: Immunology of Bats

Bats are a large and diverse group comprising approximately 20% of all living mammalian species. They are the only mammals capable of powered flight and have many unique characteristics, including long lifespans, echolocation, and hibernation, and play key roles in insect control, pollination, and seed dispersal. The role of bats as natural reservoirs of a variety of high-profile viruses that are highly pathogenic in other susceptible species yet cause no clinical disease in bats has led to a resurgence of interest in their immune systems. Equally compelling is the urgency to understand the immune mechanisms responsible for the susceptibility of bats to the fungus responsible for white syndrome, which threatens to wipe out a number of species of North American bats. In this chapter we review the current knowledge in the field of bat immunology, focusing on recent highlights and the need for further investigations in this area.

Tools to study pathogen-host interactions in bats

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Bats are important reservoir hosts for emerging zoonotic viruses.

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Viruses detected in bats are difficult to isolate using traditional cell lines.

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Bat cell lines provide critical tools to dissect host pathogen interactions.

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Little is known about immune cell populations and their responses in bats.

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Sharing reagents and cell lines will accelerate research and virus discovery.

Bats are natural reservoirs for a variety of emerging viruses that cause significant disease in humans and domestic animals yet rarely cause clinical disease in bats. The co-evolutionary history of bats with viruses has been hypothesized to have shaped the bat-virus relationship, allowing both to exist in equilibrium. Progress in understanding bat-virus interactions and the isolation of bat-borne viruses has been accelerated in recent years by the development of susceptible bat cell lines. Viral sequences similar to severe acute respiratory syndrome corona virus (SARS-CoV) have been detected in bats, and filoviruses such as Marburg virus have been isolated from bats, providing definitive evidence for the role of bats as the natural host reservoir. Although viruses can be readily detected in bats using molecular approaches, virus isolation is far more challenging. One of the limitations in using traditional culture systems from non-reservoir species is that cell types and culture conditions may not be compatible for isolation of bat-borne viruses. There is, therefore, a need to develop additional bat cell lines that correspond to different cell types, including less represented cell types such as immune cells, and culture them under more physiologically relevant conditions to study virus host interactions and for virus isolation. In this review, we highlight the current progress in understanding bat-virus interactions in bat cell line systems and some of the challenges and limitations associated with cell lines. Future directions to address some of these challenges to better understand host-pathogen interactions in these intriguing mammals are also discussed, not only in relation to viruses but also other pathogens carried by bats including bacteria and fungi.

Herpesviral capture of immunomodulatory host genes

Herpesviruses have acquired numerous genes from their hosts. Although these homologs are not essential for viral replication, they often have important immunomodulatory functions that ensure viral persistence in the host. Some of these viral molecules are called virokines as they mimic cellular cytokines of their host such as interleukin-10 (cIL-10). In recent years, many viral homologs of IL-10 (vIL-10s) have been discovered in the genome of members of the order Herpesvirales. For some, gene and protein structure as well as biological activity and potential use in the clinical context have been explored. Besides virokines, herpesviruses have also captured genes encoding membrane-bound host immunomodulatory proteins such as major histocompatibility complex (MHC) molecules. These viral MHC mimics also retain many of the functions of the cellular genes, in particular directly or indirectly modulating the activity of natural killer cells. The mechanisms underlying capture of cellular genes by large DNA viruses are still enigmatic. In this review, we provide an update of the advances in the field of herpesviral gene piracy and discuss possible scenarios that could explain how the gene transfer from host to viral genome was achieved.

Jumping species-a mechanism for coronavirus persistence and survival

Zoonotic transmission of novel viruses represents a significant threat to global public health and is fueled by globalization, the loss of natural habitats, and exposure to new hosts. For coronaviruses (CoVs), broad diversity exists within bat populations and uniquely positions them to seed future emergence events. In this review, we explore the host and viral dynamics that shape these CoV populations for survival, amplification, and possible emergence in novel hosts.

The interplay between viruses and the immune system of bats

Bats are an abundant and diverse group of mammals with an array of unique characteristics, including their well-known roles as natural reservoirs for a variety of viruses. These include the deadly zoonotic paramyxoviruses; Hendra (HeV) and Nipah (NiV)1,2, lyssaviruses3, coronaviruses such as severe acute respiratory coronavirus (SARS-CoV)4 and filoviruses such as Marburg5. Although these viruses are highly pathogenic in other species, including humans, bats rarely show clinical signs of disease whilst maintaining the ability to transmit virus to susceptible vertebrate hosts. In addition, bats are capable of clearing experimental infections with henipaviruses, filoviruses and lyssaviruses at doses of infection that are lethal in other mammals6–12. Curiously, the ability of bats to tolerate viral infections does not appear to extend to extracellular pathogens such as bacteria, fungi and parasites13. Over the past few years, considerable headway has been made into elucidating the mechanisms responsible for the ability of bats to control viral replication, with evidence for unique differences in the innate immune responses of bats14–20. However, many questions remain around mechanisms responsible for the ability of bats to co-exist with viruses, including their ability to tolerate constitutive immune activation, the triggers associated with viral spillover events and the sites of viral replication. Although bats appear to have all of the major components of the immune system present in other species, their unique ecological characteristics (including flight, high density populations and migration) combined with their long co-evolutionary history with viruses has likely shaped their immune response resulting in an equilibrium between the host and its pathogens.