Serum proteomics reveals a tolerant immune phenotype across multiple pathogen taxa in wild vampire bats

Translational lessons from the balanced immune system in bats

Bats are a natural reservoir for a wide variety of notorious viruses that are deadly to humans and other mammals but cause no or minimal clinical damage in bats. The co-evolution of bats and viruses for more than sixty million years has established unique and balanced immune defenses within bats against a number of viruses. With the COVID-19 pandemic, bats have gained greater attention as a likely reservoir of the SARS-CoV-2 ancestor virus. The coupling of omics technology and bat research opens an exciting new field to understand and translate discoveries from bats to humans, in the context of infectious disease and beyond. Here, we focus on the mechanism of immunity balance in bats, the application of omics and how this might lead to improvement of human health.

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.

Geographically widespread and novel hemotropic mycoplasmas and bartonellae in Mexican free-tailed bats and sympatric North American bat species

Bacterial pathogens remain poorly characterized in bats, especially in North America. We describe novel (and in some cases panmictic) hemoplasmas (10.1% positivity) and bartonellae (25.6% positivity) across three colonies of Mexican free-tailed bats (Tadarida brasiliensis), a partially migratory species that can seasonally travel hundreds of kilometers. Molecular analyses identified three novel Candidatus hemoplasma species most similar to another novel Candidatus species in Neotropical molossid bats. We also detected novel hemoplasmas in sympatric cave myotis (Myotis velifer) and pallid bats (Antrozous pallidus), with sequences in the latter 96.5% related to Candidatus Mycoplasma haematohominis. We identified nine Bartonella genogroups, including those in cave myotis with 96.1% similarity to Candidatus Bartonella mayotimonensis. We also detected Bartonella rochalimae in migratory Mexican free-tailed bats, representing the first report of this human pathogen in the Chiroptera. Monthly sampling of migratory Mexican free-tailed bats during their North American occupancy period also revealed significant seasonality in infection for both bacterial pathogens, with prevalence increasing following spring migration, peaking in the maternity season, and declining into fall migration. The substantial diversity and seasonality of hemoplasmas and bartonellae observed here suggest that additional longitudinal, genomic, and immunological studies in bats are warranted to inform One Health approaches.

IMPORTANCE

Bats have been intensively sampled for viruses but remain mostly understudied for bacterial pathogens. However, bacterial pathogens can have significant impacts on both human health and bat morbidity and even mortality. Hemoplasmas and bartonellae are facultative intracellular bacteria of special interest in bats, given their high prevalence and substantial genetic diversity. Surveys have also supported plausible zoonotic transmission of these bacteria from bats to humans, including Candidatus Mycoplasma haematohominis and Candidatus Bartonella mayotimonensis. Greater characterization of these bacteria across global bat diversity (over 1,480 species) is therefore warranted to inform infection risks for both bats and humans, although little surveillance has thus far been conducted in North American bats. We here describe novel (and in some cases panmictic) hemoplasmas and bartonellae across three colonies of Mexican free-tailed bats and sympatric bat species. We find high genetic diversity and seasonality of these pathogens, including lineages closely related to human pathogens, such as Bartonella rochalimae.

Geographically widespread and novel hemotropic mycoplasmas and bartonellae in Mexican free-tailed bats and sympatric North American bat species

Bacterial pathogens remain poorly characterized in bats, especially in North America. We describe novel (and in some cases panmictic) hemoplasmas (10.5% positivity) and bartonellae (25.5% positivity) across three colonies of Mexican free-tailed bats (Tadarida brasiliensis), a partially migratory species that can seasonally travel hundreds of kilometers. Molecular analyses identified three novel Candidatus hemoplasma species most similar to another novel Candidatus species in Neotropical molossid bats. We also detected novel hemoplasmas in sympatric cave myotis (Myotis velifer) and pallid bats (Antrozous pallidus), with sequences in the latter 96.5% related to C. Mycoplasma haemohominis. We identified nine Bartonella genogroups, including those in cave myotis with 96.7% similarity to C. Bartonella mayotimonensis. We also detected Bartonella rochalimae in migratory Mexican free-tailed bats, representing the first report of this human pathogen in the Chiroptera. The seasonality and diversity of these bacteria observed here suggest that additional longitudinal, genomic, and immunological studies in bats are warranted.

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.

Advances in understanding bat infection dynamics across biological scales

Over the past two decades, research on bat-associated microbes such as viruses, bacteria and fungi has dramatically increased. Here, we synthesize themes from a conference symposium focused on advances in the research of bats and their microbes, including physiological, immunological, ecological and epidemiological research that has improved our understanding of bat infection dynamics at multiple biological scales. We first present metrics for measuring individual bat responses to infection and challenges associated with using these metrics. We next discuss infection dynamics within bat populations of the same species, before introducing complexities that arise in multi-species communities of bats, humans and/or livestock. Finally, we outline critical gaps and opportunities for future interdisciplinary work on topics involving bats and their microbes.