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

Abstract

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.

Keywords: Evolutionary biology; Virology.

Graphical abstract

Figure 1

Ecological and behavioral aspects that influence the viral reservoir status of bats The order Chiroptera is composed of over 1400 bat species that span various ecological and behavioral traits. (A) For instance, bats are the only mammals capable of true flight. During flight, the core body temperature of bats, such as the little brown bat (Myotis lucifugus), rises to over 40°C (Hock, 1951). Some viruses that have been identified in bats, such as filoviruses, are capable of replicating in selected bat cells (Hypsignathus monstrosus, Epomops buettikoferi, Rousettus aegyptiacus, Artibeus jamaicensis, Tadarida brasiliensis, and Eptesicus fuscus) at temperatures attained during flight, suggesting that some viruses may have adapted to replicate or co-exist in their bat hosts even at elevated body temperatures (Miller et al., 2016). In addition, ebolavirus and bat paramyxoviruses have been detected in fecal matter and/or urine of experimentally and naturally infected bats (Swanepoel et al., 1996; Peel et al., 2019), suggesting that some viruses may be transmitted during defecation. Bats are also capable of discarding food remnants, such as fruits, which may be contaminated with virus-infected biological matter, allowing foraging animals to potentially become infected upon consumption (Chua et al., 2002; Nikolay et al., 2019). (B) The ability of bats to fly long distances may allow for the transmission of novel viruses and variants amongst bat populations and potentially into humans and other animals. (C) Yangochiropterans, like Myotis lucifugus (Kazial et al., 2008), use laryngeal echolocation to produce sounds from the mouth or nose (Neuweiler, 2000). Therefore, it might be possible for these bat species to produce aerosols while echolocating, which would potentially allow for the dissemination of viruses that replicate within the respiratory tract, such as rabies, Aravan, Khujand, and Irkut viruses (Constantine et al., 1972; Hughes et al., 2006). The relation between echolocation and aerosol production in bats remains to be mechanistically investigated. (D) Temperate bats are known to hibernate during winter, allowing viruses, such as rabies virus, to be maintained for extended periods of time (George et al., 2011). The role of hibernation and daily torpor in facilitating virus tolerance and persistence in bats remains understudied. (E) Hibernation, amongst other factors, positively influences the exceptional lifespan of bats (Wilkinson and South, 2002). The immunological consequences of aging and associated control over virus replication and persistence remain unknown for bats. (F) Bats are gregarious species and roost in multi-species colonies. Living in dense clusters may facilitate the spread of viruses and other pathogens between different bat species and within immune and naive individuals of the same species, as described for European bat lyssavirus (López-Roig et al., 2014). (G) As the order Chiroptera diverged over 64 million years ago, bats may have co-evolved with some viral families to develop fine-tuned antiviral responses that limit host damage and promote viral tolerance. The phylogenetic tree was adapted from the phylogenomic analyses performed by Lei and Dong (Lei and Dong, 2016). Created with BioRender.com.

Figure 2

The bat innate immune response In mammalian cells, infection with single stranded (ssRNA) or double-stranded RNA (dsRNA) virus is detected by pattern recognition receptors, such as Toll-like receptors (TLRs) 3, 7, and 8 within the endosome (A, B). Studies in the black flying fox (Pteropus alecto) have described the existence of TLRs 3, 7, and 8 (Cowled et al., 2011); however, computational analyses for eight different bat species (Eptesicus fuscus, Myotis brandtii, Myotis davidii, Myotis lucifugus, Pteropusalecto, Pteropus vampyrus, Rousettus leschenaultii, and Desmodus rotundus) have identified unique mutations within the binding domains of TLR7 and 8 which suggest potential functional differences (Escalera-Zamudio et al., 2015). Cytoplasmic receptors, such as retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5) may also detect dsRNA in the cytosol (C), leading to the activation of downstream adaptor proteins, such as mitochondrial antiviral signaling protein (MAVS) (D). Studies in P. alecto and E. fuscus cells have shown conserved structure, expression, and function for MDA5 and RIG-I (Cowled et al., 2012; Papenfuss et al., 2012; Banerjee et al., 2017). The downstream signaling cascade mediated by MAVS has not been reported for bats. Viral DNA (vDNA) present within endosomes or cytosol can be detected by TLR9 (E) and cytosolic DNA sensors (F), with the latter signaling through the stimulator of interferon genes (STING) (G). Within three different bat families (Phyllostomidae, Pteropodidae, and Vespertilionidae), computational analyses have discovered mutations within the binding domain of TLR9 which may alter the specificity and signaling of TLR9 in these bats (Escalera-Zamudio et al., 2015). Bats have also lost certain DNA sensors, such as the PYHIN gene family, ultimately leading to a dampened NLR-family PYRIN domain containing 3 (NLRP3)-mediated inflammasome response (Ahn et al., 2016, 2019) (H). In addition, a recent study demonstrated that bats have reduced STING activation because of a point mutation at amino acid position 358 (Xie et al., 2018), with implications for cellular responses generated against DNA virus infection and host DNA damage. Upon recognition of viral nucleic acid by TLRs, RIG-I, MDA5 and cytosolic DNA sensors, cellular kinases within the infected cell are activated (I), leading to the activation of transcription factors, like interferon (IFN) regulatory factor 1 (IRF1), IRF3 and IRF7 (J). This will ultimately lead to the induction of type I IFNs (K), such as IFNα and IFNβ, which will be secreted (L) by the infected cell to induce an antiviral state in an autocrine and paracrine manner. The existence of IRF1, 3 and 7 have been described in P. alecto and E. fuscus, and studies have demonstrated a difference in the distribution and expression pattern of IRF7 in P. alecto (Zhou et al., 2014, 2016a), enhanced antiviral activity of IRF3 (Banerjee et al., 2020d), and the regulation of alternate antiviral pathways by IRF1 and IRF7 (Irving et al., 2020). Signaling through TLRs may also lead to the activation of nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) (M), which in turn induces the expression (N) and secretion of pro-inflammatory cytokines, such as tumor necrosis factor alpha (TNFα), interleukin 8 (IL-8), and IL-1 (O). While NF-κB has been described in bats, genome wide screens performed for six bat species (Rhinolophus ferrumequinum, Rousettus aegyptiacus, Phyllostomus discolor, Myotis myotis, Pipistrellus kuhlii and Molossus molossus) have demonstrated altered NF-κB signaling, which may contribute to bats having a higher tolerance toward viruses (Jebb et al., 2020). The activation of certain pro-inflammatory cytokines, such as TNFα is dampened in E. fuscus cells due to the presence of a c-Rel binding site in the TNFα promoter (P) (Banerjee et al., 2017). In this schematic, dampened responses are indicated by the red arrows (↓). This figure is not representative of a universal bat cellular response, as it has been compiled using evidence from various studies. Much remains unknown about bat cellular responses to infection and differences between bat species and with other mammalian species. ER, endoplasmic reticulum. Created with BioRender.com.