Bats: A Unique Viral Reservoir

Bats are small mammals capable of true sustained flight. They belong to the second-largest mammalian order, Chiroptera, which comprises 22% (over 1400) of all mammalian species after rodents. They are found on all continents except Antarctica. Bats are divided into two types: Megachiroptera, which fly by vision, and Microchiroptera, which fly by echolocation and magnetoreception.

Most bats live in caves, hollow trees, and foliage, in large colonies of 100 to 100,000 individuals. Because of their feeding practices on insects, fruits, nectar, pollen, fish, etc., they are extremely important for the ecosystem, for pollination and pest control. Without bats, numerous medicinal plants would vanish.

Some unique features of Bats

Bats are rather unique mammals. Their ability to harbor viruses comes from various traits within themselves. A few of those traits are as follows: 

• Bats can fly up to 2000 km, depending on the species, in their immense migration movements. 
• During their flight, bats are metabolically active with their body temperature rising to about 41°C, with 1066 beats per minute and up to 34 times basal metabolic rate owing to energy consumption. 
• Because of this reason, the nocturnal mammals sleep and hibernate occasionally, during which their heart beats become 10 to 16 beats per minute, with the body temperature being less than 5.8°C. 
• Similarly, despite their smaller body size ratio, they are the second most common living mammal, living up to 40 years. 

Bats as a Viral Host Reservoir

The idea of viruses being in bats was formulated in the early 20^th century with the discovery of the rabies virus in bats. Since then, about 54% of zoonotic viral pathogens have been discovered in these small creatures. A few of the significant bat-borne viruses are SARS, MERS, Ebola, Hendra, Nipah, etc. These viruses are responsible for respiratory diseases, diarrhea, pneumonia, bronchitis, the common cold, and other diseases.

Coronaviruses like SARS-CoV-2 are asymptomatic in bats, unlike in civets and pangolins. However, they are not entirely immune to all illnesses. To elaborate, a few viruses, such as Tacaribe virus and Lyssavirus, can cause severe symptoms in bats, even leading to death. 

Coevolution of Bats and Viruses

The abundance of viruses in bats calls for their relationship to be rooted deep within their genome, relating to evolution. Fossil evidence suggests that bats and viruses go way back to more than 65 million years during the extinction of dinosaurs, i.e., the KT extinction event. Studies have revealed that bats have an abundance of RNA viruses rather than DNA viruses. This might be because RNA viruses are more prone to errors because of the lack of a proofreading mechanism in RNA polymerase, in contrast to DNA polymerase in DNA viruses. This leads to more variability in the antigen receptor of the virus, which enhances its viral capabilities to infect the host.

Bat Immunity Against Viruses

Several hypotheses exist regarding why bats harbor a considerable number of viruses. Since the discovery of viral pathogens in bats, the fever hypothesis has been thought to be the major assumption for bat immunity.

When the body gets infected by a certain pathogen, its primary defense mechanism is to raise temperature, to cause fever. The body increases its temperature to a suboptimal level to kill the unwanted pathogen/s. Bats innately have rising body temperatures of up to 41°C during their flight. According to the fever hypothesis, this is thought to decrease viral load and minimize infection. However, research has suggested that no viral titer variability was found in bat cells grown at 37°C and 41°C, meaning that the number of virus counts was similar in both cases. The hypothesis, however, cannot be fully discarded as it might have some kind of relationship with other aspects of bat immunity. 

Innate Defense Mechanism

• Interferon Expression: Type I Interferons (IFNs) are cytokines or signaling proteins that help the body fight against infections. In humans, INFs are inducible after infection. The same is not true for bats; bats constitutively express INFs even before stimulation with enhanced viral response. These genes are regulated by INF regulatory factors, lowering the production of inflammatory cytokines. Similarly, IFN-stimulated Genes (ISGs) also play a role in maintaining viral titer in bats without showing any sign of clinical symptoms. 
• Heat shock proteins: Bats produce enhanced levels of Heat-shock Proteins (HSPs) that tolerate high temperature and oxidative stress during flight. This may account for the rapid evolution of the virus, which tolerates mutations through the use of chaperone viral proteins. 
• Enhanced autophagy: Autophagy is the cellular process of removing and recycling impaired proteins and cell organelles. Bats are known to have enhanced autophagy with the ability to remove pathogens. 

Immune Tolerance Mechanism

The most significant immune tolerance mechanisms in bats are the variation in the cGAS STING pathway and the NLPR3 Inflammasome pathway. 

cGAS STING Pathway

The cGAS STING pathway is a component of the immune system that functions to determine the presence of cytosolic DNA to trigger the expression of inflammatory genes. 

The foreign or abnormal DNA is initially detected by the DNA sensor, cGAS (cyclic GMP-AMP Synthase). The cGAS binds to the DNA and leads to the formation of cyclic GMP-AMP (cGAMP). The produced cGAMP binds to Stimulator of interferon genes (STING) and activates the protein. This activation leads to a cascade of signaling events, which includes TBK1 and IRF3. Upon this activation, INFs and several other immune responses are induced. 

In bats, however, the STING-dependent IFN response is weak. This is because there is a point mutation in the STING protein, inducing weak TBK1 response, leading to lower production of INF.

NLPR3 Inflammasome Pathway

The NLPR3 inflammasome pathway is responsible for regulating the innate immune system and inflammatory cytokines.

In humans or mice, the NLPR3 inflammasome pathway involves the activation of NF-kB signaling via Pattern Recognition Receptor (PRR). This leads to the expression of pro-IL-1β and NLRP3. Similarly, NLRP3 assembly can also be activated via pathogens (viruses) or other abnormal danger signals. On the other hand, foreign dsDNA leads to ASC protein recruitment. The combined effect of the NLRP3 inflammasome complex and ASC proteins induces caspase-1 activation. The caspase-1 cleaves pro-IL-1β B to IL-1β B, leading to the production of IL-1β and pyroptosis of the cell. 

In bats, the inflammasome pathway is diminished. The PRR activation abnormally activates NLPR3, leading to its reduced protein function. Similarly, because of the loss of AIM2 protein (of the PYHIN family), the ASC complex is impaired. This leads to reduced activation of caspase-1. Therefore, pyroptosis is absent, and there is lower production of IL-1β B, significantly lowering inflammation in bats.

Spillover in Bats

• Roosting habitat: Bats innately have a dirty roosting habitat where thousands of bats reside in a single cave. They often excrete, which becomes a grooming area for zoonotic pathogens. The pathogens can also spread easily within the colony. 
• Physiological stress: When bats are stressed by certain stimuli, which can include low food availability, a predator, their behavior becomes abnormal. This leads to reduced immune function, more susceptibility to pathogens, and negatively-affects reproductive physiology. 
• Environmental changes: Climate change affects the physiology of the ecosystem. Bats must travel large distances to feed on insects, fruits, nectar, etc., which is not optimal. Similarly, urbanization disturbs bat habitats, therefore, promoting viral spillover.  
• Direct human exposure: Direct human exposure to bats is rare. In some parts of the world, humans hunt bats for illegal wildlife trading and even consume them. Bat excreta, known as bat guano, is also a valuable fertilizer because of its rich nitrogen. The extraction of this compound can also expose humans to bats, leading to zoonotic transmission. 
• Bridging host exposure: Bridging host exposure is more common when an intermediate animal directly contacts bats, and then the pathogen is transmitted to humans. This is an example of the SARS-CoV-2 pandemic where either civets or pangolins got infected through bats and then spread the virus to humans. 

Learning from Bats

• Immune tolerance: Bats have developed several mechanisms to combat zoonotic viral pathogens and inflammatory diseases. Studies on these mechanisms can be helpful to develop a cure for autoimmune diseases or even combat viruses. 
• Viral spillovers: Research in viral spillovers from bats or any other zoonosis helps to prevent and predict potential epidemics and pandemics in the future.
• Model species: Like mouse, zebra fish, drosophila, etc., bats have high potential to be a powerful model species to study zoonotic hosts. There is much to learn about the dynamics of zoonotic pathogens with their hosts. Bat-mouse chimaera models and mouse models with bat genes are being invented for such purposes.
• Disturbance of bat habitats: Because of the nature of habitats in bats and their onset of occurrence of numerous diseases, it is wise not to get in contact with bats or disturb their population. 

Challenges in Bat Research

• In vivo challenges: In vivo research is essential to fully understand bats and their physiology. This would require the captivity of bats, which can be extremely challenging because of their nocturnal, mysterious lifestyles and flying abilities. Extensive safety and care would be required for such intervention. 
• In vitro challenges: In vitro research could be an alternative for bat research. Growing bat cells in a laboratory setting is slowly taking shape, however, the research is still novice and requires the assimilation of tools and reagents specific for bat cells. In addition to this, because of the variation of bat immunity from humans and mice, there is a need for better research tools and antibodies, which are not readily available.  
• Larger species diversity: Because of the larger species diversity in bats, a consensus bat species is difficult to pinpoint. Findings in a certain species of bats might not work in other species. 

References

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