Immune System Modulation and Viral Persistence in Bats: Understanding Viral Spillover

Subudhi, Sonu; Rapin, Noreen; Misra, Vikram

The unique features of bat immune responses that promote viral persistence may exert evolutionary pressures on the virus as well. Bats have superseded rodents in harboring greater number of viruses and also having greater proportion of zoonotic viruses [63] . It is therefore crucial to understand how evolutionary pressure may have a role in the emergence of new viral strains. A recent study found that henipavirus genomes are best adapted to pteropid bats [64] . Adaptation of genomes refers to better capability of the virus to use host cellular machinery for its replication and protein synthesis, which is usually governed by natural selection; diversity in codon usage bias may contribute to it. Codon usage is an interspecies bias where one codon is selected over other synonymous codons in a particular species [65] . Natural selection for viral variants works by selecting codons matching host tRNA abundance. It also selects for variants with the advantage of not activating innate response genes, such as those for toll-like receptor 9. Codon bias analysis suggested that henipaviruses have the highest level of adaptation to pteropid bats. It would be interesting to study whether other viruses also show such codon bias towards their reservoir hosts. We might be able to use such codon bias studies in the future to identify reservoir hosts of spilled over viruses. Due to coevolution with the reservoir host, the viruses would have a codon bias specific towards their reservoir host. Apart from codon bias, natural selection based on receptor utilization also has a role to play in the evolution of viruses. Variation in the efficiency of bat coronaviruses to recognize human receptors show that the viral spike protein evolved in a stepwise manner to infect human cells [66] . Despite several other receptor-binding studies [67, 68] , the mechanism of adaptation to new hosts is not definitively understood.