Adaptive evolution of virus-sensing toll-like receptor 8 in bats

Abstract

Recently, bats have gained attention as potential reservoir hosts for emerging zoonotic single-stranded (ssRNA) viruses that may prove fatal for humans and other mammals. It has been hypothesized that some features of their innate immune system may enable bats to trigger an efficient early immune response. Toll-like receptors (TLRs) represent a first line defense within the innate immune system and lie directly at the host-pathogen interface in targeting specific microbe-molecular patterns. However, the direction and strength of selection acting on TLRs are largely unknown for bats. Here, we studied the selection on viral ssRNA sensing TLR8 based on sequence data of 21 bat species. The major part (63 %) of the TLR8 gene evolved under purifying selection, likely due to functional constraints. We also found evidence for persistent positive selection acting on specific amino acid sites (7 %), especially when compared to viral TLR evolution of other mammals. All of these putatively positively selected codons were located in the ligand-binding ectodomain, some coincidenced or were in close proximity to functional sites, as suggested by the crystallographic structure of the human TLR8. This might contribute to the inter-species variation in the ability to recognize molecular patterns of viruses. TLR8 evolution within bats revealed that branches leading to ancestral and recent lineages evolved under episodic positive selection, indicating selective selection pressures in restricted bat lineages. Altogether, we found that the TLR8 displays extensive sequence variation within bats and that unique features separate them from humans and other mammals.

Selective evolution of Toll-like receptors 3, 7, 8, and 9 in bats

“…On average, both the mammalian and bat TLR3, TLR7, TLR8, and TLR9 were under purifying selection, consistent with previous studies (Areal et al 2011;Cowled et al 2011;Escalera-Zamudio et al 2015;Schad and Voigt 2016;Zhang et al 2013). But there were some specific branches and sites in bat TLR7, TLR8, and TLR9 evolved under positive selection.…”

“…However, on average, all TLRs are under negative selection with a few positively selected sites in vertebrates (Alcaide and Edwards 2011;Areal et al 2011;Fornůsková et al 2013;Grueber et al 2014;Nakajima et al 2008;Roach et al 2005). The selective evolution of bat TLRs have been discussed in some previous studies on single or multiple loci (Cowled et al 2011;Escalera-Zamudio et al 2015;Schad and Voigt 2016;Zhang et al 2013). In this study, we detected specific selective branches and sites in bats with half more taxa (up to 15-34 bats in different genes were included) and obtained some different results.…”

“…The common element among TLR7, TLR8, and TLR9 was that the bat ancestor was positively selected in the long-term evolution, which was also detected by Escalera-Zamudio et al (2015). However, in a similar study for TLR8, the ancestor of bats was not detected (Schad and Voigt 2016). In the site models, 675R was specifically selected in bats while the other four positively selected sites were consistent between bats and mammals (Table 4).…”

“…For TLR8, the higher taxonomic branches have been positively selected, especially the two nodes with ω = 999. One of them was the ancestor of R. hipposideros, R. ferrumequinum, and R. euryale, which was also detected for episodic positive selection previously (Schad and Voigt 2016). The other one, the ancestor of Nyctalus spp.…”

Novel Insights Into Immune Systems of Bats

“…63% of bat TLR8 genes have evolved under purifying selection and 7% of amino acid sequences that make up the ligand-binding domain of TLR8 differ between bat and other mammalian TLR8 proteins. Moreover, TLR8 sequences vary within bat species (50). Thus, it is important to acknowledge species-specific adaptations in bats.…”