4.1. Frugivorous Bat-Identified Influenza A (H17–H18) Viruses Use MHC Class II as a Receptor
The genomes of new IAV subtypes, H17N10, which was identified in liver, intestine, lung and kidney tissues and in rectal swabs, but not in oral swabs, of frugivorous yellow-shouldered bats (Sturnira lilium, family Phyllostomidae) in Guatemala [119], and H18N11, which was identified in intestine tissue and in rectal swabs of frugivorous flat-faced bats (Artibeus planirostris, family Phyllostomidae) in Peru [120] during the period from 2009 to 2010, were reported in 2012 and in 2013, respectively (Table 1). Based on aa sequences of H1–H18 HAs, both bat H17 and H18 HAs are more similar to H1–H2, H5–H6, H8–H9, H11–H13 and H16 in group 1 than to H3–H4, H7, H10 and H14–H15 in group 2 [198]. Analysis of the crystal structures of bat H17 and H18 HAs revealed that their overall structures retain possession of typical IAV HA molecules including the receptor binding site (RBS). However, bat HAs showed no binding to any of 610 diverse structures of glycans including more than 100 sialylated glycans with either α2,3, α2,6, α2,8 or mixed linkages. Detailed structural analysis (Figure 3d) revealed that at least four residues unique to bat H17 and H18 HAs seem to substantially reject a sialylated glycan from the bat RBS. (i) Highly conserved Y98 anchoring Sia is replaced by F98 in bat HAs. (ii) Highly conserved Q226 in H1 HAs and Q/L/I226 (a determinant of sialyl linkage specificity) in H3 HAs is replaced by H226 in bat HAs. (iii) The large residue D228 in bat HAs would make a steric clash with the side chain of Sia. (iv) The other large negatively charged residue D136 in bat HAs, possibly the most important residue, would provide electrostatic repulsion to a negatively charged sialylated glycan. Both structural and functional studies on bat HAs strongly confirmed that bat H17 and H18 HAs do not bind to sialylated glycans [120].
In 2019, human leukocyte antigen DR (HLA-DR) isotype, one of the three major MHC class II isotypes, found on susceptible cell lines including MDCKII clone no. 1, human U-87MG cells, human Calu-3 cells and human haematopoietic cancer cells was shown by the following findings to be required for mediating entry of pseudotyped viruses harboring H17 HAs [63] or H18 HAs [62] into a mammalian cell: (i) knockdown of HLA-DRA or HLA-DR α-chain or cell preincubation with an HLA-DRA-targeting antibody significantly reduced both H17- and H18-pseudotyped virus infections, (ii) ectopic expression of HLA-DR in non-susceptible cell lines rendered them susceptible to both H17- and H18-pseudotyped virus infections, (iii) expression of HLA-DR from other mammals, including different bat species, pigs and mice, or from chickens makes cells susceptible to H18-pseudotyped virus infection and (iv) intranasal infection of mice with H18N11 virus led to viral replication in the upper respiratory tracts of the mice. While these findings suggested possible spread of bat H17 and H18 viruses to other vertebrates, recent studies have shown that wild-type H18N11 infection is restricted to bats and that H18N11 has poor replication in mice and ferrets [199]. The potential of these viruses to spread to and infect other animals and the tissue tropism within other animals remain unclear. Characterization of direct binding of H17 and H18 HAs to MHC-II is also required.