Study of retroviral restrictions in a Tadarida bat cell line enlightens specific early blocks and TRIM5 locus multiplication

Studies on retroviral infections in bat cells have revealed lineage-dependent differences in susceptibility level, implicating the presence of retrovirus-inhibiting mechanisms in bats. However, the limited number of bat species studied has hampered the clarification of the specific underlying mechanisms. While studying the susceptibility of bat cell lines to murine leukemia virus (MLV), we identified a specific Yangochiropteran bat cell line that exhibited a primate tripartite motif-containing protein (TRIM) 5α-like MLV restriction. Here, we characterized the restrictive nature of the TRIM5α homolog in a Yangochiropteran cell line. This cell line was restrictive to gibbon ape leukemia virus (GaLV)-a phylogenetic relative to Yinpterochiropteran gammaretroviruses-and N-tropic MLV (N-MLV) dependent on the capsid (CA). A genomic search of TRIM5α-like sequences revealed four TRIM5α homologs, none of which reproduced the strong N-MLV and GaLV restriction observed in the cell line. Instead, a chimeric Yangochiropteran TRIM5α with the RING-Bbox-coiled-coil domain from a restrictive Yinpterochiropteran TRIM5α restricted B-tropic MLV independent of the proteasome. Mutational analyses of the Yangochiropteran TRIM5α revealed two responsible gain-of-function mutations in the L1 linker. One of the mutations specifically reduced the self-ubiquitination of Yangochiropteran TRIM5α, whereas its reciprocal mutation abolished N-MLV restriction by Yinpterochiropteran bat and rhesus macaque TRIM5α, highlighting the importance of this residue in the mechanism of TRIM5α-mediated restriction. Herein, we demonstrate that mutations in L1 may induce the loss of anti-MLV activity in Yangochiropteran TRIM5α, with its B30.2 domain retaining the ability to recognize the CA core and a yet-unidentified factor restricting N-MLV in the Yangochiropteran cell line.

Bats host many RNA viruses that can potentially cause spillover to humans; however, they rarely develop severe symptoms after infection, suggesting the presence of bat-specific antiviral mechanisms. Although no infectious retroviruses have been isolated from bats, we and other groups have proposed the presence of unknown retrovirus-inhibiting mechanisms in bat cells. In this study, we focused on a particular host factor (TRIM5α) in a specific Yangochiropteran bat cell line that showed resistance to gammaretroviruses; however, its TRIM5α homolog lost its antiretroviral activity, with its ligand-interacting domain retaining the ability to recognize the retrovirus capsid. We identified specific gain-of-function mutations in Yangochiropteran TRIM5α that endow the protein with gammaretrovirus-restricting activity. This finding suggests that during Yangochiropteran evolution, these mutations may have prevented Yangochiropteran TRIM5α from inhibiting gammaretroviruses. Further understanding of retrovirus-inhibiting mechanisms in bat cells would help develop new therapeutic strategies to combat retrovirus-related diseases, such as acquired immunodeficiency syndrome.