Development of an entirely cloned cDNA-based reverse genetics system for Tofla virus of orthonairovirus.

The genus Orthonairovirus includes highly pathogenic tick-borne viruses such as the Crimean-Congo hemorrhagic fever orthonairovirus (CCHFV). A reverse genetics system is an indispensable tool for determining the viral factors related to pathogenicity. Tofla orthonairovirus (TFLV) is a recently identified virus isolated from ticks in Japan and our research has suggested that TFLV is a useful model for studying pathogenic orthonairoviruses. In this study, we successfully established a reverse genetics system for TFLV using T7 RNA polymerase. Recombinant TFLV was generated by transfecting cloned complementary DNAs encoding the TFLV genome into BSR T7/5 cells expressing T7 RNA polymerase. We were able to rescue infectious recombinant TFLV mutant (rTFLVmt) and wild-type TFLV (rTFLVpt) viruses, which exhibited indistinguishable growth kinetics in mammalian cells and pathogenicity in A129 mice compared with the authentic virus. Our approach provides a valuable method for establishing reverse genetics system for orthonairoviruses.

Convergent evolution of antiviral machinery derived from endogenous retrovirus truncated envelope genes in multiple species.

Host genetic resistance to viral infection controls the pathogenicity and epidemic dynamics of infectious diseases. Refrex-1 is a restriction factor against feline leukemia virus subgroup D (FeLV-D) and an endogenous retrovirus (ERV) in domestic cats (ERV-DC). Refrex-1 is encoded by a subset of ERV-DC loci with truncated envelope genes and secreted from cells as a soluble protein. Here, we identified the copper transporter CTR1 as the entry receptor for FeLV-D and genotype I ERV-DCs. We also identified CTR1 as a receptor for primate ERVs from crab-eating macaques and rhesus macaques, which were found in a search of intact envelope genes capable of forming infectious viruses. Refrex-1 counteracted infection by FeLV-D and ERV-DCs via competition for the entry receptor CTR1; the antiviral effects extended to primate ERVs with CTR1-dependent entry. Furthermore, truncated ERV envelope genes found in chimpanzee, bonobo, gorilla, crab-eating macaque, and rhesus macaque genomes could also block infection by feline and primate retroviruses. Genetic analyses showed that these ERV envelope genes were acquired in a species- or genus-specific manner during host evolution. These results indicated that soluble envelope proteins could suppress retroviral infection across species boundaries, suggesting that they function to control retroviral spread. Our findings revealed that several mammalian species acquired antiviral machinery from various ancient retroviruses, leading to convergent evolution for host defense.