TRIM21 Neutralizes Hazara Virus Using a Dual Mechanism of Nucleoprotein Caging and Ubiquitination.

Crimean-Congo Hemorrhagic Fever Virus (CCHFV) is a tick-borne bunyavirus with widespread and growing geographic distribution that causes severe hemorrhagic fever and death. Vaccine candidates targeting the viral nucleoprotein (NP) have shown efficacy in both mouse and non-human primate models but the mechanism of protection is unclear. Here we employ the closely related Hazara virus (HAZV) to investigate how the intracellular antibody receptor TRIM21 uses anti-NP antibodies to neutralize infection. We show that TRIM21 can detect incoming NP particles within hours of infection and that this results in a potent block to infection. Electroporated-antibody-dependent neutralization assay (EDNA) experiments reveal that TRIM21 inhibits viral transcription, protein expression and genome synthesis and reduces the production of infectious virions. Mutations and domain deletions within TRIM21 reveal that neutralization requires antibody-binding by the PRYSPRY domain but is only partially dependent on the E3 ubiquitin ligase RING domain. The data suggest a dual restriction mechanism in which NP cross-linking by TRIM21 physically interferes with NP function whilst parallel ubiquitination labels the protein for degradation. This dual mechanism is similar to that used by TRIM5 against retroviruses and suggests that antiviral TRIMs may utilize their capacity for self-assembly both for catalytic activation and viral caging.