Crimean-Congo Hemorrhagic Fever Virus Suppresses Innate Immune Responses via a Ubiquitin and ISG15 Specific Protease

Abstract

SUMMARY Antiviral responses are regulated by conjugation of ubiquitin (Ub) and interferon-stimulated gene 15 (ISG15) to proteins. Certain classes of viruses encode Ub- or ISG15-specific proteases belonging to the ovarian tumor (OTU) superfamily. Their activity is thought to suppress cellular immune responses, but studies demonstrating the function of viral OTU proteases during infection are lacking. Crimean-Congo hemorrhagic fever virus (CCHFV, family Nairoviridae) is a highly pathogenic human virus that encodes an OTU with both deubiquitinase and deISGylase activity as part of the viral RNA polymerase. We investigated CCHFV OTU function by inactivating protease catalytic activity or by selectively disrupting its deubiquitinase and deISGylase activity using reverse genetics. CCHFV OTU inactivation blocked viral replication independently of its RNA polymerase activity, while deubiquitinase activity proved critical for suppressing the interferon responses. Our findings provide insights into viral OTU functions and supports the development of therapeutics and vaccines.

ISG15: It's Complicated

“…It would be interesting to revisit these mutants to assess whether they have any impairment in deISGylase activity, or possibly pursue additional ones that might provide insight into the impact of ISGylation for these viruses. Currently, the only reverse genetics system that has been utilized to investigate the impact on ISGylation is for CCHFV [43,82]. This was able to substantiate different effects that were mediated by ubiquitination versus ISGylation in the context of a nairovirus infection by comparing a mutant lacking both Ub and ISG15 activity with one deficient in only Ub activity.…”

“…OTU activity has been associated with immune suppression and reduction in ISG15 conjugates [42e44]. In addition, a recent study has tentatively associated the deISGylase activity of nairovirus OTUs with higher levels of the L protein in CCHFV [43]. Coronavirus PLPs have also been studied relatively extensively at the molecular and cellular level.…”

“…This was able to substantiate different effects that were mediated by ubiquitination versus ISGylation in the context of a nairovirus infection by comparing a mutant lacking both Ub and ISG15 activity with one deficient in only Ub activity. Unfortunately, current insights are limited by the lack of a deISGylase-specific OTU mutant, as the most promising candidates to date have also yielded an impairment in deubiquitinase activity [43,61,83]. Similar difficulty has been encountered with the SARS-CoV PLP, with mutants typically exhibiting a moderate effect or substantially impacting both Ub and ISG15 activities [64,79].…”

Stable Occupancy of the Crimean-Congo Hemorrhagic Fever Virus-Encoded Deubiquitinase Blocks Viral Infection

“…The small (S) segment encodes the nucleoprotein (NP), the medium (M) segment encodes the glycoprotein precursor (GPC), and the large (L) segment encodes the multifunctional L protein that contains the RNA-dependent RNA polymerase (RdRp) and an ovarian tumor (OTU)-like cysteine protease domain. The CCHFV OTU domain possesses deubiquitinase (DUB) and deISGylase activity, enabling the removal of ubiquitin (Ub) and ubiquitin-like interferon (IFN)-stimulated gene 15 (ISG15) conjugates from their protein targets (7)(8)(9). Ub and ISG15 are small proteins involved in regulating cellular signaling, including innate immune responses.…”

Second International Conference on Crimean-Congo Hemorrhagic Fever

“…In contrast, growth of the OTU double mutant was severely impaired in cells producing IFN in response to CCHFV. Moreover, another recombinant virus with reduced OTU deubiquitinase activity elicited increased IFN responses, suggesting that innate immune responses are countered by OTU activity (Scholte et al, 2017). …”