Interplay of the ubiquitin proteasome system and the innate immune response is essential for the replication of infectious bronchitis virus

The role of IBV PL1pro in virus replication and suppression of host innate immune responses

BACKGROUND

Coronavirus papain-like proteases (PLpros) play a crucial role in virus replication and the evasion of the host immune response. Infectious bronchitis virus (IBV) encodes a proteolytically defective remnant of PL1pro and an active PL2pro. However, the function of PL1pro in IBV remains largely unknown. This study aims to explore the effect of PL1pro on virus replication and underlying mechanisms.

RESULTS

The recombinant viruses rIBV-ΔPL1pro and rIBV-ΔPL1pro-N were obtained using reverse genetic techniques through the deletion of the IBV PL1pro domain and the N-terminal conserved sequence of PL1pro (PL1pro-N). We observed significantly lower replication of rIBV-ΔPL1pro and rIBV-ΔPL1pro-N than wild-type IBV. Further investigation revealed that the lack of PL1pro-N in IBV decreased virus resistance to interferon (IFN) while also inducing host immune response by enhancing the production of IFN-β and activating the downstream STAT1 signaling pathway of IFNs. In addition, the overexpression of PL1pro-N significantly suppressed type I IFN response by down-regulating the expressions of genes in the IFN pathway.

CONCLUSIONS

Our data demonstrated that IBV PL1pro plays a crucial role in IBV replication and the suppression of host innate immune responses, suggesting that IBV PL1pro could serve as a promising molecular target for antiviral therapy.

A Role for the Chicken Interferon-Stimulated Gene CMPK2 in the Host Response Against Virus Infection

Virus infection can lead to the production of interferon, which activates the JAK/STAT pathway and induces the expression of multiple downstream interferon-stimulated genes (ISGs) to achieve their antiviral function. Cytidine/uridine monophosphate kinase 2 (CMPK2) gene has been identified as an ISG in human and fish, and is also known as a rate-limiting enzyme in mitochondria to maintain intracellular UTP/CTP levels, which is necessary for de novo mitochondrial DNA synthesis. By mining previous microarray data, it was found that both Avian Influenza Virus (AIV) and Newcastle Disease Virus (NDV) infection can lead to the significant upregulation of chicken CMPK2 gene. However, little is known about the function of CMPK2 gene in chickens. In the present study, the open reading frame (ORF) of chicken CMPK2 (chCMPK2) was cloned from DF-1, a chicken embryo fibroblasts cell line, and subjected to further analysis. Sequence analysis showed that chCMPK2 shared high similarity in amino acid with CMPK2 sequences from all the other species, especially reptiles. A thymidylate kinase (TMK) domain was identified in the C-terminus of chCMPK2, which is highly conserved among all species. In vitro, AIV infection induced significant increases in chCMPK2 expression in DF-1, HD11, and the chicken embryonic fibroblasts (CEF), while obvious increase only detected in DF-1 cells and CEF cells after NDV infection. In vivo, the expression levels of chCMPK2 were up-regulated in several tissues from AIV infected chickens, especially the brain, spleen, bursa, kidney, intestine, heart and thymus, and notable increase of chCMPK2 was detected in the bursa, kidney, duodenum, lung, heart, and thymus during NDV infection. Here, using MDA5 and IFN-β knockdown cells, we demonstrated that as a novel ISG, chCMPK2 could be regulated by the MDA5/IFN-β pathway. The high expression level of exogenous chCMPK2 displayed inhibitory effects on AIV and NDV as well as reduced viral RNA in infected cells. We further demonstrated that Asp135, a key site on the TMK catalytic domain, was identified as critical for the antiviral activities of chCMPK2. Taken together, these data demonstrated that chCMPK2 is involved in the chicken immune system and may play important roles in host anti-viral responses.

Current Knowledge on Infectious Bronchitis Virus Non-structural Proteins: The Bearer for Achieving Immune Evasion Function

Infectious bronchitis virus (IBV) is the first coronavirus discovered in the world, which is also the prototype of gamma-coronaviruses. Nowadays, IBV is widespread all over the world and has become one of the causative agent causing severe economic losses in poultry industry. Generally, it is believed that the viral replication and immune evasion functions of IBV were modulated by non-structural and accessory proteins, which were also considered as the causes for its pathogenicity. In this study, we summarized the current knowledge about the immune evasion functions of IBV non-structural and accessory proteins. Some non-structural proteins such as nsp2, nsp3, and nsp15 have been shown to antagonize the host innate immune response. Also, nsp7 and nsp16 can block the antigen presentation to inhibit the adapted immune response. In addition, nsp13, nsp14, and nsp16 are participating in the formation of viral mRNA cap to limit the recognition by innate immune system. In conclusion, it is of vital importance to understand the immune evasion functions of IBV non-structural and accessory proteins, which could help us to further explore the pathogenesis of IBV and provide new horizons for the prevention and treatment of IBV in the future.

Myricetin exerts its antiviral activity against infectious bronchitis virus by inhibiting the deubiquitinating activity of papain-like protease

Infectious bronchitis virus (IBV) is a causative agent that causes severe economic losses in the poultry industry worldwide. Papain-like protease (PLpro) is a nonstructural protein encoded by IBV. It has deubiquitinating enzyme activity, which can remove the ubiqutin modification from the protein in nuclear factor kappa-B (NF-κB) and interferon regulatory factor 7 (IRF7) signaling pathway, so as to negatively regulate the host's innate immune response to promote viral replication. In this study, PLpro was selected as the target to screen antiviral agents against IBV. Through protein prokaryotic expression technology, we successfully expressed the active IBV PLpro. Among the 16 natural products, myricetin showed the strongest inhibitory effect on IBV PLpro. Next, we tested the antiviral activity of myricetin against IBV and verified whether it can exert antiviral activity by inhibiting the deubiquitinating activity of PLpro. The results showed that myricetin can significantly inhibit IBV replication in primary chicken embryo kidney (CEK) cells and it can significantly upregulate the transcription levels in the NF-κB and IRF7 signaling pathways. Moreover, we verified that myricetin can increase the ubiquitin modification level on tumor necrosis factor receptor-associated factor 3 and 6 (TRAF3 and TRAF6) reduced by IBV PLpro. In conclusion, these results indicated that myricetin exerts antiviral activity against IBV by inhibiting the deubiquitinating activity of PLpro, which can provide new perspective for the prevention and treatment of IBV.

Construction, Identification and Analysis of the Interaction Network of African Swine Fever Virus MGF360-9L with Host Proteins

African swine fever virus (ASFV) is prevalent in many countries and is a contagious and lethal virus that infects pigs, posing a threat to the global pig industry and public health. The interaction between the virus and the host is key to unlocking the mystery behind viral pathogenesis. A comprehensive understanding of the viral and host protein interaction may provide clues for developing new antiviral strategies. Here, we show a network of ASFV MGF360-9L protein interactions in porcine kidney (PK-15) cells. Overall, 268 proteins that interact with MGF360-9L are identified using immunoprecipitation and liquid chromatography–mass spectrometry (LC-MS). Accordingly, gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses were conducted, and the protein–protein interaction (PPI) network was created. It was speculated that the cellular proteins interacting with MGF360-9L are involved in protein binding, metabolism, and the innate immune response. Proteasome subunit alpha type (PSMA3), 26S protease regulatory subunit 4 (PSMC1), autophagy and beclin 1 regulator 1 (AMBRA1), and DEAD-box helicase 20 (DDX20) could interact with MGF360-9L in vitro. PSMA3 and PSMC1 overexpression significantly promoted ASFV replication, and MGF360-9L maintained the transcriptional level of PSMA3 and PSMC1. Here, we show the interaction between ASFV MGF360-9L and cellular proteins and elucidate the virus–host interaction network, which effectively provides useful protein-related information that can enable further study of the potential mechanism and pathogenesis of ASFV infection.