Porcine TRIM21 Enhances Porcine Circovirus 2 Infection and Host Immune Responses, But Inhibits Apoptosis of PCV2-Infected Cells

A Interacting Model: How TRIM21 Orchestrates with Proteins in Intracellular Immunity

Tripartite motif-containing protein 21 (TRIM21), identified as both a cytosolic E3 ubiquitin ligase and FcR (Fragment crystallizable receptor), primarily interacts with proteins via its PRY/SPRY domains and promotes their proteasomal degradation to regulate intracellular immunity. But how TRIM21 involves in intracellular immunity still lacks systematical understanding. Herein, it is probed into the TRIM21-related literature and raises an interacting model about how TRIM21 orchestrates proteins in cytosol. In this novel model, TRIM21 generally interacts with miscellaneous protein in intracellular immunity in two ways: For one, TRIM21 solely plays as an E3, ubiquitylating a glut of proteins that contain specific interferon-regulatory factor, nuclear transcription factor kappaB, virus sensors and others, and involving inflammatory responses. For another, TRIM21 serves as both E3 and specific FcR that detects antibody-complexes and facilitates antibody destroying target proteins. Correspondingly delineated as Fc-independent signaling and Fc-dependent signaling in this review, how TRIM21's interactions contribute to intracellular immunity, expecting to provide a systematical understanding of this important protein and invest enlightenment for further research on the pathogenesis of related diseases and its prospective application is elaborated.

TRIM21 of Micropterus salmoides exerts antiviral roles against largemouth bass ulcer syndrome virus

Tripartite motif 21 (TRIM21), a member of the TRIM family, plays an important role in apoptosis, autophagy and ubiquitination in human, and has been proven to play antiviral roles in different organisms. In this study, the TRIM21 gene of Micropterus salmoides (MsTRIM21) was cloned, and it encoded 376 amino acids, which showed 89.3% similarity with Micropterus dolomieu and 38.3% with homo sapiens. Bioinformatics analysis revealed MsTRIM21 contained four domains: C4HC3-type RING-variant (RINGv), coiled coil, PRY and SPRY. The high expression level of MsTRIM21 could be detected in liver, stomach and muscle of healthy Micropterus salmoides, and it was significantly upregulated in head kidney, muscle, gill and brain and significantly down-regulated in the stomach of Micropterus salmoides infected with largemouth bass ulcer syndrome virus (LBUSV). The overexpression of MsTRIM21 could significantly inhibit the viral replication in vitro, evidenced by the reduction of CPE severity and the downregulation of the viral gene transcription. In addition, the overexpression of MsTRIM21 could significantly increase the expression level of interferon regulatory factor (IRF) 3, IRF7, myxovirus resistance 1 (Mx1), interferon stimulated gene 15 (ISG15), double-stranded RNA-activated protein kinase (PKR) and tumor necrosis factor α (TNF-α) in vitro, indicating the enhancement of innate immune response and inflammatory response, which may directly affect the replication of LBUSV. Thus, these results provide new lights on the roles of fish TRIM21 in innate immune response against iridovirus.

Special Issue "State-of-the-Art Porcine Virus Research in China"

China is one of the major countries involved in pig production and pork consumption [...].

Multiple Roles of TRIM21 in Virus Infection

The tripartite motif protein 21 (TRIM21) belongs to the TRIM family, possessing an E3 ubiquitin ligase activity. Similar to other TRIMs, TRIM21 also contains three domains (named RBCC), including the Really Interesting New Gene (RING) domain, one or two B-Box domains (B-Box), and one PRY/SPRY domain. Notably, we found that the RING and B-Box domains are relatively more conservative than the PRY/SPRY domain, suggesting that TRIM21 of different species had similar functions. Recent results showed that TRIM21 participates in virus infection by directly interacting with viral proteins or modulating immune and inflammatory responses. TRIM21 also acts as a cytosol high-affinity antibody Fc receptor, binding to the antibody-virus complex and triggering an indirect antiviral antibody-dependent intracellular neutralization (ADIN). This paper focuses on the recent progress in the mechanism of TRIM21 during virus infection and the application prospects of TRIM21 on virus infection.

Expression and immunogenicity analysis of the capsid proteins of porcine circovirus types 2 to 4

Porcine circovirus (PCV) comprises four types, PCV1, PCV2, PCV3, and PCV4, which belong to the Circovirus genus of the family Circoviridae. PCV1 is nonpathogenic, whereas PCV2, PCV3, and PCV4 can infect pigs and cause disease. However, due to a lack of experimental evidence, whether vaccines based on PCV capsid (Cap) can induce cross-reactivity against PCVs remains controversial. In this study, recombinant truncated capsids (rCaps) of PCV2, PCV3, and PCV4 were highly and efficiently expressed and purified, followed by the development and evaluation of antibodies against PCVs. The results showed that monovalent and trivalent antigens based on the recombinant Caps had adequate immunogenicity to stimulate specific antibodies against the corresponding protein and virus. Furthermore, antisera prepared from the recombinant Caps also cross-reacted with different PCVs. Therefore, recombinant proteins can be used as candidate antigens to develop vaccines and ELISA diagnostic kits. In addition, the antibodies prepared in this study are promising candidates for the simultaneous prevention and treatment of PCVs in the clinic.

Advances in Crosstalk between Porcine Circoviruses and Host

Porcine circoviruses (PCVs), including PCV1 to PCV4, are non-enveloped DNA viruses with a diameter of about 20 nm, belonging to the genus Circovirus in the family Circoviridae. PCV2 is an important causative agent of porcine circovirus disease or porcine circovirus-associated disease (PCVD/PCVAD), which is highly prevalent in pigs and seriously affects the swine industry globally. Furthermore, PCV2 mainly causes subclinical symptoms and immunosuppression, and PCV3 and PCV4 were detected in healthy pigs, sick pigs, and other animals. Although the pathogenicity of PCV3 and PCV4 in the field is still controversial, the infection rates of PCV3 and PCV4 in pigs are increasing. Moreover, PCV3 and PCV4 rescued from infected clones were pathogenic in vivo. It is worth noting that the interaction between virus and host is crucial to the infection and pathogenicity of the virus. This review discusses the latest research progress on the molecular mechanism of PCVs–host interaction, which may provide a scientific basis for disease prevention and control.