African Swine Fever Virus Cysteine Protease pS273R Inhibits Type I Interferon Signaling by Mediating STAT2 Degradation.

African swine fever virus (ASFV) is a large DNA virus that causes African swine fever (ASF), an acute and hemorrhagic disease in pigs with lethality rates of up to 100%. To date, how ASFV efficiently suppress the innate immune response remains enigmatic. In this study, we identified ASFV cysteine protease pS273R as an antagonist of type I interferon (IFN). Overexpression of pS273R inhibited JAK-STAT signaling triggered by type I IFNs. Mechanistically, pS273R interacted with STAT2 and recruited the E3 ubiquitin ligase DCST1, resulting in K48-linked polyubiquitination at K55 of STAT2 and subsequent proteasome-dependent degradation of STAT2. Furthermore, such a function of pS273R in JAK-STAT signaling is not dependent on its protease activity. These findings suggest that ASFV pS273R is important to evade host innate immunity. IMPORTANCE ASF is an acute disease in domestic pigs caused by infection with ASFV. ASF has become a global threat with devastating economic and ecological consequences. To date, there are no commercially available, safe, and efficacious vaccines to prevent ASFV infection. ASFV has evolved a series of strategies to evade host immune responses, facilitating its replication and transmission. Therefore, understanding the immune evasion mechanism of ASFV is helpful for the development of prevention and control measures for ASF. Here, we identified ASFV cysteine protease pS273R as an antagonist of type I IFNs. ASFV pS273R interacted with STAT2 and mediated degradation of STAT2, a transcription factor downstream of type I IFNs that is responsible for induction of various IFN-stimulated genes. pS273R recruited the E3 ubiquitin ligase DCST1 to enhance K48-linked polyubiquitination of STAT2 at K55 in a manner independent of its protease activity. These findings suggest that pS273R is important for ASFV to escape host innate immunity, which sheds new light on the mechanisms of ASFV immune evasion.

African swine fever virus I267L acts as an important virulence factor by inhibiting RNA polymerase III-RIG-I-mediated innate immunity.

ASFV is a large DNA virus that is highly pathogenic in domestic pigs. How this virus is sensed by the innate immune system as well as why it is so virulent remains enigmatic. In this study, we show that the ASFV genome contains AT-rich regions that are recognized by the DNA-directed RNA polymerase III (Pol-III), leading to viral RNA sensor RIG-I-mediated innate immune responses. We further show that ASFV protein I267L inhibits RNA Pol-III-RIG-I-mediated innate antiviral responses. I267L interacts with the E3 ubiquitin ligase Riplet, disrupts Riplet-RIG-I interaction and impairs Riplet-mediated K63-polyubiquitination and activation of RIG-I. I267L-deficient ASFV induces higher levels of interferon-ß, and displays compromised replication both in primary macrophages and pigs compared with wild-type ASFV. Furthermore, I267L-deficiency attenuates the virulence and pathogenesis of ASFV in pigs. These findings suggest that ASFV I267L is an important virulence factor by impairing innate immune responses mediated by the RNA Pol-III-RIG-I axis.

Mex3B inhibits DC-STAMP mRNA level and osteoclastogenesis.

Bone homeostasis is maintained through continuous remodeling by osteoclast-driven bone resorption and osteoblast-mediated bone formation. Osteoclasts are multinucleated giant cells (MNCs) differentiated from myeloid progenitors of the monocytic lineage. During osteoclast maturation, DC-STAMP (dendritic cell specific transmembrane protein) has been shown as a master determinant of osteoclast cell fusion. In this study, we demonstrate that Mex3B inhibits osteoclast fusion protein DCSTAMP expression and osteoclastogenesis. During differentiation of osteoclasts, the expression of Mex3B is down-regulated by cytokines such as RANKL and TNFa, resulting in relief of Mex3B-mediated down-regulation of DC-STAMP mRNA level. Our findings not only reveal critical mechanisms on regulation of DC-STAMP-mediated osteoclastogenesis, but also point to Mex3B as a potential therapeutic target for the treatment of human bone diseases.