ASFV pD345L protein negatively regulates NF-κB signaling through inhibiting IKK kinase activity
Authors: Chen Huan , Wang Zhenzhong , Gao Xiaoyu , Lv Jiaxuan , Hu Yongxin , Jung Yong-Sam , Zhu Shanyuan , Wu Xiaodong , Qian Yingjuan , Dai Jianjun 
Published: September 2021

NF-κB is a critical transcription factor in immediate early viral infection, including African swine fever virus (ASFV), playing an important role in inflammation response and expression of antiviral genes. ASFV encodes for more than 151 proteins by its own transcription machinery and possesses a great capacity to evade or subvert antiviral innate immune responses. A couple of such viral proteins have been reported, but many remain unknown. Here, we showed that pD345L, an ASFV-encoded lambda-like exonuclease, is an inhibitor of cGAS/STING mediated NF-κB signaling by blocking IKKα/β kinase activity. Specifically, we showed that overexpression of pD345L suppresses cGAS/STING induced IFNβ and NF-κB activation, resulting in decreased transcription of IFNβ and several pro-inflammatory cytokines, including IL-1α, IL-6, IL-8, and TNFα. In addition, we showed that pD345L targeted at or downstream of IKK and upstream of p65. Importantly, we found that pD345L associates with KD and HLH domains of IKKα and LZ domain of IKKβ, and thus interrupts their kinase activity on downstream substrate IκBα. Finally, we showed that pD345L inhibition of NF-κB signaling was independent of its exonuclease activity. Taken together, we concluded that pD345L blocks IKK α/β kinase activity by protein-protein interaction and thus disrupts cGAS/STING mediated NF-κB signaling.

Importance

African Swine Fever (ASF) is one of the most devastating and economically significant swine diseases caused by African swine fever virus (ASFV). Since expanding of ASFV affected areas into Asian countries, especially China, an effective vaccine is urgently needed more than ever. Therefore, it is of great significance to understand the interaction between ASFV infection and host immune responses. The NF-κB signaling plays a central role in innate and acquired immune responses. Activation of IκB kinase (IKK) complex is a key step of both canonical and non-canonical NF-κB pathways, and is commonly targeted by different viruses. But no ASFV protein has been shown to regulate IKK yet. In this study, we demonstrated that pD345L blocks IKKα/β kinase activity by protein-protein interaction and thus disrupts cGAS/STING mediated NF-κB signaling. It has been shown that conventional vaccine development approaches are proven to be inapplicable to ASFV. Neither subunit nor DNA vaccine provides efficient protection. Gene deleted live-attenuated vaccine candidates render adequate protection, but establishment of chronic or persistent infection in vaccinated animals and risk of recombination with filed strains are key challenges. To overcome these, one potential strategy would be generation of replication-defective viruses. As a lambda-like exonuclease, pD345L plays a critical role in ASFV replication and ASFV deficient in D345L cannot be rescued. Given the dual role of pD345L in virus replication and immune evasion, it may serve as a potential target for replication-defective virus vaccine development.