Short description: A research group led by Prof. DENG Hongyu from the Institute of Biophysics, Chinese Academy of Sciences (CAS), identified host protein phosphatase PPM1G as a negative regulator of STING- and MAVS-mediated innate immune responses and showed that this regulatory mechanism is hijacked by a human herpesvirus for immune evasion.

The cytosolic DNA and RNA sensing pathways play pivotal roles in innate immune defense against viral infections. Phosphorylation of the adaptor proteins, STING and MAVS, is an essential and conserved mechanism to activate the type I IFN pathway. Although STING- and MAVS-mediated cytosolic nucleic acid sensing pathways play crucial roles in antiviral immunity, excessive activation of the system is associated with sometimes fatal inflammatory diseases. Thus, the activity of the system, and in particular the activities of the innate immune adaptors, must be precisely regulated to ensure a proper and balanced innate immune homeostasis in infected host cells. How phosphorylation of STING and MAVS is regulated and how this post-translational modification is manipulated by viruses remain largely unknown.

Recently, a research group led by Prof. DENG Hongyu from the Institute of Biophysics, Chinese Academy of Sciences (CAS), identified a host protein phosphatase PPM1G which negatively regulates STING- and MAVS-mediated innate immune responses, and showed that Kaposi's sarcoma-associated herpesvirus (KSHV) hijacks PPM1G for immune evasion via its tegument protein ORF33. This study, entitled "PPM1G restricts innate immune signaling mediated by STING and MAVS and is hijacked by KSHV for immune evasion", was published online in Science Advances on November 20, 2020.

In order to successfully infect host cell and establish persistent latent infection, herpesviruses have evolved to employ various strategies to antagonize host innate immunity. Tegument proteins are components of tegument, a unique structure of herpesviral particles. They not only play essential roles in virion assembly and egress during late stage of viral lytic replication, but also participate in regulation of many cellular signaling pathways, especially immune evasion, during early stage of de novo infection. Along this line, Prof. Deng's group has previously shown that ORF33, a conserved tegument protein of herpesviruses, is essential for virion assembly of gammaherpesviruses. However, it is not yet known whether ORF33 can mediate immune evasion function.

To answer this question, the researchers first constructed an ORF33-null KSHV mutant. They found that, upon infection of host cells, the ORF33-null mutant induced higher IFNβ production than WT virus did; ORF33 interacted with STING and MAVS and impaired the recruitment of IRF3 by STING and MAVS. These results indicated that ORF33 inhibits the host innate immune responses by affecting the functions of the adaptor proteins STING and MAVS.

Mechanistically, the expression of ORF33 markedly decreased the phosphorylation levels of STING and MAVS. Interestingly, in the in vitro phosphatase assay, only ORF33 purified from mammalian cells, but not ORF33 purified from prokaryotic cells, reduced the phosphorylation levels of STING and MAVS, indicating that ORF33 may utilize host protein phosphatase(s) to dephosphorylate STING and MAVS. Indeed, using coimmunoprecipitation and mass spectrometry analysis, the researchers identified a host protein phosphatase PPM1G that is associated with ORF33. PPM1G purified from prokaryotic cells directly dephosphorylated STING and MAVS in the in vitro phosphatase assay. Moreover, ORF33 enhanced the interaction between PPM1G and STING or MAVS. These results demonstrated that ORF33 recruits PPM1G to dephosphorylate STING and MAVS, thus inhibiting their activation.

Furthermore, PPM1G inhibited host IFNβ response; consistently, PPM1G knockdown or knockout enhanced host defense against DNA and RNA viruses. These results indicated that PPM1G negatively regulates both DNA and RNA sensing pathways.

In summary, this study identified a host protein phosphatase PPM1G that serves as a negative regulator to restrict excessive activation of antiviral innate immunity. It also revealed a novel strategy of viral immune escape: KSHV tegument protein ORF33 recruits PPM1G to dephosphorylate STING and MAVS, thereby inhibiting IFN production and antiviral responses.

Professor DENG Hongyu from the Institute of Biophysics, Chinese Academy of Sciences, is the correspondence author of this research article. Dr. YU Kuai of DENG's research team is the first author, and Dr. TIAN Huabin, an Assistant Researcher, participated in this study. The work was supported by grants from the National Natural Science Foundation, the Ministry of Science and Technology, and the Chinese Academy of Sciences.

Figure 1: Mechanism diagram of PPM1G and ORF33

(Image by Dr. DENG Hongyu's group)

Article Link: https://advances.sciencemag.org/content/6/47/eabd0276

Contact: DENG Hongyu

Institute of Biophysics, Chinese Academy of Sciences

Beijing 100101, China

Email: hydeng@ibp.ac.cn

(Reported by Dr. DENG Hongyu's group)