Porcine Epidemic Diarrhea Virus nsp13 Protein Downregulates Neonatal Fc Receptor Expression by Causing Promoter Hypermethylation through the NF-κB Signaling Pathway

Abstract

Porcine epidemic diarrhea virus (PEDV) is a highly pathogenic porcine enteric coronavirus that causes severe watery diarrhea and even death in piglets. The neonatal Fc receptor (FcRn) is the only transport receptor for IgG. FcRn expressed by intestinal epithelial cells can transport IgG from breast milk to piglets to provide immune protection. Previous studies have shown that viral infection affects FcRn expression. In this study, we showed for the first time, to our knowledge, that FcRn expression can be influenced by methyltransferases. In addition, we found that PEDV inhibited FcRn protein synthesis in porcine small intestinal epithelial cells postinfection. Then, we found that PEDV interfered with the transcription of genes through aberrant methylation modification of the FcRn promoter. DNA methyltransferase 3b (DNMT3b) has been implicated in this process. Using a series of PEDV structural and nonstructural protein (nsp) expression plasmids, we showed that nsp13 plays an important role in this aberrant methylation modification. PEDV nsp13 can affect the NF-κB canonical pathway and promote DNMT3b protein expression by facilitating p65 protein binding to chromatin. PEDV caused aberrant methylation of the FcRn promoter via DNMT3b. The same phenomenon was found in animal experiments with large white piglets. IgG transcytosis demonstrated that PEDV nsp13 can inhibit bidirectional IgG transport by FcRn. In addition, the core region of nsp13 (230–597 aa) is critical for FcRn inhibition. Taken together, to our knowledge, our findings revealed a novel immune escape mechanism of PEDV and shed new light on the design and development of vaccines and drugs.

A Comprehensive View on the Protein Functions of Porcine Epidemic Diarrhea Virus

“…Promotes the replication of PEDV [36] Nsp3 PLpro, regulates the deubiquitination of RIG-I and STING, inhibits IFN-β and IFN-λ1 [37,38] Nsp4 Upregulates pro-inflammatory cytokines and chemokines expression (IL-1α, IL-1β, TNF-α, CCL2, CCL5, and CXCL8) [39] Nsp5 3C-like protease，IFN antagonist [40,41] Nsp7 Inhibits type I IFN [42,43] Nsp8 Inhibits type III IFN [22] Nsp9 Inhibits ERS-mediated apoptosis [44] Nsp10 Essential for viral replication, upregulates IL-2, IL-4, IL-10, TNF-α, and IFN-γ [45,46] Nsp12 RdRp, viral replication [47] Nsp13 HEL, inhibits bidirectional IgG transport by FcRn [48] Nsp14 ExoN, suppresses ER stress-induced GRP78, acts as NF-κB pathway antagonist, downregulates pro-inflammatory cytokines [49,50] Nsp15 EndoU, inhibits IFN-β and IRF3, downregulates CCL5, CXCL8, CXCL10, OAS, MXs, STAT1, and IRF9 [51,52] Nsp16 2 ′ -O-MTase, downregulates the activities of RIG-I/MDA5-mediated IFN-β and ISRE [53]…”

“…Consequently, this leads to abnormal methylation of the neonatal Fc receptor (FcRn) promoter, ultimately inhibiting the bidirectional transport of IgG. In addition, the essential domain for FcRn inhibition is within the core region of nsp13 (230~597 aa) [48].…”

Developing Next-Generation Live Attenuated Vaccines for Porcine Epidemic Diarrhea Using Reverse Genetic Techniques

“…The overexpression of nsp13 could significantly decrease the mRNA level of MHC-I in PEDV-infected cells, but it has no regulatory effect on other immune molecules, such as IFN-β, IL-6, and TNF-α [102]. In addition, nsp13 also down-regulates neonatal Fc receptor (FcRn) expression to facilitate the immune escape of PEDV by causing promoter hypermethylation in vitro and in vivo [103]. Nsp13 stimulates the overexpression of DNA methyltransferase 3b (DNMT3b) by activating the NF-κB signaling pathway, resulting in abnormal hypermethylation of the FcRn gene promoter region.…”