Transmissible Gastroenteritis Virus Infection Up-Regulates FcRn Expression via Nucleocapsid Protein and Secretion of TGF-β in Porcine Intestinal Epithelial Cells.

Shaoju Qian,Zitong Gao,Shaowen Li,Qigai He,Xianrong Meng,Zili Li,Yijie Cui,Kang Yang,Rui Cao

doi:10.3389/fmicb.2019.03085

Abstract

Transmissible gastroenteritis virus (TGEV) is a porcine intestinal coronavirus that causes fatal severe watery diarrhea in piglets. The neonatal Fc receptor (FcRn) is the only IgG transport receptor, its expression on mucosal surfaces is triggered upon viral stimulation, which significantly enhances mucosal immunity. We utilized TGEV as a model pathogen to explore the role of FcRn in resisting viral invasion in overall intestinal mucosal immunity. TGEV induced FcRn expression by activating NF-κB signaling in porcine small intestinal epithelial (IPEC-J2) cells, however, the underlying mechanisms are unclear. First, using small interfering RNAs, we found that TGEV up-regulated FcRn expression via TLR3, TLR9 and RIG-I. Moreover, TGEV induced IL-1β, IL-6, IL-8, TGF-β, and TNF-α production. TGF-β-stimulated IPEC-J2 cells highly up-regulated FcRn expression, while treatment with a JNK-specific inhibitor down-regulated the expression. TGEV nucleocapsid (N) protein also enhanced FcRn promoter activity via the NF-κB signaling pathway and its central region (aa 128–252) was essential for FcRn activation. Additionally, N protein-mediated FcRn up-regulation promotes IgG transcytosis. Thus, TGEV N protein and TGF-β up-regulated FcRn expression, further clarifying the molecular mechanism of up-regulation of FcRn expression by TGEV.

Highlights

Transmissible gastroenteritis virus (TGEV) infection mainly causes severe watery diarrhea, vomiting, dehydration, and mortality in suckling piglets less than 2 weeks old, as well as colossal economic losses in the worldwide pig industry (Zhao et al, 2014)
Enhanced FcRn luciferase activities tended to increase during the progression of TGEV infection from 24 to 48 hpi, and this increase was significantly different compared with mock-infected IPEC-J2 cells (Figure 1A)
In order to determine whether TGEV induces FcRn expression in IPEC-J2 cells, live or UV-inactivated TGEV at a multiplicity of infection (MOI) of 1 was used to inoculate IPEC-J2 cells, which were harvested for FcRn analysis at 12, 24, 36, and 48 hpi by RT-qPCR and Western blot (Figures 1B,C)

Summary

Introduction

Transmissible gastroenteritis virus (TGEV) infection mainly causes severe watery diarrhea, vomiting, dehydration, and mortality in suckling piglets less than 2 weeks old, as well as colossal economic losses in the worldwide pig industry (Zhao et al, 2014). ORF1a and ORF1b, located in the 5 twothirds of the viral genome, are cleaved into 16 non-structural proteins (nsp to nsp16) by the nsp and nsp proteases. These nsps are responsible for viral replication, viral transcription, and the antagonization of host innate immune responses. Inoculated fusion protein, HSV2 gD, HIV Gag fused with the Fc region of IgG, and targeted FcRn can all induce systemic and mucosal immunity to genital infections (Lu et al, 2011; Ye et al, 2011). TGEV induced FcRn expression via the NF-κB pathway in IPEC-J2 cells (Guo et al, 2016b), but the regulatory mechanisms underlying this remain unclear; we need to further explore the involvement of pattern recognition receptors (PRRs), inflammatory factors, and virus-coding proteins in the regulation of FcRn

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