Porcine IFI44L inhibits transmissible gastroenteritis virus (TGEV) replication by activating IFN-Ⅰ signaling pathway

Interferon-Induced protein 44-Like (IFI44L), a member of the interferon-stimulated genes (ISGs) family, plays a critical role in a variety of biological processes, particularly in antiviral defense and immune regulation. However, the role of human IFI44L in viral replication remains controversial. To further characterize porcine IFI44L (poIFI44L), we cloned poIFI44L for the first time and investigated its role in porcine transmissible gastroenteritis virus (TGEV) replication. The full-length poIFI44L cDNA encodes a 439-amino acid protein, containing an N-terminal TLDc domain and a C-terminal Ras-like GTPase domain. Sequence similarity to orthologs from mouse, hamster, rat, ferret, dog, rabbit, cat, horse, human, monkey, cattle and camel ranged from 48.6 % to 70.2 %. poIFI44L expression was dose-dependently upregulated in PK-15 cells by both poly (I:C) and interferon-alpha (IFNα-2a) treatments. Furthermore, TGEV infection significantly induced poIFI44L expression at both the mRNA and protein levels. Overexpression of poIFI44L in vitro resulted in significantly reduced TGEV N gene mRNA levels, N protein expression, and viral titers in a dose-dependent manner, whereas siRNA-mediated knockdown of poIFI44L enhanced viral replication. Mechanistically, poIFI44L upregulated IFN-β and ISRE promoter activities and upregulated the production of IFN-β and downstream ISGs (ISG56 and ISG60). Collectively, our data suggest that poIFI44L acts as an innate immune effector that suppresses TGEV by activating the IFN-β/ISGs signaling pathway.