# H9N2 Avian Influenza Virus Downregulates FcRY Expression in Chicken Macrophage Cell Line HD11 by Activating the JNK MAPK Pathway

## TL;DR

This study shows how H9N2 avian influenza virus reduces FcRY expression in chicken macrophages by activating the JNK pathway, contributing to immunosuppression.

## Contribution

The study identifies the JNK MAPK pathway as a novel mechanism by which H9N2 virus downregulates FcRY in chicken macrophages.

## Key findings

- H9N2 virus and its proteins NS1 and M1 downregulate FcRY expression in HD11 cells.
- Activation of the JNK pathway is responsible for the downregulation of FcRY.
- The transcription factor c-jun inhibits FcRY expression at the transcriptional level.

## Abstract

The H9N2 avian influenza virus causes reduced production performance and immunosuppression in chickens. The chicken yolk sac immunoglobulins (IgY) receptor (FcRY) transports from the yolk into the embryo, providing offspring with passive immunity to infection against common poultry pathogens. FcRY is expressed in many tissues/organs of the chicken; however, there are no reports investigating FcRY expression in chicken macrophage cells, and how H9N2-infected HD11 cells (a chicken macrophage-like cell line) regulate FcRY expression remains uninvestigated. This study used the H9N2 virus as a model pathogen to explore the regulation of FcRY expression in avian macrophages. FcRY was highly expressed in HD11 cells, as shown by reverse transcription polymerase chain reactions, and indirect immunofluorescence indicated that FcRY was widely expressed in HD11 cells. HD11 cells infected with live H9N2 virus exhibited downregulated FcRY expression. Transfection of eukaryotic expression plasmids encoding each viral protein of H9N2 into HD11 cells revealed that nonstructural protein (NS1) and matrix protein (M1) downregulated FcRY expression. In addition, the use of a c-jun N-terminal kinase (JNK) activator inhibited the expression of FcRY, while a JNK inhibitor antagonized the downregulation of FcRY expression by live H9N2 virus, NS1 and M1 proteins. Finally, a dual luciferase reporter system showed that both the M1 protein and the transcription factor c-jun inhibited FcRY expression at the transcriptional level. Taken together, the transcription factor c-jun was a negative regulator of FcRY, while the live H9N2 virus, NS1, and M1 proteins downregulated the FcRY expression through activating the JNK signaling pathway. This provides an experimental basis for a novel mechanism of immunosuppression in the H9N2 avian influenza virus.