Roles of the ERK MAPK in the regulation of proinflammatory and apoptotic responses in chicken macrophages infected with H9N2 avian influenza virus

The mitogen-activated protein kinase (MAPK) family is responsible for important signalling pathways which regulate cell activation, differentiation, apoptosis and immune responses. Studies have shown that influenza virus infection activates MAPK family members in mammals. While the extracellular signal-regulated kinase (ERK)1/2 is important for virus replication, activation of p38 controls the expression of RANTES, interleukin (IL)-8 and tumour necrosis factor (TNF)-α. In this study, we report that avian influenza virus (AIV) activates ERK, p38 and Jun-N-terminal kinases in avian species. In chicken macrophages, while ERK was required for H9N2 AIV replication, ERK regulated proinflammatory cytokines IL-1β, IL-6 and IL-8, which is distinct from what has been previously reported in mammalian cells. Moreover, ERK alone suppressed TNF-α and FasL and inhibited TNF-family-mediated extrinsic apoptosis in H9N2-infected chicken macrophages. Taken together, these findings suggest that ERK signalling may uniquely play important roles in avian host responses to AIV infection.