Anti-microbial host factor HDAC6 is antagonised by the influenza A virus through host caspases and viral PA

Histone deacetylase 6 (HDAC6), through the repertoire of its substrate proteins, plays a critical role in human physiology, and an aberrant function of HDAC6 contributes to various pathophysiological conditions. HDAC6 is also known to be an anti-microbial host factor and has been implicated in restricting or clearing the infection of various human viral and bacterial pathogens. However, the state and the mechanisms of its antagonism in infected cells are not understood. Here, we demonstrate that influenza A virus (IAV) antagonises HDAC6 by recruiting both viral and host components. We found that HDAC6 mRNA expression, and consequently, the HDAC6 polypeptide expression is downregulated in human lung epithelial cells during early stage of IAV infection but can be rescued by depleting the expression of viral polymerase acidic (PA) protein, a subunit of IAV RNA polymerase. In addition, during later stage of the infection, the HDAC6 polypeptide undergoes caspase-mediated cleavage at two sites, generating two cleaved fragments. Both these fragments disappeared when the expression of caspase 3 was depleted in infected cells, whereas only second fragment disappeared when the expression of caspase 6 was depleted. But both fragments disappeared and the level of full-length HDAC6 polypeptide was rescued when the expression of PA was depleted in infected cells. Collectively, these data indicated that IAV antagonises the HDAC6 by decreasing its expression level in infected cells, both at mRNA and polypeptide level via PA gene, which has been implicated in auxiliary functions like degradation of host mRNA and induction of apoptosis.