Abstract
NF-kappaB and inflammasomes both play central roles in orchestrating anti-pathogen responses by rapidly inducing a variety of early-response cytokines and chemokines following infection. Myxoma virus (MYXV), a pathogenic poxvirus of rabbits, encodes a member of the cellular pyrin domain (PYD) superfamily, called M013. The viral M013 protein was previously shown to bind host ASC-1 protein and inhibit the cellular inflammasome complex that regulates the activation and secretion of caspase 1-regulated cytokines such as IL-1beta and IL-18. Here, we report that human THP-1 monocytic cells infected with a MYXV construct deleted for the M013L gene (vMyxM013-KO), in stark contrast to the parental MYXV, rapidly induce high levels of secreted pro-inflammatory cytokines like TNF, IL-6, and MCP-1, all of which are regulated by NF-kappaB. The induction of these NF-kappaB regulated cytokines following infection with vMyxM013-KO was also confirmed in vivo using THP-1 derived xenografts in NOD-SCID mice. vMyxM013-KO virus infection specifically induced the rapid phosphorylation of IKK and degradation of IkappaBalpha, which was followed by nuclear translocation of NF-kappaB/p65. Even in the absence of virus infection, transiently expressed M013 protein alone inhibited cellular NF-kappaB-mediated reporter gene expression and nuclear translocation of NF-kappaB/p65. Using protein/protein interaction analysis, we show that M013 protein also binds directly with cellular NF-kappaB1, suggesting a direct physical and functional linkage between NF-kappaB1 and ASC-1. We further demonstrate that inhibition of the inflammasome with a caspase-1 inhibitor did not prevent the induction of NF-kappaB regulated cytokines following infection with vMyxM013-KO virus, but did block the activation of IL-1beta. Thus, the poxviral M013 inhibitor exerts a dual immuno-subversive role in the simultaneous co-regulation of both the cellular inflammasome complex and NF-kappaB-mediated pro-inflammatory responses.