Poxvirus protein N1L targets the I-kB kinase complex, inhibits signaling to NF-kB by the tumor necrosis factor superfamily of receptors, and inhibits NF-kB and IRF3 signaling by toll-like receptors

Abstract

Poxviruses encode proteins that suppress host immune responses, including secreted decoy receptors for pro-inflammatory cytokines such as interleukin-1 (IL-1) and the vaccinia virus proteins A46R and A52R that inhibit intracellular signaling by members of the IL-1 receptor (IL-1R) and Toll-like receptor (TLR) family. In vivo, the TLRs mediate the innate immune response by serving as pathogen recognition receptors, whose oligomerized intracellular Toll/IL-1 receptor (TIR) domains can initiate innate immune signaling. A family of TIR domaincontaining adapter molecules transduces signals from engaged receptors that ultimately activate NF- B and/or interferon regulatory factor 3 (IRF3) to induce proinflammatory cytokines. Data base searches detected a significant similarity between the N1L protein of vaccinia virus and A52R, a poxvirus inhibitor of TIR signaling. Compared with other poxvirus virulence factors, the poxvirus N1L protein strongly affects virulence in vivo; however, the precise target of N1L was previously unknown. Here we show that N1L suppresses NF- B activation following engagement of Toll/IL-1 receptors, tumor necrosis factor receptors, and lymphotoxin receptors. N1L inhibited receptor-, adapter-, TRAF-, and IKK- and IKK- -dependent signaling to NF- B. N1L associated with several components of the multisubunit I- B kinase complex, most strongly associating with the kinase, TANK-binding kinase 1 (TBK1). Together these findings are consistent with the hypothesis that N1L disrupts signaling to NF- B by Toll/IL-1Rs and TNF superfamily receptors by targeting the IKK complex for inhibition. Furthermore, N1L inhibited IRF3 signaling, which is also regulated by TBK1. These studies define a role for N1L as an immunomodulator of innate immunity by targeting components of NF- B and IRF3 signaling pathways.