Viral tricks to grid-lock the type I interferon system

Abstract

Type I interferons (IFNs) play a crucial role in the innate immune avant-garde against viral infections. Virtually all viruses have developed means to counteract the induction, signaling, or antiviral actions of the IFN circuit. Over 170 different virus-encoded IFN antagonists from 93 distinct viruses have been described up to now, indicating that most viruses interfere with multiple stages of the IFN response. Although every viral IFN antagonist is unique in its own right, four main mechanisms are employed to circumvent innate immune responses: (i) general inhibition of cellular gene expression, (ii) sequestration of molecules in the IFN circuit, (iii) proteolytic cleavage, and (iv) proteasomal degradation of key components of the IFN system. The increasing understanding of how different viral IFN antagonists function has been translated to the generation of viruses with mutant IFN antagonists as potential live vaccine candidates. Moreover, IFN antagonists are attractive targets for inhibition by small-molecule compounds.

Figure 1

Viral antagonism with the IFN circuit. A check symbol is displayed at various steps of the IFN cascade (X-axis) that are impaired by a particular virus (Y-axis). The plot is organized by viral genome type. Double symbols indicate inhibition of IFN induction or signaling at yet unknown step of the pathway. Black symbols indicate proof for viral antagonism of the indicated step in the pathway by recombinant viruses lacking the IFN antagonist. Grey symbols indicate proof by over expression and/or wild-type virus infection.

Figure 2

Schematic representation of type I IFN induction through RLRs and TLRs. Viruses and their antagonistic proteins are indicated at the steps of the IFN pathway they affect. Antagonistic proteins are shown adjacent to their targets in alphabetical order. Antagonists in red indicate proof for IFN antagonist by recombinant viruses lacking the IFN antagonist. Antagonists in blue indicate proof by over expression and/or wild-type virus infection.

Figure 3

Schematic representation of type I IFN signaling. Viruses and their antagonistic proteins are indicated at the steps of the IFN pathway they affect. Antagonistic proteins are shown adjacent to their targets in alphabetical order. Antagonists in red indicate proof for IFN antagonist by recombinant viruses lacking the IFN antagonist. Antagonists in blue indicate proof by over expression and/or wild-type virus infection.