Inverse interference: how viruses fight the interferon system.

Viruses need to multiply extensively in the infected host in order to ensure transmission to new hosts and survival as a population. This is a formidable task, given the powerful innate and adaptive immune responses of the host. In particular, the interferon (IFN) system plays an important role in limiting virus spread at an early stage of infection. It has become increasingly clear that viruses have evolved multiple strategies to escape the IFN system. They either inhibit IFN synthesis, bind and inactivate secreted IFN molecules, block IFN-activated signaling, or disturb the action of IFN-induced antiviral proteins. The molecular mechanisms involved range from a broad shut-off of the host cell metabolism to fine-tuned elimination of key components of the IFN system. Type I (alpha/beta) IFNs are produced in direct response to virus infection and double-stranded RNA (dsRNA) molecules that are sensed as a danger signal by infected cells. IFNs induce the expression of a number of antiviral proteins, some of which are again activated by dsRNA. Therefore, many viruses produce dsRNA-binding proteins to sequester the danger signal or express virulence genes that target specific components of the IFN system, such as members of the IFN regulatory factor (IRF) family or components of the JAK-STAT signaling pathway. Finally, some viruses have adopted means to directly suppress the very antiviral effector proteins of the IFN-induced antiviral state directed against them. Evidently, viruses and their host's innate immune responses have coevolved, leading to a subtle balance between virus-promoting and virus-inhibiting factors. A better understanding of virus-host interactions is now emerging with great implications for vaccine development and drug design.