RESUMO
IFNs, produced during viral infections, induce the expression of hundreds of IFN-stimulated genes (ISGs). Some ISGs have specific antiviral activity, whereas others regulate the cellular response. Besides functioning as an antiviral effector, ISG15 is a negative regulator of IFN signaling, and inherited ISG15 deficiency leads to autoinflammatory IFNopathies, in which individuals exhibit elevated ISG expression in the absence of pathogenic infection. We have recapitulated these effects in cultured human A549-ISG15-/- cells and (using A549-UBA7-/- cells) confirmed that posttranslational modification by ISG15 (ISGylation) is not required for regulation of the type I IFN response. ISG15-deficient cells pretreated with IFN-α were resistant to paramyxovirus infection. We also showed that IFN-α treatment of ISG15-deficient cells led to significant inhibition of global protein synthesis, leading us to ask whether resistance was due to the direct antiviral activity of ISGs or whether cells were nonpermissive because of translation defects. We took advantage of the knowledge that IFN-induced protein with tetratricopeptide repeats 1 (IFIT1) is the principal antiviral ISG for parainfluenza virus 5. Knockdown of IFIT1 restored parainfluenza virus 5 infection in IFN-α-pretreated, ISG15-deficient cells, confirming that resistance was due to the direct antiviral activity of the IFN response. However, resistance could be induced if cells were pretreated with IFN-α for longer times, presumably because of inhibition of protein synthesis. These data show that the cause of virus resistance is 2-fold; ISG15 deficiency leads to the early overexpression of specific antiviral ISGs, but the later response is dominated by an unanticipated, ISG15-dependent loss of translational control.
Assuntos
Citocinas/deficiência , Resistência à Doença/genética , Interferon-alfa/metabolismo , Infecções por Paramyxoviridae/imunologia , Transdução de Sinais/imunologia , Ubiquitinas/deficiência , Células A549 , Proteínas Adaptadoras de Transdução de Sinal/genética , Proteínas Adaptadoras de Transdução de Sinal/metabolismo , Animais , Chlorocebus aethiops , Técnicas de Silenciamento de Genes , Técnicas de Inativação de Genes , Humanos , Vírus da Parainfluenza 2 Humana/imunologia , Vírus da Parainfluenza 3 Humana/imunologia , Vírus da Parainfluenza 5/imunologia , Infecções por Paramyxoviridae/virologia , Processamento de Proteína Pós-Traducional/imunologia , Proteínas de Ligação a RNA/genética , Proteínas de Ligação a RNA/metabolismo , Transdução de Sinais/genética , Enzimas Ativadoras de Ubiquitina/genética , Células VeroRESUMO
The induction of IFN-beta by the paramyxovirus PIV5 (formerly known as SV5) is limited by the action of the viral V protein that targets the cellular RNA helicase mda-5. Here we show that 12 other paramyxoviruses also target mda-5 by a direct interaction between the conserved cysteine-rich C-terminus of their V proteins and the helicase domain of mda-5. The inhibition of IFN-beta induction is not species-restricted, being observed in a range of mammalian cells as well as in avian cells, and we show that the inhibition of mda-5 function is also not restricted to mammalian cells. In contrast, the V proteins do not bind to the related RNA helicase RIG-I and do not inhibit its activity. The relative contributions of mda-5 and RIG-I to IFN-beta induction are discussed.