The influenza virus PB1-F2 protein has interferon antagonistic activity

Abstract

PB1-F2 is a nonstructural protein of influenza viruses encoded by the PB1 gene segment from a +1 open reading frame. It has been shown that PB1-F2 contributes to viral pathogenicity, although the underlying mechanisms are still unclear. Induction of type I interferon (IFN) and the innate immune response are the first line of defense against viral infection. Here we show that influenza A viruses (IAVs) lacking the PB1-F2 protein induce an enhanced expression of IFN-β and IFN-stimulated genes in infected epithelial cells. Studying molecular mechanisms underlying the PB1-F2-mediated IFN antagonistic activity showed that PB1-F2 interferes with the RIG-I/MAVS protein complex thereby inhibiting the activation of the downstream transcription factor IFN regulatory factor 3. These findings were also reflected in in vivo studies demonstrating that infection with PR8 wild-type (wt) virus resulted in higher lung titers and a more severe onset of disease compared with infection with its PB1-F2-deficient counterpart. Accordingly, a much more pronounced infiltration of lungs with immune cells was detected in mice infected with the PB1-F2 wt virus. In summary, we demonstrate that the PB1-F2 protein of IAVs exhibits a type I IFN-antagonistic function by interfering with the RIG-I/MAVS complex, which contributes to an enhanced pathogenicity in vivo.

PB1-F2 Modulates Early Host Responses but Does Not Affect the Pathogenesis of H1N1 Seasonal Influenza Virus

“…An early delay in innate immune activation has been observed in lung tissues of mice infected with a sublethal mouse-adapted virus carrying the PB1-F2 N66S mutation (3), supporting a growing body of evidence from in vitro studies that PB1-F2 delays innate immune activation (5,30). Here we observed that only the N66S-carrying 1918 PB1-F2 protein was associated with inhibition of IFN-␣, IL-1␤, and IL-6 induction in ferret nasal wash cells at day 1 after infection, providing further evidence for an immunomodulatory role of full-length PB1-F2 proteins with this mutation.…”

“…In mice, introduction of the entire 1918 PB1-F2, or the N66S mutation alone, into the context of the mouse-adapted H1N1 WSN/33 strain resulted in an increased production of proinflammatory cytokines at later infection stages (3,4,19). However, further characterization of the N66S mutant revealed decreased type I IFN production during the initial phase of the infection (3,5,30).…”

Emergence of Highly Pathogenic Avian Influenza A(H5N1) Virus PB1-F2 Variants and Their Virulence in BALB/c Mice

“…PB1-F2 truncations and IFN-␤ expression. PB1-F2 from mouse-adapted viruses alters type I interferon production in some cell types (22)(23)(24)28). We transfected A549 and DF1 cells with the PB1-F2 plasmids and measured IFN-␤ expression at various time points using the TaqMan assay qRT-PCR.…”

“…Several possible mechanisms have been suggested, including mitochondrial targeting and proapoptotic activity (8,9,19), enhancing PB1 function (20), and inhibition (21)(22)(23)(24)(25)(26)(27) or enhancement (28) of the IFN-␤ response. PB1-F2 has also been linked to bacterial infections following IAV infection (29)(30)(31)(32).…”

Influenza A Virus Multiplication and the Cellular SUMOylation System

“…NS1, plays a critical role during infection by neutralizing cellular antiviral responses and increasing viral protein synthesis, while decreasing the synthesis of cellular proteins (reviewed by Hale et al [8], Lin et al [9], and Krug et al [10]). The role of neutralizing antiviral defenses has been recently shown to be shared by another viral protein produced by a limited number of viral strains: The so-called PB1-F2 protein, a 90 amino acid protein produced by an alternative ORF contained within the PB1 gene segment [11][12].…”