ISG15 counteracts Listeria monocytogenes infection.
Bottom Line: ISG15 is an interferon-stimulated, linear di-ubiquitin-like protein, with anti-viral activity.Surprisingly this induction can be type I interferon independent and depends on the cytosolic surveillance pathway, which senses bacterial DNA and signals through STING, TBK1, IRF3 and IRF7.Together, our data reveal a previously uncharacterized ISG15-dependent restriction of Listeria infection, reinforcing the view that ISG15 is a key component of the innate immune response.
Affiliation: Unité, Institut Pasteur, Paris, France.
ISG15 is an interferon-stimulated, linear di-ubiquitin-like protein, with anti-viral activity. The role of ISG15 during bacterial infection remains elusive. We show that ISG15 expression in nonphagocytic cells is dramatically induced upon Listeria infection. Surprisingly this induction can be type I interferon independent and depends on the cytosolic surveillance pathway, which senses bacterial DNA and signals through STING, TBK1, IRF3 and IRF7. Most importantly, we observed that ISG15 expression restricts Listeria infection in vitro and in vivo. We made use of stable isotope labeling in tissue culture (SILAC) to identify ISGylated proteins that could be responsible for the protective effect. Strikingly, infection or overexpression of ISG15 leads to ISGylation of ER and Golgi proteins, which correlates with increased secretion of cytokines known to counteract infection. Together, our data reveal a previously uncharacterized ISG15-dependent restriction of Listeria infection, reinforcing the view that ISG15 is a key component of the innate immune response.
No MeSH data available.
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Mentions: Since ISG15 protein levels increased relatively rapidly and ISG15 is known to be transcriptionally induced in response to interferon, we monitored transcript levels of both ISG15 and IFNB1 by quantitative real time PCR (qRT-PCR). Interestingly, we found that the two transcripts are concomitantly induced after 3 hr of infection with Listeria (Figure 1D,E). This concomitant induction led us to hypothesize that ISG15 could be induced in an interferon-independent manner during Listeria infection. In order to test this hypothesis, we used the viral protein B18R to block signaling from the interferon receptor (Chairatvit et al., 2012). The protein acts similarly to a blocking antibody. When cells are pretreated with B18R, the viral protein inhibits binding of interferon to its receptor, which is thus prevented from signaling. Following pretreatment with B18R, HeLa cells were either stimulated with interferon or infected with Listeria to assess whether the bacterial-ISG15 induction was dependent on secreted interferon signaling in an autocrine or paracrine manner. We observed that bacteria-induced ISG15 production was not diminished by B18R pretreatment in stark contrast to the interferon-induced ISG15 signal, which was almost entirely abrogated by B18R pretreatment (Figure 1F). To confirm the B18R results, we took advantage of a human fibrosarcoma cell line, 2fTGH, from which interferon-unresponsive mutants have been isolated (Pellegrini et al., 1989). The U5A clone lacks a functional IFNAR2 receptor (IFNAR2−/−) and thus is impaired in type I interferon receptor signaling (Lutfalla et al., 1995). 2fTGH and U5A cells were both highly permissive to Listeria infection (Figure 1—figure supplement 1). Strikingly in the U5A cells (defective for type I interferon binding and signaling), as in 2fTGH cells, there is still a robust ISG15 response to Listeria infection (Figure 1G).
No MeSH data available.