We previously reported that HSV-1 infection leads to inhibition of the IFN signal transduction pathway, as we observed marked suppression of IFN-induced phosphorylation of Jak1, Tyk2, Jak2, STAT1, and STAT2 (44
). These results indicate that HSV-1 inhibits the JAK/STAT pathway at a point that precedes the JAK phosphorylation step. In the present study, we showed that SOCS3, a host JAK/STAT inhibitor, is transcriptionally induced by HSV-1 infection. SOCS3 inhibits JAK phosphorylation through binding to the cytokine receptors (3
). To our knowledge, only one report showing that virus induces SOCS3 has been published to date. Namely, Bode et al. (4
) demonstrated that human hepatitis C virus core protein transcriptionally induces SOCS3, which suppresses the IFN-induced antiviral state. In the case of HSV-1 infection studied here, maximal protein levels of SOCS3 were detected a few hours after virus infection. This finding is consistent with the kinetics of the HSV-1-mediated IFN signal transduction suppression described previously (44
). It has been shown that HSV-1 activates IFN-inducible genes under experimental conditions in which de novo cellular protein synthesis was inhibited (23
). These observations are consistent with our finding that SOCS3 is synthesized de novo following HSV-1 infection. In addition, UV-treated VR3 did not induce SOCS3, which correlates with its poor inhibition of the JAK/STAT pathway (44
). In the previous report, we initially speculated that the virus-mediated inhibition of the JAK/STAT pathway requires viral protein synthesis. However, we found that SOCS3 induction occurs too rapidly (within 10 min) at the mRNA level for it to be driven by the de novo synthesis of viral proteins. Two mutant viruses that are defective in one of the tegument proteins UL41 and UL13, which are hypersensitive to IFN (37
), weakly induced SOCS3 compared to the parental wild-type strain. Accordingly, we now postulate that the induction of SOCS3 occurs after endocytosis or uncoating but not at the step of virus attachment to host cells. The UL41 gene product is an RNase, the vhs protein, which rapidly degrades host and virus mRNA and thereby causes protein synthesis shutoff (15
). The UL13 gene encodes a protein kinase whose exact function is currently unknown. However, it is proposed that the protein kinase regulates vhs activity (26
). The impairment of SOCS3 induction by these mutants should contribute to their hypersensitivity to IFN. However, we found that the mutant viruses are still able to weakly induce SOCS3. Consequently, we propose that the two tegument proteins do not contribute directly to SOCS3 induction. The poor induction of SOCS3 by these mutants may instead relate to the fact that they replicate with a lower efficacy (about 1 log unit less of virus titer) than the parental virus (data not shown).
We also suggest that the induction of SOCS3 by HSV-1 blocks the IFN production system. The IFN production system has been well characterized (5
). Initially, IFN-β is transcriptionally activated by phosphorylated IRF-3 and activated NF-κB in a JAK/STAT pathway-independent manner. Virus-induced IFN-β then stimulates the expression of IRF-7, a component of the transactivator of IFN-α genes, via the JAK/STAT pathway (5
). IRF-7 subsequently mediates the induction of IFN-α, such as human IFN-α4, and various IFN-inducible genes. This cycle results in the production of large amounts of IFN-α and the establishment of a strong antiviral state. SOCS3, which is induced by HSV-1, may suppress the JAK/STAT-dependent production of large amounts of IFN-α. We found that after HSV-1 infection, IFN-β is upregulated but IRF-7 and IFN-α4 levels are poorly induced (Fig. ). However, HSV-1 activates IFN- (namely JAK/STAT pathway-) independent signal transduction, including IRF-3 phosphorylation and upregulation of IFN-β (27
; also this study). These events were equally well induced by the wild-type virus, VR3, and the mutant viruses d41 and d13. In contrast, the IFN-dependent signal transduction activated by IFN-β is markedly suppressed after infection with VR3 but only partially blocked by infection with the mutant virus particles.
SOCS3 is an important regulator of cytokine signaling. SOCS3 induction would influence not only the IFN system but would also have a dramatic impact on the immune system in a manner that would favor HSV-1 replication. For example, SOCS3 promotes Th2 development by inhibiting interleukin-12 (IL-12)-mediated STAT4 activation in T cells (34
). It also negatively regulates IL-2 signaling (6
) and IL-2 production via CD28 signaling (19
). Furthermore, it inhibits IL-6 signaling in macrophages (8
). These SOCS3-mediated events would suppress the ability of the host to clear the virus. We conclude that the induction of SOCS3 by the virus plays a key role in host-virus interactions, as it may directly promote an active and productive infection by the virus. We are currently elucidating the details of the molecular mechanism by which SOCS3 is induced and the effects that it has on both the virus and the host.