In the present study, we examined whether IFN-λ, a newly identified member of IFN family, has the ability to inhibit HSV-1 infection of human astrocytes and neurons. In agreement with the study (Cheeran et al. 2000
), we demonstrated that both astrocytes and neurons could be efficiently infected by HSV-1, which was evidenced by the detection of HSV-1 DNA and protein expression in these cells ( and ). In the presence of IFN-λ, HSV-1 replication, however, was suppressed in these cells (). This finding is supported by a recent report (Melchjorsen et al. 2006
) showing IFN-λ1 exerts potential antiviral activity against HSV-1 replication in human macrophages and dendritic cells by repressing HSV-1 ICP27 gene transcription. The anti-HSV-1 effect by IFN-λ is IFN-λ receptor (IL-10Rβ) mediated, as antibody to IL-10Rβ could block the IFN-λ action on HSV-1. This receptor-mediated ability to respond to IFN-λ treatment by human astrocytes and neurons supports our earlier study showing that human neurons express functional IFN-λ receptors, IL-28Rα and IL-10Rβ (Zhou et al. 2009
Host cell innate immune responses play a vital role in suppressing HSV-1 infection and even infected, maintaining HSV-1 in a quiescent status. Among these responses, the production of type I IFNs is of the most importance in controlling viral replication (Hochrein et al. 2004
). It has been known for several decades that IFN-α/β, secreted by blood cells and fibroblasts, have potent anti-HSV activity (Koelle and Corey 2008
). The importance of type I IFN in the host response to HSV infection is highlighted by the findings that there is enhanced virulence of HSV in type I IFN-receptor-deficient mice (Leib 2002
), and that several HSV proteins have the specific type I IFN evasion functions. Type I IFN-mediated innate immunity also has a key role in restricting viral infections in the CNS (Paul et al. 2007
). It has been demonstrated that type I IFNs can inhibit transmission of HSV-1 from neuronal axon to epidermal cells and subsequent spread in these cells (Mikloska and Cunningham 2001
). Thus, the induction of intracellular IFN-α/β expression by IFN-λ provides a plausible mechanism for the anti-HSV-1 action of IFN-λ in astrocytes and neurons. The role of type I IFNs in IFN-λ-mediated action on HSV-1 inhibition was confirmed by the observation that antibody to IFN-α/β receptor could compromise the IFN-λ action in astrocytes () and neurons ().
To further determine the underlying mechanism of anti-HSV-1 effect by IFN-λ, we examined whether IFN-λ has the ability to activate IFN pathways in astrocytes and neurons. We found that IFN-λ treatment of astrocytes and neurons induced the expression of ISGs (MxA, OAS-1, and ISG56), which could be compromised by antibody to IFN-α/β receptor (). This finding supports the role of IFN-α/β in the IFN-λ actions on ISGs. However, because of the lack of a complete blockage of the IFN-λ action by the antibody, it is likely that other mechanism(s) are also involved in the IFN-λ action. One possibility is that IFN-λ has the ability to directly activate ISGs and other antiviral factors (Brand et al. 2005
; Li et al. 2009
). It has been shown that although IFN-λ use the receptor complex different from that of type I IFNs, the activation of IFN-λ receptor leads to the expression of type I IFN-induced antiviral genes (Dumoutier et al. 2004
). The investigation from several group showed that IFN-λ-mediated antiviral activity is linked to its ability to activate ISG3 and several antiviral genes in human hepatocytes (Doyle et al. 2006
) and several carcinoma cell lines (Dumoutier et al. 2004
). Taken together, these findings observed in different model systems support the notion that IFN-λ functionally resembles type I IFNs, inducing the expression of ISGs and resulting in the establishment of an antiviral state.
Type I IFN gene expression is tightly regulated by the transcription factors such as IFN-regulatory factors (IRFs). Therefore, we examined the effect of IFN-λ on the induction of IRFs in astrocytes and neurons. Our data that the selective induction of IRF7 expression by IFN-λ in neurons provides a sound mechanism for the positive action of IFN-λ on IFN-α expression, as IRF7 is a master regulator of IFN-α (Honda et al. 2005
). Whereas in astrocytes, in addition to IRF7, the upregulation of IRF3 and IRF5 by IFN-λ treatment was also observed. This finding was in parallel with the observation that IFN-λ induced not only IFN-α but also IFN-β expression in astrocytes, as IRF3 is a key positive regulator of IFN-β. We also determined the impact of IFN-λ on the expression of the key negative regulators in IFN-α/β signaling pathway, as IFN-λ has the ability to activate JAK-STAT pathway (Dumoutier et al. 2004
; Kotenko et al. 2003
; Sheppard et al. 2003
), which is highly regulated by type I IFNs. The suppressor of cytokine signaling (SOCS) is a group of key negative regulators of the IFN-α-triggered STAT transactivation. It has been reported that SOCS-1 and SOCS-3 could be induced by HSV-1 infection, leading to a high preference of viral replication (Yokota et al. 2005
). Therefore, we investigated whether IFN-λ treatment could inhibit SOCS expression. The finding that IFN-λ treatment selectively inhibited SOCS-1 expression provides an additional mechanism responsible for the IFN-λ action on the induction of IFN-α/β and ISGs in astrocytes and neurons.
Collectively, although the accurate mechanisms for IFN-λ-mediated suppression on HSV-1 replication in astrocytes and neurons remain to be determined, our data that IFN-λ has the ability to activate type I IFN pathway, leading to the induction of IFN-α/β and IFN-stimulated antiviral genes provide a sound mechanism responsible for the IFN-λ action on HSV-1 inhibition. Future animal and in vivo studies are necessary to determine whether IFN-λ-mediated antiviral function is beneficial for human therapy against HSV-1 infections in the CNS. In addition, further research into the fundamental and biological functions of IFN-λ also is a key to address the role of IFN-λ in the host response to viral infections and to understand the mechanisms of IFN-λ actions at both the molecular and cellular levels.