The results presented here provide novel information regardingT1-IFN induction and protection in vivo
. While the induction of T1-IFNs in vivo
following systemic (intravenous (i.v.) or intraperitoneal (i.p.) administration of viruses has been studied to a great extent 
and their ability to induce an antiviral response is well known 
, we still lack an understanding of how T1-IFNs and ISGs are temporally induced and protect from disease during the course of the many viral infections that follow a stepwise mode of LH dissemination 
. The induction of T1-IFN genes depends on cells sensing viral infection. Cells recognize Pathogen Associated Molecular Patterns (PAMPs) of viruses (in most cases nucleic acids) by means of Pathogen Recognition Receptors (PRR) expressed at the plasma membrane (e.g. Toll Like Receptor (TLR)2, TLR4), in endosomes (TLR3, TLR7, TLR9) or in the cytosol (RIG-I, MDA5, DAI). 
. Signaling through PRRs culminate in the activation of specific members of the IRF family of transcription factors, most notably IRF3, IRF7, NF-κB and c-jun which stimulate the T1-IFN promoters. During stepwise infection, T1-IFNs could act on vital target organ indirectly. For example, they could induce ISGs and help orchestrate the innate and adaptive immune response in the D-LN thereby curbing virus spread to the target organ. As we have previously shown, this is a major mechanism whereby NK 
and memory CD8+ T cells 
protect mice from mousepox. Alternatively, T1-IFNs could directly induce ISGs in the target organ and/or contribute to the recruitment of immune cells. In this case, the protection of the target organ could result from the T1-IFN produced at the primary site of infection that is distributed systemically, or from the T1-IFN produced locally in the target organ from PAMPS either distributed systemically or locally produced. Here we have used the classical ECTV model of LH spread to show that during ECTV infection, T1-IFN signaling in the liver (the target organ) strongly correlates with resistance to disease. We also show that ISG induction in the liver correlates with T1-IFN transcription in the liver but not in the D-LN suggesting that T1-IFN signaling in the liver is reliant on local T1-IFN production. Moreover, we demonstrate that the induction of T1-IFN in the liver depends exclusively on local viral replication. This suggests that PAMPS produced in the footpad or in the D-LN do not distribute systemically to the liver before virus arrival. It should be noted, however, that poxviruses excel in the number of immune evasion proteins that affect innate immunity 
. Hence, it is possible that during other viral infections where T1-IFN production and signaling is not targeted by the virus, the systemic distribution of T1-IFN may have a more important role in distant ISG induction.
Our work also impinges on our understanding of viral immune evasion. During the past few years there has been much progress towards the characterization of virally encoded immune evasion genes. While the cellular and molecular mechanisms whereby many of these evasion molecules operate are well known 
, we still have an incomplete understanding on how they subvert the immune response in vivo
. We have previously shown that the OPV T1-IFNbp is secreted from infected cells and binds back to cell surfaces 
by attaching to glycosaminoglycans at the cell membrane 
. Whether this also occurs in vivo
and is significant for viral virulence remained unknown. Our experiments reveal that the T1-IFNbp produces its evasive effect at least in part by attaching to uninfected liver cells surrounding infected foci, thereby precluding their ability to signal through the T1-IFN receptor. Of interest, the ECTV T1-IFNbp does not block IFN-β although ECTV infection induced IFN-β transcription. A remaining question is why IFN-β was insufficient to induce high levels of ISGs; our data may indicate that in vivo
, IFN-α and IFN-β have different functions.
Our results also have implications to our understanding of Ab-mediated protection from viral disease. The most commonly accepted mechanism of Ab protection is viral particle neutralization 
. Indeed, it has been suggested that this is the mechanism whereby the smallpox vaccine protects 
. However, while clinical data showed that protection from smallpox correlated with Ab neutralization, the same investigators could not find a causal association between neutralizing Ab titers and protection against smallpox 
. These findings suggests that mechanisms other than viral particle neutralization may be involved in protection by the smallpox vaccine. In support of this, Benhnia et al. recently demonstrated that mice can be protected from VACV by prior administration of Abs to the structural protein B5R and that this protection relied in complement activation 
, a well known Ab effector mechanism. Here we show that Abs can protect from and cure advanced systemic OPV disease by a previously unsuspected mechanism: inhibiting the function of an immune evasion protein. Of interest, while it is known that polyclonal antibodies in the form of convalescent sera and vaccinia immunoglobulin (VIG) are protective pre- and soon after exposure to VARV, the possible mechanisms of this protection remain unknown 
. We show that Abs to the T1-IFNbp cure mousepox even when administered as late as at 5 dpi suggesting that Abs to the T1-IFNbp may play a role in protection not only by the smallpox vaccine but also by VIG because, at least in mice, anti T1-IFNbp are present in sera following VACV vaccination 
. Thus, our experiments uncovered a novel mechanism of Ab mediated protection. We have previously shown the importance of the T-1 IFNbp in ECTV pathogenesis 
. Whether Abs to other secreted immunoregulatory viral protein could have a similar effect will likely depend on whether they play an essential role in pathogenesis or not and remains to be studied.
It is interesting to note that while resistance to primary infection with ECTV requires T1-IFN function, resistance to secondary infection does not as IFNAR1 deficient mice immunized with attenuated ECTV or VACV resisted a challenge with WT ECTV 
. This suggests that the main role of T1-IFNs in protection is to control the virus until an adaptive response is generated and thereafter become irrelevant. Hence, it is likely that Abs to the T1-IFNbp also have a similar effect.
Drugs being tested for the treatment of OPV infections are ST-246 
, which targets VACV F13L protein and its orthologs in other OPVs to inhibit the egress of extracellular virions from cells, and CMX001, an oral ether-lipid analogue of the acyclic nucleoside phosphonate Cidofovir 
, that target the viral DNA polymerase. These two drug types have been very effective for the treatment of various OPVs in several animal models 
. CMX001 has been shown to cure intranasal ECTV infection when treatment was started as late as at 5 dpi 
. Still, there is the caveat that OPVs could naturally develop resistance, or that resistant viruses could be artificially created. Indeed, VACV resistant to Cidofovir and its derivatives has been demonstrated 
. Similarly, cowpox virus resistant to ST-246 has been isolated 
. Thus, more than one or two anti-poxvirus drugs directed towards different targets are needed.10G7 mAb or similar T1-IFNbp mAbs could be exploited to treat humans against OPV infections because OPVs that affect humans encode a T1-IFNbp and at least three of them (VACV,VARV and MPXV) are inhibited by 10G7. In addition, a strategy of using mAbs to inhibit secreted immune evasion proteins important for viral pathogenesis could be explored to prevent and treat infections with any other virus that encode proteins of this kind.