To determine the potential of host factors as a means of counteracting viral immunomodulating activity and stimulating effective antiviral immunity [44] , we bioengineered the infectious clones to express a variety of type I IFNs, which were selected because of their well-documented role in PRRSV infections and activity in mediation of antiviral immunity [9, 15, [45] [46] [47] . In addition, the suppression of type I IFN expression by PRRSV infection has been well documented [15] . Shown in Figure 3 are data from the infectious clone expressing four type I IFNs (IFNα1, IFNβ, IFNδ3 and IFNω5). At 4 d post transfection with equivalent cDNA clones, virus-positive cells were detected in cells transfected with all four infectious clones but with different densities. Where IFNδ3-PRRSV propagated similarly as the control GFP-virus, the replication rates of IFN-β-, IFNα1-and IFNω5-viruses were attenuated by approximately 70% (β-type) or 90% (α1-& ω5-types) ( Figure 3A-H) . Attenuation of IFN-expressing viruses could be caused by viral genome alteration or more likely by the replication-associated expression of active IFN peptides, which was consistent with the anti-PRRSV activity of these IFN subtypes [15] ( Figure 3C ). In contrast, IFNδ3-and GFP-type viruses replicated well. To confirm these findings, we counterstained IFNα1-virus infected cells with antibodies against porcine IFNα (R&D, Minneapolis, MN, USA) and PRRSV nucleocapsid (N) protein ( Figure 4A-D) . Co-localization of IFNα-and PRRSV-labeling ( Figure 4C ) indicated that IFN polypeptides were expressed by the engineered viruses during infection. Furthermore, the IFN expressing viruses inhibited the replication of a second PRRS virus co-transfected or co-infected; and again, the intensity of the inhibition was consistent with the anti-PRRSV activity of these IFN subtypes, i.e., IFNω5IFNα1>IFN-β>IFNδ3 ( Figure 4E ). However, because the expression of IFN inhibits virus replication [22, 44] , engineered viruses with the most active IFN subtypes against PRRSV may not be good candidates for a modified live virus (MLV) vaccine. This limitation would prevent the preparation of a high titer virus as a vaccine. Several measures may be used to overcome these limitations, such as incorporating IFN subtypes that do not inhibit replication of a MLV strain but may up-regulate B-and T-cell responses [44] , or controlled expression/activation of the incorporated IFN peptides within certain temporal windows or cell types. Pigs have at least 35 functional type I IFN genes with diverse antiviral or immunoregulatory activity [47] , which provides several candidates for balancing antiviral and immunoregulatory activity to optimize the replication-competent recombinant viruses. Type I IFNs mediate antiviral responses through induction of IFN stimulated genes, such as MxA and RNase L. Direct incorporation of some ISGs (or their functional domains) into the PRRSV infectious clone provides an attractive alternative given that PRRSV-specific ISGs have been identified [15] . In addition, incorporating an exogenous gene tag (a compliance marker) at the vector backbone of a MLV will also allow differentiation of vaccinated from non-vaccinated animals [27] .