Several viral vectors, including those based on pseudorabies virus, poxvirus, adenovirus and transmissible gastroenteritis coronavirus (TGEV) have been used to express PRRSV structural proteins [22] or host immune factors [23] [24] [25] for developing anti-PRRSV immunity. For example, humoral immunity against PRRSV GP5 protein was detected in pigs immunized with fowlpoxviruscoexpressing PRRSV GP5/GP3 and porcine IL-18 [23] , and in mice immunized with adenovirusexpressing GP5/GP3 fused with swine granulocyte-macrophage colony stimulating factor (GM-CSF) [25] . In this context, we and others have proposed to use PRRSV infectious cDNA clones [26, 27] or virus replicons [28] as vectors for the expression of immune effectors that potentiate innate and adaptive immunity against a broad range of PRRSV isolates. Here we show that PRRSV infectious clones are effective vector systems to express exogenous antigens and host immune effectors. Specifically, we have constructed serial replication-competent viruses from a PRRSV infectious clone-based vector expressing indicator proteins and porcine type I interferons (IFNs). The indicator protein-expressing PRRS viruses efficiently produce progeny viruses and provide an efficient means for real-time monitoring of viral replication, thus allowing high-throughput elucidation of the role of host factors in PRRSV infection. In addition, the replication of some IFN-incorporated viruses is associated with the expression of active IFN peptides, which are capable of counteracting the subverted innate immune response and with potential to induce more effective adaptive immunity against PRRSV infection.