I thank Kim Barrymore for editing the manuscript. This work was supported by the Takeda Science Foundation, and done within the framework of the Japan Initiative for Global Research Network on Infectious Diseases (J-GRID) and the Global COE Program "Establishment of International Collaboration Centers for Zoonosis Control" of the Ministry of Education, Culture, Sports, Science, and Technology (MEXT), Japan. The work was further supported by a Grant-in-aid from the Ministry of Health, Labor, and Welfare, Japan. Replication-Competent Recombinant Porcine Reproductive and Respiratory Syndrome (PRRS) Viruses Expressing Indicator Proteins and Antiviral Cytokines Porcine reproductive and respiratory syndrome virus (PRRSV) can subvert early innate immunity, which leads to ineffective antimicrobial responses. Overcoming immune subversion is critical for developing vaccines and other measures to control this devastating swine virus. The overall goal of this work was to enhance innate and adaptive immunity following vaccination through the expression of interferon (IFN) genes by the PRRSV genome. We have constructed a series of recombinant PRRS viruses using an infectious PRRSV cDNA clone (pCMV-P129). Coding regions of exogenous genes, which included Renilla luciferase (Rluc), green and red fluorescent proteins (GFP and DsRed, respectively) and several interferons (IFNs), were constructed and expressed through a unique subgenomic mRNA placed between ORF1b and ORF2 of the PRRSV infectious clone. The constructs, which expressed Rluc, GFP, DsRed, efficiently produced progeny viruses and mimicked the parental virus in both MARC-145 cells and porcine macrophages. In contrast, replication of IFN-expressing viruses was attenuated, similar to the level of replication observed after the addition of exogenous IFN. Furthermore, the IFN expressing viruses inhibited the replication of a second PRRS virus co-transfected or co-infected. Inhibition by the different IFN subtypes corresponded to their anti-PRRSV activity, i.e., IFNω5 ° IFNα1 > IFN-β > IFNδ3. In summary, the indicator-expressing viruses provided an efficient means for real-time monitoring of viral replication thus allowing high‑throughput elucidation of the role of host factors in PRRSV infection. This was shown when they were used to clearly demonstrate the involvement of tumor susceptibility gene 101 (TSG101) in the early stage of PRRSV infection. In addition, replication‑competent IFN-expressing viruses may be good candidates for development of modified live virus (MLV) vaccines, which are capable of reversing subverted innate immune responses and may induce more effective adaptive immunity against PRRSV infection. More than 20 years after initial reports [1] [2] [3] , porcine reproductive and respiratory syndrome virus (PRRSV) continues to be a global swine industry problem with losses in the U.S. approaching $6 billion over the last decade [4] . Belonging to the arteriviridae family in the order nidovirales, PRRSV is an enveloped RNA virus containing a single positive-strand RNA genome. The 15 kb viral RNA genome consists of seven open reading frames (ORF1- 7) . ORF1 comprises about 80% of the genome and encodes proteins with protease, replicase and regulatory functions. The smaller overlapping ORF2-7 encode five minor (GP2a, GP3, GP4, 5a and E proteins) and three major (GP5, M and N proteins) structural proteins [5] [6] [7] . Several studies have shown that PRRSV possesses the capacity to subvert early innate immune responses in pigs by suppressing the production of antiviral cytokines [8] [9] [10] [11] [12] [13] [14] [15] , which also contributes to ineffective B-and T-cell responses [16] [17] [18] [19] . Superimposed on this suppressive activity is a high viral mutation rate, which has made the development of vaccines challenging [20] . Modified live vaccines (MLV) used for control of PRRSV in the U.S. are based on only two virus isolates [20, 21] . Although MLV protect against some homologous field strains, their efficacy is not satisfactory due to failure to protect against infections of heterologous strains, as well as the potential risk for reversion to virulence [20, 21] . To develop successful vaccines against PRRSV infections, particularly those by heterologous strains, it is necessary to develop novel vector systems and to extensively categorize host factors critical in the virus-host interaction.