A NOVEL INTERFERON-INDUCING PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS STRAIN: CHARACTERIZATION AND VACCINE DEVELOPMENT
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Porcine reproductive and respiratory syndrome virus (PRRSV) causes a swine infectious disease characterized by severe reproductive failure in sows and respiratory disease in pigs of all ages. Despite substantial efforts to control PRRS, no production or vaccination regimen has demonstrated sustaining success. Type I interferons (IFNs) are critical to the innate immunity against viral infections and play an important role in activation of the adaptive immune response. PRRSV appears to antagonize induction of type I IFNs. Fortunately, we discovered an atypical PRRSV strain, A2MC2, which induces type I IFNs in cultured cells. A2MC2 elicits earlier onset and higher levels of virus-neutralizing antibodies than the Ingelvac PRRS® MLV in pigs. However, moderate virulence of A2MC2 was observed in infected piglets. The objective of this project was to characterize A2MC2 and explore this unique strain for the development of an improved vaccine against PRRS. First, I attenuated this strain by serial passaging in MARC-145 cells for 90 consecutive passages. The passage 90 virus (A2MC2-P90) was avirulent and retained the capability of IFN induction. The A2MC2-P90 virus induced higher level virus-neutralizing antibodies in pigs. Secondly, I constructed an infectious cDNA clone of A2MC2. The recovered virus from the infectious clone was similar to the parental strain in growth properties and IFN induction. Gene fragment swapping demonstrated that the middle half genome of A2MC2 was essential for its IFN induction. Thirdly, I conducted studies to exam the genetic source of A2MC2 in IFN induction. Comparison of A2MC2 and other closely relevant PRRSV strain identifies five unique non-synonymous nucleotides. These five nucleotides remained unchanged in the A2MC2-P90 virus. Site-directed mutagenesis indicated that one unique nucleotide in A2MC2 genome was critical in the IFN induction as mutation of this nucleotide led to the loss of IFN induction. Together, our data demonstrate that A2MC2 is a novel strain that is worth further exploration for an improved vaccine against PRRS. The infectious clone of A2MC2 will be useful for the development of a marker vaccine by insertion of a marker sequence into the A2MC2 genome.