Recently, we identified a unique -2/-1 ribosomal frameshift mechanism in PRRSV, which yields two truncated forms of nonstructural protein (nsp) 2 variants, nsp2TF and nsp2N. Here, in vitro expression of individual PRRSV nsp2TF and nsp2N demonstrated their ability to suppress cellular innate immune responses in transfected cells. Two recombinant viruses were further analyzed, in which either nsp2TF was C-terminally truncated (vKO1) or expression of both nsp2TF and nsp2N was knocked out (vKO2). Host cellular mRNA profiling showed that a panel of cellular immune genes, in particular those involved in innate immunity, was upregulated in cells infected with vKO1 and vKO2. Compared to the wild-type virus, vKO1 and vKO2 expedited the IFN-α response and increased NK cell cytotoxicity, and subsequently enhanced T cell immune responses in infected pigs. Our data strongly implicate nsp2TF/nsp2N in arteriviral immune evasion and demonstrate that nsp2TF/nsp2N-deficient PRRSV is less capable of counteracting host innate immune responses.

In order to study the mechanism of PRRSV persistence, an in vitro model of persistence was developed by serially passaging PRRSV-infected MARC-145 cells 109 times. Viral persistence was detected to be associated with increased double-stranded (dsRNA) in the infected cells. In PRRSV infected pigs, reduced ratio of plus to minus strands of viral RNA was observed in lymphoid tissues from PRRSV persistent pigs at 52 days post infection. Viral dsRNA was mostly detected in the germinal center during persistent infection compared to the localization of dsRNA in the inter-follicular zones during acute infection. RNA array analysis of antiviral cytokines in persistently infected lymph nodes showed that the presence of dsRNA did not stimulate antiviral immunity. These results suggest that PRRSV dsRNA functions as a mediator for viral persistence. The localization of PRRSV dsRNA in the germinal center of lymphoid tissues reveals a novel mechanism for PRRSV persistence.

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