Additive inhibition of porcine reproductive and respiratory syndrome virus infection with the soluble sialoadhesin and CD163 receptors

Abstract

Porcine reproductive and respiratory syndrome (PRRS) is an economically important swine disease to the swine industry worldwide. Current PRRS vaccines are only partially effective and new vaccine development faces great challenges. Sialoadhesin (Sn) and CD163 are the two essential receptors for PRRSV infection of porcine alveolar macrophage (PAM). To investigate the feasibility of the soluble viral receptors for PRRS control, in the present study we generated recombinant adenovirus (rAd) expressing the four N-terminal Ig-like domains of porcine Sn (Sn4D), the fifth SRCR domain (SRCR5) or domains 5-9 (SRCR59) of porcine CD163 as porcine Fc (pFc) fusion proteins. Efficient expression of the soluble viral receptors in the rAd-transduced cells was confirmed by RT-PCR and Western blotting. To detect their antiviral activities, the soluble viral receptors were purified from the media of rAd-transduced cells and identified by Western blotting. The viral binding assay showed that the soluble receptors Sn4D-Fc and SRCR59-Fc, but not SRCR5-Fc and the control pFc, were able to bind to PRRSV particles. The viral infection blocking assays showed that co-treatment of PRRSV with different concentrations of Sn4D-Fc and SRCR59-Fc proteins resulted in a much higher (72.1%-77.6%) reduction in PRRSV-positive cell number than the single protein treatment (45.1%-60.0% or 44.0%-56.2%). To investigate the feasibility of delivering the soluble viral receptors to PAM, two pig cell lines were transduced with rAd-Sn4D-Fc and/or rAd-SRCR59-Fc using a transwell culture system. PAM cells were infected with PRRSV and then co-cultured with the rAd-transduced cells. Viral titration assay showed that co-cultivation of the infected PAM with rAd-Sn4D-Fc- and rAd-SRCR59-Fc-transduced cells resulted in much higher (by ∼3.5 log) reduction in the viral titers (TCID50) than that of co-cultivation with the single vector-transduced cells (by ∼1.0 log). Further studies showed that the rAd co-delivered soluble receptors Sn4D-Fc and SRCR59-Fc had dose-dependent and temporal antiviral effect against three different PRRSV strains. Since the data presented indicate an additive anti-PRRSV activity between the soluble receptors Sn4D-Fc and SRCR59-Fc, we conclude that the two rAd vectors generated will be useful for development a novel reagent for PRRS control.

Keywords: Antiviral effects; Porcine reproductive and respiratory syndrome virus; Recombinant adenoviruses; Soluble viral receptors.