| During the last decade, swine producers, veterinary practitioners, and animal health researchers have been economically and scientifically challenged by porcine reproductive and respiratory syndrome (PRRS). This disease is caused by a recently emerged virus of swine (PRRSV) that has been classified in the Arteriviridae family. Members of this family are small, cytolytic, positive-stranded, enveloped RNA viruses. The goals of these studies were to characterize the immune response to PRRSV with emphasis on cell-mediated immunity. Analysis of immune responses amongst pigs following either inoculation with virulent or attenuated PRRSV indicated that this immunity was weak and protracted. These results were confirmed in additional experiments designed to compare the immunity elicited by exposure of pigs to either attenuated or inactivated PRRSV or pseudorabies virus (PRV) vaccines. Attempts to enhance the immune response induced by attenuated PRRSV by the co-administration of an adjuvant, demonstrated to be effective with attenuated PRV vaccines, failed. Studies were then performed to address whether cytokine networks affect secondary immune responses to PRRSV. Interleukin-12 (IL-12) was shown to increase; whereas, EL-10 decreased the frequency of memory IFN-γ-secreting cells responding to recall viral antigen in vitro. Moreover, co-administration of soluble recombinant IL-12 protein or plasmids containing the cDNA encoding the protein with a PRRS MLV vaccine resulted in transiently enhanced IFN-γ T cell responses in vivo. In addition, the ability of a synthetic dsRNA polynucleotide, polyinosinic acid:polycytidylic acid (poly I:C), complexed with poly-L-lysine and carboxymethylcellulose (poly ICLC), known to be capable of inducing the production of IL-12, was tested for its ability to potentiate vaccine-induced PRRSV-specific immunity. Whereas co-administration of MLV vaccine with either rIL-12 or the EL-12 cDNA containing plasmid transiently enhanced specific cell-mediated immunity, the co-administration of MLV vaccine and poly ICLC resulted in a more sustained increase in the frequency of virus-specific IFN-γ responses. Thus, cytokines can enhance the cell-mediated immune responses induced by a PRRS MLV vaccine. |
