Characteristics of the cell-mediated immune response of swine to porcine reproductive and respiratory syndrome virus

Studies were conducted to ascertain the characteristics of the immunity induced by infection with wild-type virus or immunization with a modified-live virus (MLV) vaccine against porcine reproductive and respiratory syndrome virus (PRRSV). A striking observation resulting from this research was the protracted development of an interferon-gamma (IFN-γ) response to this virus. For instance, weak PRRSVspecific IFN-γ-secreting cells were initially detected in the blood of pigs at 3 weeks after exposure to either wild-type virus or an MLV vaccine. Only after a gradual and continuous increase during the ensuing 4 to 9 months did this antiviral immunity approach the level induced by a pseudorabies virus, MLV, by 2 to 4 weeks after vaccination. In contrast, a detectable humoral immune response was elicited against both virulent and attenuated PRRSV within 2 weeks after exposure, although a noticeable titer of virus-neutralizing antibodies was not apparent until 6 to 9 weeks later. Interestingly, the frequency of PRRSV-specific IFN-γsecreting cells remained constant after the first 10 months after infection, whereas 80% of the pigs became seronegative by 23 months after infection. To add to the complexity of the regulation of the porcine immune response to PRRSV, an attempt to enhance the immunogenicity of the PRRS MLV by coadministration of a proven stimulator of porcine immunity failed. Therefore, based on the unusual host response to PRRSV infection, the rational development of effective vaccines will require an understanding of the mechanisms that regulate the kinetics, quality, and intensity of porcine immunity against this pathogen. Given the economic losses attributed to the disease caused by this virus, this information is urgently needed.