Gradual development of the interferon-γ response of swine to porcine reproductive and respiratory syndrome virus infection or vaccination

Infection of swine with virulent porcine reproductive and respiratory syndrome (PRRS) virus induced a rapid, robust antibody response that comprised predominantly nonneutralizing antibodies and waned after approximately 3 months. In contrast, the initial onset of virus-specific interferon (IFN)-γ-secreting cells (SC) in the pig lymphocyte population remained at a fairly low level during this period and then increased gradually in frequency, plateauing at 6 months postinfection. A similar polarization of the host humoral and cellular immune responses was also observed in pigs immunized with a PRRS-modified live virus (MLV) vaccine. Even coadministration of an adjuvant that enhanced the immune response to a pseudorabies (PR) MLV vaccine failed to alter the induction of PRRS virus-specific IFN-γ SC (comprising predominately CD4/CD8α double positive memory T cells with a minority being typical CD4^-/CD8αβ⁺ T cells) and the generation of neutralizing antibodies. Moreover, unlike inactivated PR virus, nonviable PRRS virus did not elicit virus-neutralizing antibody production. Presumably, an intrinsic property of this pathogen delays the development of the host IFN-γ response and preferentially stimulates the synthesis of antibodies incapable of neutralization.