Evaluation of immune responses to porcine reproductive and respiratory syndrome virus in pigs during early stage of infection under farm conditions

Background: Porcine reproductive and respiratory syndrome virus (PRRSV) causes chronic, economically devastating disease in pigs of all ages. Frequent mutations in the viral genome result in viruses with immune escape mutants. Irrespective of regular vaccination, control of PRRSV remains a challenge to swine farmers. In PRRSV-infected pigs, innate cytokine IFN- is inhibited and the adaptive arm of the immunity is delayed. To elucidate both cellular and innate cytokine responses at very early stages of PRRSV infection, seven weeks old pigs maintained on a commercial pig farm were infected and analyzed. Results: One pig in a pen containing 25 pigs was PRRSV infected and responses from this pig and one penmate were assessed two days later. All the infected and a few of the contact neighbor pigs were viremic. At day 2 post-infection, approximately 50% of viremic pigs had greater than 50% reduction in NK cell-mediated cytotoxicity, and nearly a 1-fold increase in IFN- production was detected in blood of a few pigs. Enhanced secretion of IL-4 (in ∼90%), IL-12 (in ∼40%), and IL-10 (in ∼20%) (but not IFN-) in PRRSV infected pigs was observed. In addition, reduced frequency of myeloid cells, CD4 ^-CD8 ⁺ T cells, and CD4 ⁺CD8 ⁺ T cells and upregulated frequency of lymphocytes bearing natural T regulatory cell phenotype were detected in viremic pigs. Interestingly, all viremic contact pigs also had comparable immune cell modulations. Conclusion: Replicating PRRSV in both infected and contact pigs was found to be responsible for rapid modulation in NK cell-meditated cytotoxicity and alteration in the production of important immune cytokines. PRRSV-induced immunological changes observed simultaneously at both cellular and cytokine levels early post-infection appear to be responsible for the delay in generation of adaptive immunity. As the study was performed in pigs maintained under commercial environmental conditions, this study has practical implications in design of protective vaccines.