Infection with Porcine reproductive and respiratory syndrome virus stimulates an early gamma interferon response in the serum of pigs

Abstract

The early release of cytokines by cells involved in innate immunity is an important host response to intracellular pathogens. Gamma interferon (IFN-gamma) is an important cytokine produced during the early stages of an infection by macrophages, natural killer (NK) cells, and other cell types, and it is also a central cytokine mediator for the induction of cellular or Th1 immunity. To better understand innate and adaptive immune responses after infection with Porcine reproductive and respiratory syndrome virus (PRRSV), we investigated serum IFN-gamma concentrations and the duration of viremia. For 2 strains of atypical PRRSV, IFN-gamma was detectable in swine serum soon after infection and lasted for approximately 3 wk. Serum concentrations of IFN-gamma peaked at about 10 d after inoculation and returned to approximately baseline levels by day 22. However, individual pigs manifested short, sporadic increases in the serum concentration of IFN-gamma from 18 to 50 d after inoculation. Prior vaccination blocked the serum IFN-gamma response associated with homologous virus challenge and altered the kinetics of the response after heterologous challenge. Two other respiratory viruses of pigs, Porcine respiratory coronavirus and Swine influenza virus, do not appear to induce serum IFN-gamma. The early production of IFN-gamma in PRRSV-infected pigs might result from activation of NK cells, a response that is more characteristic of immune pathways stimulated by intracellular bacterial and protozoan infections.

Figure 1

Serum concentrations of gamma interferon (IFN-γ) in pigs post inoculation (PI) with Porcine reproductive and respiratory syndrome virus (PRRSV), strain SDSU-73 (n = 7) or strain JA-142 (n = 6), or after sham inoculation (n = 6). For all 4 figures, all serum samples were analyzed in duplicate for swine IFN-γ with a commercial enzyme-linked immunosorbent assay kit, and the values are means ± the standard error for each group.

Figure 2

Serum titers of PRRSV, expressed as the logarithm of the median tissue culture infective dose (TCID₅₀), in the same 3 groups of pigs. The asterisks indicate that the group mean titers were significantly different (P ≤ 0.05) only on days 18 and 22 PI.

Figure 3

Serum IFN-γ concentrations before and on days 5 and 10 after homologous and heterologous challenge of vaccinated and nonvaccinated pigs. On day 10 the asterisks indicate significant differences (P ≤ 0.05) between vaccinated and nonvaccinated groups whether challenged with PRRSV strain JA-142 or strain SDSU-73. The differences between the environmental control pigs and the 3 groups of vaccinated pigs were not significant on day 10. OD — optical density.

Figure 4

Serum IFN-γ concentrations after inoculation with Porcine respiratory coronavirus (PRCV) or Swine influenza virus (SIV).