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. 2009 Oct 27:6:177.
doi: 10.1186/1743-422X-6-177.

Age-dependent resistance to Porcine reproductive and respiratory syndrome virus replication in swine

Kelly L Klinge  1 , Eric M Vaughn, Michael B Roof, Elida M Bautista, Michael P Murtaugh
Affiliations

Age-dependent resistance to Porcine reproductive and respiratory syndrome virus replication in swine

Kelly L Klinge et al. Virol J. .

Abstract

Background: Porcine reproductive and respiratory syndrome virus (PRRSV) causes a prolonged, economically devastating infection in pigs, and immune resistance to infection appears variable. Since the porcine adaptive immune system is not fully competent at birth, we hypothesized that age influences the dynamics of PRRSV infection. Thus, young piglets, growing 16-20-week-old finisher pigs, and mature third parity sows were infected with virulent or attenuated PRRSV, and the dynamics of viral infection, disease, and immune response were monitored over time.

Results: Virulent PRRSV infection and disease were markedly more severe and prolonged in young piglets than in finishers or sows. Attenuated PRRSV in piglets also produced a prolonged viremia that was delayed and reduced in magnitude, and in finishers and sows, about half the animals showed no viremia. Despite marked differences in infection, antibody responses were observed in all animals irrespective of age, with older pigs tending to seroconvert sooner and achieve higher antibody levels than 3-week-old animals. Interferon gamma (IFN gamma) secreting peripheral blood mononuclear cells were more abundant in sows but not specifically increased by PRRSV infection in any age group, and interleukin-10 (IL-10) levels in blood were not correlated with PRRSV infection status.

Conclusion: These findings show that animal age, perhaps due to increased innate immune resistance, strongly influences the outcome of acute PRRSV infection, whereas an antibody response is triggered at a low threshold of infection that is independent of age. Prolonged infection was not due to IL-10-mediated immunosuppression, and PRRSV did not elicit a specific IFN gamma response, especially in non-adult animals. Equivalent antibody responses were elicited in response to virulent and attenuated viruses, indicating that the antigenic mass necessary for an immune response is produced at a low level of infection, and is not predicted by viremic status. Thus, viral replication was occurring in lung or lymphoid tissues even though viremia was not always observed.

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Figures

Figure 1
Figure 1
Effect of pig age and viral strain on PRRSV viremia. Data are group mean titers determined by limiting dilution culture on MA-104 cells.
Figure 2
Figure 2
Effect of age and viral strain on anti-PRRSV antibody response. Data are group mean sample-to-positive (S/P) ratios determined by HerdChek® PRRS 2XR ELISA.
Figure 3
Figure 3
Effect of age and viral strain on antibody responses to specific structural and nonstructural PRRSV proteins. (A) Anti-nucleocapsid antibodies. (B) Anti-nonstructural protein 2 antibodies. Data are ELISA absorbance values (mean ± 1 standard deviation) of 10 animals per group. Treatment group legend is shown in panel A.
Figure 4
Figure 4
Effect of pig age and PRRSV strain on interferon (IFN) γ-secreting cell frequencies in peripheral blood mononuclear cells (PBMC). Panels A-C, control uninfected pigs (4-5 pigs per group); D-F, pigs inoculated with attenuated ATP PRRSV (5-8 pigs per group); G-I, pigs infected with virulent JA142 (10-11 pigs per group). Panels A, D, and G, PBMC cultured without stimulation; B, E, and H, PBMC cultured with JA142 PRRSV; C, F, and I; PBMC cultured in presence of phytohemagluttinin (PHA). In each panel, piglets are open squares, finisher pigs are open circles, and sows are closed circles. Asterisk in panels F and I indicate wells with too many cells to count, i.e. >400 per well.
Figure 5
Figure 5
Effect of pig age and viral strain on IL-10 levels in serum early in infection. Data points are values from individual piglets (A), finishers (B), and sows (C) treated as indicated in the legend (box in panel A).
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