Key Messages

• Prior PRRSV infection exacerbates PRCV-induced clinical illness in co-infected pigs.
• Suppressed innate NK-cell cytotoxicity occurred in the dual-infected pigs
• Co-infection of pigs with PRRSV and PRCV resulted in elevated Th-1 and proinflammatory cytokines in pigs.

Objective

To determine how PRRS virus infection modulate immune responses specifically in relation to PRCV.

Article Brief

This study was designed to understand how infections with PRRS and PRCV modulate immune responses, and to see if they correlate with a clinical picture in pigs.178 seronegative to PRRSV, PRCV, TGEV, and PCV2 16- to 20-day-old piglets were used. After 1 week to acclimate, they were assigned to one of four groups: Control, PRRSV-infected, PRCV-infected or PRCV/PRRSV-infected. PRCV infection came 10 days after PRRS infection.

Clinical signs (body temperature, sneezing, coughing, polypnea, and anorexia) and average daily gain (ADG) were recorded during 30 days. Samples (nasal swabs and blood) were taken throughout the experimental period. Representative pro-inflammatory (IL-6), Th-1 (IL-12), and anti-inflammatory (IL-10 and TGF-b) cytokine levels in serum and lung were measured.

Clinical signs were more severe in coinfected pigs. A higher incidence of fever (p<0.05) was found in more than 70 per cent of dual virus–infected pigs, 52 per cent in PRRS-only and no fever detected in PRCV alone group. The PRRS+PRCV had a lower ADG than PRRS-only and the other groups. Growth in PRRS-only pigs was reduced compared with PRCV and control.

In comparison with the control (Mock) group, PRRSV reduced NK cell cytotoxicity by 50 to 80 per cent whereas in co-infected pigs it was reduced by 80 to 100 per cent, confirming a synergistic immunosuppressive effect. Also, co-infection caused a higher pro-inflammatory cytokines (IL-12 and IL-6).

However, dual-infection piglets had higher frequency of T-regulatory cytokines in lungs than in serum. It could indicate that the immunomodulation occurs at the infection site.

The higher T-cell and myeloid cells concentrations in the groups infected with both virus enhance the increase of Th-1 and pro-inflammatory cytokines, and could explain a higher pneumonia.

The infection with PRRSV affects innate and adaptive immune responses, which contributes to more severe consequences of a trivial (or milder) infection by PRCV.

Reference

Renukaradhya G.J., Alekseev K., Jung K., Fang Y., Saif L.J. 2010. Porcine reproductive and respiratory syndrome virus-induced immunosuppression exacerbates the inflammatory response to porcine respiratory coronavirus in pigs. Viral Immunol. 2010. 23(5):457–466.

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August 2015