Getting a Handle on Why Porcine Reproduction and Respiratory Syndrome Virus is So Evasive in Pigs

A study conducted at Virginia Tech may have uncovered some of the mechanisms involved in persistence and immune suppression by the PRRS (porcine reproductive and respiratory syndrome) virus.

PRRS virus persistence remains a major problem in the pork industry because the mechanism by which the virus evades the immune system, leading to persistent infection, has never been discovered.

The hypothesis for this study suggests that PRRS virus persistence and immune suppression is due to the ability of the virus to induce regulatory T cells. The activation of regulatory T cells results in both dampening of the immune response to both the antigen (protein or carbohydrate substance capable of inducing an immune response) that activated them and to other antigens as well.

This theory was tested by two objectives. The first objective was to determine if the number of regulatory T cells increased during a PRRS infection. The second objective aimed to determine if persistently infected pigs had higher numbers of regulatory T cells than pigs that were able to clear the infection.

To complete the first objective, pigs were inoculated with PRRS virus and the virus was assessed for its ability to induce regulatory T cells at seven and 14 days post-infection. By Day 14, an increase in a subset of these T cells was detected in both the blood and lungs.

For the second objective, a second group of pigs was inoculated with the PRRS virus and followed out to 42 days post-infection to determine if the number of regulatory T cells increased. No detectable increase in these T cells was found in the blood or lungs of pigs after 14 days post-infection.

These results suggest that regulatory T cells are induced or activated by PRRS virus early in infection, and though their development may not increase much in persistently infected pigs, they likely play a role in the development of PRRS virus by allowing the virus to evade protective immunity.

Also, non-specific dampening the immune response to other pathogens would result in the development of secondary pathogens.
Further studies will be needed to understand the role of regulatory T cells in PRRS development to enable the best vaccine adjuvants to be selected to not only protect against PRRS virus infection and persistence, but also minimize the immune-suppressive effects against other vaccines and pathogens. Knowing that PRRS virus induces regulatory T cells is crucial in the design of new vaccines.

Funding for this research project was provided by the National Pork Board.