Herd Health and Management

Immune Tools Measure Pig Disease, Vaccine Responses

U.S. Department of Agriculture (USDA) scientists at the Beltsville, MD, Agricultural Research Center (BARC), working with University of Illinois (UI) researchers, have tested new tools for measuring pig immune responses to vaccination or infection with porcine reproductive and respiratory syndrome (PRRS) virus and with the foodborne parasite Toxoplasma gondii (Tg).

The pig has immediate, non-specific or innate responses to infection and vaccination. But for long-term recovery and prevention of new infections, specific immunity is required. This involves activation of the immune system, resulting in complex interactions of immune cells and hormone-like proteins called cytokines.

Previous studies at BARC have shown that infection or vaccination with Tg leads to strong T cell-mediated responses, termed T helper 1 (Th1) responses, producing immune-stimulating proteins, or cytokines, including interferon-gamma (IFNG).

For this study, BARC and UI researchers asked if vaccination with PRRS modified live virus (MLV) leads to effective Th1 responses in 6-week-old Yorkshire x Landrace crossbred pigs. Detailed immune responses were monitored in groups of nine pigs.

UI scientists showed that the anti-PRRS virus peripheral blood cell IFNG immune response was slow in developing. An MLV booster vaccination was given eight weeks after initial vaccination and immunity response was maximized 10-13 weeks after vaccination.

In contrast, for Tg, BARC scientists showed much faster IFNG responses over a 2- to 10-day period, with pigs recovering from Tg-induced clinical symptoms within two weeks.

BARC scientists asked whether differences in protective immunity for PRRS infections could be attributed to immune factors. They used differential immune gene expression tools to measure the levels of 24 different immune gene products by real-time molecular assays on blood lymphocytes or cells related to immunity.

Gene expression data showed that pigs vaccinated with PRRS MLV had a very slow and weak immune response, with low levels of IFNG mRNA (ribonucleic acid) developing only at five weeks after vaccination. The related interleukin-12 showed no increases.

When the immune-stimulating innate markers were examined, it was clear that they had actually decreased, rather than increased. Levels of interferon-alpha (IFNA), a major innate immune marker, were down, as were levels of interleukin-8.

Thus, there was a lack of early immune activation with PRRS vaccination.

In many infections, innate cytokine stimulation is essential for speedy and effective anti-viral immune responses. The range of innate and acquired immune markers activated is critical to the level of pig response to infection.

This technology will now serve to help design better immune and vaccine approaches for prevention and control of PRRS, which the National Pork Board says costs the pork industry an estimated $600 million annually.

Additionally, BARC scientists have expanded the number of immune genes they can measure. Now more than 250 genes can be measured to assess the effect of therapeutic treatments and nutrition on infectious and metabolic diseases and vaccinations.

Researchers: Robert J. Husmann, Gabriela Calzada-Nova, William M. Schnitzlein and Federico A. Zuckermann, University of Illinois; Harry Dawson and Joseph F. Urban Jr. of the Nutrient Requirements and Functions Laboratory, Beltsville Human Nutrition Research Center, BARC, and Atabak Royaee and Joan Lunney, Animal Parasitic Diseases Laboratory, Animal and Natural Resources Institute, BARC. Contact Lunney at (301) 504-9368 or jlunney@anri.barc.usda.gov.