Porcine reproductive and respiratory syndrome (PRRS), a devastating disease in pigs, causes significant losses to the swine industry worldwide each year. The ability of the PRRS virus to persist and evade a host’s immune response has USDA Agricultural Research Service (ARS) scientists digging deeper into the molecular and cellular mechanisms of the disease.

A team of scientists led by microbiologist Laura Miller at the ARS National Animal Disease Center (NADC) in Ames, IA, along with geneticist Zhihua Jiang and his colleagues at Washington State University, looked at the effect of the PRRS virus on how genes are expressed in white blood cells of the lung called alveolar macrophages. These cells, which are located within tiny air sacs in the lungs, play an important role in fighting the disease, and are the primary targets of infection.

Miller and her colleagues in NADC’s Virus and Prion Diseases Research Unit infected macrophages with a strain of PRRS virus common in the United States and examined them at different intervals over a 24-hour period. A powerful technique called serial analysis of gene expression was used to identify “tags,” which are short segments of expressed genes, of pigs that were altered in their expression as a result of PRRS virus infection of the cells. From the tags, Jiang and Miller were able to characterize in more detail the changes in genes being expressed.

A database of cellular reactions, genetic pathways and biological processes, called a reactome, was used to generate a huge amount of data. Jiang and Miller developed a bioinformatics method to associate the early changes in gene expression with the infection process in virus-infected cells. Millions of tags were narrowed down to 699 differentially expressed genes that are involved in six biological systems, 60 functional categories and 504 genetic pathways.

Scientists were able to profile functional genes, which revealed more information about how PRRS virus replication affects host resistance. The virus took control of the cells in the course of 24 hours of infection. However, the host’s immune system started fighting back in an attempt to destroy or stop the virus.

According to Jiang and Miller, these findings, which were published in March 2013, provide the most comprehensive picture of the gene expression response to PRRS virus infection to date.

ARS is the chief intramural scientific research agency in the U.S. Department of Agriculture (USDA), and this research supports the USDA priority of promoting international food security