Identification of CD163 regions that are required for PRRSV infection but not for binding to viral envelope glycoproteins

Porcine reproductive and respiratory syndrome virus (PRRSV) is a significant pathogen in the swine industry. It is an RNA virus that primarily affects the respiratory and reproductive systems of pigs, causing respiratory distress, fever, abortion, stillbirths, and other reproductive problems. The importance of PRRSV lies in its impact on the pig industry, as it can cause significant economic losses in affected herds. The virus is highly contagious, and outbreaks can result in reduced pig growth rates, increased mortality rates, and decreased reproductive performance, leading to lower productivity and profitability for pig farmers. Additionally, the virus can lead to secondary infections, such as bacterial pneumonia, which can further complicate disease control efforts. The virus is difficult to control due to its high mutation rate and the lack of a vaccine that provides complete protection. Thus, controlling PRRSV involves implementing a comprehensive herd management program, including biosecurity measures, vaccination, and treatment of affected animals.

CD163 is a cell surface receptor protein that plays a critical role in the pathogenesis of PRRSV. CD163 is expressed on the surface of macrophages, which are one of the primary target cells for PRRSV infection. PRRSV enters the host macrophages by binding to CD163, and the virus uses the receptor to gain entry into the cell and replicate within it. The virus also downregulates CD163 expression, which helps it evade the host’s immune response and establish a persistent infection. The importance of CD163 in PRRSV infection is highlighted by the fact that pigs that lack the receptor are resistant to the virus. Moreover, researchers previously have shown that blocking CD163’s function with specific antibodies can reduce viral replication and limit disease severity in infected pigs. In addition, CD163 is also an important marker for the identification of PRRSV-infected macrophages, and it has been used as a tool for monitoring PRRSV infection and evaluating the efficacy of vaccine candidates. Because CD163 plays a crucial role in PRRSV infection by facilitating viral entry into host macrophages, understanding the specific regions of CD163 protein that are necessary for PRRSV infection to occur is essential for the development of effective control strategies against this economically important viral disease.

In a new study published in the journal Virology, Professor Raymond Rowland and Professor Alberto Brandariz-Nunez and University of Illinois at Urbana-Champaign together with Dr. Ana Stoian from University of California Davis characterized specific regions of the CD163 protein, and described how they play a crucial role in the infection process of PRRSV and by identifying these regions, researchers were able to gain important insight into the mechanism of PRRSV infection and potentially develop new strategies for controlling the virus in pigs. The research team conducted a variety of deletion mutant experiments to identify CD163 regions involved in PRRSV infection. In molecular biology studies, deletion mutant experiments are useful for studying the function of specific genes or gene regions. By removing a particular gene or genomic region, researchers can observe the effects of this deletion on the organism’s phenotype. This can help to identify the role of the deleted gene or region in various biological processes, such as development, metabolism, or disease. The authors assessed SRCR5’s contribution to PRRSV infection and looked for other CD163 domains and regions contributing to viral infection. The authors created a number of CD163 deletion mutants and tested each mutant capacity to support PRRSV-1 and PRRSV-2 infection. They showed that CD163’s capacity to serve as a PRRSV receptor is influenced by amino acid deletions, insertions, and substitutions in the SRCR4/5 interdomain region, known as AHRK. According to the authors, a disruption of the connection between the SRCR4/5 interdomain and the viral proteins may not be the cause of the detrimental effect on infection. They found that PRRSV-2 infection, rather than PRRSV-1 infection, is primarily affected by changes made in the SRCR4/5 interdomain area and that the disruption of the contact between the receptor’s domain and the matching amino acid on the viral protein does not cause the detrimental effect on infection. Moreover, the deletion of one region of CD163 PSTII domain, which contains the amino acids encoded by Exon 13, increases the possibility of producing pigs immune to the PRRSV. Depletion of the SRCR7 domain, which contains a ADHFGE peptide sequence also present in SRCR5 domain, resulted in CD163’s lack of ability to act as a PRRSV viral receptor. CD163 mutant variants that resisted infection retained the ability to interact with GP2, GP3, GP4 and GP5 viral glycoproteins. The contribution of multiple domains to viral infection but not to the binding of viral envelope proteins suggests that the viral glycoproteins  may form multiple interactions with CD163, or that receptor regions important for infection have other cellular binding partners required for PRRSV infection.

In summary, deletion mutant experiments allowed the authors to pinpoint particular CD163 areas that are necessary for PRSV-1 and PRRSV-2 infections in vitro. These findings will help the researchers to better understand viral pathogenesis and CD163-mediated PRRSV infection and contribute significantly to the construction of novel CD163 receptors that are resistant to PRRSV infection while being physically and functionally unaltered, an essential step in creating pigs who have been genetically modified to be totally immune to PRRSV.