Differential responses in placenta and fetal thymus at 12 days post infection elucidate mechanisms of viral level and fetal compromise following PRRSV2 infection

Authors: Van Goor, Angelica; PASTERNAK, ALEX - Purdue University; Walker, Kristen; HONG, LINJUN - South China Agricultural Univerisity; MALGARIN, CAROLINA - Western College Of Veterinary Medicine; MACPHEE, DANIEL - Western College Of Veterinary Medicine; HARDING, JOHN - Western College Of Veterinary Medicine; Lunney, Joan

Submitted to: BMC Genomics
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/15/2020
Publication Date: 11/4/2020
Citation: Van Goor, A.G., Pasternak, A., Walker, K.E., Hong, L., Malgarin, C., Macphee, D.J., Harding, J.C., Lunney, J.K. 2020. Differential responses in placenta and fetal thymus at 12 days post infection elucidate mechanisms of viral level and fetal compromise following PRRSV2 infection. BMC Genomics. 21:763. https://doi.org/10.1186/s12864-020-07154-0.
DOI: https://doi.org/10.1186/s12864-020-07154-0

Interpretive Summary: Porcine reproductive and respiratory syndrome virus (PRRSV) infection causes major losses to pork producers. Fetal responses to congenital PRRSV infection are variable within litters, from uninfected fetuses to dead neighbors or those with high viral levels. Our team has explored the events leading to these disparities. We collected fetuses at 12 days post infection from pregnant gilts challenged with PRRSV in the third trimester. We compared samples from fetuses with no, low or high viral load and those close to death (meconium-stained). By assessing targeted immune-related gene expression patterns and pathways in the placenta and fetal thymus we were able to elucidate molecular mechanisms involved in the resistance/tolerance and susceptibility of fetuses to PRRSV infection. We found that the anti-viral immune response was initiated only after PRRSV reached detectable levels in the fetus when a core set of interferon inducible genes were strongly upregulated in both tissues. Gene expression in the thymus revealed strong downregulation of critical immune pathways associated with high viral load. Gene expression in the placenta differentiated fetal demise. Overall, our data support the concept that fetal outcome in response to PRRSV2 infection is determined by both fetal and placental responses and is initiated only after fetal infection. These results help to reveal the complex mechanisms underpinning fetal PRRS susceptibility.

Technical Abstract: Background A pregnant gilt infected with PRRS virus (PRRSV) can transmit the virus to her fetuses across the maternal-fetal-interface (MFI) resulting in varying disease outcomes. However, the mechanisms leading to variation in fetal outcome in response to PRRSV infection are not fully understood. Our objective was to assess targeted immune-related gene expression patterns and pathways in the placenta (PLC) and fetal thymus (THY) to elucidate the molecular mechanisms involved in the resistance/tolerance and susceptibility of fetuses to PRRSV infection. Fetuses were grouped by preservation status and PRRS viral load (VL) in the fetal PLC, serum and THY: mock infected control (CTRL), no virus detected (UNINF), virus detected in the PLC only with viable (PLCO-VIA) or meconium-stained fetus (PLCO-MEC), low VL with viable (LVL-VIA) or meconium-stained fetus (LVL-MEC), and high VL with viable (HVL-VIA) or meconium-stained fetus (HVL-MEC). Results The host immune response was initiated only in fetuses with detectable levels of PRRSV. No differentially expressed genes (DEG) in either the PLC or THY were identified in UNINF, PLCO-VIA, and PLCO-MEC when compared to CTRL fetuses. Upon fetal infection, a set of core responsive IFN-inducible genes (CXCL10, IFIH1, IFIT1, IFIT3, ISG15, and MX1) were strongly upregulated in both tissues. Gene expression in the THY is a better differentiator of fetal VL; the strong downregulation of several innate and adaptive immune pathways (e.g., B Cell Development) are indicative of HVL. Gene expression in the PLC may be a better differentiator of fetal demise than the THY, based-on principle component analysis clustering and gene expression patterns. Potential PLC biomarkers of susceptibility (e.g., dysregulation of the Apoptosis Signaling and Protein Ubiquitination pathways) may be contributing to fetal demise. Conclusion Our data supports the concept that fetal outcome in response to PRRSV2 infection is determined by fetal and more significantly placental response, which is initiated only after fetal infection. This conceptual model represents a significant step forward in understanding the mechanisms underpinning fetal susceptibility to the virus. The biomarkers identified here may be used as a first step to breeding pigs for improved animal health and/or to development of anti-viral therapeutics.