Maternal and fetal thyroid dysfunction following porcine reproductive and respiratory syndrome virus2 infection

Abstract

To better understand the host response to porcine reproductive and respiratory virus-2 (PRRSV2) we evaluated circulating thyroid hormone and associated gene expression in a late gestation challenge model. Pregnant gilts were inoculated at gestation day 85 and fetal samples collected at either 12 or 21 days post-infection (dpi). A subset of fetuses was selected for analysis based on viability and viral load categorized as either uninfected-viable (UNIF), high viral load viable (HV-VIA) or high viral load meconium stained (HV-MEC) and were compared with gestational age matched controls (CON). In dams, circulating levels of total T3 and T4 decreased in the acute period following infection and rebounded by 21 dpi. A similar effect was observed in fetuses, but was largely restricted to HV-VIA and HV-MEC, with minimal decrease noted in UNIF relative to CON at 21 dpi. Gene expression in fetal heart at 12 dpi showed significant decompensatory transcription of thyroid hormone transporters (SLC16A2) and deiodinases (DIO2, DIO3), which was not observed in brain. Correspondingly, genes associated with cell cycle progression (CDK1,2,4) were downregulated in only the heart of highly infected fetuses, while expression of their inhibitor (CDKN1A) was upregulated in both tissues. Finally, expression of genes associated with cardiac stress including CAMKD and AGT were upregulated in the hearts of highly infected fetuses, and a shift in expression of MYH6 to MYH7 was observed in HV-MEC fetuses specifically. Collectively, the results suggest PRRSV2 infection causes a hypothyroid state that disproportionally impacts the fetal heart over the brain.

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

“…The question of PRRSV infection in the fetus as the cause of demise has been previously questioned based on the understanding that fetal lesions are infrequently observed [ 19 ]. However, it is now understood that the fetus responds physiologically and immunologically to PRRSV infection in the absence of pathology [ 11 , 13 , 14 , 45 ]. Previous reports show a positive association between fetal survival during PRRSV infection and reduced intrauterine growth [ 16 ], of which increased apoptosis and increased cellular senescence could contribute to reduced fetal growth.…”

“…The cause of fetal demise is not well understood with debate on whether activities at the MFI (maternal endometrium and/or fetal placenta), activities in the fetus, or both are responsible, and to what degree [ 7 ]. Studies using gene expression have found common themes during fetal infection with PRRSV including changes in innate and adaptive immunity, hypoxia, interferon signaling, apoptosis, and thyroid hormone dysregulation [ 11 – 14 ]. Our current study is part of the largest set of studies to date using the pregnant gilt challenge model to understand phenotypic and genotypic responses to PRRSV infection in fetuses and gilts [ 15 ].…”

Porcine reproductive and respiratory virus 2 infection of the fetus results in multi-organ cell cycle suppression

“…In short, total RNA was extracted and viral RNA quantified using primer and probe sequences targeting the distal region of ORF7 and a one-step qPCR assay including a standard curve comprised of linearized plasmid. Fetal serum thyroid hormone levels (T3 and T4) were each evaluated using commercially available RIA as previously described [ 15 ]. For initial evaluation, the resulting data on viral load and thyroid hormone level was used in combination with meconium staining to identify subsets of fetuses from the larger population produced by the challenge model that represent four biologically distinct, and physiologically extreme groups.…”

“…This growth is driven by progressive increases in growth promoting signals such as insulin-like growth factor 1 [ 13 ], and thyroid hormones [ 14 ] including thyroxin (T4) and its more bioactive derivative triiodothyronine (T3). Although the PRRSV infected fetus is capable of mounting an immune response [ 6 , 8 ], it creates not only supplemental energetic cost but results in a cytokine response that may interfere with normal developmental processes, including a significant disruption in fetal thyroid hormone levels [ 15 ]. Moreover, type I and II interferons, which are elevated following fetal PRRSV expression [ 6 ], have a detrimental impact on placental and fetal development and are implicated in intrauterine growth restriction (IUGR) in other models of congenital infection [ 16 , 17 ].…”

“…This system is also known to be regulated in a P53 independent manner by the transforming growth factor beta (TGFβ) superfamily, via a signaling cascade involving the corresponding transmembrane receptor and SMAD proteins [ 20 ]. We have previously observed alteration in the expression of these genes in a subset of fetal organs including the thymus at 12 days post maternal infection (dpi) and the heart and brain at 21 dpi [ 6 , 15 ]. As each fetal organ system follows a unique developmental trajectory, a more complete understanding of disruption in this system is required.…”