Altered Hippocampal Gene Expression and Morphology in Fetal Piglets following Maternal Respiratory Viral Infection

Abstract

Maternal infection during pregnancy increases the risk of neurobehavioral problems in offspring. Evidence from rodent models indicates that the maternal immune response to infection can alter fetal brain development, particularly in the hippocampus. However, information on the effects of maternal viral infection on fetal brain development in gyrencephalic species is limited. Thus, the objective of this study was to assess several effects of maternal viral infection in the last one-third of gestation on hippocampal gene expression and development in fetal piglets. Pregnant gilts were inoculated with porcine reproductive and respiratory syndrome virus (PRRSV) at gestational day (GD) 76 and the fetuses were removed by cesarean section at GD 111 (3 days before anticipated parturition). The gilts infected with PRRSV had elevated plasma interleukin-6 levels and developed transient febrile and anorectic responses lasting approximately 21 days. Despite having a similar overall body weight, fetuses from the PRRSV-infected gilts had a decreased brain weight and altered hippocampal gene expression compared to fetuses from control gilts. Notably, maternal infection caused a reduction in estimated neuronal numbers in the fetal dentate gyrus and subiculum. The number of proliferative Ki-67+ cells was not altered, but the relative integrated density of GFAP+ staining was increased, in addition to an increase in GFAP gene expression, indicating astrocyte-specific gliosis. Maternal viral infection caused an increase in fetal hippocampal gene expression of the inflammatory cytokines TNF-α and IFN-γ and the myelination marker myelin basic protein. MHCII protein, a classic monocyte activation marker, was reduced in microglia, while expression of the MHCII gene was decreased in hippocampal tissue of the fetuses from PRRSV-infected gilts. Together, these data suggest that maternal viral infection at the beginning of the last trimester results in a reduction in fetal hippocampal neurons that is evident 5 weeks after infection, when fetal piglets are near full term. The neuronal reduction was not accompanied by pronounced neuroinflammation at GD 111, indicating that any activation of classic neuroinflammatory pathways by maternal viral infection, if present, is mostly resolved by parturition.

Maternal viral infection causes global alterations in porcine fetal microglia

“…In the current study, we demonstrate that fetal microglia from a gyrencephalic species are globally altered by maternal viral infection. As we have reported previously, MIA in swine reduces overall fetal brain weight but not body weight (41), emphasizing the concentrated impact of MIA on brain development emerging within 3 wk post-maternal viral inoculation. Inflammation at the maternal-fetal interface, a likely pathway through which maternal cytokines signal to the fetal brain (14,15,52), was present even after maternal symptoms resolved.…”

“…Data from our MIA pig model confirm the manifestation of altered behaviors in neonatal piglets (40). Fetal piglets exposed to MIA also have reduced neuronal density in the hippocampus and evidence of astrogliosis before birth (41). In the current study, we show that fetal porcine microglia are indeed globally altered during MIA.…”

“…Perhaps as a compensatory response, expression of CXCR2, the beta receptor for IL-8, was acutely up-regulated in MIA microglia at 7 dpi, especially in females; while MIA no longer had an effect at 21 dpi, this sexually dimorphic expression tended to persist. Furthermore, in vitro evidence suggests that IL-6 production by microglia may suppress neurogenesis in favor of astrogenesis (79); thus, the increased expression of microglial IL6 at 7 dpi may be contributing to the increased GFAP staining and gene expression previously observed in our MIA model (41).…”

“…Microglia also promote astrocytic differentiation, in part through production of IL-6 and leukemia inhibitory factor (LIF) (79), and nitric oxide (NO) by way of NOS2 (80). We have previously shown that MIA in swine results in a reduction in estimated neuron density and increased astrocyte density in the hippocampus evident several days before birth, with no differences in cell proliferation (41). Thus, the persistent reduction in in vitro chemotactic capacity of MIA fetal microglia when stimulated with IL-8 and CCL2 is noteworthy.…”

“…Relatively Unchanged by MIA. Previous data from our laboratory indicated that MIA reduces total neuron number in the fetal hippocampus, specifically in the dentate gyrus (41). We hypothesized that altered microglial density and activation in this region could contribute to this loss.…”