Neuro-invasion by a 'Trojan Horse' strategy and vasculopathy during intrauterine flavivirus infection

Role of microglia in brain development after viral infection

Microglia are immune cells in the brain that originate from the yolk sac and enter the developing brain before birth. They play critical roles in brain development by supporting neural precursor proliferation, synaptic pruning, and circuit formation. However, microglia are also vulnerable to environmental factors, such as infection and stress that may alter their phenotype and function. Viral infection activates microglia to produce inflammatory cytokines and anti-viral responses that protect the brain from damage. However, excessive or prolonged microglial activation impairs brain development and leads to long-term consequences such as autism spectrum disorder and schizophrenia spectrum disorder. Moreover, certain viruses may attack microglia and deploy them as "Trojan horses" to infiltrate the brain. In this brief review, we describe the function of microglia during brain development and examine their roles after infection through microglia-neural crosstalk. We also identify limitations for current studies and highlight future investigated questions.

The use of Trojan-horse drug delivery system in managing periodontitis

The aim of this review is to evaluate the possibility of delivering a silver-acid complex via a Trojan-horse mechanism for managing periodontits. We theroised that the complex could be an effective treatment option for bacterial inflammatory processes in the oral cavity. Searches were conducted using MEDLINE, Embase, Web of Science Core Collection, and Google Scholar search engines. We also reviewed several reference lists of the included studies or relevant reviews identified by the search. By using Medical Subject Headings (MeSH) terminology, a comprehensive search was performed for the following keywords: silver, folic acid, periodontitis, macrophages, Trojan-horse mechanism, toxicity, and targeting. Using the keywords mentioned earlier, we selected 110 articles and after appropriate elimination the review was written based on 37 papers. Accordingly the we noted that silver isons were an effective approach to kill oral pathogens. Secondly the Trojan-horse mechanism. could be used by macrophages (as the Trojan horse) to deliver silver ions in large quantities to the inflammatory focus to kill the periodontopathogens. The Trojan-horse mechanism has never been described in the field of dentistry before. The proposed novel approach using the principle of Trojan Horse delivery of drugs/chemicals could be used to manage oral inflammatory conditions. This method can be used to supplement regular treatments.

Epigenomic and Proteomic Changes in Fetal Spleens Persistently Infected with Bovine Viral Diarrhea Virus: Repercussions for the Developing Immune System, Bone, Brain, and Heart

Bovine viral diarrhea virus (BVDV) infection during early gestation results in persistently infected (PI) immunotolerant calves that are the primary reservoirs of the virus. Pathologies observed in PI cattle include congenital defects of the brain, heart, and bone as well as marked functional defects in their immune system. It was hypothesized that fetal BVDV infection alters T cell activation and signaling genes by epigenetic mechanisms. To test this, PI and control fetal splenic tissues were collected on day 245 of gestation, 170 days post maternal infection. DNA was isolated for reduced representation bisulfite sequencing, protein was isolated for proteomics, both were analyzed with appropriate bioinformatic methods. Within set parameters, 1951 hypermethylated and 691 hypomethylated DNA regions were identified in PI compared to control fetuses. Pathways associated with immune system, neural, cardiac, and bone development were associated with heavily methylated DNA. The proteomic analysis revealed 12 differentially expressed proteins in PI vs. control animals. Upregulated proteins were associated with protein processing, whereas downregulated proteins were associated with lymphocyte migration and development in PI compared to control fetal spleens. The epigenetic changes in DNA may explain the immune dysfunctions, abnormal bone formation, and brain and heart defects observed in PI animals.

Maternal Influenza A Virus Infection Restricts Fetal and Placental Growth and Adversely Affects the Fetal Thymic Transcriptome

Maternal influenza A viral infections in humans are associated with low birth weight, increased risk of pre-term birth, stillbirth and congenital defects. To examine the effect of maternal influenza virus infection on placental and fetal growth, pregnant C57BL/6 mice were inoculated intranasally with influenza A virus A/CA/07/2009 pandemic H1N1 or phosphate-buffered saline (PBS) at E3.5, E7.5 or E12.5, and the placentae and fetuses collected and weighed at E18.5. Fetal thymuses were pooled from each litter. Placentae were examined histologically, stained by immunohistochemistry (IHC) for CD34 (hematopoietic progenitor cell antigen) and vascular channels quantified. RNA from E7.5 and E12.5 placentae and E7.5 fetal thymuses was subjected to RNA sequencing and pathway analysis. Placental weights were decreased in litters inoculated with influenza at E3.5 and E7.5. Placentae from E7.5 and E12.5 inoculated litters exhibited decreased labyrinth development and the transmembrane protein 150A gene was upregulated in E7.5 placentae. Fetal weights were decreased in litters inoculated at E7.5 and E12.5 compared to controls. RNA sequencing of E7.5 thymuses indicated that 957 genes were downregulated ≥2-fold including Mal, which is associated with Toll-like receptor signaling and T cell differentiation. There were 28 upregulated genes. It is concluded that maternal influenza A virus infection impairs fetal thymic gene expression as well as restricting placental and fetal growth.

West Nile Virus: An Update on Pathobiology, Epidemiology, Diagnostics, Control and "One Health" Implications

West Nile virus (WNV) is an important zoonotic flavivirus responsible for mild fever to severe, lethal neuroinvasive disease in humans, horses, birds, and other wildlife species. Since its discovery, WNV has caused multiple human and animal disease outbreaks in all continents, except Antarctica. Infections are associated with economic losses, mainly due to the cost of treatment of infected patients, control programmes, and loss of animals and animal products. The pathogenesis of WNV has been extensively investigated in natural hosts as well as in several animal models, including rodents, lagomorphs, birds, and reptiles. However, most of the proposed pathogenesis hypotheses remain contentious, and much remains to be elucidated. At the same time, the unavailability of specific antiviral treatment or effective and safe vaccines contribute to the perpetuation of the disease and regular occurrence of outbreaks in both endemic and non-endemic areas. Moreover, globalisation and climate change are also important drivers of the emergence and re-emergence of the virus and disease. Here, we give an update of the pathobiology, epidemiology, diagnostics, control, and “One Health” implications of WNV infection and disease.

Role of microglia in the dissemination of Zika virus from mother to fetal brain

Global Zika virus (ZIKV) outbreaks and their link to microcephaly have raised major public health concerns. However, the mechanism of maternal-fetal transmission remains largely unknown. In this study, we determined the role of yolk sac (YS) microglial progenitors in a mouse model of ZIKV vertical transmission. We found that embryonic (E) days 6.5-E8.5 were a critical window for ZIKV infection that resulted in fetal demise and microcephaly, and YS microglial progenitors were susceptible to ZIKV infection. Ablation of YS microglial progenitors significantly reduced the viral load in both the YS and the embryonic brain. Taken together, these results support the hypothesis that YS microglial progenitors serve as “Trojan horses,” contributing to ZIKV fetal brain dissemination and congenital brain defects.

ZIKV is more likely to cause fetal demise and brain malformations when the mother is infected at an early stage of pregnancy, which is the critical time window when a special type of immune cells called microglia appear in the YS and migrate to the fetal brain. YS-derived microglia are susceptible to ZIKV infection and can act as “Trojan horses” to bring ZIKV from the mother to the fetal brain.

Attenuated lymphocyte activation leads to the development of immunotolerance in bovine fetuses persistently infected with bovine viral diarrhea virus†

Bovine viral diarrhea virus continues to cost the cattle industry millions of dollars each year despite control measures. The primary reservoirs for bovine viral diarrhea virus are persistently infected animals, which are infected in utero and shed the virus throughout their lifetime. The difficulty in controlling the virus stems from a limited understanding of transplacental transmission and fetal development of immunotolerance. In this study, pregnant bovine viral diarrhea virus naïve heifers were inoculated with bovine viral diarrhea virus on day 75 of gestation and fetal spleens were collected on gestational days 82, 97, 190, and 245. Microarray analysis on splenic RNA from days 82 and 97 revealed an increase in signaling for the innate immune system and antigen presentation to T cells in day 97 persistently infected fetuses compared to controls. Reverse transcription quantitative polymerase chain reaction on select targets validated the microarray revealing a downregulation of type I interferons and lymphocyte markers in day 190 persistently infected fetuses compared to controls. Protein was visualized using western blot and tissue sections were analyzed with hematoxylin and eosin staining and immunohistochemistry. Data collected indicate that fetal immunotolerance to bovine viral diarrhea virus developed between days 97 and 190, with mass attenuation of the immune system on day 190 of gestation. Furthermore, lymphocyte transcripts were initially unchanged then downregulated, suggesting that immunotolerance to the virus stems from a blockage in lymphocyte activation and hence an inability to clear the virus. The identification of lymphocyte derived immunotolerance will aid in the development of preventative and viral control measures to implement before or during pregnancy.

NS4/5 mutations enhance flavivirus Bamaga virus infectivity and pathogenicity in vitro and in vivo

Flaviviruses such as yellow fever, dengue or Zika viruses are responsible for significant human and veterinary diseases worldwide. These viruses contain an RNA genome, prone to mutations, which enhances their potential to emerge as pathogens. Bamaga virus (BgV) is a mosquito-borne flavivirus in the yellow fever virus group that we have previously shown to be host-restricted in vertebrates and horizontally transmissible by Culex mosquitoes. Here, we aimed to characterise BgV host-restriction and to investigate the mechanisms involved. We showed that BgV could not replicate in a wide range of vertebrate cell lines and animal species. We determined that the mechanisms involved in BgV host-restriction were independent of the type-1 interferon response and RNAse L activity. Using a BgV infectious clone and two chimeric viruses generated as hybrids between BgV and West Nile virus, we demonstrated that BgV host-restriction occurred post-cell entry. Notably, BgV host-restriction was shown to be temperature-dependent, as BgV replicated in all vertebrate cell lines at 34°C but only in a subset at 37°C. Serial passaging of BgV in Vero cells resulted in adaptive mutants capable of efficient replication at 37°C. The identified mutations resulted in amino acid substitutions in NS4A-S124F, NS4B-N244K and NS5-G2C, all occurring close to a viral protease cleavage site (NS4A/2K and NS4B/NS5). These mutations were reverse engineered into infectious clones of BgV, which revealed that NS4B-N244K and NS5-G2C were sufficient to restore BgV replication in vertebrate cells at 37°C, while NS4A-S124F further increased replication efficiency. When these mutant viruses were injected into immunocompetent mice, alongside BgV and West Nile virus chimeras, infection and neurovirulence were enhanced as determined by clinical scores, seroconversion, micro-neutralisation, viremia, histopathology and immunohistochemistry, confirming the involvement of these residues in the attenuation of BgV. Our studies identify a new mechanism of host-restriction and attenuation of a mosquito-borne flavivirus.

An Acute Stress Model in New Zealand White Rabbits Exhibits Altered Immune Response to Infection with West Nile Virus

The immune competence of an individual is a major determinant of morbidity in West Nile virus (WNV)-infection. Previously, we showed that immunocompetent New Zealand White rabbits (NZWRs; Oryctolagus cuniculus) are phenotypically resistant to WNV-induced disease, thus presenting a suitable model for study of virus-control mechanisms. The current study used corticosteroid-treated NZWRs to model acute “stress”-related immunosuppression. Maximal effects on immune parameters were observed on day 3 post dexamethasone-treatment (pdt). However, contrary to our hypothesis, intradermal WNV challenge at this time pdt produced significantly lower viremia 1 day post-infection (dpi) compared to untreated controls, suggestive of changes to antiviral control mechanisms. To examine this further, RNAseq was performed on RNA extracted from draining lymph node—the first site of virus replication and immune detection. Unaffected by dexamethasone-treatment, an early antiviral response, primarily via interferon (IFN)-I, and induction of a range of known and novel IFN-stimulated genes, was observed. However, treatment was associated with expression of a different repertoire of IFN-α-21-like and IFN-ω-1-like subtypes on 1 dpi, which may have driven the different chemokine response on 3 dpi. Ongoing expression of Toll-like receptor-3 and transmembrane protein-173/STING likely contributed to signaling of the treatment-independent IFN-I response. Two novel genes (putative HERC6 and IFIT1B genes), and the SLC16A5 gene were also highlighted as important component of the transcriptomic response. Therefore, the current study shows that rabbits are capable of restricting WNV replication and dissemination by known and novel robust antiviral mechanisms despite environmental challenges such as stress.

Fetal Hepatic Response to Bovine Viral Diarrhea Virus Infection in Utero

Non-cytopathic bovine viral diarrhea virus (ncp BVDV) can cause persistent infection (PI) in animals infected in utero during early gestation. PI animals shed the virus for life and are the major source of the virus in herds. The mechanism responsible for BVDV immune tolerance in the PI fetus is unknown. We assessed the impact of BVDV infection on the fetal liver. Dams were inoculated with ncp BVDV at gestational day 75. Fetal liver samples were collected at necropsy, 7 and 14 days post-maternal-BVDV inoculation. BVDV antigen was not detected in the liver at gestational day 82 (7 days post-maternal inoculation). However, at 14 days post-maternal inoculation, BVDV was detected by immunohistochemistry in fetal Kupffer cells. Flow cytometry analysis showed a higher percentage of hepatic immune cells expressed MHC I and MHC II in BVDV-infected fetal liver (as compared to uninfected controls). Immunofluorescence was used to identify Kupffer cells, which were positive for BVDV antigen, near populations of CD3+ lymphocytes. The identification of BVDV in the fetal liver Kupffer cells at 14 days post inoculation is interesting in the context of establishment of tolerance in persistent infection. These data indicate the presence of a hepatic immune response to fetal infection.

Neutrophils in viral infection

Neutrophils are the first wave of recruited immune cells to sites of injury or infection and are crucial players in controlling bacterial and fungal infections. Although the role of neutrophils during bacterial or fungal infections is well understood, their impact on antiviral immunity is much less studied. Furthermore, neutrophil function in tumor pathogenesis and cancer treatment has recently received much attention, particularly within the context of oncolytic virus infection where neutrophils produce antitumor cytokines and enhance oncolysis. In this review, multiple functions of neutrophils in viral infections and immunity are discussed. Understanding the role of neutrophils during viral infection may provide insight into the pathogenesis of virus infections and the outcome of virus-based therapies.

Zika Virus Disease

The history of Zika virus disease serves as a paradigm of a typical emerging viral infection. Zika virus disease, a mosquito-borne flavivirus, was first isolated in 1947 in the Zika forest of Uganda. The same virus was also isolated from jungle-dwelling mosquitoes (Aedes [Stegomyia] africanus). In many areas of Africa and South Asia human infections with Zika virus were detected by both serology and virus isolation. About 80% of infections are asymptomatic, and in 20% a mostly mild disease with fever, rash, arthralgia, and conjunctivitis may occur. Fetal infections with malformations were not recorded in Africa or Asia. Zika virus was imported to northern Brazil possibly during the world soccer championship that was hosted by Brazil in June through July 2014. A cluster of severe fetal malformations with microcephaly and ocular defects was noted in 2015 in the northeast of Brazil, and intrauterine infections with Zika virus were confirmed. The dramatic change in Zika virus pathogenicity upon its introduction to Brazil has remained an enigma.

Characterization of non-lethal West Nile Virus (WNV) infection in horses: Subclinical pathology and innate immune response

Most natural West Nile virus (WNV) infections in humans and horses are subclinical or sub-lethal and non-encephalitic. Yet, the main focus of WNV research remains on the pathogenesis of encephalitic disease, mainly conducted in mouse models. We characterized host responses during subclinical WNV infection in horses and compared outcomes with those obtained in a novel rabbit model of subclinical WNV infection (Suen et al. 2015. Pathogens, 4: 529). Experimental infection of 10 horses with the newly emerging WNV-strain, WNV_NSW2011, did not result in neurological disease in any animal but transcriptional upregulation of both type I and II interferon (IFN) was seen in peripheral blood leukocytes prior to or at the time of viremia. Likewise, transcript upregulation for IFNs, TNFα, IL1β, CXCL10, TLRs, and MyD88 was detected in lymphoid tissues, while IFNα, CXCL10, TLR3, ISG15 and IRF7 mRNA was upregulated in brains with histopathological evidence of mild encephalitis, but absence of detectable viral RNA or antigen. These responses were reproduced in the New Zealand White rabbits (Oryctolagus cuniculus) experimentally infected with WNV_NSW2011, by intradermal footpad inoculation. Kinetics of the anti-WNV antibody response was similar in horses and rabbits, which for both species may be explained by the early IFN and cytokine responses evident in circulating leukocytes and lymphoid organs. Given the similarities to the majority of equine infection outcomes, immunocompetent rabbits appear to represent a valuable small-animal model for investigating aspects of non-lethal WNV infections, notably mechanisms involved in abrogating morbidity.

Innate and adaptive immune responses to in utero infection with bovine viral diarrhea virus

Infection of pregnant cows with noncytopathic (ncp) bovine viral diarrhea virus (BVDV) induces rapid innate and adaptive immune responses, resulting in clearance of the virus in less than 3 weeks. Seven to 14 days after inoculation of the cow, ncpBVDV crosses the placenta and induces a fetal viremia. Establishment of persistent infection with ncpBVDV in the fetus has been attributed to the inability to mount an immune response before 90-150 days of gestational age. The result is 'immune tolerance', persistent viral replication and shedding of ncpBVDV. In contrast, we describe the chronic upregulation of fetal Type I interferon (IFN) pathway genes and the induction of IFN-γ pathways in fetuses of cows infected on day 75 of gestation. Persistently infected (PI) fetal IFN-γ concentrations also increased at day 97 at the peak of fetal viremia and IFN-γ mRNA was significantly elevated in fetal thymus, liver and spleen 14-22 days post maternal inoculation. PI fetuses respond to ncpBVDV infection through induction of Type I IFN and IFN-γ activated genes leading to a reduction in ncpBVDV titer. We hypothesize that fetal infection with BVDV persists because of impaired induction of IFN-γ in the face of activated Type I IFN responses. Clarification of the mechanisms involved in the IFN-associated pathways during BVDV fetal infection may lead to better detection methods, antiviral compounds and selection of genetically resistant breeding animals.

End-point disease investigation for virus strains of intermediate virulence as illustrated by flavivirus infections

Viruses of intermediate virulence are defined as isolates causing an intermediate morbidity/mortality rate in a specific animal model system, involving specific host and inoculation parameters (e.g. dose and route). Therefore, variable disease phenotype may exist between animals that develop severe disease or die and those that are asymptomatic or survive after infection with these isolates. There may also be variability amongst animals within each of these subsets. Such potential variability may confound the use of time-point sacrifice experiments to investigate pathogenesis of this subset of virus strains, as uniformity in disease outcome is a fundamental assumption for time-course sacrifice experiments. In the current study, we examined the disease phenotype, neuropathology, neural infection and glial cell activity in moribund/dead and surviving Swiss white (CD-1) mice after intraperitoneal infection with various Australian flaviviruses, including West Nile virus (WNV) strains of intermediate virulence (WNVNSW2011 and WNVNSW2012), and highly virulent Murray Valley encephalitis virus (MVEV) isolates. We identified notable intragroup variation in the end-point disease in mice infected with either WNVNSW strain, but to a lesser extent in mice infected with MVEV strains. The variable outcomes associated with WNVNSW infection suggest that pathogenesis investigations using time-point sacrifice of WNVNSW-infected mice may not be the best approach, as the assumption of uniformity in outcomes is violated. Our study has therefore highlighted a previously unacknowledged challenge to investigating pathogenesis of virus isolates of intermediate virulence. We have also set a precedent for routine examination of the disease phenotype in moribund/dead and surviving mice during survival challenge experiments.

Hepatitis C Virus NS3 Mediated Microglial Inflammation via TLR2/TLR6 MyD88/NF-κB Pathway and Toll Like Receptor Ligand Treatment Furnished Immune Tolerance

Background

Recent evidence suggests the neurotrophic potential of hepatitis C virus (HCV). HCV NS3 protein is one of the potent antigens of this virus mediating inflammatory response in different cell types. Microglia being the immune surveillance cells in the central nervous system (CNS), the inflammatory potential of NS3 on microglia was studied. Role of toll like receptor (TLR) ligands Pam2CSK3 and Pam3CSK4 in controlling the NS3 mediated microglial inflammation was studied using microglial cell line CHME3.

Methods

IL (Interleukin)-8, IL-6, TNF-α (Tumor nicrosis factor alpha) and IL-1β gene expressions were measured by semi quantitative RT-PCR (reverse transcription-PCR). ELISA was performed to detect IL-8, IL-6, TNF-α, IL-1β and IL-10 secretion. FACS (Flourescent activated cell sorting) was performed to quantify TLR1, TLR2, TLR6, MyD88 (Myeloid differntiation factor 88), IkB-α (I kappaB alpha) and pNF-κB (phosphorylated nuclear factor kappaB) expression. Immunofluorescence staining was performed for MyD88, TLR6 and NF-κB (Nuclear factor kappaB). Student's t-test or One way analysis of variance with Bonferoni post hoc test was performed and p < 0.05 was considered significant.

Results

Microglia responded to NS3 by secreting IL-8, IL-6, TNF-α and IL-1β via TLR2 or TLR6 mediated MyD88/NF-κB pathway. Transcription factor NF-κB was involved in activating the cytokine gene expression and the resultant inflammatory response was controlled by NF-κB inhibitor, Ro106-9920, which is known to down regulate pro-inflammatory cytokine secretion. Activation of the microglia by TLR agonists Pam3CSK4 and Pam2CSK4 induced immune tolerance against NS3. TLR ligand treatment significantly down regulated pro-inflammatory cytokine secretion in the microglia. IL-10 secretion was suggested as the possible mechanism by which TLR agonists induced immune tolerance. NS3 as such was not capable of self-inducing immune tolerance in microglia.

Conclusion

In conclusion, NS3 protein was capable of activating microglia and the inflammatory response could be controlled via blocking the transcription factor NF-κB, or by treating the microglia with TLR ligands which likely function via secreting anti-inflammatory cytokines such as IL-10. This can have therapeutic potential in controlling HCV mediated neuroinflammation.

Mechanism of West Nile virus neuroinvasion: a critical appraisal

West Nile virus (WNV) is an important emerging neurotropic virus, responsible for increasingly severe encephalitis outbreaks in humans and horses worldwide. However, the mechanism by which the virus gains entry to the brain (neuroinvasion) remains poorly understood. Hypotheses of hematogenous and transneural entry have been proposed for WNV neuroinvasion, which revolve mainly around the concepts of blood-brain barrier (BBB) disruption and retrograde axonal transport, respectively. However, an over‑representation of in vitro studies without adequate in vivo validation continues to obscure our understanding of the mechanism(s). Furthermore, WNV infection in the current rodent models does not generate a similar viremia and character of CNS infection, as seen in the common target hosts, humans and horses. These differences ultimately question the applicability of rodent models for pathogenesis investigations. Finally, the role of several barriers against CNS insults, such as the blood-cerebrospinal fluid (CSF), the CSF-brain and the blood-spinal cord barriers, remain largely unexplored, highlighting the infancy of this field. In this review, a systematic and critical appraisal of the current evidence relevant to the possible mechanism(s) of WNV neuroinvasion is conducted.

Induction of interferon-gamma and downstream pathways during establishment of fetal persistent infection with bovine viral diarrhea virus

Development of transplacental infection depends on the ability of the virus to cross the placenta and replicate within the fetus while counteracting maternal and fetal immune responses. Unfortunately, little is known about this complex process. Non-cytopathic (ncp) strains of bovine viral diarrhea virus (BVDV), a pestivirus in the Flaviviridae family, cause persistent infection in early gestational fetuses (<150 days; persistently infected, PI), but are cleared by immunocompetent animals and late gestational fetuses (>150 days; transiently infected, TI). Evasion of innate immune response and development of immunotolerance to ncp BVDV have been suggested as possible mechanisms for the establishment of the persistent infection. Previously we have observed a robust temporal induction of interferon (IFN) type I (innate immune response) and upregulation of IFN stimulated genes (ISGs) in BVDV TI fetuses. Modest chronic upregulation of ISGs in PI fetuses and calves reflects a stimulated innate immune response during persistent BVDV infection. We hypothesized that establishing persistent fetal BVDV infection is also accompanied by the induction of IFN-gamma (IFN-γ). The aims of the present study were to determine IFN-γ concentration in blood and amniotic fluid from control, TI and PI fetuses during BVDV infection and analyze induction of the IFN-γ downstream pathways in fetal lymphoid tissues. Two experiments with in vivo BVDV infections were completed. In Experiment 1, pregnant heifers were infected with ncp BVDV type 2 on day 75 or 175 of gestation or kept naïve to generate PI, TI and control fetuses, respectively. Fetuses were collected by Cesarean section on day 190. In Experiment 2, fetuses were collected on days 82, 89, 97, 192 and 245 following infection of pregnant heifers on day 75 of gestation. The results were consistent with the hypothesis that ncp BVDV infection induces IFN-γ secretion during acute infection in both TI and PI fetuses and that lymphoid tissues such as spleen, liver and thymus, serve both as possible sources of IFN-γ and target organs for its effects. Notably, induction of IFN-γ coincides with a decrease in BVDV RNA concentrations in PI fetal blood and tissues. This is the first report indicating the possible presence of an adaptive immune response in persistent BVDV infections, which may be contributing to the observed reduction of viremia in PI fetuses.

Bovine Viral Diarrhea Virus Infections

Bovine viral diarrhea virus (BVDV) continues to be of economic significance to the livestock industry in terms of acute disease and fetal loss. Many of the lesions relating to BVDV infection have been well described previously. The virus is perpetuated in herds through the presence of calves that are persistently infected. Relationships between various species and biotypes of BVDV and host defenses are increasingly understood. Understanding of the host defense mechanisms of innate immunity and adaptive immunity continues to improve, and the effects of the virus on these immune mechanisms are being used to explain how persistent infection develops. The noncytopathic biotype of BVDV plays the major role in its effects on the host defenses by inhibiting various aspects of the innate immune system and creation of immunotolerance in the fetus during early gestation. Recent advances have allowed for development of affordable test strategies to identify and remove persistently infected animals. With these improved tests and removal strategies, the livestock industry can begin more widespread effective control programs.

Effects of in utero pestivirus infection on bovine fetal bone geometry, biomechanical properties and composition

Transplacental viral infection of the fetus can result in abnormal trabecular and cortical bone modeling in long bones through impaired bone resorption and formation. Although such infections are frequently associated with neonatal fractures in humans and animals, their effect on the biomechanical properties of the developing skeleton remain poorly understood. The goal of this study was to determine the effects of transplacental bovine viral diarrhea virus (BVDV) infection on the biomechanical properties of fetal femora. Pregnant heifers were inoculated intranasally with non-cytopathic BVDV or media alone on day 75 of gestation to produce persistently infected (PI) and control fetuses, respectively, which were then removed on days 192 and 245 of gestation. Histomorphometry, compositional analysis and 'four-point bending until failure' were performed on fetal femora. Altered cortical geometry largely accounted for differences in calculated elastic modulus (PI vs. control, and day 192 vs. day 245) and ultimate stress (day 192 vs. day 245). Fetal infection with BVDV did not significantly impair inherent biomechanical properties of bone but rather resulted in decreased periosteal apposition rates, manifested as smaller femoral mid-diaphyseal diameters. There were no differences between PI and control fetuses in cortical thickness ratio, ash density or calcium/phosphorous content; however, cortical thickness ratio decreased with fetal age. Thus even when cortical thickness ratios are similar, differences in mid-diaphyseal diameter affect the error associated with the calculation of stress and strain by classical beam theory equations.

Development of fetal and placental innate immune responses during establishment of persistent infection with bovine viral diarrhea virus

Transplacental viral infections are dependent upon complex interactions between feto-placental and maternal immune responses and the stage of fetal development at which the infection occurs. Bovine viral diarrhea virus (BVDV) has the ability to cross the placenta and infect the fetus. Infection early in gestation with non-cytopathic (ncp) BVDV leads to persistent infection. Establishment of fetal persistent infection results in life-long viremia, virus-specific immunotolerance, and may have detrimental developmental consequences. We have previously shown that heifers infected experimentally with ncp BVDV type 2 on d. 75 of gestation had transient robust up-regulation of the type I interferon (IFN) stimulated genes (ISGs) 3-15 days after viral inoculation. Blood from persistently infected (PI) fetuses, collected 115 days post maternal infection, demonstrated moderate chronic up-regulation of ISGs. This infection model was used to delineate timing of the development of innate immune responses in the fetus and placenta during establishment of persistent infection. It was hypothesized that: (i) chronic stimulation of innate immune responses occurs following infection of the fetus and (ii) placental production of the type I IFN contributes to up-regulation of ISGs in PI fetuses. PI fetuses, generated by intranasal inoculation of pregnant heifers with ncp BVDV, and control fetuses from uninfected heifers, were collected via Cesarean sections on d. 82, 89, 97, 192, and 245 of gestation. Fetal viremia was confirmed starting on d. 89. Significant up-regulation of mRNA encoding cytosolic dsRNA sensors -RIG-I and MDA5 - was detected on d. 82-192. Detection of viral dsRNA by cytosolic sensors leads to the stimulation of ISGs, which was reflected in significant up-regulation of ISG15 mRNA in fetal blood on d. 89, 97, and 192. No difference in IFN-α and IFN-β mRNA concentration was found in fetal blood or caruncular tissue, while a significant increase in both IFN-α and IFN-β mRNA was seen in cotyledons from PI fetuses on d. 192. It is concluded that fetuses respond to early gestational ncp BVDV infection by induction of the type I IFN pathway, resulting in chronic up-regulation of ISGs. Cotyledonary tissue contributes to up-regulation of ISGs by increased production of IFNs. The innate immune response might partially curtail viral replication in PI fetuses, but is not able to eliminate the virus in the absence of a virus-specific adaptive immune response.