Expression of type I interferon-induced antiviral state and pro-apoptosis markers during experimental infection with low or high virulence bovine viral diarrhea virus in beef calves

Non-cytopathic bovine viral diarrhoea virus 2 induces autophagy to enhance its replication

BACKGROUND

Bovine viral diarrhoea virus (BVDV) is an important viral pathogen that has an economic impact on the livestock industry worldwide. Autophagy is one of the earliest cell-autonomous defence mechanisms against microbial invasion, and many types of viruses can induce autophagy by infecting host cells.

OBJECTIVES

The aim of this study was to identify the role of autophagy in the pathogenesis of non-cytopathic (ncp) BVDV2 infection.

METHODS

Madin-Darby bovine kidney (MDBK) cells were treated with ncp BVDV2, rapamycin, or 3-methyladenine (MA) and ncp BVDV2 and then incubated at 37°C for 24 h. Cells were harvested, and the effects of autophagy were determined by transmission electron microscopy (TEM), confocal laser microscopy, western blotting and qRT-PCR. Apoptotic analysis was also performed using western blotting and flow cytometry.

RESULTS

In ncp BVDV2-infected MDBK cells, more autophagosomes were observed by TEM, and the number of microtubule-associated protein 1 light chain 3B (LC3B) with green fluorescent protein puncta was also increased. The ncp BVDV2-infected cells showed significantly enhanced conversion of LC3-I to LC3-II, as well as upregulation of autophagy-related proteins, including ATG5 and Beclin 1, and substantial degradation of p62/SQSTM1. These results are similar to those induced by rapamycin, an autophagy inducer. E2 protein expression, which is associated with viral replication, increased over time in ncp BVDV2-infected cells. Inhibition of autophagy by 3-MA in ncp BVDV2-infected MDBK cells downregulated the expressions of LC3-II, ATG5 and Beclin 1 and prevented the degradation of p62/SQSTM1. Moreover, the expressions of phosphorylated Akt and procaspase-3 were significantly increased in ncp BVDV2-infected cells. In addition, the mRNA level of protein kinase R (PKR) was significantly reduced in ncp BVDV2-infected cells.

CONCLUSIONS

Our results demonstrate that ncp BVDV2 infection induced autophagy in MDBK cells via anti-apoptosis and PKR suppression. Therefore, autophagy may play a role in establishing persistent infection caused by ncp BVDV.

Fetal Lymphoid Organ Immune Responses to Transient and Persistent Infection with Bovine Viral Diarrhea Virus

Bovine Viral Diarrhea Virus (BVDV) fetal infections occur in two forms; persistent infection (PI) or transient infection (TI), depending on what stage of gestation the fetus is infected. Examination of lymphoid organs from both PI and TI fetuses reveals drastically different fetal responses, dependent upon the developmental stage of the fetal immune system. Total RNA was extracted from the thymuses and spleens of uninfected control, PI, and TI fetuses collected on day 190 of gestation to test the hypothesis that BVDV infection impairs the innate and adaptive immune response in the fetal thymus and spleen of both infection types. Transcripts of genes representing the innate immune response and adaptive immune response genes were assayed by Reverse Transcription quatitative PCR (RT-qPCR) (2^−ΔΔCq; fold change). Genes of the innate immune response, interferon (IFN) inducible genes, antigen presentation to lymphocytes, and activation of B cells were downregulated in day 190 fetal PI thymuses compared to controls. In contrast, innate immune response genes were upregulated in TI fetal thymuses compared to controls and tended to be upregulated in TI fetal spleens. Genes associated with the innate immune system were not different in PI fetal spleens; however, adaptive immune system genes were downregulated, indicating that PI fetal BVDV infection has profound inhibitory effects on the expression of genes involved in the innate and adaptive immune response. The downregulation of these genes in lymphocytes and antigen-presenting cells in the developing thymus and spleen may explain the incomplete clearance of BVDV and the persistence of the virus in PI animals while the upregulation of the TI innate immune response indicates a more mature immune system, able to clear the virus.

In-vivo Activity of IFN-λ and IFN-α Against Bovine-Viral-Diarrhea Virus in a Mouse Model

Bovine-viral-diarrhea virus (BVDV) can cause significant economic losses in livestock. The disease is controlled with vaccination and bovines are susceptible until vaccine immunity develops and may remain vulnerable if a persistently infected animal is left on the farm; therefore, an antiviral agent that reduces virus infectivity can be a useful tool in control programs. Although many compounds with promising in-vitro efficacy have been identified, the lack of laboratory-animal models limited their potential for further clinical development. Recently, we described the activity of type I and III interferons, IFN-α and IFN-λ respectively, against several BVDV strains in-vitro. In this study, we analyzed the in-vivo efficacy of both IFNs using a BALB/c-mouse model. Mice infected with two type-2 BVDV field strains developed a viremia with different kinetics, depending on the infecting strain's virulence, that persisted for 56 days post-infection (dpi). Mice infected with the low-virulence strain elicited high systemic TNF-α levels at 2 dpi. IFNs were first applied subcutaneously 1 day before or after infection. The two IFNs reduced viremia with different kinetics, depending on whether either one was applied before or after infection. In a second experiment, we increased the number of applications of both IFNs. All the treatments reduced viremia compared to untreated mice. The application of IFN-λ pre- and post-infection reduced viremia over time. This study is the first proof of the concept of the antiviral potency of IFN-λ against BVDV in-vivo, thus encouraging further trails for a potential use of this cytokine in cattle.

Upregulation of the type I interferon pathway in feedlot cattle persistently infected with bovine viral diarrhea virus

Bovine viral diarrhea virus (BVDV) has a profound economic impact on the cattle industry. Calves infected in utero and born persistently infected (PI) with BVDV have increased morbidity, mortality, and reduced productivity. Further, they serve as a continual source of viral exposure to herd mates and thereby pose a significant risk to animal wellbeing and production efficiency. Understanding the mechanisms through which PI is established and maintained is therefore important in working toward finding means to prevent or mitigate losses due to infection. Early studies of acute infection suggested BVDV infection alters the host's ability to mount a type I interferon (IFN) response, thereby allowing for the establishment of PI. More recently, however, animals experimentally challenged with the virus demonstrated a chronic yet modest upregulation of the IFN pathway. To identify if the IFN or other pathways are altered due to PI by BVDV in a natural infection, the circulating blood transcriptome was analyzed from PI feedlot cattle (N = 10 BVDV1a, 8 BVDV1b, 8 BVDV2), cattle co-mingling with PI cattle but not themselves infected (N = 9), and a group of unrelated, unexposed controls (N=10). Differential expression analyses included contrasts among BVDV subtypes, and all pair-wise comparisons of PI, co-mingled non-PI, and unexposed animals. Analyses in limma-voom revealed no difference in the transcriptome based upon the BVDV genotype with which the animal was infected. However, gene expression did differ (adj P < 0.05 and |logFC|> 1) at 175 loci between the PI and co-housed, non-PI contemporaries and when compared to the unexposed controls, 489 loci were differentially expressed. Pathway analyses predict that alterations in the transcriptome of the PI cattle indicate significant upregulation of innate immune function including IFN signaling. These data support prior work suggesting IFN signaling is not completely suppressed in cattle naturally PI with BVDV.

A retrospective pathology study of two Neotropical deer species (1995-2015), Brazil: Marsh deer (Blastocerus dichotomus) and brown brocket deer (Mazama gouazoubira)

This retrospective study describes the biological and epidemiological aspects, gross and microscopical findings, and most likely causes of death (CD) in two species of Neotropical deer in Brazil. The animals were collected between 1995 and 2015 and represented 75 marsh deer (MD) and 136 brown brocket deer (BBD). Summarized, pneumonia was diagnosed microscopically in 48 MD and 52 BBD; 76 deer suffered trauma, involving dog attack (14 BBD) and vehicle-collision (14 BBD). Pulmonary edema (50 MD; 55 BBD) and congestion (57 MD; 78 BBD) were the most common findings for both species. Additionally, we diagnosed ruminal and myocardial mycosis in MD and BBD, respectively; ovarian dysgerminoma and pancreatic trematodiasis in BBD; and lesions suggestive of malignant catarrhal fever and orbiviral hemorrhagic disease in both species. The main CD in MD was: respiratory (41/75), alimentary, nutritional, trauma and euthanasia (3/75 each). Correspondingly, in BBD were: trauma (34/131), respiratory (30/131) and euthanasia (9/131). Respiratory disease was often defined by pulmonary edema and pneumonia. We provide evidence that respiratory disease, mainly pneumonia, is a critical pathological process in these Neotropical deer species. Although no etiological agents were identified, there is evidence of bacterial and viral involvement. Our results show trauma, mainly anthropogenic, as a common ailment in BBD. We propose to prioritize respiratory disease in future research focused on South American deer health aspects. We believe anthropogenic trauma may be a primary threat for populations of BBD.

Type 2 BVDV Npro suppresses IFN-1 pathway signaling in bovine cells and augments BRSV replication

Bovine viral diarrhea virus (BVDV) infection induces immunosuppression and in conjunction with bovine respiratory syncytial virus (BRSV) contributes to the bovine respiratory disease complex. Bovine turbinate cells were single or co-infected with type 2 BVDV wild-type (BVDV2-wt), its dysfunctional N^pro mutant (BVDV2-E), and/or BRSV. BVDV2-E significantly up-regulated PKR, IRF-7, TBK-1, IRF-3, and IFN-β mRNAs based on real-time Q-RT-PCR. BRSV-infected cells expressed significantly up-regulated PKR, IRF-3, IRF-7, and IFN-β mRNAs, whereas BVDV2-wt, but not BVDV2-E, abolished this up-regulation in co-infection. No significant differences were observed in MAVS, NF-κB, and PIN-1 mRNAs. A dual-luciferase reporter assay showed that BVDV2-wt significantly increased NF-κB activity compared to BVDV2-E, while BVDV2-E significantly increased IFN-β activity compared to BVDV2-wt. The BRSV titer and RNA levels significantly increased in cells co-infected with BRSV/BVDV2-wt compared to cells co-infected with BRSV/BVDV2-E or infected with BRSV alone. This data supports the synergistic action of BVDV2-wt and BRSV inhibition of IFN-1.

The core protein of a pestivirus protects the incoming virus against IFN-induced effectors

A multitude of viral factors - either inhibiting the induction of the IFN-system or its effectors – have been described to date. However, little is known about the role of structural components of the incoming virus particle in protecting against IFN-induced antiviral factors during or immediately after entry. In this study, we take advantage of the previously reported property of Classical swine fever virus (family Flaviviridae, genus Pestivirus) to tolerate a deletion of the core protein if a compensatory mutation is present in the NS3-helicase-domain (Vp447_∆c). In contrast to the parental virus (Vp447), which causes a hemorrhagic-fever-like disease in pigs, Vp447_∆c is avirulent in vivo. In comparison to Vp447, growth of Vp447_∆c in primary porcine cells and IFN-treated porcine cell lines was reduced >20-fold. Also, primary porcine endothelial cells and IFN-pretreated porcine cell lines were 8–24 times less susceptible to Vp447_∆c. This reduction of susceptibility could be partially reversed by loading Vp447_∆c particles with different levels of core protein. In contrast, expression of core protein in the recipient cell did not have any beneficial effect. Therefore, a protective effect of core protein in the incoming virus particle against the products of IFN-stimulated genes could be demonstrated.

Induction of functional interferon alpha and gamma responses during acute infection of cattle with non-cytopathic bovine viral diarrhea virus

As a part of their pathogenic mechanism, many pathogens causing persistent infections, including bovine viral diarrhea virus (BVDV), immunosuppress their hosts, often by limiting the ability to either produce, or respond to, interferon. The objective of this study was to quantify the extent to which an acute infection of cattle with a non-cytopathic strain of BVDV induces interferon responses and to establish the functionality of these responses. Functionality of responses was investigated using a bovine specific peptide array to monitor kinase-mediated signal transduction activity within peripheral blood mononuclear cells (PBMCs) at time points corresponding to the interferon gamma (IFN-γ) and alpha (IFN-α) responsive phases of acute BVDV infection. Further, with an appreciation of diverse mechanisms and levels at which pathogens modulate host cell defences, patterns of expression of IFN-γ and -α responsive genes were also quantified within PBMCs. Infection of cows with ncpBVDV2-1373 induced significant increases in levels of serum IFN-γ and IFN-α. Within the PBMCs of the infected animals, distinct patterns of kinase-mediated signal transduction activity, in particular with respect to activation of classic IFN-activated signalling pathways, such as Jak-Stat, as well as induced expression of IFN-γ and IFN-α regulated genes, support the functionality of the host interferon response.

Transcriptomic analysis of responses to cytopathic bovine viral diarrhea virus-1 (BVDV-1) infection in MDBK cells

The bovine viral diarrhea virus (BVDV) is responsible for significant economic losses in the dairy and cattle industry; however, little is known about the protective and pathological responses of hosts to infection. The present study determined the principal molecular markers implicated in viral infection through meta-transcriptomic analysis using MDBK cells infected for two hours with a field isolate of BVDV-1. While several immune regulator genes were induced, genes involved in cell signaling, metabolic processes, development, and integrity were down-regulated, suggesting an isolation of infected cells from cell-to-cell interactions and responses to external signals. Analysis through RT-qPCR confirmed the expression of more than one hundred markers. Interestingly, there was a significant up-regulation of two negative NF-κB regulators, IER3 and TNFAIP3, indicating a possible blocking of this signaling pathway mediated by BVDV-1 infection. Additionally, several genes involved in the metabolism of reactive oxygen species were down-regulated, suggesting increased oxidative stress. Notably, a number of genes involved in cellular growth and development were also regulated during infection, including MTHFD1L, TGIF1, and Brachyury. Moreover, there was an increased expression of the genes β-catenin, caprin-2, GSK3β, and MMP-7, all of which are crucial to the Wnt signaling pathway that is implicated in the embryonic development of a variety of organisms. This meta-transcriptomic analysis provides the first data towards understanding the infection mechanisms of cytopathic BVDV-1 and the putative molecular relationship between viral and host components.

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.

Acute infection with bovine viral diarrhea virus of low or high virulence leads to depletion and redistribution of WC1(+) γδ T cells in lymphoid tissues of beef calves

The objective of this study was to determine the abundance and distribution of γδ T lymphocytes in lymphoid tissue during acute infection with high (HV) or low virulence (LV) non-cytopathic bovine viral diarrhea virus (BVDV) in beef calves. This study was performed using tissue samples from a previous experiment in which thirty beef calves were randomly assigned to 1 of 3 groups: LV [n=10; animals inoculated intranasally (IN) with LV BVDV-1a (strain SD-1)], HV [n=10; animals inoculated IN with HV BVDV-2 (strain 1373)], and control (n=10; animals inoculated with cell culture medium). On day 5 post inoculation, animals were euthanized, and samples from spleen and mesenteric lymph nodes (MLN) were collected to assess the abundance of WC1(+) γδ T cells. A higher proportion of calves challenged with BVDV showed signs of apoptosis and cytophagy in MLN and spleen samples compared to the control group. A significantly lower number of γδ T cells was observed in spleen and MLN from calves in HV and LV groups than in the control calves (P<0.05). In conclusion, acute infection with HV or LV BVDV resulted in depletion of WC1(+) γδ T cells in mucosal and systemic lymphoid tissues at five days after challenge in beef calves. This reduction in γδ T cells in the studied lymphoid tissues could be also due to lymphocyte trafficking to other tissues.

Analysis of mRNA expression for genes associated with regulatory T lymphocytes (CD25, FoxP3, CTLA4, and IDO) after experimental infection with bovine viral diarrhea virus of low or high virulence in beef calves

Immunosuppression caused by bovine viral diarrhea virus (BVDV) has been associated with lymphocyte depletion, leukopenia and impairment of leukocyte function; however, no work has been done on the relationship between BVDV and regulatory T lymphocytes (Tregs). The objective of this study was to compare the mRNA expression of genes associated with Tregs (CD25, FoxP3, CTLA4, and IDO), after experimental infection of beef calves with low (LV) or high (HV) virulence BVDV. Thirty BVDV-naïve calves were randomly assigned to three groups. Calves were intra-nasally inoculated with LV (n=10, strain SD-1) or HV (n=10, strain 1373) BVDV or BVDV-free cell culture medium (control, n=10). Quantitative RT-PCR was used to determine the expression of target genes in tracheo-bronchial lymph nodes and spleen on day 5 post-infection. The mRNA expression of CD25 was up-regulated in tracheo-bronchial lymph nodes of LV (P<0.05), but not in HV compared to the control group. The expression of FoxP3 and CTLA4 was not increased in tracheo-bronchial lymph nodes of either of the BVDV-inoculated groups. A dramatic up-regulation of IDO mRNA was observed in tracheo-bronchial lymph nodes of LV (P<0.05), but not HV compared to the control calves. In conclusion, experimental infection with BVDV did not provide evidence of Treg activation based on expression of FoxP3 and CTL4. Differential expression of CD25 and IDO mRNA on day 5 post-infection with HV or LV BVDV might reflect temporal differences in transcription occurring during the immune response elicited by these viral strains, or differences in viral infectivity of the host cells.

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.

Failure of respiratory defenses in the pathogenesis of bacterial pneumonia of cattle

The respiratory system is well defended against inhaled bacteria by a dynamic system of interacting layers, including mucociliary clearance, host defense factors including antimicrobial peptides in the epithelial lining fluid, proinflammatory responses of the respiratory epithelium, resident alveolar macrophages, and recruited neutrophils and monocytes. Nevertheless, these manifold defenses are susceptible to failure as a result of stress, glucocorticoids, viral infections, abrupt exposure to cold air, and poor air quality. When some of these defenses fail, the lung can be colonized by bacterial pathogens that are equipped to evade the remaining defenses, resulting in the development of pneumonia. This review considers the mechanisms by which these predisposing factors compromise the defenses of the lung, with a focus on the development of bacterial pneumonia in cattle and supplemented with advances based on mouse models and the study of human disease. Deepening our understanding of how the respiratory defenses fail is expected to lead to interventions that restore these dynamic immune responses and prevent disease.