Enhancement of apoptosis via an extrinsic factor, TNF-alpha, in cells infected with cytopathic bovine viral diarrhea virus

Immune evasion strategies of bovine viral diarrhea virus

Bovine viral diarrhea virus (BVDV) is a significant pathogen that causes great economic losses in the global livestock industry. During the long-term interactions between BVDV and its hosts, the virus has evolved multiple strategies to evade the host's innate immunity and adaptive immunity, thereby promoting viral survival and replication. This review focuses on the most recent research on immune evasion strategies employed by BVDV, including evading type I IFN signaling pathway, evading host adaptive immunity, mediating NF-κB signaling pathway, mediating cell apoptosis and inducing autophagy. Unraveling BVDV's immune evasion strategies will enhance our understanding of the pathogenesis of BVDV and contribute to the development of more effective therapies for the prevention, control and eradication of BVDV.

Identification of differentially expressed gene pathways between cytopathogenic and non-cytopathogenic BVDV-1 strains by analysis of the transcriptome of infected primary bovine cells

The bovine viral diarrhea virus 1 (BVDV-1), belonging to the Pestivirus genus, is characterized by the presence of two biotypes, cytopathogenic (cp) or non-cytopathogenic (ncp). For a better understanding of the host pathogen interactions, we set out to identify transcriptomic signatures of bovine lung primary cells (BPCs) infected with a cp or a ncp strain. For this, we used both a targeted approach by reverse transcription droplet digital PCR and whole genome approach using RNAseq. Data analysis showed 3571 differentially expressed transcripts over time (Fold Change >2) and revealed that the most deregulated pathways for cp strain are signaling pathways involved in responses to viral infection such as inflammatory response or apoptosis pathways. Interestingly, our data analysis revealed a deregulation of Wnt signaling pathway, a pathway described in embryogenesis, that was specifically seen with the BVDV-1 cp but not the ncp suggesting a role of this pathway in viral replication.

Comprehensive analysis of circRNAs expression profiles in different periods of MDBK cells infected with bovine viral diarrhea virus

CircRNAs play an important regulatory role in the regulation of disease. However, we have a limited understanding of the role of circRNAs in the host's complex protective and pathological mechanisms of BVDV infection. Transcriptome analysis of circRNAs in host cells after BVDV infection may allow us to understand the biological functions of circRNAs in the regulation of BVDV infection. Here, we identified a total of 19,118 circRNAs from the MBDK cells (at 12 h, 24 h, and 48 h post-infection) infected with BVDV by using RNA-seq technology. We confirmed several circRNAs using RT-PCR and DNA sequencing, and qRT-PCR analysis was performed to identify several circRNAs expression and circRNAs resistance to RNase R digestion. GO (Gene Ontology) and KEGG (Kyoto Encyclopedia of Genes and Genomes) enrichment analysis showed that the host genes of differentially expressed circRNAs were involved in the regulation of cell proliferation, apoptosis, cycle and viral infection related signaling pathways. These results indicate that circRNA in host cells plays a broad regulatory role after BVDV infection and provides a valuable resource for studying circRNA biology in host cells after BVDV infection.

Analyses of long non-coding RNAs and mRNA profiling through RNA sequencing of MDBK cells at different stages of bovine viral diarrhea virus infection

BACKGROUND

Bovine viral diarrhea virus (BVDV) infection is a dynamic and complex process that leads to significant economic losses in the dairy and cattle industries. However, our understanding of the protective and pathological mechanism underlying host infection is limited.

METHODS

To determine whether BVDV regulates specific activities of the host cell, the expression of long non-coding RNA (lncRNA) during BVDV NADL infection was studied by deep sequencing.

RESULTS

A total of 1236 lncRNA transcripts and 3261 mRNA transcripts were differentially regulated at 2h, 6h, and 18h post-infection. The lncRNAs shared same characteristics with other mammals in terms of exon length, number, expression level, and conservation. The Gene Ontology (GO) enrichment and KEGG pathway analyses showed that lncRNAs regulate immune reaction during BVDV infection. Thirteen differentially expressed genes in 18 hpi were selected and independently validated by reverse-transcription qPCR.

CONCLUSIONS

The present study is the first to provide insights into the biological connection of lncRNAs and BVDV, which can be further explored for the development of antiviral prevention strategies and in understanding persistent infection between viral and host components.

Bovine Viral Diarrhea Virus Type 2 Impairs Macrophage Responsiveness to Toll-Like Receptor Ligation with the Exception of Toll-Like Receptor 7

Bovine viral diarrhea virus (BVDV) is a member of the Flaviviridae family. BVDV isolates are classified into two biotypes based on the development of cytopathic (cp) or non-cytopathic (ncp) effects in epithelial cell culture. BVDV isolates are further separated into species, BVDV1 and 2, based on genetic differences. Symptoms of BVDV infection range from subclinical to severe, depending on strain virulence, and may involve multiple organ systems and induction of a generalized immunosuppression. During BVDV-induced immune suppression, macrophages, critical to innate immunity, may have altered pathogen recognition receptor (PRR) signaling, including signaling through toll-like receptors (TLRs). Comparison of BVDV 2 strains with different biotypes and virulence levels is valuable to determining if there are differences in host macrophage cellular responses between viral phenotypes. The current study demonstrates that cytopathic (cp), noncytopathic (ncp), high (hv) or low virulence (lv) BVDV2 infection of bovine monocyte-derived macrophages (MDMΦ) result in differential expression of pro-inflammatory cytokines compared to uninfected MDMΦ. A hallmark of cp BVDV2 infection is IL-6 production. In response to TLR2 or 4 ligation, as might be observed during secondary bacterial infection, cytokine secretion was markedly decreased in BVDV2-infected MDMΦ, compared to non-infected MDMΦ. Macrophages were hyporesponsive to viral TLR3 or TLR8 ligation. However, TLR7 stimulation of BVDV2-infected MDMΦ induced cytokine secretion, unlike results observed for other TLRs. Together, these data suggest that BVDV2 infection modulated mRNA responses and induced a suppression of proinflammatory cytokine protein responses to TLR ligation in MDMΦ with the exception of TLR7 ligation. It is likely that there are distinct differences in TLR pathways modulated following BVDV2 infection, which have implications for macrophage responses to secondary infections.

Pathogenic mechanisms implicated in the intravascular coagulation in the lungs of BVDV-infected calves challenged with BHV-1

Resistance to respiratory disease in cattle requires host defense mechanisms that protect against pathogens which have evolved sophisticated strategies to evade them, including an altered function of pulmonary macrophages (MΦs) or the induction of inflammatory responses that cause lung injury and sepsis. The aim of this study was to clarify the mechanisms responsible for vascular changes occurring in the lungs of calves infected with bovine viral diarrhea virus (BVDV) and challenged later with bovine herpesvirus type 1 (BHV-1), evaluating the role of MΦs in the development of pathological lesions in this organ. For this purpose, pulmonary lesions were compared between co-infected calves and healthy animals inoculated only with BHV-1 through immunohistochemical (MAC387, TNFα, IL-1α, iNOS, COX-2 and Factor-VIII) and ultrastructural studies. Both groups of calves presented important vascular alterations produced by fibrin microthrombi and platelet aggregations within the blood vessels. These findings were earlier and more severe in the co-infected group, indicating that the concomitance of BVDV and BHV-1 in the lungs disrupts the pulmonary homeostasis by facilitating the establishment of an inflammatory and procoagulant environment modulated by inflammatory mediators released by pulmonary MΦs. In this regard, the co-infected calves, in spite of presenting a greater number of IMΦs than single-infected group, show a significant decrease in iNOS expression coinciding with the presence of more coagulation lesions. Moreover, animals pre-inoculated with BVDV displayed an alteration in the response of pro-inflammatory cytokines (TNFα and IL-1), which play a key role in activating the immune response, as well as in the local cell-mediated response.

Apoptosis in Bovine viral diarrhea virus (BVDV)-induced mucosal disease lesions: a histological, immunohistological, and virological investigation

Cattle persistently infected with a noncytopathic Bovine viral diarrhea virus (BVDV) are at risk of developing fatal "mucosal disease" (MD). The authors investigated the role of various apoptosis pathways in the pathogenesis of lesions in animals suffering from MD. Therefore, they compared the expression of caspase-3, caspase-8, caspase-9, and Bcl-2L1 (Bcl-x) in tissues of 6 BVDV-free control animals, 7 persistently infected (PI) animals that showed no signs of MD (non-MD PI animals), and 11 animals with MD and correlated the staining with the localization of mucosal lesions. Caspase-3 and -9 staining were markedly stronger in MD cases and were associated with mucosal lesions, even though non-MD PI animals and negative controls also expressed caspase-9. Conversely, caspase-8 was not elevated in any of the animals analyzed. Interestingly, Bcl-x also colocalized with mucosal lesions in the MD cases. However, Bcl-x was similarly expressed in tissues from all 3 groups, and thus, its role in apoptosis needs to be clarified. This study clearly illustrates ex vivo that the activation of the intrinsic, but not the extrinsic, apoptosis pathway is a key element in the pathogenesis of MD lesions observed in cattle persistently infected with BVDV. However, whether direct induction of apoptosis in infected cells or indirect effects induced by the virus are responsible for the lesions observed remains to be established.

Response of proinflammatory and anti-inflammatory cytokines in calves with subclinical bovine viral diarrhea challenged with bovine herpesvirus-1

The aim of this work was to investigate the susceptibility of calves infected with bovine viral diarrhea virus (BVDV) against secondary infections. For this purpose, the profile of cytokines implicated in the immune response of calves experimentally infected with a non-cytopathic strain of BVDV type-1 and challenged with bovine herpesvirus 1.1 (BHV-1.1) was evaluated in comparison with healthy animals challenged only with BHV-1.1. The immune response was measured by serum concentrations of cytokines (IL-1β, TNFα, IFNγ, IL-12, IL-4 and IL-10), acute phase proteins (haptoglobin, serum amyloid A and fibrinogen) and BVDV and BHV-1.1 specific antibodies. BVDV-infected calves displayed a great secretion of TNFα and reduced production of IL-10 following BHV-1 infection, leading to an exacerbation of the inflammatory response and to the development of more intense clinical symptoms and lesions than those observed in healthy animals BHV-1-inoculated. A Th1 immune response, based on IFNγ production and on the absence of significant changes in IL-4 production, was observed in both groups of BHV-1-infected calves. However, whereas the animals inoculated only with BHV-1 presented an IFNγ response from the start of the study and high expression of IL-12, the BVDV-infected calves showed a delay in the IFNγ production and low levels of IL-12. This alteration in the kinetic and magnitude of these cytokines, involved in cytotoxic mechanisms responsible for limiting the spread of secondary pathogens, facilitated the dissemination of BHV-1.1 in BVDV-infected calves.

Virus Distribution and Role of Thymic Macrophages During Experimental Infection With Noncytopathogenic Bovine Viral Diarrhea Virus Type 1

Thymic depletion, presence of viral antigen, and changes in distribution and cytokine production of thymic macrophages were investigated in calves experimentally infected with a noncytopathogenic bovine viral diarrhea virus type (BVDV) 1 strain. Ten clinically healthy colostrum-deprived calves were used. Eight calves were inoculated with the virus and two were used as uninfected controls. Calves were sedated and euthanized in batches between 3 and 14 days postinoculation. At necropsy, thymus samples were collected for structural, immunohistochemical, and ultrastructural study and TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling). From 6 days postinoculation, the thymic cortex was multifocally depleted with increased frequency of pyknosis and karyorrhexis, suggestive of apoptosis and confirmed by the TUNEL technique. Although the onset of lymphoid depletion was coincident with the detection of viral antigen by immunohistochemistry, the number of infected lymphocytes was very low through the experiment. There was an increase in number of macrophages in cortex and medulla, accompanied by ultrastructural changes indicative of phagocyte activation, and a decrease in cells expressing tumor necrosis factor-alpha (TNF-α) and IL-1α. These results suggest that the increase in number of these cells could be related to phagocytosis of cell debris and apoptotic lymphocytes. Furthermore, the results imply that, in contrast to the situation with classical swine fever virus, the lymphocyte apoptosis resulting from bovine viral diarrhea virus infection is not mediated by TNF-α or interleukin-1 alpha (IL-1α) production by virus-infected macrophages. This is the first study that describes this decrease in the number of thymic cells expressing TNF-α and IL-1α in cattle experimentally infected with bovine viral diarrhea virus type 1.

Apoptosis in lymphoid tissues of calves inoculated with non-cytopathic bovine viral diarrhea virus genotype 1: activation of effector caspase-3 and role of macrophages

The mechanisms responsible for lymphocyte apoptosis in bovine viral diarrhoea have not yet been clarified. Previous work suggests that bovine viral diarrhea virus (BVDV) is only directly responsible for the destruction of a small number of lymphocytes. The aim of this study was to clarify, in vivo, the role of macrophages in lymphocyte destruction through indirect mechanisms linked to the biosynthetic activation of these immunocompetent cells on ileal Peyer's patches, as well as the distribution and quantification of apoptosis. Eight colostrum-deprived calves were inoculated intranasally with a non-cytopathic strain of BVDV genotype 1 and killed in batches of two at 3, 6, 9 and 14 days post-inoculation (p.i.). The progressive depletion of Peyer's patches was found to be due to massive lymphocyte apoptosis, with an increase in cleaved caspase-3 and TUNEL-positive cells. Lymphoid depletion was accompanied, from 3 days p.i., by a significant rise in macrophage numbers both in lymphoid follicles and in interfollicular areas. Some macrophages showed signs of viral infection, together with subcellular changes indicative of phagocyte activation and, in some cases, of secretory activity. However, the number of macrophages that showed positive immunostaining for tumour necrosis factor-α and interleukin-1α, cytokines with a proven ability to induce apoptosis, remained low throughout the experiment in lymphoid follicles, where most apoptotic cells were found. These results thus appear to rule out a major involvement of macrophages and macrophage-secreted chemical mediators in the apoptosis of follicular B lymphocytes during BVDV infection.

Expression of the NS3 protease of cytopathogenic bovine viral diarrhea virus results in the induction of apoptosis but does not block activation of the beta interferon promoter

Bovine viral diarrhea virus (BVDV; genus Pestivirus) can exist as two biotypes, cytopathogenic (CP) and non-cytopathogenic (NCP). The CP form differs from NCP by the continual expression of free non-structural protein 3 (NS3). CP BVDV infection of cultured cells induces apoptosis, whereas NCP BVDV infection has been reported to block the induction of beta interferon (IFN-beta). To investigate the viral mechanisms underlying these effects, NS3 or NS2-3 proteins of NCP and CP BVDV biotypes, together with the cognate NS3 co-factor NS4A, were expressed in cells, and their effect on apoptosis and induction of IFN-beta was investigated. Expression of NS3/4A resulted in increased activity of caspase-9 and caspase-3, indicating induction of the intrinsic apoptosis pathway. Mutational analysis revealed that a protease-inactive NS3/4A was unable to induce apoptosis, suggesting that NS3 protease activity is required for initiation of apoptosis during CP BVDV infection. The ability of NS2-3 to modulate activation of the IFN-beta promoter was also investigated. These studies confirmed that, unlike the related hepatitis C virus and GB virus-B, BVDV proteases are unable to inhibit TLR3- and RIG-I-dependent activation of the IFN-beta promoter. These data suggest that BVDV NS3/4A is responsible for regulating the levels of cellular apoptosis and provide new insights regarding the viral elements associated with CP biotype pathogenesis.

Activation of extracellular signal-regulated kinase in MDBK cells infected with bovine viral diarrhea virus

Our efforts to identify the cellular signaling cascades triggered by bovine viral diarrhea virus (BVDV) infection in MDBK cells revealed marked activation of extracellular signal-regulated kinase 1/2 (ERK). Enhanced phosphorylation of ERK was detected following infection with cytopathogenic (cp) BVDV, but not with noncytopathogenic BVDV. It appears that cp BVDV-induced ERK phosphorylation is caused by oxidative stress, because ERK phosphorylation was inducible by treatment with hydrogen peroxide or serum deprivation and was attenuated by addition of antioxidants. These results indicate that BVDV infection influences the ERK signaling pathway via oxidative stress, depending on the biotype.

Inhibition of sphingosine kinase by bovine viral diarrhea virus NS3 is crucial for efficient viral replication and cytopathogenesis

Sphingosine 1-phosphate (S1P) is a bioactive sphingolipid implicated in diverse cellular functions including survival, proliferation, tumorigenesis, inflammation, and immunity. Sphingosine kinase (SphK) contributes to these functions by converting sphingosine to S1P. We report here that the nonstructural protein NS3 from bovine viral diarrhea virus (BVDV), a close relative of hepatitis C virus (HCV), binds to and inhibits the catalytic activity of SphK1 independently of its serine protease activity, whereas HCV NS3 does not affect SphK1 activity. Uncleaved NS2-3 from BVDV was also found to interact with and inhibit SphK1. We suspect that inhibition of SphK1 activity by BVDV NS3 and NS2-3 may benefit viral replication, because SphK1 inhibition by small interfering RNA, chemical inhibitor, or overexpression of catalytically inactive SphK1 results in enhanced viral replication, although the mechanisms by which SphK1 inhibition leads to enhanced viral replication remain unknown. A role of SphK1 inhibition in viral cytopathogenesis is also suggested as overexpression of SphK1 significantly attenuates the induction of apoptosis in cells infected with cytopathogenic BVDV. These findings suggest that SphK is targeted by this virus to regulate its catalytic activity.

Cytoplasmic vacuolization responses to cytopathic bovine viral diarrhoea virus

Bovine Viral Diarrhea Virus (BVDV) is a positive sense, single-stranded RNA virus which exhibits two biotypes in standard cell culture systems. The cytopathic strains of this virus (cpBVDV) induce dramatic cytoplasmic vacuolization in cell cultures, while infection with the non-cytopathic (NCP-BVDV) strains produces no overt changes in the host cells. Our results show that extensive cytoplasmic vacuolization is the earliest morphological change in response to cpBVDV infection in MDBK cells. Cells with extensive vacuolization showed no co-existing chromatin condensation, caspase activation, or loss of membrane integrity. In addition, the caspase inhibitor (zVAD-fmk), although improving cell viability of infected cells from 6.7+/-2.2% to 18.8+/-2.2%, did not prevent vacuolization. On the ultrastructural level, the virus-induced cytoplasmic vacuoles are single membrane structures containing organelles and cellular debris, which appear capable of fusing with other vacuoles and engulfing surrounding cytoplasmic materials. LysoTracker Red which marks lysosomes did not stain the virus-induced cytoplasmic vacuoles. In addition, this lysosomal dye could be observed in the cytoplasm of vacuolized cells, suggesting a lysosomal abnormality. Our data demonstrate that cpBVDV induced a novel cell death pathway in MDBK cells that is primarily associated with lysosomal dysfunction and the formation of phagocytic cytoplasmic vacuoles, and this mode of cell death is different from apoptosis and necrosis.

The relationship between the viral RNA level and upregulation of innate immunity in spleen of cattle persistently infected with bovine viral diarrhea virus

Persistent infection in cattle with bovine viral diarrhea virus (BVDV), which is classified in the genus Pestivirus, family Flaviviridae, may result in growth retardation and immunosuppression. In vitro infection with noncytopathogenic (ncp) BVDV has been shown to suppress interferon (IFN) responses, whereas ncpBVDV induces transient strong IFN responses in vivo following acute infection of naïve cattle. In this study, the innate immune response of the spleen, a crucial organ for immune system homeostasis, from PI cattle was analyzed. The transcription of five IFN- and apoptosis-related mRNAs (Mx1, iNOS, OAS-1, PKR, and TNF-alpha), which was upregulated in response to BVDV replication in our previous in vitro studies, and the level of viral RNA were quantified using real-time RT-PCR. Upregulation of Mx1, OAS-1, PKR, and TNF-alpha mRNA expression was detected in the spleens of PI cattle regardless of their age, and induction of apoptosis was also upregulated in the spleens of PI cattle compared with those of nonPI cattle. Although it cannot be excluded that the innate immune responses may be activated in response to any secondary infections in immunosuppressed cattle, the absence of any pathogenic microorganisms in the PI cattle and the statistically significant correlation between innate immune responses and the viral RNA level indicates that there may be a positive relationship between the increased level of viral RNA replication and upregulation of innate immunity in vivo.

Reactive nitrogen intermediates and monokines induce caspase-3 mediated macrophage apoptosis by anaerobically stressed Salmonella typhi

A successful pathogen manipulates its host for its own benefit. After ingestion, on reaching the intestine Salmonella encounters the resident tissue macrophages. Rather than being destroyed by these professional phagocytes after internalization, Salmonella survives intracellularly. Invasive Salmonella has been reported to induce apoptosis of macrophages as a part of its infection process, which may allow it to avoid detection by the innate immune system. However, the induction of apoptosis under different host environments, including the anaerobic stress encountered by the pathogen in the gut, remains to be examined. The present study is aimed at investigating the apoptotic potential of S. enterica serovar Typhi (S. typhi) grown under anaerobic conditions simulating the in vivo situation encountered by the pathogen. Apoptotic cell death was determined by assessment of nucleosomal DNA and flow cytometric analysis. Evaluation of the data revealed that anaerobically grown S. typhi could induce apoptosis in significantly more number of macrophages compared to the bacterial cells grown under aerobic conditions. A significantly enhanced generation of reactive nitrogen intermediates and caspase-3 activity during macrophage apoptosis induced by anaerobic S. typhi correlated with the increased generation of tumour necrosis factor-alpha, interleukin (IL)-1alpha and IL-6. The results indicate that reactive nitrogen intermediates and monokines induce caspase-3 mediated apoptosis of macrophages by S. typhi under anaerobic conditions. These findings may be relevant for clearer understanding of the Salmonella-macrophage interactions and may be of clinical importance in the development of preventive intervention against the infection.

Characterisation of bovine inducible nitric oxide synthase

Inducible nitric oxide (iNOS) is an enzyme that catalyzes the production of the reactive nitrogen intermediate nitric oxide (NO). NO is an important signalling molecule, released by numerous cells, that acts in many tissues to regulate a diverse range of physiological and biological processes, including neurotransmission, immune defence and the regulation of cell death (apoptosis). NO plays a major role in the killing of intracellular pathogens as part of the innate immune response. iNOS is known to be induced by a number of stimuli including cytokines as well as pathogens and their components. As yet, a full-length bovine iNOS sequence has only been predicted from the genome, although partial sequences from cDNA are available. Here, we have identified a 3471bp transcript for bovine iNOS, isolated from RNA from bovine alveolar macrophages stimulated with the intracellular pathogen Mycobacterium bovis. When translated this gives a protein of 1156 amino acids. Bovine iNOS shows a high degree of similarity to iNOS from other species, and also shares a common protein domain structure.

The double-stranded RNA-induced apoptosis pathway is involved in the cytopathogenicity of cytopathogenic Bovine viral diarrhea virus

Bovine viral diarrhea virus (BVDV), which is classified in the genus Pestivirus, family Flaviviridae, can be divided into two biotypes according to its ability to induce a cytopathic effect in tissue culture cells. The mechanisms through which cytopathogenic (cp) BVDV induces cell death and non-cytopathogenic (ncp) BVDV causes persistent infection without producing cell death remain unclear. Here, it was found that the overexpression of four apoptosis-related cellular mRNAs in cells infected with cpBVDV could also be caused by synthetic dsRNA. In fact, it was found that the amount of dsRNA produced by cpBVDV considerably exceeded the amount yielded by ncpBVDV. To evaluate the possible involvement of dsRNA in the induction of apoptosis, this study examined whether RNAi-mediated depletion of two dsRNA-reactive cellular factors, dsRNA-dependent protein kinase and 2′,5′-oligoadenylate synthetase 1, resulted in the prevention of cpBVDV-induced apoptosis. Although the induction of apoptosis was reduced after the suppression of either factor alone, the simultaneous silencing of both factors resulted in an almost complete inhibition of apoptosis without affecting viral titre. These results showed that dsRNA is the main trigger of apoptosis in cpBVDV-infected cells and that the cytopathogenicity of BVDV depends on the yield potential of dsRNA. In contrast, ncpBVDV yielded minimal levels of dsRNA, thereby establishing a persistent infection without inducing apoptosis. This report supports the significance of viral dsRNA as a trigger of innate immune responses.