Marek's disease virus down-regulates surface expression of MHC (B Complex) Class I (BF) glycoproteins during active but not latent infection of chicken cells

Immune escape of avian oncogenic Marek's disease herpesvirus and antagonistic host immune responses

Marek's disease virus (MDV) is a highly pathogenic and oncogenic alpha herpesvirus that causes Marek's disease (MD), which is one of the most important immunosuppressive and rapid-onset neoplastic diseases in poultry. The onset of MD lymphomas and other clinical diseases can be efficiently prevented by vaccination; these vaccines are heralded as the first demonstration of a successful vaccination strategy against a cancer. However, the persistent evolution of epidemic MDV strains towards greater virulence has recently resulted in frequent outbreaks of MD in vaccinated chicken flocks worldwide. Herein, we provide an overall review focusing on the discovery and identification of the strategies by which MDV evades host immunity and attacks the immune system. We have also highlighted the decrease in the immune efficacy of current MD vaccines. The prospects, strategies and new techniques for the development of efficient MD vaccines, together with the possibilities of antiviral therapy in MD, are also discussed.

The immune cell landscape and response of Marek's disease resistant and susceptible chickens infected with Marek's disease virus

Genetically resistant or susceptible chickens to Marek's disease (MD) have been widely used models to identify the molecular determinants of these phenotypes. However, these prior studies lacked the basic identification and understanding of immune cell types that could be translated toward improved MD control. To gain insights into specific immune cell types and their responses to Marek's disease virus (MDV) infection, we used single-cell RNA sequencing (scRNAseq) on splenic cells from MD resistant and susceptible birds. In total, 14,378 cells formed clusters that identified various immune cell types. Lymphocytes, specifically T cell subtypes, were the most abundant with significant proportional changes in some subtypes upon infection. The largest number of differentially expressed genes (DEG) response was seen in granulocytes, while macrophage DEGs differed in directionality by subtype and line. Among the most DEG in almost all immune cell types were granzyme and granulysin, both associated with cell-perforating processes. Protein interactive network analyses revealed multiple overlapping canonical pathways within both lymphoid and myeloid cell lineages. This initial estimation of the chicken immune cell type landscape and its accompanying response will greatly aid efforts in identifying specific cell types and improving our knowledge of host response to viral infection.

Evasion of the Cell-Mediated Immune Response by Alphaherpesviruses

Alphaherpesviruses cause various diseases and establish life-long latent infections in humans and animals. These viruses encode multiple viral proteins and miRNAs to evade the host immune response, including both innate and adaptive immunity. Alphaherpesviruses evolved highly advanced immune evasion strategies to be able to replicate efficiently in vivo and produce latent infections with recurrent outbreaks. This review describes the immune evasion strategies of alphaherpesviruses, especially against cytotoxic host immune responses. Considering these strategies, it is important to evaluate whether the immune evasion mechanisms in cell cultures are applicable to viral propagation and pathogenicity in vivo. This review focuses on cytotoxic T lymphocytes (CTLs), natural killer cells (NK cells), and natural killer T cells (NKT cells), which are representative immune cells that directly damage virus-infected cells. Since these immune cells recognize the ligands expressed on their target cells via specific activating and/or inhibitory receptors, alphaherpesviruses make several ligands that may be targets for immune evasion. In addition, alphaherpesviruses suppress the infiltration of CTLs by downregulating the expression of chemokines at infection sites in vivo. Elucidation of the alphaherpesvirus immune evasion mechanisms is essential for the development of new antiviral therapies and vaccines.

Revisiting cellular immune response to oncogenic Marek's disease virus: the rising of avian T-cell immunity

Marek's disease virus (MDV) is a highly oncogenic alphaherpesvirus that causes deadly T-cell lymphomas and serves as a natural virus-induced tumor model in chickens. Although Marek's disease (MD) is well controlled by current vaccines, the evolution of MDV field viruses towards increasing virulence is concerning as a better vaccine to combat very virulent plus MDV is still lacking. Our understanding of molecular and cellular immunity to MDV and its immunopathogenesis has significantly improved, but those findings about cellular immunity to MDV are largely out-of-date, hampering the development of more effective vaccines against MD. T-cell-mediated cellular immunity was thought to be of paramount importance against MDV. However, MDV also infects macrophages, B cells and T cells, leading to immunosuppression and T-cell lymphoma. Additionally, there is limited information about how uninfected immune cells respond to MDV infection or vaccination, specifically, the mechanisms by which T cells are activated and recognize MDV antigens and how the function and properties of activated T cells correlate with immune protection against MDV or MD tumor. The current review revisits the roles of each immune cell subset and its effector mechanisms in the host immune response to MDV infection or vaccination from the point of view of comparative immunology. We particularly emphasize areas of research requiring further investigation and provide useful information for rational design and development of novel MDV vaccines.

In ovo vaccination with herpesvirus of turkey enhances innate and cellular responses in meat-type chickens: Effect of vaccine dose and strain

In ovo vaccination with herpesvirus of turkey (HVT) or recombinant HVT (rHVT) is commonly used in meat-type chickens. Previous studies showed that in ovo vaccination with HVT enhances innate, cellular, and humoral immune responses in egg-type chicken embryos. This study evaluated if in ovo vaccination with HVT hastens immunocompetence of commercial meat-type chickens and optimized vaccination variables (dose and strain of HVT) to accelerate immunocompetence. A conventional HVT vaccine was given at recommended dose (RD), HVT-RD = 6080 plaque forming units (PFU), double-dose (2x), half-dose (1/2), or quarter-dose (1/4). Two rHVTs were given at RD: rHVT-A = 7380 PFU, rHVT-B = 8993 PFU. Most, if not all, treatments enhanced splenic lymphoproliferation with Concanavalin A and increased the percentage of granulocytes at day of age. Dose had an effect and HVT-RD was ideal. An increase of wing-web thickness after exposure to phytohemagglutinin-L was only detected after vaccination with HVT-RD. Furthermore, compared to sham-inoculated chickens, chickens in the HVT-RD had an increased percentage of CD3⁺ T cells and CD4⁺ T-helper cells, and increased expression of major histocompatibility complex (MHC)-II on most cell subsets (CD45⁺ cells, non-T leukocytes, T cells and the CD8⁺ and T cell receptor γδ T-cell subsets). Other treatments (HVT-1/2 and rHVT-B) share some of these features but differences were not as remarkable as in the HVT-RD group. Expression of MHC-I was reduced, compared to sham-inoculated chickens, in most of the cell phenotypes evaluated in the HVT-RD, HVT-2x and rHVT-A groups, while no effect was observed in other treatments. The effect of in ovo HVT on humoral immune responses (antibody responses to keyhole limpet hemocyanin and to a live infectious bronchitis/Newcastle disease vaccine) was minimal. Our study demonstrates in ovo vaccination with HVT in meat-type chickens can accelerate innate and adaptive immunity and we could optimize such effect by modifying the vaccine dose.

Ocular exposure to infectious laryngotracheitis virus alters leukocyte subsets in the head-associated lymphoid tissues and trachea of 6-week-old White Leghorn chickens

Infectious laryngotracheitis virus (ILTV), an alphaherpesvirus, causes acute respiratory disease primarily infecting the upper respiratory tract and conjunctiva. Administration of live attenuated ILTV vaccines via eye drop, drinking water, or by coarse spray elicits protective mucosal immunity in the head-associated lymphoid tissues (HALT), of which conjunctiva-associated lymphoid tissue (CALT) and the Harderian gland (HG) are important tissue components. The trachea, a non-lymphoid tissue, also receives significant influx of inflammatory cells that dictate the outcome of ILTV infection. The objective of this study was to evaluate leukocyte cellular and phenotypic changes in the CALT, HG and trachea following ocular infection with a virulent ILTV strain. At 1, 3, 5, 7 and 9 days post-infection, CALT, HG, and trachea of 6-week-old specific pathogen free (SPF) chickens ocularly-exposed to vehicle or virulent ILTV strain 63140 were dissociated, the cells enumerated and then phenotyped using flow cytometry. The CALT had the highest viral genomic load, which peaked on day 3. In ILTV-infected birds, the CALT had a decreased percentage of leukocytes. This was reflected by decreased numbers of MHCI⁺MHCII^-, MHCI⁺MHCII^low+, and CD4⁺ cells, while IgM⁺ and MHCI⁺MHCII^High+ expressing cell populations increased. In the HG, the most notable change in cells from ILTV-infected birds was a decrease in IgM expressing cells and histologically, an increase in Mott cells. In summary, an acute, ocular exposure to ILTV strain 63140 in young birds shifts subsets of lymphocyte populations in the CALT and HG with minimal impact on the trachea.

Highly virulent Marek's disease virus strains affect T lymphocyte function and viability of splenocytes in commercial meat-type chickens

In previous studies, we have demonstrated that very virulent plus Marek's disease viruses (vv+MDV) are highly immunosuppressive in commercial meat-type chickens. The specific objectives of this work were to evaluate if vv+MDV immunosuppression (MDV-IS) is induced by reduction of lymphocyte responsiveness and/or viability. Three experiments were conducted to (i) compare vv+MDV 686 with a partially attenuated 686-BAC; (ii) compare vv+MDV strains (648A and 686) with vMDV (GA) and vvMDV (Md5); and (iii) compare chickens vaccinated with Md5-BACΔMEQ and with CVI988 + HVT. In each experiment, spleens were collected at 28-30 days post infection and lymphocytes were isolated and investigated in three ways: their proliferative response to Concanavalin A (ConA) was analysed by MTT proliferation assay; cell death, and expression of CD45 and MHC-I was studied by flow cytometry; and MHC-IA and β-2 microglobulin (B2M) expression was evaluated by real time RT-PCR. Splenocytes of chickens inoculated with vv+MDV were severely impaired to proliferate when exposed to ConA. Furthermore, vv+MDV induced severe splenocyte death that did not occur after infection with v or vvMDV strains. Vaccination with CVI988 + HVT, and at less level with Md5-BACΔMEQ reduced these negative effects. This is in contrast to our previous results in which Md5-BACΔMEQ but not CVI988 + HVT protected against MDV-IS suggesting that although cell death and decrease lymphocyte function seem to be related to MDV virulence and certainly will be associated with immunosuppression, they might not fully explain the previously reported MDV-IS. RESEARCH HIGHLIGHTS vv+MDV induces extensive death in splenocytes in meat-type chickens 28-30 dpi. vv+MDV impairs lymphocyte function in meat-type chickens 28-30 dpi. Vaccination protects against splenocyte death and reduced lymphocyte function. Cell lysis and reduced lymphocyte function do not fully explain MDV-IS.

Influence of Marek's disease virus vaccines on chicken melanoma differentiation-associated gene 5-dependent-type I interferon signal transduction pathway with a highlight on their secondary impact on the immune responses post Newcastle disease virus vaccination

Marek's disease virus (MDV) leads to a lytic infection of B-lymphocytes in chickens, and also latently infects T-lymphocytes. Although Marek's disease vaccines have been widely in use, little is known about the innate immune response of this important livestock vaccine. In this study, we tested the effect of different commercially applied Marek's disease vaccines on the expression pattern of selected genes related to chicken interferon-alpha (chIFN-α) (melanoma differentiation associated gene 5 "MDA5″ dependent) signal transduction pathway. Both MDV serotype I (Rispens) and serotype III (Herpesvirus of turkey "HVT") vaccines could stimulate MDA5 dependent-type I interferon response as early as three days post vaccination in a dose-dependent manner. The stimulation continued up to 10 days in the instance of HVT vaccine and declined in the case of Rispens. Surprisingly, increasing the doses of the two vaccines led to dose-dependent down-regulation in the expression pattern of the investigated pathway, five and ten days post vaccination. Additionally, to shed the light on the consequent effect on the immune responses of the other viral vaccine, another experimental model based on Newcastle disease virus (NDV) vaccines was designed using HVT, HVT-VP2 and Rispens MDV vaccines. The three MDV vaccines were found to reduce chicken humoral immune response post NDV vaccination. However, only Rispens and HVT-VP2 had suppressive effects on the expression of MDA5-dependent-chIFN-α related cytokines. Consistent with this finding, the protection rate and NDV- humoral immune response post challenge with virulent NDV strain was lower in case of Rispens and HVT-VP2 vaccines.

Differentially expressed genes during spontaneous lytic switch of Marek's disease virus in lymphoblastoid cell lines determined by global gene expression profiling

Marek's disease virus (MDV), an alphaherpesvirus of poultry, causes Marek's disease and is characterized by visceral CD4+TCRαβ+ T-cell lymphomas in susceptible hosts. Immortal cell lines harbouring the viral genome have been generated from ex vivo cultures of MD tumours. As readily available sources of large numbers of cells, MDV-transformed lymphoblastoid cell lines (LCLs) are extremely valuable for studies of virus-host interaction. While the viral genome in most cells is held in a latent state, minor populations of cells display spontaneous reactivation identifiable by the expression of lytic viral genes. Spontaneous reactivation in these cells presents an opportunity to investigate the biological processes involved in the virus reactivation. For detailed characterization of the molecular events associated with reactivation, we used two lymphoblastoid cell lines derived from lymphomas induced by pRB1B-UL47eGFP, a recombinant MDV engineered to express enhanced green fluorescent protein (EGFP) fused with the UL47. We used fluorescence-activated cell sorting to purify the low-frequency EGFP-positive cells with a spontaneously activating viral genome from the majority EGFP-negative cells and analysed their gene expression profiles by RNA-seq using Illumina HiSeq2500. Ingenuity pathway analysis on more than 2000 differentially expressed genes between the lytically infected (EGFP-positive) and latently infected (EGFP-negative) cell populations identified the biological pathways involved in the reactivation. Virus-reactivating cells exhibited differential expression of a significant number of viral genes, with hierarchical differences in expression levels. Downregulation of a number of host genes including those directly involved in T-cell activation, such as CD3, CD28, ICOS and phospholipase C, was also noticed in the LCL undergoing lytic switch.

Marek's disease in chickens: a review with focus on immunology

Marek's disease (MD), caused by Marek's disease virus (MDV), is a commercially important neoplastic disease of poultry which is only controlled by mass vaccination. Importantly, vaccines that can provide sterile immunity and inhibit virus transmission are lacking; such that vaccines are only capable of preventing neuropathy, oncogenic disease and immunosuppression, but are unable to prevent MDV transmission or infection, leading to emergence of increasingly virulent pathotypes. Hence, to address these issues, developing more efficacious vaccines that induce sterile immunity have become one of the important research goals for avian immunologists today. MDV shares very close genomic functional and structural characteristics to most mammalian herpes viruses such as herpes simplex virus (HSV). MD also provides an excellent T cell lymphoma model for gaining insights into other herpesvirus-induced oncogenesis in mammals and birds. For these reasons, we need to develop an in-depth knowledge and understanding of the host-viral interaction and host immunity against MD. Similarly, the underlying genetic variation within different chicken lines has a major impact on the outcome of infection. In this review article, we aim to investigate the pathogenesis of MDV infection, host immunity to MD and discuss areas of research that need to be further explored.

Early infection with Marek's disease virus can jeopardize protection conferred by laryngotracheitis vaccines: a method to study MDV-induced immunosuppression

Marek's disease virus (MDV) is a herpesvirus that induces lymphomas and immunosuppression in chickens. MDV-induced immunosuppression (MDV-IS) is divided into two phases: early-MDV-IS occurring mainly in chickens lacking maternal antibodies (MAb) against MDV and associated with lymphoid organ atrophy; and late-MDV-IS occurring once MDV enters latency and during tumour development. Our objectives were to document the impact of late-MDV-IS on commercial poultry (meat-type chickens bearing MAb against MDV and that were vaccinated or unvaccinated against MD) and to optimize a model to study late-MDV-IS under laboratory conditions. The impact of late-MDV-IS was evaluated by assessing the effect of early infection (day of age) with a very virulent plus MDV (vv+MDV) on the efficacy of chicken-embryo-origin (CEO) infectious laryngotracheitis (ILT) virus vaccine against ILT challenge. The CEO ILT vaccine was administered in water at 14 days of age and ILT virus (ILTV) challenge was done intratracheally at 30 days of age. Development of ILT was monitored by daily evaluation of clinical signs, development of gross and histological lesions in trachea, and quantification of ILTV transcripts in trachea. Infection with vv+MDV strain 648A resulted in total abrogation of protection conferred by the CEO vaccine against ILTV challenge even in chickens vaccinated at 1 day of age with either HVT, HVT+SB-1, or CVI988. Chickens exposed to vv+MDV prior to vaccination with CEO ILTV vaccine had similar (P < 0.05) clinical scores, gross lesions, histopathologic lesion scores, and load of ILTV transcripts in trachea after ILTV challenge, as chickens that were not vaccinated with CEO ILTV vaccine.

Virus and host genomic, molecular, and cellular interactions during Marek's disease pathogenesis and oncogenesis

Marek's Disease Virus (MDV) is a chicken alphaherpesvirus that causes paralysis, chronic wasting, blindness, and fatal lymphoma development in infected, susceptible host birds. This disease and its protective vaccines are highly relevant research targets, given their enormous impact within the poultry industry. Further, Marek's disease (MD) serves as a valuable model for the investigation of oncogenic viruses and herpesvirus patterns of viral latency and persistence--as pertinent to human health as to poultry health. The objectives of this article are to review MDV interactions with its host from a variety of genomic, molecular, and cellular perspectives. In particular, we focus on cytogenetic studies, which precisely assess the physical status of the MDV genome in the context of the chicken host genome. Combined, the cytogenetic and genomic research indicates that MDV-host genome interactions, specifically integration of the virus into the host telomeres, is a key feature of the virus life cycle, contributing to the viral achievement of latency, transformation, and reactivation of lytic replication. We present a model that outlines the variety of virus-host interactions, at the multiple levels, and with regard to the disease states.

An MHC class I immune evasion gene of Marek׳s disease virus

Marek׳s disease virus (MDV) is a widespread α-herpesvirus of chickens that causes T cell tumors. Acute, but not latent, MDV infection has previously been shown to lead to downregulation of cell-surface MHC class I (Virology 282:198-205 (2001)), but the gene(s) involved have not been identified. Here we demonstrate that an MDV gene, MDV012, is capable of reducing surface expression of MHC class I on chicken cells. Co-expression of an MHC class I-binding peptide targeted to the endoplasmic reticulum (bypassing the requirement for the TAP peptide transporter) partially rescued MHC class I expression in the presence of MDV012, suggesting that MDV012 is a TAP-blocking MHC class I immune evasion protein. This is the first unique non-mammalian MHC class I immune evasion gene identified, and suggests that α-herpesviruses have conserved this function for at least 100 million years.

Expression analysis of programmed death ligand 2 in tumors caused by the avian oncovirus Marek's disease virus

PD-L2 is a ligand of the immunoinhibitory receptor PD-1. Here, we report functional and expression analyses of PD-L2 in tumor lesions and spleens from chickens infected with gallid herpesvirus 2 (GaHV-2, Marek's disease virus), which induces malignant lymphomas in chickens. We show that the expression of IFN-γ protein was decreased in PBMCs and splenocytes co-cultured with PD-L2-expressing cells and that the expression of PD-L2 mRNA was significantly higher in the spleens of infected chickens in the latent phase and in tumor lesions caused by GaHV-2. These results suggest that chicken PD-L2 has an immunoinhibitory function and is involved in the establishment of latency and tumor formation by GaHV-2.

Avian Immunosuppressive Diseases and Immunoevasion

Subclinical immunosuppression in chickens is an important but often underestimated factor in the subsequent development of clinical disease. Immunosuppression can be caused by pathogens such as chicken infectious anemia virus, infectious bursal disease virus, reovirus, and some retroviruses (e.g., reticuloendotheliosis virus). Mycotoxins and stress, often caused by poor management practices, can also cause immunosuppression. The effects on the innate and acquired immune responses and the mechanisms by which mycotoxins, stress and infectious agents cause immunosuppression are discussed. Immunoevasion is a common ploy by which viruses neutralize or evade immune responses. DNA viruses such as herpesvirus and poxvirus have multiple genes, some of them host-derived, which interfere with effective innate or acquired immune responses. RNA viruses may escape acquired humoral and cellular immune responses by mutations in protective antigenic epitopes (e.g., avian influenza viruses), while accessory non-structural proteins or multi-functional structural proteins interfere with the interferon system (e.g., Newcastle disease virus).

Sequence variations of the MHC class I gene exon 2 and exon 3 between infected and uninfected chickens challenged with Marek's disease virus

The major histocompatibility complex (MHC) among chickens has been well established as being associated with disease resistance and pathogens infection, but the genetic differences in MHC between chickens susceptible to certain infections and those chickens that remain uninfected have not been sufficiently determined. In this study, we sought the genetic basis that may underlie differences in susceptibility to infection among chickens by challenging four groups of broilers with Marek's disease virus (MDV). Over the course of the experiment, lesions began to appear between 21 and 35 days post challenge (dpc), and commercial broilers were not necessarily better than indigenous chickens in terms of disease resistance. The four groups showed neutral resistance to MDV infection validated by challenge results and evolutionary analysis of exons 2 and 3 of the MHC class I region. Several variable sites in exon 2 and exon 3 were exclusively appeared in infected chickens. Exon 3 was likely more crucial than exon 2 in disease resistance. Our observations offered a support for a potential association between promiscuous pathogens and conspicuous genetic diversity in the MHC class I region.

[Flow cytometry: is it a novel tool in microbiological diagnostics?]

Direct detection of pathogens is time- and labor-intensive. There is an increasing demand for new rapid microbiological testing methods, which would be faster and more sensitive than the conventional ones. Initially, automated methods were applied for the testing of bacteremia, urinary tract infections, characterization of antimicrobial susceptibility and quantitation of pathogen specific antibodies. Recently the nucleic acid-based detection methods have also become a routine. The molecular biological methods accelerate diagnosis, enhance specificity and provide an opportunity to identify pathogens with potential difficulties in culturing. However, they do not give any information about the immune status of the host. Yet it should also be borne in mind that detection of pathogen-specific nucleic acids is not equivalent to the presence of living microbes. The greatest advantage of FACS against these techniques is the capability to identify individual microbial cells as well. High speed FACS becomes a priority in the characterization of slow-growing microbes and identification of pathogens in mixed infections. Last but not least, it allows the monitoring of immune status and follow up of antimicrobial therapy.

Molecular characterization of immunoinhibitory factors PD-1/PD-L1 in chickens infected with Marek's disease virus

Background

An immunoinhibitory receptor, programmed death-1 (PD-1), and its ligand, programmed death-ligand 1 (PD-L1), are involved in immune evasion mechanisms for several pathogens causing chronic infections and for neoplastic diseases. However, little has been reported for the functions of these molecules in chickens. Thus, in this study, their expressions and roles were analyzed in chickens infected with Marek’s disease virus (MDV), which induces immunosuppression in infected chickens.

Results

A chicken T cell line, Lee1, which constitutively produces IFN-γ was co-cultured with DF-1 cells, which is a spontaneously immortalized chicken fibroblast cell line, transiently expressing PD-L1, and the IFN-γ expression level was analyzed in the cell line by real-time RT-PCR. The IFN-γ expression was significantly decreased in Lee1 cells co-cultured with DF-1 cells expressing PD-L1. The expression level of PD-1 was increased in chickens at the early cytolytic phase of the MDV infection, while the PD-L1 expression level was increased at the latent phase. In addition, the expression levels of PD-1 and PD-L1 were increased at tumor lesions found in MDV-challenged chickens. The expressions levels of PD-1 and PD-L1 were also increased in the spleens and tumors derived from MDV-infected chickens in the field.

Conclusions

We demonstrated that the chicken PD-1/PD-L1 pathway has immunoinhibitory functions, and PD-1 may be involved in MD pathogenesis at the early cytolytic phase of the MDV infection, whereas PD-L1 could contribute to the establishment and maintenance of MDV latency. We also observed the increased expressions of PD-1 and PD-L1 in tumors from MDV-infected chickens, suggesting that tumor cells transformed by MDV highly express PD-1 and PD-L1 and thereby could evade from immune responses of the host.

The capacity of UL49.5 proteins to inhibit TAP is widely distributed among members of the genus Varicellovirus

The lifelong infection by varicelloviruses is characterized by a fine balance between the host immune response and immune evasion strategies used by these viruses. Virus-derived peptides are presented to cytotoxic T lymphocytes by major histocompatibility complex (MHC) class I molecules. The transporter associated with antigen processing (TAP) transports the peptides from the cytosol into the endoplasmic reticulum, where the loading of MHC-I molecules occurs. The varicelloviruses bovine herpesvirus 1 (BoHV-1), pseudorabies virus, and equid herpesviruses 1 and 4 have been found to encode a UL49.5 protein that inhibits TAP-mediated peptide transport. To investigate to what extent UL49.5-mediated TAP inhibition is conserved within the family of Alphaherpesvirinae, the homologs of another five varicelloviruses, one mardivirus, and one iltovirus were studied. The UL49.5 proteins of BoHV-5, bubaline herpesvirus 1, cervid herpesvirus 1, and felid herpesvirus 1 were identified as potent TAP inhibitors. The varicella-zoster virus and simian varicellovirus UL49.5 proteins fail to block TAP; this is not due to the absence of viral cofactors that might assist in this process, since cells infected with these viruses did not show reduced TAP function either. The UL49.5 homologs of the mardivirus Marek's disease virus 1 and the iltovirus infectious laryngotracheitis virus did not block TAP, suggesting that the capacity to inhibit TAP via UL49.5 has been acquired by varicelloviruses only. A phylogenetic analysis of viruses that inhibit TAP through their UL49.5 proteins reveals an interesting hereditary pattern, pointing toward the presence of this capacity in defined clades within the genus Varicellovirus.

Down-regulation of MHC class I by the Marek's disease virus (MDV) UL49.5 gene product mildly affects virulence in a haplotype-specific fashion

Marek's disease is a devastating neoplastic disease of chickens caused by Marek's disease virus (MDV). MDV down-regulates surface expression of MHC class I molecules, although the mechanism has remained elusive. MDV harbors a UL49.5 homolog that has been shown to down-regulate MHC class I expression in other Varicelloviruses. Using in vitro assays, we showed that MDV pUL49.5 down-regulates MHC class I directly and identified its cytoplasmic tail as essential for this function. In vivo, viruses lacking the cytoplasmic tail of pUL49.5 showed no differences in MD pathogenesis compared to revertant viruses in highly susceptible chickens of the B(19)B(19) MHC class I haplotype, while there was a mild reduction in pathogenic potential of the deletion viruses in chickens more resistant to MD pathogenesis (MHC:B(21)B(21)). We concluded that the pathogenic effect of MHC class I down-regulation mediated by pUL49.5 is small because virus immune evasion possibly requires more than one viral protein.

Equus caballus major histocompatibility complex class I is an entry receptor for equine herpesvirus type 1

In this study, Equus caballus major histocompatibility complex class I (MHC-I) was identified as a cellular entry receptor for the alphaherpesvirus equine herpesvirus type 1 (EHV-1). This novel EHV-1 receptor was discovered using a cDNA library from equine macrophages. cDNAs from this EHV-1-susceptible cell type were inserted into EHV-1-resistant B78H1 murine melanoma cells, these cells were infected with an EHV-1 lacZ reporter virus, and cells that supported virus infection were identified by X-Gal (5-bromo-4-chloro-3-indolyl-beta-d-galactopyranoside) staining. Positive cells were subjected to several rounds of purification to obtain homogeneous cell populations that were shown to be uniformly infected with EHV-1. cDNAs from these cell populations were amplified by PCR and then sequenced. The sequence data revealed that the EHV-1-susceptible cells had acquired an E. caballus MHC-I cDNA. Cell surface expression of this receptor was verified by confocal immunofluorescence microscopy. The MHC-I cDNA was cloned into a mammalian expression vector, and stable B78H1 cell lines were generated that express this receptor. These cell lines were susceptible to EHV-1 infection while the parental B78H1 cells remained resistant to infection. In addition, EHV-1 infection of the B78H1 MHC-I-expressing cell lines was inhibited in a dose-dependent manner by an anti-MHC-I antibody.

Marek's disease virus influences the expression of genes associated with IFN-gamma-inducible MHC class II expression

Chickens infected with Marek's disease virus (MDV) become lifelong carriers regardless of their susceptibility to clinical disease. Therefore various viral immune-evasive mechanisms must play a role in MDV-host interactions. MDV has previously been shown to influence the expression of major histocompatibility complex (MHC) class II molecules. However, little is known about the underlying mechanisms of this phenomenon. In the present study, we studied the effect of MDV infection on the expression of several genes associated with IFN-gamma-inducible MHC class II expression at 4, 7, 14, and 21 days post-infection (dpi). There was a significant (p Collapse

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MESH Headings

• Animals
• Chickens/virology
• Gene Expression Profiling
• Genes, MHC Class II
• Immunologic Surveillance/genetics
• Interferon Regulatory Factor-1/genetics
• Interferon-gamma/immunology
• Mardivirus/immunology
• Marek Disease/genetics
• Marek Disease/immunology
• Nuclear Proteins/genetics
• Poultry Diseases/genetics
• Poultry Diseases/immunology
• Receptors, Interferon/genetics
• STAT1 Transcription Factor/metabolism
• Signal Transduction
• Time Factors
• Trans-Activators/genetics
• Interferon gamma Receptor

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Affiliation(s)

Niroshan Thanthrige-Don Department of Pathobiology, University of Guelph, Guelph, Ontario, Canada
Leah R Read
Mohamed Faizal Abdul-Careem
Hakimeh Mohammadi
Amirul I Mallick
Shayan Sharif

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Protection of chickens, with or without maternal antibodies, against IBDV infection by a recombinant IBDV-VP2 protein

The use of avian herpesviruses (Marek's disease virus, MDV) as vectors to express the capsid protein of infectious bursal disease virus (IBDV) was well established, and its protection against IBDV challenge has been evaluated previously. However, there is little data about rMDV1 expressing the VP2 protein of IBDV protecting SPF and commercial chickens against virulent IBDV (vIBDV) challenge. In this study, we constructed a stable rMDV1 expressing the VP2 protein of IBDV by inserting the coding sequence within the US10 gene of MDVl by homologous recombination and designated this as rMDVl-US10L, and evaluated effectiveness of the recombinant VP2 protein with SPF chickens and commercial chickens with maternal antibodies in vIBDV challenge. The results can be summarized as follows: (1) We constructed a rMDV1 expressing IBDV-VP2 under the control of the MDV1 glycoprotein B (gB) promoter [rMDV1-US10L]. (2) rMDV-VP2 protein was readily expressed and induced 53% protection against a vIBDV challenge in SPF chickens with 10(3)PFU/chicken, whereas 10(4)PFU induced 73% protection. (3) Vaccination of commercial chickens having maternal antibodies to rMDV1-VP2 induced 87% protection in vIBDV challenge, which was similar to results using the live vaccine, BJ87 IBDV strain, in commercial chickens. These results demonstrate that the VP2 antigen expressed in the MDV vector was an effective and stable vaccine in correlation with the vaccine efficacy against lethal IBDV challenge, and can provide a better protective effect that is likely to persist for the life of the chickens.

Cloning and functional characterization of chicken stem cell antigen 2

Stem cell antigen 2 (SCA2) is a Ly6 family member whose function is largely unknown. To characterize biological properties and tissue distribution of chicken SCA2, SCA2 was expressed in E. coli, purified, and a polyclonal antibody developed. Utilizing the polyclonal antibody, SCA2 is a 13 kDa cell surface protein anchored by a glycosyl-phosphatidylinositol (GPI) moiety. SCA2 is expressed in connective tissues of thymus and bursa based on immunohistochemistry, immunoprecipitation, and western blots. In bursal follicles, SCA2 is specifically expressed on the cortical-medullary epithelial cells (CMEC) surrounded by MHC class II presenting cells. Expression profiles of bursal cells induced by contact with SCA2-expressing cells shows down-regulation of numerous genes including CD79B, B cell linker (BLNK), spleen tyrosine kinase (SYK), and gamma 2-phospholipase C (PLCG2) that are involved in the B cell receptor (BCR) and immune response signaling pathways. These results suggest chicken SCA2 plays a role in regulating B lymphocytes.

Host responses in the bursa of Fabricius of chickens infected with virulent Marek's disease virus

The bursa of Fabricius serves as an important tissue in the process of Marek's disease virus (MDV) pathogenesis, since B cells of the bursa harbor the cytolytic phase of MDV replication cycle. In the present study, host responses associated with MDV infection in the bursa of Fabricius of chickens were investigated. The expression of MDV phosphoprotein (pp)38 antigen, MDV glycoprotein (gB) and MDV viral interleukin (vIL)-8 transcripts was at the highest at 4 days post-infection (d.p.i.) and then showed a declining trend. On the contrary, the expression of meq (MDV EcoRI Q) gene as well as the viral genome load increased gradually until day 14 post-infection. The changes in viral parameters were associated with significantly higher infiltration of macrophages and T cell subsets, particularly CD4+ T cells into the bursa of Fabricius. Of the genes examined, the expression of interferon (IFN)-alpha, IFN-gamma genes and inducible nitric oxide synthase (iNOS) was significantly up-regulated in response to MDV infection in the bursa of Fabricius. The results suggest a role for these cells and cytokines in MDV-induced responses in the bursa of Fabricius.

Expression of cytotoxicity-associated genes in Marek's disease virus-infected chickens

Cytotoxic host responses to Marek's disease virus (MDV) have been attributed to both natural killer (NK) cells and cytotoxic T lymphocytes (CTLs). However, the mechanisms of cell lysis initiated by these cytotoxic responses during MDV infection are not well defined. Therefore, the current study was aimed at elucidating the molecular mechanisms of host cytotoxic responses to MDV infection by investigating the expression of genes in the cell lysis pathway involving granzyme A. Genes encoding cytolytic proteins, NK lysin, and granzyme A were upregulated during early stages of infection, whereas the genes encoding major histocompatibility complex (MHC) class I and the DNA repair and apoptosis protein, poly(ADP-ribose) polymerase (PARP), were downregulated. These findings shed more light on the mechanisms of host response to MDV infection in chickens.

Host responses are induced in feathers of chickens infected with Marek's disease virus

Control measures are ineffective in curtailing Marek's disease virus (MDV) infection and replication in the feather follicle epithelium (FFE). Therefore, vaccinated birds which subsequently become infected with MDV, shed the virulent virus although they remain protected against disease. The present study investigated host responses generated against MDV infection in the feather. We observed that in parallel with an increase in viral genome load and viral replication in the feather, there was a gradual but progressive increase in infiltration of CD4+ and CD8+ T cells into the feather pulp of MDV-infected chickens, starting on day 4 and peaking by day 10 post-infection. Concomitant with infiltration of T cells, the expression of interleukin (IL)-18, IL-6, interferon (IFN)-gamma and major histocompatibility complex class I genes was significantly enhanced in the feather pulp of MDV-infected chickens. The finding that host responses are generated in the feather may be exploited for developing strategies to control MDV infection in the FFE, thus preventing horizontal virus transmission.

Transcriptional Analysis of Host Responses to Marek's Disease Viral Infection

This study was aimed at investigating the genes that control host responses to Marek's disease virus (MDV). Spleen tissues from infected and age-matched uninfected control chickens were examined 4, 7, 14, and 21 d postinfection for gene expression differences, using both microarray and quantitative real-time polymerase chain reaction (PCR) methodologies. Up to 51% of genes assayed during microarray analysis showed a significant change (p < or = 0.05) in expression after MDV infection, of which cell surface molecules, transcription and signal transduction molecules, housekeeping and metabolism mediators, and cytokines and cytokine receptors were most commonly differentially expressed. Setting a fold change threshold (> or =2), 14 of 84 genes showed differential expression over time. In addition, some genes showed differential expression at more than one time point. For example, the granzyme-A homolog gene remained upregulated in infected chickens, with fold changes of 7.98, 13.91, and 9.07 at 7, 14, and 21 d postinfection, respectively. Other genes that were differentially expressed at more than one time point were invariant chain, IgM, and CD3. Quantitative real-time PCR analysis was used to validate microarray results for a subset of genes showing a :2-fold change in expression. Expression of all but one gene (CD28) was confirmed. Identification of genetic mechanisms initiated by in vivo infection with MDV expands the current understanding of immune response to the virus in addition to host response elements associated with viral pathogenesis.

Marek's disease virus up-regulates major histocompatibility complex class II cell surface expression in infected cells

Many herpesviruses modulate major histocompatibility complex (MHC) expression on the cell surface as an immune evasion mechanism. We report here that Marek's disease virus (MDV), a lymphotrophic avian alphaherpesvirus, up-regulates MHC class II cell surface expression in infected cells, contrary to all other herpesviruses examined to date. This MDV-induced class II up-regulation was detected both in vitro and in vivo. This effect was not solely an indirect effect of interferon, which is a highly potent natural inducer of MHC class II expression, since MHC class II up-regulation in cultured primary fibroblast cells was confined to the infected cells only. MHC class II up-regulation was also observed in infected cells of the bursa of Fabricius during the lytic phase of MDV infection in birds and upon reactivation of MDV from latency in an MDV-transformed cell line. As MDV is a strictly cell-associated virus and requires activated T cells for its life cycle, this up-regulation of MHC class II in infected cells may contribute to virus spread within the infected host by increasing the chance of contact between productively infected cells and susceptible activated T cells.

Marek's disease virus: from miasma to model

Marek's disease virus (MDV) is an oncogenic herpesvirus that causes various clinical syndromes in its natural host, the chicken. MDV has long been of interest as a model organism, particularly with respect to the pathogenesis and immune control of virus-induced lymphoma in an easily accessible small-animal system. Recent advances in MDV genetics and the determination of the chicken genome sequence, aided by functional genomics, have begun to dramatically increase our understanding not only of lytic MDV replication, but also of the factors and mechanisms leading to latency and tumour formation. This new information is helping to elucidate cellular signalling pathways that have undergone convergent evolution and are perturbed by different viruses, and emphasizes the value of MDV as a comparative biomedical model. Furthermore, the door is now open for rational and efficient engineering of new vaccines against one of the most important and widespread infectious diseases in chickens.

Immune escape of equine herpesvirus 1 and other herpesviruses of veterinary importance

Equine herpesvirus (EHV)-1 is a pathogen of horses, well known for its ability to induce abortion and nervous system disorders. Clinical signs may occur despite the presence of a virus-specific immune response in the horse. The current review will summarize the research, on how, EHV-1-infected cells can hide from recognition by the immune system. Research findings on immune evasion of EHV-1 will be compared with those of other herpesviruses of veterinary importance.

Viral interference with MHC class I antigen presentation pathway: the battle continues

CD8+ cytotoxic T lymphocytes (CTLs) play a critical role in the defense against viral infections. In general, CD8+ CTLs recognize antigenic peptides in the context of the major histocompatibility complex (MHC) class I molecule. The MHC class I molecules are expressed on almost all the nucleated cells in the body. The trimolecular complex consisting of the class I heavy chain, beta2-microglobulin and the peptide are generated by the MHC class I antigen presentation pathway. This pathway is designed to sample the intracellular milieu and present the information to the CTLs trafficking the area. This rigorous sampling of intracellular environment enables the CTLs to quickly identify and eliminate the cells that synthesize non-self proteins as a result of a viral infection. Many viruses, including several viruses of veterinary importance, have evolved astounding strategies to interfere with the MHC class I antigen presentation pathway, as a means of evading the CTL response of the host. This review focuses on the diverse mechanisms of viral evasion of the MHC class I antigen presentation pathway with particular emphasis on viruses of veterinary importance.

Study of host-pathogen interactions to identify sustainable vaccine strategies to Marek's disease

Marek's disease virus is a highly cell-associated, lymphotropic alpha-herpesvirus that causes paralysis and neoplastic disease in chickens. The disease has been contained by vaccination with attenuated viruses and provides the first evidence for a malignant cancer being controlled by an antiviral vaccine. Marek's disease pathogenesis is complex, involving cytolytic and latent infection of lymphoid cells and oncogenic transformation of CD4+ T cells in susceptible chickens. Innate and adaptive immune responses develop in response to infection, but infection of lymphocytes results in immunosuppressive effects. The remarkable ability of MDV to escape immune responses by interacting with, and down-regulating, some key aspects of the immune system will be discussed in the context of genetic resistance. Resistance conferred by vaccination and the implications of targeting replicative stages of the virus will also be examined.

Non-major histocompatibility complex alloantigen genes affecting immunity

An alloantigen is a genetically determined cell-surface molecule detected by specific antisera. An identifying letter has been assigned to each genetic locus responsible for the 12 distinct families of alloantigens: A, B, C, D, E, H, I, J, K, L, P, and R. The genes of each system segregate independently of the other systems, except that the A and E are very closely linked (0.5 centimorgans). Selection experiments over numerous generations have revealed distinct changes in gene frequency of the A-E alloantigens, suggesting immune responses associated with susceptibility to coccidiosis, response to immunizations with SRBC, and selection for size of the bursa of Fabricius. Immune response effects of the C system of alloantigen genes are indicated by distinct gene frequency changes following selection for response to SRBC, selection for size of bursa of Fabricius, and macrophage nitrite production after lipopolysaccharide (LPS) stimulation. Immune response effects of the D system of antigens are indicated by data from genetic selection for response to immunization with SRBC, selection for bursa size, and macrophage nitrite and cytokine interleukin (IL)-6 production following LPS stimulation. Immune response effects of the I system genes are indicated by distinct gene frequency changes in lines selected for bursa size and within family comparisons for macrophage nitrite and cytokine IL-6 production following LPS stimulation. Effects of the L system, consisting of only 2 alleles, are indicated by the gene frequency changes following selection for bursa size, direct comparison of genotypes within families for monocyte phagocytosis, susceptibility to coccidiosis, outcome of Rous sarcomas, and immune responses to SRBC and Brucella abortus. Genotypes of the P alloantigen system were directly compared within families of fully pedigreed chicks with significant differences for monocyte phagocytosis. An experimental procedure for simultaneously testing for immune responses of genotypes of 9 of the alloantigen systems (A, B, C, D, E, H, I, L, and P) has been established by producing test progeny from a single cross of parent lines segregating for genes of each of the systems.

A Comprehensive Screen for Chicken Proteins that Interact with Proteins Unique to Virulent Strains of Marek's Disease Virus

Genetic resistance to Marek's disease (MD) has been proposed as a method to augment current vaccinal control of MD. Although it is possible to identify QTL and candidate genes that are associated with MD resistance, it is necessary to integrate functional screens with linkage analysis to confirm the identity of true MD resistance genes. To help achieve this objective, a comprehensive 2-hybrid screen was conducted using genes unique to virulent Marek's disease virus (MDV) strains. Potential MDV-host protein interactions were tested by an in vitro binding assay to confirm the initial two-hybrid results. As a result, 7 new MDV-chicken protein interactions were identified and included the chicken proteins MHC class II beta (BLB) and invariant (Ii) chain (CD74), growth-related translationally controlled tumor protein (TPT1), complement component Clq-binding protein (C1QBP), retinoblastoma-binding protein 4 (RBBP4), and alpha-enolase (ENO1). Mapping of the encoding chicken genes suggests that BLB, the gene for MHC class II beta chain, is a positional candidate gene. In addition, the known functions of the chicken proteins suggest mechanisms that MDV might use to evade the chicken immune system and alter host gene regulation. Taken together, our results indicate that integrated genomic methods provide a powerful strategy to gain insights on complex biological processes and yield a manageable number of genes and pathways for further characterization.

Herpesvirus of turkeys: microarray analysis of host gene responses to infection

Herpesvirus of turkeys (HVT) provides an economically important live vaccine for prevention of Marek's disease (MD) of chickens. MD, characterized by both immunosuppression and T-cell lymphoma, is caused by another herpesvirus termed Marek's disease virus (MDV). Microarrays were used to investigate the response of chicken embryonic fibroblasts (CEF) to infection with HVT. Genes responding to HVT infection include several induced by interferon along with others modulating signal transduction, transcription, scaffolding proteins, and the cytoskeleton. Results are compared with earlier studies examining the responses of CEF cells to infection with MDV.

Biocharacteristics shared by highly protective vaccines against Marek's disease

Attenuated serotype 1 Marek's disease virus strains vary widely in their protection properties. This study was conducted to elucidate which biocharacteristics of serotype 1 MDV strains are related with protection. Three pairs of vaccines, each one including a higher protective (HP) vaccine and a lower protective (LP) vaccine originating from the same MDV strain, were studied. Two other highly protective vaccines (RM1 and CVI988/BP5) were also included in the study. Comparison within pairs of vaccines showed that marked differences existed between the HP and the LP vaccines. Compared with LP vaccines, HP vaccines replicated better in vivo. Also, they induced a significant expansion of total T cells and of the helper and cytotoxic T cell lineages (CD45(+)CD3(+), CD4(+)CD8(-), CD4(-)CD8(+)) as well as a marked increase in the expression of the antigens of MhcI and MhcII on T cells. Thus, our results show that in vivo replication and early stimulation of the T-cell lineage are two characteristics shared by HP vaccines. However, comparison among the four HP vaccines that provided protection equal to that of CVI988 (RM1, CVI988/BP5, CVI988 and 648A80) revealed variability, especially regarding in vivo replication. Strains RM1 and CVI988/BP5 showed much stronger replication in vivo than the other two vaccine strains (CVI988 and 648A80). Thus, no single set of characteristics could be used to identify the most protective Marek's disease vaccines, implying, perhaps, that multiple mechanisms may be involved.

Down-regulation of surface major histocompatibility complex class I by guinea pig cytomegalovirus

Live attenuated strains of human cytomegalovirus are under development as vaccines to prevent birth defects resulting from congenital infections. These strains encode four proteins that inhibit surface expression of MHC class I, presumably to evade cytotoxic T-cell recognition and, perhaps, attenuate induction of immunity. To initiate studies of the role of class I down-regulation on congenital infection and vaccine efficacy, the ability of guinea pig cytomegalovirus to down-regulate class I was examined. Surface class I was specifically down-regulated on infected cells up to 8-fold. Sensitivity to UV irradiation and insensitivity to a viral DNA synthesis inhibitor revealed that immediate early or early viral gene(s) are responsible. Identification of these genes will permit future experiments to evaluate the role of class I down-regulation in congenital cytomegalovirus disease and its impact on vaccine efficacy. These findings should be pertinent to understanding human cytomegalovirus disease and may help guide the design of candidate vaccines.