Influence of genetic resistance of the chicken and virulence of Marek's disease virus (MDV) on nitric oxide responses after MDV infection

The pUL51 Tegument Protein Is Essential for Marek's Disease Virus Growth In Vitro and Bears a Function That Is Critical for Pathogenesis In Vivo

pUL51 is a minor tegument protein important for viral assembly and cell-to-cell spread (CCS) but dispensable for replication in cell culture of all Herpesviruses for which its role has been investigated. Here, we show that pUL51 is essential for the growth of Marek's disease virus, an oncogenic alphaherpesvirus of chickens that is strictly cell-associated in cell culture. MDV pUL51 localized to the Golgi apparatus of infected primary skin fibroblasts, as described for other Herpesviruses. However, the protein was also observed at the surface of lipid droplets in infected chicken keratinocytes, hinting at a possible role of this compartment for viral assembly in the unique cell type involved in MDV shedding in vivo. Deletion of the C-terminal half of pUL51 or fusion of GFP to either the N- or C-terminus were sufficient to disable the protein's essential function(s). However, a virus with a TAP domain fused at the C-terminus of pUL51 was capable of replication in cell culture, albeit with viral spread reduced by 35% and no localization to lipid droplets. In vivo, we observed that although the replication of this virus was moderately impacted, its pathogenesis was strongly impaired. This study describes for the first time the essential role of pUL51 in the biology of a herpesvirus, its association to lipid droplets in a relevant cell type, and its unsuspected role in the pathogenesis of a herpesvirus in its natural host. IMPORTANCE Viruses usually spread from cell to cell through two mechanisms: cell-released virus and/or cell-to-cell spread (CCS). The molecular determinants of CCS and their importance in the biology of viruses during infection of their natural host are unclear. Marek's disease virus (MDV) is a deadly and highly contagious herpesvirus of chickens that produces no cell-free particles in vitro, and therefore, spreads only through CCS in cell culture. Here, we show that viral protein pUL51, an important factor for CCS of Herpesviruses, is essential for MDV growth in vitro. We demonstrate that the fusion of a large tag at the C-terminus of the protein is sufficient to moderately impair viral replication in vivo and almost completely abolish pathogenesis while only slightly reducing viral growth in vitro. This study thus uncovers a role for pUL51 associated with virulence, linked to its C-terminal half, and possibly independent of its essential functions in CCS.

The alphaherpesvirus conserved pUS10 is important for natural infection and its expression is regulated by the conserved Herpesviridae protein kinase (CHPK)

Conserved Herpesviridae protein kinases (CHPK) are conserved among all members of the Herpesviridae. Herpesviruses lacking CHPK propagate in cell culture at varying degrees, depending on the virus and cell culture system. CHPK is dispensable for Marek's disease herpesvirus (MDV) replication in cell culture and experimental infection in chickens; however, CHPK-particularly its kinase activity-is essential for horizontal transmission in chickens, also known as natural infection. To address the importance of CHPK during natural infection in chickens, we used liquid chromatography-tandem mass spectrometry (LC-MS/MS) based proteomics of samples collected from live chickens. Comparing modification of viral proteins in feather follicle epithelial (FFE) cells infected with wildtype or a CHPK-null virus, we identified the US10 protein (pUS10) as a potential target for CHPK in vivo. When expression of pUS10 was evaluated in cell culture and in FFE skin cells during in vivo infection, pUS10 was severely reduced or abrogated in cells infected with CHPK mutant or CHPK-null viruses, respectively, indicating a potential role for pUS10 in transmission. To test this hypothesis, US10 was deleted from the MDV genome, and the reconstituted virus was tested for replication, horizontal transmission, and disease induction. Our results showed that removal of US10 had no effect on the ability of MDV to transmit in experimentally infected chickens, but disease induction in naturally infected chickens was significantly reduced. These results show CHPK is necessary for pUS10 expression both in cell culture and in the host, and pUS10 is important for disease induction during natural infection.

The sulphated polysaccharides extract ulvans from Ulva armoricana limits Marek's disease virus dissemination in vitro and promotes viral reactivation in lymphoid cells

Background

Marek’s disease (MD) is a highly contagious lymphoproliferative disease of chickens caused by an alphaherpesvirus, Marek’s disease virus (MDV). MD is presently controlled by systematic vaccination of animals, which protects efficiently against the development of clinical disease. However, MDV vaccines do not prevent the multiplication and spread of MDV field strains and may favor the emergence of strains with increased virulence. Therefore, MDV persists to be a major problem for the poultry industry and the development of new alternative strategies to control MDV is needed. Seaweed extracts have previously been shown to exert immunomodulatory and antiviral activities, especially against herpesviruses. The objective of the present study was to explore the effect of Ulva armoricana extracts on MDV infection in vitro.

Results

We could demonstrate that the ulvan extract as well as its vitamin-enriched formulation reduce the viral load by about 80% at 24 h post-infection in infected chicken fibroblasts at concentrations that are innocuous for the cells. We also observed a substantial decrease in MDV plaque size suggesting that ulvans impede MDV cell-to-cell spread in vitro. Moreover, we showed that ulvan extract could promote MDV reactivation in lymphoid cells.

Conclusions

Our data provide the first evidence that the use of the ulvan extract could be a good alternative to limit MDV infection in poultry.

The Conserved Herpesviridae Protein Kinase (CHPK) of Gallid alphaherpesvirus 3 (GaHV3) Is Required for Horizontal Spread and Natural Infection in Chickens

We have formerly identified the conserved herpesvirus protein kinase (CHPK) as essential for horizontal transmission of Marek’s disease virus (MDV). Thus far, it has been confirmed that the mutation of the invariant lysine (K) of CHPKs abrogates kinase activity and that CHPK activity is required for MDV horizontal transmission. Since CHPK is conserved among all members of the Herpesviridae, we hypothesized that CHPK, and specifically its kinase activity, is important for the horizontal transmission of other herpesviruses. To test this hypothesis, we utilized our experimental and natural infection model in chickens with MD vaccine strain 301B/1 of Gallid alphaherpesvirus 3 (GaHV3). First, we mutated the invariant lysine (K) 157 of 301B/1 CHPK to alanine (A) and determined whether it was required for horizontal transmission. To confirm the requirement of 301B/1 CHPK activity for transmission, a rescued virus was generated in which the A157 was changed back to a K (A157K). Despite both the CHPK mutant (K157A) and rescuant (A157K) viruses having replication defects in vivo, only the CHPK mutant (K157A) was unable to spread to contact chickens, while both wild-type and rescuant (A157K) viruses transmitted efficiently, confirming the importance of CHPK activity for horizontal spread. The data confirm that CHPK is required for GaHV3 transmission and suggest that the requirement of avian CHPKs for natural infection is conserved.

The Requirement of Glycoprotein C for Interindividual Spread Is Functionally Conserved within the Alphaherpesvirus Genus (Mardivirus), but Not the Host (Gallid)

Marek’s disease (MD) in chickens is caused by Gallid alphaherpesvirus 2, better known as MD herpesvirus (MDV). Current vaccines do not block interindividual spread from chicken-to-chicken, therefore, understanding MDV interindividual spread provides important information for the development of potential therapies to protect against MD, while also providing a natural host to study herpesvirus dissemination. It has long been thought that glycoprotein C (gC) of alphaherpesviruses evolved with their host based on their ability to bind and inhibit complement in a species-selective manner. Here, we tested the functional importance of gC during interindividual spread and host specificity using the natural model system of MDV in chickens through classical compensation experiments. By exchanging MDV gC with another chicken alphaherpesvirus (Gallid alphaherpesvirus 1 or infectious laryngotracheitis virus; ILTV) gC, we determined that ILTV gC could not compensate for MDV gC during interindividual spread. In contrast, exchanging turkey herpesvirus (Meleagrid alphaherpesvirus 1 or HVT) gC could compensate for chicken MDV gC. Both ILTV and MDV are Gallid alphaherpesviruses; however, ILTV is a member of the Iltovirus genus, while MDV is classified as a Mardivirus along with HVT. These results suggest that gC is functionally conserved based on the virus genera (Mardivirus vs. Iltovirus) and not the host (Gallid vs. Meleagrid).

The requirement of glycoprotein C (gC) for interindividual spread is a conserved function of gC for avian herpesviruses

We have formerly shown that glycoprotein C (gC) of Gallid alphaherpesvirus 2, better known as Marek’s disease (MD) alphaherpesvirus (MDV), is required for interindividual spread in chickens. Since gC is conserved within the Alphaherpesvirinae subfamily, we hypothesized gC was important for interindividual spread of other alphaherpesviruses. To test this hypothesis, we first generated a fluorescent protein tagged clone of Gallid alphaherpesvirus 3 MD vaccine strain 301B/1 to track virus replication in cell culture and chickens using fluorescent microscopy. Following validation of this system, we removed the open reading frame of 301B/1 gC from the genome and determined whether it was required for interindividual spread using experimental and natural infection studies. Interindividual spread of MD vaccine 301B/1 was abrogated by removal of 301B/1 gC. Rescuent virus in which 301B/1 gC was inserted back into the genome efficiently spread among chickens. To further study the conserved function of gC, we replaced 301B/1 gC with MDV gC and this virus also efficiently spread in chickens. These data suggest the essential function of alphaherpesvirus gC proteins is conserved and can be exploited during the generation of future vaccines against MD that affects the poultry industry worldwide.

Marek's Disease Virus Requires Both Copies of the Inverted Repeat Regions for Efficient In Vivo Replication and Pathogenesis

Marek's disease virus (MDV) is an oncogenic alphaherpesvirus of chickens. The MDV genome consists of two unique regions that are both flanked by inverted repeat regions. These repeats harbor several genes involved in virus replication and pathogenesis, but it remains unclear why MDV and other herpesviruses harbor these large sequence duplications. In this study, we set to determine if both copies of these repeat regions are required for MDV replication and pathogenesis. Our results demonstrate that MDV mutants lacking the entire internal repeat region (ΔIR_LS) efficiently replicate and spread from cell-to-cell in vitro However, ΔIR_LS replication was severely impaired in infected chickens and the virus caused significantly less frequent disease and tumors compared to the controls. In addition, we also generated recombinant viruses that harbor a deletion of most of the internal repeat region, leaving only short terminal sequences behind (ΔIR_LS-HR). These remaining homologous sequences facilitated rapid restoration of the deleted repeat region, resulting in a virus that caused disease and tumors comparable to the wild type. Therefore, ΔIR_LS-HR represents an excellent platform for rapid genetic manipulation of the virus genome in the repeat regions. Taken together, our study demonstrates that MDV requires both copies of the repeats for efficient replication and pathogenesis in its natural host.IMPORTANCE Marek's disease virus (MDV) is a highly oncogenic alphaherpesvirus that infects chickens and causes losses in the poultry industry of up to $2 billion per year. The virus is also widely used as a model to study alphaherpesvirus pathogenesis and virus-induced tumor development in a natural host. MDV and most other herpesviruses harbor direct or inverted repeats regions in their genome. However, the role of these sequence duplications in MDV remains elusive and has never been investigated in a natural virus-host model for any herpesvirus. Here, we demonstrate that both copies of the repeats are needed for efficient MDV replication and pathogenesis in vivo, while replication was not affected in cell culture. With this, we further dissect herpesvirus genome biology and the role of repeat regions in Marek's disease virus replication and pathogenesis.

The Tegument Protein pUL47 of Marek's Disease Virus Is Necessary for Horizontal Transmission and Is Important for Expression of Glycoprotein gC

Viral tropism and transmission of herpesviruses are best studied in their natural host for maximal biological relevance. In the case of alphaherpesviruses, few reports have focused on those aspects, primarily because of the few animal models available as natural hosts that are compatible with such studies. Here, using Marek's disease virus (MDV), a highly contagious and deadly alphaherpesvirus of chickens, we analyze the role of tegument proteins pUL47 and pUL48 in the whole life cycle of the virus. We report that a virus lacking the UL48 gene (vΔUL48) is impaired in growth in cell culture and has diminished virulence in vivo In contrast, a virus lacking UL47 (vΔUL47) is unaffected in its growth in vitro and is as virulent in vivo as the wild-type (WT) virus. Surprisingly, we observed that vΔUL47 was unable to be horizontally transmitted to naive chickens, in contrast to the WT virus. In addition, we show that pUL47 is important for the splicing of UL44 transcripts encoding glycoprotein gC, a protein known as being essential for horizontal transmission of MDV. Importantly, we observed that the levels of gC are lower in the absence of pUL47. Notably, this phenotype is similar to that of another transmission-incompetent mutant ΔUL54, which also affects the splicing of UL44 transcripts. This is the first study describing the role of pUL47 in both viral transmission and the splicing and expression of gC.IMPORTANCE Host-to-host transmission of viruses is ideally studied in vivo in the natural host. Veterinary viruses such as Marek's disease virus (MDV) are, therefore, models of choice to explore these aspects. The natural host of MDV, the chicken, is small, inexpensive, and economically important. MDV is a deadly and contagious herpesvirus that can kill infected animals in less than 4 weeks. The virus naturally infects epithelial cells of the feather follicle epithelium from where it is shed into the environment. In this study, we demonstrate that the viral protein pUL47 is an essential factor for bird-to-bird transmission of the virus. We provide some molecular basis to this function by showing that pUL47 enhances the splicing and the expression of another viral gene, UL44, which is essential for viral transmission. pUL47 may have a similar function in human herpesviruses such as varicella-zoster virus or herpes simplex viruses.

Chickens can durably clear herpesvirus vaccine infection in feathers while still carrying vaccine-induced antibodies

Marek’s disease (MD) is a major disease of chickens induced by Marek’s disease virus (MDV) associated to lethal lymphomas. Current MD vaccines protect against lymphomas, but fail to prevent infection and shedding. The control of MDV shedding is crucial in order to eradicate this highly contagious virus. Like pathogenic MDV, MD vaccines infect the feather follicles of the skin before being shed into the environment. MD vaccines constitute excellent models to study virus interaction with feathers, the unique excretion source of these viruses. Herein we studied the viral persistence in feathers of a MD vaccine, the recombinant turkey herpesvirus (rHVT-ND). We report that most of the birds showed a persistent HVT infection of feathers over 41 weeks with moderate viral loads. Interestingly, 20% of the birds were identified as low HVT producers, among which six birds cleared the infection. Indeed, after week 14–26, these birds named controllers had undetectable HVT DNA in their feathers through week 41. All vaccinated birds developed antibodies to NDV, which lasted until week 41 in 95% of the birds, including the controllers. No correlation was found between HVT loads in feathers and NDV antibody titers over time. Interestingly, no HVT DNA was detected in the spleens of four controllers. This is the first description of chickens that durably cleared MD vaccine infection of feathers suggesting that control of Mardivirus shedding is achievable by the host.

Expression of the Conserved Herpesvirus Protein Kinase (CHPK) of Marek's Disease Alphaherpesvirus in the Skin Reveals a Mechanistic Importance for CHPK during Interindividual Spread in Chickens

The Herpesviridae encode many conserved genes, including the conserved herpesvirus protein kinase (CHPK) that has multifunctional properties. In most cases, herpesviruses lacking CHPK can propagate in cell culture to various degrees, depending on the virus and cell culture system. However, in the natural animal model system of Marek's disease alphaherpesvirus (MDV) in chickens, CHPK is absolutely required for interindividual spread from chicken to chicken. The lack of biological reagents for chicken and MDV has limited our understanding of this important gene during interindividual spread. Here, we engineered epitope-tagged proteins in the context of virus infection in order to detect CHPK in the host. Using immunofluorescence assays and Western blotting during infection in cell culture and in chickens, we determined that the invariant lysine 170 (K170) of MDV CHPK is required for interindividual spread and autophosphorylation of CHPK and that mutation to methionine (M170) results in instability of the CHPK protein. Using these newly generated viruses allowed us to examine the expression of CHPK in infected chickens, and these results showed that mutant CHPK localization and late viral protein expression were severely affected in feather follicles wherein MDV is shed, providing important information on the requirement of CHPK for interindividual spread.IMPORTANCE Marek's disease in chickens is caused by Gallid alphaherpesvirus 2, better known as Marek's disease alphaherpesvirus (MDV). Current vaccines only reduce tumor formation but do not block interindividual spread from chicken to chicken. Understanding MDV interindividual spread provides important information for the development of potential therapies to protect against Marek's disease while also providing a reliable natural host in order to study herpesvirus replication and pathogenesis in animals. Here, we studied the conserved Herpesviridae protein kinase (CHPK) in cell culture and during infection in chickens. We determined that MDV CHPK is not required for cell-to-cell spread, for disease induction, and for oncogenicity. However, it is required for interindividual spread, and mutation of the invariant lysine (K170) results in stability issues and aberrant expression in chickens. This study is important because it addresses the critical role CHPK orthologs play in the natural host.

Virus-Induced Immunosuppression in Chickens

A healthy immune system is a cornerstone for poultry production. Any factor diminishing the immune responses will affect production parameters and increase cost. There are numerous factors, infectious and noninfectious, causing immunosuppression (IS) in chickens. This paper reviews the three viral diseases that most commonly induce IS or subclinical IS in chickens: Marek's disease virus (MDV), chicken infectious anemia virus (CIAV), and infectious bursal disease virus (IBDV), as well as the interactions among them. MDV-induced IS (MDV-IS) affects both humoral and cellular immune responses. It is very complex, poorly understood, and in many cases underdiagnosed. Vaccination protects against some but not all aspects of MDV-IS. CIAV induces apoptosis of the hemocytoblasts resulting in anemia, hemorrhages, and increased susceptibility to bacterial infections. It also causes apoptosis of thymocytes and dividing T lymphocytes, affecting T helper functions, which are essential for antibody production and cytotoxic T lymphocyte (CTL) functions. Control of CIAV is based on vaccination of breeders and maternal antibodies (MAbs). However, subclinical IS can occur after MAbs wane. IBDV infection affects the innate immune responses during virus replication and humoral immune responses as a consequence of the destruction of B-cell populations. Vaccines with various levels of attenuation are used to control IBDV. Interactions with MAbs and residual virulence of the vaccines need to be considered when designing vaccination plans. The interaction between IBDV, CIAV, and MDV is critical although underestimated in many cases. A proper control of IBDV is a must to have proper humoral immune responses needed to control CIAV. Equally, long-term control of MDV is not possible if chickens are coinfected with CIAV, as CIAV jeopardizes CTL functions critical for MDV control.

Marek's disease alphaherpesvirus (MDV) RLORF4 is not required for expression of glycoprotein C and interindividual spread

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus that causes Marek's disease (MD) in chickens. RLORF4 is a MDV-specific gene that is systematically deleted during attenuation of MDV in vitro. Concomitantly, the expression of glycoprotein C (gC) is diminished during attenuation, suggesting these two changes may be linked. Original studies in which RLORF4 was deleted utilized an infectious clone that lacked gC expression due to a frame-shift mutation within the gC open reading frame (UL44). Here, we utilized an infectious clone in which gC expression was restored to test our hypothesis that RLORF4 is important for expression of MDV gC, and subsequently, interindividual spread. Contrary to our hypothesis, gC expression was unaltered during both in vitro and in vivo replication of RLORF4-null MDV and was able to efficiently transmit from chicken to chicken, conclusively showing that RLORF4 does not regulate gC expression and is not required for horizontal transmission.

The Herpesviridae Conserved Multifunctional Infected-Cell Protein 27 (ICP27) Is Important but Not Required for Replication and Oncogenicity of Marek's Disease Alphaherpesvirus

The Herpesviridae conserved infected-cell protein 27 (ICP27) is essential for cell culture-based replication of most herpesviruses studied. For members of the Alphaherpesvirinae, ICP27 regulates the expression of many viral genes, including expression of pUL44 (gC), pUL47 (VP13/14), and pUL48 (VP16). These three viral proteins are dysregulated during Marek's disease alphaherpesvirus (MDV) replication in cell culture. MDV replicates in a highly cell-associated manner in cell culture, producing little to no infectious virus. In contrast, infectious cell-free MDV is produced in specialized feather follicle epithelial (FFE) cells of infected chickens, in which these three genes are abundantly expressed. This led us to hypothesize that MDV ICP27, encoded by gene UL54, is a defining factor for the dysregulation of gC, pUL47, and pUL48 and, ultimately, ineffective virus production in cell culture. To address ICP27's role in MDV replication, we generated recombinant MDV with ICP27 deleted (vΔ54). Interestingly, vΔ54 replicated, but plaque sizes were significantly reduced compared to those of parental viruses. The reduced cell-to-cell spread was due to ICP27 since plaque sizes were restored in rescued viruses, as well as when vΔ54 was propagated in cells expressing ICP27 in trans In chickens, vΔ54 replicated, induced disease, and was oncogenic but was unable to transmit from chicken to chicken. To our knowledge, this is the first report showing that the Herpesviridae conserved ICP27 protein is dispensable for replication and disease induction in its natural host.IMPORTANCE Marek's disease (MD) is a devastating oncogenic disease that affects the poultry industry and is caused by MD alphaherpesvirus (MDV). Current vaccines block induction of disease but do not block chicken-to-chicken transmission. There is a knowledge gap in our understanding of how MDV spreads from chicken to chicken. We studied the Herpesviridae conserved ICP27 regulatory protein in cell culture and during MDV infection in chickens. We determined that MDV ICP27 is important but not required for replication in both cell culture and chickens. In addition, MDV ICP27 was not required for disease induction or oncogenicity but was required for chicken-to-chicken transmission. This study is important because it addresses the role of ICP27 during infection in the natural host and provides important information for the development of therapies to protect chickens against MD.

Unraveling the role of B cells in the pathogenesis of an oncogenic avian herpesvirus

Marek's disease virus (MDV) is a highly oncogenic alphaherpesvirus that causes immunosuppression, paralysis, and deadly lymphomas in chickens. In infected animals, B cells are efficiently infected and are thought to amplify the virus and transfer it to T cells. MDV subsequently establishes latency in T cells and transforms CD4⁺ T cells, resulting in fatal lymphomas. Despite many years of research, the exact role of the different B and T cell subsets in MDV pathogenesis remains poorly understood, mostly due to the lack of reverse genetics in chickens. Recently, Ig heavy chain J gene segment knockout (JH-KO) chickens lacking mature and peripheral B cells have been generated. To determine the role of these B cells in MDV pathogenesis, we infected JH-KO chickens with the very virulent MDV RB1B strain. Surprisingly, viral load in the blood of infected animals was not altered in the absence of B cells. More importantly, disease and tumor incidence in JH-KO chickens was comparable to wild-type animals, suggesting that both mature and peripheral B cells are dispensable for MDV pathogenesis. Intriguingly, MDV efficiently replicated in the bursa of Fabricius in JH-KO animals, while spread of the virus to the spleen and thymus was delayed. In the absence of B cells, MDV readily infected CD4⁺ and CD8⁺ T cells, allowing efficient virus replication in the lymphoid organs and transformation of T cells. Taken together, our data change the dogma of the central role of B cells, and thereby provide important insights into MDV pathogenesis.

Induction of the unfolded protein response (UPR) during Marek's disease virus (MDV) infection

Marek's disease (MD) is a pathology of chickens associated with paralysis, immune suppression, and the rapid formation of T-cell lymphomas. MD is caused by the herpesvirus, Marek's disease virus (MDV). We examined endoplasmic reticulum (ER) stress and the activation of unfolded protein response (UPR) pathways during MDV infection of cells in culture and lymphocytes in vivo. MDV strains activate the UPR as measured by increased mRNA expression of GRP78/BiP with concomitant XBP1 splicing and induction of its target gene, EDEM1. Cell culture replication of virulent, but not vaccine MDVs, activated the UPR at late in infection. Pathotype-associated UPR activation was induced to a greater level by a vv + MDV. Discrete UPR activation was observed during MDV in vivo infection, with the level of UPR modulation being affected by the MDV oncoprotein Meq. Finally, ATF6 was found to be activated in vv + MDV-induced primary lymphomas, suggesting a possible role in tumor progression.

Atrophy of primary lymphoid organs induced by Marek's disease virus during early infection is associated with increased apoptosis, inhibition of cell proliferation and a severe B-lymphopenia

Marek's disease is a multi-faceted highly contagious disease affecting chickens caused by the Marek's disease alphaherpesvirus (MDV). MDV early infection induces a transient immunosuppression, which is associated with thymus and bursa of Fabricius atrophy. Little is known about the cellular processes involved in primary lymphoid organ atrophy. Here, by in situ TUNEL assay, we demonstrate that MDV infection results in a high level of apoptosis in the thymus and bursa of Fabricius, which is concomitant to the MDV lytic cycle. Interestingly, we observed that in the thymus most of the MDV infected cells at 6 days post-infection (dpi) were apoptotic, whereas in the bursa of Fabricius most of the apoptotic cells were uninfected suggesting that MDV triggers apoptosis by two different modes in these two primary lymphoid organs. In addition, a high decrease of cell proliferation was observed from 6 to 14 dpi in the bursa of Fabricius follicles, and not in the thymus. Finally, with an adapted absolute blood lymphocyte count, we demonstrate a major B-lymphopenia during the two 1st weeks of infection, and propose this method as a potent non-invasive tool to diagnose MDV bursa of Fabricius infection and atrophy. Our results demonstrate that the thymus and bursa of Fabricius atrophies are related to different cell mechanisms, with different temporalities, that affect infected and uninfected cells.

Induction of DNA Damages upon Marek's Disease Virus Infection: Implication in Viral Replication and Pathogenesis

Marek's disease virus (MDV) is a highly contagious alphaherpesvirus that infects chickens and causes a deadly neoplastic disease. We previously demonstrated that MDV infection arrests cells in S phase and that the tegument protein VP22 plays a major role in this process. In addition, expression of VP22 induces double-strand breaks (DSBs) in the cellular DNA, suggesting that DNA damage and the associated cellular response might be favorable for the MDV life cycle. Here, we addressed the role of DNA damage in MDV replication and pathogenesis. We demonstrated that MDV induces DSBs during lytic infection in vitro and in the peripheral blood mononuclear cells of infected animals. Intriguingly, we did not observe DNA damage in latently infected MDV-induced lymphoblastoid cells, while MDV reactivation resulted in the onset of DNA lesions, suggesting that DNA damage and/or the resulting DNA damage response might be required for efficient MDV replication and reactivation. In addition, reactivation was significantly enhanced by the induction of DNA damage using a number of chemicals. Finally, we used recombinant viruses to show that VP22 is required for the induction of DNA damage in vivo and that this likely contributes to viral oncogenesis.IMPORTANCE Marek's disease virus is an oncogenic alphaherpesvirus that causes fatal T-cell lymphomas in chickens. MDV causes substantial losses in the poultry industry and is also used in small-animal models for virus-induced tumor formation. DNA damage not only is implicated in tumor development but also aids in the life cycle of several viruses; however, its role in MDV replication, latency, and reactivation remains elusive. Here, we demonstrate that MDV induces DNA lesions during lytic replication in vitro and in vivo DNA damage was not observed in latently infected cells; however, it was reinitiated during reactivation. Reactivation was significantly enhanced by the induction of DNA damage. Recombinant viruses that lacked the ability to induce DNA damage were defective in their ability to induce tumors, suggesting that DNA damage might also contribute to cellular transformation processes leading to MDV lymphomagenesis.

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.

Keratinocytes derived from chicken embryonic stem cells support Marek's disease virus infection: a highly differentiated cell model to study viral replication and morphogenesis

BACKGROUND

Marek's disease is a virus disease with worldwide distribution that causes major losses to poultry production. Vaccines against Marek's disease virus, an oncogenic alphaherpesvirus, reduce tumour formation but have no effect on virus shedding. Successful horizontal virus transmission is linked to the active viral replication in feather follicle epithelial cells of infected chickens, from which infectious viral particles are shed into the environment. The feather follicle epithelium is the sole tissue in which those infectious particles are produced and no in vitro cell-systems can support this highly efficient morphogenesis. We previously characterized embryonic stem-cell-derived keratinocytes, showing they display a marker-gene profile similar to skin keratinocytes, and therefore we tested their susceptibility to Marek's disease virus infection.

FINDINGS

We show herein that keratinocytes derived from chicken embryonic stem-cells are fully permissive to the replication of either non-pathogenic or pathogenic Marek's disease viruses. All viruses replicated on all three keratinocyte lines and kinetics of viral production as well as viral loads were similar to those obtained on primary cells. Morphogenesis studies were conducted on infected keratinocytes and on corneocytes, showing that all types of capsids/virions were present inside the cells, but extracellular viruses were absent.

CONCLUSIONS

The keratinocyte lines are the first epithelial cell-line showing ectodermal specific markers supporting Marek's disease virus replication. In this in vitro model the replication lead to the production of cell-associated viral progeny. Further work will be devoted to the study of relationship between 3D differentiation of keratinocytes and Marek's disease virus replication.

Differential expression of Marek's disease virus (MDV) late proteins during in vitro and in situ replication: role for pUL47 in regulation of the MDV UL46-UL49 gene locus

Marek's disease virus (MDV) is a lymphotropic alphaherpesvirus that replicates in a highly cell-associated manner in vitro. Production of infectious cell-free virus only occurs in feather follicle epithelial (FFE) cells of infected chicken skins. Previously, we described differential expression for a core alphaherpesvirus protein, pUL47 that was found to be abundantly expressed in FFE cells of infected chickens, while barely detectable during in vitro propagation. Here, we further examined the dynamics of expression of four tegument proteins within the UL46-49 locus during in vitro and in situ replication. All four proteins examined were expressed abundantly in situ, whereas both pUL47 and pUL48 expression were barely detectable in vitro. Replacement of the putative UL47 and UL48 promoters with the minimal cytomegalovirus promoter enhanced mRNA and protein expression in vitro. Interestingly, enhanced expression of pUL47 resulted in increased UL46, UL48, and UL49 transcripts that resulted in increased pUL46 and pUL48 expression.

Antibody response toSalmonella: its induction and role in protection against avian enteric salmonellosis

Human enteritis resulting from the consumption of poultry products contaminated with serovars of Salmonella enterica remains a major public-health concern. Reducing food contamination by preventing or controlling infection in the chicken during rearing is an attractive solution. An accurate understanding of the mechanisms of immunity to Salmonella infection in the chicken will help to focus the development of vaccines for birds and prevent contaminated products from entering the human food chain. Infection is primarily restricted to the intestinal lumen when chickens are infected with S. enterica serovars Typhimurium or Enteritidis, where they persist for many weeks. High titers of Salmonella-specific antibodies are observed following infection and demonstrate a high degree of cross-reactivity against other serovars. However, depletion of B cells and, therefore, removal of the capacity for antibody production in the chicken does not exacerbate the infection following either primary or secondary challenge.

Fluorescent tagging of VP22 in N-terminus reveals that VP22 favors Marek's disease virus (MDV) virulence in chickens and allows morphogenesis study in MD tumor cells

Marek’s disease virus (MDV) is an alpha-herpesvirus causing Marek’s disease in chickens, mostly associated with T-cell lymphoma. VP22 is a tegument protein abundantly expressed in cells during the lytic cycle, which is essential for MDV spread in culture. Our aim was to generate a pathogenic MDV expressing a green fluorescent protein (EGFP) fused to the N-terminus of VP22 to better decipher the role of VP22 in vivo and monitor MDV morphogenesis in tumors cells. In culture, rRB-1B EGFP22 led to 1.6-fold smaller plaques than the parental virus. In chickens, the rRB-1B EGFP22 virus was impaired in its ability to induce lymphoma and to spread in contact birds. The MDV genome copy number in blood and feathers during the time course of infection indicated that rRB-1B EGFP22 reached its two major target cells, but had a growth defect in these two tissues. Therefore, the integrity of VP22 is critical for an efficient replication in vivo, for tumor formation and horizontal transmission. An examination of EGFP fluorescence in rRB-1B EGFP22-induced tumors showed that about 0.1% of the cells were in lytic phase. EGFP-positive tumor cells were selected by cytometry and analyzed for MDV morphogenesis by transmission electron microscopy. Only few particles were present per cell, and all types of virions (except mature enveloped virions) were detected unequivocally inside tumor lymphoid cells. These results indicate that MDV morphogenesis in tumor cells is more similar to the morphorgenesis in fibroblastic cells in culture, albeit poorly efficient, than in feather follicle epithelial cells.

Oral administration of a combination of select lactic acid bacteria strains to reduce the Salmonella invasion and inflammation of broiler chicks

One-day-old chicks are susceptible to infection by strains of Salmonella enterica subspecies. Because multistrain probiotics are suggested to be more effective than monostrain probiotics due to the additive and synergistic effects, in this study, we prepared a multistrain formula A (MFA) consisting of 4 lactic acid bacteria (LAB) strains selected by enhancing the TNF-α production for mouse macrophage 264.7 cells. The antagonistic effect of this MFA against the cecal colonization, viscera invasiveness, as well as the inflammation of 1-d-old chicks challenged with Salmonella Typhimurium were then assayed. One-day-old chicks were fed with MFA from d 1 to d 3, and on d 4, chicks were challenged with Salmonella Typhimurium (200 μL, 10(6) cfu/mL). The livers, spleens, and cecal tonsils of chicks were then removed on d 3 and 6 postinfection. Compared with the multistrain formula B (MFB) which consisted of LAB strains selected at random, the efficacy of MFA to reduce the Salmonella counts recovered from the cecal tonsils, spleens, and livers of chicks were significantly higher. Moreover, when the levels of proinflammatory cytokines, such as IL-1β, IL-6, interferon (IFN)-γ, and anti-inflmmatory cytokine, that is, IL-10, in cecal tonsils were measured by reverse-transcription real-time quantitative PCR; it was found that chicks fed with MFA for 3 d had lower levels of IL-1β, IL-6, IFN-γ and a higher level of IL-10 in the cecal tonsils of chicks as compared with those of the chicks fed with MFB or without LAB. These results suggest that multistrain probiotics consisting of LAB strains selected by immunomodulatory activity and adherence are more effective than those consisting of strains selected at random in antagonistic effect against Salmonella colonization, invasion, and the induced inflammation.

Temporal transcriptome changes induced by MDV in Marek's disease-resistant and -susceptible inbred chickens

Background

Marek's disease (MD) is a lymphoproliferative disease in chickens caused by Marek's disease virus (MDV) and characterized by T cell lymphoma and infiltration of lymphoid cells into various organs such as liver, spleen, peripheral nerves and muscle. Resistance to MD and disease risk have long been thought to be influenced both by genetic and environmental factors, the combination of which contributes to the observed outcome in an individual. We hypothesize that after MDV infection, genes related to MD-resistance or -susceptibility may exhibit different trends in transcriptional activity in chicken lines having a varying degree of resistance to MD.

Results

In order to study the mechanisms of resistance and susceptibility to MD, we performed genome-wide temporal expression analysis in spleen tissues from MD-resistant line 6₃, susceptible line 7₂ and recombinant congenic strain M (RCS-M) that has a phenotype intermediate between lines 6₃ and 7₂ after MDV infection. Three time points of the MDV life cycle in chicken were selected for study: 5 days post infection (dpi), 10dpi and 21dpi, representing the early cytolytic, latent and late cytolytic stages, respectively. We observed similar gene expression profiles at the three time points in line 6₃ and RCS-M chickens that are both different from line 7₂. Pathway analysis using Ingenuity Pathway Analysis (IPA) showed that MDV can broadly influence the chickens irrespective of whether they are resistant or susceptible to MD. However, some pathways like cardiac arrhythmia and cardiovascular disease were found to be affected only in line 7₂; while some networks related to cell-mediated immune response and antigen presentation were enriched only in line 6₃ and RCS-M. We identified 78 and 30 candidate genes associated with MD resistance, at 10 and 21dpi respectively, by considering genes having the same trend of expression change after MDV infection in lines 6₃ and RCS-M. On the other hand, by considering genes with the same trend of expression change after MDV infection in lines 7₂ and RCS-M, we identified 78 and 43 genes at 10 and 21dpi, respectively, which may be associated with MD-susceptibility.

Conclusions

By testing temporal transcriptome changes using three representative chicken lines with different resistance to MD, we identified 108 candidate genes for MD-resistance and 121 candidate genes for MD-susceptibility over the three time points. Genes included in our resistance or susceptibility genes lists that are also involved in more than 5 biofunctions, such as CD8α, IL8, USP18, and CTLA4, are considered to be important genes involved in MD-resistance or -susceptibility. We were also able to identify several biofunctions related with immune response that we believe play an important role in MD-resistance.

Differential modulation of cytokine, chemokine and Toll like receptor expression in chickens infected with classical and variant infectious bursal disease virus

Infectious bursal disease (IBD) is an important immunosuppressive disease of chickens. The causative agent, infectious bursal disease virus (IBDV), consists of two serotypes, 1 and 2. Serotype 1 consists of classic IBDV (cIBDV) and variant IBDV (vIBDV). Both of these strains vary in antigenicity and pathogenesis. The goal of this study was to compare the immunopathogenesis of cIBDV and vIBDV. Three-week-old specific pathogen free chickens were inoculated intraocularly with standard challenge strain (STC) (cIBDV) and a variant strain Indiana (IN) (vIBDV). The cIBDV produced more pronounced bursal damage, inflammatory response and infiltration of T cells as compared to vIBDV. There were significant differences in the expression of innate (IFN-α and IFN-β), proinflammatory cytokine and mediator (IL-6 and iNOS) in cIBDV- and vIBDV-infected bursas. The expression of chemokines genes, IL-8 and MIP-α was also higher in cIBDV-infected chickens during the early phase of infection. The expression of Toll like receptor 3 (TLR3) was downregulated at post inoculation days (PIDs) 3, 5, and 7 in the bursas of vIBDV-infected chickens whereas TLR3 was upregulated at PIDs 3 and 5 in cIBDV-infected bursas. In vIBDV-infected bursa, TLR7 expression was downregulated at PIDs 3 and 5 and upregulated at PID 7. However, TLR7 was upregulated at PIDs 3 and 7 in cIBDV-infected bursas. The expression of MyD88 was downregulated whereas TRIF gene expression was upregulated in cIBDV- and vIBDV-infected bursa. These findings demonstrate the critical differences in bursal lesions, infiltration of T cells, expression of cytokines, chemokines and TLRs in the bursa of cIBDV-and vIBDV-infected chickens.

Cytokine responses and inducible nitrous oxide synthase expression patterns in neonatal chicken brain microglia infected with very virulent Marek's disease virus strain YL040920

Purified and enriched brain microglia from neonatal chickens were infected with live Marek's disease virus (MDV)-both the very virulent (vv) YL040920 strain and the attenuated vaccine strain CVI988/Rispens in vitro. Although YL040920-infected microglia showed lower viral DNA loads compared with those infected with CVI988/Rispens at the same infectious dose (400 plaque-forming units for each), no significant differences in IFN-γ and IL-12p35 transcription were detected between the two MDV strains. Chicken microglia infected with live or fixed YL040920 expressed dramatically higher levels of IL-12p40, IL-8, and macrophage inflammatory protein-1β (MIP-1β) transcripts compared with those infected with CVI988/Rispens. On the other hand, CVI988/Rispens induced significantly higher levels of IFN-β transcription than YL040920, especially the live virus. Inducible nitric oxide (NO) synthase (iNOS) transcription and NO production correlated with levels of both YL040920 and CVI988/Rispens live strain infection. Moreover, fixed MDVs induced higher levels of iNOS/NO than live viruses, especially with CVI988/Rispens. This study demonstrates that chicken microglial cells can become infected with live YL040920 and CVI988/Rispens and that microglia represent cellular sources of IL-12p40, IL-12p35, IFN-γ, IFN-β, IL-8, MIP-1β, iNOS mRNA, and NO expression after MDV infection in vitro. Transcription levels of IL-12p35 and IFN-γ were associated with MDV DNA replication, whereas transcription levels of IL-12p40, IFN-β, IL-8, and MIP-1β were associated with both MDV DNA replication and expression of viral specific genes. The transcription of iNOS was responsible for expression of viral specific genes, whereas it was suppressed by viral DNA replication during infection. Although YL040920, compared with CVI988/Rispens, induced similar levels of the typical Th1-type cytokine IFN-γ in microglia, vvMDV induced significant increases in other cytokines [IL-12 (p40 and 12p35), IL-8, and MIP-1β]. More detailed investigation, as well as in vivo testing of the effects of vvMDV infection on Th1 responses, iNOS expression, and NO production in the brain of chickens should be undertaken.

Immune responses against Marek's disease virus

It is more than a century since Marek's disease (MD) was first reported in chickens and since then there have been concerted efforts to better understand this disease, its causative agent and various approaches for control of this disease. Recently, there have been several outbreaks of the disease in various regions, due to the evolving nature of MD virus (MDV), which necessitates the implementation of improved prophylactic approaches. It is therefore essential to better understand the interactions between chickens and the virus. The chicken immune system is directly involved in controlling the entry and the spread of the virus. It employs two distinct but interrelated mechanisms to tackle viral invasion. Innate defense mechanisms comprise secretion of soluble factors as well as cells such as macrophages and natural killer cells as the first line of defense. These innate responses provide the adaptive arm of the immune system including antibody- and cell-mediated immune responses to be tailored more specifically against MDV. In addition to the immune system, genetic and epigenetic mechanisms contribute to the outcome of MDV infection in chickens. This review discusses our current understanding of immune responses elicited against MDV and genetic factors that contribute to the nature of the response.

The effect of siRNA treatment on experimental equine herpesvirus type 1 (EHV-1) infection in horses

Available vaccines fail to induce lasting and protective immunity to equine herpesvirus 1 (EHV-1) associated diseases. RNA interference is a novel approach showing promise for therapeutic use in outbreak situations. This study examined the effect of small interfering RNA (siRNA) on clinical signs as well as the presence of live virus and viral DNA in nasal secretions and peripheral blood mononuclear cells (PBMCs) in horses experimentally infected with EHV-1. siRNA targeting two EHV-1 genes (glycoprotein B and the origin binding protein) was administered 12 h before and 12 h after intranasal infection with EHV-1. Control horses received siRNA targeting firefly luciferase. A significantly smaller proportion (0/10) of horses receiving siRNA targeting viral genes required euthanasia due to intractable neurologic disease as compared to horses in the control group (3/4; p = 0.01). There was no significant difference in the presence of live virus or viral DNA in the nasal secretions or PBMCs between the two groups. Future studies are necessary to define the relative contributions of host and virus factors in the development of the neurological form of the infection and to determine an optimal dosing regimen for metaphylactic or therapeutic use of siRNA for treating EHV-1 infection.

Increased DNA damage and oxidative stress in chickens with natural Marek's disease

Oxidative stress contributes to the accumulation of genomic abnormalities, prevents cellular apoptosis, and also mediates immunosuppression resulting in tumor formation. Marek's Disease provides excellent opportunities for the study of herpesvirus-induced tumors both in experimental- and natural conditions. The aim of this study was to examine the effects of Marek's Disease (MD) on basal levels of DNA strand breaks and on the oxidative-antioxidative status of chickens with MD. White-Lohmann hens-fifteen infected with Marek's Disease Virus (MDV) and fifteen healthy-of same age and sex were included in this study. MD infection was diagnosed via clinical signs, gross- and micro-pathological findings and also by detection of viral antigens in feather follicle epithelium by the indirect immunoperoxidase method. Compared with healthy controls, DNA damage was greater and levels of malondialdehyde (MDA) and plasma protein carbonyl (PCO), and plasma concentration of nitric oxide metabolites (NOx) higher in the MD group. Furthermore, total antioxidant activities (AOAs) were found lowered and glutathione (GSH) levels reduced in the MD group compared to the control group. Significantly positive correlation was found between DNA damage, MDA, PCO, and NOx in the MD group. DNA strand breaks were found negatively associated with AOA and GSH concentrations in the MD group. Our results demonstrated that oxidative stress markers and DNA damage substantially increased in chickens with MD, which indicated that increased DNA damage levels might be related to the increased oxidative stress and reduced antioxidant activity.

Marek's disease virus-induced expression of cytokine genes in feathers of genetically defined chickens

Marek's disease (MD) vaccines, although effective in reducing lymphoproliferation, cannot control infectious virus production in the feather follicle epithelium (FFE) which is the site of virus shedding. Therefore, we investigated Marek's disease virus (MDV) replication as well as the expression of cytokine genes in feathers of MDV-infected chickens belonging to genetically defined lines (N2a or B(21)/B(21) haplotype-resistant and P2a or B(19)/B(19) haplotype-susceptible). Though there was not a difference in MDV genome load and transcripts between feathers of these chicken lines at 4 and 10 days post-infection (d.p.i.), feathers of resistant chickens carried significantly lower viral genome load and transcripts at 21 d.p.i. Irrespective of genetic background of the chickens examined, MDV replication showed a significant positive correlation with the expression of IFN-gamma gene. The results imply the usefulness of genetic control approach in reducing virulent MDV transmission.

Modeling variability in immunocompetence and immunoresponsiveness

The purposes of this paper were to 1) develop a stochastic model that would reflect observed variation between animals and across ages in immunocompetence and responsiveness; and 2) illustrate consequences of this variability for the statistical power of genotype comparisons and selection. A stochastic model of immunocompetence development and responsiveness kinetics was developed. This model enabled variability in immunological variables to be taken into account in the evaluation of challenge and measurement strategies for selection. The characteristics of the variation in model output reflect those observed in the literature, to the extent that variation in the literature shows a consistent pattern; knowledge of true variation and patterns of variation in immunological variables is limited. The model created correlations between immunocompetence and immunoresponsiveness components, as well as correlations within each component across time. These correlations were generally in agreement with literature estimates, where available. The model enabled predictions of the effectiveness of selection for improved health through immunocompetence or immunoresponsiveness. It was predicted that effective selection for increased general immunocompetence to improve health should be done only when baseline immunity has matured. Further, the model implied that selection is unlikely to be successful if it is based only on a single measurement. Problems with low statistical power to detect differences between genotypes can be reduced by increasing challenge age in the experimental design, and one should ensure that the effects of maternal immunity are minimal when the challenge is done. The ability to detect differences between different groups of animals differs substantially with measurement timing because of low repeatabilities of immunocompetence and responsiveness across time. In general, the probability of detecting differences becomes higher when the challenge age is increased. Consequently, both the age at selection and the age at which information is gathered for selection must be considered carefully when designing genetic evaluations.

Vaccination against Marek's disease reduces telomerase activity and viral gene transcription in peripheral blood leukocytes from challenged chickens

We investigated whether telomerase activity and viral gene transcription were associated with protection against the RB-1B strain of Marek's disease virus (MDV) in chickens vaccinated with Rispens CVI988 or the herpes virus of turkey (HVT). Telomerase activity in peripheral blood leukocytes (PBLs) seemed to be an appropriate marker of lymphoma and levels of viral transcription were correlated with the virulence of MDV strains. Vaccinated protected birds had lower levels of telomerase activity and RB-1B viral gene transcription than unvaccinated chickens infected with RB-1B. The decrease in RB-1B viral transcription was more marked in chickens vaccinated with CVI988 than in those vaccinated with HVT. Indeed, RB-1B viral transcription was not detectable after 14 days post-challenge. In conclusion, telomerase activity and gene transcription in challenge MDV strains are potential new reliable criteria of protection in vaccinated chickens.

Horizontal transmission of Marek's disease virus requires US2, the UL13 protein kinase, and gC

Marek's disease virus (MDV) causes a general malaise in chickens that is mostly characterized by the development of lymphoblastoid tumors in multiple organs. The use of bacterial artificial chromosomes (BACs) for cloning and manipulation of the MDV genome has facilitated characterization of specific genes and genomic regions. The development of most MDV BACs, including pRB-1B-5, derived from a very virulent MDV strain, involved replacement of the US2 gene with mini-F vector sequences. However, when reconstituted viruses based on pRB-1B were used in pathogenicity studies, it was discovered that contact chickens housed together with experimentally infected chickens did not contract Marek's disease (MD), indicating a lack of horizontal transmission. Staining of feather follicle epithelial cells in the skins of infected chickens showed that virus was present but was unable to be released and/or infect susceptible chickens. Restoration of US2 and removal of mini-F sequences within viral RB-1B did not alter this characteristic, although in vivo viremia levels were increased significantly. Sequence analyses of pRB-1B revealed that the UL13, UL44, and US6 genes encoding the UL13 serine/threonine protein kinase, glycoprotein C (gC), and gD, respectively, harbored frameshift mutations. These mutations were repaired individually, or in combination, using two-step Red mutagenesis. Reconstituted viruses were tested for replication, MD incidence, and their abilities to horizontally spread to contact chickens. The experiments clearly showed that US2, UL13, and gC in combination are essential for horizontal transmission of MDV and that none of the genes alone is able to restore this phenotype.

Modeling Immunocompetence Development and Immunoresponsiveness to Challenge in Chicks

The purpose of this study was 2-fold: 1) to develop a deterministic model that describes the development of immunocompetence and the kinetics of immunoresponsiveness to a pathogenic challenge in chicks and 2) to use this model to illustrate the importance of factors in experimental design, such as type of variable measured, measurement timing, and challenge age. Difficulties in evaluating immunological variables hinder attempts to improve animal health through selection on immunological variables. In young chicks, evaluating immunological variables is additionally complicated by immune system development and maternal immunity. The evaluation of immunocompetence and immunoresponsiveness and the definition of appropriate challenge and measurement strategies may be enabled through a mathematical model that captures the key components of the immune system and its development. Therefore, a model was developed that describes the development of immunocompetence as well as the kinetics of immunoresponsiveness to a pathogenic extracellular bacterial challenge in an individual chick from 0 to 56 d of age. The model consisted of 4 components describing immunocompetence (maternal and baseline immunity) and immunoresponsiveness (acute phase and antibody response). Individual component equations generally fit published data adequately. Four scenarios that represented combinations of challenge age and measurement timing were simulated. In each scenario, the immunoresponsiveness to a particular challenge was compared for 3 different levels of baseline immunity, representing 3 broiler genotypes. It was illustrated that experimental design (type of immunoresponsiveness measured, measurement timing, and challenge age) can have an important effect on the ranking of genotypes, groups, or individuals and on the reliability of extrapolations based on this ranking. It is concluded that this model is a potentially useful tool in the definition of appropriate challenge and measurement strategies when evaluating immunocompetence and immunoresponsiveness. Further, it may be used as a generator of hypotheses on global immunological relationships to be tested experimentally.

Marek's disease virus: lytic replication, oncogenesis and control

Marek's disease (MD) is caused by a ubiquitous, lymphotropic alphaherpesvirus, MD virus (MDV). MD has been a major concern in the poultry industry owing to the emergence of increasingly virulent strains over the last few decades that were isolated in the face of comprehensive vaccination. The disease is characterized by a variety of clinical signs; among them are neurological symptoms, chronic wasting and, most notably, the development of multiple lymphomas that manifest as solid tumors in the viscera and musculature. Much work has been devoted to study MD-induced oncogenesis and the genes involved in this process. Among the many genes encoded by MDV, a number have been shown recently to affect the development of tumors in chickens, one protein directly causing transformation of cells (Meq) and another being involved in maintaining transformed cells (vTR). Other MDV gene products modulate and are involved in early lytic in vivo replication, thereby increasing the chance of transformation occurring. In this review, we will summarize specific genes encoded by MDV that are involved in the initiation and/or maintenance of transformation and will focus mostly on current vaccination and control strategies against MD, particularly how modern molecular biological methods may be used to improve strategies to combat the disease in the future.

Innate immunity, assessed by plasma NO measurements, is not suppressed during the incubation fast in eiders

Immunity is hypothesized to share limited resources with other physiological functions and may mediate life history trade-offs, for example between reproduction and survival. However, vertebrate immune defense is a complex system that consists of three components. To date, no study has assessed all of these components for the same animal model and within a given situation. Previous studies have determined that the acquired immunity of common eiders (Somateria mollissima) is suppressed during incubation. The present paper aims to assess the innate immune response in fasting eiders in relation to their initial body condition. Innate immunity was assessed by measuring plasma nitric oxide (NO) levels, prior to and after injection of lipopolysaccharides (LPS), a method which is easily applicable to many wild animals. Body condition index and corticosterone levels were subsequently determined as indicators of body condition and stress level prior to LPS injection. The innate immune response in eiders did not vary significantly throughout the incubation period. The innate immune response of eiders did not vary significantly in relation to their initial body condition but decreased significantly when corticosterone levels increased. However, NO levels after LPS injection were significantly and positively related to initial body condition, while there was a significant negative relationship with plasma corticosterone levels. Our study suggests that female eiders preserve an effective innate immune response during incubation and this response might be partially determined by the initial body condition.

Cytokine expression in chicken peripheral blood mononuclear cells after in vitro exposure to Salmonella enterica serovar Enteritidis

Cytokines are secreted proteins involved with cell recruitment and regulation of both innate and adaptive immune responses. They are essential for an effective host immune response to pathogens. The objective of this study was to determine the effect of Salmonella enterica serovar Enteritidis (S. Enteritidis) exposure and genetic line on cytokine mRNA expression level of cultured chicken peripheral blood mononuclear cells (PBMC). Interleukin-2, interleukin-6 (IL-6), CXCLi2, and transforming growth factor-beta4 (TGF-B4) messenger ribonucleic acid expression was measured by quantitative reverse transcription-PCR assays in PBMC from 3 chicken lines (broiler, Leghorn, Fayoumi) after in vitro exposure to S. Enteritidis. The PBMC were isolated from uninfected birds and cultured overnight. The next day, live pathogenic S. Enteritidis was added to half of the cultures. All cultures were harvested after 2 or 4 h of exposure. Exposure to S. Enteritidis downregulated IL-6, CXCLi2, and TGF-beta4 but not interleukin-2 mRNA expression. No significant genetic line or exposure time effects were detected. These findings demonstrate that exposure of chicken PBMC to S. Enteritidis can induce a rapid change in both proinflammatory (IL-6, CXCLi2) and antiinflammatory (TGF-beta4) cytokine gene expression.

In VivoActivation of Chicken Macrophages by Infectious Bursal Disease Virus

Infectious bursal disease virus (IBDV) infects and replicates in the dividing B lymphocytes of chickens. In the present study, the in vivo effect of IBDV infection on chicken macrophage populations and macrophage activation were examined. Specific-pathogen-free chickens were exposed to virulent IBDV and splenic macrophages were recovered during the acute phase of the disease. At 3 and 5 days post-infection (dpi), spleens of virus-exposed chickens had fewer macrophages than those of virus-free controls (p < 0.05). Confocal microscopic examination revealed cells that were positive for both KUL01 (macrophage surface marker) and R63 (IBDVVP2), indicating presence of the virus in macrophages. MQ-NCSU cells, an avian macrophage cell line, were susceptible to replication of IBDV. In addition, splenic macrophages were activated and had temporarily increased levels of mRNA transcripts of pro-inflammatory mediators, including IL-1beta, IL-6, IL-18, and iNOS. The robust expression of proinflammatory cytokine transcripts, along with a decrease in macrophage numbers, suggest that IBDV activates and may lead to a reduction of resident macrophages in vivo.

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.

Infection and activation of bursal macrophages by virulent infectious bursal disease virus

In this study the effect of infectious bursal disease virus (IBDV) on bursal macrophages during the acute phase of the infection was examined. Specific-pathogen-free (SPF) chickens were exposed to virulent IBDV and bursal adherent cells were examined by immunohistochemisrty and RT-PCR for virus infection and by real-time quantitative RT-PCR (qRT-PCR) for mRNA transcripts of proinflammatory cytokines and iNOS. Viral genome was detected in bursal macrophages at 3, 5 and 7 days post-infection (dpi). Immuno-histochemical staining revealed double positive cells for KUL01 (macrophage marker) and intracellular viral proteins, showing viral replication in bursal macrophages of infected chickens. We noted a significant decrease in the total number of bursal macrophages in infected chickens, probably due to the lysis of infected cells. However, likely due to extensive necrosis of B cells, the relative proportion of bursal macrophages was significantly higher (P<0.05) in infected birds at 3 and 5 dpi than in controls. Among the cytokines examined, IL-6 showed the greatest upregulation (100-fold increase) at 3 dpi. Expression of IL-1beta was maximum at 3 dpi whereas IL-18 expression was highest at 1 dpi. Enhanced expression of iNOS mRNA was observed at 5 dpi. Increased expression of the proinflammatory cytokines and iNOS correlated well with the presence of the inflammatory response in the infected bursa. These data suggested that B cells may not be the sole targets for the virus; macrophages and possibly other cells may serve as host for IBDV.

A strong antigen-specific T-cell response is associated with age and genetically dependent resistance to avian enteric salmonellosis

Chicken genetics and age affect resistance to enteric infection with Salmonella enterica serovar Typhimurium and were used to identify the immune responses that may contribute to rapid clearance. When birds were infected at 40 days of age, line 6(1) chickens cleared the infection more effectively than line N chickens, whereas when birds were infected at 10 days of age, both chicken lines were highly susceptible to infection. Antibody levels, T-cell responsiveness, and cytokine mRNA levels were all elevated during infection. A negative correlation between resistance and antigen-specific antibody production was observed in older chickens. However, this finding was not replicated for age-related resistance; we found that older chickens exhibited a stronger and more rapid antibody response than younger chickens. The levels of interleukin-1beta (IL-1beta) and gamma interferon (IFN-gamma) mRNA were similar in the spleens and cecal tonsils of both line 6(1) and line N chickens, except for higher levels of IL-1beta in the spleens of line 6(1) chickens at 6 days postinfection. Differences in the levels of IFN-gamma and IL-1beta 1beta mRNA between the lines were more apparent in younger chickens, but while the increases were greater than those observed in the older chickens, the clearance of enteric S. enterica serovar Typhimurium was much slower. The level of antigen-specific proliferation of splenocytes was associated with increased resistance in both experimental systems, and the strongest responses were observed in older and genetically resistant chickens. The data presented here implicate T-cell responses in the clearance of S. enterica serovar Typhimurium from the intestine of infected chickens.

Attenuation of Marek's disease virus by deletion of open reading frame RLORF4 but not RLORF5a

Marek's disease (MD) in chickens is caused by the alphaherpesvirus MD virus (MDV) and is characterized by the development of lymphoblastoid tumors in multiple organs. The recent identification and cloning of RLORF4 and the finding that four of six attenuated strains of MDV contained deletions within RLORF4 suggested that it is involved in the attenuation process of MDV. To assess the role of RLORF4 in MD pathogenesis, its coding sequence was deleted in the pRB-1B bacterial artificial chromosome clone. Additionally, RLORF5a was deleted separately to examine its importance for oncogenesis. The sizes of plaques produced by MDV reconstituted from pRB-1BdeltaRLORF5a (rRB-1BdeltaRLORF5a) were similar to those produced by the parental pRB-1B virus (rRB-1B). In contrast, virus reconstituted from pRB-1BDeltaRLORF4 (rRB-1BdeltaRLORF4) produced significantly larger plaques. Replication of the latter virus in cultured cells was higher than that of rRB-1B or rRB-1BdeltaRLORF5a using quantitative PCR (qPCR) assays. In vivo, both deletion mutants and rRB-1B replicated at comparable levels at 4, 7, and 10 days postinoculation (p.i.), as determined by virus isolation and qPCR assays. At 14 days p.i., the number of PFU of virus isolated from chickens infected with rRB-1BdeltaRLORF4 was comparable to that from chickens infected with highly attenuated RB-1B and significantly lower than that from rRB-1B-infected birds. The number of tumors and kinetics of tumor production in chickens infected with rRB-1BdeltaRLORF5a were similar to those of P2a chickens infected with rRB-1B. In stark contrast, none of the chickens inoculated with rRB-1BdeltaRLORF4 died up to 13 weeks p.i.; however, two chickens had tumors at the termination of the experiment. The data indicate that RLORF4 is involved in attenuation of MDV, although the function of RLORF4 is still unknown.