Evaluation of factors affecting vaccine efficacy of recombinant Marek's disease virus lacking the Meq oncogene in chickens

Novel lncRNA 803 related to Marek's disease inhibits apoptosis of DF-1 cells

Marek's disease (MD) is a neoplastic disease that significantly affects the poultry industry. Long non-coding RNAs (lncRNAs) are crucial regulatory factors in various biological processes, including tumourigenesis. However, the involvement of novel lncRNAs in the course of MD virus (MDV) infection is still underexplored. Here, we present the first comprehensive characterization of differentially expressed lncRNAs in chicken spleen at different stages of MDV infection. A series of differentially expressed lncRNAs was identified at each stage of MDV infection through screening. Notably, our investigation revealed a novel lncRNA, lncRNA 803, which exhibited significant differential expression at different stages of MDV infection and was likely to be associated with the p53 pathway. Further analyses demonstrated that the overexpression of lncRNA 803 positively regulated the expression of p53 and TP53BP1 in DF-1 cells, leading to the inhibition of apoptosis. This is the first study to focus on the lncRNA expression profiles in chicken spleens during MDV pathogenesis. Our findings highlight the potential role of the p53-related novel lncRNA 803 in MD pathogenesis and provide valuable insights for decoding the molecular mechanism of MD pathogenesis involving non-coding RNA.RESEARCH HIGHLIGHTS Differentially expressed lncRNAs in spleens of chickens infected with Marek's disease virus at different stages were identified for the first time.The effects of novel lncRNA 803 on p53 pathway and apoptosis of DF-1 cells were reported for the first time.

Epigenetic Factor MicroRNAs Likely Mediate Vaccine Protection Efficacy against Lymphomas in Response to Tumor Virus Infection in Chickens through Target Gene Involved Signaling Pathways

Epigenetic factors, including microRNAs (miRNAs), play an important role in affecting gene expression and, therefore, are involved in various biological processes including immunity protection against tumors. Marek's disease (MD) is a highly contagious disease of chickens caused by the MD virus (MDV). MD has been primarily controlled by vaccinations. MD vaccine efficacy might, in part, be dependent on modulations of a complex set of factors including host epigenetic factors. This study was designed to identify differentially expressed miRNAs in the primary lymphoid organ, bursae of Fabricius, in response to MD vaccination followed by MDV challenge in two genetically divergent inbred lines of White Leghorns. Small RNA sequencing and bioinformatic analyses of the small RNA sequence reads identified hundreds of miRNAs among all the treatment groups. A small portion of the identified miRNAs was differentially expressed within each of the four treatment groups, which were HVT or CVI988/Rispens vaccinated line 63-resistant birds and line 72-susceptible birds. A direct comparison between the resistant line 63 and susceptible line 72 groups vaccinated with HVT followed by MDV challenge identified five differentially expressed miRNAs. Gene Ontology analysis of the target genes of those five miRNAs revealed that those target genes, in addition to various GO terms, are involved in multiple signaling pathways including MAPK, TGF-β, ErbB, and EGFR1 signaling pathways. The general functions of those pathways reportedly play important roles in oncogenesis, anti-cancer immunity, cancer cell migration, and metastatic progression. Therefore, it is highly likely that those miRNAs may, in part, influence vaccine protection through the pathways.

Role of T Cells in Vaccine-Mediated Immunity against Marek’s Disease

Marek’s disease virus (MDV), a highly cell-associated oncogenic α-herpesvirus, is the etiological agent of T cell lymphomas and neuropathic disease in chickens known as Marek’s disease (MD). Clinical signs of MD include neurological disorders, immunosuppression, and lymphoproliferative lymphomas in viscera, peripheral nerves, and skin. Although vaccination has greatly reduced the economic losses from MD, the molecular mechanism of vaccine-induced protection is largely unknown. To shed light on the possible role of T cells in immunity induced by vaccination, we vaccinated birds after the depletion of circulating T cells through the IP/IV injection of anti-chicken CD4 and CD8 monoclonal antibodies, and challenged them post-vaccination after the recovery of T cell populations post-treatment. There were no clinical signs or tumor development in vaccinated/challenged birds with depleted CD4+ or CD8+ T cells. The vaccinated birds with a combined depletion of CD4+ and CD8+ T cells, however, were severely emaciated, with atrophied spleens and bursas. These birds were also tumor-free at termination, with no virus particles detected in the collected tissues. Our data indicated that CD4+ and CD8+ T lymphocytes did not play a critical role in vaccine-mediated protection against MDV-induced tumor development.

A Novel Effective and Safe Vaccine for Prevention of Marek's Disease Caused by Infection with a Very Virulent Plus (vv+) Marek's Disease Virus

Marek’s disease virus (MDV) is a highly contagious alphaherpesvirus that causes rapid onset lymphoma in chickens. Marek’s disease (MD) is effectively controlled using vaccination; however, MDV continues to break through vaccinal immunity, due to the emergence of highly virulent field strains. Earlier studies revealed that deletion of the meq gene from MDV resulted in an attenuated virus that protects against MD in chickens challenged with highly virulent field strains. However, the meq deleted virus retains the ability to induce significant lymphoid organ atrophy. In a different study, we found that the deletion of the vIL8 gene resulted in the loss of lymphoid organ atrophy in inoculated chickens. Here, we describe the generation of a recombinant MDV from which both meq and vIL8 genes were deleted. In vitro studies revealed that the meq and vIL8 double deletion virus replicated at levels similar to the parental very virulent plus (vv+) virus. In addition, in vivo studies showed that the double deletion mutant virus (686BAC-ΔMeqΔvIL8) conferred protection comparable to CVI988, a commercial vaccine strain, when challenged with a vv+ MDV virus, and significantly reduced lymphoid organ atrophy, when compared to meq null virus, in chickens. In conclusion, our study describes the development of a safe and effective vaccine candidate for prevention of MD in chickens.

Utilization of herpesviridae as recombinant viral vectors in vaccine development against animal pathogens

Throughout the past few decades, numerous viral species have been generated as vaccine vectors. Every viral vector has its own distinct characteristics. For example, the family herpesviridae encompasses several viruses that have medical and veterinary importance. Attenuated herpesviruses are developed as vectors to convey heterologous immunogens targeting several serious and crucial pathogens. Some of these vectors have already been licensed for use in the veterinary field. One of their prominent features is their capability to accommodate large amount of foreign DNA, and to stimulate both cell-mediated and humoral immune responses. A better understanding of vector-host interaction builds up a robust foundation for the future development of herpesviruses-based vectors. At the time, many molecular tools are applied to enable the generation of herpesvirus-based recombinant vaccine vectors such as BAC technology, homologous and two-step en passant mutagenesis, codon optimization, and the CRISPR/Cas9 system. This review article highlights the most important techniques applied in constructing recombinant herpesviruses vectors, advantages and disadvantages of each recombinant herpesvirus vector, and the most recent research regarding their use to control major animal diseases.

Marek's disease vaccine activates chicken macrophages

To provide insights into the role of innate immune responses in vaccine-mediated protection, we investigated the effect of Marek's disease (MD) vaccine, CVI988/Rispens, on the expression patterns of selected genes associated with activation of macrophages in MD-resistant and MD-susceptible chicken lines. Upregulation of interferon γ, interleukin (IL)-1β, IL-8, and IL-12 at different days post-inoculation (dpi) revealed activation of macrophages in both chicken lines. A strong immune response was induced in cecal tonsils of the susceptible line at 5 dpi. The highest transcriptional activities were observed in spleen tissues of the resistant line at 3 dpi. No increase in the population of CD3⁺ T cells was observed in duodenum of vaccinated birds at 5 dpi indicating a lack of involvement of the adaptive immune system in the transcriptional profiling of the tested genes. There was, however, an increase in the number of macrophages in the duodenum of vaccinated birds. The CVI988/Rispens antigen was detected in the duodenum and cecal tonsils of the susceptible line at 5 dpi but not in the resistant line. This study sheds light on the role of macrophages in vaccine-mediated protection against MD and on the possible development of new recombinant vaccines with enhanced innate immune system activation properties.

Early Immune Responses to Marek's Disease Vaccines

Marek's disease virus (MDV), a highly cell-associated lymphotropic α-herpesvirus, is the causative agent of Marek's disease (MD) in domestic chickens. MDV replicates in chicken cells and establishes a latent infection within CD4⁺ T cells. Although MD vaccines have been in use for several decades, the exact mechanism of vaccine-induced protection is unclear. It is believed that the innate immune system plays a role in vaccine-induced immunity against pathogenic strains of MDV. To shed light on the possible function of the innate immunity in vaccine-mediated protection, we investigated the effect of vaccination, Rispens/CVI988, on the activation of cellular components of the innate immune system by analyzing the expression pattern of select immune-related genes in the cecal tonsils (CT) and duodenum of two MD-susceptible and MD-resistant chicken lines at 3, 5, and 10 days postvaccination (dpv). The differential expression patterns of the tested genes within the CT and duodenum of vaccinated birds revealed the activation of the innate immune system in both the susceptible and resistant lines. Stronger innate immune response was induced within the CT of the vaccinated birds of the susceptible line at 5 dpv. Upregulation of some of the tested genes at 10 dpv was likely due to the activation and response of the adaptive immune system to vaccination. Immunohistochemical analysis showed no increase in the number of CD3⁺ T cells in the CT and duodenum of the vaccinated birds of either line at 5 dpv. There was, however, an increase in the macrophage populations within the duodenum of the vaccinated birds of both the susceptible and resistant lines at 5 dpv. The vaccine strain antigen was detected in the CT and duodenum of the susceptible line, but not the resistant line at 5 dpv.

Construction of recombinant Marek's disease virus (rMDV) co-expressing AIV-H9N2-NA and NDV-F genes under control of MDV's own bi-directional promoter

To qualitatively analyze and evaluate a bi-directional promoter transcriptional function in both transient and transgenic systems, several different plasmids were constructed and recombinant MDV type 1 strain GX0101 was developed to co-express a Neuraminidase (NA) gene from Avian Influenza Virus H9N2 strain and a Fusion (F) gene from the Newcastle disease virus (NDV). The two foreign genes, NDV-F gene and AIV-NA gene, were inserted in the plasmid driven in each direction by the bi-directional promoter. To test whether the expression of pp38/pp24 heterodimers are the required activators for the expression of the foreign genes, the recombinant plasmid pPpp38-NA/1.8kb-F containing expression cassette for the two foreign genes was co-transfected with a pp38/pp24 expression plasmid, pBud-pp38-pp24, in chicken embryo fibroblast (CEF) cells. Alternatively, plasmid pPpp38-NA/1.8kb-F was transfected in GX0101-infected CEFs where the viral endogenous pp38/pp24 were expressed via virus infection. The expression of both foreign genes was activated by pp38/pp24 dimers either via virus infection, or co-expression. The CEFs transfected with pPpp38-NA/1.8kb-F alone had no expression. We chose to insert the expression cassette of Ppp38-NA/1.8kb-F in the non-essential region of GX0101ΔMeq US2 gene, and formed a new rMDV named MZC13NA/F through homologous recombination. Indirect fluorescence antibody (IFA) test, ELISA and Western blot analyses indicated that F and NA genes were expressed simultaneously under control of the bi-directional promoter, but in opposite directions. The data also indicated the activity of the promoter in the 1.8-kb mRNA transcript direction was higher than that in the direction for the pp38 gene. The expression of pp38/pp24 dimers either via co-tranfection of the pBud-pp38-pp24 plasmid, or by GX0101 virus infection were critical to activate the bi-directional promoter for expression of two foreign genes in both directions. Therefore, the confirmed function of the bi-directional promoter provides better feasibilities to insert multiple foreign genes in MDV genome based vectors.

Application of a new bivalent Marek's disease vaccine does not interfere with infectious bronchitis or Newcastle disease vaccinations and proves efficacious

A possible interference after Marek's disease (MD) vaccination using an experimental bivalent vaccine, consisting of a redesigned CVI-988/Rispens-type MDV-1 strain and herpesvirus of turkeys, with vaccination against infectious bronchitis (IB) virus (IBV) or Newcastle disease (ND) virus (NDV) was examined. Day-old specific-pathogen-free chicks were concomitantly vaccinated with the bivalent MD vaccine (either intramuscularly or subcutaneously) and with commercially available vaccines against ND or IB. Afterward chickens were challenged with either lethal MD virus (MDV) or NDV strains or with a pathogenic IBV strain. After challenge, neither mortality nor notable clinical signs of MD, ND, or IB were observed in the vaccinated birds. The experimental bivalent MDV vaccine proved efficacious against lethal MDV challenge and did not affect the efficacy of the NDV or IBV vaccines. In conclusion, no signs of interference or adverse effects were detected. Thus, the vaccines can be administered concomitantly on chickens' first day of life.