Marek’s disease: The continuing struggle between pathogen and host

Pervasive Differential Splicing in Marek's Disease Virus can Discriminate CVI-988 Vaccine Strain from RB-1B Very Virulent Strain in Chicken Embryonic Fibroblasts

Marek's disease is a major scourge challenging poultry health worldwide. It is caused by the highly contagious Marek's disease virus (MDV), an alphaherpesvirus. Here, we showed that, similar to other members of its Herpesviridae family, MDV also presents a complex landscape of splicing events, most of which are uncharacterised and/or not annotated. Quite strikingly, and although the biological relevance of this fact is unknown, we found that a number of viral splicing isoforms are strain-specific, despite the close sequence similarity of the strains considered: very virulent RB-1B and vaccine CVI-988. We validated our findings by devising an assay that discriminated infections caused by the two strains in chicken embryonic fibroblasts on the basis of the presence of some RNA species. To our knowledge, this study is the first to accomplish such a result, emphasizing how relevant a comprehensive picture of the viral transcriptome is to fully understand viral pathogenesis.

A high frequency of Gallid herpesvirus-2 co-infection with Reticuloendotheliosis virusis associated with high tumor rates in Chinese chicken farms

The prevalence of Marek's disease (MD) caused by Gallid herpesvirus-2 (GaHV-2) has been increasing in chickens in China despite universal vaccination. Among the possible reasons for this trend, of Reticuloendotheliosis virus (REV) contamination in vaccines could lead to co-infection and reduce the vaccine efficacy. Here, we report the epidemiological findings of our continuous surveillance of MD, and an examination of the effects of REV and/or GaHV-2 co-infection. A total of 1230 samples were collected between 2011 and 2015 from 305 flocks covering many of the chicken-raising regions of China. Among these, 606 samples were determined to be GaHV-2-positive, 13.0% of which were found to be co-infected with REV from 18.8% of the flocks. One GaHV-2 strain (HS/1412), a REV strain (HS/1412R), and a GaHV-2 and REV-co-infected strain (HS/1412 GR) were isolated from different chickens of a GaHV-2 and REV co-infected flock. Pathogenicity tests showed that HS/1412 and HS/1412 GR caused disease in all chickens and that HS/1412R induced morbidity in 84.6% of the infected chickens. HS/1412 GR induced 100% mortality and 76.9% tumor formation, which were significantly higher frequencies than those observed with strain HS/1412 (38.5% and 15.4%, respectively) and HS/1412R (0% and 0%). These results indicate that co-infection with GaHV-2 and REV might explain the persistent, sporadic outbreaks of neoplastic disease in some commercial flocks, resulting in a significant economic burden to the poultry industry of China.

Host genetic resistance to Marek's disease sustains protective efficacy of herpesvirus of turkey in both experimental and commercial lines of chickens

Marek's disease (MD) remains a continual threat to the poultry industry worldwide as the MD virus continues evolving in virulence. MD has been controlled primarily by intensive use of vaccines since 1969. Based on the antigenic and pathogenic differences of the viruses that the vaccines were derived from, commercially available MD vaccines are classified into three categories, MDV-1, -2, and -3 vaccines. This study was designed to compare the protective efficacy of MDV-1 (CVI988/Rispens) and MDV-3 (HVT) vaccines against challenge of a very virulent plus strain of Marek's disease virus (vv+MDV) in experimental and commercial egg-layer lines of chickens under controlled experimental conditions. The two experimental lines (63 and 72) of chickens carry a uniform MHC B*2 haplotype and are known to differ in resistance to MD. One of the two commercial egg-layer lines (WL and BL) segregates for three MHC haplotypes (B*2, B*15, and B*21); the other is unclear. MD incidences of the unvaccinated groups of both experimental lines and both commercial lines were 100% or close to 100% induced by the vv+MDV, 648A. Survived day patterns of the unvaccinated groups significantly differed between the two experimental lines, but did not between the two commercial lines, which suggested the two experimental lines do differ in resistance to MD but not between the two commercial lines. At manufacturers' recommended vaccine dosage, two HVTs conveyed comparable protection for the MD resistant line 63 chickens as did both CVI988/Rispens used in this study. The two HVTs also conveyed comparable protection for both commercial lines of chickens as did one of two CVI988/Rispens (CVI988/Rispens-A). At a 2000PFU uniform dose, HVT and CVI988/Rispens again conveyed comparable protection for the MD resistant experimental line of chickens. The findings suggest vaccine protective efficacy is modulated by factors including the types and the sources of vaccines and the genetic backgrounds of chickens. The findings also suggest HVT delivers equal protection in MD resistant lines of chickens as does the industry-recognized golden standard of MD vaccine, CVI988/Rispens.

Cloning of Gallid herpesvirus 3 (Marek's disease virus serotype-2) genome as infectious bacterial artificial chromosomes for analysis of viral gene functions

Marek's disease virus serotype 2 (Gallid herpesvirus 3) is a non-pathogenic alphaherpesvirus belonging to the Mardivirus genus, used widely in live vaccines against Marek's disease. Although the complete genome sequence of the MDV-2 strain HPRS-24 has been published, very little is known about the gene functions. As a first step for carrying out functional genomic analysis of MDV-2, the full-length genome of the MDV-2 vaccine strain SB-1 was cloned as an infectious bacterial artificial chromosome (BAC) clone pSB-1. Virus reconstituted from the pSB-1 clone showed morphological and growth characteristics in cell culture very similar to the parent virus. Generation of SB-1 constructs deleted in glycoprotein E and viruses expressing Citrine-UL35 fusion protein by the application of different BAC mutagenesis techniques demonstrated the amenability of the pSB-1 clone for reverse genetics approaches to identify molecular determinants associated with different biological features of this virus. The generation of replication-competent infectious clones of SB-1, together with those of CVI988 and herpesvirus of turkey strains described previously, completes the portfolio of generating infectious BAC clones of the MD vaccine strains belonging to all the three serotypes, paving the way for the application of reverse genetics for functional analysis of immunogenic determinants of these vaccines as well as for developing novel recombinant vectors.

Targeting Marek's disease virus by RNA interference delivered from a herpesvirus vaccine

Live attenuated herpesvirus vaccines such as herpesvirus of turkey (HVT) have been used since 1970 for the control of Marek's disease (MD), a highly infectious lymphoproliferative disease of poultry. Despite the success of these vaccines in reducing losses from the disease, Marek's disease virus (MDV) strains have shown a continuing increase in virulence, presumably due to the inability of the current vaccines in preventing MDV replication. The highly specific and effective nature of RNA interference (RNAi) makes this technology particularly attractive for new antiviral strategies. In order to exploit the power of RNAi-mediated suppression of MDV replication in vivo delivered through existing vaccines, we engineered recombinant HVT expressing short hairpin RNA (shRNA) against MDV genes gB and UL29. The levels of protection induced by the RNAi-expressing HVT against virulent virus challenge were similar to the parent pHVT3 virus. However, chickens vaccinated with recombinant HVT expressing shRNA showed moderate reduction of challenge virus replication in blood and feather samples. Delivery of RNAi-based gene silencing through live attenuated vaccines for reducing replication of pathogenic viruses is a novel approach for the control of infectious diseases.

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.

Examination of the effect of a naturally occurring mutation in glycoprotein L on Marek's disease virus pathogenesis

We recently reported a comparison of glycoprotein-encoding genes of different Marek's disease virus pathotypes (MDVs). One mutation found predominantly in very virulent (vv)+MDVs was a 12-bp (four-amino acid) deletion in the glycoprotein L (gL)-encoding gene in four of 23 MDV strains examined (three were vv+MDVs and one was a vvMDV). This mutation was noted in the gL of the TK (615K) strain, but not in the RL (615J) strain of MDV. These strains have identical mutations in the meq gene characteristic of vv+MDVs but can be distinguished by the mutation in the gL-encoding gene. The TK strain was originally isolated from vaccinated chickens and appeared to confer or enhance horizontal transmission of the vaccine virus, herpesvirus of turkeys (HVT). Because the molecular basis for increased virulence of MDV field strains is unknown, we hypothesized that one mechanism might be by coreplication of MDV-1 strains with HVT and that it could be mediated by the mutation of gL, an essential component of the glycoprotein H/L complex. In this study, we compared the pathogenicity of TK (615K) and RL (615J) strains of MDV in the presence and absence of simultaneous HVT coinfection. MDV infections were monitored at the levels of viremia (for both MDV-1 and HVT), clinical signs of MD, tumor incidence, and mortality in 1) inoculated chickens, 2) chickens exposed at 1 day of age, 3) chickens exposed at 2 wk of age, and 4) chickens exposed to both TK/HVT- and RL/HVT-infected chickens at 6 wk of age. We found high incidences of clinical MD signs in all inoculated treatment groups and all chickens exposed to TK and RL viruses, regardless of the presence of HVT. The median time to death of chickens exposed to TK1HVT-infected chickens, however, was lower than the other treatment groups for contact-exposed chickens. Although this difference was not considered to be statistically significant to a rigorously interpreted degree because of the removal of chickens for sampling from the test groups, these data suggest that replication of the TK strain and HVT, when coadministered, might incrementally affect the virulence of MDV-1 strains. The strict correlation of this enhancement of virulence with the mutation in gL, however, requires additional experiments with genetically identical MDV background strains.