Microsatellite markers associated with resistance to Marek's disease in commercial layer chickens

The objective of the current study was to identify QTL conferring resistance to Marek's disease (MD) in commercial layer chickens. To generate the resource population, 2 partially inbred lines that differed in MD-caused mortality were intermated to produce 5 backcross families. Vaccinated chicks were challenged with very virulent plus (vv+) MD virus strain 648A at 6 d and monitored for MD symptoms. A recent field isolate of the MD virus was used because the lines were resistant to commonly used older laboratory strains. Selective genotyping was employed using 81 microsatellites selected based on prior results with selective DNA pooling. Linear regression and Cox proportional hazard models were used to detect associations between marker genotypes and survival. Significance thresholds were validated by simulation. Seven and 6 markers were significant based on proportion of false positive and false discovery rate thresholds less than 0.2, respectively. Seventeen markers were associated with MD survival considering a comparison-wise error rate of 0.10, which is about twice the number expected by chance, indicating that at least some of the associations represent true effects. Thus, the present study shows that loci affecting MD resistance can be mapped in commercial layer lines. More comprehensive studies are under way to confirm and extend these results.

Synthesis of chicken major histocompatibility complex class II oligomers using a baculovirus expression system

Chicken major histocompatibility complex (MHC) B21 and B19 haplotypes are associated with resistance and susceptibility to Marek's disease (MD), respectively. T-cell-mediated immune response is crucial in coordinating protection against Marek's disease virus (MDV) infection, but it has been difficult to identify and characterize antigen-specific T-cells. MHC class II tetramers and oligomers have been widely used for characterization of antigen-specific T-cells in the context of infectious and autoimmune diseases. Thus, the objective of this study was to synthesize chicken MHC class II oligomers of B21 and B19 haplotypes for the future identification of antigen-specific T-cells. To achieve this objective, full-length coding sequences of chicken MHC class II B21 and B19 molecules were amplified and the molecules were expressed as fusion proteins, carrying Fos and Jun leucine zipper (LZ), histidine-tag and biotin ligase recognition site sequences, using a baculovirus expression system. Recombinant MHC-II were loaded with self-peptides, which stabilized the heterodimer in SDS-PAGE and allowed the detection of these molecules in Western blots with a conformation-specific anti-chicken MHC class II antibody. Biotinylated MHC molecules were conjugated to streptavidin to form oligomers, which were resolved under the transmission electron microscope through immuno-gold labelling, thus confirming success of oligomerization. In conclusion, chicken MHC class II oligomers may be used in the future to study the antigen-specific CD4+ T-cell compartment.

Molecular Characterization of Major Histocompatibility Complex Class I (B-F) mRNA Variants from Chickens Differing in Resistance to Marek's Disease

In this study, the relative distributions of two alternatively polyadenylated chicken major histocompatibility complex (MHC) mRNA isoforms of approximately 1.5 and 1.9 kb were analysed in spleen cells from chickens homozygous for the MHC haplotypes B21 and B19v1 as well as in heterozygous B19v1/B21 birds. Both isoforms are likely to encode classical MHC class I (B-F) alpha chains. The B19v1 and B21 MHC haplotypes confer different levels of protection against Marek's disease (MD), which is caused by infection with MD virus (MDV). In spleen cells, MD-resistant B21 birds were shown to have the highest percentage of the 1.5 kb variant relative to the total MHC class I expression, MD-susceptible B19v1 birds the lowest and B19v1/B21 birds an intermediate percentage. Infection of 4-week-old chickens with the GA strain of MDV was shown to cause a significant increase in the relative amount of 1.5 kb transcripts in B21 birds 32 days postinfection (dpi). Alternatively polyadenylated mRNA isoforms may encode identical proteins, but differences in the 3' untranslated region (UTR) can influence polyadenylation, mRNA stability, intracellular localization and translation efficiency. It was shown that the increased 1.5 kb percentage in B21 birds 32 days postinfection may be a result of a change in the choice of poly(A) site rather than a locus-specific upregulated transcription of the BF1 gene that preferentially expresses the 1.5 kb variant. Furthermore, the 3' end of the 1.5 kb mRNA variants deriving from B19v1 and B21 chickens was characterized by Rapid Amplification of cDNA Ends (RACE) and sequencing. No potentially functional elements were identified in the 3' UTR of the RACE products corresponding to this short isoform. However, variation in polyadenylation site was observed between the BF1 and BF2 mRNA transcripts and alternative splicing-out of the sequence (exon 7) encoding the second segment of the cytoplasmic part of the mature BF2*19 molecules. This alternative exon 7 splice variant was also detected in other MD-susceptible haplotypes, but not in the MD-resistant B21 and B21-like haplotypes, suggesting a potential role of exon 7 in MHC-related MD resistance.

Identification of peptides associated with chicken major histocompatibility complex class II molecules of B21 and B19 haplotypes

Chicken major histocompatibility complex (MHC) molecules present peptides to T cells to initiate immune response. Some variants of the chicken MHC, such as B19 and B21 haplotypes, are strongly associated with susceptibility and resistance to Marek's disease, respectively. The objective of the present study was to characterize the repertoire and origin of self-peptides presented by chicken MHC class II (B-L) molecules of B19 and B21 haplotypes. Following immunoaffinity purification of B21 and B19 B-L molecules from transformed B cell lines, their associated peptides were eluted, high performance liquid chromatography-fractionated, and sequenced by tandem mass spectrometry. Four peptides were identified associated with B21 B-L molecules. These ranged from 16 to 21 residues in length and had originated from membrane-bound, cytosolic, and mitochondrial proteins. Two of these peptides were present in form of an overlapping set, which is a common characteristic of MHC II-associated peptides. The single B19-associated peptide was 17 residues long and had originated from a cytosolic source. Presentation of endogenous peptides, such as those derived from cytosolic and mitochondrial proteins, by B-L molecules is indicative of cross-sampling between MHC class I and II antigen presentation pathways. These findings facilitate future studies aimed at elucidating mechanisms of chicken MHC association with disease resistance.

Association between rate of viral genome replication and virulence of Marek's disease herpesvirus strains

The early pathogenesis of Marek's disease virus (MDV) infection is characterized by a lytic infection followed by the induction of latency. Genetically resistant N2a and susceptible P2a chickens were infected with the less virulent JM-16 or the very virulent plus (vv+) RK-1 MDV strains to examine the relationship between virulence and resistance on virus replication during 1-10 days postinfection (dpi) using real-time quantitative polymerase chain reaction (qPCR) and quantitative reverse transcriptase (qRT)-PCR assays. The numbers of copies of the viral DNA or transcripts amplified by these assays were normalized relative to cellular controls and subjected to three-way ANOVA. Viral DNA but not RNA was present in spleens at 1-3 dpi in decreasing quantities, and at 4 dpi, viral DNA started to increase concomitant with the initiation of viral transcription independently of virus strain and genetic resistance. At 6 dpi, JM-16 became latent in resistant N2a and susceptible P2a chickens with low levels of viral transcripts, but transcriptional activity increased in susceptible P2a chickens at 9 and 10 dpi. In contrast, infection with vv+ RK-1 never went into latency in both chicken lines. Viral transcripts were present from 4 to 10 dpi showing a higher and more persistent viral activity that may lead to severe damage to the lymphoid organs resulting in increased immunosuppression and increased incidence of MD. The use of qPCR and qRT-PCR to determine viral DNA load and transcriptional activity may offer an alternative to the current system of pathotyping to characterize new MDV isolates.

Marek's disease is a natural model for lymphomas overexpressing Hodgkin's disease antigen (CD30)

Animal models are essential for elucidating the molecular mechanisms of carcinogenesis. Hodgkin's and many diverse non-Hodgkin's lymphomas overexpress the Hodgkin's disease antigen CD30 (CD30(hi)), a tumor necrosis factor receptor II family member. Here we show that chicken Marek's disease (MD) lymphoma cells are also CD30(hi) and are a unique natural model for CD30(hi) lymphoma. Chicken CD30 resembles an ancestral form, and we identify a previously undescribed potential cytoplasmic signaling domain conserved in chicken, human, and mouse CD30. Our phylogeneic analysis defines a relationship between the structures of human and mouse CD30 and confirms that mouse CD30 represents the ancestral mammalian gene structure. CD30 expression by MD virus (MDV)-transformed lymphocytes correlates with expression of the MDV Meq putative oncogene (a c-Jun homologue) in vivo. The chicken CD30 promoter has 15 predicted high-stringency Meq-binding transcription factor recognition motifs, and Meq enhances transcription from the CD30 promoter in vitro. Plasma proteomics identified a soluble form of CD30. CD30 overexpression is evolutionarily conserved and defines one class of neoplastic transformation events, regardless of etiology. We propose that CD30 is a component of a critical intracellular signaling pathway perturbed in neoplastic transformation. Specific anti-CD30 Igs occurred after infection of genetically MD-resistant chickens with oncogenic MDV, suggesting immunity to CD30 could play a role in MD lymphoma regression.

Distinctive polymorphism of chicken B-FI (major histocompatibility complex class I) molecules

The major histocompatibility complex (MHC) in chickens influences disease resistance, but the mechanism is not understood. In Leghorn lines, the MHC contains 2 closely-linked class I loci, B-FI and B-FIV. Previously, we determined nucleotide sequences of well-expressed class I (B-F) genes from unique MHC haplotypes of broiler chicken lines. More recently, we identified 7 new B-F alpha1alpha2-coding sequences from less well-expressed loci by amplification of genomic DNA from unique broiler haplotypes. Phylogenetic analysis of chicken MHC class I alpha1alpha2-coding sequences resolved 2 clusters (Groups A and B), which appear to correspond to B-FIV and B-FI loci, respectively. Compared with B-FIV locus, B-FI alleles were less polymorphic overall, but nevertheless demonstrated evidence of diversifying selection. The most striking feature of B-FI alleles is a conserved, locus-specific motif in the alpha helix of the alpha1 domain, a region that is highly variable in B-FIV alleles. This distinctive pattern of allelic polymorphism resembles that of the HLA-C class I locus in the human MHC (HLA). The conservation of the alpha helix of the alpha1 domain relates to HLA-C interaction with members of the killer immunoglobulin-like receptors on natural killer (NK) cells that are specific for recognition of HLA-C molecules and function to regulate activation of NK cells. Whereas HLA-C molecules may be dominant ligands for NK cell regulation, HLA-A and -B molecules are more important in presenting antigen to cytotoxic T lymphocytes. We hypothesize that chicken B-FI molecules may be specialized to serve similar functions as HLA-C molecules.

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

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

Identification of chicken lymphocyte antigen 6 complex, locus E (LY6E, alias SCA2) as a putative Marek's disease resistance gene via a virus-host protein interaction screen

Marek's disease virus (MDV) is a naturally occurring oncogenic avian herpesvirus that causes neurological disorders and T cell lymphoma disease in domestic chickens. Identification and functional characterization of the individual factors involved in Marek's disease (MD) resistance or pathogenesis will enhance our understanding of MDV pathogenesis and further genetic improvement of chickens. To study the genetic basis for resistance to MD, a strategy that combined protein-protein interaction screens followed by linkage analysis was performed. The MDV protein US10 was used as the bait in an E. COLI two-hybrid screening of a cDNA library derived from activated splenic T cells. The chicken LY6E, also known as SCA2 and TSA1, was found to specifically interact with US10. This interaction was confirmed by an in vitro protein-binding assay. Furthermore, LY6E was found to be significantly associated with MD traits in an MD resource population comprised of commercial chickens. Previously, LY6E was implicated in two independent DNA microarray experiments evaluating differential gene expression following MDV infection. Given that LY6E is involved in T cell differentiation and activation, we suggest that LY6E is a candidate gene for MD resistance and deserves further investigation on its role in MDV pathogenesis, especially with respect to the binding of US10.

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

Nitric oxide (NO), a free radical produced by the enzyme NO synthase (NOS), is a potent antiviral agent in addition to having immune regulating functions. Recently, it was reported that chickens resistant (N2a, MHC: B21B21) to the development of Marek's disease (MD) had a greater potential to produce NO than MD-susceptible chickens (P2a, MHC: B19B19). This difference was shown by measuring NO levels in chick embryo fibroblast cultures obtained from these chickens after treatment with lipopolysaccharide and recombinant chicken interferon-gamma (IFN-gamma). To extend these results, the levels of NO in blood plasma from N2a and P2a chickens inoculated with the nonattenuated JM-16 strain of MD virus (MDV) were examined. In four out of five experiments, N2a chickens had increased NO levels at 7 days postinoculation (DPI). In contrast, P2a chickens challenged with JM-16 had a significant increase in NO in only one of four experiments, and in that experiment the increase was delayed (10 DPI) compared with N2a chickens. Attenuation abrogated MDV-induced NO in chickens. Inoculation with MDV strains ranging from mild to very virulent plus showed that the more virulent strains induced the highest level of NO in blood plasma, suggesting a role of NO in the pathogenesis of MD with more virulent strains. On the basis of quantitative real-time reverse transcription-polymerase chain reaction (RT-PCR) assays for analysis of mRNA expression, IFN-gamma does not appear to be the primary inducer of inducible (i)NOS gene expression during MDV infection. iNOS gene expression and NO production are mediated during the cytolytic phase of MDV infection on the basis of real-time RT-PCR assays with primers specific for glycoprotein B, a late gene expressed only during the cytolytic phase of MDV infection. These findings implicate NO as a factor potentially involved in increasing virulence of MDV, possibly through immune suppression.

Differences in chicken major histocompatibility complex (MHC) class Ialpha gene expression between Marek's disease-resistant and -susceptible MHC haplotypes

The expression of chicken major histocompatibility complex (MHC) class Ialpha genes was investigated in spleen cells from a panel of chickens with well-defined MHC haplotypes, and two class Ialpha transcripts of 1.9 and 1.5 kb were detected in various amounts. In BW1, B130 and B21, the two transcripts were almost equally expressed. In B2, B6, B12 and B19, the ratio between the two transcripts was 4 : 1, with the 1.9 kb transcript having the strongest expression. In B14 and B15, the 1.5 kb transcript was undetectable and the 1.9 kb transcript appeared to be exclusively expressed. Thus, haplotypes considered to have an MHC-determined resistance to Marek's disease (MD) had the highest relative amount of the 1.5 kb transcript, whereas haplotypes considered to be MD-susceptible had the lowest. In order to address a possible correlation between MHC-Ialpha transcriptional patterns and MD resistance, a larger animal material experimentally infected with MD virus (MDV) was examined. The expression of MHC class Ialpha genes was investigated in spleens as well as in other organs, 9 weeks post-infection (p.i.), from animals of the two MD-resistant haplotypes B21 and BW1 as well as from the MD-susceptible haplotype B19. In the spleen cells of infected animals, the relative amount of the 1.5 kb transcript in the haplotypes BW1 and B21 was shown to be significantly higher than that in B19. Interestingly, in infected BW1 and B21 animals, the relative amount of the 1.5 kb transcript was also significantly higher than that in healthy MHC-matched controls. In B19, no differences were detected between uninfected and infected animals. Furthermore, it was shown in BW1 and B21 that the two classical MHC-Ialpha genes located in the MHC region were both able to produce both mRNA transcripts. Hybridization experiments, using specific probes upstream and downstream of the polyadenylation signals in the 3' end of the MHC-Ialpha genes, demonstrated that alternate use of these signals is probably involved in the production of the two mRNA transcripts.

[The selecting experiment of resistance to Marek's disease of chicken]

The Marek's Disease is an acute infectious disease in poultry production. The selecting experiment through blood-group gene inspection of adult parents and attack-test by using of Marek's Disease virus (MDV) for offspring chicks was carried out. The results showed as follows: (1) The rates of positive reaction by No. 614 blood-group gene inspection of adult parents in four generations were increased due to generation increase, as F0-68.8%, F1-77.3%, F2-81.8% and F3-90.5%. (2) In the attack-test by using of MDV at 10 days of age of chicks the mortalities of the experimental group at 60 days of age of chicks whose parents have positive reaction by No. 614 blood-group reagent detect in F2-F4 generations were 25.8%, 69.2% and 29.4% respectively, while the mortalities of the control group at same age of chicks whose parents have negative reaction were 30.6%, 81.0% and 52.9% respectively. These differences of mortality were significant (P < 0.01 or P < 0.05). (3) The rates of pathological change of 9 viscera tumor of the experimental group chicks in F2-F4 generations were 87.6%, 83.3% and 58.8% respectively, while chicks in the control group were 88.1%, 84.5% and 70.6% respectively. Except individual generation and viscera, these differences between the experimental group and the control group were almost not significant (P > 0.05). The experimental results and genetic control of resistance to Marek's Disease are discussed in this paper. The new strain of chicken resistant to Marek's Disease will be bred.

Phenotypic variation among three broiler pure lines for Marek's disease, coccidiosis, and antibody response to sheep red blood cells

To identify candidate genes, chicken lines with the most divergent phenotypes are usually crossed to generate resource mapping populations, for example, either backcrossed or F2 populations. Linkage between the genetic marker and the phenotypic trait locus is then tested in the mapping population. As an initial step in the development of a mapping population from commercial broilers, the goal of the current research was to evaluate the phenotypic variation among three pure lines for antibody response to SRBC and in resistance to two economically important poultry diseases, Marek's disease (MD) and coccidiosis (Eimeria acervulina). Chicks from each line were received and separated into three experimental studies to evaluate each of their responses. In summary, broiler Line 3 had significantly lower antibody responses to SRBC immunizations compared to the other two lines, and nonvaccinated birds from Line 3 were also more susceptible to MD. With coccidiosis, the response was complex, and ranking of the lines was dependent on the age of infection, and whether it was a first or second challenge. With the first challenge, Line 1 was most susceptible at the younger age (Day 30), whereas Line 3 was susceptible at the older age (Day 58). Upon the second challenge, broiler Line 1 remained susceptible at the younger age, but Line 2 was more susceptible at the older age. Line 3 was completely resistant to the second challenge at the older age. Thus, although the broiler lines have been intensively selected for productivity and general livability, this study also demonstrates that the lines differ for immune response and disease resistance. Based on the phenotypic differences between Lines 1 and 3, they were chosen to establish a mapping population for identifying candidate genes that affect MD and coccidiosis in commercial broiler chickens.

Immunological basis of differences in disease resistance in the chicken

Genetic resistance to diseases is a multigenic trait governed mainly by the immune system and its interactions with many physiologic and environmental factors. In the adaptive immunity, T cell and B cell responses, the specific recognition of antigens and interactions between antigen presenting cells, T cells and B cells are crucial. It occurs through a network of mediator proteins such as the molecules of the major histocompatibility complex (MHC), T cell receptors, immunoglobulins and secreted proteins such as the cytokines and antibodies. The diversity of these proteins that mainly is due to an intrinsic polymorphism of the genes causes phenotypic variation in disease resistance. The well-known linkage of MHC polymorphism and Marek's disease resistance difference represents a classic model revealing immunological factors in resistance differences and diversity of mediator molecules. The molecular bases in any resistance variation to infectious pathogens are vaguely understood. This paper presents a review of the major immune mediators involved in resistance and susceptibility to infectious diseases and their functional mechanisms in the chicken. The genetic interaction of disease resistance with production traits and the environment is mentioned.

Quantitative trait locus mapping in chickens by selective DNA pooling with dinucleotide microsatellite markers by using purified DNA and fresh or frozen red blood cells as applied to marker-assisted selection

Many large, half-sib sire families are an integral component of chicken genetic improvement programs. These family structures include a sufficient number of individuals for mapping quantitative trait loci (QTL) at high statistical power. However, realizing this statistical power through individual or selective genotyping is yet too costly to be feasible under current genotyping methodologies. Genotyping costs can be greatly reduced through selective DNA pooling, involving densitometric estimates of marker allele frequencies in pooled DNA samples. When using dinucleotide microsatellite markers, however, such estimates are often confounded by overlapping "shadow" bands and can be confounded further by differential amplification of alleles. In the present study a shadow correction procedure provided accurate densitometric estimates of allele frequency for dinucleotide microsatellite markers in pools made from chicken purified DNA samples, fresh blood samples, and frozen-thawed blood samples. In a retrospective study, selective DNA pooling with thawed blood samples successfully identified two QTL previously shown by selective genotyping to affect resistance in chickens to Marek's disease. It is proposed that use of selective DNA pooling can provide relatively low-cost mapping and use in marker-assisted selection of QTL that affect production traits in chickens.

A strategy to identify positional candidate genes conferring Marek's disease resistance by integrating DNA microarrays and genetic mapping

Marker-assisted selection (MAS) to enhance genetic resistance to Marek's disease (MD), a herpesvirus-induced T cell cancer in chicken, is an attractive alternative to augment control with vaccines. Our earlier studies indicate that there are many quantitative trait loci (QTL) containing one or more genes that confer genetic resistance to MD. Unfortunately, it is difficult to sufficiently resolve these QTL to identify the causative gene and generate tightly linked markers. One possible solution is to identify positional candidate genes by virtue of gene expression differences between MD resistant and susceptible chicken using deoxyribonucleic acid (DNA) microarrays followed by genetic mapping of the differentially-expressed genes. In this preliminary study, we show that DNA microarrays containing approximately 1200 genes or expressed sequence tags (ESTs) are able to reproducibly detect differences in gene expression between the inbred ADOL lines 63 (MD resistant) and 72 (MD susceptible) of uninfected and Marek's disease virus (MDV)-infected peripheral blood lymphocytes. Microarray data were validated by quantitative polymerase chain reaction (PCR) and found to be consistent with previous literature on gene induction or immune response. Integration of the microarrays with genetic mapping data was achieved with a sample of 15 genes. Twelve of these genes had mapped human orthologues. Seven genes were located on the chicken linkage map as predicted by the human-chicken comparative map, while two other genes defined a new conserved syntenic group. More importantly, one of the genes with differential expression is known to confer genetic resistance to MD while another gene is a prime positional candidate for a QTL.

Resistance to Marek's disease virus in White Leghorn chickens: effects of avian leukosis virus infection genotype, reciprocal mating, and major histocompatibility complex

Genetic improvement for resistance to Marek's Disease (MD) in chickens continues to be of interest to the poultry industry. The aims of this study were to identify effects of the MHC on the molecular level and of avian leukosis virus (ALV) resistance status on MD mortality in two noninbred White Leghorn chicken lines that differ in B blood group type. Previously, within each of the chicken lines, sublines had been selected for resistance or susceptibility to ALV infection with Subgroups A and B. In this study, F2 offspring, obtained by crossing the two ALV-resistant or the two ALV-susceptible sublines, were tested for MD mortality after contact exposure at 1 d of age. Reciprocal matings were made in the grandparental generation. The MD mortality percentages, in an observation period of 17 wk, of F2 offspring from two hatches were 82.63 and 92.35%, respectively. Survival analysis (Cox model) was applied to assess the risk of dying from MD. No differences in MD mortality risk profiles were found between ALV-resistant and ALV-susceptible F2 offspring. Within ALV-susceptible F2 offspring, however, a reciprocal mating effect was observed in both hatches. The MHC Class I, II, and IV restriction fragment length polymorphism (RFLP) analyses were carried out on birds of the first hatch. Although two of 11 MHC class IV RFLP bands displayed a significant effect, in general, a strong association of MHC and MD mortality was not detectable.

Genetic variation in susceptibility to Marek's disease in a commercial broiler population

Two commercial broiler pure lines that were previously identified to differ in their susceptibility to Marek's disease (MD) were line-crossed to generate an F1 population. Eight F1 males were randomly mated to four or five F1 females to produce an F2 test population that would be segregating for genes affecting MD. All F2 progeny (four hatches) were pedigreed at hatch and placed in colony houses as nonvaccinated. At 5 days of age, they were challenged intraabdominally with MD virus RB1B. Clinical signs, mortality, and gross and microscopic lesions were recorded during the MD challenge. At 8 wk postchallenge, all remaining birds were euthanatized and necropsied. During the MD challenge of the first two hatches, we observed that several severely stunted broilers originated from certain families and the differences in body weight among birds appeared as early as 3 wk postchallenge. To confirm this observation, body weight at 6 wk postchallenge was determined for all surviving birds in hatches 3 and 4 (n = 242). Genetic variation in body weight among broiler sire families was apparent; the average body weight for males at this time was 2.07 kg, whereas with females, it was 1.87 kg. At least 12.2% of the broilers, including both sexes, weighed less than 1 kg ("severely stunted") at this time. The incidence of these growth-stunted birds within each broiler sire family ranged from 0 to 26% and for dam families, 0 to 60%. Correlation analyses between stunting and other MD-associated traits revealed that the incidence of stunting had a significant and positive association with paralysis (r = 0.50). Therefore, the data suggest that there may be a genetic component affecting body weight loss during MD infection. The genetic component is speculated to affect susceptibility to MD paralysis with an indirect effect on the body weight of birds. The significance of this finding is best exemplified by the identification of a broiler sire family with over 26% of its progeny affected by this MD-associated trait.

Resistance to Marek's disease herpesvirus-induced lymphoma is multiphasic and dependent on host genotype

Genotype-dependent differences in Marek's disease (MD) susceptibility were identified using 14-day-old line N and 6(1) (resistant) and 151 and 7(2) (susceptible) inbred chickens infected with HPRS-16 MD virus (MDV). All line 72 chickens developed progressive MD. Line 15I had fluctuating MD-specific clinical signs and individuals recovered. A novel histologic scoring system enabled indices to be calculated for lymphocyte infiltration into nonlymphoid organs. All genotypes had increased mean lesion scores (MLSs) and mean total lesion scores after MDV infection. These differed quantitatively and qualitatively between the genotypes. Lines 6(1) and 7(2) had a similar MLS distribution in the cytolytic phase, although scores were greater in line 7(2). At the time lymphomas were visible in line 7(2), histologic lesions in line 6(1) were regressing. AV37+ cells were present in similar numbers in all genotypes in the cytolytic phase, suggesting that neoplastically transformed cells were present in all genotypes regardless of MD susceptibility. After the cytolytic phase, AV37+ cell numbers increased in lines 7(2) and 15I but decreased in lines 6(1) and N. In the cytolytic and latent phases, in all genotypes, most infiltrating cells were CD4+. After this time, line 7(2) and 15I lesions increased in size and most cells were CD4+; line 6(1) and N lesions decreased in size and most cells were CD8+. In all genotypes, AV37 immunostaining was weak in lesions with many CD8+ cells, suggesting that AV37 antigen expression or AV37+ cells were controlled by CD8+ cells. The rank order, determined by clinical signs and pathology, for MD susceptibility (highest to lowest) was 7(2) > 15I > 6(1) > N.

The simple chicken major histocompatibility complex: life and death in the face of pathogens and vaccines

In contrast to the major histocompatibility complex (MHC) of well-studied mammals such as humans and mice, the particular haplotype of the B-F/B-L region of the chicken B locus determines life and death in response to certain infectious pathogens as well as to certain vaccines. We found that the B-F/B-L region is much smaller and simpler than the typical mammalian MHC, with an important difference being the expression of a single class I gene at a high level of RNA and protein. The peptide-binding specificity of this dominantly expressed class I molecule in different haplotypes correlates with resistance to tumours caused by Rous sarcoma virus, while the cell-surface expression level correlates with susceptibility to tumours caused by Marek's disease virus. A similar story is developing with class II beta genes and response to killed viral vaccines. This apparently suicidal strategy of single dominantly expressed class I and class II molecules may be due to coevolution between genes within the compact chicken MHC.

Marek's disease: an update on oncogenic mechanisms and control

Marek's disease (MD) is a common lymphoproliferative disease of poultry caused by a highly contagious and oncogenic herpesvirus. In spite of the widespread use of highly effective MD vaccines, recently there have been worrying trends in the evolution of MD virus pathotypes towards greater virulence. In the last few years, there has been significant progress in determining the molecular structure of MD virus and several genes that map within the repeat regions of the virus, such as Bam HI-H family, ICP 4, meq and pp38, which are potentially associated with the latency and transformation have been identified. The functions of some of these genes have provided insights into the mechanisms of MD virus-induced oncogenesis. This review summarises some of these oncogenic mechanisms and the progress in the control of MD.

Identification of the Marek's disease virus serotype 2 genes homologous to the glycoprotein B (UL27), ICP18.5 (UL28) and major DNA-binding protein (UL29) genes of herpes simplex virus type 1

We determined the nucleotide sequence of non-pathogenic Marek's disease virus serotype 2 (MDV2) strain HPRS24 glycoprotein B (gB) (UL27), ICP18.5 (UL28) and major DNA-binding protein (MDBP) (UL29) genes homologous to herpes simplex virus type 1 (HSV-1). The sequence data revealed that important motives in the proteins are conserved in MDV2 ICP18.5 and MDBP, however the sequence of viral DNA replication origin which exists in the regions between the UL29 and UL30 genes of other alphaherpesviruses was not found in the regions of the MDV2 genome. By northern blot analyses, we also demonstrated that 8.9, 5.0 and 2.6 kb transcripts were actually transcribed from the sequenced region in MDV2-infected cells. The MDV2 UL28 and UL29 genes have not been reported in other serotypes of MDV.

Development of the hot spot-combined PCR assay for detection of retroviral insertions into Marek's disease virus

A two-step PCR, the Hot Spot-combined PCR assay, was developed for the identification and characterization of recombinant viruses in Marek's disease (herpes) and retrovirus co-infections. In the first PCR the herpesvirus genomic fragment, that was recognized in previous studies as a hot spot site for retroviral integration was amplified [reviewed in Bronovskis, P., Kung, H.-J., 1996. Retrotransposition and herpesvirus evolution. Virus Genes 11, 259-270]. The products served for a second amplification step, performed in six PCR sets, using the six possible combinations of the two herpes and the retrovirus primer sets. Development of the assay employed DNA of the recombinant virus, RM1, which was created by in vitro co-cultivation of Marek's disease and reticuloendotheliosis viruses [Isfort et al., 1992. Proc. Natl. Acad. Sci. 89, 991-995; Witter et al., 1997. Avian Dis. 41, 407-421]. As the retroviral insertion site and junction sequences were determined previously [Jones et al., 1996. J. Virol. 70, 2460-2467], RMI served in the present study as a test virus for the development of the new assay. It is shown now that the Hot Spot-combined PCR can detect the retroviral insert in RM1. the MDV integration site and the insert orientation. For confirmation the herpes and retrovirus chimeric PCR products were sequenced and the results were similar to those published previously [Jones et al., 1996. J. Virol. 70, 2460-2467]. This assay might be adopted in additional systems to detect foreign inserts at suspected genomic sites.

Mapping quantitative trait loci for binary traits using a heterogeneous residual variance model: an application to Marek's disease susceptibility in chickens

A typical problem in mapping quantitative trait loci (QTLs) comes from missing QTL genotype. A routine method for parameter estimation involving missing data is the mixture model maximum likelihood method. We developed an alternative QTL mapping method that describes a mixture of several distributions by a single model with a heterogeneous residual variance. The two methods produce similar results, but the heterogeneous residual variance method is computationally much faster than the mixture model approach. In addition, the new method can automatically generate sampling variances of the estimated parameters. We derive the new method in the context of QTL mapping for binary traits in a F2 population. Using the heterogeneous residual variance model, we identified a QTL on chromosome IV that controls Marek's disease susceptibility in chickens. The QTL alone explains 7.2% of the total disease variation.

Mitochondrial PEPCK: a highly polymorphic gene with alleles co-selected with Marek's disease resistance in chickens

The gene coding for the mitochondrial phosphoenolpyruvate carboxykinase (PEPCK-M), a pivotal component in gluconeogenesis from lactate via the Cori cycle, was highly polymorphic in strains of egg-type chickens (White Leghorn) of different origins. Based on MspI restriction fragment polymorphisms a total of seven alleles could be distinguished. The allele frequencies were determined in six pairs of strains derived from different genetic base populations. Each pair consisted of two strains which differed in their susceptibility to Marek's disease (MD), a virus-induced neoplastic disease. The frequency of the most common haplotype (M2) was consistently higher in the susceptible strains than in the corresponding resistant strains (P < 0.05, Wilcoxon signed-ranks test), indicating that the observed differences were not due to random genetic drift. This result suggests that PEPCK-M may be a candidate gene which contains genetic variants affecting MD susceptibility. Variations in gluconeogenesis may affect the interplay between proliferation of neoplasia and host metabolism.

Analysis of tumor suppressor gene p53 in chicken lymphoblastoid tumor cell lines and field tumors

To determine whether there is any abnormalities of the p53 gene in chicken lymphoblastoid tumor cell lines derived from Marek's disease (MD), lymphoid leukosis, reticuloendotheliosis, and field tumors, some portions of p53 cDNA corresponding to core and C-terminal domains (nucleotide positions 277-1104 in the p53 open reading frame (ORF)) were sequenced. Several mutations were identified in both cell lines and field tumors. However, none of these mutations is localized at the "hot spot", which has been reported as the site for transformation-activating mutations. Moreover, partial cDNA clones with a 122-bp deletion in the p53 ORF were identified in two cell lines, MSB1 and MTB1 derived from MD tumors. Southern blot analysis showed that no deletion occurred in the genome of p53 in MSB1, indicating that deletion occurred at the transcriptional level. This deletion could cause a frame shift of the encoding p53 protein, possibly resulting in the generation of a functionally different p53 protein. However, we confirmed that p53 mRNA without deletion is also present in each of these cell lines. These mutations of the p53 gene and deletion in the p53 transcript may be ones of molecular changes specific to the transformation induced by MD virus.

Genetic resistance to avian viruses

Viral infections of poultry can be catastrophic in terms of both welfare and economics, and although vaccines have been very successful in combating these diseases, new forms of viruses have evolved which present increasing difficulties for vaccine control. Differences in genetic susceptibility are known to exist for many of the major viral pathogens of poultry. Consequently, an increase in the level of genetic resistance provides a possible means of enhancing protection of flocks. This is particularly feasible where specific resistance genes have been identified, as in the case of avian leukosis and Marek's disease, and the development of genetic maps of the chicken has offered new possibilities for the identification of further resistance genes. It has also become clear that there are genetic differences in the response to live attenuated vaccine viruses, and new possibilities exist to manipulate the genetics of host flocks so that the effect of vaccination can be optimised.

Class II MHC cDNAs in 15I5 B-congenic chickens

cDNA was obtained from the bursae of Fabricius of chickens from six B-congenic lines developed at this laboratory and studied for expression of class II B-LB genes. Following cDNA amplification, cloning and sequencing, genes were assigned to B-LB loci based on characteristic DNA sequences, amino acid relatedness to characterized genes, and level of expression. Genes from the B-LBI, B-LBII, and B-LBVI loci were differentially expressed in chickens with the B2, B5, B13, B15, or B21 haplotypes. Chickens of all haplotypes expressed a B-LBII gene. Additional B-LB genes expressed included: B-LBI genes in the B5 and B19 haplotypes; a B-LBI/VI recombinant gene in the B2 haplotype; and a B-LBVI gene in the B13 haplotype. The B-congenic lines have demonstrable differences in resistance to Marek's disease (MD), and in responses to MD viral vaccines. This variability in disease resistance may be correlated with polymorphisms in the expressed B-LB genes, or with differential expression of genes at different loci.

Rfp-Y region polymorphism and Marek's disease resistance in multitrait immunocompetence-selected chicken lines

Although the influence of the chicken classical MHC in resistance to many diseases is well established, the role of the recently identified, genetically independent, MHC-like region known as Rfp-Y is unclear. The objectives of this study were to analyze the frequencies of DNA polymorphisms of the Rfp-Y region in White Leghorn lines, which were divergently selected in replicate for multitrait immunocompetence, and to determine the association of these polymorphisms with Marek's disease (MD) resistance. Chicks, either with or without herpes virus of turkey (HVT) vaccination, were challenged with 500 ffu of a very virulent Marek's disease virus (Md5) at 2 d of age. The MD-related data were collected for 10 wk. PvuII-digested genomic DNA was hybridized with an Rfp-Y region-specific probe, 18.1. Three Rfp-Y polymorphisms were observed. The frequency of one Rfp-Y polymorphism was significantly different between divergently selected multitrait immunocompetence lines in one replicate only; therefore, the impact of multitrait immunocompetence selection on Rfp-Y polymorphisms is inconclusive. The PvuII defined Rfp-Y region polymorphisms had no association with either innate or vaccine-induced MD resistance to Md5 virus challenge.

Association of endogenous viral genes with quantitative traits in chickens selected for high egg production and susceptibility or resistance to Marek's disease

1. The association of endogenous viral (ev) genes with quantitative traits in 2 genetically distinct sets of White Leghorn strains were investigated. Strain S had been selected for susceptibility to Marek's disease (MD) whereas strain K had been selected for resistance to MD and high egg production and egg weight. 2. In all, 8 ev genes were typed. Ev10, ev19 and 'newB' occurred exclusively in strain S, 'newA' occurred only in strain K, and ev1, ev3, ev6 and ev8 occurred in both strains. 3. Whereas ev6 and ev8 were associated with reduction in egg production rate in strain S, in strain K, the presence of ev3 was associated with increased group specific antigen. 4. The genetic background of the chicken strain may play a role in the way certain ev genes affect traits. 5. It was thought that the position of ev genes on the chromosome may be important and their association with traits of economic importance make them potential genetic markers for uncovering quantitative trait loci.

The "minimal essential MHC" revisited: both peptide-binding and cell surface expression level of MHC molecules are polymorphisms selected by pathogens in chickens

Birds, like mammals, have a highly polymorphic MHC that determines strong allograft rejection. However, in contrast to mammals, there are a number of viral diseases for which resistance and susceptibility are determined by particular chicken MHC haplotypes. We have found that certain common chicken MHC haplotypes express only one class I molecule at high levels. The selection on a single MHC gene should be strong, in contrast to the situation in mammals. We have determined the peptide motifs for the dominant class I molecules from a number of chicken MHC haplotypes and found that they can explain the outcome of infections with a small virus. However, the strongest MHC association is the resistance of the chicken B21 haplotype to classical Marek's disease virus, a relatively large pathogen for which any MHC molecule should find peptides. In 40 chicken lines, the level of class I expression correlates with the level of MHC-determined susceptibility to Marek's disease, the most susceptible B19 with the highest expression and the most resistant B21 with the lowest expression. Thus, cell surface expression level of class I molecules appears to be a polymorphism under selection by infectious pathogens, just like peptide-binding specificity. We speculate that these expression level differences are another manifestation of the simple MHC of chickens, which in human and mouse haplotypes are averaged out.

Genetic mapping of quantitative trait loci affecting susceptibility to Marek's disease virus induced tumors in F2 intercross chickens

Marek's disease (MD) is a lymphoproliferative disease caused by the MD virus (MDV), which costs the poultry industry nearly $1 billion annually. To identify quantitative trait loci (QTL) affecting MD susceptibility, the inbred lines 6(3) (MD resistant) and 7(2) (MD susceptible) were mated to create more than 300 F2 chickens. The F2 chickens were challenged with MDV JM strain, moderately virulent) at 1 wk of age and assessed for MD susceptibility. The QTL analysis was divided into three stages. In stage 1, 65 DNA markers selected from the chicken genetic maps were typed on the 40 most MD-susceptible and the 40 most MD-resistant F2 chickens, and 21 markers residing near suggestive QTL were revealed by analysis of variance (ANOVA). In stage 2, the suggestive markers plus available flanking markers were typed on 272 F2 chickens, and three suggestive QTL were identified by ANOVA. In stage 3, using the interval mapping program Map Manager and permutation tests, two significant and two suggestive MD QTL were identified on four chromosomal subregions. Three to five loci collected explained between 11 and 23% of the phenotypic MD variation, or 32-68% of the genetic variance. This study constitutes the first report in the domestic chicken on the mapping of non-major histocompatibility complex QTL affecting MD susceptibility.

Major histocompatibility complex class II DNA polymorphisms in chicken strains selected for Marek's disease resistance and egg production or for egg production alone

The objective of this study was to investigate frequencies of major histocompatibility complex (MHC) class II restriction fragments in two groups of White Leghorn strains. Each group consisted of an unselected control, a strain selected for egg production traits, and a strain selected for egg production traits and Marek's disease (MD) resistance. PvuII-digested genomic DNA was hybridized with a chicken genomic MHC class II probe. The MHC class II DNA fragment frequencies in the selected strains differed from those in the related unselected control and in the strain selected using the same criteria from a different base population. Based on the sizes of the breeding populations, particularly those in the control strain and in the strain selected for egg production, it was considered unlikely that the observed changes of the MHC class II fragment frequencies were due to random genetic drift. The data suggested that some MHC class II bands are associated with production traits or with MD resistance, and that these associations tend to be unique to each genetic background. Hence, MHC class II genes are likely candidates for the investigation of quantitative trait loci in egg production and disease resistance traits such as those for which the studied strains were selected.

Non-association between Rfp-Y major histocompatibility complex-like genes and susceptibility to Marek's disease virus-induced tumours in 6(3) x 7(2) F2 intercross chickens

Marek's disease (MD) is a lymphoproliferative disease caused by a member of the herpesvirus family, and the best understood genetic resistance to MD involves the chicken major histocompatibility complex (MHC) B-complex. Preliminary observations have suggested that MHC-like Rfp-Y genes might also influence the incidence of MD. This study describes the differentiation and definition of unique Rfp-Y genes in inbred lines 6(3) and 7(2), lines that possess identical B-complex genes, but that are resistant or susceptible to MD, respectively. To assess if Rfp-Y genes affect susceptibility to MD, 265 6(3) x 7(2) F2 chickens were challenged with the JM strain of MD virus at 1 week of age and were evaluated for MD lesions at up to 10 weeks of age. Genotyping of the F2 chickens for Rfp-Y haplotypes was performed by restriction fragment length polymorphism analysis of genomic DNA using TaqI and a B-FIV probe. Analysis of variance and interval mapping procedures were used to determine association between the Rfp-Y haplotypes and the phenotypic MD values of the F2 chickens. The cosegregation analysis of 265 F2 chickens indicated that there was no association between Rfp-Y haplotypes and MD susceptibility. Furthermore, the fact that the Rfp-Y haplotypes fit the 1:2:1 segregation ratio and the Rfp-Y allele frequencies did not differ significantly from 0.5 in the full population or in selected subpopulations (of either 40 MD-resistant or 39 MD-susceptible chickens) also indicated that Rfp-Y haplotypes do not significantly influence MD susceptibility. We conclude that Rfp-Y haplotypes do not play a major role in determining the genetic susceptibility to MD in 6(3) x 7(2) F2 White Leghorn chickens.

Recombinant fowlpox virus vaccines against Australian virulent Marek's disease virus: gene sequence analysis and comparison of vaccine efficacy in specific pathogen free and production chickens

We have cloned and sequenced the glycoprotein genes gB, gC and gD of the Australian virulent Marek's disease virus (MDV) isolate Woodlands No. 1. The glycoprotein gB and gC sequences were identical to the homologs of other virulent MDV type 1 strains, and the glycoprotein gD sequence contained only one unique amino acid substitution. Recombinant fowlpox viruses (rFPVs) expressing the MDV glycoprotein genes were constructed and their efficacy as vaccines was evaluated in specific pathogen free (SPF) and production chickens. Vaccination with the FPV-gB recombinant protected SPF chickens from Marek's disease mortality and tumour formation following challenge with virulent MDV Woodlands No. 1. The degree of protection from Marek's disease was dependent on the vaccine dose and route of inoculation. The rFPVs expressing gC or gD did not provide protection from Marek's disease. A rFPV expressing both gB and gC did not provide enhanced protection in comparison with the rFPV-gB alone. The rFPV-gB vaccine failed to protect commercial chickens from MD mortality and provided little protection from tumour formation in comparison with the commercial herpesvirus of turkey (HVT) vaccine. The failure to provide protection against MD may be related to the impact of maternally derived immunity to MDV and FPV and possibly the genotype of the chickens.

Influence of Marek's disease virus strain AC-1 on cellular immunity in birds carrying endogenous viral genes

The effects of Marek's disease virus (MDV) strain AC-1 on humoral and cellular immune responses were investigated in two lines of chickens segregating for the endogenous viral (ev) genes ev-6 and ev-12. All birds were vaccinated at 14 days of age against Newcastle disease virus (NDV). At 3 wk of age, 48 birds per line received an intraperitoneal injection of MDV (AC-1 isolate), and 24 were injected with saline. Birds of each group were killed on days 5, 7, 12, and 14 postinfection. Data were first analyzed for each day of testing. Results showed that, for variable measured, treatment effects were similar on days 5 and 7, and on days 12 and 14. Therefore, day 5 and day 7 data, and day 12 and day 14 data were pooled and analyzed. In MDV-infected chickens, proliferative lymphocyte responses to mitogens were suppressed (P < 0.001) after the first and second week of infection, whereas responses to NDV antigen were enhanced (P < 0.001) after the first week and then reduced (P < 0.01) by the end of the second week when compared to uninfected birds. The percentage of CD4+ T cells was higher (P < 0.01), and the percentage of cells expressing major histocompatibility complex (MHC) class II antigens was lower (P < 0.001), in MDV-infected chickens than in uninfected birds. The cytotoxic activity of natural killer (NK) cells was also enhanced (P < 0.001) in MDV-infected birds when compared to uninfected birds. Antibody responses to NDV were not different among groups, and the presence of ev-6 or ev-12 genes did not influence the immune response parameters measured in both infected and uninfected chickens. In conclusion, a marked increase in the CD4+ T lymphocyte population occurred in the early stage of MDV infection in all chickens regardless of the presence of ev genes, whereas the number of cells expressing MHC class II antigen was severely reduced.

Genetic susceptibility to Marek's disease virus of local chickens in Taiwan

Marek's disease is a contagious disease in chickens caused by Marek's disease virus (MDV) infection. Invasion of very virulent MDV (vvMDV) was considered to be a major cause of vaccine break, resulting in a large economic loss in the poultry industry. Two strains of the vvMDV (strains LTB-1 and LTS-1) have been reported in Taiwan, causing early mortality and formation of lymphoid tumors in broilers and layers. In this study, we report the susceptibility of local chickens in Taiwan to inoculation with strains LTB-1 and LTS-1 at 1 day of age. Five lines of Taiwanese local chickens (lines B, D, L2, KM, and BG) were used to compare the susceptibility to vvMDV. The chicks were inoculated via intraperitoneal route at 1 day of age. All MDV-inoculated chickens showed atrophy of various lymphoid organs. Protection tests were also conducted in local chickens, using vaccine of herpesvirus of turkey (HVT) at 1 day of age followed by vvMDV challenge at 10 days of age. Among the vaccinated chickens, 0-20.0% showed early mortality, and tumor occurrences in visceral organs and in peripheral nerves were 66.7%-100% and 0-27.2%, respectively. From these results, it could be concluded that local chickens in Taiwan have different susceptibilities to vvMDVs. Furthermore, vaccination with HVT showed no protective effect against Taiwanese vvMDV isolate challenge in this experiment.

Influence of chicken genotype on protection against Marek's disease by a herpesvirus of turkeys recombinant expressing the glycoprotein B (gB) of Marek's disease virus

Two inbred lines of White Leghorn chickens which differ in B-haplotype were immunized at 2 days of age with a thymidine kinase negative (tk-ve) herpesvirus of turkeys (HVT) recombinant expressing the glycoprotein B (gB) gene of Marek's disease virus (MDV) and were challenged 6 days later with 1000 p.f.u. of the highly virulent RB1B strain of MDV. Mock-vaccinated chickens and chickens immunized with a spontaneous tk-ve HVT mutant served as controls. Genetically resistant B21 chickens were protected by immunization with the recombinant as well as by the tk-ve HVT, whereas highly susceptible B13 chickens were partially protected by the recombinant but were not protected by the tk-ve HVT. Rhode Island Red chickens (HPRS RIR), which differ from the White Leghorns at the B locus, were protected by both vaccines but the recombinant conferred a significantly higher level of protection than the tk-ve HVT. The results suggest that the gB gene of MDV serotype 1 has an important role in the induction of protective immunity against highly virulent MDV in genetically susceptible lines of chickens and that vaccinal immunity in White Leghorns might be influenced by the B haplotype.

Interferon modulation of Marek's disease virus genome expression in chicken cell lines

Lines of chicken lymphoblastoid cells were established from local lesions induced by simultaneous injection of Marek's disease virus and various stimulants of T-cell activation. Lines developed with regular medium had relatively high mean rates of spontaneous expression of viral internal antigen (6.2%). In contrast, lines developed and maintained with conditioned medium generated by mixed-lymphocyte reaction had a 62-fold reduction in the mean rate of viral internal antigen expression (0.1%). The expression rate could be modulated by the removal or re-addition of conditioned medium to the growth medium. Down regulation involved proteins classified as immediate-early (a 14-kDa polypeptide), early (a 38-kDa phosphoprotein), and late (glycoprotein B homologue) antigens, indicating that the block is very early in virus replication. Once initiated in a given cell, replication apparently proceeded unimpeded. Interferon was determined to be largely responsible for the suppressive activity of the conditioned medium, although involvement of other cytokines could not be ruled out. Also, chicken interferon from other sources, including recombinant interferon, was able to similarly suppress viral antigen expression.

Expression of the endogenous Marek's disease virus ICP4 homolog (MDV ICP4) gene is enhanced in latently infected cells by transient transfection with the recombinant MDV ICP4 gene

The ICP4 homolog of Marek's disease virus (MDV ICP4) is a possible candidate for the transactivator of the early genes. We transfected MDCC-MSB-1 (MSB-1) tumor cells with plasmid including a coding region of MDV ICP4 using cationic liposome. As carriers for intranuclear transport, high mobility group -1 and -2 proteins were bound to the plasmid DNA before forming liposomes. We detected transcripts from the plasmid 2 hr after transfection by quantitative reverse-transcriptase polymerase chain reaction (RT-PCR) analysis. We also detected abundant transcripts of endogenous ICP4 2-96 hr after transfection. These data suggested that expression of introduced MDV ICP4 gene enhanced the expression of endogenous MDV ICP4. On the other hand, quantitative PCR analysis for virus genome DNA indicated no significant alteration of copy number of virus genome in transfected MSB-1 cells, suggesting that reactivation of virus requires more than turning on MDV ICP4 gene.

Cytogenetic studies of cell lines derived from Marek's disease virus-induced local lesions

Cytogenetic analysis of cell lines derived from Marek's disease virus (MDV) induced local lesions was performed to detect the presence of a chromosomal alteration found previously in cell lines derived from MDV-induced visceral tumors. This chromosomal alteration involves an amplified region on the short arm of chromosome 1. Trypsin G-binding was performed on 12 local-lesion cell lines having various T-cell receptor phenotypes. Eight of 12 cell lines had a diploid female karyotype, and four lines were diploid males. Ten of the cell lines showed a normal G-banding pattern; two lines, however, had the short arm amplification on chromosome 1 (1p+). Thus, the 1p+ alteration does not appear to be necessary for establishment of cell lines from local lesions. The occurrence of the 1p+ alteration in Marek's disease cell lines suggests a possible role for this alteration in late stages of multi-step viral oncogenesis.

B haplotype influence on the relative efficacy of Marek's disease vaccines in commercial chickens

The objectives were to investigate whether or not B haplotypes influence vaccinal immunity against Marek's disease (MD) in commercial chickens and to evaluate whether retrospective analysis would detect the influence. This method involved evaluating the B haplotypes of turkey herpesvirus (HVT)-vaccinated sick vs normal chickens from a flock afflicted with MD symptoms. An analysis of the retrospective data disclosed that MD symptoms were present in a higher proportion of B2B19 than B2B21 chickens. A prospective study was then conducted with blood-typed chickens of the strain vaccinated with HVT or HVT + 301B bivalent MD vaccines prior to inoculation of the very virulent Md5 virus. The bivalent vaccine provided better protection than HVT alone, but with either vaccine fewer B2B21 chickens developed MD lesions. We conclude that the B haplotype influence on vaccinal immunity against MD previously demonstrated in B-congenic strains of chickens is also significant in commercial chickens and that the influence can be detected through analysis of B haplotypes in sick vs normal chickens of an affected flock.

Resistance to Marek's disease in chickens with recombinant haplotypes to the major histocompatibility (B) complex

Genetic resistance to Marek's disease (MD) is associated with the B-F region of the MHC. The resistance of chickens possessing either of two MHC haplotypes to challenge with different strains of MDV was examined. Chickens with serologically similar MHC recombinants BR2 and BR4 (both BF2-G23) were backcrossed for four generations to the highly inbred UCD-003 line (B17B17). Heterozygotes (B17BF2-G23) were mated to produce BR2BR2 and BR4BR4 homozygotes with 93% background gene uniformity. Both genotypes were highly resistant to GA-5 MDV, having an incidence of 0 and 8% MD for BR2BR2 and BR4BR4, respectively, whereas the incidence of MD in the UCD-003 birds was above 80%. Challenge with the very virulent RB-1B strain caused 10% and 31% MD in the BR2BR2 and BR4BR4 chickens, respectively, compared with 100% and 52% in the B17B17 (UCD-003) and B23B23 (New Hampshire 105) lines, respectively. Viremia levels at 5 and 6 d postinfection were significantly lower in BR2BR2 and B23B23 than in B17B17 genotypes.

Mapping of susceptibility to Marek's disease within the major histocompatibility (B) complex by refined typing of White Leghorn chickens

The major histocompatibility (B) complex of a distinct commercial pure White Leghorn chicken line was characterized using serological, biochemical and restriction fragment length polymorphism (RFLP) typing. Line B chickens displayed a high recombination frequency within the B complex. Three recombinant haplotypes were identified. The influence of these haplotypes was determined in relation to the haplotypes B19 and B21 on their resistance to Marek's disease (MD) in an experimental infection with the virus. Offspring of sires with a recombinant haplotype in combination with B19 or B21, and dams, which were homozygous B19/B19 or B21/B21 were infected. The B type of the offspring had a significant effect upon survival. Animals with B complex types B21/B21, B134/B21 and B234/B21 were relatively resistant to MD (24-32% mortality), whereas B19/B19 birds were highly susceptible (68% mortality). Animals with a recombinant haplotype B19r21 (B-G21, B-F19) were equally susceptible to MD as birds with the complete B19 haplotype. In contrast to earlier publications, resistance was not inherited as a dominant trait. Apparently, B19 was associated with a dominant susceptibility. The gene(s) associated with the B complex and involved in resistance to MD were localized within the B-F/B-L region. However, the association with a presumably non-coding subregion of B-G could not be excluded.

Influence of B-haplotype on the relative efficacy of Marek's disease vaccines of different serotypes

To determine if B-haplotype differentially influences vaccinal immunity to very virulent Marek's disease (MD) virus challenge, chickens of five 15.B-congenic lines were vaccinated with vaccines representing serotypes 1, 2, and 3. B-haplotype differentially influenced vaccinal immunity to very virulent MD virus challenge using vaccines of all three serotypes, and different MD vaccines were optimal for some B-haplotypes. Regarding specific haplotypes, the 15.B-congenic chickens with B2 and B13 developed less protection against MD than chickens with B15 following vaccination with all three serotypes of MD vaccine, whereas the chickens with B5 and B21 developed variable protection with different MD vaccines. Thus, for induction of maximum MD resistance, it may be necessary to select a vaccine appropriate for the predominant B-haplotypes of the chicken flock.

Influence of turkey herpesvirus vaccination on the B-haplotype effect on Marek's disease resistance in 15.B-congenic chickens

Eight recently developed 15.B congenic lines of chickens were tested for Marek's disease (MD) resistance by intra-abdominal injection of cell-associated preparations of MD virus of a virulent strain (JM), a very virulent strain (Md5), or Md5 after vaccination with turkey herpesvirus (HVT) strain FC126. Chickens of the 15.N congenic line (B15B21 or B21B21) were very resistant to JM-induced MD, in contrast to chickens homozygous for the B-haplotypes 2, 5, 12, 13, 15, or 19. After Md5 infection, more than 88% of the chickens in all of the congenic lines developed MD. However, when chickens were vaccinated with HVT before being inoculated with Md5, the B5 and B12 homozygotes were more resistant to MD than were the B2, B13, or B19 homozygotes, and B15 and B21 homozygotes had intermediate resistance. B5B5 and B2B5 F2 chicks inoculated with HVT and Md5 had a lower prevalence of MD than B2B2 sibs. These results demonstrate that a protocol involving HVT vaccination of chicks followed by infection with very virulent MD virus will allow the detection of B-haplotypes determining MD resistance, some of which are not detectable in unvaccinated chicks challenged with virulent MD.

Gordon Memorial Lecture. Chicken neoplasia--a model for cancer research

1. The use of animal models has been immensely important for the advancement of our knowledge of the aetiology and pathogenesis of human diseases, including neoplasia. 2. Viruses, as oncogenic agents, were first described in the early 1900s when cell-free filtrates were used experimentally to transmit leukemias and sarcomas in chickens. In more recent years, studies with avian leukosis/sarcoma viruses have led the field in attempts to establish the genetic and molecular basis of viral oncogenesis. 3. Marek's disease of chickens was the first neoplasm proven to be caused by a herpesvirus and it remains the only neoplastic disease for which an effective vaccine has been developed and deployed. It serves as an elegant model as we seek an understanding of the pathogenesis of herpesvirus-induced lymphomas at both the cellular and molecular levels.