Genetics and vaccine efficacy: host genetic variation affecting Marek's disease vaccine efficacy in White Leghorn chickens.

Marek's disease (MD) is a T-cell lymphoma disease of domestic chickens induced by MD virus (MDV), a naturally oncogenic and highly contagious cell-associated α-herpesvirus. Earlier reports have shown that the MHC haplotype as well as non-MHC genes are responsible for genetic resistance to MD. The MHC was also shown to affect efficiency of vaccine response. Using specific-pathogen-free chickens from a series of 19 recombinant congenic strains and their 2 progenitor lines (lines 6(3) and 7(2)), vaccine challenge experiments were conducted to examine the effect of host genetic variation on vaccine efficacy. The 21 inbred lines of White Leghorns share the same B*2 MHC haplotype and the genome of each recombinant congenic strain differs by a random 1/8 sample of the susceptible donor line (7(2)) genome. Chickens from each of the lines were divided into 2 groups. One was vaccinated with turkey herpesvirus strain FC126 at the day of hatch and the other was treated as a nonvaccinated control. Chickens of both groups were inoculated with a very virulent plus strain of MDV on the fifth day posthatch. Analyses of the MD data showed that the genetic line significantly influenced MD incidence and days of survival post-MDV infection after vaccination of chickens (P<0.01). The protective indices against MD varied greatly among the lines with a range of 0 up to 84%. This is the first evidence that non-MHC host genetic variation significantly affects MD vaccine efficacy in chickens in a designed prospective study.

Genetic variation at the tumour virus B locus in commercial and laboratory chicken populations assessed by a medium-throughput or a high-throughput assay.

The tumour virus B (TVB) locus encodes cellular receptors mediating infection by three subgroups of avian leukosis virus (B, D, and E). Three major alleles, TVB*S1, TVB*S3, and TVB*R, have been described. TVB*S1 encodes a cellular receptor mediating infection of subgroups B, D, and E. TVB*S3 encodes the receptor for two subgroups, B and D, and TVB*R encodes a dysfunctional receptor that does not permit infection by any of the subgroups, B, D, or E. Genetic diversity at the TVB locus of chickens was investigated in both layer and broiler commercial pure lines and laboratory lines. Genotyping assays were developed for both medium-throughput and high-throughput analysis. Of the 36 broiler lines sampled, 14 were fixed for the susceptible allele TVB*S1. Across all broiler lines, 83% of chickens were typed as TVB*S1/*S1, 3% as TVB*R/*R, and 14% as TVB*S1/*R. In the egg-layer lines, five of the 16 tested were fixed for TVB*S1/*S1. About 44% of egg-layers were typed as TVB*S1/*S1, 15% as TVB*R/*R, with the rest segregating for two or three of the alleles. In the laboratory chickens, 60% were fixed for TVB*S1/*S1, 6% for TVB*S3/*S3, 14% for TVB*R/*R, and the rest were heterozygotes (TVB*S1/*S3 or TVB*S1/*R). All commercial pure lines examined in this study carry the TVB*S1 allele that sustains the susceptibility to avian leukosis viruses B, D, and E. More importantly, the TVB*R allele was identified in multiple populations, thus upholding the opportunities for genetic improvement through selection.

Lymphoid organ size varies among inbred lines 6(3) and 7(2) and their thirteen recombinant congenic strains of chickens with the same major histocompatibility complex.

The objective was to evaluate lymphoid organ size in chickens from a series of 13 recombinant congenic strains (RCS) and their highly inbred parental lines (6(3) and 7(2)). The parental line 6(3) was selected for resistance to tumors induced by Marek's disease virus and avian leukosis viruses, whereas line 7(2) was selected for susceptibility to these tumors. Each RCS on the average contains a random one-eighth of genome from the donor line 7(2). Previous studies have shown that lines 6(3) and 7(2) differ in the size of primary lymphoid organs; i.e., the bursa of Fabricius (BF) and the lobes of the thymus (T) are smaller in line 6(3) than line 7(2). In the current study, the relative size of the T, BF, and spleen was first examined in about 15 males from each of 13 RCS and the 2 parental lines at 60 to 69 d of age. The differences of relative BF, T, and spleen size among the RCS and the parental lines 6(3) and 7(2) differed significantly (P < 0.001). Males and females from 4 RCS and the 2 parental lines were evaluated a second time, and differences in the relative sizes in lymphoid organs among the RCS and parental lines were consistent. In 2 RCS, the size of the T and BF was comparatively large as in line 7(2), leading to the conclusion that different allelic forms at 1 or more loci in these RCS regulate the size of both organs. In 2 other RCS, the BF was large compared with the T, suggesting that allelic forms at some loci in these RCS influence the BF independent of the T. The relative lymphoid organ size among the RCS appeared to cosegregate with the concentration of IgG in the plasma measured previously. The evaluation of genomic variability of these lines is underway, and the RCS are available for research on traits that differ between lines 6(3) and 7(2).

Development and validation of a PCR-RFLP assay to evaluate TVB haplotypes coding receptors for subgroup B and subgroup E avian leukosis viruses in White Leghorns.

The cellular receptor of subgroup B avian leukosis virus (ALVB) is encoded by a gene at the tumour virus B (TVB) locus. TVB alleles encode specific receptors permitting infection by exogenous ALVB or avian leukosis virus subgroup D (ALVD) as well as endogenous avian leukosis virus subgroup E (ALVE), and thus susceptibility is dominant to resistance. Two single nucleotide polymorphisms at the TVB locus have been reported distinguishing three TVB alleles (TVB*S1, TVB*S3 and TVB*R). We have developed a polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) assay using the two single nucleotide polymorphisms to define three observed allelic haplotypes and to identify the six possible TVB genotypes consisting of the three haplotypes in defined laboratory strains of chickens. One additional potential allelic haplotype and four genotypes were also briefly discussed. Chickens from parents heterozygous for different TVB alleles were challenged with Rous sarcoma viruses of subgroup ALVB and ALVE to induce wing-web tumours. Tumour incidences were evaluated between chickens of the genotypes determined with this newly developed PCR-RFLP assay. Importantly, chickens typed with this assay as TVB*S3/*S3 were resistant to infection by ALVE only, and those TVB*R/*R were resistant to both ALVE and ALVB. Furthermore, a vast majority of chickens with the susceptible TVB*S1/- genotypes developed a tumour. This PCR-RFLP assay enables a relatively rapid assessment of all six anticipated TVB genotypes in experimental strains of chickens undergoing segregation for TVB*S1, TVB*S3, and TVB*R alleles. This non-infectious assay should be further evaluated for the capacity to select and breed commercial chickens for genetic resistance to infections by ALVB, ALVD and ALVE.

Behavioral and physiological features of chickens diversely selected for resistance to Avian Disease. 1. Selected inbred lines differ in behavioral and physical responses to social stress.

To test the hypothesis that genetic variations in response to social stress modulate susceptibility to disease in poultry, aggressive behaviors induced by social stress were measured in chickens of different inbred lines selected for disease resistance (line 63) or susceptibility (lines 72 and 15I5), as well as 2 recombinant congenic strains (B and X). At 15 wk of age, roosters from each genetic line or strain were randomly assigned to pairs for intraline male-male aggression tests (n = 8 per line). Based on the results of the intraline aggression tests, the roosters were divided into 2 groups, winners and losers. At 16 wk of age, the roosters were randomly paired as winners vs. winners and losers vs. losers for interline aggression tests, i.e., line 63 vs. 72 and 15I5; line 73 vs. line 15I5; and strain X vs. strain B. Similarly, at 17 wk of age, line 63 vs. strains X and B, and line 72 vs. strains X and B were tested. The tests were conducted in a novel cage that was similar to their home cages, to provide a neutral space for both roosters being tested. Each pair was videotaped for 15 min. Male-male interaction-induced aggressive behaviors were markedly different among the genetic lines. Compared with roosters of lines 15I5 and 72, line 63 roosters generally showed fewer aggressive behaviors, including aggressive pecks and fights, as well as durations (P < 0.05). Roosters of the recombinant congenic strains X and B, each possessing a unique random 87.5% genome of line 63, exhibited low aggressive behaviors, which were similar or equal to the level of line 63 in both intraline and interline aggression tests (P = 0.05). These results may indicate that some of the gene(s) commonly carried between strains X and B as well as line 63 likely played an important role in governing their lower levels of aggression. The present chicken lines may be used as animal models for investigation of the cellular mechanisms of genetic-environmental interactions on disease resistance and stress responses.

Methods for evaluating and developing commercial chicken strains free of endogenous subgroup E avian leukosis virus.

The genome of nearly all chickens contains various DNA proviral insertions of retroviruses of subgroup E avian leukosis virus (ALVE). However, the elimination or control of ALVE gene expression is desirable to improve productivity, to improve resistance to avian leukosis virus (ALV)-induced tumours, and to develop safer live virus vaccines in chick embryos and cultured chick cells. Restriction fragment length polymorphism and polymerase chain reaction methods are used to define the presence of ALVE genes; and the expression of ALVE in chicken plasma or on cells, and the susceptibility of cells to ALVE is determined by flow cytometry using a specific (R2) antibody. ADOL line 0 chickens have been selected to be free of ALVE genes, while being resistant (i.e. lack receptors to ALVE), but susceptible to exogenous ALV (i.e. ALVA, ALVB, ALVC and ALVJ). To develop improved line 0-type chickens, ADOL line 0 was outcrossed to a commercial line that had one ALVE gene and evidence for ALVE resistance. Rous sarcoma virus (RSV) challenge was used to confirm resistance of F1 chickens to ALVE, and susceptibility of F2 breeders to ALVA and ALVB using test chicks produced by matings to line 7(2). Selected F2 breeders were resistant to ALVE, but susceptible to exogenous ALVA, ALVB, ALVC and ALVJ, based on challenge tests of progeny chick cells using an enzyme-linked immunosorbent assay. The new line, 0(1), has evidence for improved egg size, productivity, fertility and hatchability. Similar procedures may be used for development of productive ALVE free chicken lines with preferred ALV susceptibility traits.

The efficacy of recombinant fowlpox vaccine protection against Marek's disease: its dependence on chicken line and B haplotype.

Earlier studies have shown that the B haplotype has a significant influence on the protective efficacy of vaccines against Marek's disease (MD) and that the level of protection varies dependent on the serotype of MD virus (MDV) used in the vaccine. To determine if the protective glycoprotein gene gB is a basis for this association, we compared recombinant fowlpox virus (rFPV) containing a single gB gene from three serotypes of MDV. The rFPV were used to vaccinate 15.B congenic lines. Nonvaccinated chickens from all three haplotypes had 84%-97% MD after challenge. The rFPV containing gB1 provides better protection than rFPV containing gB2 or gB3 in all three B genotypes. Moreover, the gB proteins were critical, since the B*21/*21 chickens had better protection than chickens with B*13/*13 or B*5/*5 using rFPV with gB1, gB2, or gB3. A newly described combined rFPV/gB1gEgIUL32 + HVT vaccine was analyzed in chickens of lines 15 x 7 (B*2/*15) and N (B*21/*21) challenged with two vv+ strains of MDV. There were line differences in protection by the vaccines and line N had better protection with the rFPV/gB1gEgIUL32 + HVT vaccines (92%-100%) following either MDV challenge, but protection was significantly lower in 15 X 7 chickens (35%) when compared with the vaccine CVI988/Rispens (94%) and 301B1 + HVT (65%). Another experiment used four lines of chickens receiving the new rFPV + HVT vaccine or CVI988/Rispens and challenge with 648A MDV. The CVI 988/Rispens generally provided better protection in lines P and 15 X 7 and in one replicate with line TK. The combined rFPV/gB1gEgIUL32 + HVT vaccines protected line N chickens (90%) better than did CVI988/Rispens (73%). These data indicate that rFPV + HVT vaccines may provide protection against MD that is equivalent to or superior to CVI988/ Rispens in some chicken strains. It is not clear whether the rFPV/gB1gEgIUL32 + HVT vaccine will offer high levels of protection to commercial strains, but this vaccine, when used in line N chickens, may be a useful model to study interactions between vaccines and chicken genotypes and may thereby improve future MD vaccines.

Retrospective evidence that the MHC (B haplotype) of chickens influences genetic resistance to attenuated infectious bronchitis vaccine strains in chickens.

Infectious bronchitis is a respiratory disease of chickens that is caused by the coronavirus infectious bronchitis virus (IBV). Virtually all broiler and layer breeder flocks are routinely vaccinated against IBV. Two hatches of 1-day-old chicks from four lines were mistakenly vaccinated for infectious bronchitis using a moderately attenuated vaccine designed for chicks of an older age. The vaccination resulted in high mortality, and chicks from three of four lines died with signs typical of infectious bronchitis. The mortality that occurred using this less-attenuated vaccine was significantly influenced by the genetic line, and the MHC (B) haplotype in chickens of three B congenic lines. B congenic chickens possessing the B*15 haplotype were resistant in contrast to chickens possessing the B*13 or B*21 haplotypes. Chicks from two further hatches of the four lines were vaccinated appropriately with a more attenuated IBV vaccine, and only limited chick mortality was seen. These retrospective data from two repeated hatches confirm earlier data indicating chicken genes influence resistance to IBV, and indicate for the first time that genes tightly linked to the B haplotype are relevant in resistance to IBV. Due to extenuating circumstances it was not possible to verify results with chicks from F2 matings. Factors that may enhance definition of the role of the B haplotype in immune response to IBV, and the desirability for further analysis of a B haplotype-linked influence on immunity to IBV are discussed.

Identification and evaluation of major histocompatibility complex antigens in chicken chimeras and their relationship to germline transmission.

Chimeric chickens were evaluated as an intermediate for development of transgenic chickens. The transfer of Barred Plymouth Rock (BR) blastodermal cells into White Leghorn (WL) embryos results in BR-->WL chimeras, and some breeder males generate over 30% germline transmission of the BR genotype to offspring based on a feather-color trait. The objectives of the current study were to 1) identify the MHC (B haplotypes) in resident BR and WL lines, 2) establish that B antigens could be detected and quantified in red blood cells (RBC) of chimeras, 3) establish if there is a correlation in chimeras between percentage of RBC with donor B antigens and percentage germline transmission, and 4) evaluate if the MHC genotype influences chimera development. The RBC agglutination data indicated three B haplotypes were present in each line. The B*2-like, and B*19-like genes were unique to the WL line, and B*13-like and B-15-like genes were unique to the BR line, whereas a B*21-like gene was present in both lines. In adult BR-->WL chimeras, as well as 10- to 14 d-old WL-->WL chimeras, donor-type B antigens were detectable and quantifiable on RBC using flow cytometry. In BR-->WL chimeras, the percentage germline transmission was significantly correlated with the percentage of RBC with donor B antigen, as well as percentage of black feathers in the plumage. In a retrospective study using previously developed BR-->WL chimeras, the level of chimerism and germline transmission was higher in B*21/*21 type recipients, but this was not statistically significant in two prospective studies. It was concluded that MHC antigens on RBC can be used for identifying, quantifying, and selecting chicken chimeras developed by the transfer of blastodermal cells.

Concentration of immnoglobulin G in plasma varies among 6C.7 recombinant congenic strains of chickens.

Chicken Lines 63 and 72 were inbred during selection for resistance or susceptibility to viral-induced tumors. A sandwich ELISA assay was adapted to define the milligrams per milliliter of Ig-gamma (IgG) in plasma from chickens of Lines 63 and 72, as well as 19 recombinant congenic strains (RCS). Each RCS resulted from a 7(2) x 6(3) F(1) and two backcross matings using 63 as the recurrent female line. The IgG levels in the RCS were evaluated after four to seven generations of sib-matings, when each RCS was becoming inbred and fixed for a different 12.5% of the 72 genome. In three generations approximately 24-wk-old chickens of Line 72 had higher levels of plasma IgG than chickens of Line 63 (P < 0.05). None of the RCS had repeatable IgG levels comparable to Line 7(2). However, in the last two generations, two of the 18 RCS had higher IgG levels than nine with low IgG levels (P < 0.05). There was no correlation between an IgG level of a RCS and resistance to Marek's disease. It was concluded that selected RCS may be useful for identifying genes that determine differences in IgG levels, as well as for understanding the relationship between genes, IgG levels, and other traits that differ between Lines 63 and 72.

Chicken major histocompatibility complex class I definition using antisera induced by cloned class I sequences.

Alloantisera directed against chicken class I MHC (BFIV) antigens were produced by using transfected cell lines expressing cloned BFIV sequences. The cloned BFIV sequences were from haplotypes *12, *13, and *21. Two laboratory-derived class I mutant sequences (BFIV13m126 and BFIV21m78) were developed to analyze cross-reactive epitopes and to induce specific alloantisera. Antisera were tested in hemagglutination and flow cytometry assays. The antisera produced were highly specific and had minimal cross-reactivity. The antisera induced by the BF1V21m78 mutant confirmed the significance of amino acids 78 and 81 in cross-reactivity between haplotypes B*21 and B*5. The highly specific antisera were tested by hemagglutination on red blood cells of 31 different MHC haplotypes. The consistency of hemagglutination patterns and minimal cross-reactivity demonstrated that these BFIV antisera are extremely valuable in defining MHC haplotype in various chicken lines. Because of the extreme low level of recombination between the chicken class I and class II loci, identification of BFIV allele can be used to define MHC haplotype within a line. Complete identity between the transfected cell line and the chicken used to produce the antiserum is required to ensure the monospecificity.

Characterization and experimental reproduction of peripheral neuropathy in White Leghorn chickens.

A clinical neurological syndrome termed peripheral neuropathy (PN) that resembles Marek's disease (MD) occurred at low frequency in a commercial layer strain for several years. Study of chickens from six field cases showed that the PN syndrome could be distinguished pathologically from MD on the basis of several factors, including onset as early as 6 weeks, presence of B-type but not A-type lesions in peripheral nerves, and absence of visceral lymphomas. Serotype 1 MD virus could not be isolated from blood from any chicken or demonstrated in tissues by histochemistry or polymerase chain reaction assays. Moreover, the syndrome was not prevented by MD vaccination, either in the field or in laboratory trials. PN was induced in 3 to 54%of commercial line chickens inoculated at 1 or 6 days of age with whole blood or buffy coat cells from clinically affected donor chickens. Sonicated cells also induced PN, but plasma was ineffective. Chickens did not develop PN if reared in isolators without cellular transfer or when vaccinated solely against MD. However, PN was observed in 9% of 57 B*2/*19 commercial chickens reared in isolators following vaccination against MD, infectious bursal disease, Newcastle disease and infectious bronchitis, suggesting that common vaccines may predispose chickens to PN. The data confirmed a strong influence of the major histocompatibility complex (B-complex) on both naturally occurring and experimentally induced PN with the B*19 haplotype conferring susceptibility compared with other alleles. It is postulated that PN may represent an autoimmune reaction to nerve tissue that may result from response to a combination of common vaccines. These studies confirmed that PN is distinct from MD, provided criteria for its differential diagnosis, identified strategies for its control, and established a model for its experimental induction.

A review of the development of chicken lines to resolve genes determining resistance to diseases.

The resolution of genes that determine resistance to disease is described using chicken lines maintained at the Avian Disease and Oncology Laboratory (ADOL). This description includes a summary 1) of existing selected and inbred lines differing for resistance to viral-induced tumors, i.e., Marek's disease (MD) and lymphoid leukosis (LL), and of the use of inbred and line crosses to define relevant disease-resistant genes, e.g., TV, ALVE, B, R, LY4, TH1, BU1, and IGG1; 2) of the development of TVB*/ALVE congenic lines to establish the affects of endogenous virus (EV) expression on resistance to avian leukosis virus (ALV), and methods to detect ALVE expression; 3) of the development of B congenic lines to define the influence of the MHC on MD resistance and vaccinal immunity, for producing B antisera, and for evaluating DNA sequences of Class I and II genes; and 4) of the current development of 6C.7 recombinant congenic strains (RCS) to define the role of non-MHC genes influencing susceptibility to MD and LL tumors, immune competence, and epistatic effects of genes. The procedures of pedigree mating, to avoid or maintain inbreeding, and of blood-typing, to ensure genetic purity of the lines, are also described.

Avian leukosis virus subgroup J infection profiles in broiler breeder chickens: association with virus transmission to progeny.

Profiles of infection with avian leukosis virus subgroup J (ALV-J) and factors that predict virus transmission to progeny were studied. Eggs from an infected broiler breeder flock were hatched at the laboratory. The flock was reared in a floor pen, transferred to laying cages at 22 wk, and inseminated to produce fertile eggs. A cohort of 139 chickens was tested at frequent intervals over a 62-wk period for virus, viral antigens, or antibodies in plasma, cloacal swabs, egg albumen, and embryos. Virus was detected in 7% of chicks at hatch but spread rapidly so that virtually all chicks became infected between 2 and 8 wk of age. Mortality due to myeloid leukosis and related tumors was 22%. Over 40% of the chicks developed persistent infections, whereas the remainder experienced transient infections. Five types of infection profiles were recognized. Novel responses included hens that were positive for virus intermittently or started late in life to shed viral antigens into the cloaca. ALV-J was isolated from 6% of 1036 embryos evaluated between 26 and 62 wk. However, over 90% of the virus-positive embryos were produced between 29 and 34 wk of age. Of 80 hens that produced embryos, 21 produced at least one infected embryo and were identified as transmitters. All but one transmitter hen would have been detected by a combination of viremia, cloacal swab, and albumen tests conducted between 18 and 26 wk. However, virus was transmitted to embryos from hens that were not persistently viremic or that rarely shed viral group-specific antigen into the albumen of their eggs. Intermittent patterns of both antigen shedding and virus transmission to embryos were observed in some hens. These results validate current screening procedures to identify potential transmitter hens and provide some suggestions for improvement but also show that identification of all transmitter hens by such procedures is unlikely. Thus, eradication programs based solely on dam testing may be less effective than those where dam testing is combined with procedures to mitigate early horizontal transmission in progeny chicks.

Detection of endogenous avian leukosis virus envelope in chicken plasma using R2 antiserum.

Immunologic tolerance to oncogenic avian leukosis virus (ALV) is mediated, in part, by the interaction of endogenous ALV (EV) envelope and immune competent cells. A flow cytometry method is described for detecting the EV envelope in chicken plasma or serum. The method employs two types of target red blood cells (RBC) obtained from chickens lacking EV; RBC susceptible to EV infection (containing EV receptors), and those resistant to EV infection (lacking EV receptors). RBC from susceptible chickens will bind EV envelope glycoprotein (gp85) when present in plasma. The gp85-bound RBC are subsequently incubated with a highly specific chicken alloantibody, termed R2. Using flow cytometry, gp85 is detected indirectly with a fluoresceine-tagged antibody to chicken immunoglobulin; plasmas lacking gp85 are nonreactive and fluorescence remains at a background level. Because RBC from resistant chickens are nonreactive regardless of the presence or absence of EV gp85, a specific binding index was calculated to compare relative binding of EV gp85 on susceptible and resistant RBC, and thus identify chickens that express EV gp85. The specificity of the assay was demonstrated using plasma from chickens of 14 standard laboratory lines previously defined for EV envelope expression including two sets of highly congenic lines that differ in EV expression. This assay detects differences attributable to EV gp85 in chickens of commercial breeding lines of White Leghorns and broilers. Moreover, if chickens lack EV, the R2 plasma assay can differentiate between EV-susceptible and EVresistant siblings.

An acute form of transient paralysis induced by highly virulent strains of Marek's disease virus.

A novel syndrome was observed after inoculation of 3-wk-old chickens with highly virulent Marek's disease virus (MDV) strains. This syndrome was characterized by the acute onset of neurologic signs including flaccid paralysis of neck and limbs 9-10 days postinoculation, typically resulting in death 1-3 days after the onset of clinical signs. Most affected birds died, and spontaneous recovery was rare. Few if any gross tissue changes were found. Histologic brain lesions included acute vasculitis with vasogenic edema and perivascular cuffing. The syndrome was influenced by the virus strain and dose and by chicken strain and B haplotype and was prevented by vaccination with turkey herpesvirus. Chickens up to 18 wk of age were susceptible. On the basis of clinical signs and histopathology, the syndrome was determined to be an acute form of transient paralysis (TP); its more acute nature and virtual lack of spontaneous recovery differentiated this syndrome from classical TP. Affected birds were viremic, and brains were positive for viral DNA by polymerase chain reaction assays, but these tests were also positive in inoculated chickens without clinical signs and may have limited value for diagnosis. Although acute TP should occur only rarely in Marek's disease-vaccinated commercial flocks, this syndrome may be important in laboratory studies, where it could interfere with pathogenesis trials. Finally, acute TP appears to be one component in the pathogenesis of the early mortality syndrome, a previously described immunodepressive disease induced by inoculation of 1-day-old chicks with highly virulent MDV.

High resolution mapping and identification of new quantitative trait loci (QTL) affecting susceptibility to Marek's disease.

Marek's disease (MD) is a lymphoproliferative disease of chickens that costs the poultry industry approximately $1 billion annually. Genetic resistance to MD is gaining increased attention to augment vaccinal control as disease outbreaks occur more frequently. Previously, analysis of a 272 F2 White Leghorn resource population measured for many MD traits and genotyped for 78 microsatellite markers revealed two and four quantitative trait loci (QTL) with significant and suggestive association, respectively, to one or more MD associated traits. Additional genetic markers have since been scored on the MD resource population to increase QTL resolution and genome coverage. Saturation of four of the QTL regions with 17 markers revealed five new QTL while 32 markers extended the genome coverage by 400 + CM and uncovered three more QTL. QTL analysis by single-point and interval mapping algorithms agreed well when marker saturation was approximately 20 CM or less. Currently 127 genetic markers cover approximately 68% of the genome that contain up to 14 MD QTL associated to one or more MD trait; seven at the significant level and seven at the suggestive level. Individually each QTL accounts for 2-10% of the variation and, in general, resistance was dominant although the resistant allele may come from either parental line. This study suggests that a limited number of genomic regions play a major role in the genetic control of MD resistance. Markers linked to these loci may be useful for selection of MD resistant stock by the poultry industry following verification of the association within their breeding populations.

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.

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.