GENETIC AND BIOLOGICAL DETERMINANTS OF AVIAN TUMOR VIRUS SUSCEPTIBILITY
Location: Avian Disease and Oncology Laboratory
Title: Avian Disease and Oncology Laboratory (ADOL) research update
Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 16, 2010
Publication Date: August 1, 2011
Citation: Fadly, A.M., Cheng, H.H., Dunn, J.R., Heidari, M., Hunt, H.D., Lee, L.F., Silva, R.F., Zhang, H. 2011. Avian Disease and Oncology Laboratory (ADOL) research update. Proceedings One Hundred and Fourteenth Annual Meeting of the United States Animal Health Association (USAHA), November 11-17, 2010, Minneapolis, Minnesota. p. 528-530.
To meet the growing demands of consumers, the poultry industry will need to continue to improve methods of selection in breeding programs for production and associated traits. One possible solution is genome-wide marker-assisted selection (GWMAS). In brief, evenly-spaced genetic markers spanning the entire genome are genotyped (scored) on individuals to estimate their breeding value, which in theory could substantially increase the rate of genetic gain compared to traditional selection methods. To test the power of GWMAS, meat-type and egg-type chicken lines are being selected in parallel using either traditional (BLUP) or GWMAS. This year, after completing two rounds of selection, we conclude that compared to birds selected in parallel using current state-of-the-art breeding methods, genomic selection is superior for the vast majority of the traits selected including body weight and breast yield. This research strongly suggests that genomic selection is an improved breeding method. If costs for genetic testing continue to go down, then poultry breeders should be able to economically breed chickens faster using genomic selection and adapt more readily to changing consumer demands. The economic impact could be great since with 1 million meat-type birds processed per hour in the US alone, the net effect of even small improvements are large and worth millions of dollars.
Diagnosis, Surveillance and Pathotyping
Marek’s disease virus (MDV) strains with similar mutation were isolated from chicken farms in Pennsylvania in 2007 and 2010. Affected farms ranged from 13-28 miles apart; the case involved different bird strains, vaccine companies, and pullet farm and hatchery origins. The isolated MDV strains were typed as vv+, but not unusually virulent. Mutation affects specificity of T65 monoclonal antibody for differentiating field strains from Rispens. We also diagnosed MD early mortality syndrome in Connecticut backyard flocks, demonstrating high potential virus load and need for vaccination even in backyard flocks. Peripheral neuropathy was also diagnosed in 6 week-old pullets in Ohio; the case involved low incidence of leg paralysis and the presence of lymphoplasmacytic neuritis and edema.
Understanding the relationship between host genetics and MD vaccine efficacy plays an important role in developing vaccination schemes for better control of the disease. Recently, chickens from two highly inbred lines (highly resistant and susceptible) and a series of 19 recombinant congenic strains were used to evaluate the protective efficacy of two commonly used MD vaccines and a candidate recombinant vaccine termed rMd5-Meq deleted vaccine. The protective indices of the vaccine ranked from high to low; the change in the ranking order of protective indices for two of the three vaccines between the two chicken lines indicated a vaccine X chicken line interaction affecting the vaccine protective efficacy.
Marek’s disease virus immune evasion gene:
MDVs retain the ability to evade immune recognition. Identifying and removing the viral genes that are responsible for virus immune evasion will produce a more effective vaccine. We have previously shown that MDV down-regulates MHC class I, a critical protein that signals the chicken’s immune system there is a virus infection, however, the gene (s) involved have not been identified. Recently, we demonstrated that an MDV gene, termed MDV012 is capable of reducing surface expression of MHC class I on chicken cells. Our results suggest that this is the first non-mammalian MHC class I immune evasion gene identified, and that it is highly conserved in herpesviruses.
Cytokine and Chemokine Gene Expression Analysis in MDV Infection
Through cytokine and chemokine gene expression analysis, we have discovered that vv+ strains of MDV drive the immune response to a Th-2 lineage and suppression of Th-1 immunity. Th1-type adaptive immune activity is critical for the induction of a successful host antiviral immune response. Global gene expression profiling has provided evidence that highly pathogenic strains of MDV induce severe and prolonged immune suppression by repression of the transcriptional activities of many genes that are critical components of both the innate and adaptive immune responses. Among the many immune response genes down regulated by MDV, adhesion molecules are of critical importance. Suppression of these cell surface receptors impedes the transmigration of leukocytes to the site of infection and inflammation.
Using cosmid clone and bacterial artificial chromosome (BAC) technologies, we have developed a recombinant MD vaccine virus where both copies of the Meq gene were deleted. Evaluation of this vaccine, termed rMd5-meq deleted vaccine under laboratory and field conditions revealed that the vaccine is efficacious and provided better protection than the most effective commercially available vaccines. Attempts are now being made to insert gB and gJ genes from infectious laryngotracheitis virus (ILTV) into our BAC- rMd5-meq-deletd virus; if successful this new vaccines should provide protection against both vv strains of MDV and ILTV.
Screening for Recombinant Avian Leukosis Viruses
Use of genetically resistant (restrictive) chicken embryo fibroblasts (CEFs) is essential for screening for subgroups of ALVs. In susceptible CEFs dually infected with avian leukosis virus (ALV) subgroup A (ALV-A) and ALV-J, ALV-A appeared to be the dominant subgroup. Under these experimental conditions, dual infection of susceptible CEFs with ALV-A and ALV-J resulted only in either ALV-A, or ALV-J. No recombinant ALV such as ALV-A/J or ALV-J/A was detected. Use of PCR specific for envelope and LTR of subgroup of ALV following propagation on restrictive CEFs should be a useful tool in identifying recombinant ALVs, if present. Inability to detect recombination between ALV-A and ALV-J suggests that conditions used in the current experiment were not suitable for recombination. Factors that were not tested and should be considered such as multiplicity of infection, virus dose, strain and subgroup of virus.
Characterization of Various Reticuloendotheliosis Virus (REV) Isolates Obtained from Various Species Located in Different Geographical Regions in the United States
Nine reticuloendotheliosis virus (REV) isolates obtained from broiler breeders, turkeys, and prairie chickens located in three different geographical regions in the USA, and three isolates obtained from known contaminated live-virus vaccines were characterized using polymerase chain reaction (PCR) and indirect immunofluoresence (IFA) assays. All isolates were propagated in chicken-embryo-fibroblasts (CEF) obtained from a specific-pathogen-free (SPF) breeder flock. Results from sub-typing indicated that all nine isolates from broiler breeders, turkeys, and prairie chickens belonged to subtype 3, and are antigenically related to the chick-syncytial virus (CSV) strain of REV, the prototype of subtype 3 REV. In contrast, the three isolates from contaminated vaccines were classified as subtype 2, and antigenically related to spleen necrosis virus (SNV) strain of REV, the prototype of subtype 2 REV. Results from DNA sequence analysis confirmed those from sub-typing and indicated that the three REV isolates representing those from broiler breeders, turkeys, and prairie chickens are closely related to CSV of REV, with an amino acid homology of 98% or greater as compared to SNV with an amino acid homology of 95% or less. Data from this study clearly indicate that subtype 3 is the most common subtype of REV circulating in three different avian species, namely broiler breeders, turkeys and prairie chickens located in three different geographical regions in the United States.