Significant progress has been made in elucidating the mechanisms of viral membrane fusion proteins; both those that function at low, as well as those that function at neutral, pH. For many viral fusion proteins evidence now suggests that a triggered conformational change that exposes a previously cryptic fusion peptide, along with a rearrangement of the fusion protein oligomer, allows the fusion peptide to gain access to the target bilayer and thus initiate the fusion reaction. Although the topologically equivalent process of cell-cell fusion is less well understood, several cell surface proteins, including members of the newly described ADAM gene family, have emerged as candidate adhesion/fusion proteins.

Virus-cell and cell-cell fusion.

The continuation of an apparent evolutionary trend of Marek's disease virus (MDV) towards greater virulence may explain recent increased losses from Marek's disease (MD) in vaccinated flocks. To address this question, the virulence of 31 isolates of serotype 1 MDV obtained from layer or broiler flocks between 1987 and 1995 were characterized. Each isolate was cultured in duck embryo fibroblasts for four to six passages, and ascertained to be free from contamination with avian retroviruses, chicken anemia virus, and MDVs of other serotypes. The viruses, along with prototype viruses JM/102W and Md5, were tested for virulence by inoculation at 6 days of age into laboratory strain 15I5 x 7(1) chickens of three types: nonvaccinated, vaccinated with turkey herpesvirus (HVT) and bivalent (HVT + SB-1)-vaccinated. The results showed that three isolates did not differ from JM/102W and were classified in the virulent (vMDV) pathotype. Twenty-one isolates produced significantly higher levels of MD in HVT-vaccinated chickens than did the JM/102W control and were classified in the very virulent (vvMDV) pathotype. Seven isolates, five of which were isolated in 1994 or 1995, produced significantly higher levels of MD in bivalent-vaccinated chickens than did the Md5 (vvMDV) control. These isolates, provisionally designated as the vv+MDV pathotype, appeared to be at the high end of a virulence continuum. Several MD response parameters, including lymphoma mortality, early mortality with bursal/thymic atrophy, and frequency of visceral lymphomas or ocular lesions in nonvaccinated chickens were positively correlated with virulence. These findings support the continued evolution of MDV towards greater virulence.

Increased Virulence of Marek's Disease Virus Field Isolates

Avian leukosis virus-induced tumors have common proviral integration sites and synthesize discrete new RNAs: oncogenesis by promoter insertion

Publisher Summary The question that whether the unit of malignancy is a gene, or an organism that grows in highly specialized protoplasmic environments, or a molecule that multiplies by virtue of integral chemical union with a molecule of protoplasm, remains unanswered. The question of whether leukemia in the mouse is caused by either genetic or viral mechanisms has taken on a new meaning with the recognition of host genetic constitution with the viral infection in leukemogenesis. The chapter covers the early studies of murine leukemia and the genetic control of friend virus susceptibility and naturally occurring leukemia viruses. It also discusses the h2 linked genetic control of leukemogenesis and the other genes responsible for leukemogenesis. Infection with ontogenic viruses is complex and malignancy is not an inevitable result of tumor virus infection; many specific events affect the interaction of leukemia virus with cells, leading to leukemogenesis.

Genetic Control Of Murine Viral Leukemogenesis

Avian leukemia viruses: interaction with their target cells in vivo and in vitro.

SUMMARY Studies were conducted to determine whether a herpesvirus isolated from turkeys (HVT) would protect chickens against subsequent challenge with the virulent JM strain of Marek's disease herpesvirus (MDHV). HVT administered intra-abdominally at doses as low as 600 plaque-forming units per one-day-old chick gave protection against Marek's disease (MD). The virus would protect when birds were challenged with MDHV by intra-abdominal inoculation at 3 weeks or by contact exposure as early as 2 weeks postvaccination. Chickens inoculated with HVT and observed for 17 to 20 weeks did not develop lesions. Birds produced antibody and infection persisted throughout this period, however, as indicated by reisolation of the virus. Furthermore, HVT did not spread to chickens in direct contact with vaccinated birds. Thus HVT gives substantial protection against the development of MD yet is nonpathogenic and noncontagious, all of which are important characteristics of vaccine viruses.

Protection against Marek's disease by vaccination with a herpesvirus of turkeys.

SUMMARY Immunoglobulins from antiserum raised against chromatographically purified avian myeloblastosis virus (AMV) group-specific (gs) antigens were used in an enzyme-linked immunosorbent assay (ELISA). Readily discernible color was produced with 2-3 ng of AMV protein in microplate wells coated with 4 jtg of salt-precipitated immunoglobulins. When a biological assay, i.e., phenotypic mixing (PM), was the criterion for the infectious status of specimens, the ELISA consistently identified a greater percentage of virus-positive specimens than direct complement-fixation (DCF) tests. Over 95% concordance was obtained between the! ELISA and PM bioassays when meconia and whole-blood samples were tested. Moreover, three DCF(-) egg albumens from one virus shedder hen were positive by the direct ELISA. Complete agreement was found between a biological assay for endogenous virus and the ELISA when blood and albumens from inbred chickens were tested. The ELISA is a rapid and convenient alternative to the DCF test for identifying infected chickens in eradication programs, because virus-rich sources such as meconia and blood that are unsuitable for DCF can be tested directly.

An Enzyme-Linked Immunosorbent Assay For Detecting Avian Leukosis-Sarcoma Viruses

Female chickens known to be heterozygous for resistance to subgroups A and B of the avian leukosis-sarcoma viruses were mated to males known to be homozygously resistant to both. The progeny were assayed both on the chorioallantoic membrane (CAM) and in tissue culture for resistance to representative viruses of the A, B, and tentatively defined C subgroups. Segregation ratios of resistance to A and B subgroup viruses agreed with the previously suggested hypothesis of single-autosomal-recessive genes controlling resistance to each subgroup. Mixed infection on the CAM and replicate plate infection in tissue culture with subgroup A and B viruses showed that resistance to the A and B subgroups was inherited independently. Assays with viruses tentatively classified as subgroup C indicated that they were largely composed of a mixture of subgroup A and B viruses or of particles possessing the host range specificity of both. However, virus stocks of the subgroup C category, as well as some stocks classified as subgroup B, produced small numbers of pocks or foci on individuals known to be resistant to subgroup A and B viruses. It is suggested that these Rous sarcoma virus stocks carry between 1 and 10% of a true subgroup C virus.

Two Loci Controlling Genetic Cellular Resistance to Avian Leukosis-Sarcoma Viruses

In long-term field trials lasting approximately 18 months, the herpesvirus of turkeys proved safe and highly effective in preventing Marek's disease (MD). The average MD mortality was reduced from 19.2 to 2.8%, and the degree of protection averaged 80.8% in the 11 experiments in which detailed necropsies were performed. Protection lasted the entire experimental period and there was a reduction in leukosis condemnation at slaughter. The degree of protection varied from flock to flock but was greatest in flocks that had the highest incidence of MD in the unvaccinated controls. There was no apparent difference in protection between the low, intermediate, or high (respectively about 500, 2,000, or 10,000 PFU per chick) doses of vaccine. Vaccination reduced the number of birds from which MD virus could be isolated and also reduced the level of virus in the positive birds. Lymphoid leukosis mortality exceeded 10% on 2 of the farms and the occurrence of this disease was not affected by the vaccine. The incidence of other neoplasms is recorded. Vaccinated birds produced 4.0 and 14.7% more eggs than unvaccinated birds on a hen-day and hen-housed basis, respectively. In addition, vaccinated birds reached 50% egg production 2 to 8 days earlier than unvaccinated controls.

W. Okazaki

Papers

Protection against Marek's disease by vaccination with a herpesvirus of turkeys.

An Enzyme-Linked Immunosorbent Assay For Detecting Avian Leukosis-Sarcoma Viruses

Two Loci Controlling Genetic Cellular Resistance to Avian Leukosis-Sarcoma Viruses

Long-term field trials with the herpesvirus of turkeys vaccine against Marek's disease.

Field trials with the herpes virus of turkeys (HVT) strain FC126 as a vaccine against Marek's disease.