Herpes virus of turkey vaccine: viraemias in field flocks and in experimental chickens

In a field survey of viraemias due to vaccination of chickens with herpes virus of turkey, variation was encountered in titres and percentages of birds viraemic. The incidence of viraemias was much lower in sick than in healthy birds in flocks undergoing mortality from Marek's disease. In a concurrent experiment the same strain of chicken and the same commercial vaccine were used as in the field flocks affected with Marek's disease. A high incidence of viraemia and 84.6 per cent protection against Marek's disease were obtained with single vaccination at one day of age. Revaccination at 21 days of age produced no measurable benefits in the same experiment.

Contrasting characteristics of Marek's disease herpesvirus isolated from chickens with and without avian leukosis virus infection

Marek's disease herpesvirus (MDHV) isolated from chickens free of naturally occurring avian leukosis virus (ALV) infection produced characteristic foci in both chicken embryo fibroblast (CEF) and chicken kidney cell (CKC) cultures. MDHV-A, which was extracted from the feather follicle epithelium of chickens naturally infected with ALV, did not induce cytologic changes in CEF cultures, but did cause focus formation in CK cultures. ALV was detected in MDHV-A, but not in MDHV preparations. MDHV-A (reconstructed in vivo) and MDHV were further distinguished from one another by inoculation of ALV-free LSI-SPF chickens. MDHV-A elicited a high incidence of early mortality which was not accompanied by the gross tumor spectrum characteristic of Marek's disease, although extensive histologic lesions were present. The differences between MDHV and MDHV-A were not as striking in another line of ALV-free chickens (SPAFAS). By contrast, among conventional chickens with naturally occurring ALV infection, neither MDHV-A nor MDHV caused appreciable early mortality although both were highly oncogenic (gross tumor development). These observations demonstrate that the presence of an oncornavirus (ALV), detected by radioimmune but not complement fixation assays, can influence the in vitro and in vivo characteristics of an oncogenic herpesvirus (MDHV). The observations recorded here resolve some of the inconsistencies reported in the literature. Thus, the apparent failure by some to find interactions between MDHV and oncornaviruses can be ascribed to the comparatively limited sensitivity of the complement fixation assay used to detect oncornaviruses (ALV). We have shown that the presence of oncornavirus detectable by radioimmune assay, but not by complement fixation, can influence the in vitro and in vivo responses of an oncogenic herpesvirus (MDHV). Our observations relating to viral interaction do not imply that MDHV required the presence of ALV to produce disease.

The effects of sonication, freezing and lyophilization on JMV leukosis strain

JMV lymphoblastic leukemic cells were subjected to sonication, followed by freezing and lyophilization in an attempt to learn the tolerance of JMV cells to these treatments. Sonication experiments indicated that a high percentage of cell breakage (greater than 89%) is necessary for any decrease in lethality to be observed. Freezing experiments involving a wide range of cryoprotectors demonstrated 2M glycerol to be the best for JMV preservation. Subsequent freeze-drying of sonicated, frozen JMV preparations, of high titer, consistently resulted in all loss of lethality.

Dual virus maturation of both pathogenic and apathogenic Marek's disease herpesvirus (MDHV) in the feather follicles of dually infected chickens

Infectious cell-free viruses of both pathogenic and apathogenic Marek's disease herpesvirus (MDHV) were detected in feather-tip extracts of chickens dually infected with both MDHV isolants either by inoculation or by contact exposure. This indicated concurrent viral maturation of both pathogenic and apathogenic MDHV isolants in the feather-follicle epithelium, resulting in possible double shedding of the two isolants.

Dissociation of antiviral and antitumor immunity in resistance to Marek's disease

Immunization of chickens either with gluteral-dehyde-inactivated chicken kidney cells infected with Marek's disease (MD) virus or with glutaraldehyde-inactivated cells of MD lymphoma-derived continuous lymphoblastoid cell lines protected against MD. The former type of immunity was associated with an immunologic suppression of virus replication and virus antigen production after challenge with virulent virus, but lymphocytes specifically cytotoxic to cells bearing MD tumor antigens were not detected. In the latter type of immunity, virus multiplication was not affected; some evidence of the stimulation of cell-mediated antitumor immunity was found. The results supported the view that immunity to MD may be directed against either virus-specific or tumor-specific antigens and that in natural resistance to MD both mechanisms may be operative.

Herpesvirus-like particles in tumor cells of breast muscle of chickens with Marek's disease

Tumors of the breast musculature of 4 chickens with Marek's disease were examined by light and electron microscopy. By light microscopy, tumors were composed of lymphoid cells which had infiltrated and separated bundles of muscle fibers. By electron microscopy, there was margination of chromatin and duplication of the inner nuclear membrane in almost all neoplastic cells of breast tumors of all 4 chickens. In addition, in one chicken the nuclei of tumor cells contained naked herpesvirus-like particles.

[Mechanism of postvaccinal immunity in Marek's disease]

The apathogenic variant No. 38 of Marek's disease virus, Kekava strain (MDV-Kekava), at the level of the 16th and 45th passages in vitro made chickens resistant to Marek's disease when inoculated 14 days before the infection of the chickens with the pathogenic variant No. 55 of MDV-Kekava. A simultaneous administration of both variants did not protect the animals against the disease. The occurrence in variants of MDV-Kekava of genetic markets manifest upon virus passages in chick embryo fibroblast (CEF) cultures provided an opportunity to study the interactions between them in chickens, using the CEF culture for virus isolation. The results of virus isolation from the blood cells of vaccinated chickens showed interference to occur in chickens between the virus variants as the rate of isolation of the pathogenic variant was 3 times as low as that of apathogenic MDV-Kekava, and both virus variants persisted in different cells. When chickens were inoculated simultaneously with both virus variants, the recovery rate of both pathogenic and apathogenic variants from the blood cells was similar. In such cases, persistence of two virus variants in one cell is also possible. The experimental results suggest that in the mechanism of resistance of vaccinated chickens to Marek's disease a significant role may be played by interference between the viruses in which the "occupation" of target cells by vaccine virus protects them from pathogenic MDV.

A nonproducer T lymphoblastoid cell line from Marek's disease transplantable tumor (JMV)

A continuous lymphoblastoid cell line was established from a JMV tumor transplant related to Marek's disease (MD). It is designated RPL1 (JMV) lymphoblastoid cell line. This cell line contains DNA sequences complementary to MD virus DNA and has an antigen similar to MD-tumor-associated surface antigen (MATSA). However, it lacks any MD virus (MDV) rescuable in vivo or in vitro. The cell line has surface antigens typical of chicken thymus cells (T cells) and histocompatability antigens different from those of the host chicken.

Characteristics of JMV Marek's disease tumor: a nonproductively infected transplantable cell lacking in rescuable Virus

Cells of the JMV Marek's disease (MD) tumor, originally produced by rapid serial passage of MD lymphoma cells in chickens, were characterized to determine whether they were of host or donor origin and to ascertain certain virus-host cell interrelationships. Differences noted in blood group B surface alloantigens between tumor cells and host lymphocytes indicated a probable nonhost origin (i.e., transplantability) of the tumor. JMV spleen tumors contained predominantly large lymphoblasts bearing MD tumor-associated surface antigen. DNA from JMV tumor cell suspensions hybridized significantly with MD virus cRNA, which indicated that JMV cells contained at least a portion of the MD virus genome. No MD virus was rescued from JMV tumors by techniques suitable for rescue of virus from MD lymphomas. The JMV tumor cells were also devoid of MD virus-specific antigens. These properties differed markedly from those of MD lymphoma cells and make the JMV tumor cell a unique, potentially valuable, tool for further study of oncogenic herpesvirus infection and tumor immunity in the chicken.

Turkey herpesvirus infection in chickens: induction of lymphoproliferative lesions and characterization of vaccinal immunity against Marek's disease

Chickens vaccinated at hatching with high doses of turkey herpesvirus (HVT) developed viremia that peaked in titer around the 12th day and gradually declined. HVT infection also induced mild microscopic lymphoproliferative lesions in the nerves and gonads. These lesions were most prominent around the 12th day and then regressed. The fact that such lesions were also induced by HVT in cyclophosphamide-treated chicks suggests that they were T-cell-dependent. Some of the cells in early HVT lesions appeared to have morphologic properties of neoplastic cells. HVT viremia and lesions were both dose-dependent and were less in chickens with maternal antibodies against Marek's disease virus (MDV). Sequential studies on chickens vaccinated with HVT and challenged with MDV showed that chickens were protected against the earliest detectable MD viremia and lymphoproliferative lesion response attributed to MD. Also, the transient necrobiotic lesions associated with productive infection of thymic lymphocytes by MDV were totally absent in vaccinated chickens. These data provide further insight on the mechanisms by which HVT protects against MD lymphoma induction. A limited oncogenic (transforming) potential of HVT as suggested by our data would provide the basis to assume that at least one component of HVT-induced immunity may be directed against tumor-specific antigens. On the other hand, our observations that HVT protects against productive MDV infection in the thymus and against cell-associated viremia are evidence for an anti-viral immune response. These hypotheses are not mutually exclusive.

Marek's disease vaccine breaks: differences in viremia of vaccinated chickens between those with and without Marek's disease

Viremia with turkey herpesvirus (HVT) and/or Marek's disease (MD) herpesvirus (MDHV) was examined in chickens from three different commercial flocks experiencing "vaccine breaks" following vaccination with HVT vaccine against MD. In all groups of sample chickens from the flocks, the incidence of detectable HVT viremia was significantly less in the MD-affected than in the healthy penmates, and incidence of MD was significantly higher in the birds without detectable HVT viremia than in the viremic penmates. The results suggest an association between HVT viremia and protection against MD development.

[Marek's disease--characterization of the VUB-70 strain in vivo]

In the course of serial passaging of the strain of Marek's disease (MD) (VUB-70) in White Leghorns the author investigated the morbidity, mortality, development of the MD-specific changes, and the effect of sex on these indices, for the purpose of characterization of the biological properties of an isolated strain of MD in vivo. The incubation period lasted from 41 to 46 days p. inf. on the first day of life. Pullets showed greater sensitiveness towards infection not only through higher morbidity and mortality, but also due to a larger number of tumorous changes. In MD-positive animals the same proportion of macroscopic and microscopic changes was found. The average occurrence of Marek's disease amounted to 74.4 p. c. On the basis of the biological properties in vivo the VUB-70 strain was characterized as a medium pathogenic strain of the acute form of Marek's disease.

Enhancement and interference in chickens inoculated with Marek's disease herpesvirus and oncornaviruses

Enhancement of mortality rates and symptomatology was observed in isolator-held LSI-SPF chickens concurrently inoculated with MDHV and avian oncornaviruses (RAV-1, RAV-2, RAV-7, RAV-50, or REV). Interference with MD antigen production also was demonstrated in extracts of the feather follicle epithelium from chickens inoculated with both MDHV and RAV-1.

Influence of vaccination with avirulent herpesvirus on subsequent infection of chickens with virulent Marek's disease herpesvirus

Vaccination of chickens with turkey herpesvirus (HVT) or attenuated Marek's disease herpesvirus (aMDHV) blocked infection with virulent MDHV (VMDHV) for approximately 5 weeks after contact exposure. However, there was no apparent blockage of infection when challenge virus was administered intraabdominally (IA). Evidence for infection with VMDHV was based on viral isolation by in vivo assay or by detecting precipitins to "A" antigen associated with virulent virus. The HVT stimulated production of neutralizing antibody against VMDHV in a high percentage of chickens, whereas the aMDHV was a comparatively poor inducer of such antibody. Despite this difference, both of the vaccinal viruses conferred protection against development of Marek's disease.

Oncogenesis by Marek's disease herpesvirus in chickens lacking expression of endogenous (gs, chick helper factor, Rous-associated virus-O) and exogenous avian RNA tumor viruses

Chickens free of exogenous avian leukosis virus (ALV) infection, replicating endogenous ALV (Rous-associated virus-O), gs antigen, and chick helper factor were fully susceptible to induction of Marek's disease (MD) by ALV-free MD viruses. Dual infection with Rous-associated virus-2 and MD virus did not significantly alter the character of the MD lesions. Thus exogenous ALV infection was not requisite for MD virus-induced oncogenesis. Although participation of endogenous RNA tumor virus genes in MD lesion induction could not be excluded, expression of such genes in MD tumors as gs antigen was not established.

Morphogenesis of marek's disease virus in feather follicle epithelium

Three evaluative systems, immunodiffusion, fluorescent antibody (FA), and electron microscopy (EM), were used to follow the morphogenesis of Marek's disease virus in inoculated chickens. Of the three, EM and FA were the most sensitive in detecting early stages of infection. Virus particles were found in skin biopsy specimens as early as 12 days post inoculation. Immature naked particles appeared first in the nucleus; later particles were enveloped in the cytoplasm and enclosed in cytoplasmic inclusion bodies. No evidence for continued virus replication was seen in feather follicles after an initial burst of heavy virus production, which lasted several weeks. Residual virus, however, was found occasionally in cytoplasmic inclusion bodies within keratinized material near the feathers. This was believed to contribute to the long-term shedding of infectious virus into the environment.