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Reticuloendotheliosis virus (REV) strains T and CS consistently induced focus formation in a quail fibroblast cell line designated QT35 that was maintained under agarose overlay following inoculation of REV. The foci were more distinct and better circumscribed than those that developed in QT35 cultures under fluid medium. The focus formation in QT35 cultures under agarose overlay was inhibited by REV antiserum, and there were a highly significant correlation (r = 0.966, P less than 0.005) and a linear relationship between the number of foci that developed and relative virus concentration inoculated. This provides a valid focus assay for REVs.

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Reticuloendotheliosis virus (REV) was isolated in cell cultures from commercial Marek's disease (herpesvirus of turkeys) vaccine and re-isolated from the organs of vaccinated chickens. Runting and feathering abnormalities were produced when 1-day-old specific pathogen free chickens were inoculated with REV. Histopathological lesions in infected chickens were hypoplasia of the thymus, bursa and spleen, and inflammation of the proventriculus, kidneys and liver. Serological responses to REV were detected by the indirect immunoflorescence test in chickens directly inoculated with contaminated vaccine, and spread of REV infection to in-contact chickens was demonstrated by histopathological and serological investigations.

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Immune serum, obtained from animals that had survived sublethal challenge with RE virus, was very effective in achieving tumor cures in chickens infected with this virus. The therapeutic effect could also be obtained by the immunoglobulin fraction of immune serum. The serum did not influence the development of an unrelated malignancy. Serotherapy studies in Bx and Tx recipients indicate that the B-cell or T-cell system of the host does not significantly contribute to the curative activity of immune serum. Absorption studies show that the curative effect is mediated by antibodies to tumor-associated transplantation antigens on tumor cells, not by antiviral antibodies.

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Two-day-old chicks were inoculated with one or ten doses of Marek's disease (MD) vaccine originated from the herpesvirus of turkeys (HVT) and contaminated with a reticuloendotheliosis virus (REV). As a result, they presented such symptoms as abnormality in the vane of remiges, undergrowth, anemia, and leg paralysis. These symptoms were the same as those induced by the same vaccine among chicks in the field. Control chicks which had been placed in the same house as those inoculated with the vaccine exhibited no abnormal signs. A persistent infection with REV was noticed in the vaccine-inoculated group. A horizontal infection with REV was the highest in the control group, which was followed by the group inoculated with one dose and that inoculated with ten doses in the order listed. The antibody response of chicks to HVT and MD virus was also inhibited by REV.

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Over a period from spring to fall in 1974, a disease with delayed growth, anemia, abnormal feathers, and leg paralysis as main symptoms broke out in flocks of chickens inoculated with Marek's disease vaccine. A virus was isolated from affected birds in the field and the same lot of Marek's disease vaccine as inoculated into these birds. It had a common antigenicity to the T strain of reticuloendotheliosis virus (REV) and could not be discriminated from this strain on the basis of morphology or property. When chicks were inoculated with it, they presented essentially the same symptoms as the birds affected in the field. Since the disease was reproduced in this manner, it was presumed to have been caused by REV contained in the vaccine as contaminant. The virus persisted in the body for long time and also induced horizontal infection.

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We have described the in vitro isolation of type-C RNA viruses from two outbreaks of a fatal neoplastic disease in turkeys we diagnosed as reticuloendotheliosis. The virus had a density of 1.16 g/ml in sucrose gradients, had a DNA polymerase capable of using both endogenous and exogenous (synthetic) templates, and was infectious in vitro for turkey and chick cells. The culture-propagated virus was oncogenic for turkeys. The virus isolates were unrelated to avian leukosis virus and antigenically identical to reticuloendotheliosis virus (REV) strain T. Thus our studies suggested that REV is a causative agent of naturally occurring, fatal leukosis in turkeys.

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Lymphoreticular proliferative disorders occur in most species of animals including submammalian vertebrates but only occasionally affect the nervous system. Primary lymphoreticular disorders of the nervous system have been recognized in chickens as a herpesvirus-induced avian neurolymphomatosis and in mammals, patricularly in the dog, as reticulum cell sarcomas originating from perivascular mesenchymal precursor cells. Criteria have been discussed to distinguish neoplastic from inflammatory lymphoreticuloses but borderline cases exist where a clear distinction is not always possible.

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Replication of reticuloendotheliosis viruses (REV) in cultures of chicken and duck fibroblasts leads to some cell death soon after infection. This cell killing was used to develop a plaque assay for Trager duck spleen necrosis virus (TDSNV) on duck embryo fibroblasts. A normal replicative cell cycle was required for normal virus production and the development of cytopathic effects in chicken cells exposed to TDSNV. The latent period was about two days. Stationary chicken embryo fibroblasts could be infected by REV; DNA synthesis was required, but protein synthesis was not.

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Velocity sedimentation and isopycnic density gradient centrifugation indicate that reticuloendotheliosis virus has a different mass and buoyant density than members of the avian tumor virus group. The group-specific antigen of the avian tumor virus group was not detected in concentrated and purified reticuloendotheliosis virus preparations.

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