Marek's disease in turkeys: the induction of lesions and the establishment of lymphoid cell lines.

The inoculation of turkeys with large doses of a virulent strain of Marek's disease virus (GA strain), but not of two other virulent strains (HPRS-16 and JM), was found to induce a disease resembling Marek's disease of the chicken. The most prominent lesions were lymphocytic leukaemia and lymphoid and reticular hyperplasia in the spleen and the liver. These developed after a prolonged latent period and the early histological changes (lymphoid cell destruction and reticuloendothelial cell hyperplasia) reported in chickens were not observed. Twelve cell lines were established from suspensions of spleen cells or of buffy coat cells from infected turkeys. These cells expressed both Marek's disease tumour-associated surface antigen and T-cell antigens. The cells carried the Marek's disease virus genome and when inoculated into chickens induced typical Marek's disease lymphomas. Nine of the cell lines were infected with an avian leukosis virus, but three lines were free of such infection. All cell lines had normal turkey karyotypes.

Studies on the role of macrophages in Marek's disease of the chicken.

Macrophage activity in relation to Marek's disease was investigated by determining phagocytic indices in vivo and by examining virus plaque-inhibiting activity of peritoneal macrophages in vitro. No correlation was observed between phagocytic index and resistance in different genetic strains of chickens. Infection with Marek's disease virus increased both phagocytic indices and the plaque-inhibiting activity of peritoneal macrophages, more so in susceptible than in resistant chickens. There was an association between increased macrophage activity and virus replication, and it is suggested that the enhancement of macrophage activity results from activation indirectly caused by the presence of Marek's disease virus or viral antigens.

The role of histocompatibility antigens in cell-mediated cytotoxicity against Marek's disease tumour-derived lymphoblastoid cell lines.

The cytotoxic activity of spleen cells from Marek's disease (MD) virus-infected chickens against syngeneic and allogeneic tumour cell lines was compared, using MD lymphoma-derived lymphoblastoid cell lines obtained from two inbred and two outbred chicken strains. Activity was significantly greater against allogeneic than against syngeneic target cells, although some activity against syngeneic cells was detected. Cold target cell inhibition tests confirmed that unlabelled normal spleen cells could block the cytolysis of tumour cell targets bearing the same histocompatibility antigens as the spleen cells. It was concluded that the tumour-specific antigens against which the effector cells were reacting may be modified histocompatibility antigens and that the enhanced cytotoxicity seen with allogeneic target and effector cells may be an artefact analogous to an adjuvant effect.

The occurrence of chicken foetal antigen after infection with Marek's disease virus in three strains of chicken.

An indirect immunofluorescence procedure employing specific antiserum was used to demonstrate chicken foetal antigen (CFA), a membrane antigen typically present on embryonic cells. The expression of CFA by lymphocytes from the peripheral blood, spleen, thymus and bursa was found to decline rapidly after hatching; the bursa, however, contained about 30% CFA-positive cells until at least 120 days of age. After infection of a genetically susceptible strain of chickens (HPRS Rhode Island Red) with Marek's disease virus, the expression of CFA by lymphocytes in the blood and lymphoid organs was found to increase steadily. Lymphomas which developed contained, on average, 25.7% CFA-expressing cells. In contrast, a second susceptible strain (line 7) showed no increase in CFA expression (compared with uninfected controls), and lymphomas from these birds contained only 4.8% positive cells. A genetically resistant strain (line 6) also did not differ from uninfected controls. It was concluded that in Marek's disease, CFA expression is associated with malignant transformation, although transformation may occur without the appearance of CFA, and that CFA is a marker for a stage of dedifferentiation of transformed cells rather than a marker for transformation. The degree of dedifferentiation seems to vary from strain to strain.

The mechanism of genetic resistance to Marek's disease in chickens.

Genetic resistance to Marek's disease in RPL line-6 chickens is expressed not only at the level of host immunological responses against virus an tumour antigens, but also at the level of target lymphoid cells for virus infection and transformation. The nature of the target cell involved was investigated. Spleen cells from susceptible line-7 chickens adsorbed more Marek's disease virus and turkey herpesvirus in vitro than line-6 spleen cells. In the case of Marek's disease virus this was reflected in the replicative ability of the virus in vivo. Transplantation of thymus fragments from 1-day-old line-7 chickens into thymectomized line-6 chickens conferred a high degree of susceptibility on the latter, but the transplantation of spleen or fragments had no significant effect. The reverse procedure, i.e. grafting of line-6 thymi into line-7 chickens, did not diminish the susceptibility of the recipients. In each treatment group the observed titres of leukocyte-associated viraemia correlated with the susceptibility of the group to Marek's disease. Histologically the grafted thymus fragments became depleted of lymphocytes immediately after transplantation. By 6 days there was substantial recovery, apparently as a result of re-population of the thymic epithelium by host stem cells. This was confirmed by transplanting thymus fragments between individuals of opposite sexes. Karyotype analysis showed that the thymus contained lymphocytes of the sex of the recipient. However, karyotype analysis of lymphoma cells taken from recipient line-6 chickens that had received thymus grafts from line-7 birds of the opposite sex showed that, in the majority of cases, the lymphomas consisted of cells of donor origin. It is concluded that the susceptibility of line-7 chickens is largely attributable to the greater susceptibility of their T-lymphocytes to infection and transformation by Marek's disease virus, and that this susceptibility can be transferred to genetically resistant line-6 birds by adoptive transfer of the cells in the form of thymus fragments.

The effect of bursectomy on the adoptive transfer of resistance to Marek's disease.

Immunization with inactivated viral antigens protected chickens against Marek's disease. Non-immunized chickens could be protected by injections of spleen cells but not of serum from immunized, histocompatible donors. Chickens rendered agamma-globulinaemic by bursectomy and irradiation could also be immunized against Marek's disease by inoculation with viral antigens, but spleen cells from these immunized, bursectomized and irradiated donors did not confer protection on the recipients into which they were injected. It was concluded that, although in the bursectomized, immunized donors cell-mediated immunity alone was able to provide a fair degree of protection against Marek's disease, the protection afforded against the disease by spleen-cell transfer was at least partly attributable to the transfer of antibody-producing cells, and that humoral immunity, while not being an absolute requirement for resistance, is normally an important component of the resistance mechanism.

The effect of bursectomy on vaccination against Marek's disease with the herpesvirus of turkeys.

Protection against Marek's disease following vaccination with the herpesvirus of turkeys was significantly impaired but not abolished by ablation of the bursa-dependent lymphoid system by surgery and X-irradiation. It was concluded that the humoral immune system is relatively more important in vaccinal immunity induced by the herpesvirus of turkeys compared with that induced by attenuated Marek's disease virus.