Classification of Marek's disease viruses according to pathotype: philosophy and methodology.

The concept of pathotype in Marek's disease (MD) probably dates from the recognition of a more virulent form of the disease in the late 1950s (Benton & Cover, 1957). Distinctions between MD virus strains were further expanded with the description of the vv pathotype in the early 1980s and of the vv+ pathotype in the 1990s. Pathotype designations reflect important biological properties that correlate with the break-through of vaccinal immunity in the field. However, pathotyping methods applied by various laboratories have not been uniform, preventing critical comparison of results. Better uniformity of pathotyping procedures is desirable.The Avian Disease and Oncology Laboratory (ADOL) method is based on induction of lymphoproliferative lesions in vaccinated chickens. This method has been used to pathotype more than 45 isolates and is the basis for the current pathotype classification of MD virus strains. Its limitations include requirements for a specific type of chickens (15x7 ab+), large numbers of animals, and a statistical method to compare lesion responses to those of JM/102W and Md5 control strains. Because of these limitations, it has not been and is not likely to be used in other laboratories. Comparability in pathotyping can be improved by the comparison of field isolates with standard prototype strains such as JM/102W, Md5 and 648A (American Type Culture Collection) or their equivalents. Data may be generated by different in vivo procedures that measure tumour induction, neurological disease (both neoplastic and non-neoplastic lesions), or solely non-neoplastic criteria (such as lymphoid organ weights or virus replication). Methods based on neoplastic criteria, especially when generated in MD-immunized chickens, will probably correlate most closely with that of the ADOL method and be most relevant to evolution of MD virus in the field. Based on data from several trials, a modification of the ADOL method that utilizes fewer chickens and can be conducted with commercial specific pathogen free strains is proposed. The modified method is based on "best fit" comparisons with prototype strains, and is expected to provide results generally comparable with the original method. A variety of other alternative criteria (see earlier) are also evaluated both for primary pathotyping and as adjuncts to other pathotyping methods. Advantages and disadvantages of alternative methods are presented.

Comparative susceptibility of Marek's disease cell lines to chicken infectious anemia virus.

Chicken infectious anemia virus (CIAV) is known to infect and replicate in various Marek's disease chicken cell lines (MDCCs) derived from Marek's disease (MD) tumors. One line, MDCC-MSB1, has been the substrate used in most studies. We compared a total of 26 MDCCs, including two sublines of MDCC-MSB1, MSB1 (L) and MSB1 (S), four other MD tumor-derived lines, and 20 lines derived from MD virus-induced local lesions, for susceptibility to the Cux-1 and CIA-1 strains of CIAV. The cell lines represented six phenotypic groups of T cells based on the expression of CD4, CD8, and TCR-2 and -3 surface markers. Susceptibility was measured by the number of cells positive for viral antigen in immunofluorescence (IF) tests at 3-10 days postinfection. No clear-cut differences were found in susceptibility related to phenotype, although CD4-/8+ lines and CD4-/8- lines might be more susceptible than CD4+/8- lines. However, several individual lines were more susceptible to Cux-1 than the two MSB1 sublines tested. Contrary to an earlier report, cells of MDCC-CU147, a CD8+, TCR3+, local-lesion derived line, were found to be susceptible to CIA-1. In fact, CU147 was distinguished by very high susceptibility to both CIAV strains. In direct comparisons with MSB1, CU147 detected approximately 10-fold lower doses of virus. Also, virus spread was faster (P < 0.05) in CU147 than in MSB1 and other lines. Results from polymerase chain reaction (PCR) tests to detect infection in titrations were in general agreement with IF test results although PCR detected infection in a few terminal dilution cultures that were negative by IF.

Relationship between the immunosuppressive potential and the pathotype of Marek's disease virus isolates.

Isolates of Marek's disease virus (MDV) representing three pathotypes of differing virulence were compared for relative immunosuppressive properties in genetically susceptible P2a-strain and genetically resistant N2a-strain chickens. Criteria of immunosuppression were 1) persistence of early cytolytic infection (i.e., a delay or failure to enter latency) in lymphoid organs, 2) atrophy of the bursa of Fabricius and thymus as measured by organ weight proportional to body weight at 8 and 14 days postinfection (DPI), and 3) histopathologic evidence of necrosis and atrophy in lymphoid organs. No significant differences in infection level were observed among the pathotypes during the early (4-5 DPI) period of infection. However, the extent of persistent cytolytic infection at 7-8 DPI, based on numbers of tissues positive and mean scores in immunofluorescence tests, was greater (P < 0.05) for three isolates (RK1, 584A, 648A) in the highest virulence pathotype (very virulent-plus MDV [vv + MDV]) than for two isolates (JM16, GA5) in a lower virulence (virulent MDV [vMDV]) pathotype. Results from two isolates (RB1B, Md5) classified in the intermediate very virulent pathotype (very virulent MDV [vvMDV]) fell between those from the other two pathotypes. Similarly, there was a stepwise effect of viral pathotype in which the vv + MDV isolates caused the most severe damage to lymphoid organs in terms of atrophy (relative organ weights) and histopathologic changes. Organs from chickens infected with vv + MDVs showed little recovery between 8 and 14 DPI. The vMDV isolates caused the least severe damage, and lymphoid organs showed a significant return toward normal by 14 DPI; vvMDV isolates induced intermediate degrees of atrophy and recovery. The same pattern of relationship between virulence pathotype and degree of bursal and thymic atrophy was also observed in genetically resistant N2a chickens. These results suggest that the degree of immunosuppression is linked to virulence and that a simple measure of atrophic changes (relative organ weights) in the bursa of Fabricius and thymus might be useful in determining the pathotype classification of new MDV isolates. The basis for differences in immunosuppressive potential of MDV isolates needs further clarification.

Control of avian encephalomyelitis: a historical account.

Avian encephalomyelitis control methods were not developed until the 1950s although the disease had been discovered and described over 20 yr earlier. Inability to transmit the infection by other than intracerebral inoculation, lack of suitable immunologic methods, the unknowing use of immune chickens or embryos for experimental studies, and reliance on a highly adapted strain of virus rather than fresh field isolates were the main reasons for a general lack of progress. In the absence of supportive experimental data, at least two commercial breeding organizations turned to the use of a crude chicken brain-propagated virus for vaccination of breeder replacement flocks in the 1950s. This control procedure turned out to be practical and efficacious. Development of suitable embryo infection methods and immunologic tests and the chance finding that antibody-free flocks were essential for experimental studies led to the development of embryo-susceptibility tests to identify immune breeder flocks and formed the basis for another commercially applied control program, the testing and selection of only immune flocks for hatching purposes. The application of the new testing methods coupled with a switch from the adapted Van Roekel strain of virus to fresh field isolates for experimentation resulted in a rapid unraveling of the epizootiology and pathogenesis of the disease and also to the development of a safe and effective vaccine that was licensed for administration to breeder replacements in 1962.

Interferon modulation of Marek's disease virus genome expression in chicken cell lines.

Lines of chicken lymphoblastoid cells were established from local lesions induced by simultaneous injection of Marek's disease virus and various stimulants of T-cell activation. Lines developed with regular medium had relatively high mean rates of spontaneous expression of viral internal antigen (6.2%). In contrast, lines developed and maintained with conditioned medium generated by mixed-lymphocyte reaction had a 62-fold reduction in the mean rate of viral internal antigen expression (0.1%). The expression rate could be modulated by the removal or re-addition of conditioned medium to the growth medium. Down regulation involved proteins classified as immediate-early (a 14-kDa polypeptide), early (a 38-kDa phosphoprotein), and late (glycoprotein B homologue) antigens, indicating that the block is very early in virus replication. Once initiated in a given cell, replication apparently proceeded unimpeded. Interferon was determined to be largely responsible for the suppressive activity of the conditioned medium, although involvement of other cytokines could not be ruled out. Also, chicken interferon from other sources, including recombinant interferon, was able to similarly suppress viral antigen expression.

Stages of Marek's disease virus latency defined by variable sensitivity to interferon modulation of viral antigen expression.

Cytokines in conditioned medium can suppress expression of viral internal antigens (VIA) in lymphocytes latently infected with Marek's disease virus. In the present study, conditioned media produced by spleen cells stimulated with concanavalin A or by mixed-lymphocyte reaction had significantly greater (P < 0.05) VIA-suppressive activity with lymphocytes harvested from birds at 14 days post infection than with those collected at 7 days. This finding defines two stages during the latent period in which sensitivity of lymphocytes to cytokine modulation of viral expression differs. Suppression involved proteins representing immediate-early, early and late viral antigens. Physico-chemical characterization of the suppressive factor in conditioned medium was consistent with that expected of interferon. Indeed, natural interferon prepared from avian reovirus-exposed chicken embryo cells, and recombinant chicken interferon, both mimicked the activity of conditioned medium and were more suppressive with lymphocytes from the later stage of latency.

Cytogenetic studies of cell lines derived from Marek's disease virus-induced local lesions.

Cytogenetic analysis of cell lines derived from Marek's disease virus (MDV) induced local lesions was performed to detect the presence of a chromosomal alteration found previously in cell lines derived from MDV-induced visceral tumors. This chromosomal alteration involves an amplified region on the short arm of chromosome 1. Trypsin G-binding was performed on 12 local-lesion cell lines having various T-cell receptor phenotypes. Eight of 12 cell lines had a diploid female karyotype, and four lines were diploid males. Ten of the cell lines showed a normal G-banding pattern; two lines, however, had the short arm amplification on chromosome 1 (1p+). Thus, the 1p+ alteration does not appear to be necessary for establishment of cell lines from local lesions. The occurrence of the 1p+ alteration in Marek's disease cell lines suggests a possible role for this alteration in late stages of multi-step viral oncogenesis.

Pathologic and serologic characterization of a variant of duck hepatitis type I virus.

A duck hepatitis virus (DHV), isolated from ducks on a farm in Virginia in 1963, was shown to be only partially related to DHV type I (DHV-I) in cross-neutralization and in cross-protection tests. The virus, named DHV-Ia, apparently is a serologic variant of DHV-I; both viruses are serologically distinct from DHV type III. Pathologic responses to DHV-Ia were similar to those described for DHV-I infection.

Stable transfection of reticuloendotheliosis virus-transformed lymphoblastoid cell lines.

Lymphoblastoid T cell lines were established by infection of chicken splenocytes with reticuloendotheliosis virus (REV). The target cells first were cultured in interleukin-containing conditioned medium or were stimulated by concanavalin A, or both. Most cell lines were T cells expressing CD3 and one of the T cell receptors, and all cell lines were positive for major histocompatibility complex (MHC) class II antigens. Several REV-transformed cell lines were stably transfected using electroporation with a selectable plasmid, pNL1, containing the neor gene. Transfected cell lines were selected using G418 and were maintained for periods up to 137 days. Transfected cell lines were susceptible to MHC class-I restricted lysis by cytotoxic T lymphocytes from REV-infected chickens.

Gordon Memorial Lecture. Chicken neoplasia--a model for cancer research.

1. The use of animal models has been immensely important for the advancement of our knowledge of the aetiology and pathogenesis of human diseases, including neoplasia. 2. Viruses, as oncogenic agents, were first described in the early 1900s when cell-free filtrates were used experimentally to transmit leukemias and sarcomas in chickens. In more recent years, studies with avian leukosis/sarcoma viruses have led the field in attempts to establish the genetic and molecular basis of viral oncogenesis. 3. Marek's disease of chickens was the first neoplasm proven to be caused by a herpesvirus and it remains the only neoplastic disease for which an effective vaccine has been developed and deployed. It serves as an elegant model as we seek an understanding of the pathogenesis of herpesvirus-induced lymphomas at both the cellular and molecular levels.

Two distinct alpha beta T-cell lineages can be distinguished by the differential usage of T-cell receptor V beta gene segments.

Avian T cells can be divided into three subpopulations based on their expression of distinct T-cell receptors (TCR1, TCR2, and TCR3), ontogeny, and tissue distribution. The TCR1 cells appear to be the equivalent of mammalian gamma delta cells, but the derivation of cells expressing TCR2 and TCR3 has been unclear. Here we report that chickens contain two families of TCR beta variable (V) gene segments, V beta 1 and V beta 2. Furthermore, TCR2 and TCR3 represent subsets of alpha beta cells that are defined by mutually exclusive usage of these two families of V beta gene segments. Sequence comparisons of V beta 1 and V beta 2 with mammalian TCR beta V segments reveal that V beta 1 gene segments encode the conserved amino acids used to define the mammalian V beta consensus subgroup I, while V beta 2 encodes the amino acids used to define the mammalian V beta subgroup II. Although the beta chains of TCR2 and TCR3 cells are encoded by the same diversity (D), joining (J), and constant (C) region segments, V beta 1 gene segments undergo rearrangement before V beta 2 gene segments during T-cell development. This may result from the fact that TCR2 cells undergo V-DJ joining by deletional rearrangement, whereas TCR3 cells undergo V-DJ joining by inversional rearrangement. These data suggest that the TCR alpha beta cells can be divided into two distinct and evolutionarily conserved lineages based on V beta gene segment usage. The clear-cut separation of these lineages in the chicken may help to define their immunologic role.

Proliferation of chicken lymphoblastoid cells after in vitro infection with Marek's disease virus.

Activated, thymus-derived (T) lymphoblasts were exposed to Marek's disease virus and cultivated in attempts to induce in vitro transformation. After 9 to 15 days, colonies or small clusters of proliferating lymphoblasts were observed in cultures from three of a total of 122 attempts. These developed into proliferating cell cultures that resembled conventional Marek's disease (MD) lymphoblastoid cell lines in terms of growth characteristics and morphology. All proliferative cultures were unusual in that 1) the expression of viral internal antigens consistently or periodically was very high (up to 30% of all cells) and 2) the cells deteriorated and/or proliferation ceased in all cases after culture periods of 45-176 days. The proliferative cultures were all characterized as CD2+ and CD3+, Ia-bearing T cells; one was CD4+/CD8- and TCR2+, the other two were CD4-/CD8- and TCR1+. The latter two are the only cultures of MD-infected cells known to be TCR1+.

Genetic susceptibility of chicken macrophages to in vitro infection with infectious laryngotracheitis virus.

Chicken macrophages are susceptible to infection with infectious laryngotracheitis virus (ILTV), the level of infection being dependent, in part, on the genotype of the host cell. Following infection in vitro a greater proportion of macrophages from the ILTV-resistant J1(B113/113) and N1(B114/114) inbred lines of chickens were found to be positive for ILTV antigens, than macrophages from the ILTV-susceptible M1(B15/15) chickens. The proportion of ILTV-positive macrophages was found to be genetically regulated, in part by the chicken major histocompatibility complex (MHC), although alloantisera to class I and class II MHC antigens did not reduce the number of macrophages infected. Similarly, the phagocytic activity of the cultured macrophages from chickens of different MHC genotype did not correlate with their susceptibility to infection with ILTV.

Transformation of T-lymphocyte subsets by Marek's disease herpesvirus.

Marek's disease herpesvirus (MDV)-transformed lymphoblastoid tumor cell lines were characterized for the presence of the surface markers. Monoclonal antibodies were used for CD3 (T-cell receptor [TCR] complex), TCR1, TCR2, and TCR3, CD4, CD8, and Ia antigen by indirect fluorescence staining followed by microscopic examination or flow cytometry. The lymphoblastoid cell lines were obtained from tumors from chickens infected with MDV (n = 44) or from local lesions induced by inoculation of allogeneic, MDV-infected chick kidney cells (n = 56). Lymphocytes were harvested from these lesions between 4 and 16 days postinoculation and cultured in vitro to establish cell lines. All cell lines expressed Ia antigen and CD3 and/or TCR and thus are activated T cells. Most of the cell lines developed from tumors were CD4+ CD8-; only one cell line was negative for both markers. Sixteen percent of the cell lines were TCR3+, while the remainder were TCR2+. The cell lines developed from local lesions were much more heterogeneous: 45% were CD4- CD8+, 34% were CD4- CD8-, and only 21% were CD4+ CD8-. The number of TCR3+ cell lines was larger than expected for the CD4- CD8+ and CD4- CD8- cell lines, as judged from the presence of these cells in the blood. These results indicate that several subsets of T lymphocytes can be transformed by MDV, depending on the pathogenesis of infection. Activation of T cells as a consequence of the normal pathogenesis or by allogeneic stimulation seem to be a first important step in the process of transformation.

Pathogenesis of Marek's disease virus-induced local lesions. 1. Lesion characterization and cell line establishment.

Subcutaneous (wing-web) or intramuscular inoculation of chickens with allogeneic normal or Marek's disease virus (MDV)-infected chicken kidney cells induced local lesions visible by 3-4 days postinoculation (PI). Lesions were slightly larger (P less than 0.05) in infected than uninfected chickens 5 and 8 days PI. They persisted and grew past 9 days PI only when infected. Infiltrating lymphocytes in infected and uninfected early lesions were similar; they included B-cells and also T-cells with and without Ia antigen. Up to 42% of lymphocytes from infected or uninfected lesions had the surface antigen MATSA. At 3 to 6 days PI, infected lesions contained lymphocytes with viral internal antigen, especially in Ia-bearing cells and MATSA-bearing cells, but thereafter infection was latent. Cells harvested daily from local lesions induced with allogeneic MDV-infected cells were cultured; MD tumor cell lines were established from lesions as early as 4 days PI, with a total success rate of about 50% thereafter. Either transformed tumor cells were already present during the early cytolytic infection period or else appropriate target cells were present that became infected in vivo and/or in vitro and then became transformed in vitro.

Effect of reticuloendotheliosis virus and infectious bursal disease virus on Marek's disease herpesvirus latency.

Birds infected with reticuloendotheliosis virus (REV) were exposed to Marek's disease virus (MDV) to determine if the establishment of MDV latency was affected by REV-induced immunosuppression, while other chickens, already latently infected with MDV, were challenged with REV or infectious bursal disease virus (IBDV) to determine if the consequent immunosuppression caused a return to cytolytic infection. Immunosuppression was assessed by in vitro mitogen stimulation assays with spleen cells. Latently MDV-infected cells were free of viral internal antigen(s) (VIA) but could be identified by their ability to produce VIA after in vitro cultivation. The results were unexpected: chickens infected with either of these viruses had very low, and often undetectable, levels of MDV infection when compared with appropriate controls. REV infection interfered with early cytolytic MDV infection, and IBDV and REV both failed to activate latent MDV infection in the face of inferred (IBDV) or demonstrated (REV) immunosuppression by these viruses. Apparently, both viruses reduced the number of MDV infected cells since neither cytolytic nor latent infection could be demonstrated. This was based on an absence of cells with VIA either before or after cultivation and, in the case of REV infection, on failure to detect MDV-DNA using a dot-blot hybridisation technique.

Cytotoxic T lymphocytes in reticuloendotheliosis virus-infected chickens.

Cytotoxic T lymphocytes were functionally demonstrated in spleen cells from chickens 7 days post inoculation with reticuloendotheliosis virus using a Cr-release assay. Major histocompatibility complex (MHC)-restricted cytotoxicity was demonstrated using effector and target cells from two different strains of chickens of known avian MHC haplotype. Anti-viral specificity was shown and in vivo generation of MHC-restricted cytotoxicity was evaluated. Cytotoxic T cells were distinguished from macrophages and natural killer cells. Their cytotoxicity was not antibody dependent. Higher levels of cytolysis were found with cytotoxic T cells from embryonally bursectomized vs. intact chickens over a large range of effector to target cell ratios. Using monoclonal antibodies, cytotoxic T cells were further defined as Ia+ T cells by immunofluorescence, antibody plus complement-mediated lysis of effector cells and blocking of cytolysis in the Cr-release assay.

Immune response versus susceptibility to Marek's disease.

It was hypothesized that the generation of activated T cells through an efficient and rapid immune response during the early pathogenesis of Marek's disease virus (MDV) infection provides a large pool of target cells for transformation. Therefore, the correlation between genetic susceptibility to Marek's disease (MD) and in vitro mitogenic responses of lymphocytes as a measure of cell-mediated immune competence and efficiency was tested. In one series of trials, spleen cells from strains of chickens with differing levels of susceptibility to MD tumors were stimulated with graded doses of Concanavalin A (Con A) or phytohemagglutin (PHA). In a second series of trials, peripheral blood lymphocytes from individual chickens within genetic strains were tested at the same time chickens were challenged with MDV to determine susceptibility. Responsiveness was determined using one-way mixed lymphocyte reaction (MLR) tests as well as mitogen stimulation. Data from the tests comparing chicken strains supported the hypothesis in some but not all cases. The S13 chickens, which are more susceptible than P2a chickens to MD, were significantly more responsive, and highly resistant N2a chickens were significantly less responsive to Con A. In contrast, five other resistant strains were either more responsive (UCD-058, OS13) or equally responsive (UCD-140, OS5, C) to Con A when compared with P2a chickens. The PHA responses were even less predictive of MD susceptibility. No general correlation was observed between responsiveness to either mitogen or MLR tests and subsequent tumor development in trials comparing individuals within strains.

Maintenance of Marek's disease herpesvirus latency in vitro by a factor found in conditioned medium.

Chicken spleen cells latently infected with Marek's disease virus were cultured with and without conditioned medium (CM) obtained from concanavalin A-stimulated chicken spleen cell cultures. The expression of viral internal antigen(s) (VIA), which is usually associated with cultivation, was prevented or markedly reduced by the CM. This effect required the continued presence of CM, since its removal after 48 h resulted in the subsequent appearance of VIA. Although CM contains both gamma interferon (IFN-gamma) and interleukin 2, our studies suggest that the 'latency-maintaining activity' (LMF) may not be associated with either of these products of stimulated lymphocytes. However, IFN-gamma may also have had some suppressive effect. LMF appears to have an Mr greater than 10,000 and to be inactivated by heating to 90 degrees C for 5 min.