The oncologist's debt to the chicken

Avian Models for Human Carcinogenesis-Recent Findings from Molecular and Clinical Research

Birds, especially the chick and hen, have been important biomedical research models for centuries due to the accessibility of the avian embryo and the early discovery of avian viruses. Comprehension of avian tumor virology was a milestone in basic cancer research, as was that of non-viral genesis, as it enabled the discovery of oncogenes. Furthermore, studies on avian viruses provided initial insights into Kaposi's sarcoma and EBV-induced diseases. However, the role of birds in human carcinogenesis extends beyond the realm of virology research. Utilization of CAM, the chorioallantoic membrane, an easily accessible extraembryonic tissue with rich vasculature, has enabled studies on tumor-induced angiogenesis and metastasis and the efficient screening of potential anti-cancer compounds. Also, the chick embryo alone is an effective preclinical in vivo patient-derived xenograft model, which is important for the development of personalized therapies. Furthermore, adult birds may also closely resemble human oncogenesis, as evidenced by the laying hen, which is the only animal model of a spontaneous form of ovarian cancer. Avian models may create an interesting alternative compared with mammalian models, enabling the creation of a relatively cost-effective and easy-to-maintain platform to address key questions in cancer biology.

Latest Insights into Marek's Disease Virus Pathogenesis and Tumorigenesis

Marek’s disease virus (MDV) infects chickens and causes one of the most frequent cancers in animals. Over 100 years of research on this oncogenic alphaherpesvirus has led to a profound understanding of virus-induced tumor development. Live-attenuated vaccines against MDV were the first that prevented cancer and minimized the losses in the poultry industry. Even though the current gold standard vaccine efficiently protects against clinical disease, the virus continuously evolves towards higher virulence. Emerging field strains were able to overcome the protection provided by the previous two vaccine generations. Research over the last few years revealed important insights into the virus life cycle, cellular tropism, and tumor development that are summarized in this review. In addition, we discuss recent data on the MDV transcriptome, the constant evolution of this highly oncogenic virus towards higher virulence, and future perspectives in MDV research.

Artesunate derivative TF27 inhibits replication and pathogenesis of an oncogenic avian alphaherpesvirus

Nucleoside analogues have been the cornerstone of clinical treatment of herpesvirus infections since the 1970s. However, severe side effects and emergence of drug resistant viruses raise the need for alternative treatment options. We recently investigated the broad and strong antiherpesviral activity of the optimized artesunate derivative TF27 in vitro. TF27 efficiently inhibited replication of the highly oncogenic Marek's disease virus (MDV), a virus that infects chickens, causes deadly lymphomas and threatens poultry populations worldwide. In this study, we used this natural virus-host model for herpesvirus-induced cancer by infecting chickens with MDV, and evaluated the protective efficacy of TF27 and the nucleoside analogue valganciclovir (VGCV) on virus replication and tumorigenesis. We could demonstrate that both drugs reduced viral load in the blood and prevented tumor development in a large portion of the animals. Antiviral treatment also had a positive impact on body weight gain, while no negative compound-associated side effects were observed. This research provides the first evidence that the artesunate derivative TF27 and VGCV can be used in avian species and that they inhibit MDV replication and tumorigenesis. In addition, our study paves the way for promising approaches in future antiherpesviral drug development.

Clonal structure of rapid-onset MDV-driven CD4+ lymphomas and responding CD8+ T cells

Lymphoid oncogenesis is a life threatening complication associated with a number of persistent viral infections (e.g. EBV and HTLV-1 in humans). With many of these infections it is difficult to study their natural history and the dynamics of tumor formation. Marek's Disease Virus (MDV) is a prevalent α-herpesvirus of poultry, inducing CD4+ TCRαβ+ T cell tumors in susceptible hosts. The high penetrance and temporal predictability of tumor induction raises issues related to the clonal structure of these lymphomas. Similarly, the clonality of responding CD8 T cells that infiltrate the tumor sites is unknown. Using TCRβ repertoire analysis tools, we demonstrated that MDV driven CD4+ T cell tumors were dominated by one to three large clones within an oligoclonal framework of smaller clones of CD4+ T cells. Individual birds had multiple tumor sites, some the result of metastasis (i.e. shared dominant clones) and others derived from distinct clones of transformed cells. The smaller oligoclonal CD4+ cells may represent an anti-tumor response, although on one occasion a low frequency clone was transformed and expanded after culture. Metastatic tumor clones were detected in the blood early during infection and dominated the circulating T cell repertoire, leading to MDV associated immune suppression. We also demonstrated that the tumor-infiltrating CD8+ T cell response was dominated by large oligoclonal expansions containing both “public” and “private” CDR3 sequences. The frequency of CD8+ T cell CDR3 sequences suggests initial stimulation during the early phases of infection. Collectively, our results indicate that MDV driven tumors are dominated by a highly restricted number of CD4+ clones. Moreover, the responding CD8+ T cell infiltrate is oligoclonal indicating recognition of a limited number of MDV antigens. These studies improve our understanding of the biology of MDV, an important poultry pathogen and a natural infection model of virus-induced tumor formation.

Many viral infections target the immune system, making use of the long lived, highly proliferative lymphocytes to propagate and survive within the host. This characteristic has led to an association between some viruses such as Epstein Barr Virus (EBV), Human T cell Lymphotrophic Virus-1 (HTLV-1) and Mareks Disease Virus (MDV) and lymphoid tumors. We employed methods for identifying the T cell receptor repertoire as a molecular bar-code to study the biology of MDV-induced tumors and the anti-tumor response. Each individual contained a small number of large (high frequency) tumor clones alongside some smaller (lower frequency) clones in the CD4+ T cell population. The tumor infiltrating CD8+ T cell response was highly focused with a small number of large clones, with one representing a public CDR3 sequence. This data is consistent with the recognition of a small number of dominant antigens and understanding the relationship between these and protective immunity is important to improve development of new vaccination strategies. Collectively, our results provide insights into the clonal structure of MDV driven tumors and in the responding CD8+ T cell compartment. These studies advance our understanding of MDV biology, an important poultry disease and a natural infection model of virus-induced tumor formation.

Identification of the neoplastically transformed cells in Marek's disease herpesvirus-induced lymphomas: recognition by the monoclonal antibody AV37

Understanding the interactions between herpesviruses and their host cells and also the interactions between neoplastically transformed cells and the host immune system is fundamental to understanding the mechanisms of herpesvirus oncology. However, this has been difficult as no animal models of herpesvirus-induced oncogenesis in the natural host exist in which neoplastically transformed cells are also definitively identified and may be studied in vivo. Marek's disease (MD) herpesvirus (MDV) of poultry, although a recognized natural oncogenic virus causing T-cell lymphomas, is no exception. In this work, we identify for the first time the neoplastically transformed cells in MD as the CD4(+) major histocompatibility complex (MHC) class I(hi), MHC class II(hi), interleukin-2 receptor alpha-chain-positive, CD28(lo/-), phosphoprotein 38-negative (pp38(-)), glycoprotein B-negative (gB(-)), alphabeta T-cell-receptor-positive (TCR(+)) cells which uniquely overexpress a novel host-encoded extracellular antigen that is also expressed by MDV-transformed cell lines and recognized by the monoclonal antibody (MAb) AV37. Normal uninfected leukocytes and MD lymphoma cells were isolated directly ex vivo and examined by flow cytometry with MAb recognizing AV37, known leukocyte antigens, and MDV antigens pp38 and gB. CD28 mRNA was examined by PCR. Cell cycle distribution and in vitro survival were compared for each lymphoma cell population. We demonstrate for the first time that the antigen recognized by AV37 is expressed at very low levels by small minorities of uninfected leukocytes, whereas particular MD lymphoma cells uniquely express extremely high levels of the AV37 antigen; the AV37(hi) MD lymphoma cells fulfill the accepted criteria for neoplastic transformation in vivo (protection from cell death despite hyperproliferation, presence in all MD lymphomas, and not supportive of MDV production); the lymphoma environment is essential for AV37(+) MD lymphoma cell survival; pp38 is an antigen expressed during MDV-productive infection and is not expressed by neoplastically transformed cells in vivo; AV37(+) MD lymphoma cells have the putative immune evasion mechanism of CD28 down-regulation; AV37(hi) peripheral blood leukocytes appear early after MDV infection in both MD-resistant and -susceptible chickens; and analysis of TCR variable beta chain gene family expression suggests that MD lymphomas have polyclonal origins. Identification of the neoplastically transformed cells in MD facilitates a detailed understanding of MD pathogenesis and also improves the utility of MD as a general model for herpesvirus oncology.

Resistance to Marek's disease herpesvirus-induced lymphoma is multiphasic and dependent on host genotype

Genotype-dependent differences in Marek's disease (MD) susceptibility were identified using 14-day-old line N and 6(1) (resistant) and 151 and 7(2) (susceptible) inbred chickens infected with HPRS-16 MD virus (MDV). All line 72 chickens developed progressive MD. Line 15I had fluctuating MD-specific clinical signs and individuals recovered. A novel histologic scoring system enabled indices to be calculated for lymphocyte infiltration into nonlymphoid organs. All genotypes had increased mean lesion scores (MLSs) and mean total lesion scores after MDV infection. These differed quantitatively and qualitatively between the genotypes. Lines 6(1) and 7(2) had a similar MLS distribution in the cytolytic phase, although scores were greater in line 7(2). At the time lymphomas were visible in line 7(2), histologic lesions in line 6(1) were regressing. AV37+ cells were present in similar numbers in all genotypes in the cytolytic phase, suggesting that neoplastically transformed cells were present in all genotypes regardless of MD susceptibility. After the cytolytic phase, AV37+ cell numbers increased in lines 7(2) and 15I but decreased in lines 6(1) and N. In the cytolytic and latent phases, in all genotypes, most infiltrating cells were CD4+. After this time, line 7(2) and 15I lesions increased in size and most cells were CD4+; line 6(1) and N lesions decreased in size and most cells were CD8+. In all genotypes, AV37 immunostaining was weak in lesions with many CD8+ cells, suggesting that AV37 antigen expression or AV37+ cells were controlled by CD8+ cells. The rank order, determined by clinical signs and pathology, for MD susceptibility (highest to lowest) was 7(2) > 15I > 6(1) > N.

Avian leukosis virus subgroup J: a rapidly evolving group of oncogenic retroviruses

A strain of avian leukosis virus (ALV) belonging to a new envelope subgroup J was isolated in the UK in 1988 from meat-type chickens. The disease caused by the members of this subgroup has since spread very rapidly worldwide and has become one of the major problems facing the broiler meat industry. Molecular characterisation of HPRS -103, the prototype of subgroup J, has shown that it has a structure of a typical ALV with gag, pol and env genes. However the env gene was distinct from that of other ALV s and was closely related to that of novel endogenous retroviral elements designated EAV - HP. As other regions of the genome were closely related to ALV s, it is believed that ALV-J has evolved by recombination with the env sequences of EAV - HP. ALV-J has a tropism for myeloid cells, a feature that may be associated with its ability to induce myeloid leukosis. Recent data show that ALV -J isolates evolve rapidly resulting in sequence changes within the variable regions of the env gene leading to antigenic variation. Eradication programmes established for other subgroups are proving to be effective in eradicating ALV-J from infected flocks.