Use of an agar culture technique for establishing lymphoid cell lines from Marek's disease lymphomas

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.

MicroRNAs 221 and 222 target p27Kip1 in Marek's disease virus-transformed tumour cell line MSB-1

MicroRNAs (miRNAs) are a class of short RNAs that function as post-transcriptional suppressors of protein expression and are involved in a variety of biological processes, including oncogenesis. Several recent studies have implicated the involvement of miR-221 and miR-222 in tumorigenesis as these miRNAs are upregulated in a number of cancers and affect the expression of cell cycle regulatory proteins such as the cyclin-dependent kinase (cdk) inhibitor p27(Kip1). Marek's disease virus (MDV) is a highly oncogenic herpesvirus that affects poultry, causing acute neoplastic disease with lymphomatous lesions in several organs. MDV-encoded oncogenes such as Meq are directly implicated in the neoplastic transformation of T cells and have been well studied. More recently, however, the involvement of both host and virus-encoded miRNAs in the induction of MD lymphomas is being increasingly recognized. We analysed the miRNA expression profiles in the MDV-transformed lymphoblastoid cell line MSB-1 and found that endogenous miRNAs miR-221 and miR-222 were significantly upregulated. Demonstration of the conserved binding sites for these miRNAs in the chicken p27(Kip1) 3'-untranslated region sequence and the repression of luciferase activity of reporter constructs indicated that miR-221 and miR-222 target p27(Kip1) in these cells. We also found that overexpression of miR-221 and miR-222 decreased p27(Kip1) levels and that treatment with retrovirally expressed antagomiRs partially alleviated this suppression. These data show that an oncogenic herpesvirus, as in the case of many cancers, can exploit the miRNA machinery for suppressing cell cycle regulatory molecules such as p27(Kip1) in the induction and progression of T-cell lymphomas.

Effect of cyclosporin a on normal, mitogen-stimulated and Marek's disease virus-exposed and transformed chicken lymphoid cells

Studies have been made with the immunosuppressive drug cyclosporin A (CsA) to examine its value in the establishment of lymphoid tumour cell lines from Marek's disease (MD) lymphomas and from lymphoid cell cultures exposed to MD virus in vitro. CsA was shown to depress the proliferative response of normal spleen cells to phytohaemagglutinin, Concanavalin A and pokeweed mitogen, and to a lesser extent to lipopolysaccharide. Short-term proliferative responses of lymphoma cells were either not affected, depressed or stimulated by CsA. The efficiency of establishment of lymphoid cell lines from long-term cultures of lymphoma cells was not increased by CsA, and the drug had a depressive effect on the proliferation of cell lines in the lympho-cytoid stage. The majority of lymphoblastoid cell lines studied were stimulated by CsA. Interleukin 2 partially overcame the suppressive effect of CsA on the cell lines, and enhanced the stimulatory effects. Cultures of lymphoid cells exposed to MD virus in vitro were usually depressed by CsA; a few stimulatory combinations were observed, but these were not considered to be of biological significance. These results indicate that CsA suppresses normal T-cell responses in the chicken, but that some MD-associated lymphoid cells are stimulated by the drug, in some instances at least by a direct effect.

MicroRNA profile of Marek's disease virus-transformed T-cell line MSB-1: predominance of virus-encoded microRNAs

Research over the last few years has demonstrated the increasing role of microRNAs (miRNAs) as major regulators of gene expression in diverse cellular processes and diseases. Several viruses, particularly herpesviruses, also use the miRNA pathway of gene regulation by encoding their own miRNAs. Marek's disease (MD) is a widespread lymphomatous neoplastic disease of poultry caused by the highly contagious Marek's disease virus type 1 (MDV-1). Recent studies using virus-infected chicken embryo fibroblasts have identified at least eight miRNAs that map to the R(L)/R(S) region of the MDV genome. Since MDV is a lymphotropic virus that induces T-cell lymphomas, analysis of the miRNA profile in T-cell lymphoma would be more relevant for examining their role in oncogenesis. We determined the viral and host miRNAs expressed in MSB-1, a lymphoblastoid cell line established from an MDV-induced lymphoma of the spleen. In this paper, we report the identification of 13 MDV-1-encoded miRNAs (12 by direct cloning and 1 by Northern blotting) from MSB-1 cells. These miRNAs, five of which are novel MDV-1 miRNAs, map to the Meq and latency-associated transcript regions of the MDV genome. Furthermore, we show that miRNAs encoded by MDV-1 and the coinfected MDV-2 accounted for >60% of the 5,099 sequences of the MSB-1 "miRNAome." Several chicken miRNAs, some of which are known to be associated with cancer, were also cloned from MSB-1 cells. High levels of expression of MDV-1-encoded miRNAs and potentially oncogenic host miRNAs suggest that miRNAs may have major roles in MDV pathogenesis and neoplastic transformation.

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.

Adaptation of Marek's disease virus to the Vero continuous cell line

Marek's disease virus (MDV) is a highly infectious, cell-associated oncogenic herpesvirus. Production of MD vaccines has been limited to primary chicken and duck embryo fibroblast (CEF and DEF) cultures. These have a limited life span and cannot be readily stored in liquid nitrogen. Moreover, the need to prepare CEF and DEF cells on a regular basis from 10 to 11 day-old embryos derived from a flock that must be tested continuously for the presence of avian pathogens adds to the cost of vaccine production. A continuous cell line that would support MDV replication could have significant advantages for the rapid large-scale preparation of MD vaccines. In this report, we describe the adaptation to growth of CEF-grown preparations of serotype 1 and serotype 3 (herpesvirus of turkeys; HVT) strains of MDV in cells of the Vero continuous cell line. Although both viruses produced typical CPE, higher levels of infectious progeny and more extensive virus-specific immunofluorescence were obtained for HVT than for the serotype 1 virus. PCR and pulsed field electrophoresis (PFE) analysis of the DNA from Vero cells infected with either virus confirmed the presence of virus-specific DNA.

Transactivation of latent Marek's disease herpesvirus genes in QT35, a quail fibroblast cell line, by herpesvirus of turkeys

The QT35 cell line was established from a methylcholanthrene-induced tumor in Japanese quail (Coturnix coturnix japonica) (C. Moscovici, M. G. Moscovici, H. Jimenez, M. M. Lai, M. J. Hayman, and P. K. Vogt, Cell 11:95-103, 1977). Two independently maintained sublines of QT35 were found to be positive for Marek's disease virus (MDV)-like genes by Southern blotting and PCR assays. Sequence analysis of fragments of the ICP4, ICP22, ICP27, VP16, meq, pp14, pp38, open reading frame (ORF) L1, and glycoprotein B (gB) genes showed a strong homology with the corresponding fragments of MDV genes. Subsequently, a serotype 1 MDV-like herpesvirus, tentatively name QMDV, was rescued from QT35 cells in chicken kidney cell (CKC) cultures established from 6- to 9-day-old chicks inoculated at 8 days of embryonation with QT35 cells. Transmission electron microscopy failed to show herpesvirus particles in QT35 cells, but typical intranuclear herpesvirus particles were detected in CKCs. Reverse transcription-PCR analysis showed that the following QMDV transcripts were present in QT35 cells: sense and antisense meq, ORF L1, ICP4, and latency-associated transcripts, which are antisense to ICP4. A transcript of approximately 4.5 kb was detected by Northern blotting using total RNA from QT35 cells. Inoculation of QT35 cells with herpesvirus of turkeys (HVT)-infected chicken embryo fibroblasts (CEF) but not with uninfected CEF resulted in the activation of ICP22, ICP27, VP16, pp38, and gB. In addition, the level of ICP4 mRNA was increased compared to that in QT35 cells. The activation by HVT resulted in the production of pp38 protein. It was not possible to detect if the other activated genes were translated due to the lack of serotype 1-specific monoclonal antibodies.

Development and use of monoclonal antibodies to chicken fibronectin to show that the chicken hepatocellular carcinoma cell line, LMH, constitutively expresses fibronectin

Fibronectin (Fn) is a high molecular weight glycoprotein and acute phase reactant that contributes to a variety of cellular activities including proliferation and wound healing. Production of Fn is influenced by cytokines such as IL-1alpha, IL-6 and TNF -alpha, and in serum Fn levels can function as an indicator of sepsis and reticulo-endothelial function. Here we describe the production of a panel of mAb to chicken Fn and give evidence that a chicken hepatocellular carcinoma cell line, LMH, constitutively expresses Fn. A capture ELISA to measure chicken Fn was developed using an IgG1 mAb (AV62) as the capture Ab, and biotinylated AV63 (IgG2b) as the detecting Ab. This study identified a single commercially available mAb directed against human Fn that also recognised chicken Fn. By contrast, the anti-chicken Fn mAbs did not cross-react with either human or bovine Fn.

A quantitative duplex PCR technique for measuring amounts of cell-associated Marek's disease virus: differences in two populations of lymphoma cells

A duplex polymerase chain reaction (PCR) was developed to measure Marek's disease virus (MDV) load in two subpopulations of Marek's disease (MD) lymphoma cells from chickens. PCR primers were designed using the sequence of the MDV-ICP4 gene and the chicken IFNgamma gene. Each set of primers was present in the same reaction tube so that the 327 bp ICP4 product and the 420 bp IFNgamma product were co-amplified. Two different fluorescent dyes were used to 5'-end label one PCR primer of each pair to distinguish the IFNgamma and ICP4 products by colour. The IFNgamma PCR product was used as an internal standard enabling comparisons of MDV-ICP4 products between different samples. Neither duplex PCR product was preferentially amplified and both reactions were in their exponential phases when stopped. The products could be distinguished by both size and colour. MD lymphoma cells were taken ex vivo and separated on the basis of expressing a novel host surface antigen recognised by the monoclonal antibody AV37. AV37 + lymphoma cells had greater MDV-loads than AV37 lymphoma cells. The principles used here should be applicable to any cell phenotype and/or cell-associated DNA virus.

Transformation of primary chick embryo fibroblasts by Marek's disease virus

Marek's disease virus (MDV) is an alphaherpesvirus, which can mediate the malignant transformation of lymphocytes to form lymphomas in chickens. In this study, we demonstrate that MDV can transform primary chick embryo fibroblasts (CEF). The cell line derived from primary CEF infected with the GA strain of MDV was called CEM(MDV). The fibroblast nature of CEM(MDV) was verified by absence of cytokeratin type II. The CEM(MDV) phenotype differed from either primary CEF or MDV-infected CEF. CEM(MDV) were extensively vacuolated, with unusual multilamellar structures in the cytoplasm, The nuclei were considerably larger than those in primary CEF and were uniformly positive for proliferating cell nuclear antigen. The cell line was subcultured for more than 10 generations; however, CEM(MDV) did not support a fully productive MDV infection, because complete nucleocapsids were not detected and infectivity assays showed that cell line produced no infectious virus. PCR analyses demonstrated that this cell line carried both polypeptide 38 (pp38) and Meq DNA, MDV-specific genes associated with transformation. In addition, examination by laser scanning confocal microscopy revealed that CEM(MDV) constitutively produced MDV MEQ protein in nuclei and pp38 as well as glycoprotein B in the cytoplasm and on the plasma membrane. Growth in soft agar assay demonstrated that CEM(MDV) formed colonies, similar to HeLa and human melanoma cells. Retroviral insertion was not detected in DNA from the CEM(MDV) line.

Quantification of Marek's disease virus in chicken lymphocytes using the polymerase chain reaction with fluorescence detection

A quantitative assay was developed for Marek's disease virus (MDV). The assay determines the numbers of viral genomes present in samples by polymerase chain reaction (PCR) amplification of a portion of the viral genome for a restricted number of cycles. Fluorescent-tagged primers are used for the PCR amplification which allows quantification of the fluorescent product. Previously, quantitation of Marek's disease virus has required plaque assays, which are laborious and potentially error-prone, and this had limited quantitative comparisons. The PCR assay is rapid, less laborious and can be applied to high levels of accuracy, since replicate assays can be carried out relatively easily. The PCR-based assay assesses the number of viral genomes present in the sample, rather than the numbers of infected cells measured in the plaque assay, however correlation between the two assays is high, suggesting viral copy number per cell may be rather uniform. In crosses between genetically resistant and susceptible animals the PCR-based assay was correlated significantly with subsequent development of disease, and was a better predictor than the plaque assay of the likelihood of development of pathological disease in the birds studied.

Evidence that Marek's disease virus exists in a latent state in a sustainable fibroblast cell line

Previously, we reported the development of two fibroblastic cell lines (MDV OU2.1 and OU2.2) infected with Marek's disease virus (MDV). The two cell lines, in nonconfluent continuous cultures, displayed characteristics consistent with MDV existing in a latent state. However, presence of distinct plaques in confluent cell monolayers and the ability to transfer cytolytic infection to susceptible birds and primary chick embryo fibroblasts, suggest that, if latent, the virus is easily reactivated from MDV OU2 cell lines. In this report, we present evidence which supports the hypothesis that MDV genomes in MDV OU2 cells are latent. PCR analyses and in vivo experiments demonstrate that CHCC-OU2 cells stabilize MDV so that serial in vitro passage does not attenuate the virus. Following two years of active culture, MDV genomes in MDV OU2 cells are still oncogenic, similar to that seen in MDV-lymphoblastoid cell lines. Expansion of the 132-bp repeat within MDV BamHI fragments H and D, typical of highly passaged serotype 1 MDV, has not been observed beyond two copies in MDV OU2 cells. Indirect immunofluorescence assays clearly demonstrate that MDV OU2 cells do not express glycoproteins B and I when subconfluent. However, upon reaching confluence these proteins are expressed in readily detectable amounts. Using RT-PCR we demonstrate that glycoproteins E and D are highly expressed in confluent MDV OU2 cells but absent from subconfluent cells, and MDV latency-associated transcripts (LATs), which are antisense to ICP4 transcripts and have been associated with latent MDV infection, are expressed in subconfluent MDV OU2 cells. Coincident with an increase in ICP4 expression, MDV LAT expression is down regulated when MDV OU2 cells become confluent.

Production of an immune suppressor factor by Marek's disease lymphoblastoid cell lines

Culture fluids from Marek's disease (MD) lymphoblastoid cell lines have suppressive activity against normal and mitogen-stimulated chicken spleen and bursal cells and also against the homologous cell lines. Suppressive activity was also present in supernatants from spleen cells infected in vitro with MD virus. The suppressor factor from MD cell lines was non-sedimentable, trypsin sensitive, heat resistant and partially dialysable. Preliminary studies suggest it has a molecular weight of 20,000 daltons. Studies were also conducted on the effect of the prostaglandin inhibitors indomethacin and aspirin on the production and action of the suppressor factor. At low concentrations they have a stimulatory effect on the cell lines suggesting that they inhibit the effects of suppressor factor; however only small amounts of prostaglandin E2 were present in supernatants. Evidence was obtained that the suppressor factor may act indirectly by stimulating the production of prostaglandin by spleen cell cultures. The role of a suppressor factor in the immunosuppression observed in MD is discussed.

Marek's disease--a model for herpesvirus oncology

The article will review the salient features of pathogenesis of Marek's disease in terms of sequential events which occur from the time of virus entry to the development of frank lymphomas. A hypothesis will be presented which will offer a credible explanation for this specific sequence of changes. Then, various factors which influence the incidence of neoplasms will be identified and the possible mechanisms by which they are influential will be discussed. These include the variable oncogenic properties of different virus strains, the influence of host genotype, and immune responses.

Establishment of lymphoblastoid cell lines from Marek's disease primary tumors

Seven lymphoblastoid cell lines were developed from 15 Marek's disease primary lymphomas treated to remove adherent cells (46.7% success rate) and 1 cell line was developed from 15 untreated lymphomas (6.7% success rate). These cell lines are designated as MDCC-RP22 through MDCC-RP29. Tumor materials were from 151(5) x 7(1) chickens infected with either JM-111S clone or the GA-22 clone of Marek's disease virus. Factors contributing to the establishment of cell lines include, in addition to the removal of adherent cells at the initial stage of culture, the periodic adjustment and the concentration of viable cells, including the removal of dead cells. The cell lines bear T-cell surface markers and Marek's disease tumor associated surface antigen (MATSA). MDCC-RP22 and MDCC-RP23 are high virus producers.