Establishment and characterization of a T-lymphoblastoid cell line MDCC-MTB1 derived from chick lymphocytes infected in vitro with Marek's disease virus serotype 1

Identification and characterization of a novel spliced form of the meq transcript in lymphoblastoid cell lines derived from Marek's disease tumours

In tumour cell lines established from Marek's disease (MD) lymphomas L-meq is consistently expressed. It contains a 180 bp insertion encoding additional copies of the proline-rich repeat in the meq open reading frame and its product may contribute to the maintenance of MD virus (MDV) latency. In this study, we identified a novel spliced form of the meq transcript in MD-derived lymphoblastoid cell lines and in MDV-infected cells. This transcript, termed Deltameq, encodes an N-terminal 98 aa of the Meq protein and lacks part of the basic leucine zipper (bZIP) and transactivation domains. In MD cell lines, transcription of L-meq was significantly downregulated, while that of the Deltameq transcript was upregulated during apoptosis. These observations were also confirmed at the protein expression level. Reporter assays using meq- and interleukin-2 (IL-2)-promoter-driven luciferase vectors revealed that DeltaMeq suppressed transactivation by L-Meq or Meq in a dose-dependent manner. Immunoprecipitation confirmed that DeltaMeq was associated with L-Meq or Meq physically. These results suggest that DeltaMeq could be involved in apoptosis in MD cell lines as it works as a negative regulator of L-Meq and Meq by direct interaction.

The presence of the p53 transcripts with truncated open reading frames in Marek's disease tumor-derived cell lines

Several kinds of the p53 transcripts in which their open reading frames (ORFs) were truncated (ranging from 101 to 765 bp) were identified in Marek's disease (MD)-derived tumor cell lines as well as avian leukosis- and reticuloendotheliosis-derived ones, detected by nested RT-PCR and subsequent nucleotide sequence analysis. In these ORFs, regions encoding the proline-rich and DNA-binding domains of the p53 protein were frequently deleted, and many of these deletions were found to cause frame shift. Western blot analysis using anti-p53 monoclonal antibodies revealed that multiple p53 isoform proteins with various molecular weights including 45-46, 35 and 28 kDa were expressed in these tumor cell lines, though the p53 protein with a molecular weight of 49 kDa was detected in chicken embryo fibroblasts transformed by the SV40 T antigen as a control. Since no deletions were found in the p53 gene of these MD tumor cell lines, truncations in the p53 ORFs observed in this study might result from alternative splicing of the p53 gene.

Diversity (polymorphism) of the meq gene in the attenuated Marek's disease virus (MDV) serotype 1 and MDV-transformed cell lines

The meq gene encoding a 339-amino-acid bZIP transactivator protein has been identified as a candidate oncogene of Marek's disease virus serotype 1 (MDV1), which induces malignant lymphomas in chickens. We have previously reported that, in addition to meq, L-meq, in which a 180-bp sequence is inserted into the region encoding the transactivation domain of meq, is also detected in chickens experimentally infected with MDV. To further analyze the diversity in meq, PCR was performed using a primer set which specifically amplify the proline-rich repeat (PRR) region in the transactivation domain of meq. In CVI988/R6, a vaccine strain of MDV1, and JM, an MDV1 strain attenuated by prolonged passage in vitro, a major band of a 0.8 kb corresponding to L-meq as well as a minor band of 0.6 kb corresponding to meq was detected by PCR. Furthermore, extra 0.5- and 0.3-kb bands, corresponding to genes termed as short meq (S-meq), and very short meq (VS-meq), respectively, were also detected. These genes were also detected in MDV-transformed cell lines, MSB1 and MTB1. In Md5, an oncogenic MDV1, attenuated by prolonged passage in vitro, the 0.6-kb meq was consistently detected, and 0.5-kb S-meq was occasionally detected. This diversity in meq was due to the difference in the copy number of the PRR region: L-meq and meq contained 9 and 6 copies of PRR while 4 and 2 copies of PRR were present in S-meq and VS-meq, respectively. Thus, the meq gene is polymorphic in the attenuated MDV1 and the MDV-transformed cell lines, and gene products from different meq genes may have different functions from each other.

Analysis of tumor suppressor gene p53 in chicken lymphoblastoid tumor cell lines and field tumors

To determine whether there is any abnormalities of the p53 gene in chicken lymphoblastoid tumor cell lines derived from Marek's disease (MD), lymphoid leukosis, reticuloendotheliosis, and field tumors, some portions of p53 cDNA corresponding to core and C-terminal domains (nucleotide positions 277-1104 in the p53 open reading frame (ORF)) were sequenced. Several mutations were identified in both cell lines and field tumors. However, none of these mutations is localized at the "hot spot", which has been reported as the site for transformation-activating mutations. Moreover, partial cDNA clones with a 122-bp deletion in the p53 ORF were identified in two cell lines, MSB1 and MTB1 derived from MD tumors. Southern blot analysis showed that no deletion occurred in the genome of p53 in MSB1, indicating that deletion occurred at the transcriptional level. This deletion could cause a frame shift of the encoding p53 protein, possibly resulting in the generation of a functionally different p53 protein. However, we confirmed that p53 mRNA without deletion is also present in each of these cell lines. These mutations of the p53 gene and deletion in the p53 transcript may be ones of molecular changes specific to the transformation induced by MD virus.

Transcriptional analysis of Marek's disease virus (MDV) genes in MDV-transformed lymphoblastoid cell lines without MDV-activated cells

Spontaneously activated MDV is rarely included in MDV-transformed cells, while it may influence the results of transcriptional analysis. A population consisting of 10(3) MDV-transformed cells probably did not include spontaneously activated MDV, since the estimated frequency of MDV-transformed cells including activated MDV was below 0.01% according to limiting-dilution polymerase chain reaction (PCR) and the presence of the major early antigen pp38 in 6 transformed cell lines. Reverse transcriptase-PCR (RT-PCR) products corresponding to ICP27, pol, TK, US3, A41, gA, gB and UL50 genes were undetectable in 10(3) cells by Southern hybridization of the RT-PCR products. Transcripts of the VP16 and SORF2 genes were detected in the 10(3) cells of MSB-1, and the pp14 gene transcript was found in 10(3) cells of RPL-1 but not in 10(3) cells of HPRS-1, MOGA-2, MSB-1 or MTB-1. A transcript corresponding to the ICP4 sequence was detected as a 0.7 kbp RT-PCR product in 10(3) cells of these MDV cell lines but not in the retrovirus-transformed 1104B1 cell line. The transcript corresponding to the 0.7 kbp RT-PCR product suggested a splice by its size and sequence. Thus, transcriptional analysis of 10(3) MDV-transformed cells revealed that the transcript corresponding to the ICP4 sequence was a common transcript in latently infected MDV-transformed cells, while most of the genes did not transcribe in these cells.

Analysis of Marek's disease tumor-associated surface antigen on MDCC-MSB1-clo.18 cells

The physico-chemical nature of Marek's disease tumor-associated surface antigen (MATSA) on Marek's disease (MD) lymphoblastoid tumor cell line (MDCC-MSB1-clo.18) was examined by the cellular enzyme-linked immunosorbent assay (CELISA) and the sandwich enzyme-linked immunosorbent assay (ELISA) using an anti-MATSA immune serum or a monoclonal antibody (MAb) 2B9 developed against MATSA. Our results indicate that MATSA is a glycoprotein and 2B9 recognizes an antigenic site in the protein moiety of MATSA. MATSA was solubilized from MSB1-clo.18 cells by treatment with 0.5% Nonidet P-40, and purified by affinity chromatography coupling with 2B9 and further by ion exchange chromatography on diethylaminoethylcellulose (IECD). MATSA was eluted with 0.2 to 0.3 M KCl in IECD and the purity of MATSA was increased about 2,500-fold. The purified MATSA was shown to have a molecular weight (Mr) of 70,000 by SDS-PAGE. The reactivity of purified MATSA with anti-thymus cell serum was examined. MATSA was detectable by anti-thymus cell serum, although 2B9, which was used to purify MATSA from MSB1-clo.18 cells, was not reactive to cells prepared from the thymus. However, MATSA was no longer detectable after the absorption of anti-thymus cell serum by chicken bursa cells. The absorption of anti-thymus cell serum by chicken red blood cells (RBC) had no effect on the reactivity against MATSA. These results suggest that MATSA may be a lymphocyte-specific antigen modified during leukemogenesis by Marek's disease virus (MDV).

Partial transcription map of Marek's disease herpesvirus in lytically infected cells and lymphoblastoid cell lines

Marek's disease herpesvirus (MDV) can cause either a productive-restrictive or lytic infection, a latent infection or can transform thymus-derived lymphocytes. RNA was extracted from infected chicken embryo fibroblasts (CEF) or from lymphoblastoid tumour cell lines. Some of the infected CEF were treated with 200 micrograms/ml cycloheximide to identify immediate early (IE) transcripts, and others with 1 microM 1-(2-fluoro-2-deoxy-B-D-arabinofuranosyl)-5-methyluracil (FMAU), an inhibitor of herpesvirus DNA synthesis to identify early transcripts. An extensive Northern blot analysis was carried out using DNA probes spanning almost the complete MDV genome. In the lytically infected CEF at least 66 discrete transcripts were detected, ranging in size from 9.1 kb to 0.6 kb. Eleven IE transcripts were identified, of which 8 were mapped in the genome segment consisting of the IRL, IRS, US and TRS. Six transcripts were identified as early genes. In the MD lymphoblastoid cell lines MDCC-HPI, a non-producer cell line, and MDCC-CU41, a non-expression cell line, 4 and 7 transcripts were detected, respectively. These RNAs were transcribed from IE genes located mainly in the repeat sequences flanking UL and US and in US. Treatment of the lymphoblastoid cell lines with 20 micrograms/ml 5-iodo-2-deoxyuridine resulted in the additional transcription of 1 RNA species in HPI and 9 in CU41. Most of the transcripts present in lytically infected cells were also detected in MDCC-CU36, a cell line with a high percentage of antigen-positive cells (expression cell line).

RNA transcripts of Marek's disease virus (MDV) serotype-1 in infected and transformed cells

RNA was isolated from two strains of Marek's disease virus (MDV-Z and MDV-B). The virus was grown in duck embryo fibroblasts (DEF) for 96 hr, 72 hr in the presence of phosphonoacetic acid (PAA) and 24 hr in the presence of cycloheximide added at the time of infection. With the use of DNA probes representing about 80% of the MDV genome, an extensive Northern blot analysis of the RNA was carried out. A similar analysis was done with RNA extracted from the MDV-transformed cell line MSB-1. This study revealed 42, 25 and 29 discrete viral RNA transcripts in MDV-Z and MDV-B-infected DEF and in the MSB-1 cell line, respectively, ranging in size from 0.8 to 13 kb. In MDV-Z-infected DEF, there were twelve late RNA species, two early and eight immediate-early viral transcripts. In MDV-B-infected DEF there were eleven late RNA species, two early and seven immediate-early viral transcripts. The RNA species were homologous for all the probes used except the BamHI-G DNA fragment where no RNA transcripts were detected in the MSB-1 cell line. The RNA transcripts were used to produce a preliminary viral RNA map. Comparison of the location and sizes of the viral RNA transcripts in MDV-infected and MDV-transformed cells revealed several differences.