| Marek's disease virus (MDV) is an acute transforming alphaherpesvirus that causes a T-cell lymphoma, paralysis, immunosuppression and mortality in chickens. MDV establishes lytic infection in B-lymphocytes and epithelial cells, and establishes latency in and transforms activated CD+4 T-cells. A T-cell model for the study of MDV latency is not currently available. As understanding MDV latency is important to identifying key steps in lymphomagenesis. We have redefined a latency model based on a reticuloendotheliosis virus-transformed cell line (RECC-CU91) superinfected with various MDVs. MDV-harboring CU91 cells had a higher growth rate and had increased resistance to serum starvation-induced apoptosis. MDV-transformed and -harboring cell lines were comparable in: MDV reactivation, expression of latency-associated meq transcript, and in the induction of MDV lytic antigens pp38 and g13. In these studies, we identified two novel spliced meq sense and antisense transcripts. Meq is a basic-zipper protein homologous to the Jun/Fos family of transcription activators that has transactivation, transrepression, anti-apoptotic and transforming properties. Meq sense splice variants, Meq/vIL-8 and Meq/vIL-8Deltaexon3, consisted of the Meq N-terminus (100 a.a.) spliced to either exon 2 and 3, or exon 2 only of vIL-8. These splice variants had potent transcriptional repressor activity. Meq/vIL-8 induced proliferation of a chicken fibroblast cell line. The meq promoter was expressed with immediate-early/latent kinetics, and was down regulated by sodium butyrate treatment within 6 hours. Sodium butyrate treatment induces virus reactivation from latency, suggesting that a function of meq gene products may be to maintain latency. The Meq antisense promoter had late kinetics and was resistant to sodium butyrate treatment. Concomitant with our studies of meq gene products, we identified meq mutations that segregated according to MDV virulence level. Lower virulence MDVs tended to contain reiterated proline-rich repeats in Meq, whereas higher virulence MDV strains had disruptions of this proline-rich domain. We found that the transcriptional activation mediated by the different Meq isoforms correlated positively with virulence level. Increased transcriptional activity, however, did not correlate with increased cellular proliferation. Our results do suggest, that mutations in meq may play a role in the evolution of MDV virulence. |
