| Marek's disease virus (MDV) is composed of three serotypes. Marek's disease virus serotype 3 is also known as turkey herpesvirus (HVT) because it was originally isolated from turkeys. All 3 MDV serotypes induce productive and latent infections. Productive infections result in extensive viral antigen expression while latent infections have limited or no viral antigen expression in the presence of the viral genome. During a productive infection, structural viral proteins such as glycoprotein B are produced. Based on indirect immunofluorescence assays (IFA) and co-cultivation assays, expression of HVT gB was detected in lymphoid tissue, the spleen, thymus and bursa. Criteria for a latent infection was the presence of HVT genome in the absence of HVT gB expression. In situ hybridization detected HVT RNA while IFA was used to demonstrate gB was not expressed. Latent infections were found in the spleen, thymus, bursa, sciatic plexus, brachial plexus and feather tips. In lymphoid tissue, spleen, thymus and bursa, latent HVT RNA was present in CD4+ and CD8+ lymphocytes. In vivo studies indicate that a latent HVT infection occurs in the bursa but the infection is subsequently cleared from the bursa. In the brachial plexus and sciatic plexus, satellite cells have latent HVT, however, the cell type harboring latent HVT in the feather tips could not be identified. To investigate the reduction in transcription during latency, probes representing 80% of the HVT genome were used to identify the region of the genome that is transcriptionally active during latency. Only the repeat regions flanking the Unique long region, the junction between the inverted repeats flanking the unique long and unique short regions and a small portion of the unique region of the latent HVT genome was transcribed. Limited transcription during latency may be involved in controlling either the establishment, maintenance or reactivation from the latent state. |
