Tissue-specific restriction of latent turkey HVT transcription.

Using in situ hybridization, latent turkey herpesvirus (HVT) transcription was examined in lymphoid and/or nonlymphoid tissues. Blood samples were taken for virus isolation from chickens at 7 and 240 days post infection (PI) representing time points for productive and latent turkey herpesvirus infections, respectively. Spleen, thymus, sciatic and brachial nerves from infected chickens were analyzed for latent HVT transcription and HVT glycoprotein B (gB) expression at 240 days PI. Using indirect immunofluorescence, HVT gB expression was not detected in any tissues examined at 240 days PI. HVT genomic fragments from a HVT BamHI library were used as probes in in situ hybridization assays. In the spleen, thymus, sciatic and brachial nerves, latent HVT transcription occurred from the repeat regions flanking the unique long region (TRL and IRL). However, fine-mapping of this region revealed a difference in latent HVT transcriptional pattern. A SmaI map of the HVT BamHI-F fragment was made to further fine-map latent HVT transcription. A 1.6 kbp SmaI subfragment hybridized to cells infected with latent HVT in the spleen and thymus. However, the 1.6 kbp SmaI subfragment did not hybridize to cells of the brachial or sciatic nerves. In addition, a 2.0 kbp SmaI subfragment hybridized to cells in the thymus but not in the spleen, sciatic or brachial nerves. The above results suggest that latent turkey herpesvirus exhibits tissue-specific transcription.