Pathogenesis of type II avian adenovirus infection in turkeys: in vivo immune cell tropism and tissue distribution of the virus.

Hemorrhagic enteritis virus (HEV), a type II avian adenovirus, causes intestinal hemorrhages and immunosuppression in turkeys. In this study, we exposed turkeys to virulent HEV and examined fractionated spleen cells for the presence of viral DNA by in situ hybridization and amplification of DNA extracted from virus-infected cells by PCR. HEV replication was detected only in the immunoglobulin M-bearing B lymphocytes and macrophage-like cells but not in the CD4+ or CD8+ T lymphocytes. The inability to infect T cells distinguishes type II avian adenoviruses from lymphotropic mammalian adenoviruses which infect and replicate in T cells. Furthermore, these data suggested that HEV-induced immunosuppression in turkeys may be due to the effect of the virus on B lymphocytes and macrophages. We also examined tissue tropism of HEV by in situ hybridization conducted on sections of lymphoid and nonlymphoid tissues. Large numbers of HEV-positive cells were detected in spleen and cecal tonsils. Diminutive viral activity was present in the intestines, the principal site of HEV-induced pathology. Thus, intestinal pathology was not associated with local cytopathic viral replication. This result and our previous observation that cyclosporin A abrogated intestinal hemorrhaging in HEV-infected turkeys strongly suggested that intestinal lesion induced by this virus may be immune system mediated.