Dynamics of immune responses following duck Tembusu virus infection in adult laying ducks reveal the effect of age-related immune variation on disease severity.

Duck Tembusu virus (DTMUV), an emerging avian pathogenic flavivirus, is notably associated with neurological disorders and acute egg drop syndrome in ducks. We previously demonstrated that the susceptibility of ducks to DTMUV infection varies significantly with age, with younger ducks (4-week-old) exhibiting more severe disease than older ducks (27-week-old). However, the immunological mechanisms underlying these age-related differences in disease severity remain unclear. In this study, we investigated the dynamics of immune responses following DTMUV infection in adult laying ducks (27-week-old) and compared them to our previous findings on young ducks (4 weeks old). The numbers of T helper, cytotoxic T, B, and non-T and B lymphocytes, as well as neutralizing antibody levels, were measured in parallel with DTMUV loads in the blood and target organs. Our results revealed that the number of non-T and B lymphocytes/myeloid cells in 27-week-old adult laying ducks infected with DTMUV remained consistently stable throughout the observation period, in contrast to findings in 4-week-old younger ducks, where myeloid cell responses were implicated in disease progression. Regarding lymphocyte responses, unlike in 4-week-old younger ducks, only cytotoxic T lymphocyte responses in 27-week-old older ducks showed a significant negative correlation with the reduction of viremia and viral loads in target organs, indicating their role in controlling viral replication in older ducks. Additionally, 27-week-old adult laying ducks infected with DTMUV exhibited high levels of neutralizing antibodies, which were significantly correlated with reduced viral loads in blood and target organs. Overall, the presence of robust DTMUV-specific neutralizing antibody and CTL responses, along with a finely tuned myeloid cell response likely plays a significant role in controlling severe neurological outcomes in 27-week-old adult laying ducks. This study highlights the age-related differences in immune responses following DTMUV infection, which potentially contribute to the varying disease severity among ducks of different ages. Understanding the interplay between the host and DTMUV provides significant implications for disease management strategies and vaccine development.