Immunosuppressive potential of fowl adenovirus serotype 4

Fowl adenovirus serotype 4 (FAdV-4) is the causative agent of hydropericardium syndrome. To clarify the effects of FAdV-4 on immune organs in birds, we conducted a detailed examination of dynamic morphology and damage mechanisms in chickens randomly divided into 4 groups (FAdV-4, vaccination, FAdV-4 plus vaccination, and control). FAdV-4 caused the depletion of lymphocytes and subsequent growth impairment in the thymus and bursa. Chickens infected with FAdV-4 and subjected to vaccination experienced greater inhibition of antibody responses to inactivated vaccines against Newcastle disease and avian influenza virus subtype H9 than uninfected and vaccinated chickens. The mechanisms underlying adenovirus-mediated lymphoid organ damage were further investigated via transferase-mediated dUTP nick-end labeling and apoptotic genes transcription analyses. Notably, lymphocytes apoptosis in lymphoid organs and expression of specific gene transcripts was significantly upregulated after infection (P < 0.05). Furthermore, increased expression of interleukin (IL)-6, IL-8, and tumor necrosis factor (TNF)-α mRNA was observed (P < 0.05), compared to the control group. Our collective findings suggested that FAdV-4 caused structural and functional damage of immune organs via apoptosis along with induction of a severe inflammatory response.