Infectious Bursal Disease Virus-Induced Immunosuppression in the Chick Is Associated with the Presence of Undifferentiated Follicles in the Recovering Bursa

Infectious bursal disease virus (IBDV) causes an acute cytolytic infection in chicken B lymphocytes resulting in destruction of the B-cell population. Most severe depletion occurs in the bursa of Fabricius, where the immunoglobulin repertoire is developed by gene conversion. Chicks surviving IBDV infection are immunosuppressed despite repopulation of the bursa with B cells. Here we show that infection of neonatal chicks with a classical virulent IBDV strain (F52/70) causes severe bursal Bcell depletion with recovery after about one week. Two distinct types of bursal follicles developed: large reconstituted follicles and small poorly developed follicles lacking a discernible cortex and medulla. The presence of large numbers of undifferentiated follicles was associated with inability to mount antibody responses to IBDV itself and after immunization with Salmonella Enteritidis bacterin, indicating that B cells in these follicles are unable to produce peripheral B-cells with an effective immunoglobulin repertoire. Additionally a number of inflammatory foci were observed in the recovering bursa. These foci contained few B cells at the margins, but large numbers of CD4⁺ and CD8⁺ cells, scattered γδ⁺ T-cells and macrophages, and small central aggregates of dendriticlike cells expressing the CD40 antigen.