THE WORKING MECHANISM OF AN IMMUNE-COMPLEX VACCINE THAT PROTECTS CHICKENS AGAINST INFECTIOUS BURSAL DISEASE

The role of immune complexes (Icx) in B-cell memory formation and affi nity maturation allow for their potential use as vaccines. Recently, a new immune complex vaccine has been developed that is currently under field trials conducted in commercial poultry. This immune complex vac cine is developed by mixing live intermediate plus infectious bursal d isease virus (IBDV) with hyperimmune IBDV chicken serum (IBDV-Icx vacc ine). Here we have investigated the infectivity of this vaccine as wel l as the native IBDV (uncomplexed) vaccine in terms of differences in target organs, in target cells and speed of virus replication. At vari ous days after inoculation on day 18 of incubation (in ovo) with eithe r one dose of virus alone or the IBDV-Icx vaccine, the replication of IBDV and the frequency of B cells and other leucocyte populations were examined in the bursa of Fabricius, spleen, and thymus using immunocy tochemistry. With both vaccines, IBDV was detected associated with B c ells, macrophages and follicular dendritic cells (FDC) in bursa and sp leen, although complexing IBDV with specific antibodies caused a delay in virus detection of about 5 days. Most remarkable was the low level of depletion of bursal and splenic B cells in IBDV-Icx vaccinated chi ckens. Furthermore, in ovo inoculation with the IBDV-Icx vaccine induc ed more germinal centres in the spleen and larger amounts of IBDV were localized on both splenic and bursal FDC, From these results we hypot hesize that the working mechanism of the IBDV-Icx vaccine is related t o its specific cellular interaction with FDC in spleen and bursa.