Transient depletion of cortical thymocytes induced by chicken anaemia agent

Chicken anaemia agent (CAA) causes severe anaemia, loss of body weight, and hypoplasia of thymus at day 14 after inoculation of one-day-old chickens. Several reports have described an enhancement of concurrent infections with f.e. Marek's disease virus, Infectious Bursal Disease virus, and Reovirus. Immunohistochemical methods were used to describe the immunopathological lesions of the thymus that probably form the basis of the immunodeficiency caused by CAA. Monoclonal antibodies and antisera against leucocytes, T lymphocytes, CD4, B lymphocytes, mononuclear phagocytes, MHC class II, and keratin were used. At day 14 after inoculation, the thymic cortex was completely depleted of thymocytes, whereas the medulla was not. T-cell areas in the spleen also lacked T lymphocytes. In contrast the cortex still contained stromal cells with MHC class II molecules and keratin. At day 21, the cortex had completely regenerated and all clinical signs of CAA infection had disappeared. Labelling experiments with BrdU in 4-week-old control chickens demonstrated that 25% of the divided cells was detected in the medulla and 75% in the cortex. The tissue tropism of CAA may, apart from the preference for rapidly dividing cells, be directed by specific cell determinants.