IMMUNE DYSFUNCTION FOLLOWING INFECTION WITH CHICKEN ANEMIA AGENT AND INFECTIOUS BURSAL DISEASE VIRUS .1. KINETIC ALTERATIONS OF AVIAN LYMPHOCYTE SUBPOPULATIONS

The potential effect of chicken anemia agent (CAA) alone or in combination with infectious bursal disease virus (IBDV) on the immune system of young chickens was determined by measuring alterations in hematocrit values, lymphoid organ-to-body weight ratios and lymphoid cell concentrations at 4, 7, 10, 14, 17, 21, 28 and 42 days post-inoculation (PI). Lymphocyte subpopulations were identified and counted by flow cytometry using cell suspensions stained with monoclonal antibodies (Mabs) for panlymphocytes (K55), cytotoxic T-cells (CTLA3), T-helper cells (CT3), Ia-expressing cells (P2M11) and macrophages (P7). Chicken anemia agent induced a substantial but transient decrease in hematocrit value, thymus-to-body weight ratio and bursa-to-body weight ratio between 7 and 21 days PI corresponding to a generalized lymphocytopenia in the thymus, bursa and spleen. However, cytotoxic T-cell, T-helper cell and Ia-expressing cell concentrations increased in the bone marrow of birds inoculated with CAA alone or in combination with IBDV during the same time period. T-helper-to-cytoxic T-cell ratios increased in the thymus and spleen during severe lymphocytopenia, indicating a selective decrease in cytotoxic T-cells. T-helper-to-cytotoxic T-cells ratios increased in the bone marrow, indicating a selective increase in T-helper cell concentrations. The increase in Ia-expressing cells in the bone marrow may be a reflection of increased number of activated T-cells which express Ia antigen. Infectious bursal disease virus alone induced a persistent depression of Ia-expressing cells in the bursa and the spleen and no measurable change in the bone marrow lymphocyte subpopulations. Chickens inoculated simultaneously with CAA and IBDV experienced clinical signs observed in chickens inoculated with each virus separately with a prolonged acute phase prior to recovery or mortality.