Lymphocyte subpopulations in peripheral blood of sheep experimentally infected with tick-borne fever

The lymphocyte subpopulations in the peripheral blood of normal sheep and sheep experimentally infected with Cytoecetes phagocytophila, the causative agent of tick-borne fever, were analysed by flow cytometry, using a panel of monoclonal antibodies against specific lymphocyte epitopes. Experimental infection with tick-borne fever was characterised by a significant reduction in the total number of circulating lymphocytes six days after experimental infection (P less than 0.001). This lymphocytopenia was associated with a significant reduction in the number of B (LCAp220+) and T (CD5+) lymphocytes (P less than 0.001) but there was a significant increase in the number of cells which were neither T nor B (CD5-LCAp220-) cells (P less than 0.01). The reduction in the number of T lymphocytes was due to reduced numbers of circulating CD4+ (helper) T cells, CD8+ (cytotoxic/suppressor) T cells and those with the pan T cell marker (CD5+) but without CD4 or CD8 epitopes (CD4-CD8-). All lymphocytes returned to preinoculation levels 13 to 16 days after experimental infection.