Lymphocyte subpopulations in peripheral blood of lambs experimentally infected with Pasteurella haemolytica

The lymphocyte subpopulations in peripheral blood obtained from eleven lambs experimentally infected with Pasteurella haemolytica were compared with those obtained from eight control lambs by flow cytometry, using a panel of monoclonal antibodies against specific lymphocyte epitopes. Experimental infection with P. haemolytica was characterized by a transient but significant reduction in SBU-T1+ (CD5+) T cells and SBU-T4+ (CD4+ or helper) T lymphocytes (P less than 0.05) and a significant rise in lymphocytes which did not express the LCA p220 epitope and the pan T cell surface marker (CD5-LCA p220-) ("null"). The reductions in CD5+ and CD4+ lymphocytes occurred 24 h after experimental infection, returning to preinoculation levels 5 days post inoculation (DPI). Five to 9 days after experimental infection, there was a significant increase in the number of lymphocytes, which expresses the pan T cell surface marker (CD5+) but which were CD4-CD8-. Lymphocyte transformation responses to the mitogen phytohaemagglutinin (PHA) were significantly reduced 24 h after experimental infection with P. haemolytica (P less than 0.05).