Abstract
T-cell activation is dependent on nominal antigen associated with major histocompatibility complex (MHC) class II molecules and interleukin-1 (IL-1), both provided by antigen-presenting cells. We have studied the effects of Brucella abortus and recombinant bovine gamma interferon (IFN-gamma) on bovine macrophage expression of MHC class II and IL-1 molecules and subsequent T-cell proliferation in response to B. abortus. When peripheral blood mononuclear cells were cocultured with B. abortus and IFN-gamma, increasing amounts of IFN-gamma, from 1 to 100 U/ml, down regulated T-cell proliferation. Expression of MHC class II molecules on macrophages was increased independently by IFN-gamma or B. abortus treatment. Thus, class II molecule expression was not the cause of down regulation of T-cell proliferation as observed in other systems. However, B. abortus-IFN-gamma-treated macrophages obtained from overnight cultures had minimal membrane IL-1 compared with macrophages treated with B. abortus alone. Loss of membrane IL-1 required IFN-gamma and the o-polysaccharide of the lipopolysaccharide. IFN-gamma at 1 U/ml in combination with B. abortus produced a 32% decrease in T-cell response, while IFN-gamma at 100 U/ml added to B. abortus-treated cultures produced an 82% reduction in T-cell response. Membrane IL-1 levels were not altered when recombinant bovine IFN-alpha or the rough strain 45/20 of B. abortus, which lacks the o-polysaccharide, was used. Secreted IL-1 levels were unaffected by IFN-gamma and B. abortus treatment. The addition of recombinant bovine IL-1 beta (0.001 to 0.1 ng/ml) to B. abortus- and IFN-gamma-treated cultures failed to provide a signal necessary for T-cell proliferation. These data suggest that membrane IL-1 has a key role in T-cell activation in response to B. abortus. When the o-polysaccharide of B. abortus lipopolysaccharide is combined with IFN-gamma at an inappropriate time during an immune response, T-cell proliferation is prevented and cannot be restored by the addition of exogenous IL-1.