Ability of T cell subsets and their soluble mediators to modulate the replication of Mycobacterium bovis in bovine macrophages

Peripheral blood mononuclear cells (PBMCs) from cattle vaccinated with Bacillus Calmette–Guerin (BCG) were obtained and expanded in vitro by incubation with purified protein derivative. The ability of these cells to modulate the replication of virulent Mycobacterium bovis in autologous-infected macrophages was compared to cells from non-vaccinated controls. Cells from non-vaccinated animals were shown to confer a significant degree of mycobacteriostatic activity to autologous-infected macrophages. This activity was not inhibited by including a neutralizing antibody versus interferon-γ (IFN-γ), and was dependent on direct contact between PBMCs and infected macrophages. Addition of autologous PBMCs from BCG-vaccinated cattle was shown to significantly enhance macrophage resistance to M. bovis, and this increased macrophage resistance was partly abrogated by including a neutralizing antibody to IFN-γ. Addition of T cells from non-vaccinated animals to infected macrophages was associated with a modest increase in macrophage release of TNF-α and nitric oxide, whereas PBMCs from vaccinated animals increased very significantly the release of these factors. Neutralization of nitric oxide (NO), by inclusion of monomethyl-l-arginine, significantly diminished the ability of PBMCs from vaccinated animals to enhance macrophage resistance to M. bovis, but had no impact on the ability of T cells from naïve animals to modulate macrophage function. The ability of naïve cells to increase macrophage anti-M. bovis activity was largely mediated by CD4+ T cells, whereas both CD4+ T cells and CD8+ T cells conferred macrophage resistance to M. bovis in vaccinated animals. These data highlight the role of IFN-γ and NO in the immune resistance of cattle to M. bovis.