Bovine natural killer cells restrict the replication of Mycobacterium bovis in bovine macrophages and enhance 1L-12 release by infected macrophages

In this contribution, the impact of bovine natural killer (NK) cells on resistance to bovine tuberculosis as, studied, using a monoclonal antibody against bovine NKp46. NK cells cultured with A bovis-infected macrophages, but not control uninfected macrophages, proliferated and released IFN-gamma. Blood monocyte-derived macrophages were infected with virulent A bovis, and growth of intramacrophage bacteria was monitored by incorporation of tritiated uracil. Coculturing infected macrophages with autologous NK cells significantly reduced the intracellular bacterial growth. Stimulation of NK cells with interleukin-2 (IL-2) enhanced further the capacity of these cells to reduce M. bovis replication in infected macrophages. NK cells from both BCG vaccinated and unvaccinated animals mediated this intra-macrophage growth restriction at similar levels. The ability of NK cells to reduce bacterial growth was independent of the release of IFN-gamma, as blocking IFN-gamma with an antibody in vitro did not affect intra-macrophage bacterial growth. NK cells reduced bacterial. growth and also increased macrophage release of interleukin-12 (IL-12) and nitric oxide (NO) production by A bovis-infected macrophages. Neutralizing NO production by macrophages in vitro with monomethyl-L-arginine (MMLA) did not abrogate the ability of NK cells to decrease bacterial. growth in infected macrophages. Reduction of mycobacterial intramacrophage growth by NK cells was dependent on direct contact between NK cells and infected macrophages. Supernatants from NK cells failed to impact significantly on A bovis replication in infected macrophages. The reduction in bacterial growth in macrophages correlated with the induction of an apoptosis program in infected macrophages. Cell death occurred at a similar rate in infected macrophages, exposed to NK cells or not. We conclude that bovine NK cells are stimulated by and release IFN-gamma in response to infected cells and reduce M. bovis growth in infected macrophages by an unclear mechanism, and are potentially involved in innate resistance of cattle to tuberculosis. (c) 2006 Elsevier Ltd. All rights reserved.