In vitro control of Mycobacterium bovis by macrophages

Mycobacterium bovis is frequently seen inside macrophages in vivo. The outc ome of M. bovis infection depends on T cell interactions with macrophages, however mycobacteria are thought to be relatively resistant to macrophage k illing. Little is known about the immunological mechanisms which control in tracellular growth of M. bovis, and in the absence of T cell help the organ ism is capable of intracellular survival and replication. We have investiga ted the role of macrophages in controlling growth of virulent M. bovis or M . bovis BCG in vitro. At a multiplicity of infection of 5:1, macrophages fr om a range of animal species including cattle, deer, possums, ferrets and m ice restricted growth of BCG while M. bovis grew progressively. Inter-speci es variation in controlling growth of M. bovis by alveolar macrophages was observed. Pre-treatment of macrophages with interferon-gamma and lipopolysa ccharide inhibited intracellular growth of M. bovis. Addition of freshly re cruited macrophages further inhibited M. bovis, and intracellular growth wa s arrested by activated fresh macrophages. Our observations suggest that na ive macrophages can prevent BCG growth, while T cell activation in conjunct ion with freshly recruited macrophages is required for preventing growth of M. bovis. (C) 2001 Harcourt Publishers Ltd.