Macrophages are effective weapons used by our immune system to absorb and digest pathogenic intruders. Some bacteria, however, can subvert this defence mechanism and even multiply within the macrophages. Cell biologists at the University of Bonn have revealed such a strategy in a recently publication in the journal 'Traffic' (Vol. 6, No. 8, August 2005, pp.635-653). Their findings reveal that the pathogens escape the 'stomach' of the macrophages which might otherwise digest them.
Action stations in the horse's lung! A bacterium has just been inhaled into a horse's bronchial tubes, and immune cells are quickly recruited to the spot to neutralise the intruder. Macrophages, cells whose job is to devour such intruders, are attracted by substances typical of bacteria, which surround the microbe like a cloud. As soon as the immune cells have detected the intruder, they cover the bacterium with part of their own cell membrane like a hood, creating a membrane sac in which the intruder is trapped. This 'phagosome' (from Greek phagein = to eat) cuts itself off into the inside of the macrophage and is now the point on which all the macrophage's offensive weaponry is concentrated: the phagosome is flooded with oxygen radicals and acid. Another kind of membrane bags, the lysosomes, merge with the phagosome and confront the microbe with highly reactive digestive enzymes. A few hours after the first alarm bells have rung there is nothing left of the bacterium, and the potential danger has been eliminated.
Multiplication inside the killer
This is what normally happens. However, a whole range of pathogens have become specialised in tricking this very part of the defence mechanism and survive or even multiply in these macrophages which are actually supposed to kill them.