Switched on to danger

How the immune system tackles the dilemma of discriminating between harmless and virulent bacteria in mice is revealed in a study published online this week in Nature Immunology.

Stanimir Ivanov and Craig Roy looked at the mouse immune response to either pathogenic or non-pathogenic mutant Legionella bacteria. They found that cells infected with non-pathogenic bacteria default to an anti-inflammatory response, while pathogenic bacteria disrupt a key cellular molecule called mTOR. This disruption of mTOR results in a switch whereby certain inflammatory genes are selectively translated into protein and therefore a protective immune response is initiated. This simple switching mechanism allows the immune system to on the one hand effectively ‘ignore’ non-threatening bacteria and avoid needless inflammation, but on the other ramp up defenses when required.