DAP12-deficient mice (dotted line) are resistant to a lethal endotoxin challenge.

DAP12 is a transmembrane adaptor protein that is associated with an array of activating receptors on the surface of immune, brain, and bone cells. DAP12 is required for the development of bone-resorbing osteoclasts, but its role in immune cells is less clear. Some studies have suggested that DAP12-dependent signals amplify cellular activation and inflammatory cytokine production in response to invading microbes, but others argue that DAP12 signaling inhibits these functions.

Turnbull et al. now weigh in on this debate by showing that mice lacking DAP12 can fend off a systemic bacterial infection without producing shock-inducing amounts of inflammatory cytokines such as tumor necrosis factor, suggesting that DAP12 signaling normally amplifies inflammation. These data are consistent with their previous studies in which blocking the DAP12-associated receptor TREM-1, which is expressed on granulocytes and monocytes, protected mice against septic shock.

The resistance of the DAP12-deficient mice to bacterial infections is consistent with the situation in humans. Humans lacking DAP12 develop a lethal bone wasting and neurodegenerative disease, but do not seem to be more susceptible to infections. Thus far, the benefit of expressing DAP12 on immune cells that respond to bacterial infections remains a mystery.