When a bacterium enters the body, the first line of defense it encounters is the innate immune system. This rapid-response system recognizes common bacterial components such as lipoproteins, lipocarbohydrates or bacterial DNA, and sets off a chain of reactions to kill the invader and alert the adaptive immune system, which can offer a more targeted attack, if needed. Now researchers from Japan and Germany have found a specific receptor protein, called Toll-like receptor 9, that the innate immune system requires for the recognition of bacterial DNA. They report their findings in today's Nature.

Unlike the DNA of body cells, DNA from bacteria frequently contains a motif of two particular bases, C and G in its unmethylated form. The scientists bred mice that lack Toll-like receptor 9 and found that these animals, unlike their wildtype cousins, no longer showed any innate immune response to CG-containing DNA. Furthermore, these mice did not die from the shock that CG-containing DNA can induce in normal mice. Understanding the exact mechanism of how bacterial DNA stimulates the innate immune system can possibly help scientists improve the effects of DNA vaccines, which work by getting the immune system's attention.