The body’s innate immune system provides an immediate and nonspecific defence against invading pathogens. The main innate immune cells in the brain, microglia, rarely encounter such infections, but they can respond to peripheral inflammation elsewhere in the body. One intriguing feature of peripheral innate immunity is a phenomenon called immune memory: the body’s innate immune system ‘remembers’ previous exposures to microbes, and augments its responses to reinfections accordingly
1,
2. Does it follow, then, that peripheral inflammation could elicit immune memory in microglia? In
a paper in Nature, Wendeln
et al.3 provide evidence for immune memory in microglia, and show that it can alter the progression of brain disorders in mice.
There are two types of immune memory: training, which exaggerates immune responses; and tolerance, which dampens them. To test whether microglia retain an immune memory of peripheral infection, Wendeln
et al. injected lipopolysaccharide (LPS) molecules, a component of some bacteria, into the body cavities of mice. Exposing wild-type mice to two injections of LPS induced a microglial response resembling immune training, characterized by elevated levels of pro-inflammatory molecules. Four LPS exposures resulted in immune tolerance, indicated by reduced pro-inflammatory signals.
