The brain and immune system communicate mainly by the exchange of soluble factors such as cytokines. In PLoS Biology, Momma and colleagues demonstrate that such communication also occurs through the delivery of exosomes. Using an experimental system in which Cre recombinase is conditionally expressed only in hematopoietic cells, the authors observe very low, steady-state delivery of Cre to Purkinje neurons in the absence of any apparent immunocyte-neuron fusion. This delivery is notably enhanced during inflammation. Cre protein itself is not released; instead, mRNA encoding Cre is packaged into exosomes, which are then released into the serum, especially in inflammatory conditions. There is no appreciable production of such exosomes by brain-resident cells of the immune system, such as microglia; instead, the exosomes are able to traverse the blood-brain barrier directly. Neurons taking up exosomes show altered expression of microRNA, which suggests that the exosomes influence the activity of these cells in an as-yet-unspecified manner.

PLoS Biol. (3 June 2014) doi:10.1371/journal.pbio.1001874