B_1_B_cells___a_new_source_of_macrophages_in_homeostasis_and_during_inflammation. B_1_B_cells___a_new_source_of_macrophages_in_homeostasis_and_during_inflammation

The ‘classical’ model of haematopoiesis postulates that haematopoietic stem cells make an early and irrevocable choice between lymphoid and myeloid cell fates. However, recent data suggest a close developmental relationship between myeloid and lymphoid lineages. Here we first demonstrate evidence of lineage switch from B-1 B lymphocyte to a macrophage (M?), or so-called transdifferentiation, occurring in mature cells in vivo. Specifically, using transgenic B6.mb1.iCre::Rosa26-YFP mice, in which YFP expression is restricted exclusively to B cell lineage, we discovered that a significant portion (3-12 %) of tissue-resident M?s in peritoneum, pleural cavity and intestine in steady state are positive for YFP.