The layered development of mouse B and T Cells.

Traditional models of lymphopoiesis present B and T cell development as a linear process that initiates in the fetus and continues after birth in the bone marrow and thymus, respectively. However, this view of lymphocyte development is not in accord with reports, dating back several decades, indicating that the types of lymphocytes generated before and after birth differ. In this regard, selected γδ T cells, and those that utilize the Vγ3 receptor in particular, and innate-like B-1 B cells preferentially arise during fetal blood cell development. This review synthesizes data from multiple laboratories, with an emphasis on our own work using mouse models, demonstrating that innate and conventional B and T cells emerge in hematopoietic stem cell independent and dependent waves of development that are differentially regulated. This layering of lymphocyte development has implications for understanding the composition of the adult immune system and may provide insights into the origin of various lymphocytic leukemias.