The bone marrow: a nest for migratory memory T cells

It has been known for a long time that T-cell precursors generated in the bone marrow migrate to the thymus, where T-cell development occurs. However, a fact often neglected is that, under physiological conditions, mature CD4 and CD8 lymphocytes undergo extensive migration from the blood to the bone marrow and vice versa. Here, we first review several observations showing that the bone marrow can function as a secondary lymphoid organ for both CD4 and CD8 cells, as well as a preferential homing site for memory T cells. Second, we discuss evidence that, a long time after priming, memory CD8 cells proliferate more extensively in the bone marrow than they do in either secondary lymphoid or extra-lymphoid organs. Finally, we propose that the bone marrow is a central organ in mature T-cell traffic and contributes greatly to long-term cytotoxic memory, which has implications for adoptive immunotherapy and vaccine design.