We are thankful for an excellent opportunity to expand on our Review (Organization of immunological memory by bone marrow stroma. Nat. Rev. Immunol. 10, 193–200 (2010)) 1 that has been provided by the correspondence from Francesca Di Rosa (Maintenance of memory T cells in the bone marrow: survival or homeostatic proliferation? Nat. Rev. Immunol.(2016)) 2.

The traditional concept on how immunological memory is maintained proposes that memory lymphocytes circulate through the body in a quest for cells presenting them their cognate antigen. Their numbers are determined by a balance between (homeostatic) proliferation and cell death. It has been proposed that this proliferation mainly takes place in the bone marrow and is driven by interleukin-15 (IL-15), with an estimated turnover of the entire population within 2 months 3, 4, 5, 6. Memory CD8+ T cells from the bone marrow are thus considered to be part of the circulating memory T cell population.