Commentary: Maintenance of CD8(+) T Memory Lymphocytes in the Spleen but Not in the Bone Marrow Is Dependent on Proliferation

A short communication by Radbruch's lab in Eur J Immunol addresses the question of memory CD8 T lymphocyte proliferation in mouse bone marrow (BM), by using repeated injections of Cyclophosphamide (CyP) as a tool to gain information on cell division in vivo (1). The authors claim that memory CD8 T cells in the BM do not proliferate, based on the evidence that their numbers are not reduced at a single time point 3 days after stopping CyP treatment, that is expected to kill all dividing cells. Moreover, they claim that memory CD8 T cells are resident in the BM, based on the results of a combination treatment with CyP and FTY720, a S1P receptor modulator that blocks cell egress from lymphoid organs. In the same issue of Eur J Immunol, Nolte et al. commented the findings by Radbruch and coworkers, highlighting their novelty and potential implications for the clinic (2). In this commentary, we would like to offer a different perspective, from a point of view that includes previous kinetics issues on CD8 T cell renewal in the BM and on the homeostatic regulation of memory T cells after an insult.