Homeostasis, Memory and CTL Vaccines

We have long known that one of the consequences of T-cell activation is vigorous clonal expansion of responding T cells. CD8 T cells specific for a particular viral epitope, for example, can expand a few orders of magnitude over a roughly one-week period (Murali-Krishna et al., 1998; Butz and Bevan, 1998). But what becomes of these greatly expanded pools of potentially dangerous cells once a pathogen is cleared? Given the frequency with which T-cell activation must occur, the body obviously cannot maintain all the cells produced from each round of infection. Some will go on to become memory T cells, by a process that is still not well understood, but the vast majority of cells must be cleared to prepare the way for response to other infections. The way in which these cells are removed has become a topic of great interest during the past decade.