Strength of stimulus and clonal competition impact the rate of memory CD8 T cell differentiation

The developmental pathways of long-lived memory CD8 T cells and the lineage relationship between memory T cell subsets remain controversial. Although some studies indicate the two major memory T cell subsets, central memory T (T(CM)) and effector memory T (T(EM)), are related lineages, others suggest that these subsets arise and are maintained independently of one another. In this study, we have investigated this issue and examined the differentiation of memory CD8 T cell subsets by tracking the lineage relationships of both endogenous and TCR transgenic CD8 T cell responses after acute infection. Our data indicate that TCR transgenic as well as nontransgenic T(EM) differentiate into T(CM) in the absence of Ag. Moreover, the rate of memory CD8 T cell differentiation from T(EM) into the self-renewing and long-lived pool of T(CM) is influenced by signals received during priming, including Ag levels, clonal competition, and/or the duration of infection. Although some T(EM) appear to not progress to T(CM), the vast majority of T(CM) are derived from T(EM). Thus, long-lasting, Ag-independent CD8 T cell memory results from progressive differentiation of memory CD8 T cells, and the rate of memory T cell differentiation is governed by events occurring early during T cell priming.