Development of circulating CD4⁺ T-cell memory

The ability of circulating CD4⁺ T cells to retain memories of previous antigenic encounters is a cardinal feature of the adaptive immune system. Over the past two decades, since the first description of central and effector memory T cells, many studies have examined molecular mechanisms controlling CD8⁺ T-cell memory, with comparatively less research into CD4⁺ T-cell memory. Here, we review a number of seminal studies showing that circulating memory CD4⁺ T cells develop directly from effector cells; and in so doing, preserve features of their effector precursors. We examine mechanisms controlling the development and phenotypes of memory CD4⁺ T cells, and provide an updated model that accommodates both the central and effector memory paradigm and the diverse T helper cell classification system.