Human memory T cells: generation, compartmentalization and homeostasis

Most of our understanding of immunological memory comes from studies in mice. However, these studies cannot recapitulate the exposure to numerous diverse pathogens that occurs over decades in humans. But, as reviewed here, recent studies focusing on human memory T cells are revealing important features of these cells, including subset heterogeneity and spatial compartmentalization. Memory T cells constitute the most abundant lymphocyte population in the body for the majority of a person's lifetime; however, our understanding of memory T cell generation, function and maintenance mainly derives from mouse studies, which cannot recapitulate the exposure to multiple pathogens that occurs over many decades in humans. In this Review, we discuss studies focused on human memory T cells that reveal key properties of these cells, including subset heterogeneity and diverse tissue residence in multiple mucosal and lymphoid tissue sites. We also review how the function and the adaptability of human memory T cells depend on spatial and temporal compartmentalization.