Heterogeneity of CD4+ memory T cells: functional modules for tailored immunity

Phenotypic and functional heterogeneity is the hallmark of effector and memory T cells. Upon antigenic stimulation, naïve CD4(+) T cells make choices to become effector Th1, Th2 or Th17 cells, or even Treg. In addition to differences in cytokine repertoire, effector CD4(+) T cells exhibit diversity in homing, such as migration to lymph node follicles to help B cells versus migration to inflamed tissues. Upon clearance of the antigen, two major types of memory T cells remain: central memory cells, which patrol lymphoid organs, and effector memory cells that act as sentinels in peripheral tissues such as the skin and the gut. Here, we review our current understanding of CD4(+) T-cell lineage heterogeneity and flexibility, with emphasis on the human system, and propose an organization of effector and memory T cells based on distinct functional modules.