Central memory T cells mediate long-term immunity to Leishmania major in the absence of persistent parasites.

Infection with Leishmania major induces a protective immune response and long-term resistance to reinfection, which is thought to depend upon persistent parasites. Here we demonstrate that although effector CD4(+) T cells are lost in the absence of parasites, central memory CD4(+) T cells are maintained. Upon secondary infection, these central memory T cells become tissue-homing effector T cells and mediate protection. Thus, immunity to L. major is mediated by at least two distinct populations of CD4(+) T cells: short-lived pathogen-dependent effector cells and long-lived pathogen-independent central memory cells. These data suggest that central memory T cells should be the targets for nonlive vaccines against infectious diseases requiring cell-mediated immunity.