Not all CD4(+) memory T cells are long lived

Memory T cells are thought to have several properties that distinguish them from their naive precursors. They are found in parts of the body that rarely house naive cells, they respond to antigen with faster proliferation and more rapid progression to effector function, they are less sensitive to the absence of their selecting major histocompatibility complex (MHC), and, above all, they are long lived. Here we show that this last property may not be universal. Some CD4(+) T cells that have surface proteins characteristic of memory cells have the same half-life in vivo as naive cells. The description of these cells as memory cells therefore depends on our definition of the word 'memory'.