Shortening the infectious period does not alter expansion of CD8 T cells but diminishes their capacity to differentiate into memory cells.

Following a primary immune response, a portion of effector T cells gives rise to long-lived memory cells. Although primary expansion and differentiation of effector CD8 T cells is dictated by a brief exposure to Ag, it is unclear whether full memory differentiation is also programmed within the same short window. By carefully modulating the kinetics of Listeria monocytogenes infection, we analyzed the requirements for the programming of effector and memory T cell development in vivo. We find that although limiting the infectious period to the first 24-48 h does not impact the size of the primary CD8 response, the ensuing memory population is significantly diminished. This effect is particularly pronounced in the development of tissue-homing memory cells and is inversely proportional to the initial infectious dose. In contrast to CD8 responses, the differentiation of primary CD4 responses was highly dependent on the continued presence of the infection. Shortening the duration of the infection greatly reduced the development of CD4 effector responses in the spleen and prevented their trafficking to peripheral sites of infection. We propose that the stimulus received by CD8 T cells during the early stages of infection largely contribute to the differentiation of CD8 effector cells, whereas continued or distinct signals received at later stages influence their ability to differentiate into memory cells.