Activation of murine T cells by Ag or mitogens results in changes in the expression of several cell-surface adhesion molecules that alter the way in which these cells migrate and localize in tissues in vivo. As naive CD8 precursor cells in lymph nodes (LN) differentiate into effector CTL in response to a skin allograft, they up-regulate their expression of Pgp-1 (CD44), VLA-4, and LFA-1 (CD11a/18), while down-regulating L-selectin (CD62L). All cytolytic activity is therefore present in this minor population of L-selectin- Pgp-1high LN T cells. We now report that, late after rejection of minor histocompatibility Ag-disparate skin grafts, the majority of memory CD8 T cells express both L-selectin and Pgp-1 and thus would be expected to migrate via the classical route of recirculation through LN. Furthermore, restimulation of these memory cells by Ag causes them to down-regulate L-selectin, so that memory-effector cells have the same L-selectin- Pgp-1high phenotype as do primary effector cells. These results are in contrast to recent reports that murine and ovine CD4 memory cells do not express L-selectin or recirculate through LN high endothelial venules. In addition, although virgin and naive CD4 cells may be divided based upon their expression of CD45RA or CD45RB, memory CD8 cells cannot be differentiated by their expression of CD45 isoforms.

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