THE MAJORITY OF EXTRATHYMIC T-CELLS IN THE LIVER HAVE THE PHENOTYPE OF MEMORY T-CELLS

Memory T cells have been characterized by their functional and phenoty pic criteria. In the case of mice, memory T cells are known to have th e phenotype of CD45RB(low) CD44(+)L-selectin(-). Since extrathymic T c ells also express adhesion molecules (e.g., CD44(+)L-selectin(-)) simi lar to memory T cells, their relationship was intensively examined in this study. Extrathymic T cells were identified as IL-2R beta(+) inter mediate CD3 (or TCR) cells. Surprisingly, a subpopulation of IL-2R bet a(+) intermediate CD3 cells, namely, NK1.1(+) T cells with CD4(+) phen otype, were found to be CD45RB(low) T cells. Indeed, in vivo injection of anti-IL-2R beta mAb eliminated not only intermediate CD3 cells but also a considerable proportion (up to 65%) of CD45RB(low) T cells. Ev en after elimination of intermediate CD3 cells and many CD45RB(low) ce lls, remaining T cells were able to respond to recall antigens, such a s ovalbumin and allogeneic lymphocytes. In these experiments, lymphocy tes were obtained from mice pretreated with either ovalbumin or alloge neic lymphocytes. Furthermore, a sorted fraction of CD45RB(low) cells from intermediate CD3 cells was not able to respond to the recall anti gen, ovalbumin, whereas those from high CD3 cells were able to do so. Extrathymic T cells constitutively express IL-2R beta. Reflecting this property, many of them are primarily at sub-activated states. These r esults suggest that memory T cells thus far classified according to th e phenotypic criteria contain extrathymic T cells. Since extrathymic T cells increase with aging, many CD45RB(low) cells in adult mice have been found to consist of not only memory T cells but also extrathymic T cells.