Dichotomy between naive and memory CD4(+) T cell responses to Fas engagement

Engagement of Fas (APO-1, CD95), a member. of the tumor necrosis factor rec eptor superfamily, can induce apoptotic cell death. However, Fas engagement also can costimulate lymphocyte proliferation. The physiologic regulation of these two outcomes is poorly understood. Here, we have used two systems, the first in vitro and the second in vivo, to demonstrate that naive and m emory CD4(+) T cells display dichotomous responses to Fas ligation. Naive C D4(+) T cells (CD44(lo), CD45RB(+), CD62L(+)) die as a consequence of Fas l igation in the presence of anti-CD3 antibody, whereas memory T cells (CD44( hi) CD45RB(-), CD62L(-)), freshly isolated from the same starting populatio n and subjected to the same stimulation conditions, are costimulated to pro liferate by Fas ligation, In vitro,we demonstrate that CD28-mediated signal s or T helper 1 and T helper 2 differentiation cytokines alter the response of naive T cells, but not of memory T cells, to Fas ligation. In vivo expe riments in hen egg lysozyme (HEL)T cell receptor transgenic mice show that CD4(+) T cells from HEL-naive mice are killed by Fas ligation, but CD4(+) T cells from long-term MEL-exposed mice are costimulated by Fas ligation. Th us, the physiological outcome of Fas ligation in CD4(+) T cells is determin ed primarily by the antigenic history of the T cell.