The dual functions of Fas ligand in the regulation of peripheral CD8+ and CD4+ T cells

Although Fas ligand (FasL) is well characterized for its capacity to delive r a death signal through its receptor Fas, recent work demonstrates that Fa st also can receive signals facilitating antigen (Ag)-specific proliferatio n of CD8(+) T cells. The fact that the gld mutation differentially influenc es the proliferative capacity of CD8(+) and CD4(+) T cells presented the in triguing possibility that a single molecule may play opposing roles in thes e two subpopulations. The present study focuses on how these positive and n egative regulatory roles are balanced. We show that naive CD4(+) T cells ar e responsive to Fast-mediated costimulation on encounter with Ag when Fas-m ediated death is prevented. Thus, the machinery responsible for transducing the Fast positive reverse signal operates in both CD4(+) and CD8(+) T cell s. Instead, differential control of FasL expression distinguishes the role of Fast in these two T cell subpopulations. Fast costimulation occurs immed iately on T cell receptor ligation and correlates with the up-regulation of Fast expression on CD8(+) and naive CD4(+) T cells, both of which are sens itive to the Fast costimulatory signal. Conversely, Fast-initiated death oc curs late in an immune response when high levels of FasL expression are mai ntained on CD4(+) T cells that are sensitive to Fas-mediated death, but not on CD8(+) T cells that are relatively insensitive to this signal. This car eful orchestration of Fast expression during times of susceptibility to cos timulation and conversely, to death, endows Fast with the capacity to both positively and negatively regulate the peripheral T cell compartment.