Figure 5.

Model to correlate FasL expression with the dual roles this molecule plays in costimulation and death. The data presented here are most compatible with a model in which costimulation occurs early after Ag stimulation (correlating with increased FasL expression on Ag stimulation in sensitive cells), and Fas-mediated death occurs late (correlating with the maintenance of FasL expression in CD4⁺ T cells). The numbers “1” and “2” represent the sequence of the two FasL functions as they occur. The sizes of the arrows correlate with the strength of the overall effect from each signal. ∗ denotes the possibility that long-term CD4⁺ T cell lines may be less sensitive to Fas-mediated cell death (44–46), possibly because of the up-regulated levels of Fas death signaling inhibitors (44, 45).