FIGURE 1.

Experimental design. A, Schematic of the stochastic/selection model of lineage choice as applied to MHC-II-specific T cells. During positive selection, MHC-II-specific TCR signals induce DP thymocytes to randomly terminate expression of either CD4 or CD8 coreceptors, generating short-lived intermediate thymocytes bearing MHC-II-specific TCR that are destined to die if they fail to receive a TCR/coreceptor-mediated survival signal. Since CD4 is required for MHC-II-dependent TCR signaling, only short-lived MHC-II-selected thymocytes expressing CD4 coreceptors can receive a TCR-mediated survival signal and differentiate into long-lived T cells. In WT mice, MHC-II-specific thymocytes that have randomly adopted the CD8 lineage fate by terminating CD4 gene expression undergo cell death because they are unable to receive MHC-II-specific TCR survival signals (upper panel). In the present study, the E8_I-CD4 transgene was designed to only induce transgenic CD4 coreceptor expression on those positively selected thymocytes that have adopted the CD8 lineage fate. Consequently, in E8_I-CD4 transgenic mice, MHC-II-specific thymocytes that have randomly terminated CD4 gene expression would express transgenic CD4 coreceptors (shown in red) and so would receive MHC-II-specific TCR survival signals and differentiate into long-lived CD8 lineage T cells (lower panel). B, Design of the E8_I-CD4 transgene. The E8_I-CD4 transgene consisted of the mature CD8 T cell-specific 7.6-kb E8_I transcriptional enhancer element (16) ligated to the CD8α promoter (P8α) and a murine CD4 intron splicing module (intron) driving CD4 cDNA expression (28). The origin of the E8_I element and CD8α promoter are shown in a schematic representation of the Cd8 gene locus; horizontal gray bars correspond to regions of cis-enhancer elements (E8_I-E8_V) (29, 30).