Targeting CD4 coreceptor expression to postselection thymocytes reveals that CD4/CD8 lineage choice is neither error-prone nor stochastic.

The mechanism by which CD4/CD8 lineage choice is coordinated with TCR specificity during positive selection remains an unresolved problem in immunology. The stochastic/selection model proposes that CD4/CD8 lineage choice in TCR-signaled CD4(+)CD8(+) thymocytes occurs randomly and therefore is highly error-prone. This perspective is strongly supported by "coreceptor rescue" experiments in which transgenic CD4 coreceptors were ectopically expressed on thymocytes throughout their development and caused significant numbers of cells bearing MHC-II-specific TCR to differentiate into mature, CD8 lineage T cells. However, it is not known if forced coreceptor expression actually rescued positively selected thymocytes making an incorrect lineage choice or if it influenced developing thymocytes into making an incorrect lineage choice. We have now reassessed coreceptor rescue and the concept that lineage choice is highly error-prone with a novel CD4 transgene (referred to as E8(I)-CD4) that targets expression of transgenic CD4 coreceptors specifically to thymocytes that have already undergone positive selection and adopted a CD8 lineage fate. Unlike previous CD4 transgenes, the E8(I)-CD4 transgene has no effect on early thymocyte development and cannot itself influence CD4/CD8 lineage choice. We report that the E8(I)-CD4 transgene did in fact induce expression of functional CD4 coreceptor proteins on newly arising CD8 lineage thymocytes precisely at the point in thymic development that transgenic CD4 coreceptors would putatively rescue MHC-II-specific thymocytes that incorrectly adopted the CD8 lineage. However, the E8(I)-CD4 transgene did not reveal any MHC-II-selected thymocytes that adopted the CD8 lineage fate. These results demonstrate that CD4/CD8 lineage choice is neither error-prone nor stochastic.