Identification of lineage-specifying cytokines that signal all CD8+-cytotoxic-lineage-fate 'decisions' in the thymus

T cells developing in the thymus are signaled during positive selection to differentiate into either CD4+ T cells or CD8+ T cells. Singer and colleagues show that CD8+-lineage specification is signaled exclusively by cytokines, including cytokines that do not signal via the γc receptor, and that these are the only signals in the thymus that upregulate the transcription factor Runx3d to direct specification to the CD8+ lineage. T cell antigen...