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 γ c receptor, and that these are the only signals in the thymus that upregulate the transcription factor Runx3d to direct specification to the CD8 + lineage.