Number of T Reg Cells That Differentiate Does Not Increase upon Encounter of Agonist Ligand on Thymic Epithelial Cells

It has been reported that the differentiation of CD4(+)CD25(+) regulatory T cells (T reg cells) can be induced by agonist peptide/major histocompatibility complex ligands in the thymus. Exploiting a transgenic mouse line wherein expression of a particular T cell epitope can be controlled temporally and quantitatively, we found that diversion of differentiating thymocytes into the FoxP3 T reg cell pathway by this agonist ligand was essentially nonexistent. However, CD4(+)CD25(+) thymocytes were much less sensitive than their CD4(+)CD25(−) companions, by two to three orders of magnitude, to agonist-induced clonal deletion, such that their proportion increased, giving the false impression of induced differentiation. To account for these and prior observations, one can propose that differentiation along the CD4(+)CD25(+) pathway is induced by cues other than recognition of self-agonist cues, which are poorly read by thymocytes, whose T cell receptors are conducive to selection toward the conventional CD4(+)CD25(−) lineage. Thus, selective survival, rather than induced differentiation, may explain the apparent enrichment observed here and in previous studies.