What determines what you become when you grow up?

Results from previous studies have led to the hypothesis that CD4⁺CD25⁺ regulatory T (T_Reg) cells differentiate in the thymus as a consequence of high-affinity peptide–MHC–T-cell receptor (TCR) interactions. However, a report published in The Journal of Experimental Medicine provides contrasting data, and the authors suggest that encounter with a high-affinity ligand during thymocyte development might only seem to induce T_Reg-cell differentiation.

It has been shown that the proportion of T_Reg cells in mice that express a transgenic TCR and its cognate peptide ligand is greater than in mice that express the transgenic TCR alone. This has led to the hypothesis that agonist peptides induce T cells that express the cognate TCR to differentiate into T_Reg cells. So, to investigate how the dose of agonist peptide affects the efficiency of T_Reg-cell differentiation, van Santen et al. generated mice that express a TCR (known as the AND-TCR) specific for residues 88–103 of moth cytochrome C (MCC) and express I–E^k-bound MCC peptide 96–101 (MCC_96–101). Expression of the MHC-class-II-bound peptide was driven by a promoter that is inhibited in a dose-dependent manner in the presence of tetracycline, and expression was restricted to the epithelial cells of the thymus. Mice were given graded doses of tetracycline in their drinking water for their lifespan, and increased levels of thymic expression of I–E^k-bound MCC_96–101 correlated with a decrease in the percentage of CD4⁺CD8⁻ thymocytes and, within this population, a decrease in the percentage of AND-TCR⁺ cells. However, within the AND-TCR⁺CD4⁺ thymocyte subset, the proportion of cells expressing CD25 increased. These cells were phenotypically T_Reg cells: for example, they expressed high levels of mRNA encoding the transcription factor FOXP3, and they inhibited the proliferation of CD4⁺CD25⁻ T cells.

These findings seemed consistent with the idea that agonist peptides induce T cells expressing the cognate TCR to differentiate into T_Reg cells in a dose-dependent manner. However, when the numbers of AND-TCR⁺CD4⁺CD25⁺ and AND-TCR⁺CD4⁺CD25⁻ thymocytes were compared with the level of I–E^k-bound MCC_96–101, it was found that the number of AND-TCR⁺CD4⁺CD25⁺ thymocytes increased only slightly as I–E^k-bound MCC_96–101 expression increased — certainly not to the level that was expected from the increase in the percentage of these cells. This indicates that the observed increase in the percentage of AND-TCR⁺CD4⁺CD25⁺ thymocytes was a result of preferential deletion of AND-TCR⁺CD4⁺CD25⁻ thymocytes and not a result of increased numbers of AND-TCR⁺ thymocytes being directed to become T_Reg cells.

This study provides evidence that is in contrast to the current hypothesis that high-affinity agonist peptides promote thymocytes that express the cognate TCR to differentiate into T_Reg cells. Instead, the authors suggest that, in this system, other signals promote T_Reg-cell differentiation and that studies elucidating the signals that induce FOXP3 expression will probably determine which signals induce T_Reg-cell differentiation. It will be important to establish whether these observations hold true for other systems.