Peptide specificity of thymic selection of CD4⁺CD25⁺ T cells

The CD4⁺CD25⁺ regulatory T cells can be found in the thymus, but their need to undergo positive and negative selection has been questioned. Instead, it has been hypothesized that CD4⁺CD25⁺ cells mature following TCR binding to MHC backbone, to low abundant MHC/peptide complexes, or to class II MHC loaded with peripheral autoantigens. In all these circumstances, processes that are distinct from positive and negative selection would govern the provenance of CD4⁺CD25⁺ cells in the thymus. By comparing the development of CD4⁺CD25^- and CD4⁺CD25⁺ cells in mice expressing class II MHC molecules bound with one or many peptide(s), we show that the CD4⁺CD25⁺ cells appear during natural selection of CD4⁺ T cells. The proportion of CD4⁺CD25⁺ cells in the population of CD4⁺ thymocytes remains constant, and their total number reflects the complexity of selecting class II MHC/peptide complexes. Hence, thymic development of CD4⁺CD25⁺ cells does not exclusively depend on the low-density, high-affinity MHC/peptide complexes or thymic presentation of peripheral self-Ags, but, rather, these cells are selected as a portion of the natural repertoire of CD4⁺ T cells. Furthermore, while resistant to deletion mediated by endogenous superantigen(s), these cells were negatively selected on class II MHC/peptide complexes. We postulate that while the CD4⁺CD25⁺ thymocytes are first detectable in the thymic medulla, their functional commitment occurs in the thymic cortex.