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We have previously hypothesized that maintaining a balanced peripheral immune system may not be the sole responsibility of a specialized subset of T cells dedicated to immune regulation, but also a side effect of normal competition for shared resources within an intact immune system. Here we show that regulatory activity is correlated with high homeostatic expansion potential, reflecting the avidity for self-peptide:MHC complexes. Monoclonal transgenic T cells with high homeostatic expansion potential and lacking characteristics previously associated with regulatory function were able to regulate wasting disease induced by transfer of a small number of naive CD45RB(hi) CD4 T cells into lymphopenic hosts. Self-regulatory function is also found in the naive polyclonal T cell repertoire depleted of CD25(+) T cells. T cells capable of preventing immune pathology, like the transgenic T cells, express higher than average levels of CD5, an indicator of avidity for self:MHC peptide complexes. We therefore propose that dysregulated expansion of potentially pathogenic T cells in a lymphopenic environment can be prevented by members of the naive T cell repertoire, irrespective of their specificity, as a side effect of their response to homeostatic and antigenic stimulation.

CD4/CD8 lineage decision is an important event during T cell maturation in the thymus. CD8 T cell differentiation usually requires corecognition of major histocompatibility complex (MHC) class I by the T cell receptor (TCR) and CD8, whereas CD4 T cells differentiate as a consequence of MHC class II recognition by the TCR and CD4. The involvement of specific peptides in the selection of T cells expressing a particular TCR could be demonstrated so far for the CD8 lineage only. We used mice transgenic for an MHC class II-restricted TCR to investigate the role of antagonistic peptides in CD4 T cell differentiation. Interestingly, antagonists blocked the development of CD4(+) cells that normally differentiate in thymus organ culture from those mice, and they induced the generation of CD8(+) cells in thymus organ culture from mice impaired in CD4(+) cell development (invariant chain-deficient mice). These results are in line with recent observations that antagonistic signals direct differentiation into the CD8 lineage, regardless of MHC specificity.

In several experimental systems analyzing the generation of single positive (SP) thymocytes from double positive (DP) thymocytes, CD4 SP cells have been shown to appear before CD8 SP cells. This apparent temporal asymmetry in the maturation of CD4 SP and CD8 SP thymocytes could either be due to divergent molecular differentiation programs of the two T cell lineages, or merely to slower degradation kinetics of the CD4 protein. To study this question in unmanipulated in vivo differentiation, we developed a four-color flow cytometry protocol which identifies a recently activated TCRintCD69pos thymocyte population containing DP cells and early CD4 SP cells but no CD8 SP cells. We show that these TCRintCD69pos thymocytes represent a transitory stage in the mainstream alphabeta T cell lineage. The precursors of the CD8 SP cells are contained in this population as incompletely selected DP cells. Moreover, we show that expression of both coreceptors in the TCRintCD69pos population depends on transcriptional and translational activity, thus excluding differences in turnover rates of the CD4 and CD8 proteins as the cause of the asynchrony in differentiation of the CD4 and CD8 lineages.