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TCR Repertoire and Foxp3 Expression Define Functionally Distinct Subsets of CD4+ Regulatory T Cells1

Despite extensive research efforts to characterize peripheral regulatory T (Treg) cells expressing transcription factor Foxp3, their subset complexity, phenotypic characteristics, TCR repertoire and Ag specificities remain ambiguous. In this study, we identify and define two subsets of peripheral Treg cells differing in Foxp3 expression level and TCR repertoires. Treg cells expressing a high level of Foxp3 and TCRs not used by naive CD4+ T cells present a stable suppressor phenotype and dominate the peripheral Treg population in unmanipulated mice. The second Treg subset, expressing a lower level of Foxp3 and using TCRs shared with naive CD4+ T cells constitutes a small fraction of all Treg cells in unmanipulated mice and enriches Treg population with the same Ag specificities as expressed by activated/effector T cells. This Treg subset undergoes extensive expansion during response to Ag when it becomes a major population of Ag-specific Treg cells. Thus, Treg cells expressing TCRs shared with naive CD4+ T cells have a flexible phenotype and may down-regulate Foxp3 expression which may restore immune balance at the conclusion of immune response or convert these cells to effector T cells producing inflammatory cytokines.