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Cutting Edge: Human CD4^–CD8^– Thymocytes Express FOXP3 in the Absence of a TCR¹

Heli Tuovinen^*, Eliisa Kekäläinen^*, Laura H. Rossi^*, Juha Puntila and T. Petteri Arstila^2,*

^* Haartman Institute, Department of Immunology, University of Helsinki and Department of Surgery, Hospital for Children and Adolescents, Helsinki University Hospital, Helsinki, Finland

The best candidate for regulatory T (Treg) cell lineage-determining factor is currently the Forkhead box transcription factor FOXP3. FOXP3 up-regulation has been linked to TCR-mediated signals, and in mice the abrogation of TCR expression or signals also prevents FoxP3 expression. In contrast, the TCR dependence of human FOXP3 is assumed but not established. In this study we show on a single cell level that 1.4% (range 0.1–3.8%) of CD4^–CD8^– thymocytes in healthy humans express FOXP3, two thirds of them without any detectable β TCR. These TCR^–FOXP3⁺ cells were mostly CD25^– and did not express TCR or B cell, NK cell, or monocyte-associated markers. Like mature Treg cells, they were mostly CD2⁺CD127^low and expressed cytoplasmic CTLA-4. Our results suggest that in immature human thymocytes the expression of FOXP3 precedes surface TCR, in which case TCR-mediated signals cannot be responsible for the thymic up-regulation of FOXP3.

The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked advertisement in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ This work was supported by grants from the Academy of Finland, the Finnish Cultural Foundation, and the Helsinki University Science Foundation and by research funds from the Helsinki University Hospital.

² Address correspondence and reprint requests to Dr. T. Petteri Arstila, Haartman Institute, Department of Immunology, PB21, University of Helsinki, 00014 Helsinki, Finland. E-mail address: petteri.arstila{at}helsinki.fi

³ Abbreviations used in this paper: Treg, regulatory T cell; DN, CD4^–CD8^– double negative; DP, CD4⁺CD8⁺ double positive; SP, single positive.