Little is known about the molecules that control the development and function of CD4+CD25+ Tregs. Recently, it was shown that the transcription factor FOXP3 is necessary and sufficient for the generation of CD4+CD25+ Tregs in mice. We investigated the capacity of FOXP3 to drive the generation of suppressive CD4+CD25+ Tregs in humans. Surprisingly, although ectopic expression of FOXP3 in human CD4+ T cells resulted in induction of hyporesponsiveness and suppression of IL-2 production, it did not lead to acquisition of significant suppressor activity in vitro. Similarly, ectopic expression of FOXP3Δ2, an isoform found in human CD4+CD25+ Tregs that lacks exon 2, also failed to induce the development of suppressor T cells. Moreover, when FOXP3 and FOXP3Δ2 were simultaneously overexpressed, although the expression of several Treg-associated cell surface markers was significantly increased, only a modest suppressive activity was induced. These data indicate that in humans, overexpression of FOXP3 alone or together with FOXP3Δ2 is not an effective method to generate potent suppressor T cells in vitro and suggest that factors in addition to FOXP3 are required during the process of activation and/or differentiation for the development of bona fide Tregs.
Sarah E. Allan, Laura Passerini, Rosa Bacchetta, Natasha Crellin, Minyue Dai, Paul C. Orban, Steven F. Ziegler, Maria Grazia Roncarolo, Megan K. Levings