Helios, an Ikaros family transcription factor, is preferentially expressed at the mRNA and protein level in regulatory T cells. Helios expression previously appeared to be restricted to thymic-derived Treg. Consistent with recent data, we show here that Helios expression is inducible in vitro under certain conditions. To understand phenotypic and functional differences between Helios + and Helios - Treg, we profiled cell-surface markers of FoxP3 + Treg using unmanipulated splenocytes. We found that CD103 and GITR are expressed at high levels on a subset of Helios + Treg and that a Helios + Treg population could be significantly enriched by FACS sorting using these two markers. Quantitative real-time PCR (qPCR) analysis revealed increased TGF-β message in Helios + Treg, consistent with the possibility that this population possesses enhanced regulatory potential. In tumor-bearing mice, we found that Helios + Treg were relatively over-represented in the tumor-mass, and BrdU studies showed that, in vivo, Helios + Treg proliferated more than Helios - Treg. We hypothesized that Helios-enriched Treg might exert increased suppressive effects. Using in vitro suppression assays, we show that Treg function correlates with the absolute number of Helios + cells in culture. Taken together, these data show that Helios + Treg represent a functional subset with associated CD103 and GITR expression.
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