The transcription factor FOXP3 plays a key role in CD4 + CD25 + regulatory T cell function and represents a specific marker for these cells. Despite its strong association with regulatory T cell function, in humans little is known about the frequency of CD4 + CD25 + cells that express FOXP3 protein nor the distribution of these cells in vivo.
Here we report the characterization of seven anti-FOXP3 monoclonal antibodies enabling the detection of endogenous human FOXP3 protein by flow cytometry and immunohistochemistry. Flow-cytometric analysis showed that FOXP3 was expressed by the majority of CD4 + CD25 high T cells in peripheral blood. By contrast, less than half of the CD4 + CD25 int population were FOXP3 + , providing an explanation for observations in human T cells that regulatory activity is enriched within the CD4 + CD25 high pool. Although FOXP3 expression was primarily restricted to CD4 + CD25 + cells, it was induced following activation of both CD4 + and CD8 + T cell clones. These findings indicate that the frequency of FOXP3 + cells correlates with the level of expression of CD25 in naturally arising regulatory T cells and that FOXP3 protein is expressed by some activated CD4 + and CD8 + T cell clones. These reagents represent valuable research tools to further investigate FOXP3 function and are applicable for routine clinical use.