Detection of basal levels of regulatory T cells
In the next series of experiments, we analyzed the number of Tregs at baseline in the absence of antigen stimulation. We defined baseline Tregs as the total number of CD137+CD154- T cells detected at baseline, without any antigen stimulation 30. As expected, Tregs defined as CD137+ were correlated with Tregs defined by the alternative markers CD4+CD25+CD127low 38,45 (Figure 4a-b ). There was substantial variation in the number of Tregs (total CD137+) among participants (Figure 4c ), the number of CD137+ cells varing 90-fold from 925 to 86,000 per million CD4+ T cells.
We hypothesized that the size of the baseline Treg population present in each individual might influence their capacity to mount an allergen-specific T cell response following stimulation with the allergen-derived peptide pools. There was a suggestion of an inverse correlation between the number of Tregs (CD137+ unstimulated cells) and the number of Teff (cytokine+ CD154+ allergen-stimulated cells), but this was not statistically significant (Figure 4d ).