Comparison of circulating CD4+ FoxP3+ regulatory T cell (Treg) frequencies in healthy donors and cancer patients. Peripheral blood mononuclear cells (PBMC) from healthy donors and cancer patients were stained with anti-CD4 fluorescein isothiocyanate (FITC), anti-CD3 peridinin chlorophyll protein (PerCP) and anti-FoxP3 phycoerythrin (PE) and analysed by flow cytometry. To determine the frequencies of Tregs, CD4/CD3 double-positive cells were gated and further analysed for FoxP3 expression. (a) Representative fluorescence-activated cell sorter (FACS) analysis of Tregs from one healthy donor and one cancer patient, showing 5% versus 11% Tregs in PBMC, respectively. (b) Percentages of CD4+ CD25+ FoxP3+ Tregs in CD4+ T cells from nine healthy donors (mean ratio: 5·4) and 113 cancer patients, with maintained (n= 36; mean ratio: 7·5) and impaired (n= 77; mean ratio: 7·1) phosphoantigen-induced proliferation of γδ T cells. (c) Ratio of Tregs to the phosphoantigen-sensitive Vδ2+ subset of γδ T cells in the peripheral blood of cancer patients with preserved (mean: 47·0) and impaired (mean: 8·3) phosphoantigen-induced γδ T-cell proliferation. Student’s t-test was used to determine statistical significance of detected differences. P<0·05 was considered significant.