Flow cytometry-based methods for studying signaling in human CD4+CD25+FOXP3+ T regulatory cells.

T regulatory (Treg) cells have a fundamental role in the establishment and maintenance of peripheral tolerance. It is well established that Treg cells have a phenotype and function that is distinct from conventional T effector cells, although how these two T cell subsets differ in terms of molecular signaling cascades remains largely unknown. Analysis of signaling events in Treg cells using classical biochemistry has been hampered due to difficulties in isolating homogeneous populations and limited cell numbers. In order to overcome these challenges, we defined the optimal conditions for culture, in vitro expansion, and stimulation of human CD4(+)CD25(+) Treg and T effector cells to study intracellular signaling events by flow cytometry. In order to avoid the pitfalls associated with cell isolation based on CD25 expression, we developed methodology to analyze subpopulations of FOXP3 positive and negative cells from ex vivo CD4(+) T cells. In addition to examination of ex vivo cells, we optimized expansion conditions for analysis of signaling in Treg and T effector cell lines. Using these methods, we found that human FOXP3(+) Treg cells displayed a greater capacity to phosphorylate the extracellular regulated kinase (ERK) compared to T effector cells, upon TCR-mediated activation. In contrast, FOXP3(+) Treg cells showed a significantly diminished capacity to phosphorylate AKT. This methodology provides a foundation for future investigation into the molecular events that regulate the phenotype and function of Treg cells, and may ultimately lead to the identification of Treg-cell specific therapeutic targets.