['Optimization of the culture environment for the ex vivo expansion of T cells is crucial for obtaining the large doses of cells needed for cellular immunotherapy. O2 tension is a key parameter that impacts the proliferation and quality of the expanded T cells.', 'Peripheral blood mononuclear cells were stimulated with either PHA or an anti-CD3 monoclonal antibody under 5% (low) or 20% (high) O2 atmospheres. After stimulation, cells were cultured in the presence of IL-2 under either low or high O2 conditions.', 'T cells stimulated and grown under 5% O2 exhibited higher proliferation rates and a mean (n = 11) of 5.8-fold greater total expansion over T cells grown under 20% O2. Stimulation under 5% O2 produced a lasting proliferative effect even after a switch to 20% O2. Examination of apoptosis by the flow cytometry-based TUNEL assay showed a mean (n = 9) of 2.9-fold greater percentage of apoptotic cells under 20% O(2). Flow-cytometric analysis of the IL-2 receptor (CD25) showed that the normal downregulation kinetics - following stimulation-induced CD25 upregulation - were slowed under 5% O(2), such that the 5% O2 cultures had a greater number of CD25+ cells, and those CD25+ cells expressed an average (n = 6) of 41% higher levels of CD25 receptor per cell. No significant O2 tension effects were observed on other surface antigens (CD3, CD28, and CD62L) examined. The key metabolic parameters, specific glucose uptake rate, q(glu), and specific lactate production rate, q(lac), were both increased by a mean (n = 5) of 47% under 5% O2.'] Does low oxygen tension enhance the stimulation and proliferation of human T lymphocytes in the presence of IL-2?