Resistance of Human Regulatory Foxp3⁺ T Cells to Normobaric Hyperoxia Exposure Under Resting and Stimulating Conditions

Oxygen tension levels may modulate immune responses. Evidence shows that hyperoxia influences the risk of infection, autoimmunity and alloreactivity and hence is a possible therapeutic option in a number of disorders. Regulatory T cells (Tregs) play a central role in tolerance maintenance, but their behaviour under hyperoxia is largely unknown. We investigated in vitro the impact of normobaric hyperoxia on human Tregs and their cellular network. Peripheral blood mononuclear cells isolated from six healthy men were cultured under normoxia and escalating duration of normobaric hyperoxia (10 min, 1, 16, 88 h) under resting conditions and at the presence of anti-CD3/CD28 beads. Foxp3⁺ Tregs' and other T cell subsets' survival, proliferation, activation, maturation and Th1/Th2 markers were assessed by flow cytometry. We observed decreasing CD4⁺ cell survival with increasing duration of hyperoxia irrespectively of the presence of stimulators. The prevalence of CD4⁺CD45RA⁺ cells increased under stimulation (P = 0.001). In stimulated samples, the proliferation and induced Foxp3 expression decreased after 88 h of hyperoxia (both P = 0.001). In conclusion, normobaric hyperoxia up to 16 h does not induce significant changes in basic human T cell subsets, including the prevalence naturally occurring Tregs. Prolonged exposure to hyperoxia likely affects all unstimulated T cell subsets in a similar way. In stimulated T lymphocytes, the proliferation is hampered and cell death increases more evidently after prolonged hyperoxia (several days). Inducible Foxp3 expression is likely closely related to these processes. Naive CD4⁺ T cells are maintained by stimulation during exposure to hyperoxia.