004 and P = 0.001, respectively). The cell proliferation assay of CD4+ and CD8+ lymphocytes performed in stimulated samples did not show a trend from the shortest hyperoxia exposure on,
check details while we observed a decrease in proliferation with hyperoxia exposure longer than 16 h (P = 0.001, 88 h hyperoxia data compared to shorter exposures). Furthermore, we found increasing prevalence of naïve CDR45RA+CD4+ cells with duration of hyperoxia in stimulated samples (P = 0.001). The proportion of regulatory T cells (CD4+Foxp3+) in unstimulated samples did not change systematically after hyperoxia, nor did the other investigated population with regulatory properties – the NK T cells. We did not find any association between hyperoxia exposure and frequencies of CD4+ and CD8+ populations in the culture. The activation molecules (CD25, CD69, HLA-DR) and T helper (Th) 1 and Th2 chemokine receptor expressions (CXCR3, CCR4, respectively) of CD4+ T helper cells were not altered during hyperoxia. We found a decrease in prevalence of CXCR3 expressing CD4+ T (Th1) cells and increased prevalence of CCR4 expressing cells (Th2) at all time points after stimulation compared to resting cultures that was
not influenced by hyperoxia. Along with activation markers, we observed a marked increase in Foxp3 expressing CD4+ cells after stimulation in all cultures but the one with the 88-h hyperoxia exposure. Similar to proliferation assay, the escalating hyperoxia trend analysis did not show any association from the shortest hyperoxia exposure on, but we noted the very low Foxp3 expression after 88-h hyperoxia (P = 0.001, 88-h hyperoxia Protein Tyrosine Kinase inhibitor data compared to shorter exposures). The prevalence of NK cells was similar in all experimental arms. In view of experimental evidence that hyperoxia may suppress autoimmunity [8–10, 13], alloreactivity  or modify response to infection , we aimed to examine the influence of hyperoxia on prevalence of naturally Inositol monophosphatase 1 occurring Tregs and basic T cell subsets important in adaptive immune response. In unstimulated cells exposed to normobaric hyperoxia of different
duration, we found no change in relative frequency of Tregs and their cellular environment including CD4+, CD8+, the Th1, Th2 populations, naïve/memory T cells or NK T cells. This finding suggests that these cell types are similarly resistant to normobaric hyperoxia while not stimulated. This in vitro finding concerning major CD4+ and CD8+ subtypes is in line with the results of other clinical studies [19, 20] which also confirmed stable numbers of circulating CD3+, CD4+, CD8+, CD25+ and HLA-DR+ expressing lymphocytes in patients undergoing repeated hyperbaric hyperoxia therapy. While some authors reported changes of circulating CD4+/CD8+ lymphocyte absolute counts and ratio [5, 6] after a single hyperbaric hyperoxia challenge, this is likely transient as 24 h after exposure they found a partial reversal to normal values.