Krog J, Tønnesen EK, Jepsen CF, Parner E, Segadal K, Hope A, Ulvik RJ, Hokland ME. Natural killer cells as biomarkers of hyperbaric stress during a dry heliox saturation dive. Aviat Space Environ Med 2010; 81:467–74. Introduction: Diving, hyperbaric oxygen, and decompression have been described as inducers of alterations in various components of the human immune system, such as the distribution of circulating lymphocytes. Hypothetically, the monitoring of specific lymphocyte subsets during hyperbaric exposure, including T- and NK-cell subsets, can serve as biomarkers of hyperbaric stress. Methods: Eight experienced saturation divers and eight reference subjects, naive to deep saturation diving, were examined. Peripheral blood mononuclear cells were isolated before and at different points during a 19.3-d dry heliox saturation dive to 2.64 MPa (254 msw). The NK cell cytotoxicity was estimated in a 4-h ⁵¹Cr-release assay using the NK cell sensitive tumor cell-line K562 as target cells. The major lymphocyte subpopulations, with special emphasis on the NK cell subsets, were phenotypically delineated by the use of 4-color flow cytometry. Results: Although NK cell cytotoxicity increased significantly in the divers during the compression phase and the reference subjects who remained in normoxic conditions outside the chamber, the NK cell cytotoxicity was significantly higher in the divers. Discussion: This finding, together with augmentation in the absolute number of circulating NK cells in the divers due to a possible activation of specific parts of the innate cellular immune system during hyperbaric exposure, suggests the monitoring of specific immune functions can be useful as biomarkers of hyperbaric-induced inflammatory stress.