Total lymphocyte CD8 expression is not a reliable marker of cytotoxic T-cell populations in human peripheral blood following an acute bout of high-intensity exercise

Cytotoxic T-lymphocytes co-express the T-cell receptor, CD3 and the MHC I restricted antigen CD8. Although total CD8 expression is often used to identify CD8+ T-cells in blood, errors are associated with this method as some CD3 negative natural killer (NK)-cells are known to express CD8. As greater relative proportions of NK-cells are found in the blood compartment after exercise, these errors are likely to be amplified in post exercise blood samples. To test this, isolated blood lymphocytes obtained from aerobically trained male subjects before, immediately after and 1 h after an exhaustive treadmill-running protocol were surface stained for CD3, CD4, CD8, CD16, and CD56 and analysed by multi-colour flow cytometry. It was found that 25.4 ± 16.9% of all CD8+ cells at rest were CD3 negative, CD8dim+ and expressed the NK-cell markers CD16 and CD56. The magnitude of this error increased to 40.8 ± 20.7% immediately after exercise due to an influx of CD8dim+ NK-cells. Although all CD8bright+ cells expressed CD3, gating around the CD8bright+ cells only identified 79.2 ± 8.7% of the total CD3+/CD8+ T-cell population; however, the magnitude of this error did not change after exercise despite the altered proportions of CD8bright+ and CD8dim+ cells. In conclusion, total lymphocyte expression of CD8 should not be used as a single antigenic marker to identify CD8+ T-cells after an acute bout of exercise. Although there are errors associated with using CD8bright+ as a single antigenic marker to identify CD3+ T-cells, these are not amplified in response to exercise