Peripheral blood mononuclear cell proliferation and cytokine production in sheep as affected by cortisol level and duration of stress

A large number of studies recognize glucocorticoids (Gc) as suppressors of inflammation; Gc exert an important role in coordinating the magnitude and duration of host immune responses. In the present in vitro investigation, we tested incremental levels of cortisol to verify the immunosuppressive or immunopermissive role of cortisol in sheep peripheral blood mononuclear cells (PBMC) after acute and chronic stress. Phytohemagglutinin (PHA)-stimulated PBMC were cultured for 24 h and 96 h at 37°C with 5% of CO2 and varying cortisol levels: 10 ng/mL (baseline), 100 ng/mL (physiological poststressor), and 1,000 ng/mL [hyperactivated hypothalamic-pituitary-adrenal (HPA) axis]. The cell-free supernatants were collected for determination of IL-6, IL-1β, and IL-10 by ELISA, and the bromodeoxyuridine assay was performed on cells. Physiological cortisol concentration negatively affected the levels of IL-6 secreted by PBMC, resulting in increased cell proliferation after acute stress (24 h of incubation). However, physiological cortisol concentration exhibited a reduction in cell proliferation induced by increased levels of IL-6 secreted by PBMC during chronic stress (96 h of incubation). The cortisol concentration representing a hyperactivated HPA axis led to a reduction in cell proliferation after acute stress, which was probably induced by the elevated IL-10 production. Our results demonstrate that in sheep the effect of Gc on the immune system was related to the magnitude and the duration of stress. In particular, cortisol levels higher than physiological concentrations suppressed cell proliferation soon after acute stress. Instead, the physiological poststressor concentration of cortisol affected the immune responses in a bidirectional manner depending on the duration of the stressor.