In vitro dexamethasone suppression of cytokine production assay and plasma total and free cortisol responses to ACTH in healthy volunteers

Glucocorticoid sensitivity varies between individuals, influencing stress and inflammatory responses, however its relationship with the synthetic ACTH [1-24] stimulation test, is not well evaluated. To evaluate glucocorticoid sensitivity and plasma total cortisol (TC) and plasma free cortisol (PFC) responses to stimulation with 250 μg of synthetic ACTH [1-24]) in healthy adults. Prospective, single-centre observational study. Forty-eight healthy adults (24 females, 24 males) underwent basal and stimulated TC and PFC level measurements using liquid chromatography-tandem mass spectrometry. Peak cortisol values were defined as the highest TC or PFC at either 30- or 60-min. Glucocorticoid sensitivity was measured using the dexamethasone suppression of cytokine production (DSCP) assay, based on interleukin-6 (IL-6) and tumour necrosis factor-α (TNF-α) production from lipopolysaccharide-stimulated (LPS) monocytes. A whole blood sample was incubated with buffer, LPS, or LPS with dexamethasone (Dex). TNF-α and IL-6 concentrations were measured in the supernatants (ng/L). The ratio of cytokine levels between LPS + Dex and LPS indicated glucocorticoid sensitivity. Data from 45 participants were analysed. Peak TC levels ranged from 391 to 1570 nmol/L (median: 618 nmol/L, IQR: 555-729), while PFC levels ranged from 27 to 133 nmol/L (median: 62 nmol/L, IQR: 53-73). No significant associations were found between baseline or peak TC and PFC levels and glucocorticoid sensitivity ratios. Synthetic ACTH (1-24) test results did not predict glucocorticoid sensitivity. Males exhibited higher BMI (median: 26 vs. 22; p = 0.031), however no significant differences were observed in age, baseline PFC and TC levels between sexes. In this exploratory study, ACTH [1-24]-stimulated cortisol responses did not correlate with glucocorticoid sensitivity measured by the DSCP assay. Findings should be interpreted as preliminary and hypothesis-generating given current assay limitations.