The effect of free and carrier-bound cortisol on equine neutrophil function

Cortisol is a key anti-inflammatory hormone that increases in bacterial sepsis and circulates predominantly bound to cortisol binding globulin (CBG). Only unbound cortisol was believed to be biologically active, but recent evidence suggests that CBG-bound cortisol also regulates inflammation. The objective of this study was to evaluate the effects of free and CBG-bound cortisol on equine neutrophil function ex vivo. We hypothesized that CBG would enhance cortisol-mediated suppression of neutrophil pro-inflammatory responses. Neutrophils isolated from 8 foals and 6 adult horses were exposed to Staphylococcus aureus antigen (SAA) alone and with hydrocortisone (HC), CBG, or both (CBG + HC). Inflammatory cytokine (TNF-α, IL-8) and reactive oxygen species (ROS) production were measured and compared among stimulants and between ages with linear mixed-effects models. CBG and CBG + HC inhibited ROS production induced by SAA in both foal and horse neutrophils, maintaining it at levels comparable to baseline production (P ≤ 0.060-0.907). TNF-α production was not significantly different among stimulants (P = 0.284). CBG + HC significantly (P ≤ 0.016) increased IL-8 production by neutrophils in response to SAA in both foals and adults, although the response of foals was significantly greater than that of adults (P < 0.001). These findings suggest that CBG directly modulates equine neutrophil responses, but the effects are cytokine- and age-specific.