Oxidative burst and Dectin-1 triggered phagocytosis affected by Norepinephrine and Endocannabinoids - Implications for fungal clearance under stress

Abstract

A prolonged stress burden is known to hamper the efficiency of both the innate and adaptive immune systems and to attenuate the stress responses by the catecholaminergic and endocannabinoid (EC) systems. Key mechanisms of innate immunity are the eradication of pathogens through phagocytosis and the respiratory burst. We tested the concentration-dependent, spontaneous and stimulated (via TNFα and fMLP) release of reactive oxygen species (ROS) by human PMNs in vitro in response to norepinephrine (NE) and AM1241, a pharmacological ligand for the endocannabinoid receptor CB₂. We evaluated phagocytosis of Dectin-1 ligating zymosan particles and tested the cytokine response against Candida antigen in an in vitro cytokine release assay. Increasing concentrations of NE did not affect phagocytosis, yet stimulated ROS release was attenuated gradually reaching maximum suppression at 500nM. Adrenergic receptor (AR) mechanisms using non-AR-selective (labetalol) as well as specific α–(prazosin) and β–(propranolol) receptor antagonists were tested. Results show that only labetalol and propranolol were capable to recuperate cytotoxicity in the presence of NE, evidencing a β-receptor mediated effect. The CB₂ agonist, AM1241, inhibited phagocytosis at 10μM and spontaneous peroxide release by PMNs. Use of the inverse CB₂ receptor agonist SR144528 led to partial recuperation of ROS production, confirming the functional role of CB₂. Additionally, AM1241 delayed early activation of monocytes and induced suppression of IL-2 and IL-6 levels in response to Candida via lower activity of mTOR. These findings provide new insights into key mechanisms of innate immunity under stressful conditions where ligands to the sympatho-adrenergic and endocannabinoid system are released.