Um circuito Neuro-Hepático modula a atividade de macrófagos residentes no fígado

Abstract

The gastrointestinal (GI) tract is exposed to a large population of commensal microorganisms and plays a pivotal barrier and immunological role against pathogenic agents such as invasive bacteria, toxins and other pathogens. In parallel, GI tract orchestrates physiological functions in nutrient absorption, metabolism and excretion. This potential conflict is balanced by the action of neural circuits which monitor the GI lumen and mount proper reflexes to counter-act inflammatory responses and promote tissue tolerance. This study investigated the impact of the activation of such neural circuits in the liver, a key organ in the interface between GI’s immunesurveilance system and the metabolism of the organism. Following the electrical stimulation of the vagus nerve (VNS), a major nerve to GI organs, liver resident macrophages (Kupffer cells) presented morphological alterations, including membrane extensions and increased surface, characteristic of macrophage activation. Using in vivo imaging, we showed an increase in the phagocytosis of E.coligfp by Kupffer cells after VNS that was abolished by vagotomy. This result was confirmed by using colloidal 99Tc-Tin and scintillography. Additionally, VNS did not modulate IL-10, TNF-α and IL-6 levels. To address whether this was a direct effect on the liver, we submitted mice to celiac ganglion removal (CGX), splenectomy (SPX) or bilateral adrenalectomy (ADX) followed or not by VNS. We observed that the celiac ganglion, spleen and adrenal glands are not crucial for the activation of the vagal circuit in the liver, although they partly modulate this response. This direct effet was corroborated by the finding of vagus nerve fibers (stained with neurotracker DiO 488) in close contact with Kupffer cells. As acetylcholine is the main neurotransmitter associated to efferent vagus nerve fibers, we pharmacologically inhibited the cholinergic signaling pathway by using mecamylamine and methyl atropine (i.v. 1mg/Kg), which are antagonists of nicotinic and muscarinic acetylcholine receptors respectively. We observed a decrease in E.coligfp phagocytosis of Kupffer cells in mice treated with the drugs when compared with control animals. To corroborate these findings, we used an animal model of cholinergic signaling deficiency (VAChT KD hom). Compared to WT mice, Kupffer cells from VAChT KDhom animals showed reduced ability to phagocytize bacteria. The functionality of the neuroimmune reflex described in the study during an immune challenge was addressed in a non-lethal sepsis model (CLP), on which VNS resulted in a decrease in bacterial colony formation in the livers of mice submitted to CLP and induced the expression of Il-10 and Il-6, with no changes inTNFa levels. Taken together our results point to a new brain-liver circuit that regulates Kupffer cells phagocytic and secretory activities, promoting a protective, anti-inflammatory and proregenerative microenvironment within the liver.