Human macrophages chronically exposed to LPS can be reactivated by stimulation with MDP to acquire an antimicrobial phenotype

Macrophages are important in host defense and can differentiate into functionally distinct subsets named classically (M1) or alternatively (M2) activated. In several inflammatory disorders, macrophages become tolerized to prevent deleterious consequences. This tolerization reduces the ability of macrophages to respond to bacterial components (e.g., LPS) maintaining a low level of inflammation but compromising the ability of macrophages to mount an effective immune response during subsequent pathogen encounters. In this study, we aimed to reactivate human monocyte-derived macrophages chronically exposed to LPS by re-stimulation with muramyl dipeptide (MDP). We observed an undefined profile of cell surface marker expression during endotoxin tolerance and absence of TNF alpha production. Stimulating macrophages chronically exposed to LPS with LPS + MDP restored TNFa, production together with an increased production of IL1, IL6, IFN gamma, IL4, IL5 and IL10. These results suggest that macrophages chronically exposed to LPS possess a mixed M1-M2 phenotype with sufficient antimicrobial and homeostatic potential. (C) 2017 Elsevier Inc. All rights reserved.