Activation of the stress response in macrophages alters the M1/M2 balance by enhancing bacterial killing and IL-10 expression.

UNLABELLED: Macrophages (Mϕs) play an important role in the inflammatory response during injury by participating in the removal of injurious stimuli, such as bacteria, and promoting tissue healing to restore homeostasis. Mϕs can acquire distinct functional phenotypes along a spectrum between two opposite stages (M1/M2) during activation. In the present study, we induced a stress response in Mϕs via heat shock (HS) and found that it incurred an increase in phagocytosis (1.6-fold, P < 0.05) and bacterial killing (2.8-fold, P < 0.01). Upon heat stress activation, Mϕs respond to group B Streptococcus (GBS) infection with lower levels of pro-inflammatory cytokines, TNF-α (2.25-fold, P < 0.01), IL-6 (7-fold, P < 0.001), and inducible nitric oxide synthase (iNOS) (2.22-fold, P < 0.05), but higher levels of the anti-inflammatory cytokine IL-10 (3.9-fold, P < 0.01). Stressed Mϕs exposed to GBS display rapid phagosome maturation, increased extracellular trap (ET) formation and elevated cathelicidin antimicrobial peptide expression (2.5-fold, P < 0.001). These findings are consistent with a heretofore uncharacterized Mϕ activation state formed in response to stress, associated with secretion of large quantities of anti-inflammatory mediators and redirection of antimicrobial mechanisms to NADPH-oxidase-independent pathways. This "friendly activation" of Mϕs is characterized by increased bactericidal activity and more rapid and controlled resolution of the inflammatory response. KEY MESSAGES: Macrophages form a dual pro-bactericidal and anti-inflammatory state. Stress in the setting of infection triggers friendly activation in macrophages. Heat shock plus infection increases macrophage bactericidal activity. Heat shock plus infection increases macrophage extracellular trap formation. Heat shock plus infection increases macrophage production of cathelicidin and IL-10.