High mobility group 1 (HMGB1) protein is both a nuclear factor and a secreted protein. In the cell nucleus it acts as an architectural chromatin-binding factor that bends DNA and promotes protein assembly on specific DNA targets. Outside the cell, it binds with high affinity to RAGE (the receptor for advanced glycation end products) and is a potent mediator of inflammation. HMGB1 is secreted by activated monocytes and macrophages, and is passively released by necrotic or damaged cells. Here we report that Hmgb1(-/-) necrotic cells have a greatly reduced ability to promote inflammation, which proves that the release of HMGB1 can signal the demise of a cell to its neighbours. Apoptotic cells do not release HMGB1 even after undergoing secondary necrosis and partial autolysis, and thus fail to promote inflammation even if not cleared promptly by phagocytic cells. In apoptotic cells, HMGB1 is bound firmly to chromatin because of generalized underacetylation of histone and is released in the extracellular medium (promoting inflammation) if chromatin deacetylation is prevented. Thus, cells undergoing apoptosis are programmed to withhold the signal that is broadcast by cells that have been damaged or killed by trauma.

Release of chromatin protein HMGB1 by necrotic cells triggers inflammation

Sepsis is the leading cause of death in critically ill patients in the United States. Yet the individual host response to septicemia is variable, depending on the patient's immune response, age, nutritional status, and coexisting conditions, as well as on the virulence of the organism and the size of the inoculum. This review examines evolving concepts of sepsis and discusses new and potential therapies. Recent clinical advances include therapy with activated protein C, stringent control of blood glucose, and early goal-directed therapy to treat cellular oxygen deficit. Future therapies may be focused on modulating the immune response in the light of the characteristics of the specific pathogen, the genetic profile of the patient, and the duration of the disease.

https://www.nejm.org/doi/pdf/10.1056/NEJMra021333

The pathophysiology and treatment of sepsis.

Vertebrates achieve internal homeostasis during infection or injury by balancing the activities of proinflammatory and anti-inflammatory pathways. Endotoxin (lipopolysaccharide), produced by all gram-negative bacteria, activates macrophages to release cytokines that are potentially lethal. The central nervous system regulates systemic inflammatory responses to endotoxin through humoral mechanisms. Activation of afferent vagus nerve fibres by endotoxin or cytokines stimulates hypothalamic-pituitary-adrenal anti-inflammatory responses. However, comparatively little is known about the role of efferent vagus nerve signalling in modulating inflammation. Here, we describe a previously unrecognized, parasympathetic anti-inflammatory pathway by which the brain modulates systemic inflammatory responses to endotoxin. Acetylcholine, the principle vagal neurotransmitter, significantly attenuated the release of cytokines (tumour necrosis factor (TNF), interleukin (IL)-1beta, IL-6 and IL-18), but not the anti-inflammatory cytokine IL-10, in lipopolysaccharide-stimulated human macrophage cultures. Direct electrical stimulation of the peripheral vagus nerve in vivo during lethal endotoxaemia in rats inhibited TNF synthesis in liver, attenuated peak serum TNF amounts, and prevented the development of shock.

Vagus nerve stimulation attenuates the systemic inflammatory response to endotoxin

Inflammation is a local, protective response to microbial invasion or injury. It must be fine-tuned and regulated precisely, because deficiencies or excesses of the inflammatory response cause morbidity and shorten lifespan. The discovery that cholinergic neurons inhibit acute inflammation has qualitatively expanded our understanding of how the nervous system modulates immune responses. The nervous system reflexively regulates the inflammatory response in real time, just as it controls heart rate and other vital functions. The opportunity now exists to apply this insight to the treatment of inflammation through selective and reversible 'hard-wired' neural systems.

The inflammatory reflex

Excessive inflammation and tumour-necrosis factor (TNF) synthesis cause morbidity and mortality in diverse human diseases including endotoxaemia, sepsis, rheumatoid arthritis and inflammatory bowel disease. Highly conserved, endogenous mechanisms normally regulate the magnitude of innate immune responses and prevent excessive inflammation. The nervous system, through the vagus nerve, can inhibit significantly and rapidly the release of macrophage TNF, and attenuate systemic inflammatory responses. This physiological mechanism, termed the 'cholinergic anti-inflammatory pathway' has major implications in immunology and in therapeutics; however, the identity of the essential macrophage acetylcholine-mediated (cholinergic) receptor that responds to vagus nerve signals was previously unknown. Here we report that the nicotinic acetylcholine receptor alpha7 subunit is required for acetylcholine inhibition of macrophage TNF release. Electrical stimulation of the vagus nerve inhibits TNF synthesis in wild-type mice, but fails to inhibit TNF synthesis in alpha7-deficient mice. Thus, the nicotinic acetylcholine receptor alpha7 subunit is essential for inhibiting cytokine synthesis by the cholinergic anti-inflammatory pathway.

Nicotinic acetylcholine receptor alpha7 subunit is an essential regulator of inflammation.

Endotoxin, a constituent of Gram-negative bacteria, stimulates macrophages to release large quantities of tumor necrosis factor (TNF) and interleukin-1 (IL-1), which can precipitate tissue injury and lethal shock (endotoxemia). Antagonists of TNF and IL-1 have shown limited efficacy in clinical trials, possibly because these cytokines are early mediators in pathogenesis. Here a potential late mediator of lethality is identified and characterized in a mouse model. High mobility group-1 (HMG-1) protein was found to be released by cultured macrophages more than 8 hours after stimulation with endotoxin, TNF, or IL-1. Mice showed increased serum levels of HMG-1 from 8 to 32 hours after endotoxin exposure. Delayed administration of antibodies to HMG-1 attenuated endotoxin lethality in mice, and administration of HMG-1 itself was lethal. Septic patients who succumbed to infection had increased serum HMG-1 levels, suggesting that this protein warrants investigation as a therapeutic target.

https://science.sciencemag.org/content/sci/285/5425/248.full.pdf

HMG-1 as a Late Mediator of Endotoxin Lethality in Mice

Activation of the innate immune system, even by relatively innocuous stimuli, stimulates the release of cytokines (e.g. TNF) that can injure or kill the host. To maintain homeostasis, mammals have evolved a counter-regulatory response that suppresses the development of excessively robust inflammation. Fetuin, a 66-kD negative acute phase glycoprotein, was first identified in 1944. We recently discovered an anti-inflammatory role for fetuin, because it suppressed the release of TNF from lipopolysaccharide- (LPS) stimulated macrophages. Here the anti-inflammatory effects of fetuin were studied in vivo in an LPS-independent model of acute inflammation caused by administration of carrageenan. Administration of fetuin (5-500 mg/kg intraperitoneally) dose-dependently attenuated the development of paw edema as compared to either asialofetuin (500 mg/kg) or bovine albumin (500 mg/kg). TNF production in the carrageenan-injected paws was significantly inhibited by administration of fetuin (586+/-98 pg TNF/paw) as compared to either asialofetuin (1018+/-186 pg TNF/paw) or saline (1,005+/-172 pg TNF/paw). When specific anti-fetuin IgG was administered into the paw prior to the application of carrageenan, the development of edema formation was significantly increased as compared to irrelevant IgG, indicating that endogenous fetuin normally attenuates the inflammatory response. These results now reveal a previously unrecognized anti-inflammatory role of fetuin in counter-regulating the innate immune response, and suggest that it may be possible to use fetuin as an experimental anti-inflammatory agent.

Asia Frazier

Papers

HMG-1 as a Late Mediator of Endotoxin Lethality in Mice

Fetuin, a negative acute phase protein, attenuates TNF synthesis and the innate inflammatory response to carrageenan.

High mobility group-1 (hmg-1) protein is a mediator of lethal endotoxemia