Year: 2007
Type: review
Abstract: Cytokine production by the immune system contributes importantly to both health and disease.The nervous system, via an inflammatory reflex of the vagus nerve, can inhibit cytokine release and thereby prevent tissue injury and death.The efferent neural signaling pathway is termed the cholinergic antiinflammatory pathway.Cholinergic agonists inhibit cytokine synthesis and protect against cytokine-mediated diseases.Stimulation of the vagus nerve prevents the damaging effects of cytokine release in experimental sepsis, endotoxemia, ischemia/reperfusion injury, hemorrhagic shock, arthritis, and other inflammatory syndromes.Herein is a review of this physiological, functional anatomical mechanism for neurological regulation of cytokine-dependent disease that begins to define an immunological homunculus.The germ is nothing; the terrain is everything. -Louis Pasteur (on his deathbed)Historical perspective of the cytokine theory of disease For more than 1,500 years, the predominant medical doctrine of disease causation was the humoral theory of disease.As posited by Galen (Claudius Galenus of Pergamum, 129-200 AD), this theory held that disease was caused by an imbalance in bodily humors.Produced by internal organs, the humors mediated healthful effects when their levels were properly regulated and balanced.This theory was not displaced until advances in microscopy and microbiology enabled Rudolf Virchow to formulate the theory of cellular pathology and Louis Pasteur the germ theory of disease.Research into the causes of illness turned outward as a growing body of investigation focused on exogenous rather than endogenous factors in disease causation.Pasteur's deathbed remark, however, belied the notion that germs could tell the whole story.It was a tacit acknowledgment to Claude Bernard, who advocated that health was due to equilibrium in the "milieu interieur" and maintained by "a continuous and delicate compensation, established as by the most sensitive of balances" (Lessons on the phenomena of life common to animals and vegetables).Common wisdom in the 1970s dictated that the immune response to pathogens or injury was uniformly for the good.Every facet of the immune response was believed to have a role (albeit some unknown) in eradicating invaders and neutralizing the potential for damage.Certainly, or so the reasoning went, the body would not use its own cells to make potentially toxic molecules in quantities large enough to injure tissues or even kill.The development of shock, tissue injury, circulatory collapse, hemorrhagic necrosis, multiple organ failure, and other physiological calamities associated with bacterial infection were attributed to the direct action of pathogenic toxins, not the host response.A few dared to challenge this view, notably Lewis Thomas, who wrote in his classic 1975 book The lives of a cell, "When we sense lipopolysaccharide, we are likely to turn on every defense at our disposal; we will bomb, defoliate, blockade, seal off, and destroy all the tissues in the area.All of this seems unnecessary, panicdriven . . .The self-disintegration of the whole animal that follows a systemic injection can be interpreted as a well-intentioned but lethal error.The mechanism is itself quite a good one, when used with precision and restraint (1)."Scientific advances would soon prove his statement to be unerringly prescient.Early investigators exploring physiological mechanisms of fever, weight loss, and acute phase responses to acute and chronic infection led the way in changing beliefs about disease pathogenesis.They gave evidence that proteins produced by macrophages and other immune cells, not pathogens, caused tissue damage and disease syndromes in experimental animals.Studies in the 1980s of these proteins, newly named cytokines (including, for example, IL-1, IL-6, and TNF), revealed that they possessed pleiotropic biological activities that were either beneficial or injurious.From this emerged the cytokine theory of disease, the concept that cytokines produced by the immune system can cause the signs, symptoms, and damaging aftereffects of disease (Figure 1) (reviewed in ref. 2).Consider as one example the case of TNF, a cytokine implicated as a necessary and sufficient mediator of lethal septic shock caused by infection with Gram-negative bacteria.TNF is produced during Gram-negative bacteria-induced acute septic shock (3).Administration of TNF to healthy mammals (including humans) reproduces the hemodynamic, metabolic, immunological, and pathological manifestations of the disease (4), and gene knockout or pharmacological blockade of TNF activity prevents the development of lethal septic shock (5).Other pathophysiological activities ascribed to TNF were the capacity to cause fever and localized inflammation (4, 6).Clinical successes in the 1990s using drugs that specifically inhibit TNF for patients with inflammatory bowel disease or rheumatoid arthritis directly implicated a pathogenic role of this cytokine in other diseases (7-9) and validated in humans the fundamental premise of the cytokine theory of disease.Today, drugs that specifically inhibit or neutralize endogenous cytokines are in widespread use.Anti-TNF-and anti-IL-1-based therapeutics have been administered to millions of patients, many of whom enjoy a significantly improved quality of life that was simply not achievable with other treatments.Dose-response and safety studies have defined the therapeutic indexes for these Nonstandard abbreviations used: ACTH1-24, adrenocorticotropic hormone; HMGB1, high-mobility group box 1; nAChR, nicotinic AChR.
DOI: 10.1172/jci30555
Author: Kevin J. Tracey
Institution: Feinstein Institute for Medical Research
Language: English
Subfield: Neurology
Field: Neuroscience
Domain: Life Sciences
Open Access status: bronze
OpenAlex ID: W2002797282