Interactions between the immune and nervous systems play an important role in modulating host susceptibility and resistance to inflammatory disease. Neuroendocrine regulation of inflammatory and immune responses and disease occurs at multiple levels: systemically, through the anti-inflammatory action of glucocorticoids released via hypothalamic-pituitary-adrenal axis stimulation; regionally, through production of glucocorticoids within and sympathetic innervation of immune organs such as the thymus; locally, at sites of inflammation. Estrogens also play an important role in immune modulation, and contribute to the approximately 2- to 10-fold higher incidence of autoimmune/inflammatory diseases seen in females of all mammalian species. During inflammation, cytokines from the periphery activate the central nervous system through multiple routes. This results in stimulation of the hypothalamic-pituitary-adrenal axis which, in turn through the immunosuppressive effects of the glucocorticoids, generally inhibits inflammation. Recent studies indicate that physiological levels of glucocorticoids are immunomodulatory rather than solely immunosuppressive, causing a shift in patterns of cytokine production from a TH1- to a TH2-type pattern. Interruptions of this loop at any level and through multiple mechanisms, whether genetic, or through surgical or pharmacological interventions, can render an inflammatory resistant host susceptible to inflammatory disease. Over-activation of this axis, as occurs during stress, can also affect severity of infectious disease through the immunosuppressive effects of the glucocorticoids. These interactions have been clearly demonstrated in many animal models, across species, strains and diseases, and are also relevant to human inflammatory, autoimmune and allergic illnesses, including rheumatoid arthritis, systemic lupus erythematosus, Sjogren's syndrome, allergic asthma and atopic skin disease. While many genes and environmental factors contribute to susceptibility and resistance to autoimmune/inflammatory diseases, a full understanding of the molecular effects on immune responses of combinations of neuropeptides, neurohormones and neurotransmitters at all levels has opened up new therapeutic approaches and are essential for the design of future therapies based on such principles.
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- Author: EM Sternberg x
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JI Webster and EM Sternberg
The hypothalamic-pituitary-adrenal (HPA) axis is activated during many bacterial and viral infections, resulting in an increase in circulating glucocorticoid levels. This HPA axis activation and glucocorticoid response are critical for the survival of the host, as demonstrated by the fact that removal of the HPA axis (by adrenalectomy or hypophysectomy) or glucocorticoid receptor (GR) blockade enhances the severity of the infection and in some cases enhances the mortality rate. Replacement with a synthetic glucocorticoid reverses these effects by reducing the severity of the infection and provides protection against lethal effects. In addition, some bacteria and viral infections have been shown to affect the GR directly. These have been described and the implications of such an effect discussed.