Psychoneuroimmunology (PNI), also referred to as psychoendoneuroimmunology (PENI) or psychoneuroendocrinoimmunology (PNEI), is the study of the interaction between psychological processes and the nervous and immune systems of the human body. It is a subfield of psychosomatic medicine. PNI takes an interdisciplinary approach, incorporating psychology, neuroscience, immunology, physiology, genetics, pharmacology, molecular biology, psychiatry, behavioral medicine, infectious diseases, endocrinology, and rheumatology.
The main interests of PNI are the interactions between the nervous and immune systems and the relationships between mental processes and health. PNI studies, among other things, the physiological functioning of the neuroimmune system in health and disease; disorders of the neuroimmune system (autoimmune diseases; hypersensitivities; immune deficiency); and the physical, chemical and physiological characteristics of the components of the neuroimmune system in vitro, in situ, and in vivo.
History Mid-20th century studies of psychiatric patients reported immune alterations in psychotic individuals, including lower numbers of lymphocytes and poorer antibody response to pertussis vaccination, compared with nonpsychiatric control subjects. In 1964, George F. Solomon, from the University of California in Los Angeles, and his research team coined the term "psychoimmunology" and published a landmark paper: "Emotions, immunity, and disease: a speculative theoretical integration."
Origins In 1975, Robert Ader and Nicholas Cohen, at the University of Rochester, advanced PNI with their demonstration of classic conditioning of immune function, and they subsequently coined the term "psychoneuroimmunology". Ader was investigating how long conditioned responses (in the sense of Pavlov's conditioning of dogs to drool when they heard a bell ring) might last in laboratory rats. To condition the rats, he used a combination of saccharin-laced water (the conditioned stimulus) and the drug Cytoxan, which unconditionally induces nausea and taste aversion and suppression of immune function. Ader was surprised to discover that after conditioning, just feeding the rats saccharin-laced water was associated with the death of some animals and he proposed that they had been immunosuppressed after receiving the conditioned stimulus. Ader (a psychologist) and Cohen (an immunologist) directly tested this hypothesis by deliberately immunizing conditioned and unconditioned animals, exposing these and other control groups to the conditioned taste stimulus, and then measuring the amount of antibody produced. The highly reproducible results revealed that conditioned rats exposed to the conditioned stimulus were indeed immunosuppressed. In other words, a signal via the nervous system (taste) was affecting immune function. This was one of the first scientific experiments that demonstrated that the nervous system can affect the immune system.
In the 1970s, Hugo Besedovsky, Adriana del Rey and Ernst Sorkin, working in Switzerland, reported multi-directional immune-neuro-endocrine interactions, since they show that not only the brain can influence immune processes but also the immune response itself can affect the brain and neuroendocrine mechanisms. They found that the immune responses to innocuous antigens triggers an increase in the activity of hypothalamic neurons and hormonal and autonomic nerve responses that are relevant for immunoregulation and are integrated at brain levels (see review). On these bases, they proposed that the immune system acts as a sensorial receptor organ that, besides its peripheral effects, can communicate to the brain and associated neuro-endocrine structures its state of activity. These investigators also identified products from immune cells, later characterized as cytokines, that mediate this immune-brain communication (more references in).
In 1981, David L. Felten, then working at the Indiana University School of Medicine, and his colleague JM Williams, discovered a network of nerves leading to blood vessels as well as cells of the immune system. The researchers also found nerves in the thymus and spleen terminating near clusters of lymphocytes, macrophages, and mast cells, all of which help control immune function. This discovery provided one of the first indications of how neuro-immune interaction occurs.
Ader, Cohen, and Felten went on to edit the groundbreaking book Psychoneuroimmunology in 1981, which laid out the underlying premise that the brain and immune system represent a single, integrated system of defense.
In 1985, research by neuropharmacologist Candace Pert, of the National Institutes of Health at Georgetown University, revealed that neuropeptide-specific receptors are present on the cell walls of both the brain and the immune system. The discovery that neuropeptides and neurotransmitters act directly upon the immune system shows their close association with emotions and suggests mechanisms through which emotions, from the limbic system, and immunology are deeply interdependent. Showing that the immune and endocrine systems are modulated not only by the brain but also by the central nervous system itself affected the understanding of emotions, as well as disease.
Contemporary advances in psychiatry, immunology, neurology, and other integrated disciplines of medicine has fostered enormous growth for PNI. The mechanisms underlying behaviorally induced alterations of immune function, and immune alterations inducing behavioral changes, are likely to have clinical and therapeutic implications that will not be fully appreciated until more is known about the extent of these interrelationships in normal and pathophysiological states.
The immune-brain loop
PNI research looks for the exact mechanisms by which specific neuroimmune effects are achieved. Evidence for nervous-immunological interactions exist at multiple biological levels.
The immune system and the brain communicate through signaling pathways. The brain and the immune system are the two major adaptive systems of the body. Two major pathways are involved in this cross-talk: the Hypothalamic-pituitary-adrenal axis (HPA axis), and the sympathetic nervous system (SNS), via the sympathetic-adrenal-medullary system (SAM system). The activation of SNS during an immune response might be aimed to localize the inflammatory response.
The body's primary stress management system is the HPA axis. The HPA axis responds to physical and mental challenge to maintain homeostasis in part by controlling the body's cortisol level. Dysregulation of the HPA axis is implicated in numerous stress-related diseases, with evidence from meta-analyses indicating that different types/duration of stressors and unique personal variables can shape the HPA response.
The major hormones involved in the HPA axis are listed below, along with their functions.
Corticotropin-releasing hormone (CRH): has been proven to aid in regulating the behavioral, autonomic, endocrine, reproductive, cardiovascular, gastrointestinal, and metabolic functions of both the central and peripheral nervous systems. CRH also exhibits immunosuppressive effects. Adrenocorticotropic hormone (ACTH): triggers the adrenal glands to release cortisol and sex hormones, such as androgens. ACTH also plays an indirect role in the stimulation of epinephrine and norepinephrine. Cortisol: typically known as the "stress hormone", regulates the body's stress response and controls the body's metabolism and has been shown to suppress inflammation, regulate blood pressure, and regulate blood sugar. Aldosterone (ALD): controls the levels of sodium and potassium in the blood to regulate blood pressure and blood volume. ALD also plays an indirect role in regulating the pH of the blood as well as electrolyte levels. Dehydroepiandrosterone (DHEA): stimulates production of other hormones, such as testosterone and estrogen. DHEA peaks in early adulthood and decreases with age. Antidiuretic hormone (ADH): causes the kidneys to retain more water, which decreases the amount of urine that is produced). Thyrotropin-releasing hormone (TRH): promotes thyrotropin, prolactin, and growth hormone release. TRH is also known to inhibit somatostatin release. T3 & T4: T4, commonly referred to as thyroxine, is a precursor for the primary thyroid hormone, which is T3. T4 is typically inactive, meaning it does not impact cells. However, T3 is considered active and directly impacts metabolism.
Molecules called pro-inflammatory cytokines, which include interleukin-1 (IL-1), Interleukin-2 (IL-2), interleukin-6 (IL-6), Interleukin-12 (IL-12), Interferon-gamma (IFN-Gamma) and tumor necrosis factor alpha (TNF-alpha) can affect brain growth as well as neuronal function. Circulating immune cells such as macrophages, as well as glial cells (microglia and astrocytes) secrete these molecules. Cytokine regulation of hypothalamic function is an active area of research for the treatment of anxiety-related disorders.
HPA axis activity and cytokines are intrinsically intertwined: inflammatory cytokines stimulate adrenocorticotropic hormone (ACTH) and cortisol secretion, while, in turn, glucocorticoids suppress the synthesis of pro-inflammatory cytokines. Cytokines mediate and control immune and inflammatory responses. Complex interactions exist between cytokines, inflammation and the adaptive responses in maintaining homeostasis. Like the stress response, the inflammatory reaction is crucial for survival. Systemic inflammatory reaction results in stimulation of four major programs:
the acute-phase reaction: a natural reaction against infection or injury that typically involves fever, leukocytosis, increased vascular permeability, and increased serum levels of acute phase proteins. sickness behavior: a set of behavioral changes that individuals will undergo during the course of an infection, which is caused by the effects of pro-inflammatory cytokines on the brain. Examples of pro-inflammatory cytokines include IL-1 and TNF-α. the pain program: explains how cytokines contribute to pain. There are two classes of cytokines related to pain: pain-promoting and pain-reducing. Cytokines that are considered to serve pain-promoting functions are TNF-α, IL-1β, IL-6, and IFN-γ. Cytokines that are considered to serve pain-reducing functions are IL-10 and TGF-β. the stress response
These are mediated by the HPA axis and the SNS. Common human diseases such as allergy, autoimmunity, chronic infections and sepsis are characterized by a dysregulation of the pro-inflammatory versus anti-inflammatory and T helper (Th1) versus (Th2) cytokine balance. Recent studies show pro-inflammatory cytokine processes take place during depression, mania and bipolar disease, in addition to autoimmune hypersensitivity and chronic infections.
Chronic secretion of stress hormones, glucocorticoids (GCs) and catecholamines (CAs), as a result of disease, may reduce the effect of neurotransmitters, including serotonin, norepinephrine and dopamine, or other receptors in the brain, thereby leading to the dysregulation of neurohormones. Under stimulation, norepinephrine is released from the sympathetic nerve terminals in organs, and the target immune cells express adrenoreceptors. Through stimulation of these receptors, locally released norepinephrine, or circulating catecholamines such as epinephrine, affect lymphocyte traffic, circulation, and proliferation, and modulate cytokine production and the functional activity of different lymphoid cells.
Glucocorticoids also inhibit the further secretion of corticotropin-releasing hormone from the hypothalamus and ACTH from the pituitary (negative feedback). Under certain conditions stress hormones may facilitate inflammation through induction of signaling pathways and through activation of the corticotropin-releasing hormone.