The immune system
The immune system consists of various elements: The cellular and humoral immune system, the complement system, the innate non-specific immune response and the adaptive specific (formerly: acquired) immune response.
The cellular immune system consists of cells such as macrophages, dendritic cells and natural killer cells, while the humoral immune system consists of non-cellular components of body fluids.
The complement system consists of proteins in the blood that help fight pathogens.
The innate immune response is rapid and non-specific and is mediated by cells such as macrophages and natural killer cells.
The acquired immune response is specific and is mediated by B and T lymphocytes.
Glucocorticoids can influence the immune response by modulating the production of cytokines. In this way, severe or prolonged stress can lead to an imbalance in the immune response and increase susceptibility to certain diseases. Glucocorticoids are also thought to influence the TH1/TH2 shift, which is important for the balance between inflammatory responses and immune defense.
A high or low cortisol stress response can have different effects on cytokine responses in the body. A higher cortisol stress response has been associated with a lower cytokine response in some cases and a higher cytokine response in others.
1. The immune system
The following brief description of the immune system is essentially based on Wolf.1
The immune system consists of several different elements that complement and overlap each other, just like the stress systems do in relation to stress.
1.1. Cellular / humoral immune response
The cellular immune response is the part of the immune response of the adaptive immune system mediated by cytotoxic cells. These include macrophages, granulocytes, dendritic cells and natural killer cells.
The cellular immune system primarily mediates TH-1 immune responses, which are active during acute inflammation.2
The humoral immune response (humor = moisture, sap, fluid) is the immune response mediated by non-cellular components of body fluids. These include cytokines, chemokines and the complement system.
The humoral immune system primarily mediates TH-2 immune responses, which are active in chronic inflammation and allergies.2
1.2. Complement system
The complement system consists of around 30 glycoproteins that are found in the blood. When one of these binds to a pathogen, a cascade of enzyme reactions occurs, through which the proteins activate each other, which can help to combat the pathogen.3
The complement system consists of plasma proteins and proteolytic enzymes and can inactivate and destroy bacteria and viruses by means of lysis, opsonization or local inflammatory reactions.
Positive regulators:
- C1 to C9
- Mannose-binding lectin (MBL)
- Serine proteases C1r, C1s, MASP-1 to 3
Many other proteins and protein complexes are formed from parts of C1 to C5 and (by means of protease-mediated cleavage) and assemblies with C6 to C9, e.g:
- C3a
- C5a
- C4a
- Membrane attack complex
Negative regulators:
- C1 inhibitor
- Factor H
- Factor I
- C4bp
- CD35
- CD46
- CD55
- CD59
- Vitronectin
Activators:
- Properdin
- Cobra Venom Factor.
1.3. Innate immune response
The innate immune response (macrophages, dendritic cells, natural killer cells, neutrophil, basophil and eosinophil granulocytes) is rapid and non-specific.
Macrophages recognize pathogens based on the proteins on their surface and break them down by phagocytosis. The macrophages then present the resulting antigens on their surface, which activates cells of the acquired immune defense. The activated macrophages release cytokines. Cytokines are chemical messengers that trigger local inflammatory reactions.
Like macrophages, dendritic cells represent antigens and mediate between innate and acquired immune defense via activated B and T lymphocytes.
Natural killer cells (NK) can recognize virus-infected or degenerated cells. NK are activated by a complex receptor system and cause apoptosis (cell death) of the target cell by releasing various cytotoxins.
1.4. Adaptive (specific, acquired) immune response
The adaptive immune response (B and T lymphocytes and the resulting TH cells) is slower, but more effective. The adaptive immune response uses antigen-specific effector cells and memory cells to store successfully formed immune responses so that they can be called up more quickly when they are needed again. Vaccination makes use of this principle.
B lymphocytes (bone) develop from hematopoietic stem cells of the bone marrow. They develop into memory cells or antibody-producing plasma cells. They produce antigen-specific antibodies (= immunoglobulins, Ig) with or without contact with T helper cells (Th cells).
Regulatory B cells suppress the immune system.
T lymphocytes develop in the fetal liver and bone marrow and migrate with the blood to the thymus, where they mature. Through recombination of gene segments, they form specific receptors for various antigens.
Lymphocytes produce cellular glycoproteins, the “clusters of differentiation”, on their surface. Depending on the type of these glycoproteins, T lymphocytes develop into different types of T helper cells (Th cells).
The mature (still antigen-naïve = not yet specialized in certain antigens) TH cells migrate from the thymus into the lymphatic tissue (lymph follicles, lymph nodes, spleen) where they are presented with antigens by B lymphocytes, dendritic cells or macrophages, causing them to specialize in these antigens (differentiation).
1.5. Surface marker
The cells of the immune system differ according to surface markers:4
- CD4+: TH-1 (T inducer cells)
- CD8+: TH-2 (T-suppressor cells)
- CD14+: monocytes, macrophages
- CD16+, CD56+: NK cells
- CD5+, CD19+: B lymphocytes
- CD3+: T lymphocytes
- CD45+: T memory cells
1.6. Receptor classes
In order to protect the body’s own cells from the immune response, the antigen-presenting cells use different classes of receptors for antigen presentation to the TH cells. There are 3 receptor classes: CD4+ TH cells only bind to antigens presented by MHC-II, CD8+ TCH cells only bind to antigens presented by MHC I. The MHC III complexes include the complement factors C2, C4 and Bf, as well as various cytokines, e.g. tumor necrosis factor (TNF). MHC III are plasma proteins that are involved in the non-specific immune defense.
1.7. T helper cells (TH cells)
TH-0 cells are TH cells that have not yet had antigen contact. Depending on the type of antigen presented to them by the antigen-presenting cells, they develop (primarily) into TH-1 or TH-2 cells.
TH-1 cells are activated during acute inflammation42 and secrete cytokines such as IFN-γ, IL-2 and TNF-α, which activates and differentiates macrophages. Macrophages (and dendritic cells) release IL-12, which in turn promotes the formation of TH-1 cells from naïve TH-0 cells.
IL-12 and IFN-γ inhibit TH-2 cell formation.
TH-2-CD4-positive cells are activated during chronic inflammation and allergic reactions42 and interact with B lymphocytes by means of cytokines and cell-bound molecules. The B lymphocytes then produce antibodies.
Type 2 CD4-positive cells thereby promote the production of IL-4, IL-5, IL-6, IL-10, IL-13 and lymphotoxin-α.
IL-4 and IL-10 inhibit macrophage activation and thus TH-1 cell formation.
TH-1 cells and TH-2 cells therefore inhibit each other.
The immune system is normally in a functional balance between a TH-1 and a TH-2 emphasis.42 Cortisol inhibits TH-1 and promotes TH-2, while growth hormone oppositely promotes TH-2 and inhibits TH-1. Cortisol and growth hormone are subject to a staggered daily rhythm and are thus likely to modulate the regular TH-1/TH-2 alternation. If the stress systems constantly release too much or too little cortisol, this stress imbalance causes disorders of the immune system.15
TH-17 cells develop from T helper cells after their activation by antigen contact using IL-6 and TGF-β. TH-17 cells need IL-23 to survive. IL-23 is involved in the development of autoimmune diseases.
TH17 cells produce IL-17A to IL17F, TNF-α and IL-6 and inhibit TH-1 cells.
Natural regulatory T cells (nTreg) account for up to 10 % of peripheral CD4-positive T lymphocytes. They have pronounced immunomodulatory properties and support the maintenance of self-tolerance by limiting excessive attacks on intact body cells. They inhibit dendritic cells, natural killer cells, CD4+, CD8+ cells and B lymphocytes. They are involved in various autoimmune diseases.
nTreg promote the production of CD25.
IL-12 - together with IL-18 - enhance the production of IFN-γ by natural killer (NK) cells. IL-12 together with IL-18, IFN-α and IFN-γ promote the differentiation of naïve TH cells into TH-1 cells.5
Stress and glucocorticoids have direct reciprocal effects on the immune system. Stress influences certain cytokines, cytokines influence glucocorticoids and glucocorticoids influence cytokines.
2. Stress and the immune system
2.1. Cytokine reactions and cortisol stress response in healthy individuals
One study found that healthy people with a low cortisol stress response to mild stress showed higher stress responses of IL-6 and IL-1ra in the blood than those with a high cortisol stress response. At the same time, subjects with a low cortisol stress response showed lower heart rate variability, indicating poorer stress processing by the autonomic nervous system.6 This correlates with the results of another study in healthy subjects, according to which a lower cortisol stress response correlated with a prolonged IL-6 stress response.7 In contrast, a further study on healthy individuals found a tendency for an increased IL-6 stress response to be associated with an increased cortisol stress response.3
2.2. Cytokine reactions and cortisol stress response in mental disorders
A smaller study on depressed people found a positive correlation between IL-6 stress response and cortisol stress response and no correlation in healthy people.8
In fatigue, a reduction in IL-6 and TNF-alpha was found in response to stress, while healthy people showed an increase as a stress response. In fatigue, ACTH and salivary cortisol levels were also lower overall, while blood cortisol was only basally lower. This showed a negative correlation of the IL-6 and TNF-alpha stress response to the cortisol stress response in fatigue, compared to a positive correlation in healthy individuals.9
A study on psoriasis sufferers found a correlation between the immune and cytokine stress response (lymphocytes, granulocytes, CD3+, CD8+, CD16+/CD56+, CD3+/HLA-DR+) and the cortisol stress response to the TSST.10
Increased ACTH and cortisol stress responses correlated positively with increased granulocytes and natural killer cells and negatively with B lymphocytes. Furthermore, the response to TSST in psoriasis sufferers and healthy individuals showed10
- CD4+ cells increased
- CD4+ cells typically trigger elevated levels of
- IL-2
- IFN-gamma
- TNF-alpha from.
- CD4+ cells typically trigger elevated levels of
- CD25+ cells reduced, but only in psoriasis sufferers, not in healthy people
- IFN-gamma significantly increased
- IL-2 significantly increased
- IL-10 significantly reduced
- IL-4 significantly reduced
In acutely depressed people (unlike healthy people), there was a positive correlation between the stress responses of cortisol and IL-6 and between the stress responses of adrenaline, TNF-α and CRP. Healthy individuals only showed a significant correlation between ACTH and CRP stress responses.8
In contrast, no correlation was found between the stress responses of cortisol on the one hand and cytokines on the other (IL-6, TNF-α, IL-10) in both non-depressed and remitted (healthy former) depressives.3
Increased chronic stress levels correlated with increased basal blood levels of IL-6 and IL-10, but not with TNF-α, in both remitted depressives and healthy individuals. Remitted depressives had higher chronic stress levels and consequently higher basal IL-6 and IL-10 levels than non-depressives.3
The subjective perception of acute stress correlated with elevated basal blood levels of IL-6, but not of TNF-α and IL-10.3 IL-10 levels were significantly elevated during the anticipation of a stress test (TSST) in subjects with a high cortisol stress response.3
TNF-α did not differ in the stress anticipation phase or in the stress response between subjects with a high and a low cortisol stress response.3
Chronic and prolonged elevation of cortisol levels can lead to decreased HPA axis activity as a result of persistent negative feedback from the HPA axis, which triggers increased neuroinflammation and impaired cognitive function.11
While the magnitude of the cortisol stress response to an emotionally stressful movie showed no correlation to subsequent cognitive control/performance in the Stroop test, this was more strongly affected by higher stress-induced increases in the proinflammatory cytokines IL-1β, IL-6, and IL-8 than by weaker increases.12
Adrenaline and noradrenaline (which are also part of the endocrine stress response) also cause a TH1/TH-2 shift.1314
2.3. Glucocorticoids and the immune system
The reaction to glucocorticoids is only part of the stress response. Moreover, glucocorticoids are essentially only released at the end of the stress response by the 3rd stage of the HPA axis.
The stress response of some cytokines correlates with the level of the cortisol stress response, while others do not. ADHD-HI is often associated with a flattened cortisol stress response, ADHD-I very often with an excessive cortisol stress response.
In the response to antigens, the highest increase in antibodies coincides with a simultaneous increase in glucocorticoids (by a factor of 2 to 3) and the highest activity of the stress hormone-producing cells of the hypothalamus.4 IL-1 causes an immediate release of ACTH and cortisol (unlike IL-2, INF-gamma and TNF-alpha). Cortisol in turn inhibits IL-1 synthesis.15 The immune response thus appears to differ from the stress response, in which glucocorticoids are only released with a time delay at the end of the HPA axis.
In addition, lymphocytes directly produce endocrine hormones such as ACTH, beta-endorphins, TSH, GH and prolactin.4
2.1.1. Activating effect of glucocorticoids
Glucocorticoids increase the production or release of
- IL-1β
- Even small amounts of dexamethasone increase the endotoxin-induced amount of IL-1β by macrophages16
- IL-4
- By inhibiting IL-12, which reduces its IL-4-inhibiting effect1718 Against this background, the authors warn against the long-term use of glucocorticoids for allergies, as they exacerbate allergy problems.
- By stimulating the production of IL-4 by TH-2 cells19
- Study results are contradictory, see below
- IL-10
- By stimulating the production of IL-10 by TH-2 cells19
- IL-13
- By stimulating the production of IL-13 by TH-2 cells19
- IL-18
- Cushing’s patients with high cortisol levels also have high IL-18 levels5
2.1.2. Inhibitory effect of glucocorticoids
Glucocorticoids reduce the production or release of
- IFN-α by 50 to 60 %5
- IFN-γ5
- By suppressing IL-12 production in antigen-presenting cells
- By reducing the IL-12 receptor sensitivity of natural killer cells and TH-1 cells
- IL-1-Beta
- By reducing the transcription rate of the genes of these interleukins and reducing the stability of their mRNA20
- IL-25
- IL-421 (study results are contradictory, see above)
- IL-521
- IL-620
- On the other hand, maternal stress, which causes damage to the unborn child, is mediated by IL-6. The defective development of GABAergic cells in the unborn child caused by prenatal stress in the mother can be prevented by IL-6 antagonists.22
- Cortisol can increase the expression of IL-6 receptors in liver cells and induce IL-6-mediated production of acute phase proteins (APP).23
- Noradrenaline, which (like cortisol) is part of the endocrine stress response, stimulates the expression of IL-6 mRNA and the production of IL-6 in astrocytes via β2 and α1 adrenoceptors in a dose-dependent manner.24 Since the release of noradrenaline occurs before the activation of the HPA axis, the inhibition of IL-6 by cortisol could represent a negative feedback loop, comparable to the inhibition of the HPA axis after a stress response.
- IL-1217
- Which increases IL-10 production5
- Catecholamines also mediate a reduction in IL-12 over several days via β2-adrenoceptors.25
- Asthma therapy with glucocorticoid and / or β2-AR agonists probably reduces
- The ability of the antigen-presenting cells to17
- To produce IL-12
- Suppress type 2 cytokine synthesis in activated TH cells and
- To combat eosinophilia.
- As a result, these antigen-presenting cells activate naive (non-cytokine-bound) T cells, whereby
- IL-4 is increased
- IFN-γ is restricted
- As a result, glucocorticoids and β2-AR agonists are only beneficial for asthma in the short term. In the long term, they can increase susceptibility to allergic diseases. This is confirmed by the fact that glucocorticoids such as β2-AR agonists potentiate IgE production. 52627
- The ability of the antigen-presenting cells to17
- The expression of IL-12 receptors on T cells and NK28
- IL-1321
- IL-18, even if not complete5
- TNF-α5
- TGF-β-1 in glial cells (on dexamethasone)29
- Histamine
- GM-CSF (granulocyte macrophage colony-stimulating factor)21
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