Cytokines are proteins that are released by body cells in response to an activating stimulus. They bind to specific receptors and thus have a variety of effects. Currently, more than 50 cytokines are known.
1.1. Areas of action of cytokines
Cytokines have various effects:
- Activation of the immune system in inflammatory processes
- Regulation of the blood-brain barrier
- Regulation of the HPA axis hormones
Excitatory and inhibitory effects on neurotransmitter systems
1.2. Types of cytokines
Cytokines differ in
- Interleukins (IL, also called lymphokines) are cytokines released by lymphocytes
(former names: Interferon-β2 (IFNB2), B-cell stimulating factor, B-cell differentiation factor, liver cell stimulating factor)
Release is increased approx. 100-fold in response to strong and long muscle stress (6 hours). Maximum release at the end of the muscle stress, followed by a rapid decline. Release partly from muscle cells themselves. Sustained training causes a reduction in IL6 release during exercise. IL-6 probably regulates immunological and metabolic responses to exercise via the liver, adipose tissue, HPA axis and leukocytes.
IL-6R, only on liver cells and leukocytes
- SIL-6R, soluble IL-6 receptor. Via this binding to binds to the glycoprotein gp130 in cell membranes of very many cell types.
- Erythropoietin (EPO)
- Thrombopoietin (TPO)
- Leukemia Inhibitory Factor (LIF)
1.2.2. Tumor necrosis factor (TNF)
- Increases CRH
- Inhibits appetite
- Regulates fever
- Forms acute phase proteins
- C-reactive protein (CRP).
- Neutrophil granulocytes
- Increases insulin resistance
- Increases cyclooxygenase-2 activity
- Local increase causes inflammation
- Extreme increase causes shock
- Bupropion decreases TNF-alpha levels.
Interferons are antiviral proteins produced by cells in response to viral infection. They inhibit the replication of viruses.
- Alpha-interferon (IFN-α) (formerly: type I)
- Beta-interferon (IFN-β)
- Gamma interferon (IFN-γ)
- Activates macrophages by improving fusion of phagosomes with lysosomes and promoting production of bactericidal nitric oxide and reactive oxygen radicals
- Induces antimicrobial peptides
- Induces 1α-hydroxylase in macrophages
- Converts 25(OH) vitamin D3 to 1,25(OH)2 vitamin D3 without product inhibition of 1α-hydroxylase
- Tau Interferon
Chemokines are released by various cells in response to bacteria or viruses. They can chemically direct leukocytes (chemoattractants). There are 4 groups: CC, CXC. C3 and C. For a comprehensive listing, see Wikipedia.. Many chemokines have multiple names. IL-8 is another name for CXC-8.
1.3. Differentiation according to TH-1 and TH-2 affiliation
1.3.1. TH-1 cytokines
TH-1 cytokines are pro-inflammatory cytokines promoted primarily by stress hormones of the first two stages of the HPA axis (CRH and ACTH) and inhibited by cortisol.
- Macrophage inhibitory protein-1
Disease patterns with a shift of the immune system towards TH-1 are e.g.
- Rheumatoid arthritis
- Multiple sclerosis
- Type 1 diabetes mellitus
- Autoimmune thyroid disease
- Collagen- and adjuvant-induced arthritis
- Experimental allergic/autoimmune encephalomyelitis
1.3.2. TH-2 cytokines
TH-2 cytokines are anti-inflammatory cytokines promoted primarily by stress hormones of the third level of the HPA axis (cortisol).
IL-1ra (receptor antagonist)
- Soluble IL-1 Receptor
TNF-α binding protein
IL-1 binding protein
Besides, TH-2 dominance of the immune system is associated with increased histamine and IgE production.
2. Other inflammatory biomarkers
2.1. Acute Phase Proteins
2.1.1. C-reactive protein (CRP)
CRP is an acute-phase protein that becomes measurable 12 to 24 hours after the onset of an infection. It activates the complement immune system. CRP is an important indicator for the activation of the immune system.
Microglia are certain glial cells with the function of tissue macrophages. They do not arise from the bone marrow like most cells, but from yolk sac cells.
There are two types of macrophage activation: M1 and M2.
Microglial cell activation is triggered by the M1 phenotype, which is associated with an increase in IL-1β and TNF-α. In contrast, the M2 phenotype is associated with a release of anti-inflammatory cytokines.
3. Cytokines in the brain: migration and generation
3.1. Migration of cytokines into the brain
Peripheral cytokines can enter or originate in the brain through a variety of pathways:
- Direct or carrier-mediated passage of the blood-brain barrier
- Activation of endothelial cells and perivascular macrophages in brain blood vessels
- Local activation of peripheral nerves (e.g., vagus), which then transmit cytokine signals to relevant brain regions, e.g., nucleus solitaris or hypothalamus
Peripherally recruited activated immune cells (e.g. monocytes, macrophages, T cells) into the brain
Cytokines such as IFN-a, IFN-c, IL-2, IL-10, IL-1, IL-6, and TNF-a can cross the blood-brain barrier, bind to receptors of vagal sensory nerves, and stimulate the HPA axis.
Disorders of the blood-brain barrier are found in 20 to 30 % of psychiatrically affected persons. This is usually accompanied by activation of astrocytes. It is assumed that the disturbance of the blood-brain barrier is a consequence of a mild chronic inflammation.
3.2. Formation of cytokines in the brain
Cytokines are produced in the brain by
- Nerve cells
Endothelial cells and perivascular macrophages are stimulated by cytokines to express the prostaglandin-producing enzymes cyclooxygenase-2 (COX-2) and prostaglandin E synthase (PGES).
4. Measurement of cytokines: CSF / serum
Pro-inflammatory cytokines are detectable in the brain and blood. The measurement of cytokines in the blood is primarily related to their immunological effects. Cytokines that are present in the blood also enter the brain. However, the reverse is not always the case. Since neuropsychological effects, i.e., changes in behavior, are mediated in the brain, and cytokines present in the brain are not detectable to the same extent in the blood, measurement of cytokines in the blood makes little sense in terms of detecting neuropsychological influences.
IFN-α given peripherally decreases blood L-tryptophan levels but not cerebrospinal fluid L-tryptophan levels.
- A single administration of endotoxin in mice immediately increased TNF-α levels. While peripheral TNF-α decreased in serum after 9 h and in liver after 1 week, it remained elevated in brain for 10 months.
- Cytokines are partially transported across the blood-brain barrier, but are also synthesized in astrocytes and microglia directly in the brain.
IL-2 in blood does not represent the symptoms mediated by IL-2 in the brain. Thus, only CSF IL-2, but not blood IL-2, has prognostic value in relation to schizophrenia.
- The effects of IL-1-beta on IL-6 level changes in blood differ from the effects on IL-6 in cerebrospinal fluid.
- Quinolinic acid, which mediates significant behavioral deficits, showed no correlation between blood levels and cerebrospinal fluid levels in one study despite significantly elevated levels in the brain.
Incidentally, this also applies to neurotransmitters and hormones.
DHEA levels in the brain average 6.5 times blood levels, but DHEA levels in cerebrospinal fluid are only 1/20 of blood levels.
While already acute and severe neuroinflammation of the brain is not easy to measure and diagnose, chronic or low-threshold neuroinflammation is even more difficult to detect and assess. Measurements of cerebrospinal fluid are characteristic of acute meningoencephalitis, more difficult in acute encephalitis without meningitis, and very difficult in low-threshold neuroinflammation and evaluable only with high expertise. Measurements should always include simultaneously collected CSF-serum sample pairs to identify the dynamics of the exchange between the blood and cerebrospinal fluid. All immunoglobulin subclasses should always be examined and the respective quotients compared with Q-albumin. Final classification is based on the entire set of all parameters from serum, cerebrospinal fluid, and cells to more quickly detect outliers and errors in the data set. Even this approach cannot completely exclude acute encephalitis, as CSF obtained lumbally (= from the back) may have little significance with respect to the cerebral cortex or CSF from subarachnoid spaces of the brain.