Local pattern of action
Under the influence of an inducer (e.g., a pathogenic microorganism), activated lymphocytes trigger two coordinated processes simultaneously:
- The producer cell begins to synthesize cytokines.
- Specific receptors are expressed on the target cell.
This mechanism ensures a strictly localized manifestation of effects. To prevent molecules from diffusing away from the site of secretion, the body is equipped with limiting barriers. First, upon binding to the receptor, the complex is internalized into the cell and degraded. Second, many factors exist as transmembrane proteins (IL-1α, TNFα) or bind tightly to peptidoglycans of the extracellular matrix (e.g., IL-7).
Normally, the serum concentration of these substances is too low to produce biological effects. Systemic action occurs only during inflammation and always serves as a sign of pathology. Under physiological conditions, cytokines are rapidly eliminated by the kidneys (the clearance curve consists of a rapid component with a half-life of a few minutes and a slow component ranging from 30 to 120 minutes).
Key properties of the system
The system is characterized by four fundamental regularities that form a complex interaction network:
- Redundancy: Different cytokines produce the same effect. If the gene for a specific factor is turned off due to a mutation, fatal consequences usually do not occur—"backups" take over the function. For example, the absence of the T-cell growth factor IL-2 is compensated by the action of IL-15, IL-7, IL-4, or a combination of other molecules. Similarly, a defect in the IL-4 gene is compensated by the properties of IL-13. The exception is indispensable molecules, such as IL-7.
- Pleiotropy: A single factor is capable of affecting multiple different cell types. Thus, IL-1β and TNFα have a huge number of targets and participate in a mass of complex reactions.
- Synergism: The joint action of several factors produces a more powerful response than the sum of their isolated actions.
- Antagonism: Some molecules can block or suppress the effects of others at the target cell level.
Interaction and cascading
Within the network, the synthesis of certain factors is constantly regulated under the influence of others. This can occur according to the principle of:
- Autocrine amplification: Pro-inflammatory IL-1β and TNFα stimulate their own production and trigger a cascade synthesis of IL-6, IL-8, and chemokines.
- Induction: IL-12 and IL-18 directly induce the formation of IFNγ.
- Suppression: TGFβ and IL-10 inhibit the synthesis of other network participants, while IL-6 inhibits the activity of the pro-inflammatory pool.
- Mutual suppression: Cytokines of different subpopulations (e.g., IFNγ from Th1 and IL-4 from Th2) mutually block each other's production.
Multiple cross-links make the outcome of the network's operation difficult to predict. A striking clinical example: therapy with high doses of IL-2 can cause a severe side effect in the form of shock (resembling toxic shock, but without bacteremia). Paradoxically, this shock is relieved not by antibodies against IL-2 itself, but by drugs against TNFα, which is produced secondarily via a cascade mechanism.
Role in inflammation
Humoral factors of innate immunity are divided into three key groups participating in the development of the inflammatory process:
- Pro-inflammatory cytokines.
- Chemokines.
- Colony-stimulating factors.
IL-12 and IFNγ are also actively involved in the functional support of this process. At the same time, the system works both ways: cytokines not only initiate the inflammatory response, but also play a crucial role in its subsequent containment and suppression.
Briefly: answers to topical questions
Which cells are the main producers of the pro-inflammatory cytokines IL-1β and TNFα?
The main producers of the pro-inflammatory cytokines IL-1β and TNFα are activated monocytes and macrophages, predominantly at the site of inflammation. Local inflammatory macrophages are the first to begin synthesis; as leukocytes emigrate from the bloodstream, the number of producer cells increases and their spectrum expands. Additional producers of pro-inflammatory cytokines can include neutrophils, dendritic cells, activated B, NK, and T lymphocytes, as well as epithelial, endothelial, synovial, glial cells, and fibroblasts upon stimulation by microbial products and inflammatory factors. Eosinophils and mast cells are additionally specified for TNFα.
Which receptor families bind cytokines on the surface of target cells?
Cytokines bind to specific receptors on the surface of target cells, which are classified into five main families based on the structure of their polypeptide chains.
- Hematopoietin receptors (Class I) — contain 4 cysteine residues and a WSXWS sequence; bind most interleukins and colony-stimulating factors.
- Interferon receptors (Class II) — contain 4 cysteine residues, but lack the WSXWS sequence; bind interferons and IL-10.
- Tumor necrosis factor receptors (TNF family, Class III) — form trimers upon interaction with ligands; bind TNFα and lymphotoxin α.
- Interleukin-1 receptors (Class IV) — extracellular domains belong to the immunoglobulin superfamily, intracellular domain is similar to TLR.
- Immunoglobulin-like receptors (Ig-like, Class V) — extracellular domains belong to the immunoglobulin superfamily, intracellular domains possess tyrosine kinase enzymatic activity.
Properties of the cytokine network: Pleiotropy (one affects all), Redundancy (all duplicate one), Antagonism (they fight), Synergism (they cooperate).
Class and exam questions
Why can't cytokines be considered hormones?
Hormones work systemically (long-range factors). Cytokines, on the other hand, are intended exclusively for local action (short-range factors), and their appearance in high concentrations in the systemic circulation is always a sign of pathology.
What happens if a person has an inherited defect in the IL-2 gene?
Fatal impairment of T-cell proliferation will not occur due to the system's redundancy: the function of IL-2 is compensated by IL-15, IL-7, IL-4, and other molecules. However, abnormalities associated with a deficiency of regulatory T cells (Tregs) are possible.
How is the entry of cytokines into the general bloodstream prevented?
This is achieved by the rapid internalization of the receptor-cytokine complex into the cell, the fixation of molecules as transmembrane forms on the cell surface, and binding to extracellular matrix proteins. Excess amounts that have entered the blood are instantly eliminated by the kidneys.
What else to study on this topic
- Mechanisms of chemokine involvement in local inflammatory reactions
- The role of colony-stimulating factors in the regulation of innate immunity
- Clinical features of severe combined immunodeficiency (SCID)
- Interaction of Th1 and Th2 cytokines: molecular bases of mutual antagonism
- The significance of regulatory T cells (Tregs) in cytokine balance
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