Tumor necrosis factor alpha

Life Cycle and Molecular Structure

The cytokine begins its journey as a type II transmembrane protein with a mass of 27 kDa, with its N-terminus hidden inside the cell. Already at this stage, it exhibits functional activity. Subsequently, enzymes cleave the extracellular part, releasing a soluble monomer (17 kDa).

To acquire the ability to transmit signals, three such monomers spontaneously assemble into a trimer. In this bell-shaped structure, the molecules are held together by their C-termini — which carry the binding sites for target cell receptors. The basic secondary structure of the protein is β-pleated sheet, but when the environmental pH drops, it reorganizes into an α-helix, altering the cytokine's properties.

Where the Cytokine Comes From

The main "factories" for TNFα production are monocytes and macrophages. However, in the setting of mounting inflammation, many other cells join in the synthesis: neutrophils, mast cells, eosinophils, lymphocytes, as well as endothelium and epithelium.

The speed of response is ensured by a special mechanism: the cell does not always need to synthesize the protein from scratch. It can rapidly "shed" the ready molecule from the membrane or release stores from granules via exocytosis.

Receptors: Between Life and Death

To transmit a signal inside the cell, the ligand must bind to receptors and induce their trimerization. The cytokine has two types of receptors:

1. TNFRI (55 kDa). Present on almost all nucleated cells of the body. Its main feature is the presence of a cytoplasmic "death domain" (death domain). This receptor performs the lion's share of TNFα functions, and disruption of its gene causes severe immunodeficiency.
2. TNFRII (75 kDa). Located predominantly on immunocompetent cells. It has a higher affinity for the ligand, but lacks the "death domain".

When the cytokine binds to TNFRI, the cell faces a choice. It can follow the pro-apoptotic pathway via adaptor proteins TRADD and FADD (activating caspase 8). Alternatively, it can turn on the anti-apoptotic program via TRAF2/5 or RIP-1, leading to the production of transcription factors (NF-κB or AP-1) and the initiation of inflammation.

Effects: Defense or Self-Destruction?

Together with interleukin-1, this factor triggers the classic picture of inflammation: it stimulates the production of chemokines, adhesion molecules, acute-phase proteins, and phagocytosis enzymes. TNFα activates leukocytes, promotes new vessel growth and wound healing.

लेकिन यदि उत्पादन नियंत्रण से बाहर हो जाता है, तो साइटokine का dark side प्रकट होता है:

• Locally: massive tissue damage occurs, up to hemorrhagic necrosis.
• Metabolically: lipoprotein lipase is blocked. The body stops synthesizing fats, leading to extreme wasting — cachexia.
• Systemically: upon massive release into the bloodstream (e.g., due to bacterial superantigens), lethal septic shock develops.

For self-protection at the peak of inflammation, cells shed receptors into the intercellular space. These soluble forms act like sponges, soaking up excess TNFα and dampening inflammation. This principle is used in medicine today: drugs based on soluble TNFRII successfully treat chronic autoimmune pathologies.

Class and exam questions

What else to study on this topic

• Features of the TNF superfamily members (FasL, TRAIL, CD40L) and their impact on immunity.
• Detailed mechanism of the caspase cascade during apoptosis induction.
• Synergism of TNFα and IFNγ in activating phagocyte NO synthase.
• How the transcription factor NF-κB is activated and which genes it switches on.
• Clinical application of TNFα inhibitors in the treatment of autoimmune diseases.

Other topics in the "Innate Immunity" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.