Cytokines

Overview: sources and the core mnemonic

What cytokines are. Cytokines are small secreted proteins that carry signals between immune cells, usually over short distances (autocrine or paracrine). Almost every exam item about a single interleukin asks two things: which cell makes it, and what it tells its target to do. The same molecules reappear as drug targets (anti-TNF, anti-IL-6, anti-IL-2 receptor) and, when a receptor is missing, as specific immunodeficiencies (IL-12 receptor deficiency).

Hot T-bone stEAK. One mnemonic covers the first interleukins in order:

• IL-1, "Hot": fever.
• IL-2, "T": growth of T cells.
• IL-3, "bone": stimulates bone marrow stem cells.
• IL-4, "E": B-cell class switching to IgE.
• IL-5, "A": supports IgA production (TGF-β is the main IgA switch signal), plus eosinophil growth.
• IL-6, "K": "aKute"-phase protein synthesis (the usual extension of the mnemonic).

Know the cell of origin. Macrophages (and dendritic cells) secrete the acute-inflammation group, the macrophage "hot" cytokines: IL-1, IL-6, IL-8, IL-12, and TNF-α. T cells make IL-2 and IL-3. Th1 cells make IFN-γ; Th2 cells make IL-4, IL-5, IL-10, and IL-13. Regulatory T cells release IL-10 and TGF-β, and virus-infected cells release IFN-α and IFN-β.

Macrophage cytokines of acute inflammation

The trigger. When macrophage pattern-recognition receptors detect microbial products (classically gram-negative lipopolysaccharide binding CD14/TLR4), the cell releases IL-1, IL-6, and TNF-α, which together produce fever, leukocytosis, and the acute-phase response.

IL-1: the endogenous pyrogen. IL-1 causes fever, activates endothelium to express adhesion molecules, and induces chemokines that recruit leukocytes. It also stimulates bone resorption; its old name is osteoclast-activating factor. IL-6 and TNF-α are pyrogens too. All three raise the hypothalamic set point by inducing PGE2, which is why COX inhibitors lower fever.

IL-6: the acute-phase signal. IL-6 (from macrophages and Th2 cells) reprograms hepatic protein synthesis:

• Positive reactants rise: CRP, fibrinogen (a major reason the ESR climbs), hepcidin, ferritin, serum amyloid A.
• Negative reactants fall: albumin and transferrin.

Hepcidin causes degradation of the iron exporter ferroportin, trapping iron in macrophages and enterocytes; this is the mechanism of anemia of chronic disease (low serum iron, low TIBC, high ferritin). Chronically elevated serum amyloid A can deposit as AA amyloidosis.

IL-8: "clean up on aisle 8." IL-8 (CXCL8) is the major chemotactic factor recruiting neutrophils to a site of infection; other neutrophil chemoattractants include C5a, leukotriene B4, and bacterial N-formylmethionyl peptides. Contrast IL-5, which recruits eosinophils.

IL-12: the Th1 switch. Macrophages and dendritic cells secrete IL-12 in response to intracellular pathogens. It commits naive CD4 T cells to Th1 and activates NK cells, both of which then release IFN-γ.

TNF-α: shock, cachexia, granulomas. TNF-α activates endothelium, recruits leukocytes, and increases vascular permeability. In overwhelming infection it mediates septic shock (vascular leak, hypotension); chronic release in advanced cancer causes cachexia, with weight loss and muscle wasting (its older name is cachectin). It is also required to keep granulomas intact.

T-cell cytokines and helper subsets

IL-2: the T-cell growth factor. Secreted by helper T cells, IL-2 drives proliferation of helper, cytotoxic, and regulatory T cells and activates NK cells. It is a major immunosuppressive target: calcineurin inhibitors (cyclosporine, tacrolimus) block NFAT-driven IL-2 transcription, and basiliximab blocks the IL-2 receptor α chain (CD25). Recombinant IL-2 (aldesleukin) is used for metastatic renal cell carcinoma and melanoma.

IL-3: marrow support. Activated T cells secrete IL-3, which supports growth and differentiation of bone marrow stem cells, much like GM-CSF.

IL-4, IL-5, IL-13: the allergy axis. IL-4 induces Th2 differentiation and B-cell class switching to IgE (and IgG); IL-13 shares the IgE-promoting effect. IL-5 supports IgA production (TGF-β drives the IgA switch) and promotes the growth and differentiation of eosinophils. This axis drives atopy and defense against helminths.

The brakes: IL-10 and TGF-β. IL-10 (from Tregs and Th2 cells) and TGF-β inhibit activated macrophages and dendritic cells, suppress Th1 cytokines, and decrease MHC class II expression. Both help maintain regulatory T cells.

Interferons

Type I (IFN-α, IFN-β). A virus-infected cell senses viral nucleic acid such as dsRNA and releases IFN-α and IFN-β. These do not save the infected cell; they put neighboring cells into an antiviral state that interferes with viral protein synthesis, and they upregulate MHC class I.

Type II (IFN-γ). IFN-γ from Th1 and NK cells is the classic macrophage-activating cytokine: it boosts phagocytosis and the respiratory burst so macrophages can kill intracellular organisms, upregulates MHC I and II, and promotes Th1 while inhibiting Th2. Interferon-γ release assays (IGRAs) for latent TB measure IFN-γ released by T cells exposed to M. tuberculosis antigens.

Clinical correlations and cytokine-directed drugs

Receptor deficiencies. IL-12 receptor deficiency (autosomal recessive) impairs Th1 development and lowers IFN-γ, causing disseminated mycobacterial and fungal infections, sometimes first revealed after BCG vaccination; IFN-γ receptor defects produce the same picture. Both break the IL-12 → Th1 → IFN-γ → macrophage loop that controls intracellular pathogens. Loss of Th17 signaling (STAT3 mutation in hyper-IgE/Job syndrome) instead impairs neutrophil recruitment, causing cold staphylococcal abscesses.

Screen for latent TB with a PPD or IGRA before any anti-TNF biologic. TNF-α holds granulomas together; block it and contained M. tuberculosis escapes.

Match the verb: cachexia and septic shock → TNF-α; fever → IL-1 (with IL-6, TNF-α); acute-phase reactants → IL-6; neutrophil chemotaxis → IL-8; Th1 commitment → IL-12; macrophage activation → IFN-γ; antiviral state → IFN-α/β.