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Cytokine and messenger molecule of immune system/ SIRS ROLE Robbins based md pathology exam answer

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cytokines AND SIRS

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Robbins pathology cytokines systemic inflammatory response syndrome TNF IL-1 role

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https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3484588

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Cytokines: messenger molecules of the immune system

Definition: Cytokines are low-molecular-weight secreted proteins that mediate and regulate immune and inflammatory reactions. They are chemical messengers allowing communication between leukocytes and also with endothelial, epithelial, and connective-tissue cells.
Major sources
  • Activated macrophages and dendritic cells
  • T lymphocytes and NK cells
  • Mast cells
  • Endothelial and epithelial cells

General properties of cytokines

  1. Pleiotropy: One cytokine acts on several different cell types and produces different effects.
    Example: IL-4 acts on B cells, T cells, and macrophages.
  2. Redundancy: Different cytokines may produce the same effect.
    Example: TNF and IL-1 both activate endothelium and cause fever.
  3. Synergy: Two cytokines together produce a greater effect than either alone.
  4. Antagonism: One cytokine inhibits the action of another.
    Example: IFN-gamma activates macrophages, whereas IL-10 inhibits macrophage activation.
  5. Autocrine, paracrine, and endocrine action:
    • Autocrine: Acts on the cell that secreted it.
    • Paracrine: Acts on nearby cells.
    • Endocrine: Enters circulation and acts at distant sites, especially when produced in large amounts.
  6. Cytokines usually act by binding to specific high-affinity receptors, which activate intracellular signaling pathways and alter gene transcription.

Important cytokines and their actions

CytokineMain sourceImportant actions
TNFMacrophages, dendritic cells, T cells, mast cellsEndothelial activation, leukocyte recruitment, fever, shock in excess
IL-1Macrophages, dendritic cells, epithelial cellsSimilar to TNF; prominent role in fever
IL-6Macrophages and other cellsHepatic acute-phase protein synthesis, fever
ChemokinesMacrophages, endothelial cells, T cells, mast cellsLeukocyte chemotaxis and migration
IL-12Dendritic cells, macrophagesActivates NK cells; stimulates IFN-gamma production; promotes Th1 response
IFN-gammaT cells and NK cellsClassical macrophage activation and enhanced microbicidal activity
IL-17T cells, especially Th17 cellsRecruits neutrophils and monocytes
IL-2Activated T cellsT-cell proliferation and survival
IL-4Th2 cells, mast cellsIgE production; alternative macrophage activation
IL-5Th2 cellsEosinophil activation
IL-10, TGF-betaTreg cells, macrophages, other cellsSuppress inflammation and limit immune response
In innate immunity, the early cytokines include TNF, IL-1, IL-12, type I interferons, IFN-gamma, and chemokines. In adaptive immunity, cytokines from CD4+ T cells control lymphocyte proliferation, differentiation, macrophage activation, eosinophil activation, and antibody responses.
Robbins & Kumar Basic Pathology, Ch. 5: “Cytokines: Messenger Molecules of the Immune System.”

Role of cytokines in SIRS

Definition

Systemic inflammatory response syndrome (SIRS) is a generalized inflammatory reaction caused by a severe insult. It may result from:
  • Severe infection or sepsis
  • Major trauma
  • Burns
  • Acute pancreatitis
  • Extensive tissue necrosis
  • Ischemia-reperfusion injury
  • Major surgery
  • Massive transfusion
Thus, SIRS is not synonymous with infection. Infection-associated SIRS was historically central to the definition of sepsis, though modern Sepsis-3 terminology emphasizes life-threatening organ dysfunction due to a dysregulated host response.

Pathogenesis

A severe insult activates macrophages, dendritic cells, endothelial cells, complement, and coagulation pathways. Microbial products, particularly endotoxin, activate pattern-recognition receptors such as Toll-like receptors.
This leads to excessive systemic release of cytokines, chiefly:
  • TNF-alpha
  • IL-1
  • IL-6
  • IL-8 and other chemokines
  • IL-12
  • IFN-gamma
  • Other inflammatory mediators: nitric oxide, complement products, prostaglandins, platelet-activating factor, and reactive oxygen species.

Cytokine cascade

Injury/microbial products → macrophage and endothelial activation → TNF and IL-1 release → IL-6, chemokines, endothelial activation, coagulation activation → widespread inflammation → shock and multiorgan dysfunction.

Effects of major cytokines in SIRS

1. TNF-alpha

TNF is a major early mediator of SIRS.
Local effects
  • Activates vascular endothelium.
  • Increases expression of adhesion molecules, promoting leukocyte adhesion and migration.
  • Induces chemokine and cytokine production.
  • Enhances procoagulant activity of endothelium.
Systemic effects
  • Fever
  • Tachycardia
  • Hypotension
  • Reduced myocardial contractility
  • Capillary leak and edema
  • Activation of coagulation and possible disseminated intravascular coagulation
  • Insulin resistance and catabolism
  • In high concentration, can produce septic shock, multiorgan failure, and death.

2. IL-1

IL-1 has actions similar to TNF.
  • Causes fever, partly by inducing prostaglandin synthesis in the hypothalamus.
  • Activates endothelial cells.
  • Promotes leukocyte recruitment.
  • Amplifies cytokine production.
  • Contributes to hypotension and systemic metabolic disturbances.

3. IL-6

IL-6 is an important mediator of the acute-phase response.
  • Stimulates hepatocytes to synthesize acute-phase proteins:
    • C-reactive protein
    • Fibrinogen
    • Serum amyloid A
  • Produces fever and constitutional symptoms.
  • Increased serum IL-6 often reflects severity of systemic inflammation.

4. Chemokines, especially IL-8

  • Recruit and activate neutrophils.
  • Cause widespread neutrophil accumulation in tissues.
  • Activated neutrophils release proteases, oxidants, and NETs, causing endothelial and parenchymal injury.

5. IL-12 and IFN-gamma

  • Activate NK cells and macrophages.
  • Increase macrophage microbicidal activity.
  • When excessive, add to the systemic inflammatory injury.

Mechanisms of organ injury in SIRS

  1. Widespread endothelial activation and injury
    Causes vasodilatation, increased vascular permeability, edema, and leukocyte adhesion.
  2. Hypotension and reduced tissue perfusion
    TNF, IL-1, and nitric oxide cause systemic vasodilatation. Capillary leak reduces effective intravascular volume.
  3. Activation of coagulation
    Cytokine-activated endothelium becomes procoagulant. Microvascular thrombi further impair perfusion and can cause DIC.
  4. Neutrophil-mediated tissue damage
    Neutrophils release reactive oxygen species, lysosomal enzymes, and proteases, damaging lungs, kidneys, liver, and other organs.
  5. Metabolic abnormalities
    Cytokines cause insulin resistance, hyperglycemia, protein catabolism, and impaired myocardial function.
  6. Multiorgan dysfunction syndrome (MODS)
    Persistent cytokine-mediated inflammation and microvascular dysfunction may result in:
    • ARDS
    • Acute kidney injury
    • Hepatic dysfunction
    • Encephalopathy
    • DIC
    • Myocardial depression
    • Shock
A review of SIRS describes TNF-alpha and IL-1beta as early cytokines that trigger further mediator release, while persistent cytokine elevation is associated with poor outcomes and organ dysfunction (SIRS cytokine review).

Compensatory anti-inflammatory response

SIRS is not solely pro-inflammatory. The body also releases anti-inflammatory mediators:
  • IL-10
  • TGF-beta
  • Soluble TNF receptors
  • IL-1 receptor antagonist
Initially these responses limit excessive inflammation. However, if the anti-inflammatory response becomes excessive or prolonged, it may cause immunosuppression, impaired microbial clearance, and susceptibility to secondary infections.

Short exam conclusion

Cytokines are protein messengers that coordinate innate and adaptive immune responses. In SIRS, excessive release of TNF-alpha, IL-1, IL-6, chemokines, and other mediators causes generalized endothelial activation, vasodilatation, capillary leakage, leukocyte-mediated injury, coagulation abnormalities, shock, and multiorgan dysfunction. TNF and IL-1 are the principal early mediators, whereas IL-6 is central to the acute-phase response. Anti-inflammatory cytokines such as IL-10 and TGF-beta attempt to control the response but may later lead to immunosuppression.
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