cytokines, chemokines and hypersensitivity reactions

week 6 clinical immunology

cytokines

• small proteins released in response to an activating stimulus via specific receptors

• many cytokines are called interleukin

• different to hormones as:

  • function relates to immune system (immunomodulating agens)

  • concs change by many orders of magnitude

  • most act over a short distance

  • may be produced by and act on a variety of cells

a broad range of cells can produce cytokines

• all immune cells are responsive to at least some cytokines

• cytokines may be produced by more than one type of cell

• cytokines act through receptors

• cytokines may act in autocrine, paracrine and endocrine mofes

• all nucleated cells can produce type I interferon

• endothelial cells are particularly sensitive to inflammatory cytokines as they regulate vascular adhesion and permeability

cytokine mode of action

• autocrine: a cytokine may bind via receptors of the same cell that secreted it

• paracrine: a cytokine may bind to receptors on target cells near the cell that secreted it

• endocrine: a cytokine may target cells in distance parts of the body

cytokines induce the acute-phase response

• TNF-α, IL-1β and IL-6 activate hepatocytes to synthesise acute-phase proteins and activate bone marrow endothelium to release neutrophils

• these acute phase proteins act as opsonins

• TNF-α, IL-1β and IL-6 are endogenous pyrogens, raising body temp

• induce synthesis of prostaglandin E2, which acts on the hypothalamus altering the body’s temperature regulation and on muscle and fat cells

• alter energy mobilisation to increase body temp

biological effects of cytokines

• pleiotropy: cytokine has different effects on different target cells

• redundancy: >2 cytokines mediate similar functions

• synergy: combined effect of 2 cytokines is greater than the addictive effect of the individual cytokines

• antagonism: 1 cytokine inhibits or offsets the effects of another cytokine

• cascade induction: action of 1 cytokine on a target cell infuces the cell to produce 1 or more other cytokines which in turn may induce other target cells to produce other cytokines

cytokines can be grouped by structure into families

• cytokine receptors belong to families of receptor proteins, each with a distinctive structure

• heterodimeric class I cytokines receptors have an α chain that often defines the ligand specificity of the recepto

• they may share with other receptors a common β and γ chain that confers the intracellular signalling function

• may cytokine receptors signal through the JAK-STAT pathway

haemopoietin superfamily of cytokines

• includes erythropoietin and interleukins (IL-6) and GM-CSF

  • IL-6 is released by activated macrophages and modulates the immune response, can trigger fever- active phase reponse and neutrophil production from bone marrow

  • IL-6 doesn’t activate endothelial cell expression of adhesion molecules or trigger overt inflammation

• receptors are tyrosine kianse-associated receptors that form dimers when their cytokine ligand binds

• dimerisation initiates cellular signalling from the tyrosine kinases associated with the cytoplasmic domains of the receptor

• haematopoietin receptors all signal through the JAK-STAT pathway

IL-1α is a damage associated molecular pattern (DAMP)

• IL-1α is released by damaged or necrotic cells

• IL-1α is not active in the native state (proteolytic cleavage outside the cell)

• functions as an alarmin

• active IL-1α binds the IL-1 receptor and triggers inflammation in the same way that IL-1β does

• dual function cytokine

  • nuclear localisation/TF activity

  • signal transduction

IL-1β requires caspase activation

• specific inhibitors of caspase-1 reduce the secretion of mature IL-1β, while precursor IL-1β accumulates inside the cell

• primary stimulus increases expression of inactive pro- IL-1β

• inflammasomes are activated by NLRPs, resulting in caspase-1 activation

• caspase-1 cleavage of pro-IL-1β yields mature, active IL-1β

• IL-1β leaves the cell to trigger inflammation in neighbouring cells

TNF family

• >17 cytokines

• many members are transmembrane proteins- homotrimers

• TNF-α, initially expressed as a trimeric membrane bound cytokine but can be released from the membrane

• TNF receptor I (TNFR-I) is expressed on a wide range of cells including endothelial cells and macrophages

• TNFR-II is expressed largely by lymphocytes

• signalling uses members of the TRAF family to activate the non canonical NFκb pathway

TNF-α is a potent inflammatory cytokine

• produced mainly by activated macrophages

• in lower quantities by some CD4+ T cells, NK cells, neutrophils, mast cells and eosinophils

• TNF-α is initially expressed as a transmembrane homotrimer, which can be cleaved by the protease TNF-α converting enzyme (TACE) to release soluble TNF-α

• able to induce fever, apoptotic cell death, cachexia and the acute phase response

interferon-γ promotes intracellular defences

• IFN-γ is produced predominantly by NK cells, Th1 and cytotoxic T cells

• IFN-γ promotes:

  • microbicidal activity of macrophages

  • Th1 polarisation of CD4+ T cells

  • increased MHC expression by APCs

  • class switching to opsonising and complement fixing isotypes

normal inflammatory response

• balance between pro- (IL-10, IL-1RA, TGF-β) and anti-inflammatory cytokines (produced hours after production of pro-inflammatory cytokines)

• infection stimulates macrophages to release cytokines and chemokines that initiate an inflammatory response

systemic inflammatory response syndrome (SIRS)

• SIRS is an inflammatory state affecting the whole body which can lead to shock, disseminated intravascular coagulation and ultimately multiple organ failure

• proinflammatory mediators enter systemic circulation, acting on cells of tissues far away from original inflammatory process

• sepsis- bacterial infection of the blood

TNF-α triggers local containment of infection but induces shock when released systemically

• local release of TNF-α vs systemic release

• in both cases, TNF-α acts on blood vessels

  • ↑ blood flow

  • ↑ vascular permeabilioty to fluid, proteins and cells

  • endothelial adhesiveness for leukocytes and platelets

• blood clots prevent spread of infection via blood, accumulated fluid and cells drain to regional lymph nodes, initiates adaptive immune response

• systemic infection or sepsis with bacteria: TNF- α is released into blood by macrophages in the liver and spleen, acts in a similar way on all small blood vessels in the body

• result is shock, disseminated intravascular coagulation with depletion of clotting factors and consequent bleeding, multiple organ failure and death

chemokines are a subset of cytokines

• chemoattractant cytokines recruit cells along a chemotactic gradient

• heparin-binding proteins

  • homeostatic (constitutively produced in some tissues to regulate leukocyte trafficking)

    or

  • inflammatory (attracting immune cells to the site of inflammation)

4 classes of chemokines

• classification based on the locations of the cysteine residues that yield structural disulphide bonds (CXC (α), CC (β), CX₃C, C)

• many CC and CXC chemokines play a role in inflammation

• CC chemokines (β) tend to recruit monocytes

• CXC chemokines (α) tend to recruit neutrophils

• CC chemokines: monocyte chemoattract protein-1) MCP-1 or CCL2 induces monocytes to leave the bloodstream and enter the surrounding tissue

• the only CX₃C chemokines discovered to date is CX₃CL1

cytokine receptors fall within five families

IL-2R

• class 1: conserved cysteines (double black lines) and WSXWS motifs (red lines)

• IL-2R is trimeric:

  • α chain: only expressed by activated T cells (also called TAC: T cell activation antigen)

  • β and γ chains belong to the class I cytokines receptor family (CCCC, WSXWS)

• IL-2R occurs in 3 forms

• the forms have different affinities for IL-2

• activated T cells express 5×10³ high affinity receptors

• NK cells: consitutively expressed β and γ chains provide intermediate IL-2 affinity and subsequent activation

cytokine receptor

• cytokine receptors of the haematopoietin superfamily are associated with the JAK family of tyrosine kinases, which activate STAT TFs

• JAK (janus kinase): 2 tandem kinase-like domains

macrophage polarisation

• the process by which undifferentiated macrophages produce distinct functional phenotypes as a reaction to specific microenvironment stimuli and signals

• classically activated M1 macrophages

• alternatively activated M2 macrophages

M1 and M2 macrophages

TH1 cells are the principal T cell helpers for macrophages

• some pathogens are not killed by macrophages

• peptides derived from such microorganisms can be displayed by MHC class II molecule to TH1 cells

• TH1 cells synthesise membrane-associated proteins and secrete cytokines that enhance the macrophages antimicrobial defences

• classical macrophage activation: M1 macrophage

M1 macrophage (activated byt Th1 cells)

• activated macrophages increase expression of CD40 and of TNF receptors and secrete TNF-α

• CD40 ligand and TNF-α synergise with IFN- γ secreted by Th1 cells to induce classicial or M1 macrophage activation whhich is characterised by production of NO and superoxide

• B7 molecuoles are upregulated in response to binding CD40L and TNF-α

• increased MHC class II expression in response to IFN-γ enables a positive feed-forward loop that enhances activation of Th1 cells

  
  

Th2 cells recruit and activate M2 macrophages via IL- 4 and IL-13

• macrophages activated by Th2 cells have increased effectiveness in eradicating helminths and promote tissue repair responses

• upregulated expression of IL-4 and IL-13 receptors

• expression or arginase-1 results in secretion of orthinine and proline

• M2 macrophages can repress local tissue inflammation through IL-1 receptor antagonist (IL-1RA) and a decoy IL-1 receptor (IL-1RII)

• M2 macrophages also produce the anti-inflammatory cytokine IL-10 and TGF-

CD4+ T cells can polarise into 4 major subsets

• subsets are elicited by different classes of pathogens

• defined on basis of different combinations of cytokines they secrete

IL-2 = universal activator of T cell proliferation

• IL-2 acts in an autocrine manner

• IL-2 is switched on when memory T cells get activated

• lack of IL-2R is the cause of severe combined immunodeficiency

4 major types of hypersensitivity

type I hypersensitivity (immediate hypersensitivity)

• most common immune disorder

• underlying causes of allergy

• related consitions include asthma, hay fever

• mediated by allergen-specific IgE ABs

• class-switching to IgE is dependent on Th2 cytokines

development of type I hypersensitivity

• IgE molecules bind to Fc receptors (FcεRI/CD23) on the surfaces of mast cells and basophils

• cross-linking of surface-bound IgE molecules generates intracellular signals via CD23

  • leads to mast cell/basophil degranulation

• vascular endothelial cell junctions loosen, increasing vascular permeability with subsequent fluid accumulation in tissues

• smooth muscle contraction accelerated fluid distribution from central trunk of body into peripheral tissues

characteristics of type I immediate and late phase reactions

repeated type I hypersensitivity reactions can cause tissue damage

• allergic responses can resolve quickly after exposure to allergen leaving little tissue damage

• repeated immediate hupersensitivy reactions within tissue can result in tissue remodelling and organ dysfunction

• approx 1 in 5 have atopy

anaphylactic shock is an extreme variant of type I hypersensitivity

• allergen may be delivered via an insect sting or absorption across intestinal tract

• if individual has high traces of IgE, there is a danger of anaphylactic shock

• mast cells are activated systemically by circulating antigen

  • causes vascular dilation and plasma leakage into tissues throughout the body, resulting in low blood pressure

  • large quantities of mucus may eb secreted into airways, airway smooth muscle contraction caused by mast cell derived mediators may cause difficulty breathing

  • current therapies: adrenaline, anti histamines

mechanism of action used to treat type I hypersensitivity

type II hypersensitivity is mediated by IgM/IgM targeting self antigens

• ABs may be generated that bind to self antigens

• is target is an immoblile tissue antigen, response is referred to as type II hypersensitivity

• pathology is restricted to tissue where the target antigen is expressed

3 mechanisms of type II hypersensitivity:

1. cells may be damaged by complement activation caused by the cross-linking of ABs

2. cells may be opsonised by ABs and targeted for phagocytosis

3. AB dependent celluar cytotoxicity (ADCC) is mediated by IgG ABs bound to antigen on the surface of tagrte cells

   • recognised by FcYRII on NK cells which degranulate and kill target cell

• the binding of cellular receptors by ABs may interfere with their normal function so cause disease

autoimmune haemolytic anaemia (AIHA)

• medsiated by IgG (causing phagocytosis of RBC) or IgM (causing complement lysis of RBC) but it’s not clear why these ABs rise

• RBCs lifetime reduced to just several days in serious cases

• AIHA is generally self limiting in children but can be more serious and require long term immunosuppression

Goodpasture’s Syndrome (GPS) / Anti-Glomerular Basement membrane disease

• ABs attack the basement membrane in the lungs/kidneys

• target host antigen is the α-3 subunit of type IV collagen

• can result in bleeding from the lungs and kidney failure

• treatment:

  • immunosuppressant drugs (corticosteroids)

  • plasmapheresis (ABs are removed from the circulation)

diseases caused by type III hypersensitivity

type III hypersensitivity is mediated by antigen-AB complexes

• large quantities of self or non self antigen reach the blood and form complexes with specific IgG or IgM ABs

• such circulating immune complexes are commonly generated at a low level during infections and part of normal body processes

• these immune complexes are cleared quickly from the circulation by the liver/spleen with help of complement binding

systemic lupus erytheamatosus

• autoimmune disease driven by immune complexes formed between autoABs and host cell nuclear antigens

• common symptoms: rashes, arthritis, glomerulonephritis

• has both genetic and environmental risk factors

• affects women more commonly than men

serum sickness

• caused by injection of large quantities of a poorly catabolised foreign antigen

• occurs 7-10 days after injection

• ABs form immune complexes with their antigens, activate complement and bind Fc receptors on leukocytes (tissue damage)

• symptoms:

  • urticaria (rash): histamine from mast cell degranulation (via FcγRIII by IgG immune complexes and anaphylatoxins C3a and C5a released due to complement activation)

  • injury to tissues/organs (skin, kidneys, nerves)

diseases caused by type IV hypersensitivity

type IV hypersensitivity mediated by CD4+ or CD8+ T cells (delayed type hypersensitivity)

• often caused by cytokines from activated CD4+ Th cells, particularly Th1 and Th17 cells

• CD8+ T cell responses may also promote a type IV hypersensitivity response

• responses are directed against a protein or peptide epitope to which effector T cells have already been generated

• CD4+ T cell responses are believed to play a key role in many autoimmune diseases

• often a long lag phase between exposure to the antigen and the resulting inflammatory response

delayed type hypersensitivity can fall under 3 major categories

• contact

  • T cell responses to environmental antigens from skin contact

  • 2-3 days max reaction time

• granulomatous

  • T cell cytokines promote formation of giant cells from macrophages

  • 2-3 weeks

• tuberculin

  • T cell response to injected tuberculin

  • 2-3 days

type V hypersensitivity: stimulatory reactions

• ABs activate receptors and unwanted outcomes

  • grave’s disease

  • myasthenia gravis

Grave’s Disease

• Grave’s hyperthyroidism is caused by the stimulation of thyroid stimulating hormone (TSH) receptor by TSH receptor autoABs

• approx 35-50% of patients with Grave’s have clinical eye involvement

• most Grave’s opthalmology patients have an increase in both orbital fat and extraocular muscle volumes

myasthenia gravis

• long term neuromuscular disease that leads to varying degrees of skeletal muscle weakness

• commonly affected muscles: eyes, face and swallowing

• ABs inhibit binding of Ach to Ach receptor, inhibiting transmission of signals from nerve endings to muscle fibres