Induced Innate Responses Flashcards
What are cytokines?
- Small proteins, about 25kDA released by various cells in the body in response to an activating stimulus.
- Help induce and maintain inflammation
- Can act in an autocrine, paracrine or endocrine manner.
- Activation of PRRs expressed by macrophags and DCs induce cytokine production that activate the cellular arm of the innate immune system.
Describe the IL-1 family cytokines
- Contains 11 members., notably IL-1a, IL-1B and IL-18
- Most members of this family produced as inactive pro-proteins that are cleaved (removing an N-terminal peptide) to produce the mature cytokine
- IL-1B and IL-18 produced by macrophases in response to TLR signaling are cleaved by caspase 1
- The IL-1 family receptors have extracellular regions composed of Ig-like domains and they signal as dimers though TIR domains in their cytoplasmic tails. They signal through the NFkB pathway.
Describe the haematopoetin family cytokines
- Includes non-immune system growth and differentiation factors such as EPO and growth hormone, as well as ILs with roles in innate and adaptive immunity
- IL-6 is a member, as is GM-CSF
- This family cytokines act through dimeric receptors that fall into several structural subfamilies characterized by functional similarties and genetic linkage
- One chain defines the ligand specificity for the receptor, whereas the other confers the intracellular signaling function. They signal through the JAK/STAT pathway.
Describe the interferon receptors
- Small family of hterodimeric receptors that recognice the type 1 IFNs as well as some cytokines, such as IL-4 , that are produced by T cells
- Also signal through the JAK/STAT pathway.
Describe the TNF family cytokines
TNF Family:
• TNF-a is the prototype
• Contains more than 17 cytokines
• Many members are transmembrane proteins, which limits their range of action – although some can be released under certain circumstances
• Usually found as homotrimers of a membrane-bound subunit, although some heterotrimers occur.
• Ligand binding induces trimerisation of the receptors, which contain death domains in their cytoplasmic tails, and signal though death domain containing receptors such as FADD.
• TNF-a is initially expressed as a trimeric membrane-bound cytokine but can be released. The effects of TNF-a are mediated by either of two TNF receptors
• TNFR-1 is expressed on a wide range of cells, including endothelial cells and macrophages
• TNFR-II is expressed by lymphocytes
Describe the functional effects of cytokines
- IL-1B → produced by proteolytic cleaveage rather than gene transcription, so faster!
- Local = activates vascular endothelium and lymphocytes, local tissue destruction.
- Systemic = fever, production of IL-6
- TNF-a →
- Local = activates vascular endothelium, increases vascular permeability, leads to increased entry of IgG and complement. Induces local inflammatory response that helps contain infections.
- Systemic = fever, shock
- IL-6 →
- Local = lymphocyte activation, increased antibody production
- Systemic = fever, APP production
- CXCL8 →
- Local = chemotactic factor, recruits neutrophils, basophils and T cells
- IL-12 →
- Local = Activates NK cells, induces the differention of CD4 into TH1 cells
What are chemokines?
- Among the cytokines released in the earliest phases of infection are members of a family of chemoattractant cytokines known as chemokines.
- Theres induce directed chemotaxis, resulting in the movement of cells towards the source of the chemokine.
- All chemokines are related in amino acid sequence
- Their receptors are GPCRs
- Signalling pathways stimulated by chemokines cause changes in cell adhesiveness and the cell cytoskeleton that lead to directed migration.
- Not just secreted by immune cells, but in the immune system they function mainly as chemoattractants for leukocytes, recruiting monocytes, neutrophils, and other effector cells of innate immunity.
What are the two groups of chemokines?
- CC chemokines → two adjacent cysteines
- Promote the migration of monocytes and lymphocytes
- CCL2: attracts monocytes to become tissue macrophages
- CCL5: degranultes basophils
- CXC chemokines → the two cysteine residues are separated by a single amino acids
- Promotes neutrophil migration
- CXCL8: mobilises, activates and degranulates neutrophils
Describe the selectins
- Membrane glycoproteins with distal lectin-like domains that bind specific carbohydrate groups
- Induced on activated endothelium
- Bind to the sialyl-lewis proteins (fucosylated ligands on passing leukocytes)
- Endothelial activation is driven by TNF-a, which induces the externalization of granluse called Weibel-Palade bodies
- These contain pre-formed P-selectin
- Shortly after P-selectin gets to the surface, mRNA encoding E-selectins is synthesized, and within 2 hours the endothelial cell is expressing mainly E-selectin
Describe the ICAMs and integrins
• The next step in leukocyte recruitment depends on tighter adhesion – due to the binding of ICAMs (intercellular adhesion molecules) on the enodhtelkum to integrins on leukocytes
• ICAMs are single pass TM proteins that belong to the Ig-like superfamily
• Intregrins are composed of two TM chains, an alpha and beta → the integrins important for extravasation are LFA-1 and CR3
• Both LFA-1 and CR3 bind ICAM-1 and ICAM-2
• In the absence of infection, monocytes are continuously entering tissues where they become resident macrophages → mediated by ICAM-2, expressed at low levels by unactivated endothelium
• Strong adhesion is promoted by the induction of ICAM-1 on inflamed endothelium, together with a conformational change in LFA-1 and CR3
− Integrins can switch between an active and inactive state
− In the activated state, integrin molecule is linked via the intracellular protein talin to the actin cytoskeleton
− Chemokines bind receptors on the leukocyte, generating intracellular signals that cause talin to bind to the cytoplasmic tails of LFA-1 and CR3, forcing the extracellular regions to assume an active binding conformation
• People with leukocyte adhesion deficiencies suffer from recurrent bacterial infections and impaired healing of wounds.
Describe the process of leukocyte extravasation
- Under normal conditions, leukocytes travel in the centre of small blood vessels where the flow is fastest
- In sites of inflammation, the vessels are dilated and the slower blood flow allows leukocytes to interact with the endothelium
- P-selectin appears on the endothelium within a few minutes of exposure to leukotriene B4, histamine, C5a or TNF-a (after release from Wiebel-Palade bodies)
- E-selectin appears a few hours later (after mRNA transcription)
- Selectins interact with Sialyl-lewis X proteins on neutrophils → weak binding allows leukocyte to roll along the endothelium surface.
- LFA-1 and CR3 biind ICAM-1 → usually bind only weakly, but CXCL8 or other chemokines bind to chemokine receptors on the leukocyte, triggering the conformational change in LFA-1 and CR3 that allow for firm attatchment.
- LFA-1 interacting with PECAM expressed both on the leukocyte and at the intercellular junctions of endothelial cells enable the phagocyte to extravasate between the endothelial cells.
- The leukocyte then penetrates the BM with the aid of enzymes that break down the ECM → diapedesis.
- Chemokines sucha s CXCL8 & CCL2 produced at the site of infection encourage the migration of the leukocytes through the tissues
• Neutrophil influx usually peaks within the first 6 hours of an inflammatory response, whereas monocytes can be recruited later through CCL2.
How does TNF-a control local infections?
- TNF-a acting on endothelial cells stimulates the expression of adhesion molecules and aids the extravasation of monocytes and neutrophils
- Another important action is to stimulate endothelial cells to express proteins that trigger blood clotting in the local small vessels, occluding them and cutting off blood flow → important in preventing the pathogen entering the bloodstream and spreading to other organs
- Instead, the fluid carries the pathogen via the lymph to the LNs where an adaptive immune response can be initiated
What happens during systemic infection?
- Once an infection has spread to the bloodstream, the same mechanisms by which TNF-a so effectively contains local infection instead become castastrophic
- TNF-a cleaved by TACE and released from the membrane to act as a soluble cytokine
- Sepsis (infection in the blood) is accompanied by a massive release of TNF-a from macrophages → causes vasodilation
- Vasodilation leads to a loss of blood pressure, increased vascular permeability, loss of plasma volume and eventually to septic shock
- This septic shock triggers blood closing in smaller vessel throughout the body → disseminated intravascular coagulation. Leads to the massive consumption of clotting proteins, so the patients blood cannot clot appropriately
- DIC frequently leads to failure of kidneys, liver, heart and lungs
Describe how TNFa, IL-6 and IL-1B contribute to fever
- One of these is elevation of body temp, caused by TNF-a, IL-1B and IL-6 → these cytokines termed endogenous pyrogens because they cause fever.
- Cause fever by inducing synthesis of prostaglandin E2 by the enzyme COX-2 (expression of which is induced by these cytokines)
- Prostaglandin E2 then acts on the hypothalamus, resulting in an increase in heat production by brown fat, and increased vasoconstriction – decreasing heat loss through the skin
- Exogenous pyrogens include LPS – these can cause fever by inducing production of exogenous pyrogens, and directly inducing COX-2
- Fever is generally beneficial to host defense, as pathogens grow better at low temps
Describe the acute phase response initated by TNFa, IL-6 and IL-1B
- Action of cytokines in liver hepatocytes change the profile of proteins they secrete → these proteins are called acute phase proteins
- C-reactive protein →
- member of the pentraxin family
- binds to certain bacterial and fungal cell wall polysaccharides
- acts as an opsonin
- also activates complement by bindint to C1q
- Mannose binding lectin →
- recognizes mannose residues on microbial surfaces
- acts as an opsonin
- activates complement
- Surfactant proteins SP-A and SP-D →
- Found along with macrophages in the alveolar fluid of the lung
- Promote phagocystosis of opportunistic respiratory pathogens
- Thus, within a day or two, the acute phase response provides the host with several proteins with the functional properties of antibodies, but able to bind a broad range of pathogens.
- However, unlike anitbodies, they have no structural diversity, and are made in response to any stimulus that triggers release of TNF-a, IL-1B and IL-6 → synthesis is specifically induced and targeted
