Resolution of Inflammation and tissue repair Flashcards
What happens after the pathogen is gone? ( explain using T cells as focus)
- T Cells recruited to the tissue continue to need signals including polarising cytokines, to stay committed to being effector cells
> If the cytokine signal disappears or alters the T cells change phenotype or undergo apoptosis.
» The trigger for cytokine changes is a reduction in the level of stimulus or pathogen in the tissue
….Less IL-12 via macrophages
What are the 3 phases in an immune response?
During an immune response how do the cells and cytokines change?
- At the start of an infection = acute inflammation
- Pathogen persists = adaptive immune response is triggered with an antigen specific T cell response
- As the pathogen starts to be cleared = body needs to reduce the levels of inflammation and repair any damage
How are Treg cells stimulated to reduce inflammation?
- Pathogen is removed and the signal driving pro-inflammatory cytokine release disappears, Toll like receptor stimulation reduces.
- The tissue starts producing TGFβ to stimulate repair.
> When T cells enter the tissue the predominant cytokines drive them to become Tregs
» These cells can inhibit effector T cells and change macrophage phenotype to anti-inflammatory
Macrophage phenotypes change?
What is the aggressive macrophage phenotype?
What stimulates it?
What does it do?
M1 > Sources of pro-inflammatory cytokines and IFN gamma give a very aggressive macrophage
> Releases:
- Pro-inflammatory
- Involved in host defence,Phagocytose and present antigen
- Produce nitric oxide
There are M2 - ‘Alternative macrophages’.
1 -What happens when macrophages receive different cytokines, such as IL-13 or IL-4 in the presence of high levels of Th2 cells?
What do they produce?
2 -What happens if macrophages encounter apoptotic cells or high levels of IL-10?
What do they produce?
1-
- Mediate wound healing and blood vessel growth (angiogenesis)
- Don’t produce nitric oxide
- Reduced MHCII presentation
- Inhibit T cell responses
2-
- Secrete repair mediators including growth factors
- Activate fibroblasts and stromal cells,
- Recruit Tregs
- Increase angiogenesis
What do MMP/ IL-1RA do?
- IL-1RA : Soluble receptor to IL-1 to stop it from interacting with its receptor on blood vessels.
- Matrix metalloproteinases:
Break up components of ECM… Help remodel damaged tissue and repair it
How does apoptotic neutrophils and the reduction of cytokine expression from reduced TLR signalling initiate the change in the tissue?
- The presence of the pathogen and pro-inflammatory cytokines (particularly TNFα) maintains the neutrophils.
- Infection is cleared the stimulus to produce pro- inflammatory cytokines is reduced and the signal to survive is lost.
> Neutrophils then undergo apoptosis - Apoptotic neutrophils secrete ‘find me, eat me’ signals which attract macrophages and inhibit further neutrophil recruitment.
- Macrophages recognise the phosphatidylserine which is placed on the outer membrane of the cell during apoptosis
> Phagocytosis of the apoptotic neutrophils changes the macrophage phenotype and its starts producing anti-inflammatory factors associated with resolution - High levels of TGFβ in the tissue induce recruited T cells to become regulatory T cells
Summarise inflammation briefly from Injury/infection leading to the release of DAMPS/PAMPS > Treg stimulation.
Define tissue repair.
What are the 3 main processes?
What 3 cells drive it?
> The restoration of tissue architecture and function after an injury
- Inflammation - Blood vessels constrict sealing themselves off. Intact vessels dilate to allow WBCs and immune proteins in
- Proliferation – Granulation tissue forms. New vessels grow to supply the oxygen and collagen is deposited.
- Maturation – The wound has closed and is remodelled to increase tensile strength
How a tissue responds to damage will depend on the tissue type.
What are the 3 tissue types?
Give examples when explaining.
- Continuously dividing tissues
> Can regenerate with minimal issues
(Haematopoietic, Epithelia) - Quiescent tissues
> Can regenerate with minimal issues
(Liver, smooth muscle, fibroblasts) - Non dividing tissues
> Repair leaves evidence such as a scar or thickening
(Cardiac and skeletal muscle)
How a tissue responds to damage will depend on the damage.
What happens during acute inflammation and chronic inflammation?
- What is the ECM? Why do we have integrins?
- Describe the structure of the ECM. ( What are the 3 components and their functional role)
> Store for growth factors and cytokines – Including TGF-β in its latent form as well as proteases which when activate promote repair.
The integrins allow cells to attach to the ECM and provide a scaffold for cells to migrate across
What are:
- Matrix metalloproteinases & Tissue inhibitor of metalloproteinases
-Specify the effects of MMPs on ECM.
- Matrix metalloproteinases:
- Family of zinc containing proteases.
- Synthesised as inactive enzymes (zymogens) which need to be cleaved into an active enzyme. They can activate each other in a cascade
- Can be grouped by function into- collagenases, gelatinases, stromelysins, matrilysins and membrane type MMPs
- They have various affects on the ECM:
> Degradation to clear areas > Release bound angiogenic factors
> Expose integrin binding sites - TIMPs : Four different TIMPs they can inhibit all of the MMPs
-Inhibitors are important in order to ensure a balance so that the ECM isnt continuously degraded so that repair can occur.
What is the function of the following growth factors involved in tissue repair?
Using skin as an example, what are the 2 ways wounds can be classified?
1- First/Primary intention
> Small size with easily approximated margins These heal with minimal scarring
2- Second/ Secondary intention
> Larger wounds with more epithelial destruction and more collagen deposition.
> Wound contraction eases the problem.
> Specialised smooth muscle like cells called myofibroblasts appear at the wound edge and drag the edges towards each other.
> Can decrease wound size by up to 80% These commonly heal with visible scars
What is the first stage immediately after wounding?
> Inflammation
- blood loss is minimised and the immune system is involved in clearing debris and pathogens
Describe what happens during stage 1 after injury.. Inflammation.
- Immediately Damaged vessels constrict and are capped to prevent blood loss, platelets form a clot
- Kinin system of plasma proteins is activated by the vessel wounding, releasing bradykinin which increases vascular permeability to allow plasma proteins into site.
> Also causes pain stimulating receptors on nerves to encourage us to immobilise the injured part. - Inflammatory cytokines contribute to dilation of the intact vasculature to allow neutrophil recruitment.
> Formation of inflammatory exudate. - Infective cause if present is removed. Debris is broken down and removed/liquified
- Resident macrophages and platelets are an early source of growth factors
including – Epidermal growth factor and transforming growth factor α (TGFα has the same function as EGF to promote Epithelial cell growth)
What happens after inflammation?
> Proliferation
- Inflammation subsides and granulation tissue is formed. - Granulation tissues is a ‘soup’ of fibroblasts secreting collagen and new blood vessels forming which will eventually recede and be remodeled over time
Describe the process of proliferation.
- Increase in macrophages in the tissue site which clear the wound and start to secrete growth factors
- On days 1-3 post wounding fibroblasts start to secrete Type III collagen to immediately repair the ECM
- Days 3-5 TGFβ stimulates a rapid increase in collagen deposition and inhibits its degradation
- The epithelial cells continue to proliferate and migrate to close the wound
- During this phase the rapid proliferation requires more oxygen and there is rapid growth of new blood vessels – Angiogenesis. This will supply the oxygen and nutrients needed for the repair
Describe the process of angiogenesis.
The formation of new blood vessels
- Essential for providing oxygen to damaged tissues to allow repair.
What process occurs after proliferation?
> Maturation
- Involves further collagen deposition and remodelling of the tissue to increase the strength of the repair
Describe the process of maturation.
What happens longterm?
In the first month after wounding repair plateaus at what percentage of original strength?
- Proliferation can leave the tissue site slightly raised in the final phase the architecture of the tissue needs to be remodelled back to near normal
- Leukocytes have left the wound site and fibroblasts proliferate
- Excess blood vessels will recede and be trimmed leaving a few to mature
- The number of fibroblasts increases as does their rate of collagen synthesis
- Fibroblasts start to secrete predominantly type I collagen.
- Over the next few weeks the ECM remodels with a balance of MMPs breaking down components and TIMPs inhibiting this process.
> Long term the wound will continue to remodel with increased collage synthesis and post synthesis modification including cross linking increased fibre size and strength.
70-80% of original strength
What 2 factors influence how well a tissue heals?
> Internal and external factors can influence how well a tissue heals
- Local Factors
* Size of the wound
* Presence of foreign bodies
* Location - highly vascularised areas heal quicker
* Infection – Beta haemolytic streptococcus organism prevent healing.
Bacterial endotoxins can stimulate collagenase secretion - Systemic factors
* Diabetes - patients have less granulation tissue, less collagen deposition and defects in collagen maturation
* Poor nutrition – e.g. Vit C needed for collagen synthesis low levels can result in unstable poorly linked collagen
* Compromised blood flow – cardiac insufficiency or arteriosclerosis
* Certain drugs – e.g. glucocorticoids inhibit inflammation and collagen formation
* Inflammation – uncontrolled inflammation will prevent the resolution of wounding and
cause too much damage
What causes fibrosis and scarring?
- Dependson the wound size and depth.
> Scars form from collagen deposition raising the skin, this will then slowly start to flatten as the wound matures.
> Pathological scarring can occur in some cases and is related to genetic and environmental facotrs
What are hypertrophic scars?
- Occur within 4 weeks of injury and can regress over time
- Linked to an aberrant autocrine loop with myofibroblasts secreting excess TGFβ.
- Too much collagen formed
What are keloids?
- Appear around a year after an injury some can grow years later.
- Scars grow outside of wound boundary with a mass of fibrous tissue.
- High numbers of fibroblasts.
- Patients can have pathologically high levels of VEGF
