L1 + L2: The foreign body response

Question 1

Two types of local interactions between biomaterial and tissue

Answer 1

1. Effect of material on host tissues: blood-material interactions, toxicity, modification of healing, inflammation, infection, tumorigenesis.
2. Effect of environment on materials: (physical-mechanical effects) wear, fatigue, corrosion, stress-corrosion cracking, (biological effects) tissue absorption of implant constituents, enzymatic degradation, calcification.

Question 2

Factors that influence the host response

Answer 2

1. Biomaterial-related factors: composition (material), degradability, mechanical properties, sterility, antigenicity, active ingredients (drugs).
2. Host-related factors: age, gender, anatomic location, previous interventions, comorbidities, immune response, medications.

Question 3

What are the lines of the immune system?

Answer 3

First line of defense: mechanical barriers (skin, mucus membranes).
Second line of defense: innate immune system
Third line of defense: adaptive immune system

Question 4

When do the innate immune response and adaptive immune response occur after the injection?

Answer 4

Innate: response quickly, intense
Adaptive: slowly the first time, memory

Question 5

Phagocytes =

Answer 5

= cells that protect the body by ingesting (phagoctytosis) harmful particles, bacteria, and dead or dying cells.

Question 6

Two types of phagocytes

Answer 6

Granulocytes
Monocytes

Question 7

Granulocytes=

Answer 7

“Eat or detonate”
Robust cells, weird looking nucleus
Short-lived; hours.

A type of immune cell that has granules (small particles) with enzymes that are released during infections, allergic reactions, and asthma. Neutrophils, eosinophils, and basophils are granulocytes. A granulocyte is a type of white blood cell.

Question 8

Monocytes

Answer 8

“Eat or encapsulate”
Precursor of macrophages
Nucleus looks like a kidney

Question 9

How do phagocytes know what to eat?

Answer 9

Pattern-recognition receptors (PRRs)
or via integrins

Question 10

Two types of PRRs

Answer 10

1. PAMPs (=Pathogen-Associated Molecular Patterns)
   - Exogenous; outside your body (bacteria)
2. DAMPs (=Damage-Associated Molecular Patterns) = alarmins:
   - Endogenous; inside your body (stuff inside a cell, intracellular parts, e.g. when cells die, this will come out.
   - E.g.: Necrotic cells → cytoplasmic & nuclear components (e.g. heat shock protein (HSP), high-mobility group protein 1 (HMGP1), ATP).
   - Damaged extracellular matrix → ECM fragments.

Question 11

Cascade of the host response to biomaterial implants

Answer 11

1. Implantation/injury
2. Blood-material interactions (protein adsorption. coagulation, complement system)
3. Acute inflammation (neutrophils)
4. Chronic inflammation (macrophages)
5. Granulation tissue
6. FBR
7. Fibrosis/ fibrous encapsulation

Question 12

3 things happen during injury

Answer 12

1. Release of histamine by mast cells → vasolidation of capilary vessels (blood vessels will open up, increase blood flow, cells can go through the walls). Splinter; finger becomes swollen and red.
2. Release of cytokines:(by activated platelets, endothelial cells, mast cells, local tissue macrophages) → activate endothelium & attract immune cells (‘homing signals’ attracts endothelium. Chemokines are cytokines that attract immune cells.
3. Danger signals (DAMPs) (by necrotic cells and damaged cells) → activate immune cells.

Question 13

5 hallmark signs of inflammation

Answer 13

• Pain (‘do not use this body part anymore’)
• Heat (helps cells recover (metabolism increases), stops pathogens)
• Redness (due to increased blood flow)
• Swelling (due to vasodilation (blood plasma → hemostasis))
• (Sometimes: loss of function)

Question 14

What is leukocyte extravasation and what are the 4 steps?

Answer 14

Leukocyte extravasation= the movement of leukocytes out of the circulatory system and towards the site of tissue damage or infection. There are 4 steps:
1. Endothelial activation: resident macrophages in the affecte tissue relese cytokines (IL-1, TNF-a and chemokines). This cause the endothelial cells to express cellular adhesion molecules (such as selectins). Circulating leukocytes are localised towards the site of injury.
2. Rolling adhesion/ weak adhesion: leukocytes slow down and begin rolling along the inner surface of the vessel wall. Bonds are formed and broken between selectins and their ligands.
3. Tight/ firm adhesion: chemokines released by macrophages activate the rolling leukocytes and cause surface integrin molecules to switch to a high-affinity state. This causes immobilization of the leukocytes.
4. Transmigration: leukocytes are spread out over the endothelial cells. Leukocytes pass through the gaps between endothelial cells. Diapedesis= blood vessel escape.

Question 15

What are the 2 cascades during the second step of blood-material interactions?

Answer 15

1. Complement cascade/ opsonization
2. Coagulation cascade

Question 16

What is the Vroman effect and what is the result?

Answer 16

= the process of competitive protein adsorption to a surface by blood serum proteins. The highest mobility proteins arrive first and are later replaced by less mobile proteins that have a higher affinity for the surface. Depending on material properties: different proteins will bind.

Result; It forms an interface between implant and tissue. Always!!!! (Host cells typically interact with adsorbed protein layer, not the biomaterial itself)

Question 17

How is the complement cascade also called and how does it work?

Answer 17

Complement cascade: also known as opsonization.

Opsonin= ‘prepare for eating’. It is an “eat me” sign. Coating of foreign material (or pathogen) with ‘opsonins’ in serum (such as antibodies). So that it is marked for phagocytosis by phagocytes (neutrophils, monocytes/macrophages). Without opsonin, the negatively-charged cell walls of the pathogen and phagocyte repel each other.

Functions: it triggers membrane attack, phagocytosis, and inflammation.
2 major opsonins: immunoglobin G (IgG) and complement C3b bind to the material.

Question 18

What is the goal of the coagulation cascade, how is it also called, and what are the 3 pathways that it consists of?

Answer 18

Coagulation cascade= a series of steps in response to bleeding caused by tissue injury, where each step activates the next (such as platelet activation) and ultimately produced a blood clot.

Also known as secondary hemostasis.

It consists of 3 pathways:
1. Intrinsic pathway; Activated by contact. The tissue gets exposed to the blood (directly). That triggers the activation of factor XII → factor X → thrombin + fibrinogen = fibrin.
2. Extrinsic pathway; Activated by cell damage to endothelial tissue. This leads to the release of tissue factor (TF) and finally to factor X → thrombin. Thrombin + fibrinogen = fibrin
3; Common pathway. Uses factors X, V, II, I, and XIII.

Important cofactors: Vitamin K, Calcium & phospholipids.

Question 19

Provisional matrix

Answer 19

Acts as a sustained release system, supplying a rich mixture of substances that initiate tissue repair, recruit inflammatory cells and fibroblasts, and facilitate ongoing wound healing processes. It forms a protein layer on the material. It is dependent on material properties!

Question 20

Provisional matrix formation = … + … + …

Fibrin = … + …

Answer 20

Provisional matrix formation: complement (opsonins) + platelet activation + coagulation.

Fibron = thrombin + fibrinogen.

Question 21

When is the peak of neutrophils?

Answer 21

After 3 days

Question 22

How does neutrophil recruitment work? What is the result?

Answer 22

Recruitment through cytokines (via extravasation= leakage of fluid out of a blood vessel). Recruitment is initiated by changes on the surface of the endothelium (inflammatory mediators that are released from leukocytes when they come into contact with pathogens). It can also be activated by PPR-mediated detection of pathogens that increase the expression of adhesion molecules.

Result: they attack the foreign material (“eat or denotate”) –> ROS, proteases –> collateral damage to the tissue –> call in the cavalry (attract monocytes (e.g. via MCP-1 and MIP-1b) –> mobilize more neutrophils (e.g. via IL-8).

Question 23

Neutrophils interact with the provisional matrix via … or …

Answer 23

Via integrins or PRRs.

Question 24

Monocytes can differentiate into …

Answer 24

macrophages

Question 25

2 functions of macrophages:

Answer 25

1. Breakdown of foreign material; phagocytosis & degradation –> maintain inflammation via pro-inflammatory cytokines & chemokines (TNF-a, IL-1b, IL-6, MCP-1).
2. Coordinate new tissue formation: they tell other cells what to do (regulation). They are the werkplaats chefs.

Question 26

Explain how macrophage plasticity works

Answer 26

Macrophages change to more or less inflammatory (= polarisation state) depending on the environment and morphology, this determines their function. They can take different functions; can also be contradicting. Signals come from other cells. E.g.: T helper cell → secrete specific factors → more pro-inflammatory (when there is still a threat). When the threat is gone: stop inflammation.

Question 27

How does macrophage-fibroblast cross-talk (the triangle macrophage - tissue repair - inflammation) work? Starting with inflammation.

Answer 27

1. Inflammation; reqruitement & activation (IL-b, IL-1b).
2. Communication with T cells; activation (IL-4, IL-13, IFNy).
3. Communication & activation of fibroblasts (TGF-b, PDGF are strong stimulants).
4. ECM synthesis; forming granulation tissue (danger signals are cleared, then the process needs to be stopped otherwise you get scar formation).
5. Inflammation lowers (IL-10!) & leasion sealed; shift in macrophage function (IL-10 stops producing TGF-b, PDGF, IL-1b).
6. Stop ECM synthesis; ECM remodeling (MMPs).

Question 28

4 hallmarks of granulation tissue

Answer 28

1. Loose connective tissue (mainly coll III)
2. Highly vascularized
3. Immune cells
4. Proliferating fibroblasts

Question 29

To what size can macrophages eat? What happens when the biomaterial is too big?

Answer 29

They cannot eat > 5-10 um. Then they fuse together in giant cells.

Question 30

What stimulates FBGCs?

Answer 30

IL-4, IL-13, and cell-cell contact.

Question 31

What is frustrated phagocytosis?

Answer 31

Phagocytosis → occurs when the foreign body has surface molecules that trigger receptors on the surface of the macrophages.
Frustrated phagocytosis → occurs when the macrophage is incapable of engulfing and consuming some mass of material considerably larger than its size. “if we cannot eat it, let’s break it down”.

How? After macrophage fusion phagocytosis lower –> degradation increases (ROS, proteases) & stop producing pro-inflammatory factors and increase fibroblast-reqruiting factors.