Chronic inflammation and wound healing

Question 1

List 5 causes of chronic inflammation

Answer 1

1. Persistent infection (most common cause).
2. Infection with viruses, mycobacteria, parasites and fungi.
3. Autoimmune disease.
4. Foreign material.
5. Carcinoma.

Question 2

Name 3 chronic inflammatory cells

Answer 2

1. Lymphocyte.
2. Plasma cell.
3. Macrophage.

Question 3

Where do macrophages come from?

Answer 3

Macrophages are derived from stem cells in the bone marrow and circulate in the blood stream as monocytes. The half-life of a monocyte is about a day whereas macrophages can live in tissues for months to years.

Question 4

Describe classical and alternative macrophage activation and the outcomes for each of these pathways

Answer 4

In classical macrophage activation, M1 macrophages are activated by bacteria or by T-cells (via interferon gamma) killing the bacteria and secreting cytokines which stimulate inflammation.

In alternative macrophage activation, M2 macrophages are activated by T-cells (producing IL-4 + 13). These M2 macrophages promote tissue repair by secreting growth factors which promote angiogenesis, fibroblast proliferation and collagen synthesis.

Question 5

Describe the origin and activation of CD4+ Helper T-cells

Answer 5

T-cells are produced in the bone marrow and mature in the thymus. There are two main types of T-cell - CD4+ Helper T-cells and CD8+ cytotoxic T-cells. T-cells use a T-cell receptor complex for surveillance (TCR + CD3). The T-cell receptor complex recognises antigen bound to MHC-II on an antigen presenting cell and needs a second signal for activation. This second signal is provided by B7 on an antigen presenting cell binding to CD28 on the CD4+ T-cell.

Question 6

Describe the activation of CD8+ Cytotoxic T-cells

Answer 6

Intracellular antigen derived from proteins in the cytoplasm is processed and presented on MHC class I molecules which is expressed by all nucleated cells and by platelets. The T-cell receptor with its CD8 co-receptor binds to this complex. IL-2 from CD4 + T-helper 1 cells provides the second activation signal. In that way cytotoxic T cells are activated for killing. Killing occurs via one of two methods. The first is through secretion of perforin and granzyme. Perforin creates pores that allow granzyme to enter the target cell. The second is by binding of FAS ligand to FAS on a target cell. The result for the cell which is attacked is apoptosis or programmed cell death.

Question 7

Describe the origin and activation of B-lymphocytes

Answer 7

B-lymphocytes are produced in the bone marrow. These cells undergo immunoglobulin gene rearrangement to become naive B cells which express IgM and IgD. On binding of antigen to these immunoglobulins, there is maturation of the B-lymphocytes into IgM or IgD-secreting plasma cells. The CD40 receptor on the B cell binds CD40 ligand on helper T-cells providing the second activation signal. The helper T cells then secrete IL-4 and IL-5 which mediates B cell isotype switching, hypermutation and maturation to plasma cells.

Question 8

What is a granuloma?

Answer 8

A collection of activated macrophages/epithelioid histiocytes. Granulomas may be caseating or non-caseating ie. may show caseous necrosis or not.

Question 9

List 6 causes of granulomatous inflammation

Answer 9

1. Foreign bodies.
2. Sarcoidosis (Non-caseating granulomas).
3. Crohns disease.
4. Cat scratch disease.
5. Mycobacterial infection. Tuberculosis typically causes caseating granulomas.
6. Fungal infection.

Question 10

How are granulomas formed?

Answer 10

Macrophages process and present antigen on their surfaces in association with MHC II molecules to CD4+ helper T-cells. Macrophages then secrete interleukin 12 which causes CD4+ helper T cells to differentiate into the interferon gamma-secreting TH-1 cell subtype. Interferon gamma converts macrophages into epithelioid histiocytes and giant cells. This interferon gamma production can be detected clinically to help make a diagnosis of tuberculosis.

Question 11

Replacement of damaged tissue with native tissue depends on the regenerative capacity of the tissue. List the 3 types of tissue based on their regenerative capacity and give examples of each tissue type.

Answer 11

1. Labile. Labile tissues have stem cells that continuously cycle. Bowel mucosa, skin and bone marrow.
2. Stable. Normally quiescent but can regenerate if necessary. Liver.
3. Permanent. Lack significant regenerative potential. Myocardium, skeletal muscle and neurons.

Question 12

What is repair and when does it happen?

Answer 12

Repair is replacement of damaged tissue with a collagen rich or fibrous scar. This occurs when regenerative stem cells are lost or when the tissue which is injured lacks regenerative capacity eg. heart muscle which undergoes ischaemic necrosis.

Question 13

What is granulation tissue?

Answer 13

Granulation tissue formation is seen in the initial phase of repair and granulation tissue consists of proliferated capillaries (which provide nutrients) fibroblasts (which deposit type 3 collagen) and myofibroblasts (which mediate wound contraction).

Question 14

What are the layers of an ulcer?

Answer 14

The first layer of an ulcer contains neutrophils, fibrin, and red blood cells. Deep to this layer there is a layer of granulation tissue which comprises a proliferation of blood vessels and fibroblasts. These fibroblasts secrete collagen and the layer beneath the granulation tissue layer comprises predominantly fibroblasts with associated collagen representing scar tissue.

Question 15

List 5 important growth factors which are necessary for repair and their actions

Answer 15

1. Transforming growth factor (TGF) alpha. Epithelial cell and fibroblast proliferation.
2. TGF-beta. Fibroblast proliferation and inhibition of inflammation.
3. Platelet-derived growth factor (PDGF). Growth of endothelium, smooth muscle and fibroblasts.
4. Fibroblast growth factor (FGF). Angiogenesis.
5. Vascular endothelial growth factor (VEGF). Angiogenesis.

Question 16

What are the four phases of normal wound healing?

Answer 16

Normal wound healing processes can be divided into 4 overlapping phases:

1. Coagulation or clotting phase.
2. Inflammatory phase.
3. Proliferative phase or granulation tissue formation phase.
4. Remodelling phase.

Question 17

What happens in the coagulation and inflammatory phases of wound healing?

Answer 17

Blood-borne cells, that is neutrophils, macrophages and platelets play critical roles. These cells provide growth factors which are needed for recruitment of epithelial cells and connective tissue cells into the wound bed.

Question 18

What happens in the proliferative phase of wound healing?

Answer 18

The proliferative phase starts approximately 3 days after injury and is characterized by increased levels of epithelial and fibroblast proliferation and migration. There is also extracellular matrix synthesis in response to autocrine and paracrine growth factors and angiogenesis or new blood vessel growth. Because of the presence of blood vessels, the tissue has a granular appearance termed granulation tissue.

Question 19

What happens in the remodelling phase of wound healing?

Answer 19

Approximately 1 to 2 weeks after injury, myofibroblasts that are present within the granulation tissue begin to remodel the extracellular matrix which is followed by apoptosis of resident cells which leads to the formation of an acellular scar.

Question 20

What is the difference between wound healing by primary and secondary intention?

Answer 20

In primary intention, wound edges are brought together with sutures which leads to minimal scarring. In secondary intention, the edges are not brought together and granulation tissue will fill the gap and then myofibroblasts contract the wound in which case scar tissue will form.

Question 21

What is the difference between a keloid scar and a hypertrophic scar?

Answer 21

A hypertrophic scar is defined as excess production of scar tissue that is localised to the area of the wound.

A keloid scar is exuberant production of scar tissue that is out of proportion to the wound size. Keloids are associated with excess type 3 collagen deposition and are more common in black people. They often affect the ear lobes, face and upper extremities.

Question 22

Name one vitamin and two minerals which are important for wound healing and how these contribute to the wound healing process.

Answer 22

Vitamin C, copper and zinc.

Deficiencies of vitamin C and copper will delay healing as these two elements are involved in the formation of collagen crosslinks.

Zinc deficiency is also associated with poor wound healing as zinc is a co-factor for collagenase which is required to replace type 3 collagen with type 1 collagen.

Question 23

List 5 reasons other than mineral/vitamin deficiency for impaired wound healing.

Answer 23

1. Presence of foreign bodies.
2. Infection.
3. Poor blood supply.
4. Diabetes.
5. Malnutrition.

Question 24

Describe the pathogenesis of the 4 types of hypersensitivity reaction and give one example of a disease caused by each type.

Answer 24

Type I reactions are immediate allergic reactions and these are caused by preformed IgE antibodies activating mast cells and basophils. Examples of type 1 reactions include anaphylaxis, food or pollen allergies and asthma.

Type II hypersensitivity reactions are referred to as cytotoxic, as they involve antibodies that are specific to particular tissues within the body and cause destruction of cells in these tissues. Examples of type II reactions include autoimmune haemolytic anaemia (where red cells are the target) and pemphigus vulgaris (where desmosomes are attacked).

Type III hypersensitivity reactions are immune complex-mediated with tissue damage caused by antigen-antibody complex deposition. Examples of type III diseases include systemic lupus erythematosus, glomerulonephritis and vasculitis.

Type IV hypersensitivity reactions are delayed and cell-mediated and are the only hypersensitivity reaction that involves sensitized T-lymphocytes rather than antibodies. Examples of diseases which are type IV reactions include rheumatoid arthritis, psoriasis and allergic contact dermatitis.

Question 25

Summarise the pathogenesis of autoimmune disorders.

Answer 25

Autoimmune disorders are characterised by immune mediated damage to self-tissues and they involve a loss of self-tolerance. The disorders are more common in women and it is thought that this may be due to the presence of oestrogen which is known to reduce apoptosis in B-cells allowing them to persist and to damage self. The cause of autoimmune disease is likely to be an environmental trigger in genetically programmed people and they are clinically characterised by progressive disease with relapses and remissions. The diseases often show overlapping features and this is partially explained by the idea of epitope spreading. This means that the disease starts out with an autoimmune reaction to a single epitope and with time new B or T-cell reactions develop to other antigens.

Question 26

Give 3 examples of autoimmune diseases which are associated with autoantibody production and list the autoantibodies which characterise each one.

Answer 26

1. Systemic lupus erythematosus (SLE). Anti-dsDNA antibodies.
2. Sjögrens syndrome. Anti-SSA/Ro and anti-SSB/La antibodies.
3. Systemic sclerosis/Scleroderma. Antibodies to DNA topoisomerase I (anti-Scl-70).