Lecture 26 - Rheumatoid Arthritis - Synovium Flashcards
1
Q
What are the layers of healthy synovium?
A
- Intima (internally)
* Subintima (externally)
2
Q
Describe the intima
A
- Interface between joint cavity and subintima
- 1-3 cell layers deep
- Made up of synoviocytes
3
Q
Describe the subintima
A
- Connective tissue
- Becomes denser closer to the joint capsule
- Contains blood vessels, lymphatics, and nerves
4
Q
Describe the supply to the synovium
A
Subintima receives:
• Blood supply
• Lymphatic vessels
• Nerve innervation
5
Q
What are the different types of synovial tissue?
A
- Areolar
- Fibrous
- Fatty
6
Q
Describe Areolar synovium
A
- Intima
- Continuous layer of lining cells
- Often see villi formation
7
Q
Describe fatty synovium
Where is it found?
A
• Mostly found in fat pads
- Under the intima are adipocytes
- Superficial net of capillaries among the adipocytes
8
Q
Where is Fibrous synovium found?
A
Layer of cells on a ligament or tendon
9
Q
What are the functions of healthy synovium?
A
- Facilitation of movement
• Non-adherent
• Provides lubricants that minimise wear and tear - Synovial fluid production
- Chondrocyte nutrition
• Solutes from synovial fluid diffuse to chondrocytes
10
Q
What are the lubricants in the synovium?
A
- Hyaluronan
* Lubricin
11
Q
What is hyaluronan?
What are its functions?
A
- Huge polysaccharide
- Maintains synovial fluid viscosity
- Shock absorption
12
Q
What is Lubricin?
What are its functions?
A
• Mucin-like proteoglycan
- Protects cartilage surfaces from protein deposition and adhesion
- Inhibits synovial cell overgrowth
13
Q
What are the cell populations in the healthy Intima?
A
- Type A synoviocyte
• Macrophage-like
• 20% - Type B synoviocyte
• Fibroblast-like
• 80%
14
Q
What are the functions of Type A synoviocytes?
A
- Phagocytosis
- Clearance of debris in joint
- Recognition of immune complexes through expression FcγR
15
Q
What are the functions of Type B synoviocytes?
A
Production of:
1. Cartilage
• Collagen
• Fibronectin
- Synovial fluid components:
• Hyaluronan
• Lubricin
16
Q
What is the target for tissue inflammation in RA?
At what point in disease does it become inflamed?
A
The synovium
Becomes inflamed very early in disease
17
Q
Describe the Intima when inflamed
A
• Hyperplasia of synoviocytes
→ up to 12 cells thick
• Proliferation of Type A synoviocytes (outnumber type B)
18
Q
Describe the processes occurring in the synovium when inflamed
A
- Hyperplasia of synoviocytes
- Infiltration of inflammatory cells
- Neovascularisation
- Ectopic lymphoid neogenesis
- Deposition of fibrin
19
Q
Which inflammatory cells infiltrate the synovium in RA?
What brings about the infiltration?
Where do they infiltrate into?
A
- Which cells:
• Macrophages
• T cells
• B cells
less commonly: neutrophils
- Recruitment:
• Release of chemokines from inflamed synovial tissue
• Expression of adhesion molecules to retain the cells - Where:
• Infiltrate into the Subintima
20
Q
Where does neovascularisation occur in RA?
A
In the subintima
21
Q
Describe ectopic lymphoid neogenesis
A
Occurs in synovitis in RA
Germinal centres form in the synovium
22
Q
What is the Pannus?
Of what is it composed?
What does it do?
A
- Inflamed synovial tissue
- Creeps over the cartilage and bone tissue of the joint (v. important)
• Hypoxic microenvironment
Contains:
• Fibroblast-like cells
• Macrophages
• Fewer immune cells than peripheral inflamed synovial tissue
Function:
• Release of factors that destroy articular cartilage and bone
23
Q
What is the oxygen status of the Pannus?
A
Hypoxic
24
Q
What happens to the ratio of the various synoviocytes in RA inflammation?
A
Type A outnumber Type B
25
Describe the phenotype of Type A synoviocytes in RA
"Activated phenotype"
• Expression of phagocytic markers
• Increased expression of MHC II → increased Ag presentation
• Production of:
- Pro-inflammatory cytokines
- Chemokines
• Possible transdifferentiation into osteoclasts
26
Which cells produce pro-inflammatory cytokines in inflamed synovium?
Which cytokines are they?
Activated Type A synoviocytes
* TNF
* IL-1
* IL-6
27
What is the function of osteoclasts?
Bone resorption
28
Describe the role of Type B synoviocytes in RA
```
Production of:
• Pro-inflammatory cytokines
• Chemokines
• Matrix-metalloproteinases (MMP)
• Factors that bring about bone destruction
• Factors that inhibit bone formation
```
29
Which cells produce chemokines in inflamed synovium?
* Type A synoviocytes
| * Type B synoviocytes
30
What is the function of MMPs?
(Matrix-metalloproteinases)
| • Degradation of the ECM
31
Which factors bring about bone destruction?
* TNF
| * RANKL
32
Which factors inhibit bone formation?
* TNF
* DKKs
* sFRPs
33
Which is the predominant lymphocyte in inflamed synovium?
Th17
34
Describe the role of T cells in RA inflammation
Th17:
• Produce IL-17
• Express RANKL
Treg:
• Present, but non-functional
• Reduced expression of IL-10 and IL-4
35
Which cytokine skews Th differentiation into Th17?
IL-6
36
Which cytokine does Th17 produce?
| What are the effects of this?
```
1. IL-17
• Matrix degradation
• Activation of osteoclasts
Induces other cells to express:
• RANKL
• Pro-inflammatory cytokines
```
2. IL-22
• Activation of osteoclasts
37
Describe the role of B cells in RA inflammation
Describe their presence
• Presence is variable (depends of stage of disease?)
```
Function:
1. Production of auto-antibodies:
• ACPA
• RF
• anti-collagen type II antibodies
```
2. Antigen presentation to CD4+ T cells
→ cytokine release
3. RANKL secretion
38
What type of molecule is RANKL?
Cytokine, member of the TNF superfamily
39
Which general categories of molecules are release by cells in inflamed synovium?
* Cytokines
* Chemokines
* Growth factors
40
Where is TNF?
* Initially membrane bound
* Cleaved by TACE (TNF-alpha converting enzyme)
* Now soluble
41
What are the receptors for TNF?
| Compare expression of these
1. TNFRI:
• Constitutive expression
2. TNFRII:
• Induced expression
42
What are the sources of TNF in RA synovium?
Predominantly: Activated macrophages
* Activated fibroblast-like synoviocytes
* T cells
43
What are the functions of TNF in RA?
* Osteoclast activation
* Osteoblast inhibition
* Proinflammatory cytokine release
* Recruitment of inflammatory cells
* Angiogenesis
44
What are the isoforms of IL-1?
Compare their locations
1. IL-1α
• Cytosolic form
• Stored in cytoplasm
2. IL-1β
• Inducible form
• Secreted and cleaved into active form by ICE
45
Which molecule activates IL-1β?
ICE: IL-1 converting enzyme
IL-1β → active form
46
Which molecules are important in the regulation of IL-1?
Describe how they function
1. Soluble IL-1 receptors: sIL-1R
• Act act decoy receptors
• Bind soluble IL-1 in extra-cellular space, so that it can't bind the real receptor
2. IL-1 receptor antagonist: IL-1Ra
• Binds the membrane bound IL-1R so that IL-1 can't bind
47
What are the sources of IL-1 in RA synovium?
• Macrophages
* Type B synoviocytes
* Endothelial cells
* Neutrophils
* Lymphocytes
48
What are the roles of IL-1 in RA?
1. Cell activation:
• Leukocyte activation
• Endothelial activation
• Fibroblast activation
2. Proinflammatory expression of cytokines and chemokines
3. MMP production by chondrocytes and synovial fibroblasts
4. Production of factors required for osteoclast differentiation
→ bone resorption
49
Describe the location and mode of action of IL-6
• Present in extra-cellular space in soluble form
```
1. Binds:
• Membrane bound IL-6R
• Soluble IL-6R
2. IL-6:IL-6R associate with gp130 homodimer
3. Response in cell
```
50
What are the sources of IL-6 in RA synovium?
• Type B synoviocytes
* Macrophages
* T cells
51
High levels of pro inflammatory cytokines in RA are detectable in...
* Synovial fluid
* Serum
(IL-6, TNF, IL-1)
52
What are the roles of IL-6 in RA?
* Acute phase response in liver → systemic inflammation
* B cell Ig production
* Differentiation into Th17
* Pro-inflammatory cytokine production in fibroblasts and macrophages
* Osteoclast differentiation via RANKL
53
What does the acute phase response in the liver bring about?
| What brings about this response
IL-6 → Acute phase Response → Systemic inflammation
54
Which animal models are used to research RA?
What have animal models told us?
Murine models:
• CIA
• h.TNF.Tg
Why:
• Hard to study RA pathogenesis in humans
What murine models have shown us:
• Identification of cellular and molecular mechanisms driving RA
• Enables pre-clinical studies
• Validation of therapeutic targets for the treatment of RA
55
What is the CIA model?
Describe how it is performed and the progression to disease
Collagen-induced arthritis
Day 0: Immunisation of foreign source of Type II collagen / FCA (by intradermal injection)
→ Systemic activation of lymphocytes
→ anti-collagen Ab production
Day 21: booster injection at base of tail
→ Local synovitis
→ Inflammatory infiltrate into synocium
Day 30:
→ Significant local synovitis
→ Destruction of cartilage and bone
56
What is FCA?
Freunds Complete Adjuvant
Part of the solution used to immunise mice against collagen in the CIA rodent model
57
What are the pros and cons of the CIA model?
```
1. Pros:
• Symmetrical arthritis affecting knees and paws
• Synovitis
• Dependent on B and T cells
• TNF and IL-1 expression elevated
• RF reported to be produced
```
2. Cons:
• Disease susceptibility dependent on expression of certain classes of MHC class II
→ Only certain mice are susceptible: DBA/1
• Anti-collagen Ab produced, unlike in humans
• Timing of disease onset is variable, large experiment groups required
58
What is the hTNF.Tg mouse?
What does the mouse experience?
What does this tell us about RA?
Genetic mouse model of RA
Mouse over expresses human TNF
* Arthritis of knees and paws
* Synovial hyperplasia
* Inflammatory cell infiltrate
* Full blown arthritis by 10 weeks
* Pannus formation
* Cartilage destruction
* Focal bone erosion
* System bone loss
* Disease severity reduced with TNF blockage
* KO of IL-1 or IL-1R results in no disease!
Conclusions:
• RA disease process driven by TNF
59
What are the pros and cons of the hTNF.Tg mouse?
1. Pros:
• Reliable, robust arthritis
• Arthritis is chronic
• Allowed investigation of TNF inhibition in treatment
2. Cons:
• TNF dependent, but the other cytokines are not so important in this model
• Not dependent on T and B cells
60
In general what are the pros and cons of rodent models of RA?
1. Pros:
• Pre-clinical identification
• Validation of therapeutics targets
2. Cons:
• Most animal models are dependent on IL-1
IL-1 is not so important in human disease:
→ mAb against IL-1 not so effective, but this may have just been an issue with the mAb
→ TNF blockade very powerful
61
Which molecule prevents protein deposition on articular cartilage?
Lubricin
62
Which mouse strain is most susceptible to RA disease in the CIA model?
DBA/1
63
Which cells make the synovial fluid?
Synovial membrane, specifically, the type B synoviocytes
64
Drawbacks of murine models of RA:
"Most animal models are dependent on ... expression"
IL-1
