Lecture 25 - Synovium in Health and Rheumatoid Arthritis Flashcards
Synovium
Thin membrane that extends from skeletal tissue at the interface between cartilage and bone, and lines the capsule of diarthroidal joints
Layers of synovium
1)
2)
1) Intima
2) Subintima
Intima 1) 2) 3) 4)
1) Inner layer of synovium.
2) Made up of synoviocytes (type 1 and 2)
3) 1-3 cells deep
4) Interface between subintima and joint cavity
Subintima
1)
2)
3)
1) Lies between the joint capsule and intima
2) Becomes more dense as it approaches the joint capsule, bone, cartilage
3) Contains blood vessels, lymphatic vessels , nerves
Three different types of synovial tissue
1) Areolar
2) Fibrous
3) Fatty
Areolar synovial tissue
1)
2)
3)
1) Continuous layer of synoviocytes lining it
2) Capillaries immediately below intima
3) Contains small arterioles, venules, lymphatic vessels
Fibrous synovial tissue
1)
2)
1) Layer of cells on a ligament or tendon
2) Hard to distinguish histologically from fibrocartilage
Fatty synovial tissue
1)
2)
1) Mostly found in fat pads
2) Underlying intima is a network of capillaries in between adipocytes
Functions of a healthy synovium 1) 2) 3) 4)
1) Facilitates movement
2) Produces synovial fluid
3) Supplies chondrocytes with nutrition
4) Type 1 chondrocytes phagocytose material
How does a healthy synovium facilitate joint movement
1)
2)
3)
1) Synovium is highly deformable and movable
2) Provides lubricants
3) Non-adherent
Lubricants secreted by synovium
1)
2)
1) Lubricin
2) Hyaluronan
Lubricin
1)
2)
3)
1) Mucin-like proteoglycan
2) Protects bone and cartilage surfaces from protein deposition and cell adhesion
3) Inhibits synovial cell overgrowth
Hyaluronan 1) 2) 3) 4)
1) High molecular weight polysaccharide
2) Maintains synovial fluid viscosity
3) Effective shock absorber
4) Prevents leakage of synovial fluid
How do chondrocytes receive nutrition?
Solute diffusion through synovial fluid
Cartilage is avascular
Proportion of type A synoviocytes to type B in healthy synovium
20% type A, 80% type B
Type B synoviocytes
1)
2)
3)
1) Fibroblast-like synoviocytes
2) Produce lubricin and hyaluronan
3) Produce collagen and fibrin
Type A synoviocytes
1)
2)
3)
1) Macrophage-like synoviocytes. Tissue-resident macrophages
2) Clear debris from joint
3) Express FcgammaR
Surface marker used to identify type B synoviocytes
CD55
Surface marker used to identify type A synoviocytes
CD68
When in RA does synovial inflammation present?
Very early
Characteristics of RA synovium 1) 2) 3) 4) 5)
1) Intima expands from 1-3 cells thick to 12 cells thick (hyperplasia)
2) Infiltration of inflammatory cells into subintima (T cells, B cells, neutrophils)
3) Neovascularisation
4) Ectopic lymphoid neogenesis
5) Deposition of fibrin in active disease
Pannus
In RA where the inflamed synovium creeps over the bone and cartilage
Features of pannus 1) 2) 3) 4) 5)
1) Differs histologically from inflamed synovium away from bone and cartilage
2) Rich in fibroblasts
3) Macrophages
4) Fewer immune cells than inflamed synovium away from bone and cartilage
5) Hypoxic micro-environment
What do cells within the pannus do?
Release factors that destroy articular cartilage and bone
Type A synoviocytes in RA synovium 1) 2) 3) 4) 5)
1) Outnumber type B synoviocytes
2) Express active phenotype (high phagocytic marker expression, high MHCII expression)
3) Release pro-inflammatory cytokines (TNFa, IL-1, IL-6)
4) Release chemokines
5) Might trans-differentiate into osteoclasts
Type B synoviocytes in RA synovium 1) 2) 3) 4) 5) 6)
1) Mediators in inflammation in RA joint
2) Release pro-inflammatory cytokines (TNFa, IL-1, IL-6)
3) Chemokine release
4) Release matrix-degrading enzymes that break down cartilage (EG: matrix-metalloproteinases)
5) Release factors that lead to bone destruction (TNF, RANKL)
6) Release factors that inhibit bone reformation (TNF, DKKs, sFRPs)
Predominant lymphocyte in RA synovium
CD4+ T cell
Role of Th17 in RA
1)
2)
1) Express RANKL
2) IL-17 induces release of other pro-inflammatory cytokines, and RANKL release
What induces Th17 differentiation in RA?
IL-6 release
Role of Treg in RA
1)
2)
1) Present in synovial tissue, but non-functional
2) Don’t release IL-14, IL-10
Role of B cells in RA 1) 2) 3) 4) 5)
1) Variable presence in RA patients
2) Local production of autoantibodies (RF, ACPA)
3) Source of RANKL
4) Can present antigens to CD4+ T cells
5) Produce antibodies in response to T cell activation by APCs
Cytokines that play a critical role in RA pathogenesis
IL-1, IL-6, TNFa
How is TNFa released?
1)
2)
3)
1) Initially a membrane-bound protein
2) Cleaved from cell membrane by TNFa converting enzyme (TACE)
3) Both membrane-bound and soluble forms are active
Types of TNFa receptors
1)
2)
1) TNFRI - Constitutively expressed
2) TNFRII - Induced expression
Roles of TNFa in RA 1) 2) 3) 4) 5) 6) 7) 8)
1) Pro-inflammatory cytokine release
2) Hepcidin production
3) Osteoclast activation
4) Chondrocyte activation
5) Angiogenesis
6) Leukocyte accumulation
7) Endothelial cell activation
8) Chemokine release
Pro-inflammatory cytokines induced by TNFa in RA 1) 2) 3) 4)
1) IL-1
2) IL-6
3) IL-23
4) GM-CSF
What are hepricidins?
Acute phase proteins made in the liver
EG: C-reactive protein
What does TNFa induce chondrocytes to do in RA?
Produce matrix-metalloproteinases –> Leads to cartilage destruction
What does TNFa make leukocytes do in RA?
Increase MHCII expression
Two isoforms of IL-1
IL-1alpha, IL-1beta
IL-1alpha
Cytosolic form of IL-1
Stored in cytophasm of some cells
IL-1beta
1)
2)
1) Inducible form
2) Secreted, then cleaved into active form by IL-1 converting enzyme (ICE)
How is IL-1 activity regulated?
By endogenous inhibitors
IL-1 endogenous inhibitors
1)
2)
1) Soluble IL-1 receptors (act as decoy receptors)
2) IL-1 receptor antagonist (competes for IL-1R binding with IL-1)
What determines the effect of IL-1 signalling?
Balance between IL-1, and IL-1 inhibitors (soluble IL-1 receptors, IL-1 receptor antagonist)
Sources of IL-1 in RA 1) 2) 3) 4) 5)
1) Macrophages
2) Type B synoviocytes
3) Endothelial cells
4) Lymphocytes
5) Neutrophils
IL-1 roles in RA 1) 2) 3) 4)
1) Activates leukocytes, synovial fibroblasts, endothelial cells
2) Induces expression of chemokines, cytokines
3) Induce metalloproteinase production by type B synoviocytes
4) Induce expression of RANKL
Unsuccessful RA treatment that targeted IL-1R
Anakinra (IL-1R antagonist)
IL-6 receptors
Either soluble or membrane-bound
How does IL-6 induce effects?
IL-6/IL-6R complex bind to gp130 homodimer on cell surface
IL-6 sources in RA
1)
2)
3)
1) Macrophages
2) Type 2 synoviocytes
3) T cells
IL-6 effects in RA 1) 2) 3) 4) 5
1) Increases acute-phase protein release from liver
2) Increases Ig release by B cells
3) Promotes Th17 differentiation
4) Increases cytokine production by type B synoviocytes, macrophages
5) Induces RANKL
Why is the study of human RA pathogenesis limited?
Only observe people with active disease.
Most common animal model of RA
Collagen-induced arthritis
How is collagen-induced arthritis induced?
Immunise mouse with foreign source of collagen II, Freund’s complete adjuvant at base of tail
How do genetics affect collagen-induced arthritis?
A lot.
DBA/1 mouse strain most susceptible.
Inducible in C57B6, but less severe disease
Rodent collagen-induced arthritis timeline
1)
2)
3)
1) Day 0 - Intradermal injection at base of tail with foreign type II collagen, Freund’s complete adjuvant.
Leads to systemic B cell, T cell activation, collagen Ab production
2) Day 21 - Booster of foreign type II collagen, Freund’s complete adjuvant.
Leads to local synovitis, invasion of synovium with inflammatory cells
3) Day 30+ - Significant local inflammation within affected joint, destruction of bone and cartilage
Pros of collagen-induced arthritis model 1) 2) 3) 4) 5)
1) Symmetrical inflammation affecting knees and paws.
2) Synovial inflammation leading to destruction of bone and cartillage
3) B and T cell infiltration, high expression of MHCII
4) TNFa, IL-1b levels elevated. If either of these are inhibited, reduces symptoms
5) Rheumatoid factor is produced
Cons of collagen-induced arthritis
1)
2)
3)
1) Susceptibility is dependent on certain HLA types. Therefore only certain mouse strains can be used
2) Timing to disease is variable. Therefore large numbers of mice are required
3) Anti-collagen antibodies are produced, which aren’t normally present in RA
Transgenic mouse for RA model
hTNF.Tg mouse
Human TNF transgenic mouse
hTNF.Tg mouse features 1) 2) 3) 4)
1) Genetically-engineered to overexpress human TNF
2) Develops arthritis spontaneously, affecting knees and paws
3) Blocking TNF reduces disease severity
4) Dependent on IL-1/IL-1R expression
Timeline of hTNF.Tg mouse disease
1)
2)
3)
1) 3-4 weeks of age - Synovial hyperplasia, inflammatory cell infiltrates
2) 10 weeks - Arthritis
3) Inflammation, pannus formation, cartilage destruction, focal bone erosion, systemic bone loss
Pros of hTNF.Tg mouse model
1)
2)
3)
1) Chronic arthritis
2) Reliable, robust development of arthritis (synovial inflammation, cartilage, bone loss)
3) Useful to test effect of TNF inhibition on bone loss
Cons of hTNF.Tg mouse model
1)
2)
1) Even though IL-1 is essential to arthritis, this is a TNF-driven model. Could discount the effects of other cytokines
2) Arthritis not dependent on T or B cells, which are important in RA
Common issue with animal RA models
Very dependent on IL-1 expression.
IL-1 inhibition in animals has been very effective at controlling RA, but less effective in humans.
