HC36: Immune cells in rheumatoid arthritis

Rheumatoid arthritis

Symptoms:

Rheumatoid arthritis (RA) is a rheumatic autoimmune disease characterized by inflamed joints:

• Painful joints
• Swollen, warm joints
• Impaired functioning
• Stiffness
• Tiredness

Process:

There are several stages of rheumatoid arthritis:

1. Normal
2. Arthralgia
   • Painful joints
3. Onset arthritis
   • Joints become inflamed
   • Undifferentiated arthritis → doesn’t necessarily lead to rheumatoid arthritis
4. Established rheumatoid arthritis
   • Leads to either slowly or rapidly progressive rheumatoid arthritis
5. Slowly progressive rheumatoid arthritis
6. Rapidly progressive rheumatoid arthritis

The time frame in which this happens is unknown. It may be longer than 10 years.

Diagnosis:

Rheumatoid arthritis is a heterogenous disease → it is difficult to diagnose. The following classification criteria is used to diagnose rheumatoid arthritis:

• Joint distribution (0-5)
  • 1 medium/large joint → 0
  • 2-10 medium joints → 1
  • 1-3 small joints → 2
  • 4-10 small joints → 3
  • >10 joints, of which at least 1 is small → 5
• Serology (0-3)
  • Neither Rf nor ACPA positive → 0
  • At least one test is low positive → 2
  • At least one test is high positive → 3
• Symptom duration (0-1)
  • <6 weeks → 0
  • >6 weeks → 1
• Acute phase reactants (0-1)
  • Neither CRP nor ESR abnormal → 0
  • Abnormal CRP or abnormal ESR → 1

In case 6 or more points are present, RA is definite.

Pathophysiology:

Rheumatoid arthritis is inflammation of the synovial membrane of the joint, characterized by:

• Influx of adaptive and innate immune cells
• Extensive angiogenesis
  • New blood vessels are formed
• Synovial hyperplasia

Inflammation

Symptoms:

Inflammation is a reaction of the immune system to triggers. It is characterized by:

• Dolor
• Calor
• Rubor
• Tumor
• Loss of function

The aim of inflammation is to remove harmful agents and repair the damage.

Process:

Healthy skin is not inflamed. Inflammation starts as follows:

1. Macrophages in tissues recognize pathogens
2. Pro-inflammatory cytokines are secreted by the macrophage
   • IL-1b→ fever and production of IL-6
   • TNF-a→ fever, mobilization of metabolites and shock
   • IL-6 → fever and induces acute-phase protein production by hepatocytes
   • CXCL8 → chemotactic factor, recruits neutrophils and T-cells to the site of infection
   • IL-12 → activates NK-cells and induces the differentiation of CD4 T-cells into Th1-cells
3. Vasodilation and increased vascular permeability allow fluid, proteins and inflammatory cells to leave and enter tissue
4. The infected tissue becomes inflamed, causing redness, heat, swelling and pain
5. Activated inflammatory cells undergo apoptosis and clearance
   • This is a tightly regulated event to prevent chronic inflammation
6. Apoptotic neutrophils promote the recruitment of monocytes
7. Monocytes are attracted to the tissue by lipid mediators → differentiate to macrophages
8. Macrophages digest apoptotic neutrophils and produce anti-inflammatory cytokines → no new immune cells enter the tissue

The inflammation stops when the harmful agent is removed. This also is the case in allergies.

Immune cells

Rheumatoid arthritis is also an inflammation. In this case, the damaging factors are auto-antibodies which cannot be removed. This causes a chronic infection:

1. An unknown trigger sets up initial focus of inflammation in the synovial membrane, attracting leukocytes into the tissue
2. Autoreactive CD4 T-cells activate macrophages → production of pro-inflammatory cytokines and sustained inflammation
3. Cytokines induce production of MMPs and RANK-ligands by fibroblasts
   • MMPs create proteins
   • RANK-ligands are ligands for osteoclasts
4. Osteoclasts destroy bones, resulting in joint destruction

It is unclear which immune cells are important in case of rheumatoid arthritis:

• Genetic studies → HLA and T-cells are important risk factors
• Biological treatment → rituximab affects B-cells and abatacept affects T-cells
  • Rituximab: lessens the symptoms → B-cells play a part in the pathogenesis
  • Abatacept: blocks the co-stimulation of T-cells
• Serology → presence of auto-antibodies secreted by B-cells

In conclusion, B- and T-cells are probably important for rheumatoid arthritis.

B-cells

B-cell antibodies:

Rheumatoid arthritis specific auto-antibodies are directed against post-translational modified human proteins:

• ACPA: recognizes citrullinated proteins
• AAPA: recognizes acetylated proteins
• Anti-Carp: recognizes carbamylated proteins

ACPA stands for anti-citrulline protein antibody. These antibodies recognize citrullinated proteins. Citrullination is the enzymatic conversion in a protein of the aminoacid arginine protein into the aminoacid citrulline by the enzyme PAD. Citrullination also occurs in healthy individuals, for example during smoking or inflammation.

Although citrullination also occurs in healthy individuals, only rheumatoid arthritis patients make antibodies against citrullinated proteins → ACPAs. Presence of ACPAs in RA can be a predictive factor for the development of a more severe form. Molecular characteristics of ACPAs change shortly before the start of the disease.

Possible pathogenic mechanisms of ACPAs are:

• Activate osteoclasts
• Bind to FcgR to active immune cells
• Activate complements
• Induce netosis

Stages in the development of seropositive rheumatoid arthritis are:

1. Risk of developing RA
   1. Genetic susceptibility and environmental challenges
   2. Loss of immune tolerance at mucosal sites
   3. Antibody production
2. ACPA-associated disease
   1. Epitope spreading and increasing antibody titres
   2. Osteoclast activation and IL-6 production → pain, bone loss and sick leave
3. ACPA-positive RA
   1. Second hit → pain, inflammation and erosions

Naturally, these auto-antibodies are produced by B-cells. Many B-cells are present in the synovium of affected joints. There are 3 possible pathogenic mechanisms of B-cells in RA:

• Auto-antibody production
  • Mainly IL-6
• Cytokine production
• Antigen presentation and T-cell activation

Clinical interventions:

Rituximab is a possible B-cell therapy in rheumatoid arthritis. It is an anti-CD20 antibody. CD20 is primarily found on the cell surface of B-cells → binding of rituximab to B-cells induces cell death. Rituximab is not effective in all patients → because RA is a heterogenous disease, there are also patients without the presence of auto-antibodies. For some patients, B-cells are not involved.

T-cells

The prevalence of RA in the general population is approximately 1%. However, the concordance rate for RA for monozygotic twins is between 12-15%. Twin pair studies show that genetic factors have a substantial impact on the susceptibility to RA. The estimated genetic contribution to RA is 50-60%.

T-helper cells:

The HLA-locus is the most important genetic risk factor for RA, but only in case of ACPA-positive disease. HLA presents an antigen to T-cells, which stimulate the B-cells in secreting ACPAs → naïve B-cells require help from Th2 cells to become antibody secreting plasma cells:

1. An antigen binds to the B-cell receptor → becomes an antigen presenting cell
2. A Th2-cell expresses a CD40 ligand and cytokines
3. The B-cell is activated → proliferates and differentiates to antibody-secreting plasma cells

T-helper cells are activated by antigen presenting cells such as dendritic cells, B-cells and macrophages. These cells secrete cytokines which cause both the pro-inflammatory and anti-inflammatory immune response. Thus, the T-cells require 3 signals from the antigen-presenting cell to be activated → co-stimulation:

• Binding of the MHC-II-complex to the T-cell receptor
• Binding of the B7 protein to CD28 on the cell surface
• Binding of cytokines to the T-cell receptors

These signals make T-cell activation, survival and differentiation possible.

Clinical interventions:

Abatacept is a drug which binds to the B7 protein of the antigen presenting cell → hinders co-stimulation. This causes the T-cell to become anergic → the immune system stops functioning. Abatacept isn’t a favorable drug to use because it blocks the entire immune system.

There also are various drugs which inhibit the signaling pathway of cytokines:

• Tocilizumab: blocks the IL-6 receptor
• Anakinra: an antagonist of the IL-1 receptor
• Infliximab: a humanized anti-TNF-aantibody → blocks TNF