Role of tumor necrosis factor-alpha in rheumatoid arthritis: a review
Abstract
Tumor necrosis factor-alpha (TNF-α) is a proinflammatory cytokine that plays a pivotal role in regulating the inflammatory response in rheumatoid arthritis (RA). Although it is controversial whether TNF-α genes are associated with RA susceptibility, they are well known to mediate RA pathogenesis. We review in depth the history, formation and biological action, TNF receptor, role in mediating pathogenesis in RA and mode of action, of anti-TNF-α drugs.
INTRODUCTION
Researchers continue to search for genetic clues into rheumatoid arthritis (RA), a chronic inflammatory joint disease. The susceptibility to RA is complex, comprising genetic and environmental factors. It is a disease defined by well accepted criteria, but its clinical features and molecular pathways involved are heterogeneous. In this genetically complex disease, the response to various treatments varies considerably between different patients, which poses problems in drug development and in predicting responsiveness to a given drug. This heterogeneous response to treatment is seen also for new targeted therapies with tumour necrosis factor (TNF) blockade.
TNF-α
Tumor necrosis factor (TNF) has an extremely broad spectrum of biological activities. Cytotoxicity to tumour cell lines was one of the first functions to be discovered which led to its name, tumour necrosis factor.1 TNF-α is produced mainly by monocytes and macrophages, but also by B-cells, T-cells and fibroblasts. It is one of the key cytokine molecules that causes inflammation in RA.2 It is an autocrine stimulator as well as a potent paracrine inducer of other inflammatory cytokines, including interleukin-1 (IL-1), IL-6, IL-8 and granulocyte monocyte-colony stimulating factor (GM-CSF).3–5 TNF-α is also known to stimulate fibroblasts to express adhesion molecules such as intracellular adhesion molecule 1 (ICAM-1).6
TNF-α plays a dominant role in rheumatoid synovitis (Table 1). In cultures of synovial cells from patients with RA, blocking TNF-α with antibodies significantly reduced the production of IL-1, IL-6, IL-8, and GM-CSF.4 Hence, the blockade of TNF-α may have a more global effect on inflammation than the blockade of other cytokines present in high concentration in synovial fluids, such as IL-1.
| Cell type | TNF-α action |
|---|---|
| Macrophages | Increases proliferation, increases cytokine production |
| Activated T-cell | Enhances proliferation, increases interleuken (IL)-2 receptor |
| B-cell | Increases proliferation, increases differentiation |
| Synovial lining cell | Induces proliferation, induces synthesis of IL-1, granulocyte monocyte-colony stimulating factor, stromelysin, collagenase, prostaglandins |
| Endothelial cells | Induces expression of intracellular adhesion molecule 1, vascular cell adhesion molecule 1, endothelial leucocyte adhesion molecule-1 (ELAM-1), IL-8 |
The gene coding for human TNF-α is located on the short arm of chromosome 6 within the major histocompatibility gene complex. A 1.7kb TNF messenger ribonucleic acid (mRNA) codes for a 26-kDa type II transmembrane protein with 233 amino acids. The mature soluble TNF is generated by proteolytic cleavage of the C-terminal extracellular region of the molecule and consists of 157 amino acids. Human TNF-α is not glycosylated and contains a single intramolecular disulphide bridge.
TNF-α exists as a trimer under physiological conditions. The monomer is an elongated molecule, measuring approximately 6 nm in length and 3 nm in width. Three monomers associate with their long axes orientated along a 3-fold axis of symmetry to form a compact bell-shaped trimer. Studies have indicated that the residues involved in receptor binding occur on both sides of each cleft between the subunits.
TNF-α receptors are present on almost all nucleated cells. Two distinct membrane receptors that have been identified and cloned are tumour necrosis factor-receptor 1 (TNF-R1) and tumour necrosis factor-receptor 2 (TNF-R2). Both these receptors are typical transmembrane proteins with extracellular and intracellular domains of about equal size and a single transmembrane domain. The approximate molecular weight for TNF-R1 is 55–60 kDa and the gene encoding 455 amino acids is located on chromosome 12p13. The gene for TNF-R2, which has an apparent molecular weight of 70–80 kDa, codes for 461 aminoacids and is located on chromosome 1p36. The difference in the apparent molecular weight between the two receptors is mainly due to variation in glycosylation. Both receptors bind the membrane-associated and soluble forms of TNF-α, although most cellular responses to soluble TNF-α are mediated by TNF-R1 and the stimulation of cells with the transmembrane form of TNF-α after cell-to-cell contact acts via both TNF receptors.
The TNF-R2 receptor is believed to have a primary role in stimulating the proliferation of T-cells and in suppressing TNF-α mediated inflammatory responses, whereas the TNF-R1 receptor appears to be critical in triggering host defense and inflammatory responses.2
Both TNF receptors are members of the TNF receptor superfamily. The members of this family possess characteristic cysteine-rich pseudorepeats in the extracellular domains. Most members of this family seem to play an important role in the regulation of the immune response and in the development pathways that regulate the generation of cells involved in these responses.1
Several models have been proposed for the molecular events in TNF and its receptors. Flavia Bazzoni and Bruce Beutler have discussed three models – trimerization hypothesis, expanding network hypothesis and molecular-switch hypothesis. Of these, the molecular-switch hypothesis is the most favoured model, as it involves conformational changes within the cytoplasmic domain of the receptor, leading to signal transduction.7
Soluble TNF receptors are found in high concentrations in the synovial fluid and serum of patient with RA.8 Nevertheless, an excess of TNF-α relative to the concentration of soluble TNF receptors prolongs joint inflammation. Neutralization of the excess amount of cytokines is a potential target to control inflammation in RA. Several lines of treatment may be devised to neutralize cytokines. Recombinant soluble cytokine receptors may be used to help suppress inflammation. Another approach is to use antibodies against cytokines. The type of antibody needs to be designed carefully to achieve clinical efficacy. Receptor antagonism may also be used as an alternative strategy to intercept signal transduction. Such molecules are grouped under a broad category called ‘biological agents’.
‘Biological’ technically means a substance that is a product of a biological system and functionally as an agent that targets a specific biological molecule.9 Currently there are several biological agents used clinically in the therapeutic approach to RA. The available licensed drugs for anti-TNF-α therapy are etanercept, infliximab and adalimumab. These drugs differ not only in structure and mechanism of action, but also in pharmacokinetics and mode of delivery.
RA AND TNF
Rheumatoid arthritis (RA) is the most commonly occurring inflammatory arthritis, affecting approximately 0.5–1.0% of the world population.10 Most patients have a progressive course that eventually leads to considerable functional disability. A wide array of factors including geographical location, age, gender, infection, oxidative stress and genetic predisposition are known to be involved in the list of causative agents of RA.11,12 Reactive oxygen and nitrogen species have been shown to cause tissue injury and pathophysiological consequences in chronic inflammatory conditions such as RA and other rheumatic diseases.13 Our previous studies indicate an altered antioxidant status in RA patients of southern Indian origin.14,15 The basic pathology in the synovium of RA is hyperplasia, increased vascularity and inflammatory cell infiltration. The principal cells among the infiltrates are activated CD4+ T-cells which produce cytokines such as IL-1, IL-6 and TNF-α. TNF-α is a potent cytokine involved in normal inflammatory and immune response. Individuals with RA have high levels of TNF-α in the synovial fluid and it plays an important role in inflammation and joint destruction that are hallmarks of RA. Anti-TNF-α therapy induces a shift in the cytokine equilibrium producing more anti-inflammatory cytokines. Studies have demonstrated dramatic improvement in synovial inflammation in RA patients after treatment with neutralizing anti-TNF-α Abs or soluble TNF receptors, and decreased joint destruction after treatment with IL-1Ra. Immunosuppressive and anti-inflammatory cytokines, including TGFβ, IL-10 and IL-1Ra are highly and consistently expressed during RA synovitis. Production of these cytokines has been proposed to reflect the patient's attempts to contain or control inflammation and achieve homeostasis.16
ANTI-TNF THERAPIES
Etanercept
Etanercept is a fusion protein made up of two recombinant TNF-R2 soluble TNF receptors fused with the Fc portion of human IgG1. The dimeric structure of etanercept makes it approximately 1000 times as efficient as the monomeric soluble TNR-R2 receptor at neutralizing TNF-α.17 It is usually administered subcutaneously. Infectious and injection site reactions such as erythema, pain, swelling and itching are common. Etanercept may be given with or without methotrexate.
Infliximab
This is a chimeric (25% mouse and 75% human) monoclonal antibody that binds with high affinity and specificity to human TNF-α. It is administered intravenously and the common side-effects include upper respiratory tract infection, nausea, headache, sinusitis, rash and cough.
The ‘mouse’ part of infliximab may act as an immunogen, causing allergic reactions during infusions or stimulating the production of neutralizing antibodies with associated reduction in efficacy even with the coprescription of methotrexate.18
Adalimumab
This is a fully humanized antibody produced by recombinant DNA technology that binds to TNF-α. It is administered subcutaneously and can be used alone or in combination with methotrexate or other disease-modifying antirheumatic drugs (DMARDs). Rare adverse effects include infectious, neurological effects and certain malignancies of the lymphoid system.
In established RA, studies on infliximab, adalimumab and etanercept demonstrated a strong ability to both suppress disease and significantly retard (and even fully stop) radiological progression. In early RA, etanercept has been shown to be both effective and have a sustained effect with long-term therapy.19–25
ASIA-PACIFIC EXPERIENCE WITH TNF BLOCKERS
In a preliminary study conducted in China, it was shown that treatment of RA with infliximab plus methotrexate was more efficacious than methotrexate alone. The positive effects appeared to be sustainable with significant improvement in American College of Rheumatology (ACR) 20 and ACR 50 responses at week 18.26
An article reviewing the clinical experience of using etanercept to treat RA at the Hospital for Rheumatic Diseases at Hanyang University in Korea showed that etanercept led to a significant improvement in RA without any serious adverse events. The results of this study indicate that etanercept was both safe and effective in Korean RA patients.27
The Philippine experience indicates superior clinical efficacy of infliximab and manageable adverse events among Filipinos with rheumatic diseases.28
Etanercept showed better efficacy in patients with adult RA in Taiwan than Western countries.29
The therapeutic goods administration of Australia has approved the use of etanercept for the treatment of RA, juvenile chronic arthritis, and psoriatic arthritis. It has approved infliximab for the treatment of RA, juvenile chronic arthritis, ankylosing spondylitis, Crohn's disease and recently psoriatic arthritis, accepting the clinical efficacy of these drugs.30
Limited experience with infliximab in India suggests that it is efficacious in both spondyloarthropathy and RA. Efficacy of both infliximab and etanercept in Indian patients was essentially comparable to that in the published literature.31–37 Biological therapies represent one of the major advances in the treatment of RA in the past decade. The TNF-α inhibitors represent a major breakthrough in the treatment of RA. Most Asian countries have started marketing these drugs and studies have been published indicating their clinical efficacy.
FUTURE DEVELOPMENTS
Research work is in progress in our institute to find the levels of expression of TNF-α gene, its product and existence of possible genetic TNF variants in RA patients. The individual polymorphisms within the TNF-α gene may be important in disease severity and for predicting responses.38,39 The goal of our current study is to correlate the level of gene expression with its plasma product and also to identify TNF-α gene polymorphism. Our study being the first of its kind in our population would give useful information about the levels of TNF-α and the genetic profile of the gene which would greatly enhance the understanding of our population's susceptibility and also the response to current treatment strategies. We also intend to compare our results with the existing literature to plan our future studies in elucidating the mechanism of action of TNF-α in our population, thus providing insight into using TNF-α as a target molecule in therapy.
CONCLUSION
An intricate network of molecules is involved in initiation, perpetuation and regulation of the inflammatory process in RA, of which TNF-α plays an important role. In recent years we have seen an explosive development of biological therapies for RA. The present knowledge about TNF-α could be exploited to understand more about the disease process with the aim of correcting cytokine imbalance through anticytokine treatment. These drugs offer the potential to decrease disease activity and improve quality of life in a majority of RA patients.
This review has been presented at the International conference and workshop on genetics: The basis and diagnosis of genetic disorders.40
