**3.1. TNF-α**

TNF-α is a proinflammaotory cytokine and played a key role in RA with its potential to degrade cartilage [76] and bone [77] *in vitro*. It has been shown in an experiment that dissociated RA synovial mononuclear cell cultures that TNF-α as well as other proinflammatory cytokines (IL-1, IL-6, GM-CSF, and IL-8) [78–81] were produced in a five-day culture [82, 83]. When the activity of TNF-α was blocked in these cultures, the spontaneous production of both IL-1 protein and IL1B mRNA was remarkably decreased and IL-1 bioactivity was neutralized [82]. This is the evidence that the secretion of all these cytokines is a network and controlled by hierarchy of their expressions.

Soluble TNF receptors are found in high concentrations in the synovial fluid and serum of patient with RA [84]. RA patients are found to have high levels of TNF-α in the synovial fluid. This plays an important role in inflammation and joint destruction, both of which 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. They also suggest decreased joint destruction after treatment with IL-1Ra [85].

In a first clinical trial of a TNF-α blocking agent for the treatment of 20 active RA patients were initiated. Infliximab (Remicade), a chimeric antibody specific for human TNF-α was used. Signs and symptoms of the RA disease were substantially reduced with the treatment with infliximab together with decreased levels of CRP in the serum [86]. Other multicentric placebo-controlled trials were also confirmed the therapeutic efficacy of infliximab when coadministrated with methotrexate. This led eventually to FDA approval of the drug for the treatment of RA [87, 88]. After two-year of clinical trial, it was observed that there was a retardation or arrest of both joint space narrowing and bone erosion due to infliximab and methotrexate therapy [89]. There were two other drugs etanercept (Enbrel) and adalimumab (Humira) which are functioned as TNF-α blockers were used in the treatment of RA.

TNF-α is now considered as controlling a wide variety of effector functions relevant to the pathogenesis of RA, including endothelial cell activation and chemokines production which causes accumulation of leukocytes [90]; osteoclast and chondrocyte activation, promoting articular destruction. These all are RA disease pathogenesis spectrum which explains the broad role of TNF-α blockade in patients. Further, improved therapies targeting TNF-α would be a potential therapeutics for the treatment of RA.

## **3.2. Interleukin-1**

Each member of the IL-1 family binds with high affinity to specific receptors. Binding of IL-1α or IL-1β to type I IL-1 receptors (IL-1RI), can is enhanced by an accessory protein, IL-1R-AcP, leads to intracellular signal transduction and regulation of gene expression and hence cellular responses [91]. The extent of response of IL-1β in the rheumatoid joint depends on a few factors such as (1) IL-1β and IL-1Rα have similar affinity for IL-1RI on synoviocytes, chondrocytes and other cells and hence, the relative concentrations of IL-1β and IL-1Rα are important in determining the level of cell activation and biological responses, (2) a greater number of IL-1RII reduces the amount of IL-1β and IL-1Rα, that is available for binding to IL-1RI. Similarly, soluble IL-1 receptors found in synovial fluid and in the circulation also decrease the amount of these cytokines available to interact with IL-1RI. The response of IL-1β, as well as to other proinflammatory cytokines, is regulated by various anti-inflammatory and immunomodulatory cytokines, including IL-4, IL-10, IL-11, IL-13 and transforming growth factor-β [91, 92].

IL1 causes inflammatory cells to move into the joints and the synovium in RA patients. An unspecified antigenic trigger is thought to activate the production of IL1 in joints by macrophages (lymphocytes, monocytes and transformed fibroblasts) [54, 93]. These cells secrete proteases and proteoglycans as cellular signals, that may result in pannus formation, which accumulates in the joints. Destructive enzymes can enter and destroy cartilage and ultimately degrade and erode bone. Importantly, specifically blocking IL1 is a targeted, rational treatment against the destructive functions of IL1 in RA [54, 94].

Evidence from experimental studies in animal models of arthritis and from an x randomized controlled trials in patients with RA indicates that IL-1 plays an important role in RA pathogenesis, and that IL-1 inhibition with anakinra is effective in slowing further radiographic progression of the disease and hence models significantly reduces bone erosions and cartilage degradation [14]. It is important to elucidate that, whether slowing radiographic progression with these biological therapies will significantly improve long-term outcomes in RA.
