Pathophysiology of rheumatoid arthritis

Rheumatoid arthritis (RA) is a chronic, systemic autoimmune disorder that primarily affects the joints but can also have widespread systemic manifestations. The pathophysiology of RA involves an intricate interplay between genetic factors, environmental triggers, immune system dysregulation, and inflammatory pathways that lead to joint damage and systemic complications.

Genetic susceptibility plays a significant role in the onset of RA. Although no single gene is responsible for the disease, certain genetic markers are associated with an increased risk. The most notable example is the link between RA and the human leukocyte antigen (HLA) system, particularly the HLA-DRB1 alleles (MacGregor et al., 2000). These alleles contribute to RA susceptibility by presenting arthritogenic peptides to T cells, initiating an immune response.

Environmental factors, such as smoking, are known to interact with genetic predispositions in the development of RA (Klareskog et al., 2006). Smoking can lead to the citrullination of proteins, a process in which the amino acid arginine is converted to citrulline. This modification may create neoantigens that are recognized as foreign by the immune system, thus triggering autoimmunity in genetically susceptible individuals.

The immune response in RA is characterized by the production of autoantibodies including rheumatoid factor (RF) and antibodies against citrullinated proteins (ACPA). The presence of ACPA has high specificity for RA and is related to disease severity (Schellekens et al., 1998). These autoantibodies form immune complexes that contribute to inflammation and joint damage by activating complement pathways and recruiting inflammatory cells to the joints.

In the synovium of RA patients, the normal homeostatic balance is disrupted due to the proliferation of synovial fibroblasts and the infiltration of immune cells like T cells, B cells, macrophages, and dendritic cells (Firestein, 2003). The activation of these immune cells produces cytokines that play central roles in the inflammatory cascade of RA.

TNF-? has been identified as a major mediator of inflammation in RA, contributing to the activation of other inflammatory cells, production of additional pro-inflammatory cytokines, and expression of adhesion molecules that lead to joint damage (Brennan et al., 1992).

RA is associated with osteoclast activation, which leads to bone erosion and joint destruction due to the interaction between RANKL and its receptor RANK on osteoclast precursors (Lacey et al., 1998).

The synovium in RA produces enzymes like matrix metalloproteinases (MMPs) that degrade cartilage and contribute to the destruction of joint structures (Burrage et al., 2006).

Despite advances in understanding the pathophysiology of RA, the disease\'s initial events remain elusive due to the interplay between genetic predispositions, environmental factors, immune dysregulation, and inflammatory processes. Ongoing research aims to unravel these relationships to develop more targeted therapies for RA.

Autoantibodies like RF and ACPAs are involved in the pathogenesis of RA, where they not only induce inflammation but also contribute to bone resorption processes (Harre et al., 2012).

The complement system and toll-like receptors on synovial cells play crucial roles in RA pathogenesis by exacerbating inflammation and recognizing endogenous ligands that amplify inflammatory responses (Holers, 2014; Green et al., 2011).

Enhanced macrophage polarization into pro-inflammatory M1-type cells contributes to inflammation and tissue damage in the RA joint. These cells produce cytokines that amplify inflammatory responses (Murray et al., 2014).