Cytokines and the immunopathology of the spondyloarthropathies

Reactive arthritis. Immune-mediated synovitis or joint infection

Reactive arthritis is one form of the seronegative Spondyloarthropathies. Susceptibility to reactive arthritis is closely linked to individuals who have the genetic predisposition to the HLA-B27 allele (gene form). Although there is a reactive-inflammatory joint reaction present, the synovium is not damaged by infectious agents (bacteria, fungi, or virus). This article discusses the pathogenesis of reactive arthritis.

Immune features of seronegative and seropositive arthritis in early synovitis studies

Synovitis of recent onset is a challenging problem, both from a diagnostic and a mechanistic point of view. The role of the immune system in mediating the systemic and synovial inflammatory response remains an area of active investigation. Studies in early synovitis cohorts have confirmed the relatively specific association of rheumatoid factor positive polyarthritis with a number of autoantibodies, particularly anticyclical citrullinated peptide (CCP) antibodies, antifilaggarin antibodies (AFA), and anti-Sa antibodies. Immunopathologic studies of synovial tissue samples from patients with early synovitis have generally suggested quantitative rather than qualitative differences between various forms of synovitis. In particular, Th1 cytokines appear to predominate in rheumatoid arthritis and psoriatic synovitis, while Th2 cytokines are more often detectable in the synovium of reactive arthritis patients. This latter observation is consistent with an immune response profile that favors persistence of intracellular organisms.

Pathways of cell activation in spondyloarthropathies

The initiating event in cell activation is unknown in most autoimmune diseases. The role of infection is clear in some cases, especially in reactive arthritis; however, there is little evidence of a specific organism in other spondyloarthropathies. Common pathways of cell-cell interaction and activation manifest in inflammation, but subtle differences may exist. The presence of T cells, macrophages, and B-lymphocytes suggest an autoimmune mechanism; the arthritogenic peptide theory has been proposed. Furthermore, the association of spondyloarthropathies with HLA-B27 suggests it may be important in synovial T-cell activation. Other cell types involved in the process of bone and cartilage destruction, including fibroblasts and osteoclasts, may also be activated. Endothelial activation and angiogenesis may be a critical primary event in these diseases. Finally, trauma (physical or psychological) in the form of stress may be an important factor; the nervous system and neuropeptides may play a role in cell activation and initiation of arthritis.