The spondyloarthropathies are a group of arthritides which share common features and include ankylos ing spondylitis, reactive arthritis (including Reiter's syndrome), enteropathic arthritis (the arthritis associated with inflammatory bowel disease), psoriatic arthritis and unclassified spondyloarthropathy.
The shared features include arthritis of the axial skeleton and particularly the sacroiliac joints, peripheral arthritis and enthesitis (the inflammation of the transitional tissue between ligament and bone and, similarly, tendon and bone). The inflammatory lesions may evolve with development of bony structures which may ankylose a joint through a vet undefined process. Also shared are extraarticular features, including inflammatory diseases of the eye, heart, lung, skin and mucocutaneous tissues, gastrointestinal and genitourinary tracts. Both conjunctivitis and anterior uveitis are seen in the eye. The skin is involved with the obvious psoriasiform rash, as well as the closely related lesion of keratoderma blennorrhagicum, and with oral ulcerations. Cardiac-lesions include an aortitis which may lead to valvular insufficiency, particularly with long-standing disease and conduction disturbances with various forms of heart block.
Whereas pulmonary fibrosis in other rheumatic diseases more frequently involves the lower segments of the lung, those seen in the spondyloarthropathies involve the upper lobes and may further compromise the patient's respiratory status, which may be affected by involvement of the costovertebral and costo-chondral joints. Lesions of the gastrointestinal tract that are seen with inflammatory bowel disease and dysentery may also be seen in other spondyloarthropathies without clinical symptoms. In addition to the urethritis seen in Reiter's subset of the reactive arthritis, there is an increased occurrence of prostatitis in the spondyloarthropathies.
Immunologically, there is a definite association of the spondyloarthropathies with the MHC class I molecule HLA-B27, particularly in ankylosing spondylitis, where more than 90% of white patients possess this antigen. Recently, there have been advances which have better defined the role of HLA-B27 in the pathogenesis of the spondyloarthropathies. For example, transgenic rat and mice models exist whose cells express human HLA-B27 antigen with animal/human ^-microglobulin. When these genes are significantly expressed, the animals develop articular and extraarticular lesions similar to the human diseases.
Environmental influences have also been defined, as animals raised in sterile conditions do not develop disease but, when allowed to reassociate with their normally-raised litter mates, subsequently develop disease. Studies are ongoing to identify which of the common enteric bacteria poses the biggest risk for development of HLA-B27-associated disease.
The structure and sequence of the HLA-B27 molecules have also been recently defined. The peptide-binding region of HLA-B27 interacts with nonapep-tides which may derive from gram-negative enteric bacteria. Of importance is that the HLA-B27 has the typical MHC class I molecular structure with parallel helical chains that form a peptide-binding groove and a ß-pleated sheet forming the floor. The HLA-B27 molecule itself possesses a similar sequence of amino acids to the enteric bacterial nonapeptidc. The differences in the amino acid sequences of the currently identified nine different HLA-B27 molecules arc being investigated to determine how they influence the risk for development of disease, as identified by epidemiologic studies.
Although there have been significant advances in the understanding of the immunology of the spondyloarthropathies, as yet it is unclear how a chronic inflammatory process develops and is perpetuated.
See also: Antinuclear antibodies; Autoimmune disease, spontaneous experimental models; Rheumatoid arthritis, animal models; Rheumatoid arthritis, human; Sjögrens syndrome; Skin, autoimmune diseases; Systemic lupus erythematosus, experimental models; Systemic lupus erythematosus (SLE), human.
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