Psoriatic arthritis and the spectrum of syndromes related to the SAPHO (synovitis, acne, pustulosis, hyperostosis, and osteitis) syndrome

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Abstract
During the past year, the increasing use of nuclear magnetic resonance imaging techniques, with their ability to delineate cartilage and ligamentous structures and to identify edema, are providing a radical improvement in ascertainment of musculoskeletal abnormalities, although their significance remains incompletely delineated. A second theme has come from the study of spondyloarthropathies in different ethnic groups and societal environments, revealing that the Northern European and North American form of the disease, with its powerful association with HLA-B27, is little evident in the rest of the world's population and that different susceptibility genes and environmental factors operate in other regions and peoples. Related to this theme is the compelling evidence of the marked influence of HIV infection on the development of spondyloarthropathies in Africa. Two areas of immune recognition are discussed as examples of emerging fields that may provide useful paradigms for the experimental approach to mechanisms in psoriatic arthritis. One of these is the three-cell model of CD8 T-cell interaction, in which a dendritic cell presents a peptide from an immunogenic protein to both a CD4 and CD8 T-cell clone, providing a cognitive interaction that disrupts tolerance and results in the expansion of the cytotoxic T-cell clone. In this respect, the combination of an activated dendritic cell, together with enhanced availability of arthritogenic microbial antigens caused by microbial persistence, are interesting candidates to explore as the basis of the HIV-associated rheumatic diseases. The second area of immune recognition is the growing understanding of the outline of the solution to the problem of the association of a spondyloarthropathy with several HLA alleles, eg, HLA-B27 and HLA-B39.