Immunoscintigraphy as an adjunct to diagnostic imaging in septic sacroiliitis. Report of a case

HLA-B27 and genetic predisposing factors in spondyloarthropathies

With the mapping of the human genome having been completed, our ability to investigate and ideally better understand the genetic basis of rheumatic diseases is advancing at a rapid pace. Substantial evidence strongly favors a direct role for HLA-B27 in genetic susceptibility to ankylosing spondylitis and related spondyloarthropathies, although the underlying molecular basis has yet to be identified. HLA-B27 contributes only 16 to 50% of the total genetic risk for the disease, clearly indicating that other genes must be involved. However, no other putative disease genes have yet been absolutely proven. Potential genes include MHC (HLA class II, low molecular weight proteasome [LMP], transporter associated with antigen processing (TAP), tumor necrosis factor [TNF]-alpha, and major histocompatibility complex class I chain-related gene A (MICA), as well as non-MHC genes (IL-1RA, IL-6, IL-10, and CYP2D6). Genome-wide screens have identified other chromosomal areas of interest: 1p, 2q, 6p, 9q, 10q, 16q, and 19q. However, different studies have given conflicting results. HLA-B27 itself is a serologic specificity, which encompasses 25 different alleles that encode 23 different products (proteins): HLA-B*2701 to B*2723. These alleles may have evolved from the most widespread subtype, B*2705, and two of them, B*2706 in Southeast Asia and B*2709 in Sardinia, seem not to be associated with ankylosing spondylitis. The distinction between the disease associated and nonassociated subtypes may provide clues to the actual role of B27 in disease pathogenesis.

Presentation of HLA class I-derived peptides: potential involvement in allorecognition and HLA-B27-associated arthritis

Some 25 years ago, when purified HLA class I allotypes were first being analyzed, of major concern was that the papain used for solubilization might produce a mess of proteolytic fragments that would prove impossible to separate and sequence. Those fears proved unfounded (Parham et al. 1975), and the homogeneity of the preparations was sufficient to allow crystallization and determination of the three-dimensional structure (Bjorkman et al. 1987). Ironically the least ordered region of the electron density map provoked the most interest because it gave a first view of the diverse peptides bound by an MHC molecule. With this image a second chapter of HLA class I biochemistry began, its charge to determine the structures of bound peptides and their influence on the immune system. The extraordinary polymorphism of HLA class I heavy chains now seems quite manageable compared to the vast complexity of the peptides, and our present ignorance as to which ones are important for health and disease. The comparative weakness of most HLA class I associations with disease has made HLA-B27 an especially favored target for investigation, and more is known of the structure and peptide-presenting function of HLA-B27 than for any other HLA-B allotype (López de Castro 1994). Much of this information relates to the native HLA-B27 molecule and has been collected in the belief that disease is a direct consequence of its antigen-presenting function. If one subscribes to the relevance of the transgenic rodent models, this position has almost become untenable. For rats and mice 'non-functional' forms of HLA-B27 are the agents of disease, raising the possibility that B27-associated arthritis is induced by HLA class II presentation of a B27-derived peptide, a variant of the mechanism advanced for the classical HLA class II-associated diseases: type 1 diabetes, multiple sclerosis and rheumatoid arthritis (Gregersen et al. 1987, Roudier et al. 1989, Cucca & Todd 1996, Hall & Bowness 1996). Such speculation invites the obvious question as to whether other diseases associated with HLA class I and chronic inflammation, HLA-C and psoriasis for example (Tiilikainen et al. 1980, Yanagisawa et al. 1995), result from class II presentation of class I peptides.