Significance of intestinal inflammation in the pathogenesis of spondylarthropathies

The concept of spondylarthropathy (SpA) gathers together a group of chronic diseases with common clinical, biological, genetic and therapeutic characteristics. The concept forms a distinct entity, different from other rheumatic diseases. The target organs are not only the joint, but also the axial skeleton, the enthesis, the eye, the gut, the urogenital tract, the skin and sometimes the heart. The prevalence of this entity in the general population is estimated 1%, equal to the prevalence of rheumatoid arthritis. Genetical predisposition (HLA-B27) is one of the clues to the pathogenesis of the disease. Since reactive arthritis is induced by specific urogenital or enterogenic bacteria, and since the gut is implied in different forms of spondylarthropathies, especially in IBD, it was clear that the gut could play an important role by permitting exogenous factors to enter the body. This hypothesis was the rationale for investigating the gut in the spondylarthropathies by performing ileo-colonoscopies. In the first ileo-colonoscopic studies of SpA patients, histological signs of gut inflammation were found in a relatively great number of patients, mostly not presenting any clinical intestinal manifestations. These lesions were not seen in other inflammatory joint diseases. Further ileo-colonoscopic studies confirmed the strong relationship between gut and joint inflammation. In patients in whom a second ileo-colonoscopy was performed, remission of the joint inflammation was always connected with a disappearance of the gut inflammation, whereas persistence of locomotor inflammation was mostly associated to the persistence of gut inflammation. The hypothesis was proposed that some patients with a spondylarthropathy had a form of subclinical Crohn's disease in which the locomotor inflammation was the only clinical expression. This hypothesis was confirmed in prospective long-term studies in which the ileo-colonoscopied patients were reviewed 2 to 9 years later:about 6% of SpA patients not presenting any sign of Crohn's disease at first investigation but demonstrating gut inflammation on biopsy, developed full-blown Crohn's disease. By performing electronmicroscopy it was described that in patients with SpA the number of membranous (M) cells, which are scarce in normal ileum, is increased in number in inflamed mucosa. They showed a thin rim of cytoplasm covering groups of lymphocytes. In chronic inflammatory lesions necrotic M-cells, rupture of M-cells and lymphocytes entering the gut lumen was observed. The bursting of M-cells at the top of the lymphoid follicles leads to interruption of the gut epithelial lining and gives the luminal content access to the lymphoid tissue. This can be responsible for an exponential increase of local antigen stimulation. Accelerated luminal antigen presentation through a break in the epithelial layer, together with cytokines released from activated monocytes, might induce a second line of defense aiming at elimination of the massive antigen penetration into the mucosa. The postulated switch from secretory local immunity to a systemic type of local immune reaction could have different consequences:the local down-regulation of J chain in the IgA immunocytes could shift the production of polymeric IgA to monomers, jeopardizing secretory immunity; the disproportionate increase of IgG-producing cells could favor further inflammation and tissue damage through complement activation and arming of the killer cells, and cause autoimmune responses locally and in target organs at a distance (e.g. joint organs). The discovery of subclinical gut inflammation in the SpA had therapeutic consequences. Sulphasalazine (SASP) has been proven to be an active drug in the treatment of IBD. Since the gut could play a crucial role in SpA, it was logic to use this drug in the treatment of this disease. Multiple open and double-blind studies have proven the effectiveness of this drug in SpA.