Circulating and Mucosal Antibodies to Citrullinated Antigens in Rheumatoid Arthritis

Abstract

Rheumatoid arthritis (RA) is an autoimmune disease characterized by joint inflammation and subsequent destruction of cartilage and bone. The etiology is largely unknown, although genetic as well as environmental factors are involved. The manifestations and consequences of RA differ between individuals. This makes it important to find early markers for the disease course, in order to enable the most suitable treatment. IgG antibodies to cyclic citrullinated peptides (CCP) have high specificity for RA, but only around 60% of RA patients test positive for IgG anti-CCP. The aim of this thesis was to evaluate the usefulness of serum IgA anti-CCP as a diagnostic maker compared to IgG anti-CCP, and to assess IgA versus IgG anti-CCP status in relation to smoking habits and genetic background. Another aim was to evaluate signs of mucosal immunization by analyzing salivary IgA anti-CCP. IgA anti-CCP was present in a subgroup of RA patients with high levels of IgG anti-CCP and a slightly more severe disease course. Similar results were found regarding IgA class antibodies to modified citrullinated vimentin (MCV). IgG anti-MCV had slightly higher sensitivity for RA than IgG anti-CCP, thus identifying a group of IgG anti-CCP negative patients with an unfavourable disease course. However, the lower diagnostic specificity of IgG anti-MCV limits its usefulness. Among 63 patients with established RA, salivary IgA anti-CCP was found in 22% and was associated with a more favourable outcome regarding erosive joint disease at follow-up. IgA anti-CCP in serum was strongly associated with smoking, and the earlier known interaction between smoking and shared epitope (SE) was here shown to be valid only for subjects positive for IgA anti-CCP in combination with IgG anti-CCP. In conclusion, IgG anti-CCP is still the most useful serologic marker of RA, but IgA anti-CCP should be further investigated as a prognostic marker. The association between smoking and IgA anti-CCP strongly indicates a pathogenic role for smoking and IgA anti-CCP, supporting the possibility that RA may originate from chronic airway irritation. The less erosive disease in patients positive for salivary IgA anti-CCP indicates a protective role of secretory IgA anti-CCP