Antibodies as pathogenic factors and biomarkers in rheumatoid arthritis

Abstract

Ever since the evolution of an adaptive immune system capable of creating immune receptors that may recognize self-antigens, we have been at risk of autoimmunity. There are over 100 different types of autoimmune diseases targeting almost every available tissue from head to toe, with joints and connective tissues being a common target. In addition to autoimmune diseases, infections and degenerative joint diseases can cause joint inflammation making differential diagnosis between them difficult.

The most common autoimmune disease to afflict the joints is rheumatoid arthritis (RA), affecting nearly 1% of the world population, predominantly women. The etiology of RA is not known, although it involves interaction between multiple genes and environmental risk factors. It is characterized by chronic inflammation of the joints, which without successful treatment can lead to joint destruction. One of the hallmarks of RA is the presence of autoantibodies, often observed in serum several years before any symptoms of disease. The two classes of autoantibodies focused on today are rheumatoid factors (RF) and anti-citrullinated protein antibodies (ACPA), the latter being a highly specific biomarker for a large subset of RA-patients. The ACPA have greatly aided in diagnosing RA in many patients. Yet their function and origin are still not known. Nevertheless, a subset of patients still lacks a specific biomarker.

All studies in this thesis have autoantibodies in arthritis as a common theme, and four of them use a bead-based multiplex platform established during the PhD-project. In Study I, we explored the hypothesized link between periodontitis induced by the oral pathogen P.gingivalis, and its effect on arthritis progression and the production of ACPA. This study revealed a citrulline specific antibody response against P.gingivalis peptidyl arginine deiminase derived peptide, although the link to the arthritis development could not be confirmed. In Study II, we synthesized a library of triple helical peptides (THP) as a tool to characterize antibodies against type II collagen (CII). The peptides were tested in two cohorts of RA patients, as well as on monoclonal antibodies (mAb), and in collagen induced arthritis. The THPs were subsequently used in Study III to elucidate the specificity and function of antibodies against type XI collagen (CXI), revealing a shared epitope between CXI and CII in mice, rats and humans with arthritis. In addition, the THPs were also used in Study IV to explore the cross-reactivity of a joint-reactive mouse ACPA, demonstrating a molecular mechanism of how an ACPA can trigger arthritis. For Study V, the specificity of several human ACPA were dissected with a bead based multiplex assay and compared to polyclonal responses in two RA cohorts. Crystal structures of the ACPA revealed for the first time the structural basis of how human ACPA bind citrulline residues on different peptides.

The data presented in this thesis provide further evidence that the major determinant of the arthritogenicity of antibodies lies in their ability to cross-react to joint proteins. Dissecting these specificities may lead to the establishment of new clinical biomarkers.