Structural and Functional Studies on Posttranslational Modifications of Collagen type II in Rheumatoid Arthritis

Abstract: The molecular mechanisms behind the development and progression of rheumatoid arthritis are not known in fine details. Both humoral and cellular responses against collagen type II in joint cartilage seems to be important for the disease development. Especially posttranslational modifications on collagen type II are important in regulation of molecular mechanisms implicated in the disease. In this thesis we tried to identify molecular principles of anti-collagen immunity and its relation with pathogenicity by mainly X-ray crystallography. The crystal structure of anti-collagen antibody CIIC1 showed that it not only binds to collagen type II but also cross reacts with other immunoglobulins which is the typical behavior of rheumatoid factors. The arthritogenic and collagen specific antibody CIIC1 with this dual behavior suggests that RFs specific for joint proteins may form large immune complexes in the joints and simultaneously trigger inflammation and create a vicious cycle that produces new RFs in the way to a complete RA. The second article from the thesis describes the recognition of a citrullinated peptide from collagen type II by the arthritis enhancing antibody ACC4. The structure provides important clues explaining the higher antigenicity of the citrullinated peptide therefore the high titer of the ACPA antibodies in RA. In addition, we also showed that the citrullinated CII epitopes recognized by ACC4 and other ACC antibodies exist on the inflamed cartilage and in the synovial fluid which suggests that ACPA antibodies play a role in a vicious cycle that keeps inflammation continuing. Inflammation causes the formation of citrullinated antigenic epitopes or peptides which drives the formation of new ACPA antibodies eventually paves the way to a chronic inflammation and RA. The last two papers describe our experimental design to understand the role of cellular response against collagen type II and against another posttranslational modification which is glycosylation on collagen type II. In this context, we reported, in paper III, the experimental procedure that resulted in successful crystallization of MHC class II Aq presenting immunodominant CII peptide. In paper IV, experimental design for efficient production of Aq restricted collagen specific T cell receptors HCQ3 and HCR2 in insect cells by fusing to leucine zippers were described.