Arthritogenic and immunogenic properties of modified autoantigens

Abstract

Rheumatoid arthritis is a chronic inflammatory joint disorder with autoimmune features in which both genetic and environmental factors are potential contributors to the arthritogenic process. In this thesis work we aimed to increase the understanding of early pathogenic events underlying the development of this complex disease by investigating the arthritogenic and immunogenic properties of modified autoantigens in experimental settings as well as in RA patients.

Since CII reactivity as well as citrulline reactivity has been implicated in the pathogenesis of RA and since posttranslational modifications (PTMs) offer a possible explanation to the loss of tolerance to autoantigens, we were interested in investigating how cartilage reactivity could be provoked through inflammation-associated modifications of CII, and furthermore, to explore the role of citrullinated proteins in arthritis development. In order to facilitate these studies we utilised an experimental animal model that mimics RA in several aspects, namely CIA in rats.

Our studies revealed a pH-dependent conformation of CII, where CII at neutral pH formed a characteristic fibrillary network, whereas CII at low pH arranged into simple triple helices. This loss of structural order correlated with increased arthritogenic properties of CII. In addition, we could demonstrate that chlorination of CII could break tolerance and induce arthritis in rats, even when sub-optimal antigen-doses were used.

Citrullination of CII also enhanced its arthritogenicity. Furthermore, we were able to demonstrate that citrullination of synovial proteins as well as expression of PAD4 were consequences of, and correlated with, the degree of synovitis. Each of these modifications has been suggested to occur at a site of inflammation, hence it is possible that a subclinical joint inflammation could result in such protein modifications and thereby break natural tolerance mechanisms and confer chronicity to an otherwise transient arthritis.

This thesis work also includes a large population-based case control study, in which the relationship between the major genetic risk factor, the best associated environmental trigger and the most specific serological marker for RA was investigated (i.e. HLA-SE, smoking and antiCCP antibodies, respectively). We could identify a gene environment interaction between smoking and HLA-SE in the development of anti-CCP antibody positive RA. Furthermore we were able to demonstrate the presence of citrullinated proteins in the lungs of smokers, but not in non-smokers. Based on these data we have formulated a hypothesis for the etiology of RA in a subgroup of patients - smoking may trigger HLA-SE-restricted arthritogenic immune reactions to autoantigens modified by citrullination.

In conclusion, in experimental settings we have demonstrated that PTMs of endogenous proteins can affect tolerance mechanisms and with respect to cartilage antigens, also enhance their arthritogenicity. Furthermore, for one of these modifications, namely citrullination, we have determined the kinetics of the expression pattern in joints during arthritis development. In addition, we have identified an association between the best recognised environmental and genetic risk factors, smoking and HLA-SE, in the development of citrulline reactivity in RA.