Clinical and pathophysiological significance of the autoimmune response to citrullinated proteins in rheumatoid arthritis

Abstract

Rheumatoid arthritis (RA) is the most frequent human autoimmune disease, affecting about 1% of the adult population worldwide. A better knowledge of the autoimmune mechanisms involved is essential. We identified the epithelial targets of various autoantibodies specifically associated to RA, as variants of (pro)filaggrin. We also showed that these targets correspond to deiminated (“citrullinated”) proteins, of which arginyl residues have been posttranslationally transformed into citrullyl residues by a peptidylarginine deiminase (PAD). Moreover, we and others established that citrullyl residues are indispensable elements of the epitopes recognized by these autoantibodies but only in the context of specific aminoacid sequences. We also demonstrated that these autoantibodies to citrullinated proteins (ACPA) are secreted by plasma cells of the synovial tissue and that their major targets correspond to citrullinated forms of the α- and β-chains of fibrin, abundant in the tissue. These results have allowed the development of new efficient immunochemical methods for the detection of ACPA. Some of them are already commercially available. These new methods have permitted the high diagnostic value of ACPA which are present very early in the course of the disease, and also their prognostic value, to be confirmed. ACPA detection should therefore prove to be also a very valuable tool to guide the choice of therapeutic strategies, from the earliest stages of the disease. The synthesis of ACPA in the rheumatoid synovial tissue and the existence therein of a specific antigenic target constitute a strong argument for the involvement of this specific immunological conflict in the pathophysiology of RA. Indeed, it could lead to activation of effector mechanisms with pro-inflammatory effects, thus to formation in the tissue of new fibrin deposits, secondarily citrullinated. We therefore, propose a new pathophysiological model accounting for the self-maintenance and chronicity of rheumatoid inflammation. Numerous questions about the pathophysiological significance of the autoimmune response to deiminated proteins in RA remain to be answered to confirm this model.

Introduction

Rheumatoid arthritis (RA) affects about 1% of the adult population worldwide. This rheumatologic autoimmune disease is characterized by chronic inflammation of the synovial joints often with systemic manifestations. Immune cells infiltrate the synovial tissue in which synovial cells, as well as capillaries and venules, are hyperplastic. As a whole, they constitute the rheumatoid pannus which erodes subjacent cartilage and bone. Although pain, stiffness and swelling often occur insidiously, the disease can also rapidly progress towards irreversible articular destruction and serious functional disability, quite often despite the use of the large panel of available treatments. RA aetiology is still unknown. Significant advances have been made in the understanding of the immunopathological mechanisms involved but, to date, no single element has been identified as being responsible for the autoimmune inflammatory process. It appears that only a combination of several factors allows the disease to occur and explains its recurrent and chronic character. Many cellular elements or soluble factors, taken to be playing an active role within the rheumatoid joint, must be included in the immunopathogenic models of RA, as well as in the design of therapeutic strategies.