How citrullination invaded rheumatoid arthritis research |
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Authors: | Walther J van Venrooij Ger JM Pruijn |
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Institution: | 1.Department of Biomolecular Chemistry, Institute for Molecules and Materials and Nijmegen Centre for Molecular Life Sciences, Radboud University, PO Box 9101, NL-6500 HB, Nijmegen, The Netherlands |
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Abstract: | Citrullination and the immune response to citrullinated proteins have been
fundamental for the early recognition of rheumatoid arthritis by serological tests
and a better understanding of its pathophysiology. In the first years after the
initial publications, the focus was on the antibodies directed to citrullinated
proteins. It is now realized that citrullinating enzymes and citrullinated proteins
may have important roles in the maintenance of the inflammatory processes in the
joints. There is also accumulating evidence for a direct role of citrullination in
tissue destruction in the rheumatoid synovium. Here we will discuss the development
and importance of anti-citrullinated protein antibodies in rheumatoid arthritis as
well as recent findings implicating citrullination in the pathophysiology of
rheumatoid arthritis.The first indication that patients with rheumatoid arthritis (RA) produce antibodies to
a specific autoantigen was published in 1964 by two Dutch scientists, Nienhuis and
Mandema. The exact nature of this antigen, the so-called perinuclear factor, remained
unclear for decades. In 1978, the target of seemingly unrelated RA-specific
autoantibodies (that is, keratin) was identified. Almost 15 years later, Guy
Serre’s group convincingly showed that both antigens were identical to the
cytokeratin filament-aggregating protein filaggrin (reviewed in 1]). Our own previously published results had shown that the newly made
precursor of filaggrin in cultured buccal mucosa cells (that is, profilaggrin) did not
react with RA antibodies 2]. This prompted us to consider the possibility that a post-translational
modification of filaggrin, absent on newly made profilaggrin, was required for the
formation of the antigenic target of these antibodies. Since 1994, we have tested
several likely modifications using synthetic peptides. Indeed, citrullination, the
enzymatic conversion of peptidylarginine into peptidylcitrulline, turned out to be
essential to make peptides reactive with RA autoantibodies. We subsequently developed an
enzyme-linked immunosorbent assay with citrullinated peptides and confirmed that the
anti-peptidylcitrulline activity was specific for RA 3]. Our further work was directed to the development of the CCP2 test, using
cyclic citrullinated peptides (CCPs) selected from random peptide libraries 4].The discovery of CCP/protein as the most prominent RA-specific antigen had great impact
on RA diagnostics and our understanding of RA pathophysiology. The following milestones
can be noted (see 5] also).1. After decades of intensive research by many groups, a specific diagnostic
test for RA had finally been developed. The CCP2 test has a specificity of more than
95%, is very sensitive (~75%), and is still considered the gold standard in RA
autoantibody testing. Since 2010, anti-citrullinated protein antibodies (ACPAs) have
been included in the new American College of Rheumatology/European League Against
Rheumatism classification criteria for RA.2. Recently, an international reference preparation for ACPAs was evaluated
by the International Committee for the Standardization of Autoantibodies in Rheumatic
and Related Diseases 6]. It is available for the scientific community via the Centers for Disease
Control and Prevention (Atlanta, GA, USA).3. A positive CCP2 test predicts the development of RA, often years before
clinical confirmation (reviewed in 5]). It appears that time to RA diagnosis is shorter in patients with high
anti-CCP2 titers at enrollment as compared with those with low titers 7].4. ACPA-positive RA is characterized by a more severe disease course. Early
treatment of ACPA-positive individuals appears to be very effective.5. ACPA-negative patients (about 25% of the total RA population) generally
display a much milder course of disease. About 35% of such ACPA-negative patients
produce anti-carbamylated protein antibodies. Interestingly, the chemical product of
carbamylation (that is, lysine converted to homocitrulline) is structurally very similar
to citrulline 8].6. Specific human leukocyte antigen (HLA) genes (DRB1 shared epitope (SE)
alleles) not only are the most important genetic risk factor for RA but also are
strongly associated with the production of ACPAs.7. The best known environmental risk factor for RA, cigarette smoking, is a
risk factor only for ACPA-positive and not for ACPA-negative RA 9]. There is increasing evidence that smoking acts as a trigger for
anti-citrulline immunity and does so mainly in the context of certain HLA genes and
certain other genetic risk factors.8. ACPAs and citrullinated antigens form immune complexes which stimulate the
inflammatory process. Continuous production of such immune complexes ultimately results
in the chronic inflammation, characteristic for RA (Figure ).Open in a separate windowCitrullination-related immunity and pathophysiology in rheumatoid
arthritis. In genetically susceptible individuals, an environmental factor
may initiate a primary inflammation, which can occur in various tissues, and
trigger the immune response to citrullinated proteins (left). The resulting
anti-citrullinated protein/peptide antibodies (ACPAs) are distributed through the
circulation and may form immune complexes with citrullinated proteins produced in
an inflamed synovium, thereby boosting the inflammatory process. This will be
associated with the infiltration and activation of neutrophils, macrophages, and
lymphocytes; cell death; extracellular DNA trap formation; the activation and
release of peptidylarginine deiminases (PADs); de novo citrullination;
and diversification of the ACPA response. Besides the common
inflammation-associated mediators of tissue destruction (not shown), ACPAs and
PADs can be directly involved in these processes. HLA, human leukocyte
antigen. |
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