The Joy of Citrulline: New Insights into the Diagnosis, Pathogenesis, and Treatment of RA

[Guest guest]

Journal of Rheumatology

August 2004

Editorial

The Joy of Citrulline: New Insights into the Diagnosis, Pathogenesis,

and Treatment of Rheumatoid Arthritis

JONATHAN HILL, BSc,

Graduate Student, Department of Microbiology and Immunology;

EWA CAIRNS, PhD,

Associate Professor of Medicine and Microbiology and Immunology,

Department of Medicine and Microbiology and Immunology,

University of Western Ontario;

DAVID A. BELL, MD,

Professor of Medicine and Microbiology and Immunology,

Department of Medicine, Division of Rheumatology,

University of Western Ontario,

London, Ontario Canada

Address reprint requests to Dr. D.A. Bell, Rheumatology Centre,

Monsignor Roney Building, St. ph's Health Centre, 268 Grosvenor

Street, London, Ontario N6A 4V2, Canada. E-mail:

david.bell@...

While the cause of rheumatoid arthritis (RA) is not known, it is

generally acknowledged to be a chronic autoimmune disorder, and over the

past 40 years, beginning with the discovery of rheumatoid factor,

evidence has accumulated to support this notion. However, rheumatoid

factor is neither specific nor completely sensitive for the diagnosis of

RA, and there is no uniform consensus regarding its role in the

pathogenesis of the disease. Other self-antigen targets of the immune

response in RA have been identified, including collagen II (CII)1, the

most abundant protein of articular cartilage; human cartilage

glycoprotein 39 (gp39)2; and proteoglycan3. While these antigens can

induce arthritis in genetically susceptible mouse strains, evidence for

their participation in human RA is not clearly defined.

There has been considerable recent interest in the observation that a

very high proportion of patients with RA have IgG antibodies to

citrullinated peptides. Such anti-citrulline antibodies appear

relatively early in RA4, are highly specific for this disease (98%)5,

and can be measured by quite reproducible, readily available assay

systems. While these are relatively recent observations, it is of intere

st that such disease-specific autoantibodies were first described in RA

more than 40 years ago as antiperinuclear factors6. A series of studies

by several different laboratories have established that the target of

these antibodies is filaggrin, a protein expressed in the late stages of

terminal differentiation of epithelial cells of mammalian skin and

esophagus. It was also established that these autoantibodies target

posttranslationally modified or citrullinated filaggrin7,8.

The posttranslational process of citrullination involves the deimination

of arginines in certain polypeptides and is catalyzed by the Ca2+

dependent enzyme peptidylarginine deiminase (PAD) (see review9). The

biochemical outcome of this process is the conversion of positively

charged arginines to polar but uncharged citrullines. These changes in

the property of citrullinated peptides make them targets of IgG

antibodies in RA. This altered property of arginine-containing peptides

also allows them to bind with 100-fold higher affinity to the positively

charged peptide-anchoring pocket known as P4 in MHC class II molecules

expressing the shared epitope (SE) (e.g., HLA-DRB1*0101, 0401, and

0404)10. We have recently demonstrated this directly with purified MHC

class II molecules expressing the SE and in mice transgenic for

HLA-DRB1* 0401 (DR4-IE tg mice). The HLA-DRB1*0402 allele, which is

protective for RA, has a negatively charged P4 anchoring pocket that can

bind arginine11 and likely citrulline. This is predicted to result in

peptide MHC class II ligands that should negatively select T cells with

this specificity in the thymus. T cells with high avidity for this

complex should therefore not be expressed in peripheral lymphoid tissues

(Figure 1). This may explain why patients with the HLA-DRB1*0402 allele

do not appear to develop RA. When DR4-IE tg mice were immunized with

certain citrullinated peptides, they produced CD4 Th1 responses thought

to be important participants of the immune response in RA. These

observations imply that the immune response to citrullinated peptides is

influenced by MHC class II genes, which encode the SE. This is

consistent with other studies in human RA patients showing a strong

correlation between anti-citrulline antibodies and the SE12. These

experimental observations therefore link 2 commonly observed features of

RA: the high frequency of expression of the SE and the common and highly

specific occurrence of anti-citrulline antibodies.

One may ask what relevance these findings have to the pathogenesis of

RA. Several experimental observations suggest that the immune responses

to citrulline could play a significant role in the pathogenesis of RA

inflammation. First, the synovial tissue of patients with RA has been

shown to contain citrullinated proteins (citrullinated alpha and beta

chains of fibrinogen, and citrullinated vimentin)13,14. Citrullinated

proteins appear to be localized to the cytoplasm of synovial

monocyte/macrophage-like cells and interstitial deposits in the deep

synovial tissue of RA patients13,15. Similarly, in some experimental

models of arthritis, citrullinated proteins can be identified in the

synovial tissue16, suggesting that inflammation may upregulate this

process, perhaps by increasing the activity of PAD. In an appropriate

host (i.e., who expresses the SE), these citrullinated proteins could be

targets of the local immune response in the joint. A second observation

supporting the role of citrullinated proteins in the pathogenesis of RA

derives from recent studies in our laboratory17, indicating that

arthritis resembling RA can be induced in mice transgenic for the SE by

the administration of citrullinated fibrinogen. Arthritis could not be

induced in nontransgenic mice, nor in transgenic mice given

noncitrullinated or unmodified fibrinogen.

Finally, it has been recently observed that a cluster of single

nucleotide polymorphisms (SNP) on chromosome 1p36, localized at the

PADI4 gene, is associated with RA in the Japanese population18. PADI4

mRNA containing the RA associated SNP were more stable than those from

the nonsusceptible haplotype. This implies that the mRNA transcript

produced from the susceptible PAD haplotype may persist, possibly

leading to increased PAD production and more citrullinated protein.

The foregoing observations suggest that genetic factors influencing the

expression of PAD could be present in patients with RA. Those RA

patients with this PAD haplotype could generate increased quantities of

citrullinated peptides, which, in a genetically susceptible host with

the MHC class II SE, could lead to the activation of Th cells and IgG

anti-citrulline antibodies. Since citrullinated proteins are generated

in inflamed synovial tissue16, these antigens could be targeted by the

immune system and further provoke the inflammatory process leading to

chronic, persistent synovitis. If immune responses to citrulline play an

important role in the initiation and perpetuation of RA, then novel

forms of therapy targeting these pathways could be developed. One

obvious target could be to suppress or block the activity of one or more

PAD isoenzymes responsible for producing citrullinated peptides within

the synovium. Other approaches could include manipulating the CD4 Th

responses that are driving the production of IgG anti-citrulline

antibodies. These studies, now being addressed in animal models of

arthritis, open a new chapter in our understanding of the pathogenesis

of RA and invite the development of new strategies for its management.

http://www.jrheum.com/subscribers/04/08/1471.html

I'll tell you where to go!

Mayo Clinic in Rochester

http://www.mayoclinic.org/rochester

s Hopkins Medicine

http://www.hopkinsmedicine.org