Anti-citrullinated protein antibody
Encyclopedia
Anti-citrullinated protein antibodies (ACPA) or anti-cyclic citrullinated protein antibodies (anti-CCP) are autoantibodies (antibodies
directed against one or more of an individual’s own proteins) that are frequently detected in the blood of rheumatoid arthritis
patients. The main epitope
for these antibodies is filaggrin
, and there is cross-reactivity between ACPA and anti-keratin and anti-perinuclear factor.
During inflammation, arginine residues in proteins such as vimentin
can be enzymatically converted into citrulline ones (a process called citrullination
), and, if their shapes are significantly altered, the proteins may be seen as antigens by the immune system, thereby generating an immune response. ACPAs have proved to be powerful biomarkers that allow the diagnosis of rheumatoid arthritis (RA) to be made at a very early stage.
In July 2010, the 2010 ACR/EULAR Rheumatoid Arthritis Classification Criteria were introduced. These new classification criteria include ACPA testing, and overruled the "old" ACR criteria of 1987
and are adapted for early RA diagnosis.
and filaggrin
was investigated. Subsequent studies demonstrated that autoantibodies from RA patients react with a series of different citrullinated antigens, including fibrinogen, deiminated Epstein-Barr Virus Nuclear Antigen 1 and vimentin
, which is a member of the intermediate filament family of proteins. Several assays for detecting ACPAs were developed in the following years, employing mutated citrullinated Vimentin (MCV-assay), filaggrin-derived peptides (CCP-assay) and viral citrullinated peptides (VCP-assay).
In 2010, ACPA testing has become substantial part of The 2010 ACR-EULAR classification criteria for rheumatoid arthritis.
s, for example CCP-assays, they only provide a sensitivity comparable with that of rheumatoid factor
(RF). Moreover, analysis of the correlation of anti-CCP antibody titre with RA disease activity yielded conflicting results.
Unfortunately, these artificial antigens are not expressed in the affected tissue, and therefore are probably not directly involved in the pathogenesis of RA.
In the more recent time, novel test systems utilizing ACPA have been developed. Citrullinated vimentin is a very promising autoantigen in RA, and a very suitable tool for studying this systemic autoimmune disease
. Vimentin is secreted and citrullinated by macrophages in response to apoptosis, or by pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha).
A newly developed ELISA
system utilises genetically modified citrullinated vimentin (MCV
), a naturally occurring isoform of vimentin to optimize the performance of the test. Noteworthy are the findings of a recently published study that highly valuates anti-MCV test systems for diagnosing rheumatoid arthritis in anti-CCP-negative patients.
Given that ACPA are more specific than rheumatoid factor, they are used to distinguish various causes of arthritis. Novel assays may be useful for monitoring disease activity and effects of RA therapy.
The reference ranges for blood tests of anti-citrullinated protein antibodies are:
|Negative>
Low/weak positive
Moderate positive
High/strong positive
Unit
>-
| < 20
20 – 39
40 - 59
> 60
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...
directed against one or more of an individual’s own proteins) that are frequently detected in the blood of rheumatoid arthritis
Rheumatoid arthritis
Rheumatoid arthritis is a chronic, systemic inflammatory disorder that may affect many tissues and organs, but principally attacks synovial joints. The process produces an inflammatory response of the synovium secondary to hyperplasia of synovial cells, excess synovial fluid, and the development...
patients. The main epitope
Epitope
An epitope, also known as antigenic determinant, is the part of an antigen that is recognized by the immune system, specifically by antibodies, B cells, or T cells. The part of an antibody that recognizes the epitope is called a paratope...
for these antibodies is filaggrin
Filaggrin
Filaggrin is a filament-associated protein that binds to keratin fibers in epithelial cells.-Profilaggrin:Filaggrin monomers are tandemly clustered into a large, 350kDa protein precursor known as profilaggrin. In the epidermis, these structures are present in the keratohyalin granules in cells of...
, and there is cross-reactivity between ACPA and anti-keratin and anti-perinuclear factor.
During inflammation, arginine residues in proteins such as vimentin
Vimentin
Vimentin is a type III intermediate filament protein that is expressed in mesenchymal cells. IF proteins are found in all metazoan cells as well as bacteria. IF, along with tubulin-based microtubules and actin-based microfilaments, comprise the cytoskeleton...
can be enzymatically converted into citrulline ones (a process called citrullination
Citrullination
Citrullination or deimination is the term used for the post-translational modification of the amino acid arginine in a protein into the amino acid citrulline. This reaction, shown below, is performed by enzymes called peptidylarginine deiminases...
), and, if their shapes are significantly altered, the proteins may be seen as antigens by the immune system, thereby generating an immune response. ACPAs have proved to be powerful biomarkers that allow the diagnosis of rheumatoid arthritis (RA) to be made at a very early stage.
In July 2010, the 2010 ACR/EULAR Rheumatoid Arthritis Classification Criteria were introduced. These new classification criteria include ACPA testing, and overruled the "old" ACR criteria of 1987
ACR score for rheumatoid arthritis
ACR score is a scale to measure change in rheumatoid arthritis symptoms. It is named after the American College of Rheumatology.Different degrees of improvement are referred to as ACR20, ACR50, ACR70...
and are adapted for early RA diagnosis.
History
The presence of autoantibodies against citrullinated proteins in rheumatoid arthritis patients was first described in the mid-1970s when the biochemical basis of antibody reactivity against keratinKeratin
Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails...
and filaggrin
Filaggrin
Filaggrin is a filament-associated protein that binds to keratin fibers in epithelial cells.-Profilaggrin:Filaggrin monomers are tandemly clustered into a large, 350kDa protein precursor known as profilaggrin. In the epidermis, these structures are present in the keratohyalin granules in cells of...
was investigated. Subsequent studies demonstrated that autoantibodies from RA patients react with a series of different citrullinated antigens, including fibrinogen, deiminated Epstein-Barr Virus Nuclear Antigen 1 and vimentin
Vimentin
Vimentin is a type III intermediate filament protein that is expressed in mesenchymal cells. IF proteins are found in all metazoan cells as well as bacteria. IF, along with tubulin-based microtubules and actin-based microfilaments, comprise the cytoskeleton...
, which is a member of the intermediate filament family of proteins. Several assays for detecting ACPAs were developed in the following years, employing mutated citrullinated Vimentin (MCV-assay), filaggrin-derived peptides (CCP-assay) and viral citrullinated peptides (VCP-assay).
In 2010, ACPA testing has become substantial part of The 2010 ACR-EULAR classification criteria for rheumatoid arthritis.
Clinical significance
In a comparative study (in 2007), various detection kits had a sensitivity between 69.6% and 77.5% and a specificity between 87.8% and 96.4%. Despite the excellent performance of these immunoassayImmunoassay
An immunoassay is a biochemical test that measures the presence or concentration of a substance in solutions that frequently contain a complex mixture of substances. Analytes in biological liquids such as serum or urine are frequently assayed using immunoassay methods...
s, for example CCP-assays, they only provide a sensitivity comparable with that of rheumatoid factor
Rheumatoid factor
Rheumatoid factor is an autoantibody most relevant in rheumatoid arthritis. It is defined as an antibody against the Fc portion of IgG. RF and IgG join to form immune complexes that contribute to the disease process...
(RF). Moreover, analysis of the correlation of anti-CCP antibody titre with RA disease activity yielded conflicting results.
Unfortunately, these artificial antigens are not expressed in the affected tissue, and therefore are probably not directly involved in the pathogenesis of RA.
In the more recent time, novel test systems utilizing ACPA have been developed. Citrullinated vimentin is a very promising autoantigen in RA, and a very suitable tool for studying this systemic autoimmune disease
Autoimmune disease
Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body actually attacks its own cells. The immune system mistakes some part of the body as a pathogen and attacks it. This may be restricted to...
. Vimentin is secreted and citrullinated by macrophages in response to apoptosis, or by pro-inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-alpha).
A newly developed ELISA
ELISA
Enzyme-linked immunosorbent assay , is a popular format of a "wet-lab" type analytic biochemistry assay that uses one sub-type of heterogeneous, solid-phase enzyme immunoassay to detect the presence of a substance in a liquid sample."Wet lab" analytic biochemistry assays involves detection of an...
system utilises genetically modified citrullinated vimentin (MCV
Mutated citrullinated Vimentin
Detection of autoantibodies against mutated citrullinated vimentin is part of RA diagnostics , especially in sera negative for rheumatoid factor . Anti-MCV antibodies are member of ACPA family, a group of the so called antibodies to citrullinated protein/peptide antigens.Rheumatoid arthritis is an...
), a naturally occurring isoform of vimentin to optimize the performance of the test. Noteworthy are the findings of a recently published study that highly valuates anti-MCV test systems for diagnosing rheumatoid arthritis in anti-CCP-negative patients.
Given that ACPA are more specific than rheumatoid factor, they are used to distinguish various causes of arthritis. Novel assays may be useful for monitoring disease activity and effects of RA therapy.
The reference ranges for blood tests of anti-citrullinated protein antibodies are:
>-
| < 20