Anti–citrullinated protein antibody
Anti-citrullinated protein antibodies (ACPAs) are autoantibodies (antibodies to an individual’s own proteins) that are directed against peptides and proteins that are citrullinated. They are present in the majority of patients with rheumatoid arthritis. Clinically, cyclic citrullinated peptides (CCP) are frequently used to detect these antibodies in patient serum or plasma (then referred to as anti–citrullinated peptide antibodies).
During inflammation, arginine amino acid residues can be enzymatically converted into citrulline residues in proteins such as vimentin, by a process called citrullination. 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.
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 keratin 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. 
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 immunoassays, 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.
However, novel test systems utilizing ACPA have been developed. Citrullinated vimentin is a very promising autoantigen in RA, and a 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. However, data from all around the world vary substantially. Anti-CCP is also very useful in the early diagnosis of rheumatoid arthritis in high-risk groups, such as relatives of RA patients, although Silman and co-workers found that the concordance rate of developing RA was 15.4% among identical (monozygotic) twins and was 3.6% among fraternal (dizygotic) twins.
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||EU|
- Puszczewicz M, Iwaszkiewicz C (May 2011). "Role of anti-citrullinated protein antibodies in diagnosis and prognosis of rheumatoid arthritis." (PDF). Arch Med Sci. 7 (2): 189–94. doi:10.5114/aoms.2011.22067. PMID 22291756.
- Raptopoulou A, Sidiropoulos P, Katsouraki M, Boumpas DT (2007). "Anti-citrulline antibodies in the diagnosis and prognosis of rheumatoid arthritis: evolving concepts". Crit Rev Clin Lab Sci. 44 (4): 339–63. doi:10.1080/10408360701295623. PMID 17558653.
- Aletaha D, Neogi T, Silman AJ, et al. (September 2010). "2010 rheumatoid arthritis classification criteria: an American College of Rheumatology/European League Against Rheumatism collaborative initiative" (PDF). Ann. Rheum. Dis. 69 (9): 1580–8. doi:10.1136/ard.2010.138461. PMID 20699241.
- Young BJ, Mallya RK, Leslie RD, Clark CJ, Hamblin TJ (July 1979). "Anti-keratin antibodies in rheumatoid arthritis". Br Med J. 2 (6182): 97–9. doi:10.1136/bmj.2.6182.97. PMC . PMID 111762.
- Sebbag M, Simon M, Vincent C, et al. (June 1995). "The antiperinuclear factor and the so-called antikeratin antibodies are the same rheumatoid arthritis-specific autoantibodies". J. Clin. Invest. 95 (6): 2672–9. doi:10.1172/JCI117969. PMC . PMID 7539459.
- Vossenaar ER, Després N, Lapointe E, et al. (2004). "Rheumatoid arthritis specific anti-Sa antibodies target citrullinated vimentin". Arthritis Research & Therapy. 6 (2): R142–50. doi:10.1186/ar1149. PMC . PMID 15059278.
- Pratesi F, Tommasi C, Anzilotti C, Chimenti D, Migliorini P (March 2006). "Deiminated Epstein-Barr virus nuclear antigen 1 is a target of anti-citrullinated protein antibodies in rheumatoid arthritis". Arthritis Rheum. 54 (3): 733–41. doi:10.1002/art.21629. PMID 16508937.
- Bang H, Egerer K, Gauliard A, et al. (August 2007). "Mutation and citrullination modifies vimentin to a novel autoantigen for rheumatoid arthritis". Arthritis Rheum. 56 (8): 2503–11. doi:10.1002/art.22817. PMID 17665451.
- cyclic citrullinated peptide at the US National Library of Medicine Medical Subject Headings (MeSH)
- Nishimura K, Sugiyama D, Kogata Y, et al. (June 2007). "Meta-analysis: diagnostic accuracy of anti-cyclic citrullinated peptide antibody and rheumatoid factor for rheumatoid arthritis". Annals of Internal Medicine. 146 (11): 797–808. doi:10.7326/0003-4819-146-11-200706050-00008. PMID 17548411.
- Coenen D, Verschueren P, Westhovens R, Bossuyt X (March 2007). "Technical and diagnostic performance of 6 assays for the measurement of citrullinated protein/peptide antibodies in the diagnosis of rheumatoid arthritis". Clin. Chem. 53 (3): 498–504. doi:10.1373/clinchem.2006.078063. PMID 17259232.
- van Gaalen F, Ioan-Facsinay A, Huizinga TW, Toes RE (1 November 2005). "The devil in the details: the emerging role of anticitrulline autoimmunity in rheumatoid arthritis". J. Immunol. 175 (9): 5575–80. doi:10.4049/jimmunol.175.9.5575. PMID 16237041.
- Greiner A, Plischke H, Kellner H, Gruber R (June 2005). "Association of anti-cyclic citrullinated peptide antibodies, anti-citrullin antibodies, and IgM and IgA rheumatoid factors with serological parameters of disease activity in rheumatoid arthritis". Ann. N. Y. Acad. Sci. 1050: 295–303. doi:10.1196/annals.1313.031. PMID 16014545.
- Asaga H, Yamada M, Senshu T (February 1998). "Selective deimination of vimentin in calcium ionophore-induced apoptosis of mouse peritoneal macrophages". Biochem. Biophys. Res. Commun. 243 (3): 641–6. doi:10.1006/bbrc.1998.8148. PMID 9500980.
- Mor-Vaknin N, Punturieri A, Sitwala K, Markovitz DM (January 2003). "Vimentin is secreted by activated macrophages". Nat. Cell Biol. 5 (1): 59–63. doi:10.1038/ncb898. PMID 12483219.
- Soós L, Szekanecz Z, Szabó Z, et al. (August 2007). "Clinical evaluation of anti-mutated citrullinated vimentin by ELISA in rheumatoid arthritis". J. Rheumatol. 34 (8): 1658–63. PMID 17611988.
- Iwaszkiewicz C, Puszczewicz M, Białkowska-Puszczewicz G (January 2015). "Diagnostic value of the anti-Sa antibody compared with the anti-cyclic citrullinated peptide antibody in rheumatoid arthritis.". Int J Rheum Dis. 18 (1): 46–51. doi:10.1111/1756-185X.12544. PMID 25488711.
- "Anti-cyclic citrullinated peptide antibodies and rheumatoid factor in rheumatoid arthritis patients and relatives from Brazil.". Rheumatology (Oxford). 49 (8): 1590–3. Aug 2010. doi:10.1093/rheumatology/keq134. PMID 20457731.
- "Twin concordance rates for rheumatoid arthritis: results from a nationwide study.". BR J Rheumatology. 32 (10): 903–7. Oct 1993. doi:10.1093/rheumatology/32.10.903. PMID 8402000.
- Avouac J, Gossec L, Dougados M (July 2006). "Diagnostic and predictive value of anti-cyclic citrullinated protein antibodies in rheumatoid arthritis: a systematic literature review". Ann. Rheum. Dis. 65 (7): 845–51. doi:10.1136/ard.2006.051391. PMC . PMID 16606649.
- Nicaise Roland P, Grootenboer Mignot S, Bruns A, et al. (2008). "Antibodies to mutated citrullinated vimentin for diagnosing rheumatoid arthritis in anti-CCP-negative patients and for monitoring infliximab therapy". Arthritis Research & Therapy. 10 (6): R142. doi:10.1186/ar2570. PMC . PMID 19077182.
- chronolab.com > Autoantibodies associated with rheumatic diseases > Reference ranges Retrieved on 29 April 2010