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Diagnostic and clinical utility of antibody testing in rheumatoid arthritis


By Ilana Heckler, PhD R Americans.1


heumatoid Arthritis (RA) is a chronic autoimmune disease that causes joint pain, stiffness, swelling and decreased joint motility. RA affects an estimated 1.28-1.36 million RA mostly affects women at the age of 40-50 years.2


Symptoms typically appear on both the left and right sides of the body symmetrically in the extremities such as hand and finger joints. While there is no cure for RA, treatment is possible to minimize symptoms. Common treatments aim to reduce inflam- mation and pain. Physical and occupational therapy can be used to increase mobility and strength of joints.3


deformation of the joints may lead to serious limitations. The presence of specific autoantibodies is a hallmark of RA.4


RA is characterized by the production of rheumatoid factors (RFs) and antibodies against citrullinated proteins, or anti- citrullinated protein antibodies (ACPAs). Antibody production results in the formation of immune complexes and subsequent joint inflammation. RA-associated antibodies precede the onset of symptoms and predict disease course.4


RF is found in serum


and synovial fluid (SF) of most RA patients and is associated with a more aggressive disease course. ACPAs are highly specific for RA and are associated with a severe, erosive destructive disease course.4


As a majority of RA patients exhibit antibodies against RFs and ACPAs, they represent useful markers of the disease.4


In


this article, we will describe how the detection of RA-specific antibodies can be used for the diagnosis of RA. Additionally, we will discuss the advantages of antibody detection for disease subtyping and treatment monitoring.


Diagnostic criteria for RA


Since the prognosis of RA is dependent on prompt treatment, early diagnosis is critical. In 2010, the American College of Rheu- matology (ACR) and the European League Against Rheumatism (EULAR) developed an international classification criterion for RA.5


These criteria recommend the determination of both RFs


and ACPAs, along with clinical symptoms. Clinical and labora- tory findings are weighted according to a point system and the diagnosis RA is either excluded, uncertain or definitive, depending on the score.5


Role of ACPA and RF levels in the diagnosis of RA RF was the first antibody to be discovered in RA patients. RF exists as IgM, IgG and IgA subtypes but is most commonly measured as the IgM form.6


(60-90%) and a variable specificity (48-92%).7


is limited because RF can be found in healthy populations and other autoimmune diseases.7


can be increased at high titers. The role of RF in diseases pathogenesis has been demonstrated previously.7


The specificity of RF testing Several


proposed mechanisms of RF-induced pathogenesis involve the potentiation of inflammation and antigen trapping in joints.8 ACPA levels are detected in two thirds of RA patients and have a high diagnostic specificity (98%).8


ACPA levels increase


during the transition from early undifferentiated arthritis to RA and can be detected in patient serum up to 14 years prior


36 DECEMBER 2021 MLO-ONLINE.COM to the onset of symptoms.9 In RA, ACPAs can be present in


different isotypes including IgG, IgA, IgM and IgE but are mainly IgG. To detect ACPA diagnostically, artificial cyclic citrullinated peptides (CCPs) are used as target antigens. Second-generation ELISAs have been developed for the de- tection of antibodies directed against citrullinated epitopes (CCP2) and demonstrate high sensitivities (69.6%-77.5%) and specificities (87.8%-96.4%) for RA.10


Without treatment,


citrullinated vimentin, a member of the ACPA family, can be measured using an ELISA with the Sa antigen.11


Antibodies against Anti-Sa


antibodies have been proposed to be a promising marker of RA disease severity.12


Anti-CCP antibodies exhibit a higher predictive value for RA compared to RF (79% vs 56%).13


predictive indicators for the development of erosions and the degree of radiological progression in RA.14


Both antibodies are useful One study found


a mechanistic link by which RF enhances the pathogenicity of ACPA-immune complexes in RA.15


The combined presence


of ACPA and IgM-RF was found to mediate increased proin- flammatory cytokine production in vitro and is associated with increased systemic inflammation and disease activity in RA.15 An additional use for antibody testing is for monitoring RA risk in patients with Porphyromonas gingivalis infection. Por- phyromonas gingivalis mediates citrullination of host peptides and, therefore, the generation of RA-associated antibodies such as anti-CEP-1 (discussed below).16


Other novel target antigens in RA Additional antigenic targets have been identified in RA. Anti- bodies against citrullinated -enolase peptide-1 (anti-CEP-1) have been observed in 37%–62 % of patients with RA.17


CEP-1


testing has been shown to have a high specificity (97.6%) for RA. Anti-CEP-1 antibodies are associated with an erosive disease course and with interstitial lung diseases.18


While CEP-1 testing


RF testing has a high sensitivity RF specificity


may not be able to replace CCP2 testing, it may be useful as a supplementary test for confirmation of serological findings and for disease subtyping. Anti-CEP-1 antibodies occur in a subtype of RA associated with smoking and genetic factors.19 Anti-CEP-1 is an early marker that can predict the onset of symptoms ≥10.5 years.20 Carbamylation is a post-translational modification respon- sible for the conversion of lysine into homocitrulline. In some people, extensive carbamylation can trigger an autoimmune response against carbamylated proteins. Antibodies against proteins that contain homocitrulline residues (anti-CarP) have been detected in 45% of RA patients and can occur in patients who are negative for ACPAs.21


specific for RA as they are not found in healthy patients or those with other inflammatory rheumatic conditions.22


Anti-CarP antibodies are highly Anti-CarP


antibodies can be detected more than 10 years before disease onset and, therefore, are useful for early disease identification.4 Several additional RA-associated antibodies have been described including anti-PAD4, anti-BRAF, anti-RA-33, and others directed at post-translationally modified proteins.4


PAD4


is responsible for the conversion of arginine into citrulline. PAD4 antibodies have been found to occur in RA patients and are as-


Photo Courtesy of EUROIMMUN US


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