Detection of IgG4-Specific Autoantibodies in Rheumatoid Arthritis Serum Samples

Abstract

Background: Rheumatoid arthritis (RA) is a chronic multi-system autoimmune disease characterized by inflammatory synovitis. Autoantibodies such as rheumatoid factor (RF) and anti-citrullinated protein antibodies (ACPA) have been implicated in the pathogenesis of RA, and are currently important criteria for diagnosis within the 2010 American College of Rheumatology (ACR)/European League Against Rheumatism (EULAR) classification criteria.1 Yet, many patients diagnosed with RA do not have measurable circulating ACPA or RF which may result in delayed diagnosis and treatment. After IgG1, IgG4 is the second most predominant isotype among ACPA and RF; however it is not detected in currently available diagnostic assays. Recent data have demonstrated that patients deemed “sero-negative” based on standard assays may have high titers of IgG4-specific ACPA and/or RF. Objectives: The purpose of this study is to quantitate and compare IgG1- to IgG4-specific anti-CCP antibodies and rheumatoid factor in patients with rheumatoid arthritis. We will determine the frequency of IgG4 autoantibodies, and examine whether they have a differential expression among RA patients. We will also correlate their presence with disease activity, anti-rheumatic drug therapy, and levels of serum cytokines. Ultimately, this work may help to determine if a diagnostic test that detects IgG4 isotype of ACPA and RF will aid in earlier diagnosis and better characterization of rheumatoid arthritis. Methods: To explore our objectives, we have initiated a cross-sectional study with the goal of enrolling 1,000 patients with a confirmed diagnosis of rheumatoid arthritis, based on the 2010 ACR/EULAR classification criteria.1 We are collecting clinical information about each patient including demographics, current treatments, clinical disease activity, laboratory values, and radiographic results. Concurrently, we are collecting serum samples from each patient that will be analyzed for 1) total levels of IgG4 and IgG1; 2) total ACPA and RF; 3) levels of IgG1-specific and IgG4-specific ACPA and RF; and 4) cytokine levels (IL-6, TNF, IL-1, IL-17, IFNy, IL-21, and G-CSF). Results: To date, we have recruited 102 RA patients including 70 females (68.6%) and 32 males (31.4%). Their ages range from 24 to 85 years (mean 58.4 ± 12.4 years). Acute phase reactant levels were available for 98 of the 102 patients, allowing calculation of the disease activity score using 28 joints (DAS28). The mean DAS28 was 3.67 ± 1.0, which is within the moderate disease activity range. The proportion of patients in each disease category was: remission (12.2%), low disease activity (21.4%), moderate disease activity (61.2%), and high disease activity (6.1%). Based on their medical records, at any point in time, 46.1% (n=47) of the recruited subjects had positive RF titers vs. 39.2% (n=40) without RF; 58.8% (n=60) had ACPA vs 26.5% (n=27) without ACPA. For 14.7% (n=15) of the subjects, RF and/or ACPA were either unknown or untested. Of patients with RF, 91.4% (n=43) also had ACPA; of patients with ACPA, 71.7% % (n=43) also had RF. Of the patients tested for both, 27.9% (n=24) were negative for both RF and ACPA. Conclusion: Subject recruitment and data collection are well underway for this large cross-sectional study that will shed light to the role of IgG4- specific autoantibodies in the pathogenesis and diagnosis of rheumatoid arthritis. Reference: 1Aletaha D Neogi T, Silman A, et al. 2010 Rheumatoid arthritis classification criteria: An American College of Rheumatology/European League Against Rheumatism collaborative initiative. Arthritis Rheum 2010; 62(9):2569-81/Ann Rheum Dis. 2010; 69:1580-8.