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Objective: To better understand the contribution of autoantibodies in RA and the biology of their responses, we evaluated the avidity of the anti-carbamylated protein (anti-CarP) antibody response. Methods: The avidity of anti-CarP antibody, ACPA and anti-tetanus toxoid IgG were determined using elution assays. Anti-CarP IgG avidity was measured in sera of 107 RA patients, 15 paired SF and serum samples and 8 serially sampled sera before and after disease onset. Results: The avidity of anti-CarP IgG is low compared with the avidity of anti-tetanus toxoid IgG present in the same sera. Likewise, although less pronounced, anti-CarP also displayed a lower avidity as compared with the avidity of ACPA IgG. No difference in anti-CarP IgG avidity is observed between ACPA positive or ACPA negative patients. Anti-CarP IgG avidity is higher in anti-CarP IgM-negative compared with IgM-positive individuals. Furthermore, the anti-CarP avidity in serum is higher than in SF. Using samples of individuals that over time developed RA we observed no anti-CarP avidity maturation in the years before disease onset. In contrast to ACPA avidity, the anti-CarP avidity is not associated with severity of joint destruction. Conclusion: The anti-CarP response is of overall low avidity, even lower than the ACPA IgG avidity, and does not show apparent avidity maturation before or around disease onset. Overall, isotype switch and avidity maturation seem to be uncoupled as isotype switch occurs without avidity maturation, pointing towards a commonality in the regulation of both autoantibody responses as opposed to the pathways governing recall responses.

Objective: To better understand the contribution of autoantibodies in RA and the biology of their responses, we evaluated the avidity of the anti-carbamylated protein (anti-CarP) antibody response. Methods: The avidity of anti-CarP antibody, ACPA and anti-tetanus toxoid IgG were determined using elution assays. Anti-CarP IgG avidity was measured in sera of 107 RA patients, 15 paired SF and serum samples and 8 serially sampled sera before and after disease onset. Results: The avidity of anti-CarP IgG is low compared with the avidity of anti-tetanus toxoid IgG present in the same sera. Likewise, although less pronounced, anti-CarP also displayed a lower avidity as compared with the avidity of ACPA IgG. No difference in anti-CarP IgG avidity is observed between ACPA positive or ACPA negative patients. Anti-CarP IgG avidity is higher in anti-CarP IgM-negative compared with IgM-positive individuals. Furthermore, the anti-CarP avidity in serum is higher than in SF. Using samples of individuals that over time developed RA we observed no anti-CarP avidity maturation in the years before disease onset. In contrast to ACPA avidity, the anti-CarP avidity is not associated with severity of joint destruction. Conclusion: The anti-CarP response is of overall low avidity, even lower than the ACPA IgG avidity, and does not show apparent avidity maturation before or around disease onset. Overall, isotype switch and avidity maturation seem to be uncoupled as isotype switch occurs without avidity maturation, pointing towards a commonality in the regulation of both autoantibody responses as opposed to the pathways governing recall responses.

Background: The presence of autoantibodies is considered an important characteristic of rheumatoid arthritis (RA); therefore, both anticitrullinated protein antibodies (ACPA) and rheumatoid factor (RF) are included in the 2010 classification criteria for rheumatoid arthritis (RA). However, a considerable number of RA patients lack both these autoantibodies. Recently, several novel autoantibodies have been identified but their value for the classification of RA patients is unclear. Therefore, we studied the value of novel autoantibodies using the presence of anticarbamylated protein (anti-CarP) antibodies as an example for predicting RA development in patients with undifferentiated arthritis (UA). Methods: There were 1352 UA patients included in the Leiden Early Arthritis Clinic (EAC) cohort according to the 1987 criteria. When the 2010 criteria were used, there were 838 UA patients. Of these, we evaluated whether they fulfilled the 1987 or 2010 criteria after 1 year, respectively. Logistic regression analyses were performed with RA as outcome and ACPA, RF, and anti-CarP antibodies as predictors. Analyses were repeated after stratification for ACPA and RF. Results: Thirty-three percent of the 1987-UA patients and 6% of the 2010-UA patients progressed to RA during the first year of follow-up. For the 1987-UA patients, anti-CarP antibodies were associated with progression to RA, an association which remained when a correction was made for the presence of ACPA and RF (odds ratio (OR) 1.7, 95% confidence interval (CI) 1.2-2.4). After stratification for ACPA and RF, anti-CarP antibodies were associated with progression to RA only for ACPA- and RF-negative patients (OR 2.1, 95% CI 1.3-3.7). For the 2010-UA patients, anti-CarP antibo