The fine specificity of IgM anti-citrullinated protein antibodies (ACPA) is different from that of IgG ACPA

Abstract

Introduction: The antigen recognition pattern of immunoglobulin M (IgM) could, when directed against protein antigens, provide an indication of the antigenic moieties triggering new B cells. The half-life of IgM is short and memory B cells against T-cell-dependent protein antigens typically produce IgG and not IgM antibodies. In this study, we analyzed whether a difference exists between the fine specificity of IgM versus IgG anti-citrullinated protein antibodies (ACPAs).

Methods: We determined the fine specificity of IgM and IgG ACPAs in 113 ACPA-positive rheumatoid arthritis patients with IgM cyclic citrullinated peptide 2 (CCP2) levels above 100 AU/ml. Fine specificity was assessed by performing ELISA using citrullinated peptides derived from vimentin, fibrinogen-β, fibrinogen-α and α-enolase, as well as citrullinated proteins fibrinogen and myelin basic protein. The arginine counterparts were used as controls.

Results: Recognition of defined citrullinated antigens by IgM ACPA was confined to samples that also displayed recognition by IgG ACPA. However, the IgM ACPA response displayed a more restricted antigen recognition profile than IgG ACPA (OR = 0.26, P < 0.0001).

Conclusion: Our data show that several defined citrullinated antigens are recognized only by IgG ACPA, whereas others are also recognized by IgM ACPA. These observations suggest that not all citrullinated antigens are able to activate new B cells despite concurrent recognition by IgG ACPA.

Figure 1

The fine specificity of immunoglobulin G (IgG) and IgM anticitrullinated protein antibodies (ACPAs). The absorbance (Abs) at 415 nm of ACPA IgG (A) and IgM (B) ACPA fine specificity. ***P < 0.001. Data for gout controls from three ELISA plates are depicted. α-eno = α-enolase; Arg = arginine; CCP2 = cyclic citrullinated peptide 2; Cit = citrulline; Fib = fibrinogen; MBP = myelin basic protein; Vim = vimentin.

Figure 2

Positivity of immunoglobulin G (IgG) and IgM anticitrullinated protein antibodies (ACPAs). The citrullinated epitopes recognized by IgG and IgM ACPAs in each individual. The gray area represents positive reactivity, and the white area represents lack of reactivity. α-eno = α-enolase; CCP = cyclic citrullinated peptide; Fib = fibrinogen; MBP = myelin basic protein; Vim = vimentin.

Figure 3

Stability of the immunoglobulin M anticitrullinated protein antibody response. Sera taken from 18 patients at baseline (BL) and at 1, 2 and 5 years of follow-up were analyzed by ELISA for reactivity against cyclic citrullinated peptide 2 (CCP2), fibrinogen (Fib)-α and Fib-β. (A) through (C) Graphs displaying the immunoglobulin M (IgM) reactivity to the indicated peptides over time. The absorbance of citrulline (cit) reactivity minus absorbance of the arginine (arg) reactivity is shown. (D) Table showing the number of positive patients at baseline (BL) as well as the conversion toward negative or positive during one of the follow-up (FU) time points analyzed.