Background/Purpose:

Seropositivity for rheumatoid factors and anti-citrullinated (Cit) protein antibodies (ACPA) are the strongest predictor for clinical response to rituximab (RTX) in rheumatoid arthritis (RA). ACPAs are routinely detected using cyclic Cit peptides (CCP). However, the wide variety of non-cross reactive epitopes recognized by ACPA may explain why fluctuations in anti-CCP antibodies are not a reliable measure of response to therapies. We therefore investigated the kinetics of individual IgG- and IgA-ACPA in relation to clinical improvement (during B cell depletion) and recurrence of symptoms following RTX using multiplex antigen array. 

Methods:

16 patients with active RA (DAS28 ≥ 5.1; median duration 20 years) undergoing initial cycles of RTX were included. All achieved B cell depletion in the peripheral blood (<5 cells/μl) and clinically responded (ΔDAS28≥1.2) within 5 months (median=3). Follow-up to re-treatment or relapse was 4-13 months. IgG and IgA antibodies binding to a custom, bead-based, antigen array comprising CCP and 29 RA-associated Cit-antigens and 22 corresponding native antigens, based on those in inflamed synovium, was assessed using mean fluorescence intensity (MFI). To compensate for number of binding sites per antigen, MFI were Z-normalized in serial studies, with Z score >1 ‘positive’. Spearman’s Rank, T-tests used for data analysis.

Results:

Pre-RTX, IgG- and IgA-binding (MFI) to Cit antigens were strongly correlated (R²=0.75; p<0.0001) but MFI of IgA-Cit were approximately 10 fold lower. Mean % change in ACPA from baseline to clinical improvement (during depletion phase), was significantly greater for IgA-Cit (-43%) than IgG-Cit (-21%) (p=0.003), largely due to 24% (16/65) of IgG-Cit antibodies, but only 3/47 IgA-Cit, showing a transient increase following RTX. Amongst IgG- and IgA-Cit antibodies that fell following RTX, some increased prior to relapse (n=28 and 27 respectively) whereas others continued to fall (n=21 and 17). In those rising to relapse after an initial fall, 13/21 different Cit antigens recognized by IgG also bound IgA. Although putative ‘new’ specificities (baseline Z score ≤1; relapse Z score >1) were recognized by 9 IgG- and 10 IgA-Cit, most (7 IgG; 6 IgA) were detectable at low levels before RTX. Cit-fibrinogen derived targets were consistently detected in all different ACPA patterns post-RTX, but no particular specificities were identified.

Conclusion:

After RTX, ACPA showed a variety of profiles, presumably due to the ability of RTX to deplete or modify the function of different parent B cell clones. Some IgG-Cit (rarely IgA-Cit) rose after RTX, despite peripheral B cell depletion, suggesting that IgG-B cells were more resistant, or that the half-lives of IgG or IgA plasma cells vary. Location within protective niches in tissues may also aid survival.  Expansion of pre-existing memory B cells appeared to accompany disease resumption after RTX with levels of many autoantibody specificities present pre-RTX rising with B cell reconstitution or impending relapse. Dissection of the ACPA response using multiplex bead array revealed diverse kinetics of ACPA-committed B cells which could be exploited for more efficient B cell directed therapies.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/b-cell-depletion-with-rituximab-in-patients-with-rheumatoid-arthritis-multiplex-bead-array-reveals-kinetics-of-igg-and-iga-autoantibodies-to-citrullinated-antigens/