Background/Purpose:

The most important immunological event in rheumatoid arthritis (RA) is the development of anti-citrullinated protein autoantibodies (ACPAs). ACPAs are present in 2/3 of patients. The mechanisms leading to the production of ACPAs are unknown. ACPAs are produced in the absence of identified T cell responses specific for each citrullinated protein. Peptidyl arginine deiminase 4 (PAD4), which binds numerous proteins and citrullinates them is the target of autoantibodies in early RA. This suggests a model for the emergence of ACPAs in the absence T cells specific for each citrullinated antigen: anti-citrullinated protein autoantibodies could arise because PADs are recognized by T cells which help the production of autoantibodies to proteins being citrullinated by PADs, according to a “hapten/carrier” model. Here, we tested this model in mice.

Methods:

We used C3H mice which express a particular IEb^k whose third hypervariable region is highly homologous to that of RA-associated HLA-DRB1*04:01 allele and DBA/2 mice whose IEb^dis similar to that of non RA-associated HLA-DRB1*04:02. Mice were immunized subcutaneously with PADs or phosphate buffered saline (PBS) in Freund’s complete adjuvant (CFA). Three booster injections of PAD or PBS in Freund’ incomplete adjuvant (IFA) were given subcutaneously 15, 35 and 55 days later. Sera from primed mice were: 1) tested for anti-PAD antibodies by ELISA. 2) tested for T cell responses to native or citrullinated fibrinogen 65 days after PAD immunization. 3) tested for anti-citrullinated fibrinogen antibodies by ELISA using fibrinogen peptides under citrullinated and native form.

Results:

C3H mice immunized with human PAD2 or PAD4 developed antibodies and T cells to PADs and IgG antibodies to citrullinated peptides from fibrinogen, in the absence of T cell response to fibrinogen. To test whether the observed hapten carrier effect applies to immunization with self-proteins, we immunized C3H mice with murine PAD2 or PAD4 and looked for antibodies to peptides from fibrinogen under native or citrullinated form. The hapten carrier effect also occurred in the self-situation. Finally, to analyze the effect of the MHC background on hapten carrier immunization, we immunized DBA/2 mice whose IEb^dis similar to that of non RA-associated HLA-DRB1*04:02. DBA/2 mice failed to develop antibodies to citrullinated fibrinogen peptides.

Conclusion:

T cell immunization to PAD proteins triggers ACPAs through a hapten carrier mechanism in which the carrier is PAD which performs citrullination and the hapten any protein being citrullinated by PAD.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/induction-of-anti-citrullinated-protein-antibodies-by-peptidyl-arginine-deiminase-immunization-a-new-model-for-the-development-of-anti-citrullinated-protein-antibodies-in-rheumatoid-arthritis/