{"doi":"10.3389/fimmu.2023.1203506","title":"Citrullination of C1-inhibitor as a mechanism of impaired complement regulation in rheumatoid arthritis","abstract":"Background Dysregulated complement activation, increased protein citrullination, and production of autoantibodies against citrullinated proteins are hallmarks of rheumatoid arthritis (RA). Citrullination is induced by immune cell-derived peptidyl-Arg deiminases (PADs), which are overactivated in the inflamed synovium. We characterized the effect of PAD2- and PAD4-induced citrullination on the ability of the plasma-derived serpin C1-inhibitor (C1-INH) to inhibit complement and contact system activation. Methods Citrullination of the C1-INH was confirmed by ELISA and Western blotting using a biotinylated phenylglyoxal probe. C1-INH-mediated inhibition of complement activation was analyzed by C1-esterase activity assay. Downstream inhibition of complement was studied by C4b deposition on heat-aggregated IgGs by ELISA, using pooled normal human serum as a complement source. Inhibition of the contact system was investigated by chromogenic activity assays for factor XIIa, plasma kallikrein, and factor XIa. In addition, autoantibody reactivity to native and citrullinated C1-INH was measured by ELISA in 101 RA patient samples. Results C1-INH was efficiently citrullinated by PAD2 and PAD4. Citrullinated C1-INH was not able to bind the serine protease C1s and inhibit its activity. Citrullination of the C1-INH abrogated its ability to dissociate the C1-complex and thus inhibit complement activation. Consequently, citrullinated C1-INH had a decreased capacity to inhibit C4b deposition via the classical and lectin pathways. The inhibitory effect of C1-INH on the contact system components factor XIIa, plasma kallikrein, and factor XIa was also strongly reduced by citrullination. In RA patient samples, autoantibody binding to PAD2- and PAD4-citrullinated C1-INH was detected. Significantly more binding was observed in anti-citrullinated protein antibody (ACPA)-positive than in ACPA-negative samples. Conclusion Citrullination of the C1-INH by recombinant human PAD2 and PAD4 enzymes impaired its ability to inhibit the complement and contact systems in vitro . Citrullination seems to render C1-INH more immunogenic, and citrullinated C1-INH might thus be an additional target of the autoantibody response observed in RA patients.","journal":"Frontiers in Immunology","year":2023,"id":371317,"datarank":0.0,"base_score":0.0,"endowment":0.0,"self_citation_contribution":0.0,"citation_network_contribution":0.0,"self_endowment_contribution":0.0,"citer_contribution":0.0,"corpus_percentile":null,"corpus_rank":null,"citation_count":6,"citer_count":0,"citers_with_citation_signal":0,"citers_with_endowment":0,"datacite_reuse_total":0,"is_dataset":false,"is_dataset_confidence":0.9603,"is_data_producer":false,"deposit_databanks":null,"is_oa":true,"file_count":0,"downloads":0,"has_version_chain":false,"published_date":"2023-01-01","fair_score":null,"fair_percentile":null,"algorithm_id":"datarank_citation_only_1hop_v6","ranking_scope":"data_only","authors":[{"id":1131380,"name":"Sara C. Nilsson","orcid":null,"position":1,"is_corresponding":false},{"id":1131381,"name":"David Eikrem","orcid":null,"position":2,"is_corresponding":false},{"id":1130925,"name":"Karin Fromell","orcid":"0000-0001-8905-8791","position":3,"is_corresponding":false},{"id":298028,"name":"Carsten Scavenius","orcid":"0000-0002-4304-0681","position":4,"is_corresponding":false},{"id":1130926,"name":"Leonie M. Vogt","orcid":"0000-0002-2843-9002","position":5,"is_corresponding":false},{"id":1130927,"name":"Ewa Bielecka","orcid":"0000-0002-2631-6711","position":6,"is_corresponding":false},{"id":271717,"name":"Jan Potempa","orcid":"0000-0002-3600-7461","position":7,"is_corresponding":false},{"id":298029,"name":"Jan J. Enghild","orcid":"0000-0001-9292-9172","position":8,"is_corresponding":false},{"id":41948,"name":"Bo Nilsson","orcid":"0000-0003-0057-2730","position":9,"is_corresponding":false},{"id":417407,"name":"Kristina Nilsson Ekdahl","orcid":"0000-0001-7888-1571","position":10,"is_corresponding":false},{"id":1130928,"name":"Meliha C Kapetanovic","orcid":"0000-0002-7853-514X","position":11,"is_corresponding":false},{"id":41935,"name":"Anna M. Blom","orcid":"0000-0002-1348-1734","position":12,"is_corresponding":false},{"id":1080833,"name":"Myriam Martin","orcid":"0000-0003-1526-8678","position":0,"is_corresponding":true}],"reference_count":57,"raw_metadata":null,"created_at":"2026-07-19T01:15:45.330525Z","pmid":"37426666","pmcid":null,"fwci":null,"citation_percentile":null,"influential_citations":0,"oa_status":null,"license":null,"views":0,"total_file_size_bytes":0,"version_count":0,"fair_f":null,"fair_a":null,"fair_i":null,"fair_r":null,"fair_zscore":null,"fair_rationale":null,"fair_model":null,"fair_agent_version":null,"fair_fulltext_source":null,"fair_has_llm":null,"fair_computed_at":null,"clinical_trials":[],"software_tools":[],"db_accessions":[],"linked_datasets":[],"topics":[]}