Post-translationally modified proteins bind and activate complement with implications for cellular uptake and autoantibody formation

Introduction: Autoimmune diseases, such as rheumatoid arthritis (RA), are characterized by the presence of autoantibodies including those targeting self-proteins modified by post-translational modifications (PTMs). The complement system is known for its role in innate immune defense, but also in clearing debris and induction of antibody responses. We therefore hypothesized that complement could directly bind to PTMs and target PTM-modified proteins for clearance, or stimulate (chronic) inflammation and development of anti-PTM autoimmunity. Methods: Six PTMs were investigated: nitration (Nt), citrullination (Cit), carbamylation (Ca), acetylation (Ac), malondialdehyde-acetaldehyde adducts (MAA) and advanced glycation end-products (AGE). We used mass spectrometry and plate-bound assays to analyze binding of serum proteins to PTM-modified proteins. The impact of complement activation on cellular uptake was studied in phagocytosis assays. The relationship between complement SNPs, and presence of anti-PTM autoantibodies was analyzed in 587 RA patients. Results: Mass spectrometry analysis revealed a strong binding of complement to proteins modified with Ca, Ac, MAA and AGE but not to Nt and Cit. These observations were confirmed by plate-bound assays revealing that Ca-, MAA- and AGE-modified proteins activated the classical pathway, without involving antibodies. Ac activated the lectin pathway through ficolin-3. Complement activation on Ca-, Ac-, MAA- and AGE-coupled beads enhanced phagocytosis. SNPs in complement genes, associated with higher complement activity, were strongly associated with the presence of anti-PTM antibodies in RA patients. Conclusion: Proteins containing the PTMs Ca, Ac, MAA or AGE activate complement. These complement opsonized PTMs increase phagocytosis and may lead to the development of anti-PTM antibodies.