Monomeric CRP is prevalent in the blood of patients with rheumatoid arthritis and correlates with clinically relevant markers.

Introduction: Elevated C-reactive protein (CRP) is a hallmark of rheumatoid arthritis (RA) flares. CRP exists as pentameric (pCRP) and monomeric (mCRP) isoforms with distinct functions, with pCRP generally pro-inflammatory and mCRP more immunomodulatory. Standard clinical assays do not distinguish between these forms, and it remains unclear whether elevated CRP in RA reflects structural variation. We investigated CRP isoform distribution and function in RA using biomolecular and biophysical approaches. Methods: Patients with RA (n=39) were recruited from the University College London Hospital. Serum CRP was requantified by commercial ELISA. Western blotting using ELISA antibodies determined detectable CRP isoforms. mCRP was assessed using a specific monoclonal antibody by ELISA and western blot. Molecular dynamics simulations of CRP were conducted at 30 °C, 37 °C, and 50 °C to evaluate structural stability and dissociation. CRP stability was further assessed under NaCl and CaCl Results: Commercial ELISA correlated with clinical CRP measurements (p=0.002, r=0.39) but did not detect mCRP by ELISA or by Western blot. Presence of mCRP was confirmed in RA serum using an mCRP-specific antibody by ELISA and Western blot and was significantly elevated compared to controls. mCRP levels correlated with anti-CCP antibodies (p=0.02, r=0.4). pCRP dissociation was unaffected by ionic gradients. Simulations showed reduced pCRP stability at 37 °C compared to 30 °C and 50 °C, suggesting increased structural fluctuation. Higher mCRP levels were associated with suppression of CD11c expression by flow cytometry. Discussion: Standard assays fail to detect mCRP in RA. Elevated mCRP and its associations with autoimmunity and cellular markers suggest it may contribute to disease mechanisms and represent a clinically relevant biomarker.