Novel Biomarker for Rheumatoid Arthritis: Serum Peptidylarginine Deiminase 4-Potentiating Activity in an Anti-Citrullinated Protein Antibody-Positive At-Risk Population

Enzymes like peptidylarginine deiminase 4 (PAD4) generate citrullinated epitopes that are crucial in triggering and driving the anti-citrullinated protein antibody (ACPA) response in rheumatoid arthritis. Here, we assessed whether serum PAD4-potentiating activity is associated with subsequent arthritis development among ACPA-positive individuals during the pre-clinical phase. The study included 267 ACPA-positive individuals from the Karolinska Risk-RA cohort, among whom 101 (37.8%) developed arthritis within the observational period (range 22-60 months). We measured the ability of serum to potentiate PAD4 activity by quantifying the generation of citrullinated products using high-performance HPLC. Study participants were then classified as PAD4_act-positive or -negative, and PAD4 status analysed in relation to onset of arthritis, clinical characteristics (ESR, CRP and DAS28) and established risk factors (HLA-DRB genotype, smoking). PAD4_act-positivity was identified in 35 individuals, with a significant proportion (21, p = 0.0049) developing arthritis. Kaplan-Meier analysis demonstrated that individuals with PAD4_act-positivity had increased rate of progressing to arthritis (p = 0.0022). Additionally, PAD4_act-positive individuals were more likely to possess at least one copy of the HLA-DRB shared epitope allele (p = 0.045). Notably, PAD4_act-positivity did not associate with pre-clinical CRP, ESR, DAS28, or smoking status. These findings suggest that serum PAD4-potentiating activity may help stratify arthritis risk and time to onset among ACPA-positive at-risk individuals.