Immunoprecipitation-mass spectrometry reveals known and novel (NatC, <i>BCS1L</i> ) antinuclear antibodies in rheumatoid arthritis

OBJECTIVE: In a fraction of patients with rheumatoid arthritis (RA), antinuclear antibodies (ANA) are present. However, in most ANA-positive samples from patients with RA, no autoantigens can be identified. We aimed to characterise the ANA landscape in RA and to identify known and novel autoantigens using an unbiased immunoprecipitation-mass spectrometry (IP-MS) approach. METHODS: Sera from 51 ANA-positive, DMARD-naïve patients with RA were analysed by IP-MS using HeLa nuclear or cytoplasmic extracts (depending on the ANA staining pattern). Immunoprecipitated proteins were digested and identified using liquid chromatography coupled with MS. The prevalence of new or unfamiliar autoantigens was assessed in 43 healthy individuals and 88 ANA-positive individuals with a systemic rheumatic disease (systemic lupus erythematosus, Sjögren's disease, systemic sclerosis and idiopathic inflammatory myopathy). RESULTS: Autoantigens were detected in 10 of 51 RA samples (in 2/29 with titre 1/160, in 4/12 with titre 1/320 and in 4/10 with titre ≥1/640). Autoantigens identified included SSA-Ro60, SSB-La, Ku, fibrillarin, components of the mitochondrial multienzyme complex, protein argonaute 1, 2, 3 and T-cell restricted intracellular antigen 1 and TIA-1-related protein. Two novel autoantigens were discovered: the N-terminal acetyltransferase C (NatC) complex and mitochondrial chaperone BCS1. Neither NatC nor BCS1 was detected in healthy individuals or individuals with other systemic rheumatic diseases. CONCLUSION: IP-MS revealed a diverse spectrum of autoantigens in patients with ANA-positive RA, including previously unreported targets. These findings expand our knowledge of RA-associated autoimmunity and highlight antibodies to NatC and BCS1 as potential novel biomarkers. Further studies are needed to assess their clinical relevance.