Helicobacter pylori is a Gram-negative bacterium that infects around half of the global population. Reports indicate that H. pylori infection can exacerbate rheumatoid arthritis (RA), but the potential underlying mechanisms remain unclear.

Protein citrullination is catalysed by the protein arginine deiminase (PAD) family. Infection of a gastric epithelial cell line (GES-1) with H. pylori upregulated the expression of PAD4 and increased PAD4 enzymatic activity compared with uninfected GES-1 cells. In addition, levels of PAD4 were higher in the sera of H. pylori-infected individuals with RA compared with uninfected individuals with RA.