PAD4 deletion in synovial macrophages exacerbates pathology in inflammatory arthritis

Citrullination by peptidylarginine deiminase (PAD) enzymes is a post-translational protein modification implicated in the etiopathogensis of rheumatoid arthritis. Of the five known PAD isoforms, PAD4 is expressed in the nucleus of innate immune cells including synovial macrophages, and studies have shown citrullination of intracellular proteins such as transcription factors and histones can polarize cell phenotype. Here, we report how systemic and local deletion of PAD4 in synovial macrophages from PAD4^-/- or PAD4^f/fCre^LysM mice results in activation of both shared and distinct gene modules in four populations of synovial macrophages. Furthermore, severity of KBxN serum transfer arthritis was increased in PAD4^f/fCre^LysM mice compared to controls. Furthermore, macrophages isolated from the hindjoints of arthritic PAD4^f/fCre^LysM displayed increased expression of inflammatory genes compared to macrophages from arthritic PAD4^-/- mice. In addition, we report that local intra-articular administration of PAD inhibitor BB-Cl-amidine increased arthritis severity, while systemic administration had no effect. These findings indicate an anti-inflammatory role for intracellular citrullination in synovial macrophages, and highlight the potential confounding effects of extracellular and systemic PAD4 loss of function.