Psoriatic arthritis synovial histopathology: commentary on the article by Kruithof and colleagues

The excellent article by Kruithof and colleagues stimulates many questions while carefully dissecting differentiating features in synovial histopathology that characterize rheumatoid arthritis (RA) and spondyloarthropathy (SpA), in particular psoriatic arthritis (PsA) [1]. Using a semi-quantitative scoring system, the authors identified a number of features characteristic of RA synovium, including the lining layer thickness, CD83+ dendritic cells, positive staining for intracellular citrullinated proteins (44%) and positive staining for MHC/HC gp-39 peptide complexes (46%). In the SpA group as a whole and in the PsA subgroup alone, increased vascularity and neutrophil numbers distinguished from RA. CD163+ macrophages were also increased in SpA. Interestingly, no significant differences were seen between oligoarticular PsA versus polyarticular PsA. The authors conclude that the synovitis in PsA, both oligoarticular and polyarticular, resembles SpA more than RA.These observations have a number of important implications. First, although this has been disputed [2], it may be possible to diagnose RA based on the positive staining as already stated. In Kruithof and colleagues' study, positive staining for intracellular citrullinated proteins and positive staining for MHC/HC gp-39 peptide complexes were seen only in RA, although each were present in <50%. Second, the synovitis in PsA shows similar features to other SpA patients, both ankylosing spondylitis and undifferentiated SpA.Previous studies have compared PsA with RA [3], although Kruithof and colleagues' study is the first to include other SpA patients in the comparison. In the previous study [3], an increase in vessel number was also a distinguishing feature from RA, as were lower macrophage numbers and a reduction in E-selectin expression. Kruithof and colleagues were unable to confirm these findings, which have been confirmed by others [4], but this may relate to issues of patient selection and to methods of quan