MPO and rheumatoid arthritis: Immunostaining for Myeloid peroxidase (MPO neutrophil marker) showed profound infiltration of neutrophils into the synovium from patients undergoing joint replacements in 58% of RA, in 88% of PsA, and 26% of OA patients, while no neutrophils were found in the synovium of non-inflammatory controls (Table 2).