Class switched memory B cells are enriched in the synovium after rituximab treatment

A 49-year-old female was diagnosed with polyarticular, rheumatoid factor-positive juvenile idiopathic arthritis at the age of 15. Advanced destruction required bilateral knee joint replacement 8 years later. Persistent inflammatory activity led to rituximab treatment (1,000 mg on days 1 and 15) in an advanced disease stage. Peripheral blood analyses showed immediate B-cell depletion. Synovial tissue histology 8 months later in the early reconstitution phase revealed scattered follicular-like infiltrates including CD20⁺ B cells. Flow cytometry of homogenized tissue showed CD19⁺ B cells predominantly representing an IgD^-CD27⁺ class switched memory phenotype, which was contrasted by >90% IgD⁺CD27^- peripheral blood B cells. This observation, which indicates either local persistence or early B-cell repopulation and a composition of synovial B cells discordant to peripheral blood, could have an impact on the future development of monitoring strategies after rituximab treatment in arthritis.