Objectives: The B-cell chemokine, CXCL13, is a proposed serum biomarker of synovitis in RA. Its behaviour in the context of B-cell depletion therapy and reconstitution was studied during treatment of RA with rituximab.
Methods: Serum samples from 20 RA patients were analysed for CXCL13, RF-IgM and anti-CCP by ELISA before and 2 and 6 months following rituximab treatment. B cells were monitored by flow cytometry. Gene expression in blood and synovial biopsies was determined by qPCR.
Results: Patients with detectable B cells at 6 months had significantly higher levels of CXCL13 and RF-IgM and slightly higher levels of anti-CCP throughout the study, including at baseline, compared with patients with undetectable B cells at 6 months. Conversely, 10 of 12 patients with high baseline CXCL13 had detectable circulating B cells at 6 months, whereas no B cells could be detected at 6 months in patients with low baseline CXCL13. Synovial CXCL13 expression at baseline correlated significantly with serum CXCL13 levels, and the synovium of patients with high serum CXCL13 expressed elevated levels of IL-1β, IL-8, MMP1 and MMP3. In addition, synovial CXCL13 expression correlated significantly with several synovial inflammatory markers.
Conclusions: Serum CXCL13 is predictive of the rate of B-cell repopulation following a course of rituximab in RA. Serum CXCL13 correlates with synovial CXCL13 measured at a single joint, suggesting synovitis as an important source of circulating CXCL13. Within the synovium, CXCL13 expression is highly correlated with markers of synovitis.
Trial registration: ClinicalTrials.gov, http://clinicaltrials.gov/, NCT00147966.