CXCL13 and rheumatoid arthritis: Further, a recent report of a small synovial biopsy study of RA patients undergoing rituximab therapy showed a correlation between synovial tissue expression of CXCL13 and levels of CXCL13 protein in the serum (r = 0.6) [44] that suggests CXCL13 expression in the rheumatoid synovium is a major source of serum CXCL13.