At baseline, constitutive STAT3 phosphorylation was commonly found in circulating leukocytes from patients with recent-onset RA: CD4+ T cells from 14 patients (56%), CD8+ T cells from 13 patients (52%), CD19+ B cells from 7 patients (28%), and CD14+ monocytes from 12 patients (48%) were pSTAT3+. Here, CD19 is linked to rheumatoid arthritis.