Bruton's tyrosine kinase (BTK) transmits crucial survival signals from the B cell receptor (BCR) in B cells. Pharmacologic BTK inhibition effectively diminishes disease symptoms in mouse models of autoimmunity; conversely, transgenic BTK overexpression induces systemic autoimmunity in mice. We undertook this study to investigate BTK expression and activity in human B cells in the context of autoimmune disease.

Using intracellular flow cytometry, we quantified BTK expression and phosphorylation in subsets of peripheral blood B cells from 30 patients with rheumatoid arthritis (RA), 26 patients with primary Sjögren's syndrome (SS), and matched healthy controls.

In circulating B cells, BTK protein expression levels correlated with BTK phosphorylation. BTK expression was up‐regulated upon BCR stimulation in vitro and was significantly higher in CD27+ memory B cells than in CD27−IgD+ naive B cells. Importantly, BTK protein and phospho‐BTK were significantly increased in B cells from anti–citrullinated protein antibody (ACPA)–positive RA patients but not in B cells from ACPA‐negative RA patients. BTK was increased both in naive B cells and in memory B cells and correlated with frequencies of circulating CCR6+ Th17 cells. Likewise, BTK protein was increased in B cells from a major fraction of patients with primary SS and correlated with serum rheumatoid factor levels and parotid gland T cell infiltration. Interestingly, targeting T cell activation in patients with primary SS using the CTLA‐4Ig fusion protein abatacept restored BTK protein expression in B cells to normal levels.

These data indicate that autoimmune disease in humans is characterized by enhanced BTK activity, which is linked not only to autoantibody formation but also to T cell activity.