B-cell antigen receptor stimulation activates the human Bruton's tyrosine kinase, which is deficient in X-linked agammaglobulinemia.

X-linked agammaglobulinemia (XLA) is an inherited human immunodeficiency disease, characterized by an arrest in B-cell development, which results in a dramatic decrease in immunoglobulin production. The gene product defective in XLA has been identified as a cytoplasmic protein tyrosine kinase, named Bruton's tyrosine kinase (Btk). The dramatic XLA phenotype indicates a critical role for Btk in the regulation of B-cell development. However, neither external stimuli leading to Btk activation nor any of its in vivo substrates have thus far been identified, and the mechanism of disease induction remains unexplained. We report here that stimulation of the B-cell antigen receptor (membrane immunoglobulin) on mature B-cells induces tyrosine phosphorylation of Btk in vivo, accompanied by an increase in its kinase activity in vitro. These results place Btk in the B-cell receptor signal transduction pathway, which is known to be essential in driving B-cell differentiation.