Posttranscriptional regulation of Bruton's tyrosine kinase expression in antigen receptor-stimulated splenic B cells

Mutation of Bruton's tyrosine kinase (Btk) causes human X-linked agammaglobulinemia and murine X-linked immunodeficiency syndrome (xid), Quantitative aspects of B lymphocyte development and function have been demonstrated to depend on Btk level in vivo by using a murine transgenic model system. A sensitive intracellular immunofluorescent assay was developed to measure Btk protein on a per cell basis to test the hypothesis that its dosage is dynamically regulated during B cell development or functional responses, Marrow-derived hematopoietic stem cells, common lymphoid progenitor cells, and developing B and myeloid lineages expressed Btk protein at comparable levels, Resting peripheral B lineage cells had a significantly lower amount of Btk than marrow-derived cells in both wild-type and rid mice. Activation of the B cell antigen receptor up-regulated Btk protein level 10-fold within several hours by a phosphatidylinositol 3-kinase-dependent, posttranscriptional mechanism, In contrast, the protein level of Btk R28C in activated B lymphocytes from rid mice remained low. Bypass of the antigen receptor signaling pathways by treatment of cells with phorbol myristic acid and ionomycin rescued up-regulation of Btk protein in rid splenic B cells. These combined results suggest that certain receptor signals mediated by Btk regulate the level of expression of Btk protein in responding B lymphocytes to potentiate signal transduction, Dynamic regulation of Btk protein dosage is an additional mechanism to modulate B lymphocyte immune functions.