Expression of Bruton's tyrosine kinase protein within the B cell lineage

Abstract

Defects in the gene encoding Bruton's tyrosine kinase (Btk), normally expressed in B cells, cause X-linked agammaglobulinemia (XLA). The phenotype of XLA is characterized by a lack of circulating B cells and immunoglobulin. It has been suggested that B cell maturation from the pre-B cell stage to more mature stages is dependent on the appropriate expression of this gene. The Btk mRNA is expressed in B cells and myeloid cells, but protein expression in relation to B cell maturation has not been determined. Moreover, expression of the Btk protein has so far only been investigated in human Epstein-Barr virus-transformed B cell lines, and in murine splenocytes and B cell lines. We have developed an antiserum which recognizes the human Btk protein and shown that normal human tonsillar B cells, peripheral blood monocytes and myeloid cells express the protein, whereas tonsil-derived T cells do not. We also show that the protein is present in early and mature human B cell lines, but is absent in terminally differentiated plasma cell lines. Furthermore, expression is reduced or absent in three B lineage cell lines derived from two patients with defined genetic mutations in Btk and suffering from XLA.

The protein product of the c-cbl protooncogene is phosphorylated after B cell receptor stimulation and binds the SH3 domain of Bruton's tyrosine kinase.

“…The identification of Btk as a member of the nonreceptor tyrosine kinase family, a family of proteins already established as being involved in hematopoietic signal transduction, fits well with the disease phenotype of XLA. The Btk protein is expressed in early and mature human B cell lines but is absent in terminally differentiated plasma cell lines, which is consistent with the requirement of a functional Btk protein for normal B cell differentiation (4)(5)(6). There is now evidence from a variety of sources to suggest that Btk is one of the many tyrosine kinases involved in B cell receptor (BCK) signal transduction.…”

Impaired Ca2+ mobilization by X-linked agammaglobulinaemia (XLA) B cells in response to ligation of the B cell receptor (BCR)

“…In contrast, two pre-B cell lines Nalm 6 and 697 expressing surface mwLC gave detectable calcium responses. These lines have been shown by us to express mwLC and cytoplasmic Btk, but no light chain [8]. Nalm 6 has been reported previously to give a calcium response on cross-linking of mwLC [19], but in another study no calcium response was obtained with 697 [42].…”

“…The expression of selected B lineage markers and immunoglobulin gene rearrangements of most of the lines have been described previously [8]. BLCL-276 is an Epstein-Barr virus (EBV)-transformed B cell line expressing surface IgM (m 2 l 2 ) that was derived from a patient with XLA [21] (kindly provided by Dr M. de Weers, Leiden, The Netherlands).…”

“…It is expressed in B lineage cells from the pro-B to the B cell stages, but not in plasma cells, and is necessary for normal B cell development. It is also expressed in myeloid cells [6][7][8].…”

Inhibitors of B-Cell Receptor Signaling for the Treatment of Chronic Lymphocytic Leukemia

“…Btk is primarily expressed in hematopoietic cells, and is important in B-lymphocyte development and differentiation [15,16]. It is required for B-cell maturation, and is overexpressed in a number of B-cell malignancies including CLL.…”