Bruton’s Tyrosine Kinase and SLP-65 Regulate Pre-B Cell Differentiation and the Induction of Ig Light Chain Gene Rearrangement

Abstract

Bruton’s tyrosine kinase (Btk) and the adapter protein SLP-65 (Src homology 2 domain-containing leukocyte-specific phosphoprotein of 65 kDa) transmit precursor BCR (pre-BCR) signals that are essential for efficient developmental progression of large cycling into small resting pre-B cells. We show that Btk- and SLP-65-deficient pre-B cells have a specific defect in Ig λ L chain germline transcription. In Btk/SLP-65 double-deficient pre-B cells, both κ and λ germline transcripts are severely reduced. Although these observations point to an important role for Btk and SLP-65 in the initiation of L chain gene rearrangement, the possibility remained that these signaling molecules are only required for termination of pre-B cell proliferation or for pre-B cell survival, whereby differentiation and L chain rearrangement is subsequently initiated in a Btk/SLP-65-independent fashion. Because transgenic expression of the antiapoptotic protein Bcl-2 did not rescue the developmental arrest of Btk/SLP-65 double-deficient pre-B cells, we conclude that defective L chain opening in Btk/SLP-65-deficient small resting pre-B cells is not due to their reduced survival. Next, we analyzed transgenic mice expressing the constitutively active Btk mutant E41K. The expression of E41K-Btk in Ig H chain-negative pro-B cells induced 1) surface marker changes that signify cellular differentiation, including down-regulation of surrogate L chain and up-regulation of CD2, CD25, and MHC class II; and 2) premature rearrangement and expression of κ and λ light chains. These findings demonstrate that Btk and SLP-65 transmit signals that induce cellular maturation and Ig L chain rearrangement independently of their role in termination of pre-B cell expansion.

Discovery of a Potent, Covalent BTK Inhibitor for B-Cell Lymphoma

“…1,2 BTK signals downstream of the B-cell receptor (BCR) where it regulates survival, activation, proliferation, differentiation, and maturation of B-cells. 3,4 BTK is predominately expressed in hematopoietic lineage cells with the exception of T, NK, and plasma cells. BTK was originally linked to diseases following the discovery of mutations in BTK that result in X-linked agammaglobulinemia (XLA), an immune disorder where patients fail to produce mature B-cells.…”

Biology and novel treatment options for XLA, the most common monogenetic immunodeficiency in man

“…Importantly, germline transcription has been implicated in regulation of accessibility of Ig loci to the V(D)J recombinase, because it precedes or coincides with V(D)J recombination (see [93] for review). In Btk/ SLP-65 double-deficient pre-B cells, both k and l L chain germline transcripts are severely reduced [92]. Conversely, expression of the constitutive active Btk mutant E41K in early Ig H chain-negative pro-B cells induced both cell surface marker changes that signify cellular differentiation and premature rearrangement and expression of Ig light chains.…”

“…Intravenous preparations became readily available in the late 1970s and mid 1980s. Nowadays, both intravenous or subcutaneous infusion allows Adapted from [92].…”

“…Therefore, Btk is critical for an efficient transit through the small pre-B cell compartment, thereby regulating cell surface phenotype changes during the developmental progression of cytoplasmic Ig µ H chain expressing pre-B cells into immature IgM + B cells. Moreover, Btk-deficient pre-B cells have a specific defect in Ig l L chain germline transcription ( Figure 2B) [92], that is transcription of unrearranged Ig gene segments. Importantly, germline transcription has been implicated in regulation of accessibility of Ig loci to the V(D)J recombinase, because it precedes or coincides with V(D)J recombination (see [93] for review).…”

Malignant transformation of Slp65-deficient pre-B cells involves disruption of the Arf-Mdm2-p53 tumor suppressor pathway

“…30 It is therefore conceivable that Bcl-2 overexpression, which is commonly associated with enhanced malignancy of hematologic tumors, may increase tumor incidence in Slp65 Ϫ/Ϫ mice. To investigate this, we compared the incidence of pre-B cell leukemia in a panel of Slp65 Ϫ/Ϫ Btk Ϫ/Ϫ mice and E-2-22 Bcl-2 Tg Slp65 Ϫ/Ϫ Btk Ϫ/Ϫ mice.…”