Signal transduction through mu kappa B-cell receptors expressed on pre-B cells is different from that through B-cell receptors on mature B cells

Maintenance of Surrogate Light Chain Expression Induces Developmental Delay in Early B Cell Compartment

The production of a mature B cell requires passage through a number of developmental checkpoints. The pre-BCR plays a critical role in passage through the pro-B cell/pre-B cell checkpoint, and thus plays a central role in regulating the differentiation of a B cell. Due to the significance of this receptor, it is imperative that pre-BCR expression and function are precisely regulated. In this study, we have investigated a system in which the regulation of the pre-BCR is altered. We have found that continued expression of components of the pre-BCR (lambda5) resulted in a delay in the kinetics of B cell maturation. Pro-B cells from normal mouse bone marrow retrovirally infected with lambda5 exhibited a delay in differentiation. As compared with wild-type cells at the same time point, there is a reduction in the presence of cell surface markers that indicate developmental progression, and there is a 6- to 16-fold decrease in the production of Ig-positive cells in B cell maturation assays. The capacity to alter B cell progression by modifying and extending pre-BCR expression argues that the receptor and its associated signals play a unique role in directing developmental outcomes.

Deficiency of BLNK hampers PLC-gamma2 phosphorylation and Ca2+ influx induced by the pre-B-cell receptor in human pre-B cells

B-cell linker protein (BLNK) is a component of the B-cell receptor (BCR) as well as of the pre-BCR signalling pathway, and BLNK(-/-) mice have a block in B lymphopoiesis at the pro-B/pre-B cell stage. A recent report described the complete loss or drastic reduction of BLNK expression in approximately 50% of human childhood pre-B acute lymphoblastic leukaemias (ALL), therefore we investigated BLNK expression in human pre-B ALL cell lines. One of the four cell lines tested, HPB-NULL cells, was found to lack BLNK expression, and we used these human pre-B ALL cell lines that express and do not express BLNK to investigate the intracellular signalling events following pre-BCR cross-linking. When pre-BCR was cross-linked with anti-micro heavy-chain antibodies, significant phosphorylation of intracellular molecules, including Syk, Shc, ERK MAP kinase, and AKT, and an activation of Ras were observed without regard to deficiency of BLNK expression, suggesting that BLNK is not required for pre-BCR-mediated activation of MAP kinase and phosphatidyl-inositol 3 (PI3) kinase signalling. By contrast, phospholipase C-gamma2 (PLC-gamma2) phosphorylation and an increase in intracellular Ca(2+) level mediated by pre-BCR cross-linking were observed only in the BLNK-expressing cells, indicating that BLNK is essential for PLC-gamma2-induced Ca(2+) influx. Human pre-B cell lines expressing and not expressing BLNK should provide an in vitro model for investigation of the role of BLNK in the pre-BCR-mediated signalling mechanism.

Engagement of the human pre-B cell receptor generates a lipid raft-dependent calcium signaling complex

Pre-B cell receptor (pre-BCR) expression is critical for B lineage development. The signaling events initiated by the pre-BCR, however, remain poorly defined. We demonstrate that lipid rafts are the major functional compartment for human pre-B cell activation. A fraction of pre-BCR was constitutively raft associated, and receptor engagement enhanced this association. These events promoted Lyn activation and Igbeta phosphorylation and led to the generation of a raft-associated signaling module composed of tyrosine phosphorylated Lyn, Syk, BLNK, PI3K, Btk, VAV, and PLCgamma2. Formation of this module was essential for pre-BCR calcium signaling. Together, these observations directly link the previously identified genetic requirement for the components of this module in B lineage development with theirfunctional role(s) in human preBCR signaling.