Mimicry of pre-B cell receptor signaling by activation of the tyrosine kinase Blk

Involvement of SLP-65 and Btk in tumor suppression and malignant transformation of pre-B cells

Signals from the precursor-B cell receptor (pre-BCR) are essential for selection and clonal expansion of pre-B cells that have performed productive immunoglobulin heavy chain V(D)J recombination. In the mouse, the downstream signaling molecules SLP-65 and Btk cooperate to limit proliferation and induce differentiation of pre-B cells, thereby acting as tumor suppressors to prevent pre-B cell leukemia. In contrast, recent observations in human BCR-ABL1(+) pre-B lymphoblastic leukemia cells demonstrate that Btk is constitutively phosphorylated and activated by the BCR-ABL1 fusion protein. As a result, activated Btk transmits survival signals that are essential for the transforming activity of oncogenic Abl tyrosine kinase.

Accessibility control of V(D)J recombination

Mammals contend with a universe of evolving pathogens by generating an enormous diversity of antigen receptors during lymphocyte development. Precursor B and T cells assemble functional immunoglobulin (Ig) and T cell receptor (TCR) genes via recombination of numerous variable (V), diversity (D), and joining (J) gene segments. Although this combinatorial process generates significant diversity, genetic reorganization is inherently dangerous. Thus, V(D)J recombination must be tightly regulated to ensure proper lymphocyte development and avoid chromosomal translocations that cause lymphoid tumors. Each genomic rearrangement is mediated by a common V(D)J recombinase that recognizes sequences flanking all antigen receptor gene segments. The specificity of V(D)J recombination is due, in large part, to changes in the accessibility of chromatin at target gene segments, which either permits or restricts access to recombinase. The chromatin configuration of antigen receptor loci is governed by the concerted action of enhancers and promoters, which function as accessibility control elements (ACEs). In general, ACEs act as conduits for transcription factors, which in turn recruit enzymes that covalently modify or remodel nucleosomes. These ACE-mediated alterations are critical for activation of gene segment transcription and for opening chromatin associated with recombinase target sequences. In this chapter, we describe advances in understanding the mechanisms that control V(D)J recombination at the level of chromatin accessibility. The discussion will focus on cis-acting regulation by ACEs, the nuclear factors that control ACE function, and the epigenetic modifications that establish recombinase accessibility.

IL-7: a key regulator of B lymphopoiesis

Interleukin-7 plays important roles in the B cell developmental pathway including events leading to commitment, survival, proliferation, and maturation. Because of its central role in adult murine B lymphopoiesis, IL-7 is frequently used to generate B cell progenitors in vitro. We have shown that differentiation of IL-7-responsive cells in these cultures is influenced by CD45, pre-B cell receptor, and other downstream signaling molecules. A common, but often overlooked aspect of IL-7 containing cultures is the routine maturation of cells to the sIgM(+) stage. The production of B cells in IL-7 containing cultures is balanced by cell death, since such cells fail to survive for long without additional stimulation.

B cell signaling and tumorigenesis

The proliferation and differentiation of lymphocytes are regulated by receptors localized on the cell surface. Engagement of these receptors induces the activation of intracellular signaling proteins that transmit the receptor signals to distinct targets and control the cellular responses. The first signaling proteins to be discovered in higher organisms were the products of oncogenes. For example, the kinases Src and Abelson (Abl) were originally identified as oncogenes and were later characterized as important proteins for signal transduction in various cell types, including lymphocytes. Now, as many cellular signaling molecules have been discovered and ordered into certain pathways, we can better understand why particular signaling proteins are associated with tumorigenesis. In this review, we discuss recent progress in unraveling the molecular mechanisms of signaling pathways that control the proliferation and differentiation of early B cells. We point out the concepts of auto-inhibition and subcellular localization as crucial aspects in the regulation of B cell signaling.

Igα/Igβ Complexes Generate Signals for B Cell Development Independent of Selective Plasma Membrane Compartmentalization

Ligand-induced BCR association with detergent-resistant plasma membrane compartments (lipid rafts) has been argued to be essential for initiating and/or sustaining Igalpha/Igbeta-dependent BCR signaling. Because a fraction of the BCR and an even larger fraction of the preBCR associates with lipid rafts in the apparent absence of ligand stimulation, it has been proposed that raft-associated receptor complexes mediate the ligand-independent basal signaling events observed in resting B lineage cells. However, there is no direct evidence that localization of Igalpha/Igbeta-containing complexes to detergent-resistant membrane compartments is absolutely required for the signaling events that drive B cell development. To address these issues we have designed surrogate preBCR/Igalpha/Igbeta complexes that are incapable of ligand-induced aggregation and that are preferentially targeted to either raft or nonraft compartments. An analysis of their ability to promote the preBCR-dependent proB-->preB cell transition of murine B cell progenitors revealed that expression of these surrogate receptor complexes at levels that approximate that of the conventional preBCR can drive B cell development in a manner independent of both aggregation and lipid raft localization.

NF-kappaB and the immunoglobulin kappa gene enhancer

Enhancers regulate lineage choice and the developmental timing of antigen receptor gene rearrangements. The transcription factor NF-kappaB has been implicated as a key component of the recombination and transcription activation potential of the immunoglobulin kappa chain gene intronic enhancer. Here, I discuss the implications of the new observation that an NF-kappaB binding site-mutated enhancer in the correct biological context does not appear to affect kappa gene expression.

Ligand-independent tonic signaling in B-cell receptor function

The random and inherently imprecise process of V(D)J recombination is the foundation for generation of the B-cell receptor (BCR). Signals must be generated to trigger selective processes that retain cells expressing a functional BCR, and these signals must be antigen-independent to insure an unbiased and diverse pool of newly formed B cells. Moreover, BCR expression, and presumably signaling, is essential for the continued survival of the B cell. Although BCR signaling is generally thought to depend upon ligand-induced aggregation, recent studies argue that some aspects of BCR signaling occur independently of antigen, and, furthermore, these non-induced or 'tonic' signals are linked to specific cellular processes operating at multiple stages of B-cell development. The potential co-existence of tonic and induced signaling suggests a unique aspect of BCR complexes, or at least an aspect of receptors that has previously been under-appreciated.