The Pro-B- and Pre-B-Cell Checkpoints
المؤلف:
Hoffman, R., Benz, E. J., Silberstein, L. E., Heslop, H., Weitz, J., & Salama, M. E.
المصدر:
Hematology : Basic Principles and Practice
الجزء والصفحة:
8th E , P234-235
2025-12-20
64
B-cell progenitors progress through two critical checkpoints as they mature into B lymphocytes. The first occurs at the pro-B- to pre-B cell transition. The second transition occurs as pre-B cells mature into B cells.
The Pro-B- to Pre-B-Cell Transition
cHaPter 22 B-cell DeveloPment 235 The key event during the pro-B- to pre-B-cell transition is the rearrangement and expression of Ig heavy chain genes. Ig heavy chain gene rearrangements are productive in only around one-third of pro-B cells, so the majority of pro-B cells do not navigate this transition. Those cells with nonproductive Ig gene rearrangements undergo apoptosis and are eliminated from the BM by resident macrophages and stromal cells.
However, if Ig heavy chain recombination is productive and µ heavy chain protein is expressed, it appears on the surface of the pre-B cells in association with two additional molecules referred to as the surrogate light chains. The surrogate light chain proteins, Vpre-B and λ5, are encoded by genes located on chromosome 16 in mice and on chromosome 22 in humans and are non-covalently linked to one another. λ5 in turn is covalently linked to the CH1 domain of the µ heavy chain via a carboxyl-terminal (C-terminal) cysteine. One role of the surrogate light chains is to select heavy chains that will ultimately be capable of pairing with conventional light chains. If this does not occur, then these cells will likely be deleted.
The μ heavy chain-surrogate light chain complex is addition ally associated with two transmembrane proteins, Igα (CD79a) and Igβ (CD79b), and the entire complex is referred to as the pre-B-cell receptor (pre-BCR). The intracellular tails of both Igα and Igβ contain immunoreceptor tyrosine activation motifs (ITAMs) critical to the signaling function of the pre-BCR (Fig. 1, upper panel). Lipid rafts that contain mediators of intracellular signaling such as Lyn are constitutively associated with the pre-BCR in human pre-B cells. Cross-linking of the pre-BCR leads to an increase in Lyn kinase activity, phosphorylation of the Igβ chain, and recruitment and activation within the pre-BCR complex of additional signaling intermediates, including spleen tyrosine kinase (Syk), B-cell linker protein (BLNK), phosphoinositide-3 kinase (PI3K), Bruton’s tyrosine kinase (Btk), VAV, and phospholipase C-γ (PLCγ2). These events lead to calcium flux and activation of signaling cascades within the pre-B cell (see Fig. 1, lower panel).
Fig1. THE PRE-B-CELL RECEPTOR AND B-CELL RECEPTOR AND ASSOCIATED SIGNALING INTERMEDIATES. Top, μ heavy chain protein in pre-B cells is associated with the surrogate light chains v-pre-B and λ5 (left) to form the pre-B-cell receptor (pre-BCR). In newly produced B lymphocytes, μ heavy chain is associated with conventional light chains (right) to form the BCR. Associated with heavy chain in both pre-B and B cells are two additional transmembrane proteins, Igα and Igβ, that contain immunoreceptor tyrosine activation motifs (ITAMs) critical to the signaling function. Bottom, Expression of the pre-BCR (or possibly its binding to a stromal ligand) or binding of antigen to the mature BCR, respectively, initiates the assembly of a lipid raft, BCR-associated “signalosome” composed of multiple signaling molecules, ultimately leading to transcriptional events that promote cell proliferation, survival, and differentiation. ERK, Extracellular-signal-regulated kinase; Ig, immunoglobulin; JNK, Janus kinase; NF κB, nuclear factor kappa-light-chain enhancer of activated B cells; NFAT, nuclear factor of activated T cells; SHIP, src homology 2-containing inositol phosphatase; SHP, src homology 2-containing protein tyrosine phosphatase.
These signaling pathways are crucial in developing pre-B cells. One of the best examples of this requirement is the prototypical humoral immunodeficiency, X-linked agammaglobulinemia (XLA). XLA results from mutations within the gene segments that encode the nonreceptor tyrosine kinase, Btk. In males who express a defective Btk protein, pre-B-cell clonal expansion is markedly depressed, and there is an almost complete loss of immature B cells in the BM and in secondary lymphoid organs. As a result, affected males develop recur rent bacterial infections early in life because of a profound decrease in circulating Ig. A nearly identical clinical phenotype has been observed in persons with mutations in additional components of the pre-BCR signaling complex, including the μ–heavy chain, λ5, Igα, Igβ, the key B-cell adaptor protein BLNK, and the lipid kinase PLCγ2.
How signaling through the pre-BCR is initiated is unclear. It has been suggested that this occurs by binding of the extracellular portion of the pre-BCR to an environmental ligand. The identification of such ligands has been difficult, although galectin-1 has been suggested to function in this capacity. It has also been proposed that constitutive signaling occurs after pre-BCR surface expression. Structural studies which suggest that the pre-BCR constitutively assembles as an oligomer provide a potential mechanism for this behavior.
The Pre-B to B-Cell Transition
Pre-BCR expressing cells undergo several rounds of proliferation in response to the stromal cell derived cytokine IL-7, and this expands the size of the clone that expresses a particular μ heavy chain. However, IL-7R signaling inhibits Rag gene expression. Thus, in order to reactivate the recombinatorial machinery and initiate light chain gene recombination, downregulation of the IL-7 response must occur. There is general agreement that this is accomplished by movement of pre-BCR expressing cells from IL-7 high to IL-7 areas of the bone marrow.
Different mechanisms for how this is achieved have been proposed, but one current model is that pre-B cells are more motile compared to pro-B cells and thus spend a reduced amount of time in contact with IL-7 producing stromal cells (Fig. 2). This occurs because, in contrast to pro-B cells that tightly adhere to stromal cells via expression of adhesion molecules that include focal adhesion kinase (FAK) and very late antigen 4 (VLA-4), these receptors are expressed at lower levels of pre-B cells which thus allows them to move away from the IL-7 secreting cells. As a result, IL-7R signaling is attenuated and pre-BCR signaling predominates. This represses proliferation, likely through activation of the RAS–extracellular-signal-regulated kinase (ERK) pathway as well as via limiting PI3K activity through BLNK mediated signals leading to expression of transcription factors, including E2A, IRF4, and PAX5, that induce cell cycle exit, RAG expression and light chain gene recombination (see Fig. 2). As with heavy chain recombination, functional light chain gene rearrangements do not occur in all pre-B cells. However, when they are successful, the heavy and light chains assemble and are expressed on the surface of a newly produced B lymphocyte as the mature B-cell receptor (BCR).
Fig2. THE HEMATOPOIETIC MICROENVIRONMENT. (A) Cross-section of bone showing elements of the medullary circulation and the marrow sinusoids. Stromal cells that support developing blood cells are located in the intersinusoidal spaces. (B) Pro-B cells are tightly adherent to IL-7 producing stromal cells via their expression of adhesion molecules such as focal adhesion kinase and vascular cell adhesion molecule-1. Following the successful expression of Ig μ heavy chain and assembly of the pre-B-cell receptor (pre-BCR), pre-B cells must ultimately attenuate IL-7 receptor signaling in order to recombine Ig light chain genes and mature into BCR expressing B lymphocytes. A current model proposes that they are less adherent to stromal cells compared to pro-B cells and as a result migrate away from IL-7 rich areas. This results in attenuated IL-7 receptor signaling while pre-BCR signaling in these cells induces cell cycle exit, RAG expression, and light chain gene recombination. (A, From Dorshkind K. Regulation of hematopoiesis by bone marrow stromal cells and their products. Annu Rev Immunol. 1990;8:111. Reprinted by permission from the Annual Review of Immunology. B, Based on concepts reviewed and depicted in reference 14.)
Events that disrupt the pre-B-cell transition such as activating lesions in IL7 receptor subunits or downstream mediators significantly enhance the risk for development of pre-B acute lymphoblastic leukemia. An improved understanding of these changes will likely provide new therapeutic targets to inhibit leukemia cell growth.
The B-Cell Receptor
Once light chain expression occurs, a complete Ig molecule is expressed on the surface of newly produced B cells. These cells express IgM, but others may co-express cell surface IgD. This occurs because the primary heavy chain transcript includes the rearranged VDJ heavy chain complex, the μ and δ C regions, and the intron separating these exons. If RNA processing results in association of the Cμ region with the VDJ complex, the B cell expresses IgM. Alternatively, if the Cμ exon is deleted along with the heavy chain intron, the VDJ complex and the Cδ exon become contiguous and the B cell expresses IgD.
The complex of cell surface Ig, along with Igα and Igβ, is referred to as the BCR (see Fig. 1). The cytoplasmic tail of the Ig heavy chain is relatively short, and, as noted above, both Igα and Igβ contain an ITAM in their intracellular tails that is required for signal transduction following antigen binding to the BCR. Antigen engagement initiates assembly of a lipid raft, BCR-associated “signalosome,” composed of multiple signaling molecules that include tyrosine kinases, serine/threonine kinases, lipid kinases, lipases, phosphatases, and linkers and adaptors. This signalosome mediates a cascade of intracellular signals that includes the initiation of calcium influx. Additional calcium-dependent and -independent downstream signals that include the mitogen-activated protein (MAP) kinase cascade (c-jun N-terminal kinase [JNK], p38, ERK) and activation of key transcription factors that include JUN, c-fos, nuclear factor of activated T cells (NFAT), and nuclear factor kappa-light-chain enhancer of activated B cells (NF-κB) in turn mediate transcriptional events leading to cell proliferation, survival, and differentiation. The level and duration of receptor activation and hence transcriptional output are further modified by a series of cell surface co-receptors or “response modifiers” that bind to complement or to receptors on the surface of stromal cells, activated T cells, or other populations present in secondary lymphoid organs.
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