Requirement for B cell linker protein (BLNK) in B cell development

Cutting edge: B cell linker protein is dispensable for the allelic exclusion of immunoglobulin heavy chain locus but required for the persistence of CD5+ B cells

The pre-B cell receptor (pre-BCR) and the BCR are required for B lymphopoiesis and for the allelic exclusion of Ig genes. Mice lacking B cell linker (BLNK) protein that is a component of the BCR signaling pathway have impaired B cell development. In this report, we show that allelic exclusion is intact in BLNK(-/-) mice harboring a V(H)12 transgene. This differs from mice lacking the tyrosine kinase Syk that is upstream of BLNK in BCR signaling and contrasts with mice lacking SLP-76 that is the equivalent adaptor molecule in TCR-signal transduction. We also show that, whereas most wild-type V(H)12-expressing B cells are CD5(+), the majority of the splenic V(H)12-expressing BLNK(-/-) B cells are CD5(-). A small population of V(H)12-expressing, BLNK(-/-) CD5(+) B cells is detectable in the peritoneal cavity of younger but not older mice. This suggests that BLNK deficiency affects not only the generation but also the persistence of B-1 cells.

Involvement of LAT, Gads, and Grb2 in compartmentation of SLP-76 to the plasma membrane

B cell linker protein (BLNK) and Src homology 2 domain-containing leukocyte protein of 76 kD (SLP-76) are adaptor proteins required for B cell receptor (BCR) and T cell receptor function, respectively. Here, we show that expression of SLP-76 cannot reconstitute BCR function in Zap-70(+)BLNK(-) B cells. This could be attributable to inability of SLP-76 to be recruited into glycolipid-enriched microdomains (GEMs) after antigen receptor cross-linking. Supporting this idea, the BCR function was restored when a membrane-associated SLP-76 chimera was enforcedly localized to GEMs. Moreover, we demonstrate that addition of both linker for activation of T cells (LAT) and Grb2-related adaptor downstream of Shc (Gads) to SLP-76 allow SLP-76 to be recruited into GEMs, whereby the BCR function is reconstituted. The Gads function was able to be replaced by overexpression of Grb2. In contrast to SLP-76, BLNK did not require Grb2 families for its recruitment to GEMs. Hence, these data suggest a functional overlap between BLNK and SLP-76, while emphasizing the difference in requirement for additional adaptor molecules in their targeting to GEMs.

Cutting edge: essential role of phospholipase C-gamma 2 in B cell development and function

Cross-linking of the B cell Ag receptor (BCR) induces the tyrosine phosphorylation of multiple cellular substrates, including phospholipase C (PLC)-gamma 2, which is involved in the activation of the phosphatidylinositol pathway. To assess the importance of PLC-gamma 2 in murine lymphopoiesis, the PLC-gamma 2 gene was inducibly ablated by using IFN-regulated Cre recombinase. Mice with a neonatally induced loss of PLC-gamma 2 function displayed reduced numbers of mature conventional B cells and peritoneal B1 cells and defective responses in vitro to BCR stimulation and in vivo to immunization with thymus-independent type II Ags. In contrast, T cell development and TCR-mediated proliferation were normal. Taken together, PLC-gamma 2 is a critical component of BCR signaling pathways and is required to promote B cell development.

Signal transduction elements of the B cell antigen receptor and their role in immunodeficiencies

The primary function of B lymphocytes is to contribute to the elimination of foreign antigens by producing large amounts of soluble antibodies. The activation of B cells through their antigen receptor triggers a dynamic network of intracellular signaling proteins. The recent identification of the cytoplasmic adaptor protein SLP-65 (also called BLNK or BASH) provided insight in how the antigen receptor-regulated protein tyrosine kinases couple to downstream signaling cascades, including the mobilization of Ca2+ ions, activation of mitogen-activated kinases and reorganization of the cytoskeleton architecture. While these events have been mostly studied in mature B cells, it is now clear that the components of the antigen receptor and its downstream effector elements play also a central role during early and late B cell development, and in the apoptotic elimination of B cells with reactivity to self-antigens. Thus, genetic defects affecting the expression of antigen receptor subunits or its intracellular signaling proteins can interfere with B cell development and activation, and can cause severe antibody deficiencies in mouse and man. In this article I summarize our current picture of the B cell antigen receptor, how the extracellular signal is transported into the cell interior, and how dysregulation of these processes contribute to immune defects.

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.

Ras mediates effector pathways responsible for pre-B cell survival, which is essential for the developmental progression to the late pre-B cell stage

Ras is essential for the transition from early B cell precursors to the pro-B stage, and is considered to be involved in the signal cascade mediated by pre-B cell antigen receptors. To examine the role of p21(ras) in the late stage of B cell differentiation, we established transgenic mice (TG) expressing a dominant-inhibitory mutant of Ha-ras (Asn-17 Ha-ras) in B lineage cells at high levels after the early B cell precursor stage. Expression of p21(Asn-17) (Ha-ras) was associated with a prominent reduction in the number of late pre-B cells, but had little effect on proliferation of early pre-B cells. Inhibition of p21(ras) activity markedly reduced the life span of pre-B cells, due, at least in part, to downregulation of the expression of an antiapoptotic protein, Bcl-xL. Thus, the apparent role for p21(ras) activity in pre-B cell survival may explain the decreased numbers of late pre-B cells in Asn-17 Ha-ras TG. Consistent with this possibility, overexpression of Bcl-2 in Asn-17 Ha-ras TG reversed the reduction in the number of late pre-B cells undergoing immunoglobulin light chain gene (IgL) rearrangement and progressing to immature B cells. These results suggest that p21(ras) mediates effector pathways responsible for pre-B cell survival, which is essential for progression to the late pre-B and immature B stages.

Phospholipase Cgamma2 is essential in the functions of B cell and several Fc receptors

Many receptors activate phospholipase Cgamma1 or -gamma2. To assess the role of PLCgamma2, we derived enzyme-deficient mice. The mice are viable but have decreased mature B cells, a block in pro-B cell differentiation, and B1 B cell deficiency. IgM receptor-induced Ca2+ flux and proliferation to B cell mitogens are absent. IgM, IgG2a, and IgG3 levels are reduced, and T cell-independent antibody production is absent. The similarity to Btk- or Blnk-deficient mice demonstrates that PLCgamma2 is downstream in Btk/Blnk signaling. FcRgamma signaling is also defective, resulting in a loss of collagen-induced platelet aggregation, mast cell FcepsilonR function, and NK cell FcgammaRIII and 2B4 function. The results define a signal transduction pathway broadly utilized by immunoglobulin superfamily receptors.

Fates of human B-cell precursors

Development of mammalian B-lineage cells is characterized by progression through a series of checkpoints defined primarily by rearrangement and expression of immunoglobulin genes. Progression through these checkpoints is also influenced by stromal cells in the microenvironment of the primary tissues wherein B-cell development occurs, ie, fetal liver and bone marrow and adult bone marrow. This review focuses on the developmental biology of human bone marrow B-lineage cells, including perturbations that contribute to the origin and evolution of B-lineage acute lymphoblastic leukemia and primary immunodeficiency diseases characterized by agammaglobulinemia. Recently described in vitro and in vivo models that support development and expansion of human B-lineage cells through multiple checkpoints provide new tools for identifying the bone marrow stromal cell–derived molecules necessary for survival and proliferation. Mutations in genes encoding subunits of the pre-B cell receptor and molecules involved in pre-B cell receptor signaling culminate in X-linked and non–X-linked agammaglobulinemia. A cardinal feature of these immunodeficiencies is an apparent apoptotic sensitivity of B-lineage cells at the pro-B to pre-B transition. On the other end of the spectrum is the apoptotic resistance that accompanies the development of B-lineage acute lymphoblastic leukemia, potentially a reflection of genetic abnormalities that subvert normal apoptotic programs. The triad of laboratory models that mimic the bone marrow microenvironment, immunodeficiency diseases with specific defects in B-cell development, and B-lineage acute lymphoblastic leukemia can now be integrated to deepen our understanding of human B-cell development.

Fates of human B-cell precursors

Development of mammalian B-lineage cells is characterized by progression through a series of checkpoints defined primarily by rearrangement and expression of immunoglobulin genes. Progression through these checkpoints is also influenced by stromal cells in the microenvironment of the primary tissues wherein B-cell development occurs, ie, fetal liver and bone marrow and adult bone marrow. This review focuses on the developmental biology of human bone marrow B-lineage cells, including perturbations that contribute to the origin and evolution of B-lineage acute lymphoblastic leukemia and primary immunodeficiency diseases characterized by agammaglobulinemia. Recently described in vitro and in vivo models that support development and expansion of human B-lineage cells through multiple checkpoints provide new tools for identifying the bone marrow stromal cell–derived molecules necessary for survival and proliferation. Mutations in genes encoding subunits of the pre-B cell receptor and molecules involved in pre-B cell receptor signaling culminate in X-linked and non–X-linked agammaglobulinemia. A cardinal feature of these immunodeficiencies is an apparent apoptotic sensitivity of B-lineage cells at the pro-B to pre-B transition. On the other end of the spectrum is the apoptotic resistance that accompanies the development of B-lineage acute lymphoblastic leukemia, potentially a reflection of genetic abnormalities that subvert normal apoptotic programs. The triad of laboratory models that mimic the bone marrow microenvironment, immunodeficiency diseases with specific defects in B-cell development, and B-lineage acute lymphoblastic leukemia can now be integrated to deepen our understanding of human B-cell development.

Functional dissection of BCR signaling pathways

Signal transduction by the BCR is critical for progression through developmental checkpoints as well as for immune responses. Recent results obtained in mice deficient either in an adaptor molecule, BLNK (alternatively named SLP-65 or BASH), or in phosphatidylinositol 3-kinase have revealed similar - though not identical - phenotypes to those of Btk(-/-) mice, suggesting a functional link between BLNK, Btk and phosphatidylinositol 3-kinase.

Adapter proteins in lymphocyte antigen-receptor signaling

Adapter molecules contain discrete modular domains that direct specific intermolecular interactions to orchestrate assembly of signaling complexes. A number of adapter proteins play critical roles in both positive and negative regulation of antigen-receptor signaling, influencing lymphocyte development and function.

Tec family kinases in lymphocyte signaling and function

The Tec kinases are required for full Ca(2+) mobilization in lymphocytes. Recent data suggest that this process occurs via a multiprotein complex that includes LAT and SLP-76 in T cells and BLNK/SLP-65 in B cells. Mutational analyses have revealed critical roles for Tec kinases in lymphocyte development and function.

In vitro and in vivo macrophage function can occur independently of SLP-76

Expression of SH2 domain-containing leukocyte-specific phosphoprotein of 76 kDa (SLP-76), a hematopoietic cell-specific adapter protein, is required to couple Syk family tyrosine kinase activation to downstream mediators such as phospholipase C (PLC)-gamma following TCR, platelet collagen receptor and mast cell Fc epsilon R stimulation. In addition to T cells, mast cells and platelets, SLP-76 is expressed in monocytes and macrophages. To determine the role of SLP-76 in Fc gamma R-stimulated signaling pathways in macrophages, we examined cultured bone marrow-derived macrophages (BMM) from SLP-76(-/-) and wild-type mice. In this study, we show that Fc gamma R cross-linking rapidly induces tyrosine phosphorylation of SLP-76 in wild-type BMM. Surprisingly, however, BMM from SLP-76(-/-) mice activate ERK2 and phosphorylate PLC-gamma 2 following Fc gamma R ligation. Furthermore, SLP-76(-/-) BMM display normal Fc gamma R-dependent phagocytic function and reactive oxygen intermediate production. SLP-76(-/-) and SLP-76(+/+) BMM secrete comparable levels of IL-12 in response to lipopolysaccharide and IFN-gamma. To examine macrophage function in vivo, SLP-76(-/-) mice were challenged i.v. with Listeria monocytogenes. SLP-76(-/-) mice survive and efficiently contain the acute phase of infection similar to wild-type mice but exhibit a stable chronic infection attributed to the lack of mature T cells. These data show that, although SLP-76 is required to couple Syk family PTK activity to downstream mediators and effector functions in Fc gamma R-induced pathways in some cell types, activation of Fc gamma R-dependent pathways occurs independently of SLP-76 in BM

Regulation of B cell function by linker proteins

Studies over the past few years have demonstrated the importance of linker or adaptor proteins in the signaling pathways activated by the B cell antigen-receptor. These proteins direct the appropriate subcellular localization of enzymatic complexes, amplify signaling pathways and integrate the functions of distinct signaling complexes. Many of the recently identified linker proteins function through these distinct mechanisms to upregulate the BCR signaling pathway. In addition, linker proteins facilitate the influences of co-receptors that augment or dampen the BCR signaling pathway.

The B cell-restricted adaptor BASH is required for normal development and antigen receptor-mediated activation of B cells

B cell antigen receptor signals development, activation, proliferation, or apoptosis of B cells depending on their condition, and its proper signaling is critical for activation and homeostasis of the immune system. The B cell-restricted adaptor protein BASH (also termed BLNK/SLP-65) is rapidly phosphorylated by the tyrosine kinase Syk after BCR ligation and binds to various signaling proteins. BASH structurally resembles SLP-76, which is essential for T cell development and T cell receptor signaling. To evaluate the role for BASH in B cell development and function in vivo, we disrupted BASH alleles in embryonic stem cells by means of homologous recombination and used these cells to complement lymphocyte-incompetent blastocysts from RAG2-deficient mice. In the resultant chimeric mice, T cell development was apparently normal, but B cell development was impaired, and a normally rare population of large preB cells expressing preB cell receptor dominated in the bone marrow in place of small preB cells, although they were mostly noncycling. In addition, the mature B cell populations in the periphery and the bone marrow profoundly decreased in size, as did B-1 cells in the peritoneal cavity, and serum Ig was severely reduced. The BASH-deficient B cells scarcely proliferated or up-regulated B7-2 in response to BCR ligation and poorly proliferated upon CD40 ligation or lipopolysaccharide stimulation. This phenotype indicates that BASH is critical for preB cell receptor signaling inducing proliferation of large preB cells and the following differentiation, for peripheral B cell maturation, and for BCR signaling inducing activation/proliferation of B cells.

B cell development and activation defects resulting in xid-like immunodeficiency in BLNK/SLP-65-deficient mice

Engagement of the B cell receptor (BCR) leads to the activation of tyrosine kinases and other signaling molecules that ultimately determine the type and magnitude of the B lymphocyte's cellular response. The adaptor protein BLNK/SLP-65 plays a pivotal role in BCR signal transduction by coupling Syk activation to downstream elements such as Grb2, phospholipase C-gamma, Vav and Nck. We have generated BLNK(-/-) mice to determine the physiological role of this protein in B cell development and activation. BLNK(-/-) mice exhibit an incomplete block in B cell development with a severe inhibition of pro-B to pre-B cell differentiation. BLNK(-/-) sIgM(+) cells can develop, seed the peripheral lymphoid tissues and accumulate in numbers overtime. However, these mutant B cells failed to mature and are non-responsive to BCR cross-linking in terms of proliferation and up-regulation of activation markers such as CD69 and CD86 (B7-2). In addition, the CD5(+) subset of B cells is absent. The immune response to T cell-independent antigen but not T cell-dependent antigen is also impaired. Overall, the phenotype of BLNK(-/-) mice bears a striking resemblance to that of xid mice which is the murine model of human XLA that has a mutation in Bruton's tyrosine kinase. This raises the interesting possibility that mutation in BLNK/SLP-65 may be responsible for certain human immunodeficiencies.

An essential role for BLNK in human B cell development

The signal transduction events that control the progenitor B cell (pro-B cell) to precursor B cell (pre-B cell) transition have not been well delineated. In evaluating patients with absent B cells, a male with a homozygous splice defect in the cytoplasmic adapter protein BLNK (B cell linker protein) was identified. Although this patient had normal numbers of pro-B cells, he had no pre-B cells or mature B cells, indicating that BLNK plays a critical role in orchestrating the pro-B cell to pre-B cell transition. The immune system and overall growth and development were otherwise normal in this patient, suggesting that BLNK function is highly specific.