Sialic Acid-Binding Immunoglobulin-like Lectin G Promotes Atherosclerosis and Liver Inflammation by Suppressing the Protective Functions of B-1 Cells

Atherosclerosis is initiated and sustained by hypercholesterolemia, which results in the generation of oxidized LDL (OxLDL) and other metabolic byproducts that trigger inflammation. Specific immune responses have been shown to modulate the inflammatory response during atherogenesis. The sialic acid-binding immunoglobulin-like lectin G (Siglec-G) is a negative regulator of the functions of several immune cells, including myeloid cells and B-1 cells. Here, we show that deficiency of Siglec-G in atherosclerosis-prone mice inhibits plaque formation and diet-induced hepatic inflammation. We further demonstrate that selective deficiency of Siglec-G in B cells alone is sufficient to mediate these effects. Levels of B-1 cell-derived natural IgM with specificity for OxLDL were significantly increased in the plasma and peritoneal cavity of Siglec-G-deficient mice. Consistent with the neutralizing functions of OxLDL-specific IgM, Siglec-G-deficient mice were protected from OxLDL-induced sterile inflammation. Thus, Siglec-G promotes atherosclerosis and hepatic inflammation by suppressing protective anti-inflammatory effector functions of B cells.

Signalling thresholds and negative B-cell selection in acute lymphoblastic leukaemia

B cells are selected for an intermediate level of B-cell antigen receptor (BCR) signalling strength: attenuation below minimum (for example, non-functional BCR) or hyperactivation above maximum (for example, self-reactive BCR) thresholds of signalling strength causes negative selection. In ∼25% of cases, acute lymphoblastic leukaemia (ALL) cells carry the oncogenic BCR-ABL1 tyrosine kinase (Philadelphia chromosome positive), which mimics constitutively active pre-BCR signalling. Current therapeutic approaches are largely focused on the development of more potent tyrosine kinase inhibitors to suppress oncogenic signalling below a minimum threshold for survival. We tested the hypothesis that targeted hyperactivation--above a maximum threshold--will engage a deletional checkpoint for removal of self-reactive B cells and selectively kill ALL cells. Here we find, by testing various components of proximal pre-BCR signalling in mouse BCR-ABL1 cells, that an incremental increase of Syk tyrosine kinase activity was required and sufficient to induce cell death. Hyperactive Syk was functionally equivalent to acute activation of a self-reactive BCR on ALL cells. Despite oncogenic transformation, this basic mechanism of negative selection was still functional in ALL cells. Unlike normal pre-B cells, patient-derived ALL cells express the inhibitory receptors PECAM1, CD300A and LAIR1 at high levels. Genetic studies revealed that Pecam1, Cd300a and Lair1 are critical to calibrate oncogenic signalling strength through recruitment of the inhibitory phosphatases Ptpn6 (ref. 7) and Inpp5d (ref. 8). Using a novel small-molecule inhibitor of INPP5D (also known as SHIP1), we demonstrated that pharmacological hyperactivation of SYK and engagement of negative B-cell selection represents a promising new strategy to overcome drug resistance in human ALL.

A recurrent dominant negative E47 mutation causes agammaglobulinemia and BCR(-) B cells

Approximately 90% of patients with isolated agammaglobulinemia and failure of B cell development have mutations in genes required for signaling through the pre–B cell and B cell receptors. The nature of the gene defect in the majority of remaining patients is unknown. We recently identified 4 patients with agammaglobulinemia and markedly decreased numbers of peripheral B cells. The B cells that could be detected had an unusual phenotype characterized by the increased expression of CD19 but the absence of a B cell receptor. Genetic studies demonstrated that all 4 patients had the exact same de novo mutation in the broadly expressed transcription factor E47. The mutant protein (E555K) was stable in patient-derived EBV-transformed cell lines and cell lines transfected with expression vectors. E555K in the transfected cells localized normally to the nucleus and resulted in a dominant negative effect when bound to DNA as a homodimer with wild-type E47. Mutant E47 did permit DNA binding by a tissue-specific heterodimeric DNA-binding partner, myogenic differentiation 1 (MYOD). These findings document a mutational hot-spot in E47 and represent an autosomal dominant form of agammaglobulinemia. Further, they indicate that E47 plays a critical role in enforcing the block in development of B cell precursors that lack functional antigen receptors.

Siglec-G is a B1 cell–inhibitory receptor that controls expansion and calcium signaling of the B1 cell population

B1 cells are an important cell population for the production of natural antibodies and for antibacterial immunoglobulin responses. Here we identified the mouse protein Siglec-G as a B1 cell inhibitory receptor. Siglec-G was expressed in a B cell-restricted way, with large amounts present in B1 cells. When overexpressed, Siglec-G inhibited B cell receptor-mediated calcium signaling. Siglec-G-deficient mice had massive expansion of the B1a cell population, which began early in development and was B cell intrinsic. Siglec-G-deficient mice had higher titers of natural IgM antibodies but not a higher penetrance of IgG autoantibodies. Siglec-G-deficient B1 cells showed a strongly enhanced calcium signaling. Our results demonstrate that Siglec-G-dependent negative regulation exists in B1 cells, which may explain the naturally muted signaling response of B1 cells.

AID-/-mus-/- mice are agammaglobulinemic and fail to maintain B220-CD138+ plasma cells

The terminal stage of B cell differentiation culminates in the formation of plasma cells (PC), which secrete large quantities of Igs. Despite recent progress in understanding the molecular aspect of PC differentiation and maintenance, the requirement for the synthesis of secretory Igs as a contributing factor has not been explored. To address this issue, we generated activation-induced cytidine deaminase (AID)/secretory mu-chain (mus) double-knockout mice, in which a normally diverse repertoire of B cell receptors is retained, yet B cells are unable to synthesize secretory Igs. These mice possess polyclonal B cells but have no serum Igs. Following immunization in vivo, PCs, identified by CD138 expression and loss of the B220 marker, were starkly reduced in number in spleen and bone marrow of AID(-/-)mus(-/-) agammaglobulinemic mice compared with wild-type mice. Upon mitogenic stimulation in vitro, AID(-/-)mus(-/-) B cells differentiated into plasmablasts to some extent, but showed reduced survival compared with wild-type B cells. We found no evidence that this reduced survival was attributable to accumulation of membrane IgM. Our results indicate that the synthesis of secretory Igs is a requirement for maintenance of B220(-)CD138(+) PCs.

E2A Expression Stimulates Ig Hypermutation

Ig hypermutation is limited to a region of approximately 2 kb downstream of the transcription start sites of the Ig loci. The process requires transcription and the presence of Ig enhancer sequences, and is initiated by the activation-induced cytidine deaminase (AID)-mediated deamination of cytidine bases. It remains unknown why AID causes mutations selectively in the Ig genes and not in most other transcribed loci of B cells. In this study, we report that the inactivation of the E2A gene strongly reduces the rate of Ig L chain mutations in the chicken B cell line DT40 without affecting the levels of surface Ig or AID expression. The defect is complemented by the expression of cDNAs corresponding to either of the two E2A splice variants E12 or E47. The results suggest that E2A-encoded proteins enhance Ig hypermutation by recruitment of AID to the Ig loci.

Transformation of BCR-deficient germinal-center B cells by EBV supports a major role of the virus in the pathogenesis of Hodgkin and posttransplantation lymphomas

In classic Hodgkin lymphoma (HL) and posttransplantation lymphoproliferative disease (PTLD), 2 malignancies frequently associated with Epstein-Barr virus (EBV), the tumor cells often appear to derive from B-cell receptor (BCR)-deficient and therefore preapoptotic germinal center (GC) B cells. To test whether EBV can rescue BCR-less GC B cells, we infected human tonsillar CD77+ GC B cells in vitro with EBV. More than 60 monoclonal lymphoblastoid cell lines (LCLs) were established. Among these, 28 cell lines did not express surface immunoglobulin (sIg). Two of the sIg-negative cell lines carry obviously destructive mutations that have been introduced into originally functional V(H) gene rearrangements during the process of somatic hypermutation. Quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) showed that in most other lines the sIg deficiency was not simply the result of transcriptional down-regulation, but it was rather due to posttranscriptional defects. These findings strongly support the idea that EBV plays a central role in the pathogenesis of classic HL and PTLD by rescuing BCR-deficient, preapoptotic GC B cells from apoptosis, and that EBV infection renders the cells independent from survival signals normally supplied by a BCR. The monoclonal LCLs represent valuable models for early stages of lymphoma development in classic HL and PTLD.

Latent membrane protein 2A, a viral B cell receptor homologue, induces CD5+ B-1 cell development

The latent membrane protein 2A (LMP2A) of EBV plays a key role in regulating viral latency and EBV pathogenesis by functionally mimicking a constitutively active B cell Ag receptor. When expressed as a B cell-specific transgene in mice, LMP2A drives B cell development, resulting in the bypass of normal developmental checkpoints. In this study, we have demonstrated that expression of LMP2A in transgenic mice results in B cell development that exclusively favors B-1 cells. This switch to B-1 cell development occurs at the pre-B-cell stage of normal B cell development in the bone marrow, a B cell stage much earlier than appreciated for B-1 commitment. This finding indicates that all pre-B cells have the capacity to assume a B-1 cell phenotype if they encounter the appropriate signal during normal development. Furthermore, these studies offer insight into EBV latency and pathogenesis in the human host.

Impairment of B cell receptor-mediated Ca2+ influx, activation of mitogen-activated protein kinases and growth inhibition in CD72-deficient BAL-17 cells

CD72 is a 45 kDa B cell-specific type II transmembrane protein of the C-type lectin superfamily. It was originally defined as a receptor-like molecule that regulates B cell activation and differentiation; however, its precise function remains unclear since more recent functional analyses, including a gene targeting study, suggest that CD72 may serve as a negative or a positive regulator of B cell signaling. In the present study, we analyzed the cell-autonomous function of CD72 in B cell receptor (BCR) signaling using CD72-deficient cells generated from mature BAL-17 cells. We found that BCR-mediated phosphorylation of CD19, Btk, Vav and phospholipase Cgamma2 and association of CD19 with phosphatidylinositol-3 kinase were impaired in CD72-deficient cells. Inositol trisphosphate synthesis was normally induced initially but ablated at 1 min of stimulation in CD72-deficient cells. In the event, Ca(2+) release from intracellular stores remained intact, though influx of extracellular Ca(2+) was severely impaired in CD72-deficient cells. Furthermore, BCR-evoked activation of mitogen-activated protein kinases (MAPKs), extracellular signal-regulated kinase and c-Jun NH(2)-terminal kinase, and growth inhibition in BAL-17 cells were blocked in the absence of CD72. Significantly, these effects were largely reversed by re-expression of CD72. Thus, CD72 appears to exert a positive effect on BCR signaling pathways leading to Ca(2+) influx and MAPK activation, which in turn may determine the fate of BAL-17 cells.

Interaction of murine precursor B cell receptor with stroma cells is controlled by the unique tail of lambda 5 and stroma cell-associated heparan sulfate

Efficient clonal expansion of early precursor B (pre-B) cells requires signals delivered by an Ig-like integral membrane complex, the so-called pre-B cell receptor (pre-BCR). A pre-BCR consists of two membrane micro H chains, two covalently associated surrogate L chains, and the heterodimeric signaling transducer Igalphabeta. In contrast to a conventional Ig L chain, the surrogate L chain is a heterodimer composed of the invariant polypeptides VpreB and lambda5. Although it is still unclear how pre-BCR signals are initiated, two recent findings support a ligand-dependent initiation of pre-BCR signals: 1) a pre-BCR/galectin-1 interaction is required to induce phosphorylation of Igalphabeta in a human precursor B line, and 2) soluble murine as well as human pre-BCR molecules bind to stroma and other adherent cells. In this study, we show that efficient binding of a soluble murine pre-BCR to stroma cells requires the non-Ig-like unique tail of lambda5. Surprisingly however, a murine pre-BCR, in contrast to its human counterpart, does not interact with galectin-1, as revealed by lactose blocking, RNA interference, and immunoprecipitation assays. Finally, the binding of a murine pre-BCR to stroma cells can be blocked either with heparin or by pretreatment of stroma cells with heparitinase or a sulfation inhibitor. Hence, efficient binding of a murine pre-BCR to stroma cells requires the unique tail of lambda5 and stroma cell-associated heparan sulfate. These findings not only identified heparan sulfate as potential pre-BCR ligands, but will also facilitate the development of appropriate animal models to determine whether a pre-BCR/heparan sulfate interaction is involved in early B cell maturation.

Phosphorylation abnormalities: NZB mice exhibit a B-cell signalling defect

NZB mice demonstrate common and consistent abnormalities in B-cell activation and signalling. One of the hallmark characteristics of lupus disease is the prevalent hypergammaglobulinaemia, composed primarily of anti-nuclear antibodies. In addition to the hyperproliferation seen in mice exhibiting disease, the B cells also demonstrate a marked degree of hyperactivity in response to B-cell receptor occupancy. This points to an intrinsic defect in the signalling pathways regulating the response to an activation event. Correspondingly, B cells of NZB mice exhibit a significant lack of phosphatase activity, both at baseline and in response to stimulation. This is directly reflected by a higher level of phosphorylation of tyrosine residues. Individually, SAPK and SHIP-1, both players in the B-cell receptor signalling cascade, are also found to be abnormally phosphorylated in the NZB mouse.

B cell progenitors are arrested in maturation but have intact VDJ recombination in the absence of Ig-alpha and Ig-beta

Ig-alpha and Ig-beta mediate surface expression and signaling of diverse B cell receptor complexes on precursor, immature, and mature B cells. Their expression begins before that of the Ig chains in early progenitor B cells. In this study, we describe the generation of Ig-alpha-deficient mice and their comparative analysis to mice deficient for Ig-beta, the membrane-IgM, and recombination-activating gene 2 to determine the requirement of Ig-alpha and Ig-beta in survival and differentiation of pro-B cells. We find that in the absence of Ig-alpha, B cell development does not progress beyond the progenitor stage, similar to what is observed in humans lacking this molecule. However, neither in Ig-alpha- nor in Ig-beta-deficient mice are pro-B cells impaired in V(D)J recombination, in the expression of intracellular Ig micro-chains, or in surviving in the bone marrow microenvironment. Finally, Ig-alpha and Ig-beta are not redundant in their putative function, as pro-B cells from Ig-alpha and Ig-beta double-deficient mice are similar to those from single-deficient animals in every aspect analyzed.

Expression of a V region-less B cell receptor confers a tolerance-like phenotype on transgenic B cells

Neoplastic B cells from H chain disease patients express a truncated B cell receptor (BCR), comprising a membrane Ig that lacks part of its extracellular domain. It has been speculated that deletion of the Ag binding domain would confer a constitutive activity on the BCR, as it has been shown for oncogenic growth factor receptors. A V region-less BCR has constitutive activity, because in transgenic mice it causes inhibition of endogenous H chain gene rearrangements and relieves the requirement for surrogate L chain in pre-B cell development. However, it has been speculated that normal Ag receptors also display constitutive activity. Here we show that transgenic B cells expressing a membrane H chain disease protein on their surface are phenotypically and functionally similar to B cells developing in the presence of their cognate Ag and that cells with normal levels of mutant BCR are eliminated in spleen via a bcl-2 sensitive pathway while progressing toward the mature stage. In contrast, cells with lower levels of mutant receptors develop as mature B cells. These findings support the view that the truncated BCR has a constitutive activity that mimics ligand binding, in analogy to what has been shown for oncogenic growth factor receptors.

An alternatively spliced form of CD79b gene may account for altered B-cell receptor expression in B-chronic lymphocytic leukemia

Several functional anomalies of B-chronic lymphocytic leukemia (B-CLL) cells may be explained by abnormalities of the B-cell receptor (BCR), a multimeric complex formed by the sIg homodimer and the noncovalently bound heterodimer Igalpha/Igbeta (CD79a/CD79b). Because the expression of the extracellular Ig-like domain of CD79b has been reported to be absent in the cells of most CLL cases, we have investigated the molecular mechanisms that may account for this defect. Peripheral blood lymphocytes (PBL) from 50 patients and two cell lines (MEC1, MEC2) obtained from the PBL of one of them were studied. MEC1, MEC2, and 75% of CLL cases did not express detectable levels of the extracellular Ig-like domain of CD79b, which was nevertheless present in greater than 80% CD19(+) cells from normal donors. In healthy subjects the expression of CD79b was equally distributed in CD5(+) and CD5(-) B-cell subsets. Reverse transcription-polymerase chain reaction (RT-PCR) analysis of CD79b RNA from all patients and from MEC1 and MEC2 cell lines consistently yielded two fragments of different size (709 bp and 397 bp). The 709-bp band corresponds to CD79b entire transcript; the 397-bp band corresponds to an alternatively spliced form lacking exon 3 that encodes the extracellular Ig-like domain. Both fragments were also visible in normal PBL. The expression of the 397-bp fragment was increased in normal activated B cells, while no difference was seen between CD5(+) and CD5(-) B cells. To obtain a more accurate estimate of the relative proportions of the two spliced forms, a radioactive PCR was performed in 13 normal and 22 B-CLL samples and the results analyzed using a digital imager. The mean value of the CD79b to the CD79b internally deleted ratio was 0.64 +/- 0.20 SD in normal donors and 0.44 +/- 0.27 SD in B-CLL (P =.01). Direct sequencing of 397-bp RT-PCR products and of genomic DNA corresponding to exon 3 from MEC1, MEC2, their parental cells, and five fresh B-CLL samples did not show any causal mutation. Single-strand conformation polymorphism analysis of exon 3 performed in 18 additional B-CLL cases showed a single abnormal shift corresponding to a TGT --> TGC polymorphic change at amino acid 122. We propose a role for the alternative splicing of CD79b gene in causing the reduced expression of BCR on the surface of B-CLL cells. As normal B cells also present this variant, the mechanism of CD79b posttranscriptional regulation might reflect the activation stage of the normal B cell from which B-CLL derives.

Immune cell signaling defects in lupus: activation, anergy and death

Recent studies have identified novel aberrations in antigen receptor-mediated signaling events in lymphocytes from patients with systemic lupus erythematosus. Here, we propose that in lupus lymphocytes, the receptor-mediated increase in protein tyrosine phosphorylation and cytoplasmic free Ca2+ responses, along with T-cell receptor zeta chain deficiency, might explain the previously described diverse and conflicting immunoregulatory defects in human lupus.

Cell-marker studies in CLL with monoclonal OKT antisera and lactic dehydrogenase isoenzyme analysis

In 16 patients with B-type CLL, the T and B cell compartment was analysed using the monoclonal anti-T-cell antisera OKT1, 3, 4 and 8 and the lactic dehydrogenase enzyme marker. Pertinent findings were: the presence on the CLL cells in 15 out of 16 patients of the antigen determined by the OKT1 antiserum, and, in all patients, a lower total LDH content on a per cell basis combined with a higher percentage-activity in the LDH-3 band as compared to the normal B-cell. Furthermore, the T-cell compartment was also disturbed in these patients, as the ratio of OKT4+ to OKT8+ cells was significantly depressed accompanied by a significant decrease of the LDH-1 percentage-activity in favour of LDH-3 and 4. These findings argue for the B-cell being immature and confirm the recent evidence that the T-cell compartment is changed in B-CLL.