The B-cell receptor controls fitness of MYC-driven lymphoma cells via GSK3ß inhibition.

Similar to resting mature B cells, where the B-cell antigen receptor (BCR) controls cellular survival, surface BCR expression is conserved in most mature B-cell lymphomas. The identification of activating BCR mutations and the growth disadvantage upon BCR knockdown of cells of certain lymphoma entities has led to the view that BCR signalling is required for tumour cell survival. Consequently, the BCR signalling machinery has become an established target in the therapy of B-cell malignancies. Here we study the effects of BCR ablation on MYC-driven mouse B-cell lymphomas and compare them with observations in human Burkitt lymphoma. Whereas BCR ablation does not, per se, significantly affect lymphoma growth, BCR-negative (BCR-) tumour cells rapidly disappear in the presence of their BCR-expressing (BCR+) counterparts in vitro and in vivo. This requires neither cellular contact nor factors released by BCR+ tumour cells. Instead, BCR loss induces the rewiring of central carbon metabolism, increasing the sensitivity of receptor-less lymphoma cells to nutrient restriction. The BCR attenuates glycogen synthase kinase 3 beta (GSK3ß) activity to support MYC-controlled gene expression. BCR- tumour cells exhibit increased GSK3ß activity and are rescued from their competitive growth disadvantage by GSK3ß inhibition. BCR- lymphoma variants that restore competitive fitness normalize GSK3ß activity after constitutive activation of the MAPK pathway, commonly through Ras mutations. Similarly, in Burkitt lymphoma, activating RAS mutations may propagate immunoglobulin-crippled tumour cells, which usually represent a minority of the tumour bulk. Thus, while BCR expression enhances lymphoma cell fitness, BCR-targeted therapies may profit from combinations with drugs targeting BCR- tumour 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.

Siglec-G Deficiency Leads to Autoimmunity in Aging C57BL/6 Mice.

Siglec-G, a member of the sialic acid-binding Ig-like lectin (Siglec) family, is expressed on B cell and dendritic cell surfaces. It acts as an inhibitory coreceptor and modulates B cell activation, especially on B1 cells, as Siglec-G-deficient mice show mainly a B1 cell-restricted phenotype resulting in increased B1 cell numbers. Although higher B1 cell numbers are discussed to be associated with autoimmunity, loss of Siglec-G does not result in autoimmune disease in BALB/c mice. However, there is evidence from Siglec-G × CD22 double-deficient mice and Siglec-G(-/-) mice on an autoimmune-prone MRL/lpr background that Siglec-G is important to maintain tolerance in B cells. In this study, we analyzed the role of Siglec-G in induction and maintenance of B cell tolerance on C57BL/6 background and in the FcγRIIb-deficient background. We find that aging Siglec-G-deficient and Siglec-G × FcγRIIb double-deficient mice develop an autoimmune phenotype with elevated autoantibody levels and mild glomerulonephritis. Aging Siglec-G-deficient mice have elevated numbers of plasma cells and germinal center B cells, as well as a higher number of activated CD4 T cells, which likely all contribute to autoantibody production. Additional loss of the inhibitory receptor FcγRIIb in Siglec-G(-/-) mice does not result in exacerbation of disease. These results indicate that Siglec-G is important to maintain tolerance in B cells and prevent autoimmunity.

Siglec-G deficiency leads to more severe collagen-induced arthritis and earlier onset of lupus-like symptoms in MRL/lpr mice.

Siglec-G is a member of the sialic acid-binding Ig-like lectin (Siglec) family expressed on all B cells. Siglec-G-deficient mice show a large expansion of the B1 cell compartment, demonstrating the crucial role of Siglec-G as an inhibitory receptor on this cellular subset. Although Siglec-G-deficient mice did not develop spontaneous autoimmunity, mice double-deficient for Siglec-G and the related Siglec protein CD22 did show autoimmunity at an older age. In this study, we addressed the question of whether loss of Siglec G on its own affects disease severity in animal models of rheumatoid arthritis and systemic lupus erythematosus. Siglec-G-deficient mice showed moderately increased clinical severity and higher inflammation of the knee joints following collagen-induced arthritis, when compared with control mice. The Siglec-G-deficient mouse was also backcrossed to the autoimmune prone MLR/lpr background. Although both Siglec-G-deficient and control MRL/lpr mice developed a lupus-like disease, Siglec-G-deficient MRL/lpr mice showed an earlier occurrence of autoantibodies; a higher lymphoproliferation of B and T cells; and an earlier onset of disease, as shown by proteinuria and glomerular damage in the kidney. Moreover, Siglec-G-deficient female mice showed a significantly reduced survival compared with female control MRL/lpr mice. Thus, the loss of the inhibitory receptor Siglec-G led to a moderate exacerbation of disease severity and early onset in both collagen-induced arthritis and spontaneous lupus nephritis in MRL/lpr mice.

A balance between B cell receptor and inhibitory receptor signaling controls plasma cell differentiation by maintaining optimal Ets1 levels.

Signaling through the BCR can drive B cell activation and contribute to B cell differentiation into Ab-secreting plasma cells. The positive BCR signal is counterbalanced by a number of membrane-localized inhibitory receptors that limit B cell activation and plasma cell differentiation. Deficiencies in these negative signaling pathways may cause autoantibody generation and autoimmune disease in both animal models and human patients. We have previously shown that the transcription factor Ets1 can restrain B cell differentiation into plasma cells. In this study, we tested the roles of the BCR and inhibitory receptors in controlling the expression of Ets1 in mouse B cells. We found that Ets1 is downregulated in B cells by BCR or TLR signaling through a pathway dependent on PI3K, Btk, IKK2, and JNK. Deficiencies in inhibitory pathways, such as a loss of the tyrosine kinase Lyn, the phosphatase Src homology region 2 domain-containing phosphatase 1 (SHP1) or membrane receptors CD22 and/or Siglec-G, result in enhanced BCR signaling and decreased Ets1 expression. Restoring Ets1 expression in Lyn- or SHP1-deficient B cells inhibits their enhanced plasma cell differentiation. Our findings indicate that downregulation of Ets1 occurs in response to B cell activation via either BCR or TLR signaling, thereby allowing B cell differentiation and that the maintenance of Ets1 expression is an important function of the inhibitory Lyn → CD22/SiglecG → SHP1 pathway in B cells.

Chronic active B-cell-receptor signalling in diffuse large B-cell lymphoma.

A role for B-cell-receptor (BCR) signalling in lymphomagenesis has been inferred by studying immunoglobulin genes in human lymphomas and by engineering mouse models, but genetic and functional evidence for its oncogenic role in human lymphomas is needed. Here we describe a form of 'chronic active' BCR signalling that is required for cell survival in the activated B-cell-like (ABC) subtype of diffuse large B-cell lymphoma (DLBCL). The signalling adaptor CARD11 is required for constitutive NF-kappaB pathway activity and survival in ABC DLBCL. Roughly 10% of ABC DLBCLs have mutant CARD11 isoforms that activate NF-kappaB, but the mechanism that engages wild-type CARD11 in other ABC DLBCLs was unknown. An RNA interference genetic screen revealed that a BCR signalling component, Bruton's tyrosine kinase, is essential for the survival of ABC DLBCLs with wild-type CARD11. In addition, knockdown of proximal BCR subunits (IgM, Ig-kappa, CD79A and CD79B) killed ABC DLBCLs with wild-type CARD11 but not other lymphomas. The BCRs in these ABC DLBCLs formed prominent clusters in the plasma membrane with low diffusion, similarly to BCRs in antigen-stimulated normal B cells. Somatic mutations affecting the immunoreceptor tyrosine-based activation motif (ITAM) signalling modules of CD79B and CD79A were detected frequently in ABC DLBCL biopsy samples but rarely in other DLBCLs and never in Burkitt's lymphoma or mucosa-associated lymphoid tissue lymphoma. In 18% of ABC DLBCLs, one functionally critical residue of CD79B, the first ITAM tyrosine, was mutated. These mutations increased surface BCR expression and attenuated Lyn kinase, a feedback inhibitor of BCR signalling. These findings establish chronic active BCR signalling as a new pathogenetic mechanism in ABC DLBCL, suggesting several therapeutic strategies.

SIGLEC-G deficiency increases susceptibility to develop B-cell lymphoproliferative disorders.

The sialic-acid-binding immunoglobulin-like lectin SIGLEC-G is a negative regulator of B-cell receptor-mediated calcium signaling. Its deficiency leads to reduced turnover and increased proliferation and survival of murine B-1a cells. Siglecg(-/-) mice show a premature expansion of polyclonal CD5(+) B cells in the spleen and the peritoneal cavity. Here we studied the fate of B lymphocytes in Siglecg(-/-) mice over time. We demonstrate that in aging animals SIGLEC-G deficiency promotes progressive accumulation of monoclonal B lymphocytes and increases the susceptibility to develop B-cell lymphoproliferative disorders. Lymphoid tumors arising in aged Siglecg(-/-) mice are monoclonal and histologically heterogeneous as they include diffuse large B-cell lymphoma, follicular lymphoma, and medium-to-large B-cell monomorphic lymphoma but surprisingly not chronic lymphocytic leukemia. The tumors express high levels of BCL-2 and are transplantable. In keeping with these findings we have also observed a remarkable down-regulation of the human ortholog SIGLEC10 in human B-cell lymphoma and leukemia cell lines. Taken together, these observations indicate that the down-regulation of negative B-cell receptor regulators such as SIGLEC-G/SIGLEC10 may represent another mechanism relevant to the pathogenesis of B-cell lymphomas.

CD22 x Siglec-G double-deficient mice have massively increased B1 cell numbers and develop systemic autoimmunity.

CD22 and Siglec-G are inhibitory coreceptors for BCR-mediated signaling. Although CD22-deficient mice show increased calcium signaling in their conventional B2 cells and a quite normal B cell maturation, Siglec-G-deficient mice have increased calcium mobilization just in B1 cells and show a large expansion of the B1 cell population. Neither CD22-deficient, nor Siglec-G-deficient mice on a pure C57BL/6 or BALB/c background, respectively, develop autoimmunity. Using Siglec-G x CD22 double-deficient mice, we addressed whether Siglec-G and CD22 have redundant functions. Siglec-G x CD22 double-deficient mice show elevated calcium responses in both B1 cells and B2 cells, increased serum IgM levels and an enlarged population of B1 cells. The enlargement of B1 cell numbers is even higher than in Siglecg(-/-) mice. This expansion seems to happen at the expense of B2 cells, which are reduced in absolute cell numbers, but show an activated phenotype. Furthermore, Siglec-G x CD22 double-deficient mice show a diminished immune response to both thymus-dependent and thymus-independent type II Ags. In contrast, B cells from Siglec-G x CD22 double-deficient mice exhibit a hyperproliferative response to stimulation with several TLR ligands. Aged Siglec-G x CD22 double-deficient mice spontaneously develop anti-DNA and antinuclear autoantibodies. These resulted in a moderate form of immune complex glomerulonephritis. These results show that Siglec-G and CD22 have partly compensatory functions and together are crucial in maintaining the B cell tolerance.

Siglec-G regulates B1 cell survival and selection.

Siglec-G is a negative regulator of BCR-mediated signaling in B1a cells. This population of B cells is highly increased in Siglec-G-deficient mice, but the mechanism of this expansion is not known so far. In this study, we demonstrate that Siglecg(-/-) B1a cells show a lower level of spontaneous apoptosis and a prolonged life span. Mechanistically, the lower apoptosis could result from higher expression levels of the transcription factor NFATc1 in Siglec-G-deficient B1a cells. Interestingly, Siglecg(-/-) B1a cells display an altered BCR repertoire compared with wild-type B1a cells. As the BCR repertoire and the VDJ composition of Igs of Siglecg(-/-) B1a cells resembles more the Abs produced by adult bone marrow-derived B cells rather than canonical fetal liver-derived B1a cells, this suggest that the selection into the B1a cell population is altered in Siglec-G-deficient mice.

Absence of mature peripheral B cell populations in mice with concomitant defects in B cell receptor and BAFF-R signaling.

Generation of mature B lymphocytes from early (T1) and late transitional (T2) precursors requires cooperative signaling through BCR and B cell-activating factor receptor 3 (BR3). Recent studies have shown that BCR signaling positively regulates NF-kappaB2, suggesting BCR regulation of BR3 signaling. To investigate the significance of signal integration from BCR and BR3 in B cell development and function, we crossed Btk-deficient mice (btk(-/-)), which are developmentally blocked between the T2 and the mature follicular B cell stage as a result of a partial defect in BCR signaling, and A/WySnJ mice, which possess a mutant BR3 defective in propagating intracellular signals that results in a severely reduced peripheral B cell compartment, although all B cell subsets are present in relatively normal ratios. A/WySnJ x btk(-/-) mice display a B cell-autonomous defect, resulting in a developmental block at an earlier stage (T1) than either mutation alone, leading to the loss of mature splenic follicular and marginal zone B cells, as well as the loss of peritoneal B1 and B2 cell populations. The competence of the double mutant T1 B cells to respond to TLR4 and CD40 survival and activation signals is further attenuated compared with single mutations as evidenced by severely reduced humoral immune responses in vivo and proliferation in response to anti-IgM, LPS, and anti-CD40 stimulation in vitro. Thus, BCR and BR3 independently and in concert regulate the survival, differentiation, and function of all B cell populations at and beyond T1, earliest transitional stage.

Activation-induced cytidine deaminase expression and activity in the absence of germinal centers: insights into hyper-IgM syndrome.

Somatic hypermutation normally occurs as a consequence of the expression of activation-induced cytidine deaminase (AID) by Ag-activated, mature B cells during T cell-dependent germinal center responses. Nonetheless, despite their inability to express CD154 and initiate GC responses, patients with type 1 hyper-IgM syndrome (HIGM1) support populations of IgM(+)IgD(+)CD27(+) B cells that express mutated Ig genes. The origin of these mutated B cells is unknown; the IgM(+)IgD(+)CD27(+) cells do not express AID and appear to acquire mutations independent of stringent selection by Ag. Here, we demonstrate that immature/transitional 1 B cells from the bone marrow of CD154-deficient mice express AID and acquire Ig mutations that lack the hallmarks of antigenic selection via BCR signaling. Comparable levels of AID expression was found in developmentally immature B cells recovered from murine fetal liver and from human immature/transitional 1 B cells recovered from umbilical cord blood. AID expression in human fetal liver was also robust, approaching that of human tonsil tissue and the human germinal center B cell line, Ramos. These observations led us to conclude that AID expression in developing human B cells is the origin of the mutated IgM(+)IgD(+)CD27(+) B cells present in HIGM1 patients, and we propose that both mice and humans share a latent, AID-dependent pathway for the preimmune diversification of B lymphocytes that is more prominent in chicken, sheep, and rabbits.

Siglec-G Deficiency Ameliorates Hyper-Inflammation and Immune Collapse in Sepsis via Regulating Src Activation.

Hyper-inflammation during acute phase and sequential hypo-inflammation during immunosuppressive phase in macrophages/monocytes lead to multiorgan failure syndrome and immune collapse of sepsis, in which toll-like receptor (TLR)-triggered inflammatory responses play a major role. Here, we reported that Siglecg deficiency attenuated TLR4-triggered pro-inflammatory cytokine production and increased anti-inflammatory cytokine [interleukin-10 [IL-10]] production in vivo and in vitro at both acute and immunosuppressive phases. Siglecg deficiency also protected mice from lipopolysaccharide (LPS)-induced sepsis with less inflammation in the lung and less tissue destruction in the spleen. Siglec-G inhibited proto-oncogene tyrosine-protein kinase Src (Src) activation via recruiting and activating tyrosine phosphatase Src homology region 2 domain-containing phosphatase-1 (SHP1) through immunoreceptor tyrosine-based inhibitory motif (ITIM) domain. Src could inhibit TLR4-induced inflammatory cytokines and promote anti-inflammatory cytokine IL-10. Mechanical investigation showed that Src could interact with and phosphorylate STAT3. Src could also promote HIF1α degradation through activating GSK3ß. Our study reveals that Siglec-G orchestrates TLR-induced inflammation, which outlines that blocking Siglec-G or activating Src may be a promising strategy for both acute and chronic inflammatory diseases.

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.

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 pre­B 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.

Growth inhibition of myeloma cells by anti-idiotype antibodies in the absence of membrane-bound immunoglobulin.

Immunoglobulins are expressed as membrane-bound or secreted forms. Plasma cells produce little or no membrane immunoglobulin but secrete immunoglobulin molecules in large amounts. Immunoglobulin idiotypes of malignant B cells are tumor-specific antigens that may be targeted for immunotherapy. Thus, idiotype vaccination is being evaluated in clinical trials to control residual disease in multiple myeloma and non-Hodgkin's lymphoma. It is traditionally considered that anti-idiotype antibodies are not effective against plasma cell tumors, because the large amounts of immunoglobulin molecules secreted by the tumors block anti-idiotype antibodies, and because the absence of membrane immunoglobulin on the surface of these tumor cells renders them resistant to the effect of anti-idiotype antibodies. While the obstacle of abundant circulating idiotype may be obviated by reducing tumor burden to minimal residual disease, the absence of membrane immunoglobulin has been considered as a limiting factor that prevents tumor eradication by anti-idiotype antibodies. We demonstrate here that murine plasmacytoma cells can produce small amounts of membrane immunoglobulin M (IgM) heavy chains. However, the latter are precursor molecules that do not reach the cell surface. Although membrane-bound IgM is absent, the cells stain positively for surface IgM, reflecting molecules of the secreted form in the process of secretion. In spite of the relatively low levels of secreted immunoglobulin on the cell surface, anti-idiotype antibodies are effective in retardation of tumor growth in vivo. Thus, while there is no doubt that idiotype-specific cell-mediated responses are very important, myeloma patients in complete remission may additionally benefit from idiotype-specific humoral responses.

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.

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.

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.