Antigen-independent regulation of cytoplasmic calcium in B cells with a 12-kDa B-cell growth factor and anti-CD19

Recombinant human CD19-ligand protein as a potent anti-leukaemic agent

We report the cloning and characterization of a novel 54-kDa high-mobility group (HMG)-box protein as the ligand for the human pan-B cell co-receptor CD19 (CD19-L), which interacts with the extracellular domain of CD19 in trans. CD19-L is the first CD19-specific recombinant human protein with potent anti-leukaemic activity against B-lineage acute lymphoblastic leukaemia (ALL), the most common form of childhood cancer and the second most common form of acute leukaemia in adults. Soluble recombinant CD19-L protein exhibited exquisite specificity for the extracellular domain of CD19 and strong binding to the surface of B-lineage leukaemia/lymphoma cells. Engagement of CD19 co-receptor on B-lineage ALL cells with CD19-L perturbed the CD19-associated signalling network, altering the expression levels of multiple genes directly involved in regulation of apoptosis, and triggered rapid apoptotic cell death in a CD19-specific manner. The identification of human CD19-L may lead to therapeutic innovation for B-lineage ALL and other B-lineage lymphoid malignancies as well as B-cell lymphoproliferative states and systemic autoimmunity.

SLP-65 phosphorylation dynamics reveals a functional basis for signal integration by receptor-proximal adaptor proteins

Understanding intracellular signal transduction by cell surface receptors requires information about the precise order of relevant modifications on the early transducer elements. Here we introduce the B cell line DT40 and its genetically engineered variants as a model system to determine and functionally characterize post-translational protein modifications in general. This is accomplished by a customized strategy that combines mass spectrometric analyses of protein modifications with subsequent mutational studies. When applied to the B cell receptor (BCR)-proximal effector SLP-65, this approach uncovered a differential and highly dynamic engagement of numerous newly identified phospho-acceptor sites. Some of them serve as kinase substrates in resting cells and undergo rapid dephosphorylation upon BCR ligation. Stimulation-induced phosphorylation of SLP-65 can be early and transient, or early and sustained, or late. Functional elucidation of conspicuous phosphorylation at serine 170 in SLP-65 revealed a BCR-distal checkpoint for some but not all possible B cell responses. Our data show that SLP-65 phosphorylation acts upstream for signal initiation and also downstream during selective processing of the BCR signal. Such a phenomenon defines a receptor-specific signal integrator.

Histamine as an autocrine regulator of leukemic cell proliferation

We examined leukemic lymphocyte precursors from ALL patients as well as immortalized ALL cell lines for cytoplasmic histamine expression. The histamine levels ranged from 10.8 pg/10(6) cells to 82.2 pg/10(6) cells in ALL cell lines (N=4) and from 12.5 pg/10(6) cells to 1235.4 pg/10(6) cells for primary leukemic cells from ALL patients (N=13). The presence of histamine in the cytoplasm of these ALL cells was also confirmed by immunostaining using a polyclonal rabbit anti-histamine antibody. Notably, the histamine receptor blocker diphenhydramine inhibited the clonogenic growth of ALL cells by >90% prompting the hypothesis that histamine may be an autocrine regulator of ALL cell proliferation. Our study suggests that histamine receptor blockers may therefore be useful for the treatment of therapy-refractory ALL.

Qualitative regulation of B cell antigen receptor signaling by CD19: selective requirement for PI3-kinase activation, inositol-1,4,5-trisphosphate production and Ca2+ mobilization

Genetic ablation of the B cell surface glycoprotein CD19 severely impairs the humoral immune response. This requirement is thought to reflect a critical role of CD19 in signal transduction that occurs upon antigen C3dg coligation of antigen receptors with CD19 containing type 2 complement receptors (CR2). Here we show that CD19 plays a key accessory role in B cell antigen receptor signaling independent of CR2 coligation and define molecular circuitry by which this function is mediated. While CD19 is not required for antigen-mediated activation of receptor proximal tyrosines kinases, it is critical for activation of phosphatidylinositol 3-kinase (PI3-kinase). PI3-Kinase activation is dependent on phosphorylation of CD19 Y484 and Y515. Antigen-induced CD19-dependent PI3-kinase activation is required for normal phosphoinositide hydrolysis and Ca2+ mobilization responses. Thus, CD19 functions as a B cell antigen receptor accessory molecule that modifies antigen receptor signaling in a qualitative manner.

Impaired CD19 expression and signaling, enhanced antibody response to type II T independent antigen and reduction of B-1 cells in CD81-deficient mice

The tetraspanin CD81 is ubiquitously expressed and associated with CD19 on B lymphocytes and with CD4 and CD8 on T lymphocytes. Analysis of mice with disrupted CD81 gene reveals normal T cells but a distinct abnormality in B cells consisting of decreased expression of CD19 and severe reduction in peritoneal B-1 cells. CD81-deficient B cells responded normally to surface IgM crosslinking, but had severely impaired calcium influx following CD19 engagement. CD81-deficient mice had increased serum IgM and IgA and an exaggerated antibody response to the type II T independent antigen TNP-Ficoll. These results suggest that CD81 is important for CD19 signaling and B cell function.

Membrane-associated CD19-LYN complex is an endogenous p53-independent and Bc1-2-independent regulator of apoptosis in human B-lineage lymphoma cells

CD19 receptor is expressed at high levels on human B-lineage lymphoid cells and is physically associated with the Src protooncogene family protein-tyrosine kinase Lyn. Recent studies indicate that the membrane-associated CD19-Lyn receptor-enzyme complex plays a pivotal role for survival and clonogenicity of immature B-cell precursors from acute lymphoblastic leukemia patients, but its significance for mature B-lineage lymphoid cells (e.g., B-lineage lymphoma cells) is unknown. CD19-associated Lyn kinase can be selectively targeted and inhibited with B43-Gen, a CD19 receptor-specific immunoconjugate containing the naturally occurring protein-tyrosine kinase inhibitor genistein (Gen). We now present experimental evidence that targeting the membrane-associated CD19-Lyn complex in vitro with B43-Gen triggers rapid apoptotic cell death in highly radiation-resistant p53-Bax- Ramos-BT B-lineage lymphoma cells expressing high levels of Bcl-2 protein without affecting the Bcl-2 expression level. The therapeutic potential of this membrane-directed apoptosis induction strategy was examined in a scid mouse xenograft model of radiation-resistant high-grade human B-lineage lymphoma. Remarkably, in vivo treatment of scid mice challenged with an invariably fatal number of Ramos-BT cells with B43-Gen at a dose level < 1/10 the maximum tolerated dose resulted in 70% long-term event-free survival. Taken together, these results provide unprecedented evidence that the membrane-associated anti-apoptotic CD19-Lyn complex may be at least as important as Bcl-2/Bax ratio for survival of lymphoma cells.

Immunoglobulin recombinase gene activity is modulated reciprocally by interleukin 7 and CD19 in B cell progenitors

Bone marrow stromal cells promote B cell development involving recombinase gene-directed rearrangement of the immunoglobulin genes. We observed that the stromal cell-derived cytokine interleukin 7 (IL-7) enhances the expression of CD19 molecules on progenitor B-lineage cells in human bone marrow samples and downregulates the expression of terminal deoxynucleotidyl transferase (TdT) and the recombinase-activating genes RAG-1 and RAG-2. Initiation of the TdT downregulation on the first day of treatment, CD19 upregulation during the second day, and RAG-1 and RAG-2 downmodulation during the third day implied a cascade of IL-7 effects. While CD19 ligation by divalent antibodies had no direct effect on TdT or RAG gene expression, CD19 cross-linkage complete blocked the IL-7 downregulation of RAG expression without affecting the earlier TdT response. These results suggest that signals generated through CD19 and the IL-7 receptor could modulate immunoglobulin gene rearrangement and repertoire diversification during the early stages of B cell differentiation.

CD19 antigen in leukemia and lymphoma diagnosis and immunotherapy

The CD19 antigen plays an important role in clinical oncology. In normal cells, it is the most ubiquitously expressed protein in the B lymphocyte lineage. CD19 expression is induced at the point of B lineage commitment during the differentiation of the hematopoietic stem cell, and its expression continues through preB and mature B cell differentiation until it is finally down-regulated during terminal differentiation into plasma cells. CD19 expression is maintained in B-lineage cells that have undergone neoplastic transformation, and therefore CD19 is useful in diagnosis of leukemias and lymphomas using monoclonal antibodies (mAbs) and flow cytometry. Interestingly, CD19 is also expressed in a subset of acute myelogenous leukemias (AMLs) indicating the close relationship between the lymphoid and myeloid lineages. Because B lineage leukemias and lymphomas rarely lose CD19 expression, and because it is not expressed in the pluripotent stem cell, it has become the target for a variety of immunotherapeutic agents, including immunotoxins. Treatment of non-Hodgkin's lymphoma (NHL) and acute lymphocytic leukemia (ALL) with anti-CD19 mAbs coupled to biological toxins has proven to be effective in vitro and in animal models, and has shown some promising results in Phase I clinical trials. Recently, the analysis of anti-CD19 effects on lymphoma cell growth has highlighted a novel mechanism of immunotherapy. Engagement of cell surface receptors like CD19 by mAbs can have anti-tumor effects by the activation of signal transduction pathways which control cell cycle progression and programmed cell death (apoptosis).

Biotherapy of B-cell precursor leukemia by targeting genistein to CD19-associated tyrosine kinases

B-cell precursor (BCP) leukemia is the most common form of childhood cancer and the second most common form of acute leukemia in adults. Human BCP leukemia was treated in a severe combined immunodeficient mouse model by targeting of the tyrosine kinase inhibitor Genistein (Gen) to the B cell-specific receptor CD19 with the monoclonal antibody B43. The B43-Gen immunoconjugate bound with high affinity to BCP leukemia cells, selectively inhibited CD19-associated tyrosine kinases, and triggered rapid apoptotic cell death. At less than one-tenth the maximum tolerated dose more than 99.999 percent of human BCP leukemia cells were killed, which led to 100 percent long-term event-free survival from an otherwise invariably fatal leukemia. The B43-Gen immuno-conjugate might be useful in eliminating leukemia cells in patients who have failed conventional therapy.

The gene coding for the B cell surface protein CD19 is localized on human chromosome 16p11

The CD19 gene codes for one of the earliest markers of the human B cell lineage and is a target for the B lymphoid-specific transcription factor BSAP (Pax-5). The transmembrane protein CD19 has been implicated in controlling proliferation of mature B lymphocytes by modulating signal transduction through the antigen receptor. In this study, we have employed Southern blot and fluorescence in situ hybridization analyses to localize the CD19 gene to human chromosome 16p11.

B cell antigen receptor cross-linking induces tyrosine phosphorylation and membrane translocation of a multimeric Shc complex that is augmented by CD19 co-ligation

The SH2 domain-containing transforming Shc protein has been implicated in mitogenic signaling via several surface receptors through p21ras. Following tyrosine phosphorylation by either receptor or non-receptor tyrosine kinases, Shc may interact with the adaptor protein Grb2, which is linked to Sos1, a guanine nucleotide exchange factor for human ras. Ligation of the antigen receptor complex on B cells (BCR) is known to activate various intracellular signaling pathways, which may accumulate in mitogenic responses. With respect to the initial steps, the activation of BCR-associated non-receptor tyrosine kinases appears to be indispensible. In this report we show that Shc proteins become tyrosine phosphorylated after BCR ligation on both transformed and normal human B cells. This is accompanied by the association of Shc with Grb2 proteins and a yet unidentified 145-kDa tyrosine phosphorylated protein. Subcellular fractionation revealed that this activation-induced multimeric Shc complex rapidly translocates towards the plasma membrane. Co-ligation of the BCR with the CD19 molecule results in a marked increase of these events, whereas CD19 cross-linking alone does not induce Shc tyrosine phosphorylation or translocation. Thus, in B cells the Shc complex may represent a molecular junction between the BCR and the mitogenic p21ras cascade.

Epstein-Barr virus latent membrane protein 2A blocks calcium mobilization in B lymphocytes

LMP2A is expressed in latent Epstein-Barr virus (EBV) infection and interacts with LMP1 and members of the src tyrosine kinase family in the plasma membrane. Since tyrosine kinase mediate receptor-induced changes in intracellular free calcium, the effect of LMP2A on receptor-mediated intracellular calcium mobilization was evaluated by stably expressing LMP2A in an EBV-negative Burkitt tumor cell line (BJAB) or in LMP1-converted BJAB cells. LMP2A significantly blocked calcium mobilization following class II, CD19, or immunoglobulin M cross-linking. LMP2A effects were partially reversed in LMP1-converted cell lines. These results are compatible with LMP2A acting in latent B-lymphocyte infection to downmodulate LMP1 effects on cell growth or to inhibit induction of lytic EBV infection in specific human tissues following receptor ligation.

Factors modifying survival pathways of germinal center B cells. Glucocorticoids and transforming growth factor-beta, but not cyclosporin A or anti-CD19, block surface immunoglobulin-mediated rescue from apoptosis

The tendency for germinal center (GC) B cells to enter apoptosis is suppressed on engaging antigen receptor with immobilized anti-immunoglobulin; cross-linking of surface CD40 by monoclonal antibodies provides an additional signal for rescuing GC cells from programmed death. These observations are believed to reflect events that, in vivo, would allow for the selection of centrocytes which have undergone somatic mutation on Ig V-region genes to generate antigen receptor of high affinity. The purpose of the present study was to identify factors capable of modifying the survival pathways of GC cells. Transforming growth factor-beta, at an optimal concentration of 1 ng/ml, was found to inhibit surface immunoglobulin (sIg)-mediated rescue of GC cells but had no influence on survival promoted through CD40. Both routes of rescue were blocked by the glucocorticoid prednisolone at pharmacological concentrations (ID50 = 10(-7) M). Cyclosporin A, an antagonist of sIg-mediated signaling in resting B cells, failed to block rescue of GC cells through either of the receptor-activated pathways. Antibody to CD19--which also suppresses the activation of resting B cells--not only left GC cell rescue undiminished, but rather provided a modest survival signal of its own; interferon-alpha behaved similarly while interferon-gamma failed to influence GC cell survival in either direction.

The promoter of the CD19 gene is a target for the B-cell-specific transcription factor BSAP

The CD19 protein is expressed on the surface of all B-lymphoid cells with the exception of terminally differentiated plasma cells and has been implicated as a signal-transducing receptor in the control of proliferation and differentiation. Here we demonstrate complete correlation between the expression pattern of the CD19 gene and the B-cell-specific transcription factor BSAP in a large panel of B-lymphoid cell lines. The human CD19 gene has been cloned, and several BSAP-binding sites have been mapped by in vitro protein-DNA binding studies. In particular, a high-affinity BSAP-binding site instead of a TATA sequence is located in the -30 promoter region upstream of a cluster of heterogeneous transcription start sites. Moreover, this site is occupied by BSAP in vivo in a CD19-expressing B-cell line but not in plasma or HeLa cells. This high-affinity site has been conserved in the promoters of both human and mouse CD19 genes and was furthermore shown to confer B-cell specificity to a beta-globin reporter gene in transient transfection experiments. In addition, BSAP was found to be the only abundant DNA-binding activity of B-cell nuclear extracts that interacts with the CD19 promoter. Together, this evidence strongly implicates BSAP in the regulation of the CD19 gene.

The promoter of the CD19 gene is a target for the B-cell-specific transcription factor BSAP

The CD19 protein is expressed on the surface of all B-lymphoid cells with the exception of terminally differentiated plasma cells and has been implicated as a signal-transducing receptor in the control of proliferation and differentiation. Here we demonstrate complete correlation between the expression pattern of the CD19 gene and the B-cell-specific transcription factor BSAP in a large panel of B-lymphoid cell lines. The human CD19 gene has been cloned, and several BSAP-binding sites have been mapped by in vitro protein-DNA binding studies. In particular, a high-affinity BSAP-binding site instead of a TATA sequence is located in the -30 promoter region upstream of a cluster of heterogeneous transcription start sites. Moreover, this site is occupied by BSAP in vivo in a CD19-expressing B-cell line but not in plasma or HeLa cells. This high-affinity site has been conserved in the promoters of both human and mouse CD19 genes and was furthermore shown to confer B-cell specificity to a beta-globin reporter gene in transient transfection experiments. In addition, BSAP was found to be the only abundant DNA-binding activity of B-cell nuclear extracts that interacts with the CD19 promoter. Together, this evidence strongly implicates BSAP in the regulation of the CD19 gene.

Activation of human B cells through the CD19 surface antigen results in homotypic adhesion by LFA-1-dependent and -independent mechanisms

Addition of CD19 monoclonal antibodies (mAb) to highly purified tonsillar B cells resulted in homotypic adhesion and the formation of cell clusters. This response was completely blocked by antibody to LFA-1, indicating an LFA-1-dependent adhesion mechanism. In contrast, aggregate formation by B cells activated with phorbol myristate acetate (PMA) was only partially inhibited by anti-LFA-1 antibody, and those formed in response to PMA plus CD19 antibody were not inhibited at all, suggesting aggregation of activated B cells stimulated with CD19 antibody was LFA-1 independent. This was confirmed with B-cell lines. The pre-B-cell line Nalm-6 formed aggregates in response to CD19 antibody which were not inhibited with anti-LFA-1. In addition, CD19 antibody induced aggregate formation by an Epstein-Barr virus (EBV)-transformed B-cell line derived from an LFA-1-deficient donor. These results suggest that different adhesion molecules may operate at different stages of B-cell activation, and that CD19 may be important in cell-cell interactions involved in regulation of antibody responses.

Interleukin 7 receptor ligation stimulates tyrosine phosphorylation, inositol phospholipid turnover, and clonal proliferation of human B-cell precursors

Functional interleukin 7 (IL-7) receptors are expressed on the surface of multiphenotypic, biphenotypic, and immature B-lineage human lymphoid precursor cells with germ-line immunoglobulin heavy-chain genes but not on more mature B-lineage lymphoid cells with rearranged and/or expressed immunoglobulin heavy-chain genes. Thus, IL-7 may have an important regulatory role during the earliest stages of human B-cell ontogeny. The engagement of the surface IL-7 receptors on immature B-cell precursor cells with recombinant human IL-7 (rhIL-7) results in enhanced tyrosine phosphorylation of multiple phosphoproteins, stimulates inositol phospholipid turnover and DNA synthesis, and promotes their clonal proliferation. These effects are (i) specific for rhIL-7, since rhIL-3, rhIL-4, rhIL-5, rhIL-6, and recombinant human granulocyte colony-stimulating factor do not elicit similar activities on IL-7 receptor-positive human pro-B cells; and (ii) mediated by IL-7 receptors, since they are not observed in IL-7 receptor-negative B-lineage lymphoid cell populations. rhIL-7-induced tyrosine phosphorylation on the 35-, 53-, 55-, 62-, 69-, 76-, 94-, 150-, 170-, and 190-kDa substrates as well as rhIL-7-induced stimulation of inositol phospholipid turnover are abrogated by the tyrosine kinase inhibitor genistein. These results demonstrate that the IL-7 receptor on immature human B-cell precursor populations is intimately linked to a functional tyrosine kinase pathway and tyrosine phosphorylation is an important and perhaps mandatory step in the generation of the IL-7 receptor-linked transmembrane signal.

Inhibition of B cell proliferation with anti-CD19 monoclonal antibodies: anti-CD19 antibodies do not interfere with early signaling events triggered by anti-IgM or interleukin 4

The 95-kDa antigen recognized by the anti-CD19 panel of monoclonal antibodies is found on the surface of most cells of the B cell lineage. Anti-CD19 antibodies inhibit B cell proliferation in response to anti-Ig plus interleukin 4 (IL4), but enhance the response to mitogenic concentrations of either phorbol 12-myristate 13-acetate (PMA) or Epstein-Barr virus. This dichotomy in the effect of anti-CD19 antibodies suggested that the inhibitory action may be directed at the transmembrane signaling pathways utilized by anti-IgM and IL4. To investigate this hypothesis, an attempt was made to determine the mechanism of signal transduction utilized by the CD19 antigen, and elucidate its effect on transmembrane signaling invoked by anti-immunoglobulin and IL4. Binding of anti-CD19 antibody to B cells did not promote activation of either the phosphoinositide or cAMP signaling pathways. In addition, anti-CD19 antibody did not inhibit phosphatidylinositol bisphosphate (PIP2) hydrolysis induced by anti-IgM or IL4, nor did it interfere with cAMP induction by IL4. We also found that anti-CD19 antibody inhibited PMA plus calcium ionophore-induced B cell proliferation. This evidence indicates that anti-CD19 mAb interrupts the signaling cascade at a point distal to receptor-mediated breakdown of PIP2 and/or activation of adenyl cyclase. This conclusion was fully consistent with experiments in which anti-CD19 antibody was shown to inhibit DNA but not RNA synthesis, and the observation that anti-CD19 antibody must be present between 6 h and 20 h after the initiation of the culture suggesting that anti-CD19 mAb exerts its inhibitory effect in late G0 or G1, after the initial signaling events.

B-cell monoclonal antibodies and their use in clinical oncology

The use of MAbs directed against B-cell markers has identified considerably more heterogeneity within B-cell neoplasms than was evident by standard morphologic and histochemical techniques. Using markers specific for lineage and state of differentiation, it is possible to correlate malignant B cells to their normal cellular counterparts. Considering the complexity of normal B-cell ontogeny, differentiation, and function, it is not surprising that these malignancies reflect this diversity. Hopefully, with increasing characterization of the normal function of cell surface molecules, as well as the subpopulations of normal cells to which these malignancies correspond, we will have a better understanding of the biologic and clinical behavior of these malignancies.

Signaling through CD19, Fc receptors or transforming growth factor-beta: each inhibits the activation of resting human B cells differently

To understand further the roles that negative regulatory signals may play in B cell immune responses, we compared three inhibitors of B cell proliferation: cross-linking CD19 with monoclonal antibody (mAb), signaling through Fc receptors by intact anti-mu mAb, and transforming growth factor-beta (TGF-beta). Each agent was tested for its ability to block proliferation and specific activation events induced in human tonsilar B cells activated by either cross-linking surface immunoglobulin, signaling through CD20, or direct activation of protein kinase C (PKC) by phorbol 12-myristate 13-acetate. We found that each inhibitor was functionally distinct. Both anti-CD19 mAb and anti-mu mAb inhibited anti-immunoglobulin activated cells and anti-CD20-activated cells, but neither inhibited cells activated by phorbol 12-myristate 13-acetate. TGF-beta, on the other hand, inhibited equally profoundly cells activated by each of the three regimens. These results suggest that TGF-beta blocks B cell activation at a step following the activation of PKC, whereas both signaling through CD19 and Fc receptor block early steps in the PKC activation pathway. Signaling through anti-CD19 mAb was unique in that proliferation of anti-immunoglobulin-activated cells was reduced on day 3 and then augmented subsequently. With all other inhibitory combinations the block was permanent. We conclude that each of these three inhibitors has unique important functions and therefore suggest that the effectiveness of negative signaling in B cell immune regulation will depend on the combinations of specific inhibitors modulating a specific activation program.

Functional properties of CD19+ B lymphocytes positively selected from buffy coats by immunomagnetic separation

Here we report that human B lymphocytes can be positively selected directly from buffy coats applying the anti-CD19 antibody AB1 coupled to magnetic beads. This isolation protocol is highly efficient and the isolated cell population is of very high purity and viability. As judged by cell cycle analysis and various parameters for cell activation, the cells are still in a resting state after isolation. Furthermore, different functional assays have shown that the isolation procedure does not interfere with either activation or proliferation/differentiation of CD19 selected cells as compared to negatively isolated cells. As a consequence of cross-linking during the isolation process, the CD19 antigen is temporarily down-regulated as measured by AB1 binding. Despite this decreased expression, monoclonal antibodies to the CD19 antigen nevertheless inhibited anti-mu plus B cell growth factor induced B cell activation as reported also for negatively isolated cells. Taken together, the presented data strongly suggest that B cells isolated through the CD19 antigen can be used in critical functional assays.

Detection and characterization of human high molecular weight B cell growth factor receptors on leukemic B cells in chronic lymphocytic leukemia

Human high molecular weight-B cell growth factor (HMW-BCGF) (60 kD) stimulates activated normal B cells, B cell precursor acute lymphoblastic leukemia (BCP-ALL) cells, hairy cell leukemia (HCL) cells, prolymphocytic leukemia (PLL) cells, and chronic lymphocytic leukemia (CLL) cells. The expression of human high molecular weight B cell growth factor (HMW-BCGF) receptors (R) on clonal populations of leukemic B cells in CLL was studied by ligand binding assays using 125I-labeled HMW-BCGF as well as by immunofluorescence/flow cytometry and Scatchard analyses using an anti-HMW-BCGF R monoclonal antibody (MAb), designated BA-5. There was a high correlation between HMW-BCGF R expression and responsiveness to HMW-BCGF. 60% of CLL cases constitutively expressed HMW-BCGF R and showed a marked proliferative response to HMW-BCGF in [3H]TdR incorporation assays as well as colony assays. Similarly, HCL cells, PLL cells, and activated normal B cells expressed functional HMW-BCGF R, as determined by ligand binding assays using 125I-HMW-BCGF, [3H]TdR incorporation assays, and reactivity with BA-5 MAb. Scatchard analyses indicated the existence of approximately 3,000 HMW-BCGF R/cell on HMW-BCGF responsive CLL cells with an apparent Ka value of 4.6 X 10(7) M-1. The concentrations of HMW-BCGF required for maximum stimulation of CLL cells were two to three orders of magnitude lower than those needed for half maximal receptor occupancy, indicating that only a small fraction of HMW-BCGF R need to be occupied to stimulate leukemic CLL B cells. Crosslinking of surface bound 125I-HMW-BCGF (60 kD) with the bivalent crosslinker DTSSP to its binding site on fresh CLL cells identified a 150-kD HMW-BCGF/HMW-BCGF R complex, suggesting an apparent molecular weight of 90 kD for the receptor protein. The growth stimulatory effects of HMW-BCGF on clonogenic CLL cells did not depend on accessory cells or costimulant factors. The anti-HMW-BCGF R monoclonal antibody BA-5 disrupted HMW-BCGF/HMW-BCGF R interactions at the level of clonogenic CLL cells and inhibited HMW-BCGF-stimulated CLL colony formation in vitro. To our knowledge, this study represents the first detailed analysis of expression, function, and structure of HMW-BCGF R on B lineage CLL cells.

Signal transduction via MHC class II antigens on B lymphocytes

The role of the MHC class II antigens in the activation of resting human B lymphocytes (B-Go) was examined with respect to both early and late events in the activation process. The (Ca2+)i induced by anti-IgM was enhanced in the presence of, or following pre-incubation with, an anti-MHC class II DR antibody (D1.12). Pre-incubation with a sepharose conjugated antibody (Seph.-D1.12) augmented the proliferation of B-Go in response to a sub-optimal concentration of anti-IgM. The 2D PAGE profile of B-Go differed from that of in vivo activated B lymphocytes. The 2D PAGE profile of B-Go activated by Seph.-D1.12 was not identical to the profile of B-Go activated by either anti-IgM or PMA. These data suggest that the activation of B-Go via the class II antigens shares part of the pathway of anti-IgM induced activation but does not follow an identical pathway.

Differential responsiveness of human B lymphocytes to phorbol ester and calcium ionophore based on their state of activation

For tonsil B cells of a particular high density (below 65% Percoll), both phorbol myristate acetate (PMA) (5 ng/ml) and calcium ionophore A23187 (500 nM) were required to induce RNA synthesis, significant DNA synthesis also occurring in the presence of 12,000 MW B-cell growth factor (BCGF). In contrast, PMA alone, even at 1 ng/ml, was a sufficient stimulus to induce strong DNA synthesis in low-density B cells (45-50% Percoll) and strong proliferative responsiveness to BCGF in intermediate-density B cells (55-65% Percoll). In these latter B-cell populations, A23187 (500 nM), acted synergistically with non-mitogenic PMA doses to induce strong DNA synthesis, the PMA dose required being 5-50 times lower in low-density B cells (0.1-1 ng/ml) than in intermediate-density B cells (5 ng/ml). Preactivation for 30 hr with anti-Ig antibodies plus BCGF, known to drive B cells into late G1, rendered high-density B cells responsive to PMA (1-10 ng/ml) with high, dose-related DNA synthesis. These data indicate that the B-cell mitogenicity of a given nanomolar dose of PMA depends on the more advanced state of activation of B cells. It was also found that the above optimal dose of A23187 (500 nM) paradoxically inhibited the PMA-induced DNA synthesis of low-density B cells and in vitro preactivated high-density B cells. Data obtained with low-density B cells suggest that a calcium influx during the PMA-induced proliferative phase of B cells may provide a negative signal for the DNA synthesis.

Activation of the phosphatidylinositol metabolic pathway by low molecular weight B cell growth factor

The possible role of phosphatidylinositol breakdown in the induction of proliferation of human activated B cells by low molecular weight B cell growth factor (LMW-BCGF) was examined. LMW-BCGF was found to induce a rapid rise in the concentration of inositol trisphosphate (InsP3) in [3H]inositol-loaded B cell blasts, obtained by prior anti-mu antibody activation. A concomitant decrease in the concentration of phosphatidylinositol 4,5-bisphosphate could be detected at the same time. Maximum generation of InsP3 occurred within 15-30 s after the addition of the LMW-BCGF ligand to the activated B cells, then was followed by a slow decrease and return to control values. The amount of InsP3 generated by phosphatidylinositol hydrolysis was dependent on the concentration of LMW-BCGF. This effect was only detected in B cells already preactivated by a first signal such as anti-mu antibody and not in resting unstimulated B cells. In contrast, under similar conditions, interleukin 2, another B cell growth-promoting lymphokine, did not alter the rate of formation of the various phosphatidylinositol breakdown products. An augmentation of the [Ca2+]i concentration was also detected in activated B cells upon addition of LMW-BCGF and this increase could be blocked by TMB-8, a specific inhibitor of endoplasmic reticulum calcium release. Hydrolysis of phosphoinositides thus represents an essential component in the mechanism of transduction of the signal provided by LMW-BCGF.

Mitogenic stimulation of human B lymphocytes via the inositol phospholipid 'dual pathway' of signalling requires persistent activation of both second messenger arms

Highly purified resting tonsillar B lymphocytes mount an efficient mitogenic response on exposure to appropriate combinations of tumour-promoting phorbol esters and calcium ionophores, agents that mimic the biochemical second messengers generated through the cross-linking of surface immunoglobulins. By using agents that bind reversibly we show here that both signals need to be applied continuously in order for cells to proceed optimally to DNA synthesis. The data are consistent with the notion that, in the absence of 'help' from T lymphocytes or antigen-presenting cells, a chronic, persistent stimulation through antigen receptors is necessary to elicit a significant B-cell response.

Immunobiologic differences between normal and leukemic human B-cell precursors

The early stages of normal human B-cell differentiation were studied by flow cytometry and cell sorting based on expression of CD10 (CALLA) and CD19 antigens in fetal liver. Both CD10+ CD19+ and CD10+ CD19- precursor populations proliferated in vitro to form B-cell precursor colonies under stimulation from low molecular weight B-cell growth factor (L-BCGF) or recombinant interleukin 3 but did not respond to high molecular weight B-cell growth factor (H-BCGF). The colonies derived from the CD10+ CD19- fraction showed induction of CD19 expression in 10-50% of growing cells, suggesting that CD10 expression precedes CD19 expression in B-cell ontogeny. This hypothesis was corroborated by less-differentiated marker profiles of the progeny of CD10+ CD19- B-cell precursors as compared to CD10+ CD19+ B-cell precursors in BCGF-stimulated cultures and by higher percentages of CD10+CD19- versus CD10-CD19+ B-cell precursors. CD19 crosslinking on normal fetal liver or bone marrow B-cell precursors was associated with an increase in cytoplasmic calcium concentration, but was inhibitory for colony formation. Leukemic B-cell precursors from acute lymphoblastic leukemias (ALLs) differed from normal B-cell precursors in their in vitro proliferative responses, since (i) they responded not only to L-BCGF and rIL-3 but also to H-BCGF and (ii) their proliferation was stimulated rather than inhibited by CD19 crosslinking. A clonogenic leukemic counterpart for the CD10+CD19- normal B-cell precursor population does not exist among malignant cells from B-cell precursor ALL patients, suggesting that the CD19 receptor may be involved in leukemogenesis of human B-cell precursor ALL.

Interleukin 4 and soluble CD23 as progression factors for human B lymphocytes: analysis of their interactions with agonists of the phosphoinositide "dual pathway" of signalling

Human B lymphocytes pre-activated for 24 h with a combination of phorbol dibutyrate [P(Bu)2] and ionomycin were found to provide excellent targets for assessing the detailed action of B cell progression factors. Both recombinant interleukin 4 (IL 4) and affinity-purified 25-kDa fragment of the CD23 molecule (sol-CD23) were shown to be active in this assay. While the progression activity of IL 4 was enhanced by continued co-culture with P(Bu)2, that of sol-CD23 was found to be more strictly dependent upon such a joint application with the phorbol ester. Similar requirements were observed for triggering cell-cycle progression in the pre-activated B cells when using a stimulating CD23 antibody. Ionomycin, in contrast to P(Bu)2, did not augment either IL 4 or sol-CD23 in these assays but did enhance significantly the progression activity of an anti-CDw40 antibody. When added to B cells concomitantly with, or prior to, a high dose of phorbol ester, IL 4 unexpectedly down-regulated the subsequent mitogenic response to this agent whereas, when added 24 h later, IL 4 up-regulated such stimulations. The latter sequence of additions resulted in a particularly dramatic induction of CD23 at the B cell surface, much more so than seen when B cells were incubated with either IL 4 alone or with IL 4 and P(Bu)2 together. This up-regulation of surface CD23 was, in turn, mirrored by the appearance of large amounts of the soluble form of the molecule in such cultures. The findings are discussed with reference to possible mechanisms through which IL 4 and CD23 interact to exert their multiple actions on B cell regulatory pathways.