Cellular signalling mechanisms in B lymphocytes

Sequential administration of GM-CSF (Sargramostim) and IL-2 +/- autologous vaccine as adjuvant therapy in cutaneous melanoma: an interim report of a phase II clinical trial

Granulocyte-macrophage colony-stimulating factor (GM-CSF) and interleukin-2 (IL-2) are 2 cytokines with distinct mechanisms of action that complement one another in the adjuvant management of melanoma. Forty-five patients with high-risk melanoma were enrolled in an open-label, single-arm, phase II clinical trial to examine the safety, tolerability, and effectiveness of this combination. After potentially curative surgery, each patient received 12 months of GM-CSF 125 microg/m2/d subcutaneously (SC) for 14 days followed by IL-2, 9 million IU/m2/d SC for 4 days (given every other cycle from months 7-12), followed by 10 days of no treatment. In addition, patients who had tumors yielding an adequate number of live cells received autologous melanoma vaccines. For months 13-24, patients received only GM-CSF 250 microg/m2 twice weekly. This is an interim analysis based on the 45 enrolled patients with a median of 15.9 months follow-up (range, 1-50 months). Thirty-two patients are alive: 9 of 13 with stage IV resected melanoma, 16 of 25 with stage III disease, and 7 of 7 with stage II disease. Twelve died of the disease, and one due to stroke. Adjuvant use of sequential GM-CSF and IL-2 +/- autologous vaccine was well tolerated with good patient compliance and seemed to benefit high-risk patients with surgically resected melanoma.

Signal transduction of somatostatin in human B lymphoblasts

Somatostatin (SST) and somatostatin receptors (SSTR) are widely distributed in lymphoid tissues. Here, we report on the stimulatory effects of SST in Epstein-Barr virus-immortalized B lymphoblasts. By RT-PCR, we demonstrated the exclusive expression of the somatostatin receptor isoform 2A (SSTR2A) in B lymphoblasts. Addition of SST rapidly increased the cytosolic free calcium concentration [Ca(2+)](i) maximally by about 200 nM, with an EC(50) of 1.3 nM, and stimulated the formation of inositol phosphates. Furthermore, SST increased binding of guanosine 5'-O-(3-thiotriphosphate) by 50% above basal. These effects were partly inhibited by pertussis toxin (PTX), which indicates the involvement of PTX-sensitive G proteins. We provide further evidence that Galpha(16,) a PTX-insensitive G protein confined to lymphohematopoietic cells, is involved in the otherwise unusual coupling of SSTR2A to phospholipase C activation. In addition, SST activated extracellular regulated kinases and induced a 3.5-fold stimulation of DNA synthesis and a 4.4-fold stimulation of B lymphoblast proliferation, which was accompanied by an enhanced immunoglobulin formation. Thus SST exerts a growth factor-like activity on human B lymphoblasts.

Characterization of the transcriptional repressor RBP in Epstein-Barr virus-transformed B cells

RBP, a transcriptional repressor, is intricately involved in Epstein-Barr virus (EBV) transformation of human B cells. The EBV nuclear proteins EBNA-2, -3, -4 and -6 all utilize RBP to regulate the transcription of both cellular and viral genes. This study investigates the isoforms of the RBP protein in Burkitt's lymphoma (BL) cells and in EBV-transformed lymphoblastoid cell lines (LCLs). Two-dimensional gel electrophoresis showed the presence of two different cellular isoforms of RBP; the molecular masses and isoelectric points of these two isoforms corresponded to RBP-Jkappa and RBP-2N. Fractionation studies and green fluorescent protein (GFP)-tagged expression studies demonstrated that both RBP isoforms were located predominantly in the cell nucleus. Interestingly, GFP-tagged RBP-Jkappa showed diffuse, uniform nuclear staining, whereas GFP-tagged RBP-2N showed a discrete nuclear pattern, demonstrating differences between the two isoforms. Within the nuclear fraction of EBV-negative BL cells, RBP existed both in a free form and bound to chromatin, whereas in LCLs the intranuclear RBP was predominantly chromatin-bound. Expression of the EBV latent proteins was found to lead to the sequestering of RBP from the cytoplasm into the cell nucleus and to an increase in the chromatin-bound forms of RBP.

Growth factor-like action of lysophosphatidic acid on human B lymphoblasts

Proliferation and immunoglobulin secretion of B lymphocytes are regulated by specific antigens and numerous accessory immunomodulatory factors. Lysophosphatidic acid (LPA) is a glycerophospholipid mediator that is released from activated blood platelets, attains high levels in serum, and exerts potent stimulatory effects on, e.g., neutrophils, monocytes, and T lymphocytes. LPA is also generated by a secretory, cytokine-inducible phospholipase A2 present in high concentrations in inflammatory exudates and septic states. We investigated effects of LPA on human Epstein-Barr virus-immortalized B lymphoblasts, a model for immunoglobulin-secreting B cells. Intracellular Ca2+ was determined with fura 2 and the formation of inositol 1,4, 5-trisphosphate by anion-exchange chromatography. LPA stimulated an increase in inositol 1,4,5-trisphosphate levels and induced a transient rise in intracellular free Ca2+ concentration from 105 +/- 17 to 226 +/- 21 nM. This Ca2+ signal resulted from Ca2+ mobilization and Ca2+ influx and was subject to homologous desensitization. Pertussis toxin inhibited these responses by approximately 70%. Furthermore, LPA stimulated a 27.5% increase in guanosine 5'-O-(3-thiotriphosphate) binding to permeabilized B lymphoblasts, which suggests the direct activation of pertussis toxin-sensitive G proteins by LPA. LPA stimulated a strong increase in the specific phosphorylation of the mitogen-activated protein kinase (immunoblot analysis) that was prevented by the MEK inhibitor PD-98059. Finally, LPA triggered a 2-fold increase in DNA synthesis ([3H]thymidine incorporation) and a 2-fold increase in B lymphoblast number and evoked a 20- to 50-fold increase in immunoglobulin formation. By RT-PCR we detected specific mRNA transcripts for the recently cloned human LPA receptor. Thus our data suggest that LPA behaves as a B cell growth factor.

The role of the Fgr tyrosine kinase in the control of the adhesive properties of U937 monoblastoid cells and their derivatives

In humans, expression of the cellular proto-oncogene c-fgr is normally restricted to mature cells of the myeloid lineage, mantle zone B cells and various myeloid and B-cell lines. Previous studies of the monoblastoid cell line, U937, showed that c-fgr expression increased following differentiation, but its role in monocytes and related cells has not been defined in functional terms. We therefore investigated the role of c-fgr in U937 cells transfected with the c-fgr gene such that its expression could be manipulated independent of differentiation. Induction of the transfected c-fgr gene by cadmium ions did not affect cell proliferation, responses to phorbol 12-myristate 13-acetate (PMA), dihydroxycholecalciferol (DHCC), tumour necrosis factor-alpha (TNF-alpha) or retinoic acid, or phagocytosis of antibody-coated sheep red blood cells. However, there was increased surface expression of CD54 (intracellular adhesion molecule-1; ICAM-1) and CD102 (ICAM-2) and decreased surface expression of CD50 (ICAM-3) compared with cells that had been transfected with plasmid only and treated in the same way. These findings suggest that the product of the c-fgr gene may be important in control of relative adhesive properties of mature monocytic cells.

Pertussis toxin pretreatment alters the in vivo cell division behaviour and survival of B lymphocytes after intravenous transfer

Pertussis toxin (PT), produced by the causative agent of whooping cough, Bordetella pertussis, contributes to the immune dysfunction seen in infected patients. Treatment of laboratory animals with purified toxin reproduces many of the biological effects exhibited in the disease state, which include lymphocytosis, adjuvant effects for IgE secretion and delayed-type hypersensitivity reactions. In previous studies, we have demonstrated that PT pretreatment of intravenously transferred lymphocytes not only results in them being held up in the blood, but also causes a profound alteration in their positioning within the spleen. Pertussis toxin pretreated lymphocytes fail to traverse the layer of marginal zone macrophages encircling the white pulp, resulting in their exclusion from the lymphoid area of the spleen. Using a novel flow cytometric assay of cell division, the studies presented here show that a significant proportion of B, but not T, lymphocytes underwent proliferation after intravenous transfer of donor splenic lymphocytes to syngeneic recipients. This proliferation was markedly reduced by PT pretreatment of lymphocytes before transfer. In contrast, the in vitro proliferative responses of B lymphocytes to anti-IgM, LPS and antibody engagement of CD40 were unimpaired by exposure to the same levels of PT. Furthermore, the rate of in vivo decay of transferred B cells was accelerated by pretreatment with PT. Together, these data suggest PT impairs the receipt of signals which promote survival and proliferation of B cells, due to altered recirculation and positioning of lymphocytes.

Regulatory elements of the mb-1 gene encoding the Ig-alpha component of the human B-cell antigen receptor

The mb-1 gene encodes the Ig-alpha component of the B-cell antigen receptor. It is specifically expressed in pre-B and mature B cells but not in plasma cells losing membrane Ig (mIg) expression. We looked for transcriptional regulatory elements within a 12 kb genomic fragment. A strong promoter activity was found in a 591 bp fragment harboring consensus binding sites for known transcription factors including Ets, EBF/BlyF, LyF1/micro B and Spl. It was able to drive transcription of a reporter gene in the absence of any additional enhancer and was mostly active in B lymphocytes not in plasma cells or T cells. Although no fragment from the mb-1 gene displayed enhancer activity in combination with either the SV40, a Ig VH or a Ig VL promoter, a 1078 bp fragment corresponding to the 5' part of the gene behaved as a strong enhancer in either orientation in constructs driven by the mb-1 promoter itself. Deletions within this fragment allowed to delineate shorter sequences with enhancer activity upstream the first exon. The tissue-restricted, promoter-restricted and stage-specific activity of this 5' flanking region suggests that it is the main regulatory element of the mb-1 gene.

Anti-immunoglobulin M activates nuclear calcium/calmodulin-dependent protein kinase II in human B lymphocytes

We and others have previously shown that the nuclear protein, Ets-1, is phosphorylated in a calcium-dependent manner after ligation of immunoglobulin (Ig) M on B lymphocytes. As this phosphorylation was independent of protein kinase C activity, we tested whether a calcium/calmodulin-dependent protein kinase (CaM kinase) might phosphorylate the Ets-1 protein after elevation of intracellular free calcium concentrations. The dephosphorylated form of Ets-1 has been shown to bind to chromatin, suggesting that the operative kinase should be detectable in the nucleus. We prepared nuclear extracts from two human B cell lines in which increased intracellular free calcium levels correlated with increased phosphorylation of the Ets-1 protein. Activity of the CaM kinases was determined using a synthetic peptide substrate both in the absence and presence of an inhibitor specific for the CaM kinase family, KN-62. Stimulation of cells with anti-IgM led to increased activity of a nuclear kinase that could phosphorylate the peptide, and this activity was reduced by 10 microM KN-62. Kinase activity was reduced in lysates preadsorbed using an antibody specific for CaM kinase II. Two-dimensional phosphopeptide maps of the Ets-1 protein from cells incubated with ionomycin or anti-IgM contained two unique phosphopeptides that were absent in untreated cells. Incubation of isolated Ets-1 protein with purified CaM kinase II produced phosphorylation of peptides that migrated identically to those found in cells incubated with either anti-IgM or ionomycin. These data suggest a model of signal transduction by the antigen receptor on B lymphocytes in which increased intracellular free calcium can rapidly activate nuclear CaM kinase II, potentially resulting in phosphorylation and regulation of DNA-binding proteins.

Synergy between signal transduction pathways is obligatory for expression of c-fos in B and T cell lines: implication for c-fos control via surface immunoglobulin and T cell antigen receptors

Expression of the protooncogene c-fos is controlled by three main regulatory pathways involving kinase C, cAMP, and calcium. Kinase C mediates its effects via phosphorylation of serum response factor (SRF) which interacts with the serum response element (SRE); cAMP and calcium mediate their effects via phosphorylation of CREB (cAMP regulatory element binding protein) presumably by activation of a protein kinase A or calmodulin-regulated kinase. We have examined the function of these elements in Burkitt's lymphoma cells (Ramos and Daudi) as well as a T lymphocytic cell line (Jurkat). We have found that stimulation of any one of these pathways alone has little or no effect on c-fos induction. However, kinase C activation (PMA stimulation) combined with either cAMP (forskolin plus MIX) or calcium stimulation (ionophore) leads to greatly enhanced c-fos induction. By contrast, cAMP in the presence of calcium shows no synergy in c-fos induction. Okadaic acid augments PMA- as well as calcium-mediated activation of c-fos, and has little or no effect when combined with cAMP. The main difference between Ramos (B cells) and Jurkat (T cells) in the regulation of c-fos is that cAMP plus calcium is strongly synergistic in Jurkat and is without effect in Ramos. Analysis of AP-1 activity using gel mobility shift assays confirms that the requirements for synergy in c-fos mRNA induction are paralleled by requirements for synergy in induction of AP-1 activity. Signaling in B cells due to anti-Ig stimulation involves both kinase C activation and release of intracellular calcium, and results in c-fos mRNA induction. Our results indicate that synergy between the kinase C activation and calcium is needed for efficient c-fos induction since neither of these two alone induces c-fos well. That synergy of signaling pathways is relevant for the anti-Ig induction of c-fos is supported by the fact that cAMP-inducing agents and okadaic acid further enhance anti-Ig induction of c-fos. These results suggest that cell-specific patterns of synergy are an essential feature for c-fos induction and may be relevant for c-fos control through B and T cell antigen receptors.

Enhanced G protein activation in immortalized lymphoblasts from patients with essential hypertension

Epstein-Barr virus-immortalized B lymphoblasts obtained from hypertensive patients with enhanced Na+/H+ exchanger activity (HT cells) proliferate distinctly faster upon serum stimulation than those from normotensive controls with low exchanger activity (NT cells) (Rosskopf, D., E. Frömter, and W. Siffert. 1993. J. Clin. Invest. 92:2553-2559). Stimulation with platelet-activating factor (PAF) as well caused an enhanced proliferation of HT cells. In analyzing possible differences in signal transduction between the immortalized NT and HT lymphoblasts, we observed that cell stimulation with PAF and somatostatin caused a twofold higher increase in [Ca2+]i in HT than in NT cell lines. This difference was completely abrogated by pertussis toxin (PTX) treatment. Furthermore, PAF-stimulated formation of inositol 1,4,5-trisphosphate (IP3) was twofold enhanced in HT cell lines. On the other hand, PAF receptor density and affinity, total cellular phospholipase C activity, expression of PTX-sensitive G proteins, and control binding of the stable GTP analogue, guanosine 5'-[gamma-thio]triphosphate (GTP gamma S), to membrane G proteins were not different in NT and HT cell lines. However, PAF- and mastoparan-stimulated binding of GTP gamma S to G proteins, which was fully PTX-sensitive, was 2.5-fold higher in HT than NT cell lines. These data suggest an enhanced receptor-mediated activation of PTX-sensitive G proteins despite unchanged receptor and G protein expression. Thus, this study not only suggests that enhanced signal transduction and cell proliferation are abnormalities in a certain group of patients with essential hypertension but also explains these findings as a result of an enhanced G protein activation in this common disorder.

A role for second messengers in the control of activation-associated modification of the surface of Trichinella spiralis infective larvae

The involvement of second messengers in the control of activation-induced changes to the surface of Trichinella spiralis infective larvae was investigated using membrane-permeant photo-activatable 'caged' compounds to alter intracellular levels of inositol trisphosphate (IP3), calcium ions (Ca2+) and cyclic AMP (cAMP). Activation of larvae by incubation in culture medium containing trypsin and bile was followed by the loss of the surface coat labelled with the fluorescent PKH26 lipid probe and this correlated with the reciprocal acquisition of surface lipophilicity detected using the fluorescent lipid probe octadecanoyl aminofluorescein (AF18). Optimal surface coat shedding and AF18 insertion was also achieved following photolysis of caged mediators liberating IP3, Ca2+ or cAMP within the parasite. Chelation of Ca2+, however, abolished the effects of larval activation. Nevertheless, addition of cAMP (but not IP3) to Ca(2+)-depleted larvae overcame this inhibition and restored AF18 insertion to levels achieved by activated parasites. Therefore, the existence of a linear second messenger pathway involving the sequential release of IP3, Ca2+ and then cAMP is likely.

Interaction of signaling molecules with human Fc gamma RIIb1 and the role of various Fc gamma RIIb isoforms in B-cell regulation

The low-affinity type-IIb IgG Fc-binding receptors (Fc gamma RIIb) are expressed on B cells. When cross-linked with mIgM Fc gamma RIIb are known to down-regulate B-cell activation by interrupting signal transduction upstream from G-protein-activated events. We have studied Fc gamma RII isoforms expressed on resting and activated B cells and the interaction of Fc gamma RIIb1 with molecules transducing the antigen receptor-mediated signals. Expression of Fc gamma RII isoforms was studied by reverse transcription and polymerase chain reaction. Resting B cells express both Fc gamma RIIb2 and Fc gamma RIIb1 isoforms. Activation with anti-IgM or IL-4 induces the splicing of Fc gamma RIIb1 mRNA, while the alternative splicing of Fc gamma RIIb2 mRNA is down-regulated, resulting in the surface expression of Fc gamma RIIb1. Functional differences were found between the two isoforms in inhibiting B-cell activation, suggesting that Fc gamma RIIb2 might influence the threshold of signals necessary for activation of resting B cells, while Fc gamma RIIb1 may regulate in later phases of antibody response. To explore the mechanism by which Fc gamma RII may uncouple antigen receptor-mediated signal transduction, we have investigated the association of signaling molecules with Fc gamma RII. Beside the protein tyrosine kinase (PTK) fyn, protein kinase C (PKC) was found to be co-isolated with Fc gamma RIIb1, suggesting a tight connection between these kinases and Fc gamma RII. We suggest that PKC might be responsible for the activation-induced phosphorylation of Fc gamma RII on serine residues.(ABSTRACT TRUNCATED AT 250 WORDS)

The alternative splicing of human Fc gamma RII mRNA is regulated by activation of B cells with mIgM cross-linking, interleukin-4, or phorbolester

The human type two IgG binding receptors (Fc gamma RII) are encoded by three genes (Fc gamma RIIA, -B and C) resulting in at least six protein isoforms generated by alternative mRNA splicing. Surface expression of Fc gamma RII has been shown to be modulated during B cell activation, although data characterizing the isoform(s) expressed are not available. The extracellular as well as the transmembrane domains of various Fc gamma RII are highly homologous. Only the intracellular domains vary between the different Fc gamma RII isoforms, suggesting differences in signal transduction. Using reverse transcriptase and polymerase chain reaction of mRNA obtained from resting tonsil B cells, we show that the majority of Fc gamma RII mRNA species to be of b2 type, although b1 type and a low level of Fc gamma RIIa type are also present. Culturing the cells for 18 h in the presence of 2.5 U/ml interleukin-4 or 10 micrograms/ml affinity-purified anti-IgM F(ab')2 fragments induced a switch in alternative splicing, resulting in a significant increase of Fc gamma RIIb1 mRNA expression, while the synthesis of Fc gamma RIIb2 mRNA was down-regulated. Stimulation of B cells with 100 ng/ml phorbol 12-myristate 13-acetate induced similar alteration, although only after 48-h treatment. The accumulation of Fc gamma RIIb1 and the reduction of both Fc gamma RIIb2 and Fc gamma RIIa mRNA in activated cells is accompanied by the enhanced expression of Fc gamma RII on the cell surface, representing most probably the Fc gamma RIIb1 isoform. Heat-aggregated IgG inhibited the anti-IgM-induced proliferation of resting but not that of activated B cells, suggesting that aggregation of Fc gamma RIIb2 constitutively expressed on resting B cells might be responsible for the prevention of inadequate activation of resting B cells.

The interleukin-2 and interleukin-4 receptors studied by molecular modelling

BACKGROUND

Interleukin-2 (IL2) and interleukin-4 (IL4) are members of the four-helix bundle family of cytokines, whose receptors show similarity to each other and to the growth hormone receptor fold. These proteins help to control, among other things, the rate of clonal expansion of lymphocytes, and thus play an important role in the regulation of the immune system. They are therefore of interest as transmembrane signalling proteins, as well as potential pharmaceutical targets.

RESULTS

We have modelled structures of the extracellular components of the IL2 and IL4 receptors based on the structure of the complex of human growth hormone with its receptor, and incorporating the recently discovered shared gamma c chain. The models provide possible explanations for several experimental observations, including those from site-directed mutagenesis around the binding sites. Receptor residues that may be close to important side chains on IL2 and IL4 are identified and possible effects of their mutation are discussed. A comparison is made between the models and the growth hormone complex, and between the gamma c chain bound to IL2 and to IL4.

CONCLUSIONS

The models offer structural explanations for observed behaviour such as the effects of mutation of the A- and D-helices of the cytokines. In addition, they may be of use in the identification of residues which may interact in the ligand-receptor interfaces, and which would therefore be worthy of further investigation.

Epstein-Barr virus, infectious mononucleosis, and posttransplant lymphoproliferative disorders

PTLD may be considered as an "opportunistic cancer" in which the immunodeficiency state of the host plays a key role in fostering the environment necessary for abnormal lymphoproliferation. The following discussion reflects our own current thoughts regarding events which may result in PTLD and its sequelae. Many of the individual steps have not been rigorously proved or disproved at this point in time. Following transplantation and iatrogenic immunosuppression, the host:EBV equilibrium is shifted in favor of the virus. Most seronegative patients will become infected either via the graft or through natural means; seropositive patients will begin to shed higher levels of virus and may become secondarily superinfected via the graft. There is a "grace" period of approximately one month posttransplant before increased viral shedding begins. PTLD is almost never seen during this interval. In many cases infection continues to be silent whereas in rare individuals there is an overwhelming polyclonal proliferation of infected B lymphocytes. This is the parallel of infectious mononucleosis occurring in patients with a congenital defect in virus handling (X-linked lymphoproliferative disorder). It is possible that transplant patients with this presentation also suffer a defect in virus handling. In other cases excessive iatrogenic immunosuppression may paralyze their ability to respond to the infection. With CsA and FK506 regimens, individual tumors may occur within a matter of months following transplant. The short time of incubation suggests that these are less than fully developed malignancies. It may be that local events conspire to allow outgrowth of limited numbers of B-lymphocyte clones. A cytokine environment favoring B-lymphocyte growth may be one factor and differential inhibition by the immuno-suppressive drugs of calcium-dependent and -independent B-cell stimulation may be another. In addition, there is some evidence that CsA itself may inhibit apoptosis within B cells. Since most patients do not develop PTLDs, an additional signal(s) for B-cell stimulation may be required. Indeed, it is possible that the virus may simply serve to lower the threshold for B-cell activation and/or provide a survival advantage to these cells. The ability of individual cell clones to evade a weakened immune system may set into play a Darwinian type of competition in which the most rapidly proliferating cells with the least number of antigenic targets predominate. In this regard, differences in host HLA types may determine the repertoire of viral antigens which are subject to attack.(ABSTRACT TRUNCATED AT 400 WORDS)

X-linked agammaglobulinemia--gene cloning and future prospects

The btk gene has recently been identified as the causative gene in X-linked agammaglobulinemia (XLA). This has opened up many new possibilities for the treatment of this B-cell immunodeficiency. Christine Kinnon and colleagues review the high degree of sequence of homology of btk to the non-receptor tyrosine kinases and speculate on putative roles for this gene in B-cell development.