B-lymphocyte calcium influx

Dynamic changes in cytoplasmic calcium concentration dictate the immunological fate and functions of lymphocytes. During the past few years, important details have been revealed about the mechanism of store-operated calcium entry in lymphocytes, including the molecular identity of calcium release-activated calcium (CRAC) channels and the endoplasmic reticulum (ER) calcium sensor (STIM1) responsible for CRAC channel activation following calcium depletion of stores. However, details of the potential fine regulation of CRAC channel activation that may be imposed on lymphocytes following physiologic stimulation within an inflammatory environment have not been fully addressed. In this review, we discuss several underexplored aspects of store-operated (CRAC-mediated) and store-independent calcium signaling in B lymphocytes. First, we discuss results suggesting that coupling between stores and CRAC channels may be regulated, allowing for fine tuning of CRAC channel activation following depletion of ER stores. Second, we discuss mechanisms that sustain the duration of calcium entry via CRAC channels. Finally, we discuss distinct calcium permeant non-selective cation channels (NSCCs) that are activated by innate stimuli in B cells, the potential means by which these innate calcium signaling pathways and CRAC channels crossregulate one another, and the mechanistic basis and physiologic consequences of innate calcium signaling.

Developmental-associated differences in membrane cholesterol content account for the differential responses of transitional immature and mature B cells to B cell receptor engagement (83.1)

Tolerance-sensitive transitional immature B cells maintain significantly lower membrane unesterified cholesterol levels than mature splenic B cells. In addition, the relatively low level of plasma membrane-associated cholesterol in transitional immature B cells impairs compartmentalization of their B cell receptor (BCR) into cholesterol-enriched domains following BCR aggregation and reduces their ability to sustain certain aspects of BCR signaling as compared with mature B cells. By increasing the level of plasma membrane associated cholesterol in transitional immature B to that seen in mature B cells, we observe an decrease in apoptosis post-BCR stimulation as compared to unmanipulated transitional immature B cells. Furthermore, we have generated a stable genetic mouse model in which cholesterol is constitutively produced in B cells, thus increasing the level of plasma membrane associated cholesterol in transitional immature B cells in vivo. These studies establish an unexpected difference in the lipid composition of peripheral transitional immature and mature B cells and point to a determining role for development-associated differences in cholesterol content for the differential responses of these B cells to BCR engagement.

This work was supported by Training Grant AI055428 and Grants AI43620 and AI32592 from the National Institutes of Health.

Rapid B cell receptor-induced unfolded protein response in nonsecretory B cells correlates with pro- versus antiapoptotic cell fate

The adaptive unfolded protein response (UPR) is essential for the development of antibody-secreting plasma cells. B cells induced by lipopolysaccharide (LPS) to differentiate into plasma cells exhibit a nonclassical UPR reported to anticipate endoplasmic reticulum stress prior to immunoglobulin production. Here we demonstrate that activation of a physiologic UPR is not limited to cells undergoing secretory cell differentiation. We identify B cell receptor (BCR) signaling as an unexpected physiologic UPR trigger and demonstrate that in mature B cells, BCR stimulation induces a short lived UPR similar to the LPS-triggered nonclassical UPR. However, unlike LPS, BCR stimulation does not induce plasma cell differentiation. Furthermore, the BCR-induced UPR is not limited to cells in which BCR induces activation, since a UPR is also induced in transitional immature B cells that respond to BCR stimulation with a rapid apoptotic fate. This response involves sustained up-regulation of Chop mRNA indicative of a terminal UPR. Whereas sustained Chop expression correlates with the ultimate fate of the BCR-triggered B cell and not its developmental stage, Chop-/- B cells undergo apoptosis, indicating that CHOP is not required for this process. These studies establish a system whereby a terminal or adaptive UPR can be alternatively triggered by physiologic stimuli.

Membrane cholesterol content accounts for developmental differences in surface B cell receptor compartmentalization and signaling

Recent studies argue for an important role for cholesterol in maintaining plasma membrane heterogeneity and influencing a variety of cellular processes, including signaling, adhesion, and permeability. Here, we document that tolerance-sensitive transitional immature B cells maintain significantly lower membrane unesterified cholesterol levels than mature-stage splenic B cells. In addition, the relatively low level of cholesterol in transitional immature B cells impairs compartmentalization of their B cell receptor (BCR) into cholesterol-enriched domains following BCR aggregation and reduces their ability to sustain certain aspects of BCR signaling as compared with mature B cells. These studies establish an unexpected difference in the lipid composition of peripheral transitional immature and mature B cells and point to a determining role for development-associated differences in cholesterol content for the differential responses of these B cells to BCR engagement.

MMTV Env encodes an ITAM responsible for transformation of mammary epithelial cells in three-dimensional culture

Expression of immunoreceptor tyrosine-based activation motif (ITAM)-containing signaling proteins is normally restricted to hematopoietic tissues. The basal activity of ITAM-containing proteins is mediated through negative regulation by coreceptors restricted to hematopoietic tissues. We have identified an ITAM signaling domain encoded within the env gene of murine mammary tumor virus (MMTV). Three-dimensional structures derived in vitro from murine cells stably transfected with MMTV env display a depolarized morphology in comparison with control mammary epithelial cells. This effect is abolished by Y>F substitution within the Env ITAM, as well as inhibitors of Syk and Src protein tyrosine kinases. Env-expressing cells bear hallmarks of cell transformation such as sensitivity to apoptosis induced by tumor necrosis factor (TNF)–related apoptosis-inducing ligand (TRAIL) or TNFα, as well as down-regulation of E-cadherin and Keratin-18. Human normal mammary epithelial cells expressing MMTV Env also develop transformed phenotype, as typified by growth in soft agar and Matrigel invasion. These disruptions are abrogated by Y>F substitutions. We conclude that ITAM-dependent signals are generated through MMTV Env and trigger early hallmarks of transformation of mouse and human mammary epithelial cells. Therefore, these data suggest a heretofore unappreciated potential mechanism for the initiation of breast cancer and identify MMTV Env and ITAM-containing proteins in human breast tumors as probable oncoproteins.

Igα/Igβ Complexes Generate Signals for B Cell Development Independent of Selective Plasma Membrane Compartmentalization

Ligand-induced BCR association with detergent-resistant plasma membrane compartments (lipid rafts) has been argued to be essential for initiating and/or sustaining Igalpha/Igbeta-dependent BCR signaling. Because a fraction of the BCR and an even larger fraction of the preBCR associates with lipid rafts in the apparent absence of ligand stimulation, it has been proposed that raft-associated receptor complexes mediate the ligand-independent basal signaling events observed in resting B lineage cells. However, there is no direct evidence that localization of Igalpha/Igbeta-containing complexes to detergent-resistant membrane compartments is absolutely required for the signaling events that drive B cell development. To address these issues we have designed surrogate preBCR/Igalpha/Igbeta complexes that are incapable of ligand-induced aggregation and that are preferentially targeted to either raft or nonraft compartments. An analysis of their ability to promote the preBCR-dependent proB-->preB cell transition of murine B cell progenitors revealed that expression of these surrogate receptor complexes at levels that approximate that of the conventional preBCR can drive B cell development in a manner independent of both aggregation and lipid raft localization.

NF-κB inducible genes BCL-X and cyclin E promote immature B-cell proliferation and survival

B-cell receptor (BCR) ligation induces proliferation and survival in mature B-cells but conversely, can lead to apoptosis in immature B-cells. We have previously shown that c-Rel, a member of the NF-kappaB transcription factor family, is essential for mature B-cell survival and proliferation via regulation of the anti-apoptotic molecule Bcl-X and cell cycle genes E2F3a and cyclin E. Here, we report that c-Rel-deficient mature B-cells are rendered sensitive to BCR-induced growth arrest and apoptosis in a manner that strongly resembles the phenotypic response of immature B-cells to BCR signaling. We further demonstrate that BCR-stimulated immature B-cells are defective in NF-kappaB activation, but that introduction of two downstream c-Rel target genes, Bcl-X and cyclin E, can restore survival and proliferation to these cells. Our studies therefore suggest that specific blockade of NF-kappaB activation may be responsible for the growth arrest and apoptosis of BCR-activated immature B-cells.

Positive and negative selection during B lymphocyte development

Our laboratory is interested in a variety of issues related to lymphocyte development. More specifically, we have focused on the processes that regulate the decision to commit to the B lymphocyte (B cell) lineage, then the subsequent signals that are involved in maintaining this commitment to the B cell lineage. These signals result in the positive selection of those B cells that properly execute the complex genetic changes associated with B cell development, then trigger the elimination of B cells that are responsive to self-antigens and, therefore, possess the potential to mediate autoimmune disease. Our general experimental approach has been to address these issues from the perspective of signal transduction. Our goal is to define the biochemical and genetic processes that are integrated in order to accomplish these selection processes. To do so, we employ in vivo animal models as well as more defined in vitro studies, using both primary and transformed cell lines. For the past several years, we have been primarily interested in the precise mechanisms by which the B cell antigen receptor (BCR), and intermediate forms of this receptor, regulate these complex developmental processes. We have used the ongoing studies described below as two representative examples of how we are approaching these issues and some of the insights that we have made. To place both of these studies in context, we will begin with a brief introduction into B cell development.

Laser-generated waves and wakes in rotating ion crystals

Locally excited plasma waves are generated in a Coulomb crystal by "pushing" with radiation pressure on a rotating cloud of laser-cooled 9Be+ ions. The waves form a stationary wake that is directly imaged through the dependence of the ion fluorescence on Doppler shifts, and theoretical calculations in a slab geometry are shown to accurately reproduce these images. The technique demonstrates a new method of exciting and studying waves in cold ion clouds.

Immunobiology of the immature B cell: plasticity in the B-cell antigen receptor-induced response fine tunes negative selection

The immature and transitional immature B-cell stages define an important window in B-cell development, as it is at this point that cells committed to the B-cell lineage first express the clonotypic B-cell antigen receptor (BCR) and cells expressing self-reactive specificities may be identified and eliminated. The intrinsic susceptibility of the immature B cell to negative selection following BCR engagement distinguishes these cells functionally from mature-stage B cells in which BCR cross-linking leads to activation. Our laboratory has been interested in determining the molecular events responsible for the distinct and disparate responses of immature and mature B cells to antigen receptor signaling in order to understand the molecular basis of negative selection of developing B cells. These studies have indicated that developmentally regulated mechanisms, intrinsic to the B cell, regulate the differential responsiveness of the immature and mature stage B cell to antigen. However, the "hard-wired" BCR-induced apoptotic response of the immature B cell can be modified by the microenvironmental context in which the antigen is encountered. This plasticity fine tunes the BCR-induced response of the immature B cell by regulating the mechanism of negative selection and, under defined circumstances, allowing for recruitment into an immune response.

A dominant-negative mutant of c-Jun inhibits cell cycle progression during the transition of CD4(-)CD8(-) to CD4(+)CD8(+) thymocytes

While Jun/Fos-containing transcription factors are known to be necessary for many TCR-mediated events in mature T cells, relatively little is known about their roles in thymocyte development. We have generated transgenic mice that express a trans-dominant-negative mutant of c-Jun (TAM-67) specifically in thymocytes. Expression of TAM-67 inhibited the up-regulation of AP-1-responsive genes such as c-jun and IL-2 in stimulated thymocytes from transgenic mice. In addition, altered thymocyte development in TAM-67-expressing mice was revealed by a decrease in thymic cellularity ( approximately 50%) which could be accounted for primarily by a reduction in the number of CD4(+)CD8(+) thymocytes, a large percentage of which retained CD25. The decrease in the number of CD4(+)CD8(+) thymocytes did not appear to be due to an enhanced rate of apoptosis but rather to a decrease in the number of CD4(-)CD8(-)CD25(-) cells in the S + G(2)/M stages of the cell cycle. These results indicate that Jun/Fos-containing transcription factors promote the proliferative burst that accompanies the transition from the CD4(-)CD8(-) to the CD4(+)CD8(+) stage of thymocyte development.

Antigen receptor-induced signal transduction imbalances associated with the negative selection of immature B cells

Signals transduced through the B cell Ag receptor (BCR) drive B cell development. However, BCR-induced responses are developmentally regulated; immature B cells are tolerized following antigenic exposure while mature B cells are triggered to proliferate and differentiate. This differential responsiveness allows for the negative selection of self-reactive immature B cells while simultaneously allowing for clonal expansion of mature B cells in response to foreign Ags. Intrinsic differences in BCR-induced signal transduction at various stages of development may account for this functional dichotomy. We had previously demonstrated that the BCR-induced proliferation of mature B cells is accompanied by an increase in intracellular calcium levels and polyphosphoinositide bis phosphate (PIP2) hydrolysis. In contrast, immature B cells that undergo BCR-induced apoptosis increase intracellular calcium in the relative absence of PIP2 hydrolysis. Since PIP2 hydrolysis leads to the generation of diacylglycerol, a cofactor for protein kinase C (PKC) activation, these data suggested that an "imbalance" in BCR-induced signal transduction resulting from a relative inability to activate PKC may play a role in the susceptibility of immature B cells to BCR-induced apoptosis. In support of this hypothesis, we demonstrate that PKC activation can rescue immature B cells from BCR-induced apoptosis. Furthermore, the susceptibility of immature B cells to BCR-induced apoptosis is recapitulated in mature B cells that are either PKC depleted or are stimulated in the presence of PKC inhibitors, suggesting that an uncoupling of PKC activation from BCR-induced signaling is responsible for the apoptotic response of immature B cells.

Antigen Receptor-Induced Signal Transduction Imbalances Associated with the Negative Selection of Immature B Cells

Signals transduced through the B cell Ag receptor (BCR) drive B cell development. However, BCR-induced responses are developmentally regulated; immature B cells are tolerized following antigenic exposure while mature B cells are triggered to proliferate and differentiate. This differential responsiveness allows for the negative selection of self-reactive immature B cells while simultaneously allowing for clonal expansion of mature B cells in response to foreign Ags. Intrinsic differences in BCR-induced signal transduction at various stages of development may account for this functional dichotomy. We had previously demonstrated that the BCR-induced proliferation of mature B cells is accompanied by an increase in intracellular calcium levels and polyphosphoinositide bis phosphate (PIP2) hydrolysis. In contrast, immature B cells that undergo BCR-induced apoptosis increase intracellular calcium in the relative absence of PIP2 hydrolysis. Since PIP2 hydrolysis leads to the generation of diacylglycerol, a cofactor for protein kinase C (PKC) activation, these data suggested that an “imbalance” in BCR-induced signal transduction resulting from a relative inability to activate PKC may play a role in the susceptibility of immature B cells to BCR-induced apoptosis. In support of this hypothesis, we demonstrate that PKC activation can rescue immature B cells from BCR-induced apoptosis. Furthermore, the susceptibility of immature B cells to BCR-induced apoptosis is recapitulated in mature B cells that are either PKC depleted or are stimulated in the presence of PKC inhibitors, suggesting that an uncoupling of PKC activation from BCR-induced signaling is responsible for the apoptotic response of immature B cells.

Effects of carbaryl, permethrin, 4-nonylphenol, and copper on muscarinic cholinergic receptors in brain of surrogate and listed fish species

We investigated the regulation of the muscarinic cholinergic receptor (MChR) in brain from seven species of fish, two surrogates and five threatened or endangered species exposed to a series of chemicals as a measure of compensatory response among species. Fish were classified as either cold water (rainbow trout-surrogate, apache trout, lahanton trout) or warm water (fathead minnow-surrogate, razorback sucker, bonytail chub, colorado squawfish) and were exposed to chemicals shown to affect cholinergic pathways (carbaryl and permethrin) and two chemicals whose relationships to the cholinergic system is less clear (4-nonylphenol and copper). Downregulation of MChR occurred in all warm water species, except colorado squawfish, and at carbaryl concentrations similar to those causing downregulation observed in rainbow trout. Permethrin exposure resulted in downregulation in fathead minnow and razorback sucker, but the concentrations required for observation of this phenomenon were much greater than observed in cold water species. Copper exposure caused a decrease in brain MChR in rainbow trout and apache trout, whereas 4-nonylphenol exposure resulted in a decrease in brain MChR in all three cold water species. Our results indicate that surrogates are useful in assessing sublethal physiological responses to chemicals with a known mechanism of action such as carbaryl and support use of surrogates for assessing physiological responses to chemicals with diverse, less clear mechanisms of action.

A positive role for thymus-derived steroids in formation of the T-cell repertoire

T cells undergo rigorous selection processes in the thymus that are necessary to prevent T cells with either autoreactive or nonfunctional T-cell receptors (TCRs) from entering the periphery. Although both positive and negative selection depend on TCR-mediated signals, the means by which a thymocyte interprets these signals to result in survival or death is not understood. Glucocorticoids are known to induce thymocyte apoptosis at high concentrations, but at lower concentrations glucocorticoids can antagonize TCR-mediated deletional signals and allow survival of thymocytes and T cell hybridomas. Interestingly, transgenic mice in which the expression of the glucocorticoid receptor has been downmodulated specifically in thymocytes have abnormal thymocyte differentiation, indicating that glucocorticoids play a significant role in T-cell development. Furthermore, we have demonstrated the presence of steroidogenic enzymes in the thymic epithelium and can show that, in vitro, these cells readily synthesize pregnenolone, the first product in the steroidogenic pathway, and deoxycorticosterone. Inhibition of local glucocorticoid biosynthesis in thymi from TCR transgenic mice during fetal thymic organ culture (FTOC) revealed significant alterations in the process of thymocyte selection. These data suggest that glucocorticoids do not simply suppress the immune system but rather are necessary for thymocyte survival and differentiation.

Thymocyte glucocorticoid resistance alters positive selection and inhibits autoimmunity and lymphoproliferative disease in MRL-lpr/lpr mice

Thymus-derived glucocorticoids antagonize T cell receptor (TCR)-induced thymocyte apoptosis, allowing the survival (positive selection) of cells bearing TCRs that recognize self antigens with low-to-moderate avidity. Here we demonstrate that expression of an antisense glucocorticoid receptor transgene in thymocytes of spontaneously autoimmune MRL-lpr/lpr mice causes the loss of specific TCR Vbeta-bearing T cells that are normally positively selected in this strain. These transgenic mice had lower autoantibody production and milder symptoms of autoimmune disease than MRL-lpr/lpr controls and had markedly reduced accumulation of the TCR+Thy-1+CD4-CD8-B220+ T cells that are the hallmark of the lpr mutation. Thus, decreased glucocorticoid signaling in thymocytes alters the T cell repertoire and greatly diminishes autoimmunity in MRL-lpr/lpr autoimmune mice.

Lack of a role for Jun kinase and AP-1 in Fas-induced apoptosis

Cross-linking of Fas (CD95) induces apoptosis, a response that has been reported to depend upon the Ras activation pathway. Since many examples of apoptosis have been reported to involve AP-1 and/or the AP-1-activation pathway. Since many examples of apoptosis have been reported to involve AP-1 and/or the AP-1-activating enzyme Jun kinase (JNK), downstream effectors of Ras or Ras-like small GTP-binding proteins, we evaluated the role of these molecules in Fas-mediated apoptosis. Although cross-linking of Fas on Jurkat T cells did result in JNK activation, increased activity was observed relatively late, being detectable only after 60 min of stimulation. Expression of a dominant negative form of SEK1 that blocked Fas-mediated induction of JNK activity had no effect on Fas-mediated apoptosis. Furthermore, maximally effective concentrations of anti-Fas did not cause JNK activation if apoptosis was blocked by a cysteine protease inhibitor, suggesting that under these conditions, activation of JNK may be secondary to the stress of apoptosis rather than a direct result of Fas engagement. Despite the activation of JNK, there was no induction of AP-1 activity as determined by gel shift assay or induction of an AP-1-responsive reporter. The lack of a requirement for AP-1 induction in Fas-mediated death was further substantiated with Jurkat cells that were stably transfected with a dominant negative cJun, TAM-67. While TAM-67 effectively prevented AP-1-dependent transcription of both the interleukin-2 and cJun genes, it had no effect on Fas-induced cell death, even at limiting levels of Fas signaling. Thus, induction of JNK activity in Jurkat cells by ligation of Fas at levels sufficient to cause cell death is likely a result, rather than a cause, of the apoptotic response, and AP-1 function is not required for Fas-induced apoptosis.

Regulation of thymocyte development by glucocorticoids

It is generally believed that the avidity of the T cell receptor for self antigen/MHC determines the fate of a thymocyte. However, it is not understood how the thymocyte distinguishes a survival signal (positive selection) from a death signal (negative selection). Recent studies from our laboratory have explored the role that thymus-produced glucocorticoids may play in influencing thymocyte development. It appears that glucocorticoids are important and necessary at several points during thymocyte differentiation and that they may regulate antigen-specific T cell development.

Cross-talk between the T cell antigen receptor and the glucocorticoid receptor regulates thymocyte development

The fate of an immature thymocyte, life or death, is largely determined by the ligand-specificity of its T cell antigen receptor (TCR). The default pathway for thymocytes bearing TCRs with subthreshold avidity for self-antigens is death (death by neglect). Thymocytes bearing TCRs with high avidity for self also undergo apoptosis (negative selection). Those thymocytes with intermediate avidities, or that perhaps recognize self-peptides that have partial agonist or antagonist properties, survive and differentiate into mature immunocompetent T cells (positive selection). How TCR avidity is interpreted as a "rescue" signal or a death signal is unknown. Based upon a T cell hybridoma model, our laboratory proposed that glucocorticoids, which themselves are potent inducers of thymocyte apoptosis, antagonize TCR-mediated thymocyte deletion and allow positive selection to occur. In fact, epithelial cells in the thymus proved to be a source of steroid production, and interference with steroid synthesis in fetal thymic organ culture resulted in a greatly enhanced sensitivity of thymocytes to TCR-mediated apoptosis. Transgenic mice with reduced glucocorticoid receptor (GR) levels were produced by tissue-specific expression of GR antisense. Thymocytes in these mice had high levels of spontaneous apoptosis, and were exquisitely sensitive to deletion induced by cross-linking the TCR. Moreover, there was a very large (> or = 90%) loss of CD4+CD8+ thymocytes, signifying a block at the CD4-CD8- to CD4+CD8+ transition, perhaps due to apoptosis of cells upon engagement of the pre-TCR in the absence of an antagonizing glucocorticoid stimulus. The molecular mechanism of the antagonism is currently being investigated. These data indicate that there is cross-talk in thymocytes between the TCR and glucocorticoid signaling pathways resulting in apoptosis, and that locally produced steroids, in a paracrine fashion, participate in setting the TCR avidity thresholds that determine whether developing thymocytes survive or die, and therefore help to mold the antigen-specific T cell repertoire.

A targeted glucocorticoid receptor antisense transgene increases thymocyte apoptosis and alters thymocyte development

The exquisite sensitivity of thymocytes to steroid-induced apoptosis, the steroidogenic potential of thymic epithelial cells, and the ability of steroid synthesis inhibitors to enhance antigen-specific deletion of thymocytes in fetal thymic organ cultures suggest a role for glucocorticoids in thymocyte development. To address this further, transgenic mice that express antisense transcripts to the glucocorticoid receptor (GR) specifically in immature thymocytes were generated. The consequent hyporesponsiveness of thymocytes to glucocorticoids was accompanied by a reduction in thymic size, primarily owing to a decrease in the number of CD4+CD8+ cells. While an enhanced susceptibility to T cell receptor (TCR)-mediated apoptosis appeared to be partially responsible for this reduction, thymocyte loss could also be detected before thymocytes progressed to the CD4+CD8+ TCR alpha beta-expressing stage. These results suggest that glucocorticoids are necessary for survival and maturation of thymocytes, and are consistent with a role for steroids in both the transition from CD4-CD8- to CD4+CD8+ cells and the survival of CD4+CD8+ cells stimulated via the TCR.

Muscarinic cholinergic receptors in brain and atrial membranes of adult brook trout (Salvelinus fontinalis) measured by radioligand binding techniques

Muscarinic cholinergic receptors were measured by radioligand binding techniques in crude membrane particulate preparations of brain and atrial tissues from laboratory reared brook trout (Salvelinus fontinalis). The radioligand [3H]N-methyl scopolamine was used to determine number and affinity of receptors in saturation experiments. The affinity of the radioligand did not differ in brain and atrial preparations (96 +/- 8 and 60 +/- 4 pM, respectively). However, the number of binding sites was greater in atrium compared with brain (269 +/- 19 and 166 +/- 7 fmol/mg protein, respectively). The rank order of potency of competing drugs in inhibition experiments was similar for antagonists with atropine > or = scopolamine > pirenzepine. Pirenzepine, an M1-selective drug had a 3-fold higher affinity in brain than atrium. The agonists oxotremorine and carbachol each bound to two sites in both tissues. In contrast, pilocarpine bound to only one site in brain and two in atrial tissue. These results are compared with those observed in other nonmammalian species and discussed with reference to conservation of proteins that serve important cellular roles.

To be or not to be: mutually antagonistic death signals regulate thymocyte apoptosis

Recognition of self-antigens by immature thymocytes results in either activation-induced apoptosis (negative selection) or survival (positive selection). While it is believed that T cell receptor avidity plays a role in determining the outcome, the mechanisms responsible for this life or death decision are not known. Recent data concerning the mutual antagonism between activation- and glucocorticoid-induced apoptosis have prompted an examination of the potential interaction of these two signaling pathways in the regulation of antigen-specific selection.

Role of endogenous retroviruses in autoimmune diseases

Retroviruses have been implicated in the pathogenesis of murine and human lupus; however, many positive findings have been followed by alternative explanations. Initial findings implicating xenotropic retroviruses were subsequently invalidated. The first solid demonstration that endogenous retroviruses mediate disease was the study of SL/Ni mice. Here budding ecotropic retroviral particles from arterial smooth muscle cells caused an antibody response to the particles with subsequent complement deposition. Our laboratory has focused on derangements in endogenous MCF retroviral expression. We found that lupus-prone NZB, BXSB and MRL strains have a marked increase in expression of Mpmv RNA in their thymuses while bone marrow expression did not differ from normal strains. Sequence analysis demonstrated mutations in the NZB endogenous retroviruses which could alter expression. A phosphorothioate antisense oligonucleotide to the initiation sequence of Mpmv caused lymphocyte activation in vivo in normal mice, providing further evidence for in vivo effects of Mpmv and potential for pathological abnormalities in lupus-prone strains.

Constitutive expression of interferon gamma-inducible protein 10 in lymphoid organs and inducible expression in T cells and thymocytes

Interferon gamma-inducible protein 10 (IP-10), a member of a family of small proinflammatory chemotactic polypeptides, is expressed in interferon gamma-stimulated keratinocytes, macrophages, fibroblasts, and endothelial cells. Here we report that IP-10 is also expressed by activated but not resting T hybridoma cells, normal T cells, and thymocytes. Although resting lymphocytes did not synthesize IP-10, surprisingly high levels of IP-10 transcripts were found in lymphoid organs (spleen, thymus, and lymph nodes). Thymic and splenic stromal cells were found to express constitutively high levels of both IP-10 mRNA and protein, accounting for the high level of spontaneous expression in lymphoid tissue. Therefore, in addition to its role as a proinflammatory cytokine, IP-10 may participate in T cell effector function and perhaps T cell development.

Signaling for death of lymphoid cells

The induction of programmed cell death in lymphocytes is a common response to a wide variety of physiological and pharmacological stimuli. While there is still much to be learned about the transmembrane signals that lead to programmed cell death, progress has been made in identifying new cell surface molecules (e.g. APO-1/Fas) that may regulate the physiological induction of lymphocyte death, molecules whose expression inhibits apoptosis (e.g. Bcl-2), and the antagonism of activation-induced cell death in T-cell hybridomas and thymocytes by members of the steroid receptor superfamily.

Mouse mammary tumor proviral gene expression in cells of the B lineage

Mouse mammary tumor proviruses (MMTV) use a common enhancer/promoter region to accommodate their transcription in two different cellular environments. In mammary tissue, transcription is regulated through the hormone response element located in the 5' LTR. In B cells, transcription is hormone independent, can be stimulated following B cell activation, and is distinct from the transcription of other known inducible genes, including immunoglobulin. The open reading frame (ORF) in the viral 3' LTR has at least two functions. Its gene product(s) acts as a viral superantigen, but also has autoregulatory properties, leading to MMTV transactivation. We propose a scheme suggesting that MMTV evolved to use the B cell as an intermediary in its viral life cycle.

Molecular aspects of systemic lupus erythematosus: murine endogenous retroviral expression

Systemic lupus erythematosus is an immune-mediated disease in which the etiology is unknown. Full-length (8.4 kb), type C, modified polytropic (Mpmv) retroviral transcripts from the thymus are characteristic of murine lupus. Reciprocal bone marrow transplantation studies determined that this thymic expression maps to the pre-T bone marrow stem cell. In vitro and in vivo oligonucleotide antisense work suggest that type C retroviruses play a role in immune activation. This paper summarizes our studies of endogenous retroviruses in murine lupus.

The biological effects of IgM hexamer formation

The inducible B cell lymphoma, CH12, and its in vitro adapted subclone, CH12-LBK, produce immunoglobulins of identical sequence, specificity and isotype, with equivalent affinities for the hapten trimethyl ammonium. However, the hemolytic efficiencies of the antibody secreted by the two cell lines are quite different. Antibody preparations from lipopolysaccharide-stimulated CH12 cells lyse erythrocytes six- to ten times more effectively than antibody preparations of the same concentration from CH12-LBK cells. Both cell lines secrete polymeric IgM, but while CH12-LBK cells secrete predominantly the canonical pentameric IgM, CH12 cells secrete a mixture of pentamers and hexamers. High-efficiency complement-dependent cytolysis is associated with hexameric IgM, which has a specific activity that is approximately twenty times higher than that of the pentameric form. J chain protein is found in the secreted IgM of both cell lines, but is associated only with the pentameric IgM and not with the hexameric form, nor with any intermediate polymers smaller than a pentamer. A deficit in, or the inaccessibility of, J chain protein appears to facilitate hexamer formation. These experiments confirm previously published data showing that J chain is not necessary either for assembly or secretion of polymeric IgM, and suggest instead that J chain may be important in regulating the lytic efficiency of polymeric IgM by controlling the IgM pentamer/hexamer ratio. The experiments further suggest a mechanism, in addition to isotype switching and somatic mutation, by which the biological efficiency of antibodies from a single clone of B cells can be regulated.

Lipopolysaccharide and dexamethasone induce mouse mammary tumor proviral gene expression and differentiation in B lymphocytes through distinct regulatory pathways

Endogenous mouse mammary tumor virus (MMTV) proviral transcripts are up regulated during the normal course of B-lymphocyte differentiation. We report here that the regulatory mechanisms which lead to increased levels of MMTV transcripts in differentiating, lipopolysaccharide (LPS)-stimulated normal B cells and in the inducible B-cell lymphoma line CH12 are at least partially distinct from those controlling increases in immunoglobulin and J-chain gene expression. In studies designed to characterize the stimulatory pathways leading to MMTV expression in CH12 cells, we found that stimulation with either LPS or dexamethasone (Dex), a transcriptional activator of MMTV genes, induced not only MMTV expression but also differentiation to antibody secretion. Only Dex-induced and not LPS-induced MMTV expression and differentiation were inhibited by the glucocorticoid antagonist RU486, demonstrating that Dex and LPS stimulate B cells by distinct molecular pathways. Therefore, in B cells, MMTV expression can be regulated via either the conventional hormone receptor-dependent pathway or a hormone receptor-independent pathway. Furthermore, these results suggest that steroid stimulation of B cells can lead to alterations in the expression of other results suggest that steroid stimulation of B cells can lead to alterations in the expression of other steroid-responsive genes that can become involved in the process of B-cell differentiation.

Molecular events in B lymphocyte differentiation. Inducible expression of the endogenous mouse mammary tumor proviral gene, Mtv-9

We have previously shown that the steady state levels of transcripts encoded by an endogenous mouse mammary tumor virus (MMTV) increase during LPS-induced differentiation of both normal B lymphocytes and an inducible B cell lymphoma, CH12. A large body of evidence suggests that MMTV expression is primarily limited to mammary tissues and that expression in cell lines from nonmammary tissues is accompanied by viral amplification and alterations in the transcriptional control regions of the viral long terminal repeat. We have, therefore, carefully characterized MMTV expression in CH12 cells and in other cells of the B lineage in order to determine if the expression of MMTV transcripts in differentiating B cells results from the "abnormal" transcriptional regulation seen in other nonmammary tissue. In this manuscript, we present evidence that MMTV transcripts are expressed in a variety of cells of the B lineage and that the levels of constitutive expression vary among the different cells. On the other hand, T cell lymphomas lacking amplified MMTV do not contain proviral transcripts, suggesting that MMTV transcription may be preferentially expressed in B lymphocytes. We demonstrate that MMTV transcripts are up-regulated during cytokine-mediated as well as LPS-mediated differentiation, and that most, if not all, expression is due to the activity of a single proviral gene, Mtv-9, in CH12 cells. Furthermore, the expression of MMTV transcripts in CH12 cells neither requires nor is accompanied by amplification of the provirus. Sequence analysis demonstrates that the U3 region of the expressed Mtv-9 long terminal repeat contains neither deletions nor insertions, and the well-characterized enhancer and promoter sites in the glucocorticoid response element which are known to be involved in transcriptional regulation of MMTV in mammary tissues have not been disrupted. These data suggest that the Mtv-9 locus behaves as a normal somatic gene which is differentially regulated during B cell development and differentiation. Unlike the events which lead to MMTV expression in other nonmammary tissues, B cells may express transcription factor(s) which are capable of inducing expression of endogenous MMTV proviral genes during the natural course of differentiation. Analysis of the mechanisms which control the expression of this gene should be useful in characterizing the molecular events which govern B cell differentiation.

Molecular events in B lymphocyte differentiation. Inducible expression of the endogenous mouse mammary tumor proviral gene, Mtv-9

We have previously shown that the steady state levels of transcripts encoded by an endogenous mouse mammary tumor virus (MMTV) increase during LPS-induced differentiation of both normal B lymphocytes and an inducible B cell lymphoma, CH12. A large body of evidence suggests that MMTV expression is primarily limited to mammary tissues and that expression in cell lines from nonmammary tissues is accompanied by viral amplification and alterations in the transcriptional control regions of the viral long terminal repeat. We have, therefore, carefully characterized MMTV expression in CH12 cells and in other cells of the B lineage in order to determine if the expression of MMTV transcripts in differentiating B cells results from the "abnormal" transcriptional regulation seen in other nonmammary tissue. In this manuscript, we present evidence that MMTV transcripts are expressed in a variety of cells of the B lineage and that the levels of constitutive expression vary among the different cells. On the other hand, T cell lymphomas lacking amplified MMTV do not contain proviral transcripts, suggesting that MMTV transcription may be preferentially expressed in B lymphocytes. We demonstrate that MMTV transcripts are up-regulated during cytokine-mediated as well as LPS-mediated differentiation, and that most, if not all, expression is due to the activity of a single proviral gene, Mtv-9, in CH12 cells. Furthermore, the expression of MMTV transcripts in CH12 cells neither requires nor is accompanied by amplification of the provirus. Sequence analysis demonstrates that the U3 region of the expressed Mtv-9 long terminal repeat contains neither deletions nor insertions, and the well-characterized enhancer and promoter sites in the glucocorticoid response element which are known to be involved in transcriptional regulation of MMTV in mammary tissues have not been disrupted. These data suggest that the Mtv-9 locus behaves as a normal somatic gene which is differentially regulated during B cell development and differentiation. Unlike the events which lead to MMTV expression in other nonmammary tissues, B cells may express transcription factor(s) which are capable of inducing expression of endogenous MMTV proviral genes during the natural course of differentiation. Analysis of the mechanisms which control the expression of this gene should be useful in characterizing the molecular events which govern B cell differentiation.

Characterization of a presecretory phase in B-cell differentiation

We have identified and characterized an inducible in vitro subclone of the CH12 B-cell lymphoma, CH12-LBK, which appears to represent a transitional phase in the B-cell differentiation pathway. This phase, which we call the "presecretory" phase, falls between replicating B cells that are not secreting antibodies and B cells that secrete antibody at a high rate. Presecretory cells are characterized by abundant steady-state levels of immunoglobulin and joining (J) chain transcripts and of protein but low levels of mouse mammary tumor virus envelope transcripts and low rates of immunoglobulin secretion. Additional stimulation is required for presecretory cells to differentiate into cells that secrete antibodies at a high rate. The existence of cells with this phenotype suggests that high-level expression of immunoglobulin and J-chain protein does not necessarily commit a B cell to polymerize and secrete multimeric immunoglobulin. Rather, other gene products, expressed after immunoglobulin and J-chain transcripts have been upregulated late in B-cell differentiation, appear responsible for inducing high rates of antibody secretion.

Molecular events during B lymphocyte differentiation. Induction of endogenous mouse mammary tumor proviral envelope transcripts after B cell stimulation

We have identified a gene whose expression appears to be associated with a late stage in the differentiation of B lymphocytes into antibody secreting cells, as shown by using the inducible B cell lymphoma, CH12. Restriction mapping and partial sequencing of a cDNA clone isolated by subtraction analysis demonstrated that the clone, SC34, represents an envelope (env) gene transcript of a mouse mammary tumor virus (MMTV). In CH12 cells and in normal B cells, levels of MMTV RNA were increased after stimulation with LPS. The env transcript was the predominant MMTV RNA species and increased more dramatically than did levels of the genomic transcript. In differentiating CH12 cells, env transcripts increased as much as 20-fold above levels found in replicating, antibody nonsecreting CH12 cells. The major increase in expression appeared to be associated with B cell differentiation and not replication. By Southern blot analysis, only bands characteristic of endogenous proviruses were found in CH12, indicating that viral sequences were not amplified in this cell line. Restriction mapping indicated that the SC34 cDNA clone was a product of the Mtv-9 locus. Mtv-9 previously was shown to encode a complete MMTV provirus on chromosome 12, on which Ig heavy chain genes also are located. Increases in MMTV transcripts followed distinct kinetics and were quantitatively different from changes in immunoglobulin gene products. The expression of env RNA appears to more accurately reflect differentiation to antibody secretion in CH12 cells than does the expression of immunoglobulin gene transcripts.

Molecular events during B lymphocyte differentiation. Induction of endogenous mouse mammary tumor proviral envelope transcripts after B cell stimulation

We have identified a gene whose expression appears to be associated with a late stage in the differentiation of B lymphocytes into antibody secreting cells, as shown by using the inducible B cell lymphoma, CH12. Restriction mapping and partial sequencing of a cDNA clone isolated by subtraction analysis demonstrated that the clone, SC34, represents an envelope (env) gene transcript of a mouse mammary tumor virus (MMTV). In CH12 cells and in normal B cells, levels of MMTV RNA were increased after stimulation with LPS. The env transcript was the predominant MMTV RNA species and increased more dramatically than did levels of the genomic transcript. In differentiating CH12 cells, env transcripts increased as much as 20-fold above levels found in replicating, antibody nonsecreting CH12 cells. The major increase in expression appeared to be associated with B cell differentiation and not replication. By Southern blot analysis, only bands characteristic of endogenous proviruses were found in CH12, indicating that viral sequences were not amplified in this cell line. Restriction mapping indicated that the SC34 cDNA clone was a product of the Mtv-9 locus. Mtv-9 previously was shown to encode a complete MMTV provirus on chromosome 12, on which Ig heavy chain genes also are located. Increases in MMTV transcripts followed distinct kinetics and were quantitatively different from changes in immunoglobulin gene products. The expression of env RNA appears to more accurately reflect differentiation to antibody secretion in CH12 cells than does the expression of immunoglobulin gene transcripts.

Complete coding region sequence of E beta k cDNA clones: lack of polymorphism in the NH2-terminus between E beta k and E beta b molecules

Sequence analysis of two independently isolated E beta k cDNA clones revealed that the E beta k molecule is identical to the E beta b molecule at the NH2-terminus. These data resolve a discrepancy between previously published amino acid and nucleotide sequences and indicate that the E beta NH2-terminus is not as polymorphic as was once believed.

Comparative sequence analysis of cDNA clones encoding I-A molecules of the CH12 B cell lymphoma: nucleotide differences do not account for their "defective" function in B cell stimulation

The CH12 B cell lymphoma can be stimulated to secrete antibody by helper T cells that interact with I-EK but not I-AK molecules expressed on its membrane. Both molecules present antigen to the appropriate T cells. We have analyzed the mRNA by Northern blot analysis and obtained partial sequences of cDNA clones encoding A alpha and A beta of the I-A molecule to determine if deletions or mutations in the cytoplasmic or transmembrane domains account for the "defect" in triggering following interaction with I-A restricted helper T cells. The results provide no evidence for structural alterations in either A alpha or A beta which could account for these observations. The implications of these findings on the role of class II molecules in B cell activation is discussed.

Autoantibodies to nuclear, cytoplasmic, and cytoskeletal antigens in renal allograft rejection

Autoantibodies (AA) detected by indirect immunofluorescence on HEp-2 cells and lymphocyte panel reactive antibodies (PRA) were measured pretransplant and within 2 weeks after transplant nephrectomy in a group of 21 consecutive renal allograft recipients with irreversible graft rejection. No patient had clinical evidence or history of autoimmune disease. Although 43% of patients had AA and 29% had a positive PRA pretransplant, 90% had AA and 100% had a positive PRA post-transplant nephrectomy (P less than 0.0032, 0.00001, respectively). Analysis of AA detected following graft failure revealed that all were of IgG class, and more than half the patients had multiple patterns including speckled nuclear, cytoplasmic, perinuclear, mitotic spindle apparatus, and smooth muscle staining. Of 11 patients retested up to one year later, all showed a persistence of previously detected AA in both pattern and titer, although none of the patients had any other signs or symptoms of autoimmune disease. A control group of 21 transplant recipients with functioning grafts was similarly tested revealing that 35% had AA and 29% had positive PRA pretransplant, but only 14% had AA and 14% positive PRA posttransplant. The presence of post-transplant AA or PRA was not associated with antecedent delayed graft function (ATN), rejection episodes, treatment with antithymocyte globulin, the interval posttransplantation, or subsequent graft outcome. However, pretransplant AA and PRA in both groups were associated with prior graft loss, but not with other variables examined. These findings indicate that serologically detectable sensitization to lymphocyte (HLA) antigens and the presence of persistent, multiple autoantibodies to nuclear, cytoplasmic, and cytoskeletal antigens is a frequent occurrence following graft loss. Furthermore, the development of AA following graft rejection appears to be unrelated to the prior presence or subsequent development of clinical autoimmune disease.