Cellular effects and metabolic stability of N1-cyclic inosine diphosphoribose and its derivatives.

BACKGROUND AND PURPOSE: Recently, a number of mimics of the second messenger cyclic ADP-ribose (cADPR) with replacement of adenosine by inosine were introduced. In addition, various alterations in the molecule ranging from substitutions at C8 of the base up to full replacement of the ribose moieties still retained biological activity. However, nothing is known about the metabolic stability and cellular effects of these novel analogues. EXPERIMENTAL APPROACH: cADPR and the inosine-based analogues were incubated with CD38, ADP-ribosyl cyclase and NAD-glycohydrolase and metabolism was analysed by RP-HPLC. Furthermore, the effect of the analogues on cytokine expression and proliferation was investigated in primary T-lymphocytes and T-lymphoma cells. KEY RESULTS: Incubation of cADPR with CD38 resulted in degradation to adenosine diphosphoribose. ADP-ribosyl cyclase weakly catabolised cADPR whereas NAD-glycohydrolase showed no such activity. In contrast, N1-cyclic inosine 5'-diphosphoribose (N1-cIDPR) was not hydrolyzed by CD38. Three additional N1-cIDPR analogues showed a similar stability. Proliferation of Jurkat T-lymphoma cells was inhibited by N1-cIDPR, N1-[(phosphoryl-O-ethoxy)-methyl]-N9-[(phosphoryl-O-ethoxy)-methyl]-hypoxanthine-cyclic pyrophosphate (N1-cIDP-DE) and N1-ethoxymethyl-cIDPR (N1-cIDPRE). In contrast, in primary T cells neither proliferation nor cytokine expression was affected by these compounds. CONCLUSIONS AND IMPLICATIONS: The metabolic stability of N1-cIDPR and its analogues provides an advantage for the development of novel pharmaceutical compounds interfering with cADPR mediated Ca2+ signalling pathways. The differential effects of N1-cIDPR and N1-cIDPRE on proliferation and cytokine expression in primary T cells versus T-lymphoma cells may constitute a starting point for novel anti-tumor drugs.

Cyclic ADP-ribose production by CD38 regulates intracellular calcium release, extracellular calcium influx and chemotaxis in neutrophils and is required for bacterial clearance in vivo.

Cyclic ADP-ribose is believed to be an important calcium-mobilizing second messenger in invertebrate, mammalian and plant cells. CD38, the best-characterized mammalian ADP-ribosyl cyclase, is postulated to be an important source of cyclic ADP-ribose in vivo. Using CD38-deficient mice, we demonstrate that the loss of CD38 renders mice susceptible to bacterial infections due to an inability of CD38-deficient neutrophils to directionally migrate to the site of infection. Furthermore, we show that cyclic ADP-ribose can directly induce intracellular Ca++ release in neutrophils and is required for sustained extracellular Ca++ influx in neutrophils that have been stimulated by the bacterial chemoattractant, formyl-methionyl-leucyl-phenylalanine (fMLP). Finally, we demonstrate that neutrophil chemotaxis to fMLP is dependent on Ca++ mobilization mediated by cyclic ADP-ribose. Thus, CD38 controls neutrophil chemotaxis to bacterial chemoattractants through its production of cyclic ADP-ribose, and acts as a critical regulator of inflammation and innate immune responses.

Reciprocal regulation of polarized cytokine production by effector B and T cells.

Although B cells produce cytokines it is not known whether B cells can differentiate into effector subsets that secrete polarized arrays of cytokines. We have identified two populations of "effector" B cells (Be1 and Be2) that produce distinct patterns of cytokines depending on the cytokine environment in which the cells were stimulated during their primary encounter with antigen and T cells. These effector B cell subsets subsequently regulate the differentiation of naïve CD4+ T cells to TH1 and TH2 cells through production of polarizing cytokines such as interleukin 4 and interferon gamma. In addition, Be1 and Be2 cells could be identified in animals that were infected with pathogens that preferentially induce a Type 1 and Type 2 immune response. Together these results suggest that, in addition to their well defined role in antibody production, B cells may regulate immune responses to infectious pathogens through their production of cytokines.

CD38 signaling in B lymphocytes is controlled by its ectodomain but occurs independently of enzymatically generated ADP-ribose or cyclic ADP-ribose.

CD38 is a type II transmembrane glycoprotein that is expressed by many cell types including lymphocytes. Signaling through CD38 on B lymphocytes can mediate B cell activation, proliferation, and cytokine secretion. Additionally, coligation of CD38 and the B cell Ag receptor can greatly augment B cell Ag receptor responses. Interestingly, the extracellular domain of CD38 catalyzes the conversion of NAD+ into nicotinamide, ADP-ribose (ADPR), and cyclic ADPR (cADPR). cADPR can induce intracellular calcium release in an inositol trisphosphate-independent manner and has been hypothesized to regulate CD38-mediated signaling. We demonstrate that replacement of the cytoplasmic tail and the transmembrane domains of CD38 did not impair CD38 signaling, coreceptor activity, or enzyme activity. In contrast, independent point mutations in the extracellular domain of CD38 dramatically impaired signal transduction. However, no correlation could be found between CD38-mediated signaling and the capacity of CD38 to catalyze an enzyme reaction and produce cADPR, ADPR, and/or nicotinamide. Instead, we propose that CD38 signaling and coreceptor activity in vitro are regulated by conformational changes induced in the extracellular domain upon ligand/substrate binding, rather than on actual turnover or generation of products.

Mice deficient for the ecto-nicotinamide adenine dinucleotide glycohydrolase CD38 exhibit altered humoral immune responses.

CD38 is a membrane-associated ecto-nicotinamide adenine dinucleotide (NAD+) glycohydrolase that is expressed on multiple hematopoietic cells. The extracellular domain of CD38 can mediate the catalysis of NAD+ to cyclic adenosine diphosphoribose (cADPR), a Ca2+-mobilizing second messenger, adenosine diphosphoribose (ADPR), and nicotinamide. In addition to its enzymatic properties, murine CD38 has been shown to act as a B-cell coreceptor capable of modulating signals through the B-cell antigen receptor. To investigate the in vivo physiological function(s) of this novel class of ectoenzyme we generated mice carrying a null mutation in the CD38 gene. CD38-/- mice showed a complete loss of tissue-associated NAD+ glycohydrolase activity, showing that the classical NAD+ glycohydrolases and CD38 are likely identical. Although murine CD38 is expressed on hematopoietic stem cells as well as on committed progenitors, we show that CD38 is not required for hematopoiesis or lymphopoiesis. However, CD38-/- mice did exhibit marked deficiencies in antibody responses to T-cell-dependent protein antigens and augmented antibody responses to at least one T-cell-independent type 2 polysaccharide antigen. These data suggest that CD38 may play an important role in vivo in regulating humoral immune responses.

Arrest of B lymphocyte terminal differentiation by CD40 signaling: mechanism for lack of antibody-secreting cells in germinal centers.

Despite extensive research, the role of CD40 signaling in B cell terminal differentiation remains controversial. Here we show that CD40 engagement arrests B cell differentiation prior to plasma cell formation. This arrest is manifested at a molecular level as a reduction in mRNA levels of secretory immunoglobulin gene products such as mu(s) and J chain as well as the loss of the transcriptional regulator BLIMP-1. Furthermore, the inhibition of B cell differentiation by CD40 engagement could not be overcome by either mitogens or cytokines, but could be reversed by antibodies that interfere with the CD40/gp39 interaction. These data suggest that secretory immunoglobulin is not produced by B cells that are actively engaged by gp39-expressing T cells.

CD38: a new paradigm in lymphocyte activation and signal transduction.

CD38 is a type II transmembrane glycoprotein that is extensively expressed on cells of hematopoietic and non-hematopoietic lineage. Although the intracellular domain of CD38 is not homologous to any known proteins, the extracellular domain of CD38 is structurally related to enzymes in the ADP-ribosyl cyclase family. The structural homology between CD38 and the cyclase family members extends to functional homology, as the extracellular domain of CD38 can mediate the catalysis of beta-NAD+ into nicotinamide, ADP-ribose (ADPR) and, to a lesser extent, into cyclic ADPR-ribose (cADPR). Extensive investigation in other systems has shown that cADPR is an important regulator of intracellular Ca2+ release. Since engagement of CD38 on hematopoietic cells with anti-CD38 Abs has been shown to have potent effects on a number of in vitro cellular responses, we have speculated that cADPR might control CD38-mediated signal transduction. However, it has been difficult to understand how a mediator which is typically an intracellular signaling molecule could potentiate its effects from an extracellular location, thus posing a dilemma which pertains to all ecto-enzymes and the mechanisms by which they regulate signal transduction and cellular processes. This review describes the biologic properties of murine CD38, its role in humoral immunity, and its signal transduction properties in B lymphocytes. We suggest that signaling through CD38 represents a new paradigm in lymphocyte signal transduction and is predicated upon extracellular, rather than intracellular, crosstalk.

Signaling through CD38 augments B cell antigen receptor (BCR) responses and is dependent on BCR expression.

mAbs directed against the ectoenzyme CD38 will induce B cell proliferation in normal resting B lymphocytes, but cannot induce proliferation in B cells that are unresponsive to B cell Ag receptor (BCR) cross-linking. Using the CD38- murine B cell line A20 we have examined the relationship between CD38- and BCR-mediated signaling after transfection of wild-type or mutant CD38 molecules. Although association between CD38 and the BCR was not detectable, co-cross-linking of CD38 and the BCR gave rise to a synergistic response, and expression of CD38 lowered the threshold for BCR-induced responses. Generation of Ig loss variant clones established that coexpression of the BCR was required for CD38-mediated signal transduction. The cytoplasmic tail of Ig alpha or Ig beta rescued CD 38 responsiveness in the CD38+Ig- cells provided that the chimeric molecules were coligated with CD38. Separate experiments indicated that the cytoplasmic tail of CD38 is not required for CD38 signaling. The anti-CD38-induced response was dependent on the influx of extracellular calcium but was not accompanied by detectable tyrosine phosphorylation of any cellular proteins. Together, these data demonstrate that the CD38 molecule can influence BCR-induced responses and that CD38 signaling is dependent on the BCR complex, perhaps to utilize a functional cytoplasmic tail(s) for intracellular signaling.

Expression of murine CD38 defines a population of long-term reconstituting hematopoietic stem cells.

Using a monoclonal antibody to murine CD38, we showed that a population of adult bone marrow cells that expressed the markers Sca-1 and c-kit but lacked the lineage markers Mac-1, GR-1, B220, IgM, CD3, CD4, CD8 and CD5 could be subdivided by the expression of CD38. We showed that CD38high c-kit+ Sca-1+, linlow/-cells sorted from adult bone marrow cultured with interleukin-3 (IL-3), IL-6, and kit-L produced much larger colonies in liquid culture at a greater frequency than their CD38low/- counterparts. In addition, we found that CD36low/ - cells contained most of the day-12 colony-forming units-spleen (CFU-S) but were not long-term reconstituting cells, whereas the population that expressed higher levels of CD38 contained few, but significant, day-12 CFU-S and virtually all the long-term reconstituting stem cells. Interestingly, the CD38high Sca-1+ c-kit+ linlow/- cells isolated from day-E14.5 fetal liver were also found to be long-term reconstituting stem cells. This is in striking contrast to human hematopoietic progenitors in which the most primitive hematopoietic cells from fetal tissues lack the expression of CD38. Furthermore, because antibodies to CD38 could functionally replace antibodies to Thy-1.1 in a stem cell purification procedure, the use of anti-CD38 may be more generally applicable to the purification of hematopoietic stem cells from mouse strains that do not express the Thy-1.1 allele.

Signaling through murine CD38 is impaired in antigen receptor-unresponsive B cells.

CD38 is a 42-kDa membrane associated enzyme which converts NAD into cyclic ADP-ribose (cADPR), a Ca(2+)-mobilizing second messenger, and ADP-ribose (ADPR). Agonistic antibodies to murine CD38 deliver a potent growth co-stimulus to mature splenic B lymphocytes. In this report we demonstrate a striking relationship between CD38-mediated mitogenesis and the ability of surface IgM to promote B cell proliferation. Tolerized B lymphocytes obtained from a double-transgenic mouse model of B cell tolerance do not proliferate in response to antigen stimulation through the Ig receptor or to agonistic anti-CD38 antibodies. Similarly, B-1 cells isolated from the peritoneal cavity of normal mice, and splenic B cells isolated from newborn mice were also unresponsive to both anti-IgM and anti-CD38 stimulation. All of these CD38-unresponsive B cells expressed normal levels of cell surface CD38 and responded to numerous other stimuli. CD38 immunoprecipitated from these B cell populations was normal in size and effectively hydrolyzed NAD, suggesting that the defect in CD38 signaling likely occurs downstream of CD38 itself. Signaling through CD38 and IgM does not always have identical effects on B cells since anti-CD38 cannot deliver inhibitory growth or differentiation signals to normal B cells or immature B cell lines. Nevertheless, the correlative data with these multiple B cell models of unresponsiveness suggests that the signaling pathway utilized by CD38 and IgM intersect, possibly sharing at least one of the crucial components of the Ig receptor signaling cascade.

CD38 unresponsiveness of xid B cells implicates Bruton's tyrosine kinase (btk) as a regular of CD38 induced signal transduction.

CD38 is a 42 kDa membrane-associated ectoenzyme expressed by a large proportion of human and mouse lymphocytes. Agonistic antibodies to CD38 induce a strong proliferative response in lymphocytes additionally co-stimulated with other growth co-factors such as IL-4, IL-2 plus accessory cells or sub-mitogenic doses of endotoxin. We show here that B lymphocytes from unstimulated X-linked immunodeficient (xid) mice are unresponsive to CD38 stimulation, both in terms of proliferative response and surface antigen modulation. This CD38 unresponsiveness is evident in the presence of excess quantities of, and normal responses to, the accessory growth co-stimulants required for this response. CD38 molecules expressed on xid B cells are normal in terms of expression levels, size and enzymatic activity, suggesting that CD38 unresponsiveness reflects a down-stream signaling defect. In light of the recent proposal that the xid gene encodes a tyrosine kinase called Bruton's tyrosine kinase (btk), these data suggest that btk is either an integral component or an indirect regulator of the CD38-induced signal transduction pathway.

Formation and hydrolysis of cyclic ADP-ribose catalyzed by lymphocyte antigen CD38.

CD38 is a 42-kilodalton glycoprotein expressed extensively on B and T lymphocytes. CD38 exhibits a structural homology to Aplysia adenosine diphosphate (ADP)-ribosyl cyclase. This enzyme catalyzes the synthesis of cyclic ADP-ribose (cADPR), a metabolite of nicotinamide adenine dinucleotide (NAD+) with calcium-mobilizing activity. A complementary DNA encoding the extracellular domain of murine CD38 was constructed and expressed, and the resultant recombinant soluble CD38 was purified to homogeneity. Soluble CD38 catalyzed the formation and hydrolysis of cADPR when added to NAD+. Purified cADPR augmented the proliferative response of activated murine B cells, potentially implicating the enzymatic activity of CD38 in lymphocyte function.

Expression cloning of a cDNA encoding a novel murine B cell activation marker. Homology to human CD38.

A rat mAb (NIM-R5) has recently been prepared against a novel murine B cell activation marker. We report here isolation of a cDNA (1-19) encoding the B cell-derived protein recognized by NIM-R5 antibody. This cDNA contains an open reading frame that encodes a polypeptide of 304 amino acids with a predicted molecular weight of 34,500. The existence of a 22-amino acid hydrophobic region located 23 amino acids from the amino terminal of the deduced protein, together with four potential N-linked glycosylation sites, characterize the deduced protein encoded by I-19 cDNA as a typical type II transmembrane glycoprotein. Although I-19 cDNA appears to encode a novel murine protein, its nucleotide sequence and deduced amino acid sequence show approximately 70% homology to the previously reported sequence of human CD38, suggesting that I-19 cDNA encodes either the mouse homologue of CD38 or a closely related protein. Northern blot analysis of the expression of this cDNA product in a variety of cell types, together with immunoprecipitation of the recombinant protein expressed in BaF3 cells, indicated that I-19 cDNA encodes not only the epitope recognized by NIM-R5 but also a protein that is indistinguishable biochemically and in terms of distribution from the murine B cell activation marker recognized by NIM-R5 antibody. Chromosomal mapping studies have localized this locus to the proximal region of mouse chromosome 5. We anticipate that the availability of probes for the murine B cell activation marker recognized by NIM-R5, and the recombinant protein itself, will greatly aid efforts to define the role of this molecule in murine B cell development.

Interleukin-5 (IL-5) and IL-6 define two molecularly distinct pathways of B-cell differentiation.

Interleukin-5 (IL-5) and IL-6 have both been reported to act as B-cell differentiation factors by stimulating activated B cells to secrete antibody. However, it has not been possible to directly compare the effects of these two lymphokines because of the lack of a suitable B-cell line capable of responding to both. We have identified a clonal, inducible B-cell lymphoma, CH12, that has this property. Both IL-5 and IL-6 can independently stimulate increases in steady-state levels of immunoglobulin and J-chain mRNA and proteins, and they both induce the differentiation of CH12 into high-rate antibody-secreting cells. Nevertheless, there are significant differences in the activities of these two lymphokines. First, while IL-6 acts only as a differentiation factor, IL-5 also augments the proliferation of CH12 cells. Second, the differentiation stimulated by IL-5 but not by IL-6 is partially inhibited by IL-4. Inhibition of IL-5-induced differentiation was not at the level of IL-5 receptor expression, since IL-4 did not inhibit IL-5-induced proliferation. Third, IL-5 but not IL-6 stimulated increased mouse mammary tumor proviral gene expression in CH12 cells. These results demonstrate that while both IL-5 and IL-6 may act as differentiation factors for B cells, they induce differentiation by using at least partially distinct molecular pathways. Our results also establish that B cells characteristic of a single stage of development can independently respond to IL-4, IL-5, and IL-6.

MHC class II limits the functional expression of endogenous superantigens in B cells.

The open reading frames of the 3' long terminal repeat of mouse mammary tumor viruses (MMTV) encode superantigens. When expressed with MHC class II molecules on the surface of B cells, superantigens stimulate T cells expressing receptors with particular V beta elements. By using B cell lymphomas as model systems, we demonstrate that stimuli, including certain lymphokines (IL-2 and IL-5) and LPS, that lead to increased MMTV transcript levels in B cells did not increase functional superantigen presentation. Furthermore, stimulation of BCL1 and CH12 cells with LPS resulted in a pronounced reduction in superantigen presentation, even though MMTV transcript levels were increased. In contrast to these effects, functional superantigen expression was increased under conditions in which class II levels were increased, even when MMTV levels remained constant, indicating that the levels of newly synthesized class II are important for functional superantigen expression. These data together imply that the amount of cell surface class II limits superantigen presentation and/or that superantigens can associate only with a limited pool of class II.

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.

Major histocompatibility complex-restricted recognition of retroviral superantigens by V beta 17+ T cells.

It has been established that at least some V beta 17+ T cells interact with an endogenous superantigen encoded by the murine retrovirus, Mtv-9. To analyze the role of major histocompatibility complex (MHC) class II molecules in presenting the Mtv-9 encoded superantigen, vSAG-9 to V beta 17+ hybridomas, a panel of nine hybridomas was tested for their ability to respond to A20/2J (H-2d) and LBK (H-2a) cells which had been transfected with the vSAG-9 gene. Whereas some of the hybridomas recognized vSAG-9 exclusively in the context of H-2a, other hybridomas recognized vSAG-9 exclusively in the context of H-2d or in the context of both H-2d and H-2a. These results suggest that: (a) the class II MHC molecule plays a direct role in the recognition of retroviral superantigen by T cells, rather than serving simply as a platform for presentation; and, (b) it is likely that components of the TCR other than V beta are involved in the vSAG-9/TCR/class II interaction.

Regulated expression of mouse mammary tumor proviral genes in cells of the B lineage.

We evaluated the expression of mouse mammary tumor proviral (MMTV) transcripts during B cell ontogeny and compared levels of RNA in B lymphocytes and B cell lines with levels in other cells of the hematopoietic lineage and in a mammary cell line. We demonstrate that MMTV transcripts are expressed as early as the pro-B cell stage in ontogeny and are expressed at basal constitutive levels throughout most of the B cell developmental pathway. The level of MMTV expression in B cells is similar to constitutive levels in mammary tissues and two to three orders of magnitude greater than in activated T cells. Levels of MMTV transcripts in B cells are not solely due to positional effects. Transient transfection assays showed that MMTV upregulation resulted from transcriptional activation of the viral LTR, indicating that there are specific and inducible transcription factors that regulate MMTV expression in B cells. MMTV transcripts could not be upregulated in pre-B cell lines but could be induced in some mature B cell lines. There was a correlation between the ability to stimulate B cells to secrete antibody and the ability to induce upregulated MMTV expression. Evidence is presented that suggests that the principal transcription factors involved in MMTV expression do not include the B cell factors OTF-2 or NF-kappa B, but rather are likely to be novel factors that are induced during differentiation to antibody secretion. A hypothesis for why mammary tumor viruses are well adapted for expression in cells of the B lineage is proposed, and the implications of this for the documented influence of MMTV gene products on the T cell repertoire are discussed.

Endogenous superantigen expression is controlled by mouse mammary tumor proviral loci.

Superantigens are defined by their ability to stimulate T cells based predominantly on their V beta expression and ability to delete T cells in the thymus when expressed endogenously. We show here that the expression of one endogenous superantigen, Etc-1, is controlled by the expression of the open reading frame region of the 3' long terminal repeat of the mouse mammary tumor proviral gene, Mtv-9. We show that Mtv-8 controls a superantigen with similar specificity, and that both Mtv-8 and Mtv-9 stimulate some V beta 17+ T cells. A third provirus, Mtv-6, controls a superantigen with specificity for V beta 3. Data presented raise the possibility that endogenous superantigens may compete for class II molecules in a single B cell.