Dissociation between IFN-alpha-induced anti-viral and growth signaling pathways

The ability of IFN-alpha to induce an anti-viral state in a wide variety of cell types as well as to inhibit cellular growth has long been appreciated. It is less clear, however, whether both these effects lie downstream of a common signaling pathway. In this study we have taken advantage of an atypical human myeloma cell line (KAS-6/1) displaying a dramatic proliferative response to IFN-alpha in an effort to resolve the signaling requirements for IFN-alpha-induced anti-viral and growth regulatory effects. Thus, we have analyzed the ability of IFN-alpha to induce a number of known receptor-initiated events in this cell line and have compared these responses with those exhibited by a cell lineage- and maturation stage-matched myeloma cell line (ANBL-6) that displays typical IFN-alpha responsiveness. Despite the widely contrasting effects of IFN-alpha on cellular proliferation, IFN-alpha was shown to be comparable in its ability to induce the expression of early response genes as well as induce resistance to viral infection in both cell lines. By contrast, the effects of IFN-alpha on the activation of mitogen-activated protein kinase (MAPK) were strikingly distinct. Finally, although inhibition of MEK and MAPK activation had no effect on the induction of the anti-viral response, it completely blocked IFN-alpha-stimulated proliferation of the KAS-6/1 cells. In summary, our analysis of the role of the MAPK and anti-viral signaling pathways using these two cell lines suggests that the anti-viral and growth regulatory effects of IFN-alpha display a differential requirement for activation of the MAPK pathway.

Differential myeloma cell responsiveness to interferon-alpha correlates with differential induction of p19(INK4d) and cyclin D2 expression

Interferon-alpha (IFN-alpha) has been used as therapy for the treatment of a variety of viral diseases and malignancies including multiple myeloma. The effectiveness of interferon-alpha in treating multiple myeloma, however, has been somewhat variable, and the mechanism(s) accounting for this is not well understood. As a means to examine the basis for the differential effectiveness of this cytokine, we have analyzed IFN-alpha-mediated modulation of the cell cycle in two human myeloma cell lines. These two cell lines, ANBL-6 and KAS-6/1, display dramatically different outcomes in response to this cytokine. Although IFN-alpha inhibited the growth of ANBL-6 cells by blocking cell cycle progression from G0/G1 to S phase, IFN-alpha stimulated cell cycle progression in KAS-6/1 cells. Moreover, the effects of IFN-alpha on cell cycle progression correlated with the phosphorylation status of the retinoblastoma protein. Of interest, IFN-alpha increased cyclin D2 expression and cyclin-dependent kinase activity in the KAS-6/1 cells but not in the ANBL-6 cells. To determine whether the differential effects of IFN-alpha on myeloma cell cycle progression could also result from differences in the expression of cyclin-dependent kinase inhibitors, we examined the effects of IFN-alpha on the induction of cyclin-dependent kinase inhibitors with broad regulatory function (p21 and p27) and those with specificity for G1-associated cyclin-cyclin-dependent kinase complexes (p15, p16, p18, and p19). Although we failed to detect an effect of IFN-alpha on expression levels of p21, p15, p16, or p18, IFN-alpha treatment of the ANBL-6 cell line resulted in induction of p19 expression, whereas it was without effect on the KAS-6/1 cell line. These results suggest that heterogeneity in IFN-alpha-mediated growth effects in myeloma cells correlates with differential induction of cyclin D2 and p19(INK4d) expression.

A role for insulin-like growth factor in the regulation of IL-6-responsive human myeloma cell line growth

Insulin-like growth factors (IGF-I, IGF-II) have long been recognized as important mitogens in many types of malignancies. Because the role of IGFs in growth control of myeloma cells has not been extensively examined, we have used a panel of IL-6-responsive myeloma cell lines to address this issue. Initial studies demonstrated that IGF-I and IGF-II significantly enhanced DNA synthesis by each of the four cell lines, even when assayed in the absence of IL-6. The specificity of the IGF response was confirmed using an IGF-I receptor Ab, and additional studies demonstrated that IGF responsiveness did not result from induction of autocrine IL-6 expression. When IL-6 responsiveness was assayed, three of four cell lines synthesized DNA in response to IL-6 alone; however, the magnitude of responsiveness was greatly enhanced by addition of IGFs. Similar results were obtained when proliferation and cell cycle progression were analyzed. By contrast, the KP-6 cell line was responsive to IL-6 only when IGF was present. Finally, we analyzed the effects of IGF-I on normal B lymphocytes. IGF, however, did not stimulate B cell DNA synthesis, suggesting that IGF responsiveness may represent a key difference between normal and malignant B cells. In summary, these studies suggest that IGFs may play an important role in multiple myeloma by virtue of their ability to directly stimulate tumor cell growth as well as modulate the magnitude of IL-6-driven growth.

A role for insulin-like growth factor in the regulation of IL-6-responsive human myeloma cell line growth

Insulin-like growth factors (IGF-I, IGF-II) have long been recognized as important mitogens in many types of malignancies. Because the role of IGFs in growth control of myeloma cells has not been extensively examined, we have used a panel of IL-6-responsive myeloma cell lines to address this issue. Initial studies demonstrated that IGF-I and IGF-II significantly enhanced DNA synthesis by each of the four cell lines, even when assayed in the absence of IL-6. The specificity of the IGF response was confirmed using an IGF-I receptor Ab, and additional studies demonstrated that IGF responsiveness did not result from induction of autocrine IL-6 expression. When IL-6 responsiveness was assayed, three of four cell lines synthesized DNA in response to IL-6 alone; however, the magnitude of responsiveness was greatly enhanced by addition of IGFs. Similar results were obtained when proliferation and cell cycle progression were analyzed. By contrast, the KP-6 cell line was responsive to IL-6 only when IGF was present. Finally, we analyzed the effects of IGF-I on normal B lymphocytes. IGF, however, did not stimulate B cell DNA synthesis, suggesting that IGF responsiveness may represent a key difference between normal and malignant B cells. In summary, these studies suggest that IGFs may play an important role in multiple myeloma by virtue of their ability to directly stimulate tumor cell growth as well as modulate the magnitude of IL-6-driven growth.

Differential human multiple myeloma cell line responsiveness to interferon-alpha. Analysis of transcription factor activation and interleukin 6 receptor expression

Although IFN-alpha is commonly used as maintenance treatment for multiple myeloma patients, its effectiveness is varied. In this study, we have used a panel of IL-6 responsive myeloma cell lines that vary remarkably in responsiveness to IFN-alpha. Three cell lines were growth arrested by IFN-alpha; however, IFN-alpha significantly stimulated growth of the fourth cell line, KAS-6/1. Our studies have focused on elucidating the mechanism of differential IFN-alpha responsiveness. First, we have shown that IFN-alpha-stimulated growth of the KAS-6/1 cells did not result from induction of autocrine IL-6 expression. Second, analysis of Stats 1, 2, and 3 and IFN regulatory factor-1 (IRF-1) and IRF-2 activation failed to reveal differences between the IFN-alpha growth-arrested or growth-stimulated cells. Third, although IFN-alpha treatment of the IFN-alpha growth-inhibited cell lines reduced IL-6 receptor (IL-6R) expression, IFN-alpha also reduced KAS-6/1 IL-6R expression. Finally, although IFN-alpha treatment reduced IL-6R numbers on each cell line, analysis of Stat protein activation revealed that the receptors were still functional. We conclude that myeloma cell responsiveness to IFN-alpha is heterogeneous and that mechanisms of IFN-alpha-mediated growth inhibition other than IL-6R downregulation must exist in myeloma. Identification of these mechanisms may allow development of agents that are more universally effective than IFN-alpha.

Growth regulatory pathways in myeloma. Evidence for autocrine oncostatin M expression

Expression of autocrine growth factors by myeloma cells is an important mechanism that may contribute to tumor expansion. IL-6 is one of several cytokines that uses the signal transducer gp130 as a receptor component. Of these cytokines, those that have been shown to be paracrine growth factors for some myeloma cells include IL-6, IL-11, ciliary neurotrophic factor, leukemia inhibitory factor, and oncostatin M (OSM). Only IL-6, however, has been identified as an autocrine growth factor for myeloma cells. In this study we used a panel of three IL-6-responsive myeloma cell lines to investigate the expression of other autocrine growth factor loops. Initial studies employing neutralizing mAbs to IL-6 or gp130 revealed that the growth of the DP-6 and KP-6 cell lines was inhibited by both mAbs, whereas the growth of the KAS-6/1 cell line was inhibited only by the anti-gp130 mAb. Anti-OSM neutralizing mAb also inhibited KAS-6/1 cell growth. Autocrine OSM production by the KAS-6/1 cells was confirmed using a sensitive ELISA. Although the anti-OSM mAb had no significant effects on KP-6 and DP-6 cell growth, OSM was detected in DP-6 supernatants. These results suggest that OSM production and responsiveness by myeloma cells are distinct phenotypes and not necessarily related in all myeloma cells. Finally, we analyzed the significance of OSM-mediated myeloma cell growth by assessing the effects of OSM on normal, in vitro-generated plasmablasts. OSM markedly enhanced plasmablast Ig secretion but did not affect growth. Thus, the nature of the response elicited by OSM in myeloma cells is distinct from its effects on normal B lineage cells. Moreover, because gp130-mediated signaling results in myeloma cell growth, autocrine expression of any gp130-utilizing cytokine has the potential to significantly augment tumor expansion.

A role for phosphatidylinositol 3-kinase in generating T cell help for B cell growth and differentiation

Evidence supporting the importance of the 3-phosphoinositide signaling pathway in lymphocyte activation is rapidly accumulating. In our study, we assessed the effects of two PI 3-kinase inhibitors, wortmannin and LY294002, on T cells as a means to analyze the role of the PI 3-kinase-signaling pathway in the generation of T cell help for B cell growth and differentiation. For these studies, B cells were cocultured with CD3-activated mitomycin C-treated T cells to induce B cell responsiveness. Of interest, wortmannin or LY294002 pretreatment of the T cell population significantly inhibited T cell-dependent induction of B cell proliferation and differentiation. The failure of wortmannin-treated CD3-activated mitomycin C-treated T cells to provide help in driving the differentiation of B cells to Ig-secreting cells could not be corrected by the addition of exogenous IL-2. Further studies designed to elucidate the mechanism by which wortmannin-treated T cells failed to provide B cell help indicated that wortmannin and LY294002 significantly inhibited the induction of CD40 ligand and, to a lesser extent, intercellular adhesion molecule-1 expression. These results suggest that the PI 3-kinase-signaling pathway, or other wortmannin- and LY294002-sensitive pathways, may be important for the induction of expression of crucial interaction molecules, such as CD40 ligand, on T cells and thus indicates that D-3 phosphoinositides play a pivotal role in regulating T cell-dependent B cell activation.

Growth regulatory pathways in myeloma. Evidence for autocrine oncostatin M expression

Expression of autocrine growth factors by myeloma cells is an important mechanism that may contribute to tumor expansion. IL-6 is one of several cytokines that uses the signal transducer gp130 as a receptor component. Of these cytokines, those that have been shown to be paracrine growth factors for some myeloma cells include IL-6, IL-11, ciliary neurotrophic factor, leukemia inhibitory factor, and oncostatin M (OSM). Only IL-6, however, has been identified as an autocrine growth factor for myeloma cells. In this study we used a panel of three IL-6-responsive myeloma cell lines to investigate the expression of other autocrine growth factor loops. Initial studies employing neutralizing mAbs to IL-6 or gp130 revealed that the growth of the DP-6 and KP-6 cell lines was inhibited by both mAbs, whereas the growth of the KAS-6/1 cell line was inhibited only by the anti-gp130 mAb. Anti-OSM neutralizing mAb also inhibited KAS-6/1 cell growth. Autocrine OSM production by the KAS-6/1 cells was confirmed using a sensitive ELISA. Although the anti-OSM mAb had no significant effects on KP-6 and DP-6 cell growth, OSM was detected in DP-6 supernatants. These results suggest that OSM production and responsiveness by myeloma cells are distinct phenotypes and not necessarily related in all myeloma cells. Finally, we analyzed the significance of OSM-mediated myeloma cell growth by assessing the effects of OSM on normal, in vitro-generated plasmablasts. OSM markedly enhanced plasmablast Ig secretion but did not affect growth. Thus, the nature of the response elicited by OSM in myeloma cells is distinct from its effects on normal B lineage cells. Moreover, because gp130-mediated signaling results in myeloma cell growth, autocrine expression of any gp130-utilizing cytokine has the potential to significantly augment tumor expansion.

A role for phosphatidylinositol 3-kinase in generating T cell help for B cell growth and differentiation

Evidence supporting the importance of the 3-phosphoinositide signaling pathway in lymphocyte activation is rapidly accumulating. In our study, we assessed the effects of two PI 3-kinase inhibitors, wortmannin and LY294002, on T cells as a means to analyze the role of the PI 3-kinase-signaling pathway in the generation of T cell help for B cell growth and differentiation. For these studies, B cells were cocultured with CD3-activated mitomycin C-treated T cells to induce B cell responsiveness. Of interest, wortmannin or LY294002 pretreatment of the T cell population significantly inhibited T cell-dependent induction of B cell proliferation and differentiation. The failure of wortmannin-treated CD3-activated mitomycin C-treated T cells to provide help in driving the differentiation of B cells to Ig-secreting cells could not be corrected by the addition of exogenous IL-2. Further studies designed to elucidate the mechanism by which wortmannin-treated T cells failed to provide B cell help indicated that wortmannin and LY294002 significantly inhibited the induction of CD40 ligand and, to a lesser extent, intercellular adhesion molecule-1 expression. These results suggest that the PI 3-kinase-signaling pathway, or other wortmannin- and LY294002-sensitive pathways, may be important for the induction of expression of crucial interaction molecules, such as CD40 ligand, on T cells and thus indicates that D-3 phosphoinositides play a pivotal role in regulating T cell-dependent B cell activation.

Phosphatidylinositol 3-kinase activation in normal human B lymphocytes

A variety of signals, mediated via either the B cell Ag receptor (BCR), or non-BCR molecules such as CD40 or cytokine receptors, have been shown to be crucial for the regulation of B cell survival, growth, and differentiation. Although it is clear that a variety of signaling pathways can be activated in B cells in a stimulus-dependent manner, it remains unknown whether differential activation of these signaling pathways is the underlying mechanism controlling B cell fate, i.e., growth vs differentiation. Initial studies reported here indicated that stimulation of highly purified peripheral blood B cells with the polyclonal B cell activators Staphylococcus aureus and CD40 ligand (CD40L) resulted in the rapid induction of phosphatidylinositol 3-kinase (PI 3-kinase) activity. Moreover, pretreatment of B cells with wortmannin, a specific inhibitor of PI 3-kinase, resulted in a complete block in induction of PI 3-kinase activity. The effects of wortmannin, as well as a second PI 3-kinase inhibitor, LY294002, on the induction of both B cell growth and differentiation were therefore investigated. Although these PI 3-kinase inhibitors variably inhibited B cell DNA synthesis in a stimulus-dependent manner, both drugs effected a near-complete block of the ability of each of these stimuli to induce Ig production. Furthermore, separation of B cells into naive IgD+ and postswitch IgD- B cells failed to reveal differential sensitivity of these populations to wortmannin. These results suggest that activation of PI 3-kinase, or other wortmannin- and LY294002-sensitive targets, is a crucial event that occurs during the differentiation of normal human B lymphocytes. The differential sensitivity of B cell responses to inhibitors of PI 3-kinase supports the notion that distinct signal transduction pathways are involved in differentiation vs proliferation of normal human B lymphocytes.

Phosphatidylinositol 3-kinase activation in normal human B lymphocytes

A variety of signals, mediated via either the B cell Ag receptor (BCR), or non-BCR molecules such as CD40 or cytokine receptors, have been shown to be crucial for the regulation of B cell survival, growth, and differentiation. Although it is clear that a variety of signaling pathways can be activated in B cells in a stimulus-dependent manner, it remains unknown whether differential activation of these signaling pathways is the underlying mechanism controlling B cell fate, i.e., growth vs differentiation. Initial studies reported here indicated that stimulation of highly purified peripheral blood B cells with the polyclonal B cell activators Staphylococcus aureus and CD40 ligand (CD40L) resulted in the rapid induction of phosphatidylinositol 3-kinase (PI 3-kinase) activity. Moreover, pretreatment of B cells with wortmannin, a specific inhibitor of PI 3-kinase, resulted in a complete block in induction of PI 3-kinase activity. The effects of wortmannin, as well as a second PI 3-kinase inhibitor, LY294002, on the induction of both B cell growth and differentiation were therefore investigated. Although these PI 3-kinase inhibitors variably inhibited B cell DNA synthesis in a stimulus-dependent manner, both drugs effected a near-complete block of the ability of each of these stimuli to induce Ig production. Furthermore, separation of B cells into naive IgD+ and postswitch IgD- B cells failed to reveal differential sensitivity of these populations to wortmannin. These results suggest that activation of PI 3-kinase, or other wortmannin- and LY294002-sensitive targets, is a crucial event that occurs during the differentiation of normal human B lymphocytes. The differential sensitivity of B cell responses to inhibitors of PI 3-kinase supports the notion that distinct signal transduction pathways are involved in differentiation vs proliferation of normal human B lymphocytes.

Establishment and characterization of three myeloma cell lines that demonstrate variable cytokine responses and abilities to produce autocrine interleukin-6

A consensus regarding myeloma cell growth factor responsiveness and ability to produce autocrine interleukin (IL)-6 has not yet been obtained. In this study, we have established three new human myeloma cell lines (DP-6, KAS-6/1 and KP-6) from patients with aggressive disease. Extensive characterization of these cell lines revealed considerable heterogeneity at several levels. Growth factor responsiveness was initially addressed. Although the potent myeloma cell growth factor, IL-6, induced the proliferation and allowed for the expansion of all three cell lines, a panel of other cytokines elicited heterogeneous responses in each cell line. IL-3, IL-10, IL-11, insulin-like growth factor-I and tumor necrosis factor-alpha also stimulated DNA synthesis in all three cell lines; however, the magnitude of the response was generally lower than that observed in cultures containing IL-6. Transforming growth factor-beta, by contrast, uniformly inhibited the growth of all three cell lines. IL-1alpha and IL-1beta induced the proliferation of the DP-6 cells, but had minimal effects on the KAS-6/1 and KP-6 cells. Interferon (IFN)-alpha stimulated DNA synthesis in the KAS-6/1 cells, but inhibited the proliferation of the DP-6 and KP-6 cells. By comparison, IFN-gamma induced the growth of the KAS-6/1 and DP-6 cells, but inhibited the KP-6 cells. The gp130-associated cytokines, IL-11, leukemia inhibitory factor and oncostatin M, stimulated the growth of the KAS-6/1 cells, but had minimal effects on the DP-6 and KP-6 cells. The cell lines were also analyzed for IL-6 expression. RT-PCR analysis demonstrated that all three cell lines expressed IL-6 mRNA. However, when culture supernatants were tested using a sensitive IL-6 ELISA or IL-6 bioassay only the DP-6 and KP-6 cells were shown to be secreting biologically active IL-6. In summary, although all three of these cell lines were established from myeloma patients, the heterogeneity observed between these cell lines was considerable and may reflect, as well as provide tools to study, the heterogeneity observed in clinical disease.

Differential activation of a calcium-dependent endonuclease in human B lymphocytes. Role in ionomycin-induced apoptosis

The state of B cell maturation profoundly influences the outcome, i.e., activation, growth arrest, or programmed cell death, of a variety of stimuli, including the calcium ionophore, ionomycin. Initial studies confirmed the observation that cell lines representative of immature B cells, i.e., Burkitt lymphoma cell lines, were induced to undergo apoptosis in response to ionomycin, whereas more mature B cell lines did not, and instead underwent cell cycle arrest in the G1 interval. To understand this differential outcome, we have focused on comparing the expression and activation of an endonuclease(s) in cells induced by ionomycin to undergo programmed cell death (Ramos) with cells resistant to ionomycin-induced programmed cell death (Ly1). Our results demonstrated that a low m.w. fraction of an endogenous Ca2+/Mg(2+)-dependent endonuclease was activated in Ramos cells, but not in activated Ly1 cells, following the addition of ionomycin. Of interest, however, low m.w. endogenous endonuclease(s) activity was induced when isolated Ly1 cell nuclei were treated with exogenous calcium instead. Use of field inversion gel electrophoresis further indicated that cleavage of DNA into large m.w. (> 50 kbp) DNA fragments does not precede ionomycin-induced internucleosomal cleavage in Ramos cells or in ionomycin-resistant Ly1 cells. In summary, these data support the conclusion that ionomycin-induced apoptosis involves the activation of a latent, low m.w., calcium-responsive endonuclease and suggest that control of endonuclease depression may contribute to cell-specific regulation of calcium ionophore-induced apoptosis.

Differential activation of a calcium-dependent endonuclease in human B lymphocytes. Role in ionomycin-induced apoptosis

The state of B cell maturation profoundly influences the outcome, i.e., activation, growth arrest, or programmed cell death, of a variety of stimuli, including the calcium ionophore, ionomycin. Initial studies confirmed the observation that cell lines representative of immature B cells, i.e., Burkitt lymphoma cell lines, were induced to undergo apoptosis in response to ionomycin, whereas more mature B cell lines did not, and instead underwent cell cycle arrest in the G1 interval. To understand this differential outcome, we have focused on comparing the expression and activation of an endonuclease(s) in cells induced by ionomycin to undergo programmed cell death (Ramos) with cells resistant to ionomycin-induced programmed cell death (Ly1). Our results demonstrated that a low m.w. fraction of an endogenous Ca2+/Mg(2+)-dependent endonuclease was activated in Ramos cells, but not in activated Ly1 cells, following the addition of ionomycin. Of interest, however, low m.w. endogenous endonuclease(s) activity was induced when isolated Ly1 cell nuclei were treated with exogenous calcium instead. Use of field inversion gel electrophoresis further indicated that cleavage of DNA into large m.w. (> 50 kbp) DNA fragments does not precede ionomycin-induced internucleosomal cleavage in Ramos cells or in ionomycin-resistant Ly1 cells. In summary, these data support the conclusion that ionomycin-induced apoptosis involves the activation of a latent, low m.w., calcium-responsive endonuclease and suggest that control of endonuclease depression may contribute to cell-specific regulation of calcium ionophore-induced apoptosis.

Introduction of an activated N-ras oncogene alters the growth characteristics of the interleukin 6-dependent myeloma cell line ANBL6

Multiple myeloma (MM) is a late-stage B-cell cancer with an unknown etiology. Activating mutations of the N-ras and K-ras oncogenes occur with a high frequency in myeloma and, therefore, may play a role in the pathogenesis of the disease. To study the role of N-ras-activating mutations in the regulation of myeloma tumor growth, we introduced a constitutively active N-ras cDNA containing a glutamine to arginine (CAA-CGA) amino acid substitution at codon 61 into the interleukin 6 (IL-6)-dependent myeloma cell line ANBL6. Expression of the mutant N-ras cDNA resulted in significant IL-6-independent growth, as well as augmentation of growth at suboptimal concentrations of IL-6. The IL-6-independent growth pattern was not the result of activation of autocrine IL-6 production in the mutant N-ras-expressing population because neutralizing antibodies to the IL-6 receptor and to IL-6 had no effect on the rate of DNA synthesis in the absence of IL-6. Furthermore, mutant N-ras expression decreased the percentage of cells undergoing apoptosis in the absence of IL-6. These data suggest that activating mutations of the ras oncogenes may result in growth factor independence accompanied by a suppression of apoptosis in MM. Therefore, the use of therapies designed to block IL-6 action in MM may have less of an impact on tumors bearing activated ras mutations.

Expression and function of Fas (APO-1/CD95) in patient myeloma cells and myeloma cell lines

Cross-linkage of the Fas antigen induces programmed cell death in many normal and malignant lymphoid cells by a process known as apoptosis. In this study, we examined the sensitivity of myeloma cell lines and patient plasma cells to a cytolytic anti-Fas monoclonal antibody (MoAb). Eight of 10 myeloma cell lines were induced to undergo programmed cell death by anti-Fas MoAb as determined by DNA fragmentation and morphologic changes. Of the two myeloma cell lines that were resistant to anti-Fas treatment, one did not express the Fas antigen. Only the U266 cell line expressed Fas, but was not killed by the anti0Fas MoAb. To extend these studies, we have examined the expression and function of Fas in freshly isolated CD38hiCD45neg-int plasma cells from patients with multiple myeloma (MM), monoclonal gammopathy of undetermined significance (MGUS), and primary amyloidosis (AL). By three-color flow cytometry, we found Fas expression in CD38hiCD45neg-int plasma cells from all patient groups to be variable, as Fas was expressed in 15 of 28 MM, 3 of 6 MGUS, and 2 of 7 AL patients. In morphologic studies of apoptosis, Fas-positive myeloma cells in patient bone marrow mononuclear cell (MNC) cultures appeared to be resistant to anti-Fas-mediated apoptosis. By contrast, purified myeloma cells from the same patient were sensitive to anti-Fas treatment, suggesting the presence of a protective factor(s) in unseparated MNC cultures that may inhibit Fas-induced apoptosis of plasma cells. Of interest, serum from normal individuals and myeloma patients also protected myeloma cell lines from undergoing Fas-mediated apoptosis. These studies show that Fas expression in myeloma cell lines and CD38hiCD45neg-int patient plasma cells is variable and may reflect a variance in the maturation status of the various plasma cell populations. Moreover, Fas-mediated killing of patient cells and myeloma cell lines was also variable, which may be influenced, in part, by the presence of a soluble protective factor.

Role of IL-12 in human B lymphocyte proliferation and differentiation

The role of IL-12 in human peripheral blood B cell responsiveness was examined. To analyze the ability of IL-12 to directly mediate B cell growth and/or differentiation, FACS-purified (> 99% pure) B cells were studied and a polyclonal B cell-activating system utilizing Cowan I Staphylococcus aureus was used. Whereas IL-2 is highly effective in this system in promoting both B cell growth and differentiation, IL-12 was observed only to augment modestly B cell growth and to be ineffective by itself as a B cell differentiation factor for S. aureus-stimulated B cells. However, IL-12 markedly enhanced Ig secretion when added in the presence of IL-2. Moreover, when the ability of IL-12 to augment IL-2-dependent B cell Ig secretion was compared with the ability of several known auxiliary B cell differentiation factors, IL-12 was observed to be the most potent cytokine that could costimulate with IL-2. Analysis of IL-12-stimulated B cell cultures failed to reveal outgrowth of T cells and NK cells. In addition, assessment of IFN-gamma levels in IL-12-driven B cell culture supernatants and analysis of IFN-gamma effects on B cell responses added additional support to the conclusion that IL-12 directly modulates B cell function. Finally, these results suggest that IL-12 is a potent constimulus of B cell differentiation and that the signals conveyed by IL-12 seem to be qualitatively distinct from the differentiative signals delivered by other cytokines such as IL-2.

Role of IL-12 in human B lymphocyte proliferation and differentiation

The role of IL-12 in human peripheral blood B cell responsiveness was examined. To analyze the ability of IL-12 to directly mediate B cell growth and/or differentiation, FACS-purified (> 99% pure) B cells were studied and a polyclonal B cell-activating system utilizing Cowan I Staphylococcus aureus was used. Whereas IL-2 is highly effective in this system in promoting both B cell growth and differentiation, IL-12 was observed only to augment modestly B cell growth and to be ineffective by itself as a B cell differentiation factor for S. aureus-stimulated B cells. However, IL-12 markedly enhanced Ig secretion when added in the presence of IL-2. Moreover, when the ability of IL-12 to augment IL-2-dependent B cell Ig secretion was compared with the ability of several known auxiliary B cell differentiation factors, IL-12 was observed to be the most potent cytokine that could costimulate with IL-2. Analysis of IL-12-stimulated B cell cultures failed to reveal outgrowth of T cells and NK cells. In addition, assessment of IFN-gamma levels in IL-12-driven B cell culture supernatants and analysis of IFN-gamma effects on B cell responses added additional support to the conclusion that IL-12 directly modulates B cell function. Finally, these results suggest that IL-12 is a potent constimulus of B cell differentiation and that the signals conveyed by IL-12 seem to be qualitatively distinct from the differentiative signals delivered by other cytokines such as IL-2.

Sequence, expression and function of an mRNA encoding a soluble form of the human interleukin-6 receptor (sIL-6R)

Soluble receptors have been shown to be potent immunomodulators of their respective ligands. Since IL-6 is a central growth factor for myeloma cells, an sIL-6R may modulate IL-6 activity. We have previously reported a novel IL-6R mRNA from myeloma cells that exhibits a 94-nt deletion of the entire transmembrane domain from codons 356 (G-TG) to 387 (AG-G). The transmembrane domain deletion results in a shift in the translational reading frame with the insertion of 10 new amino acids followed by a stop codon. Sequence analysis shows the ligand-binding domain of the sIL-6R to be identical to that of the membrane-bound IL-6R up to the transmembrane domain deletion. The sIL-6R cDNA was expressed in QT-6 fibroblasts and PA-1 ovarian cells using the expression vector pCDM8. Supernates were immunoprecipitated with anti-IL-6R antibody and cells transfected with the sIL-6R cDNA produced a single band with a molecular weight of 50-55 kDa. This molecular weight corresponds to the size of the sIL-6R protein observed in normal human urine. Supernates were collected from mock or sIL-6R transfected PA-1 cells after 48 hours and assayed for their ability to stimulate or suppress the growth of an IL-6 dependent cell line, ANBL-6. Soluble IL-6R alone had no effect on the growth of the ANBL-6 cells. However, the growth of ANBL-6 cells by sIL-6R was potentiated in the presence of IL-6 and could be blocked by anti-IL-6 antibody. The above results suggest that, in the presence of IL-6, sIL-6R associates with gp130 leading to signal transduction and cell growth.

Inhibition of human B lymphocyte cell cycle progression and differentiation by rapamycin

In this study, we have analyzed the effects of the immunosuppressive agent rapamycin on the activation of highly purified normal human B lymphocytes. When the polyclonal activators Staphylococcus aureus (SA) and soluble CD40 ligand (CD40L) were used to stimulate B cells, rapamycin inhibited both interleukin 2 (IL2)-dependent and -independent proliferation, as well as IL2-dependent differentiation into antibody-secreting cells. Cell cycle analysis indicated that rapamycin inhibited the progression of SA+IL2-stimulated B cells past the mid-G1 phase of the cell cycle. To begin to identify rapamycin-sensitive signaling events essential for B cell activation, we examined the effects of rapamycin on p34cdc2 and p33cdk2 kinase activities. SA+IL2 stimulation induced the activation of both cyclin-dependent kinases. Of interest, rapamycin abrogated the activation of both p34cdc2 and p33cdk2. Our results indicate therefore that rapamycin inhibits a number of SA- and CD40L-inducible events that may be necessary for both entry into S phase and for permitting subsequent B cell differentiation. These studies emphasize the utility of this drug as a tool to begin to dissect the activation pathways utilized by human B cells, as well as to provide implications for the therapeutic use of rapamycin in vivo.

CD40 expression in malignant plasma cells. Role in stimulation of autocrine IL-6 secretion by a human myeloma cell line

Myeloma is a neoplasia characterized by the accumulation of malignant plasma cells in the bone marrow. In these studies, we have demonstrated that CD40 is expressed in human myeloma cells and have used a recently established IL-6-dependent myeloma cell line, ANBL-6, to examine the potential function of CD40 expression in myeloma cells. In addition to its expression on the ANBL-6 cells, we show that CD40 is expressed on freshly isolated myeloma cells from seven of seven patients tested. To address the role of CD40 expression in myeloma cells, we have examined the responsiveness of the ANBL-6 cell line to a CD40-specific mAb, G28-5. This cell line has previously been shown to proliferate only in response to IL-6. Of interest in this study, G28-5 also induced proliferation of the ANBL-6 cells. This proliferation was substantially inhibited by an IL-6-neutralizing mAb. Analysis of ANBL-6 cell culture supernatants by ELISA demonstrated that G28-5-stimulated cells secreted significant levels of IL-6, whereas unstimulated cell culture supernatants contained undetectable levels of IL-6. Furthermore, CV-1/EBNA cells expressing the human CD40 ligand also induced the proliferation of the ANBL-6 cell line, an effect that was inhibited by the anti-IL-6 mAb. Lastly, RNA blot analysis demonstrated an increase in IL-6 message in G28-5-stimulated ANBL-6 cells over unstimulated cells. These results indicate that the primary mechanism of anti-CD40-stimulated proliferation of the ANBL-6 cells is the induction of autocrine IL-6 production. Moreover, these data suggest that the expression of CD40 in malignant plasma cells may play a role in tumor cell expansion, possibly by stimulating the induction of autocrine IL-6 secretion.

CD40 expression in malignant plasma cells. Role in stimulation of autocrine IL-6 secretion by a human myeloma cell line

Myeloma is a neoplasia characterized by the accumulation of malignant plasma cells in the bone marrow. In these studies, we have demonstrated that CD40 is expressed in human myeloma cells and have used a recently established IL-6-dependent myeloma cell line, ANBL-6, to examine the potential function of CD40 expression in myeloma cells. In addition to its expression on the ANBL-6 cells, we show that CD40 is expressed on freshly isolated myeloma cells from seven of seven patients tested. To address the role of CD40 expression in myeloma cells, we have examined the responsiveness of the ANBL-6 cell line to a CD40-specific mAb, G28-5. This cell line has previously been shown to proliferate only in response to IL-6. Of interest in this study, G28-5 also induced proliferation of the ANBL-6 cells. This proliferation was substantially inhibited by an IL-6-neutralizing mAb. Analysis of ANBL-6 cell culture supernatants by ELISA demonstrated that G28-5-stimulated cells secreted significant levels of IL-6, whereas unstimulated cell culture supernatants contained undetectable levels of IL-6. Furthermore, CV-1/EBNA cells expressing the human CD40 ligand also induced the proliferation of the ANBL-6 cell line, an effect that was inhibited by the anti-IL-6 mAb. Lastly, RNA blot analysis demonstrated an increase in IL-6 message in G28-5-stimulated ANBL-6 cells over unstimulated cells. These results indicate that the primary mechanism of anti-CD40-stimulated proliferation of the ANBL-6 cells is the induction of autocrine IL-6 production. Moreover, these data suggest that the expression of CD40 in malignant plasma cells may play a role in tumor cell expansion, possibly by stimulating the induction of autocrine IL-6 secretion.

Coexistence of aneuploid subclones within a myeloma cell line that exhibits clonal immunoglobulin gene rearrangement: clinical implications

A new human myeloma cell line, ANBL-6, was established and characterized at the genotypic and phenotypic levels. The cells exhibit a clonally rearranged immunoglobulin gene locus and resemble plasma cells morphologically. The ANBL-6 cells also exhibited an absolute dependence on exogenous interleukin 6 for growth. Of interest, DNA ploidy analysis suggested the existence of a near-diploid as well as a near-tetraploid population in this cell line. Cytogenetic studies confirmed the existence of two aneuploid karyotypes and further revealed a clonal relationship between the two karyotypes, as evidenced by numerous shared structural abnormalities. To determine whether the near-diploid cells functioned as stem cells for the near-tetraploid population, the near-diploid population was separated via flow cytometry and recultured prior to ploidy analysis. This population was observed to remain predominantly near-diploid over time, suggesting that these cells did not function as stem cells for the near-tetraploid population. However, the near-tetraploid cells did exhibit a growth advantage in vitro. Moreover, sequential ploidy analysis performed retrospectively on fresh bone marrow cells from the patient also suggested that there was an expansion of the near-tetraploid population during clinical relapse. These results suggest that both populations are self-regenerating and reflect the consequences of clonal evolution in the myeloma tumor. The coexistence of clonally related subclones with shared chromosomal abnormalities, however, suggests that the near-tetraploid subclone was derived from the near-diploid subclone at an unknown time during tumorigenesis.