Immunoprophylaxis of RSV infection: advancing from RSV-IGIV to palivizumab and motavizumab

Antibodies mediate humoral immune responses and play key roles in the defense of viral infection by the recognition, neutralization, and elimination of viruses from the circulation. For the prevention of respiratory syncytial virus (RSV) infection, the natural immune response to RSV from pooled human plasma has been harvested and successfully developed as a prophylactic polyclonal RSV hyperimmune globulin, RespiGam (RSV-IGIV; MedImmune, Gaithersburg, MD). The success of RSV-IGIV validated the immunoprophylaxis approach for RSV prevention and led to the development of Synagis (palivizumab; MedImmune, Gaithersburg, MD), a humanized monoclonal antibody (mAb) that binds to the RSV F protein. Palivizumab is a potent anti-RSV mAb that is about 50-fold more potent than RSV-IGIV, and since obtaining regulatory approval in 1998 it has been used extensively to help prevent severe RSV disease in high-risk infants and children. However, a very small number of patients receiving the drug do not appear to be adequately protected. To further improve protection against RSV, we have applied a directed evolution approach to enhance the binding of palivizumab to F protein by manipulation of both the on and off rates. These efforts have yielded a more potent second-generation mAb, motavizumab, which is currently under study in phase III clinical trials. Most recently, a third generation mAb, Numax-YTE, has been generated with the intent to extend the serum half-life of the mAb in humans. If successfully developed, this drug may offer the opportunity for less frequent dosing, obviating the need for the monthly treatments that are required with palivizumab. The development of these anti-RSV approaches exemplifies the accelerated pace of drug development made possible with cutting-edge antibody engineering technologies.

Serum levels of sCD137 (4-1BB) ligand are prognostic factors for progression in acute myeloid leukemia but not in non-Hodgkin's lymphoma

CD178 (Fas/APO-1 ligand) and CD137 ligand (CD137L) have previously been described in sera of patients with various malignancies and play an important role in the pathogenesis of various diseases. Recently, we demonstrated that low levels of soluble (s) CD137L and high levels of sCD178 correlate significantly with a long progression free survival in patients with myelodysplastic syndrome (MDS). In this study, we correlated sCD137L and sCD178 levels in sera of 42 samples of patients with acute myeloid leukemia (AML) and 46 samples of patients with non-Hodgkin's lymphoma (NHL) with stages, subtypes, and the clinical course of the diseases and determined cut-off values with maximum probability for significant differentiation between cases with higher/lower probability for progress free survival. In contrast to patients with MDS, surprisingly no correlation between sCD178 levels and different subtypes and stages or with prognosis in AML or NHL were observed. Regarding sCD137L, NHL-patients displayed lower levels compared with AML. Statistically significant higher median levels of sCD137L are present in patients with undifferentiated AML (M1/M2, 1,470 pg/mL), poor cytogenetic risk (288 pg/mL) and higher levels of BM-blasts (186 pg/mL) compared with patients with monocytoid AML (M4/M5, 89 pg/mL), intermediate cytogenetic risk (59 pg/mL) and lower levels of BM-blasts (14 pg/mL) respectively. Furthermore, in AML patients sCD137L levels correlate significantly with the probabilities to achieve complete remission (CR), stay in CR or with progress of the disease. Taken together, our data demonstrate that sCD137L can be used as a prognostic factor not only in MDS but also in AML.

4-1 BB ligand--just another costimulating molecule?

Initially, scientific interest in the 4-1BB/4-1BB Ligand system focused on the role of the 4-1BB (CD137) receptor in the costimulation of T cells. More recently, evidence is accumulating that 4-1BBL is more than "just" the ligand for a costimulatory molecule. In this review we discuss the functional properties of 4-1BB Ligand such as its preference for CD8 positive T cells and the differences to costimulation via the B7/CD28 system. Furthermore, the available data regarding its ability to transduce signals bidirectionally, i.e. also back into the ligand bearing cell, its release as a soluble form following shedding from the cell surface, and its role in the interaction of tumor cells with the immune system are reviewed.

Retinoic acid and vitamin E modulate expression and release of CD178 in carcinoma cells: consequences for induction of apoptosis in CD95-sensitive cells

CD178 (CD95-ligand) is expressed on several tumor cells and likely influences the interaction of the tumor with the host immune system. However, little is known about the mechanisms that regulate its expression on the cell surface. We have evaluated the ability of various compounds and cytokines to regulate cell surface expression and release of soluble CD178 in various carcinoma cell lines. Vitamin E succinate (VES) and retinoic acid (RA) were found to reduce CD178 surface expression, whereas interferon-gamma stimulated a slight upregulation. At 48 h, the regulation of surface CD178 by VES and RA arose from a small decrease in CD178 mRNA and to a greater extent due to an increase in the release of soluble CD178; the latter was blocked by addition of a metalloproteinase inhibitor. Accordingly, VES and RA treatment diminished the ability of tumor cells to kill CD95-sensitive cells and this effect was markedly reduced by the presence of a metalloproteinase inhibitor. Our results indicate that, in vitro, CD178 expression on the cell surface of tumor cells can be regulated by agents that alter both expression and release of the ligand. In vivo, such treatments may play an important role in the outcome of tumor sensitivity or resistance to host immune mechanisms.

Soluble CD137 (4-1BB) ligand is released following leukocyte activation and is found in sera of patients with hematological malignancies

Expression of CD137 ligand (4-1BBL), a member of the TNF family of proteins, has been reported on several types of APCs, various carcinoma cells, and can be induced on activated T cells. In this study, we report that the soluble ligand was released constitutively at low levels from leukocytes and at higher levels following cellular activation. Release from cells was blocked by addition of a metalloproteinase inhibitor which concomitantly caused the accumulation of 4-1BBL on the cell surface. In addition, we show that a soluble form of 4-1BBL was present at high levels in the sera of some patients with various hematological diseases, but only at low levels in healthy donors. Soluble 4-1BBL was active in that it competed with recombinant 4-1BBL for binding to the 4-1BB receptor and was able to costimulate IL-2 and IFN-gamma release from peripheral T cells. These results indicate that the release of soluble 4-1BBL from the cell surface is mediated by one or more sheddases and likely regulates 4-1BB-4-1BBL interactions between cells in vivo. Cleavage of 4-1BBL to an active soluble form would alter both proximal and distal cellular responses, including cell survival and costimulatory or inflammatory responses, that are mediated through the 4-1BB pathway. This, in turn, would likely alter disease progression or outcome.

Recombinant human Fas ligand induces alveolar epithelial cell apoptosis and lung injury in rabbits

This study investigated whether recombinant human soluble Fas ligand (rh-sFasL) induces apoptosis of primary type II pneumocytes in vitro and lung injury in vivo. Type II cells isolated from normal rabbit lung expressed Fas on their surface and became apoptotic after an 18-h incubation with rh-sFasL. Fas expression in normal rabbit lungs was localized by immunohistochemistry to alveolar and airway epithelia and alveolar macrophages. The administration of 10 microg of rh-sFasL into the right lungs of rabbits resulted 24 h later in both significantly more bronchoalveolar lavage fluid total protein and significantly more tissue changes compared with those in the left lungs, which received rh-sFasL plus Fas:Ig (a fusion protein that binds and blocks sFasL). Tissue changes included thickening of the alveolar walls, neutrophilic infiltrates, apoptotic (terminal deoxynucleotidyltransferase-mediated dUTP nick end labeling-positive) cells in the alveolar walls, and increased expression of interleukin-8 by alveolar macrophages (as determined by immunohistochemistry). We conclude that the alveolar epithelium of normal rabbits expresses Fas and that sFasL induces lung injury and inflammation in rabbits.

Cell cycle phase-specific survival of CD95 ligand-challenged Jurkat cells: upregulation of heat-shock response

An important means of regulating T-cell function occurs via physical deletion (cytolysis) of unnecessary/unwanted T cells. Among cytolytic pathways, CD95 (Fas)-based killing plays a prominent role. Although activation of T cells results in rapid upregulation of surface CD95 expression, sensitivity to CD95-based killing lags behind. To assess determinants of resistance to CD95-based killing, we used Jurkat cells as a model. Analysis of the 10% survivors of a LD(90) dose of CD95 ligand (CD95L) at 24 h demonstrated them to arise preferentially from the S + G2/M phases of the cell cycle and to remain clustered in S + G2/M without undergoing cell division. Protein immunoblot, immunocytochemistry, and RT-PCR analyses demonstrated that hsp72 was markedly upregulated in CD95L survivors within hours of CD95L challenge, indicative of a heat-shock response. Indeed, exposure of Jurkat cells to bona fide heat shock did markedly upregulate hsp72 and, upon subsequent CD95L challenge, did greatly enhance cell survival with persistent clustering to S + G2/M. These findings collectively suggest that in response to a CD95L insult, development of a heat-shock response above some critical threshold level can protect against lethality. This raises the possibility that exaggerated and/or protracted heat-shock responses under in vivo conditions may favor the survival of T cells (including autoaggressive T cells) that otherwise would be destined to die via a CD95-based pathway.

Stimulation of inflammatory responses in vitro and in vivo by lipophilic HMG-CoA reductase inhibitors

The enzyme 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase catalyses the rate limiting step in cholesterol biosynthesis and is markedly inhibited by the statin family of drugs. The effect of statins on lipid lowering is clearly defined, but the ability of the drugs to directly regulate inflammatory functions has not been well explored. In this report, we show that there are differences among the statins in their capacity to induce proinflammatory responses both in human monocytes in vitro, and in leukocytes in mice in vivo. Treatment of human monocytes with lipophilic statins alone stimulated the production of MCP-1, IL-8, TNF-alpha and IL-1 beta and markedly sensitized the cells to subsequent challenge with inflammatory agents. Lipophilic statins also increased the production of reactive oxygen species in monocytes. In contrast, pretreatment of cells with the hydrophilic pravastatin did not induce these heightened inflammatory responses. Furthermore, treatment of mice with lipophilic statins caused a markedly higher influx of leukocytes into the inflamed peritoneal cavity following challenge with thioglycollate. Overall, these results demonstrate that the lipophilic statins influence a regulatory pathway in monocytes that controls cytokine production and that the statins induce different pro-inflammatory responses both in vitro and in vivo.

Differential susceptibility to CD95 (Apo-1/Fas) and MHC class II-induced apoptosis during murine dendritic cell development

Disappearance of antigen presenting cells (APC) from the lymph node occurs following antigen specific interactions with T cells. We have investigated the regulation of CD95 (Apo-1/Fas) induced apoptosis during murine dendritic cell (DC) development. Consistent with the moderate levels of CD95 surface expression and low, or absent, MHC class II expression, immature DC in bone marrow cultures were highly sensitive to CD95 induced apoptosis, but insensitive to class II mediated apoptosis. In contrast, mature splenic, epidermal and bone marrow derived DC were fully resistant to CD95 induced cell death, but sensitive to class II induced apoptosis. Although caspase 3 and 8 activation was detected in immature DC undergoing CD95L-induced apoptosis, the pan-caspase inhibitor zVAD-fmk did not inhibit the early events of CD95-induced mitochondrial depolarisation or phosphatidyl serine exposure and only partially inhibited the killing of immature DC. In contrast, zVAD-fmk was completely effective in preventing CD95L mediated death of murine thymocytes. Collectively, these data do not support a major role of CD95: CD95L ligation in apoptosis of mature DC, but rather emphasise the existence of distinct pathways for the elimination of DC at different stages of maturation.

Inhibitors of HMG-CoA reductase sensitize human smooth muscle cells to Fas-ligand and cytokine-induced cell death

Hydroxymethylglutaryl CoA (HMG CoA) reductase inhibitors, or statins, have been shown to reduce atherosclerotic cardiovascular morbidity and mortality. Atherosclerotic plaque lesions can be chronically inflamed and vulnerable to rupture or stable and less rupture-prone. Human smooth muscle cells (SMC) are critically important in maintaining the stability of atherosclerotic plaques. This stability may be greatly influenced by pro-inflammatory mediators such as IFN-gamma, TNF-alpha, and Il-1beta and Fas ligand (FasL) that are present in human atheroma. The purpose of the present study was to examine the effect of the statins on apoptosis of SMC. We have found that SMC are normally resistant to Fas or cytokine-induced apoptosis, but can be sensitized to these agents with pharmacological concentrations of some statins. Simvastatin and lovastatin strongly sensitized the cells to apoptotic agents while atorvastatin was less effective. In contrast to the lipophilic statins, the hydrophilic statin pravastatin did not induce this sensitization of SMC to apoptosis. Treatment of SMC with either mevalonate, the product of the HMG-CoA reductase, or geranylgeranylpyrophosphate, a down stream intermediate, prevented lipophilic statin-induced sensitization to apoptosis. These results suggest that prenylation of one or more proteins is critically involved in regulating the sensitivity of SMC to apoptotic stimuli. Our data support the emerging evidence that through this pathway the various statins may have effects which are beyond a simple lowering of the levels of circulating cholesterol.

Constitutive expression of functional 4-1BB (CD137) ligand on carcinoma cells

Members of the TNF superfamily, including Fas, Fas ligand, and CD40, have been shown to be expressed on tumor cells. In the studies described in this work, we report that another family member, the ligand for 4-1BB (CD137), is expressed on various human carcinoma cell lines, on cells of solid tumors derived from these cell lines, and cells obtained from human tumors. Expression of 4-1BB ligand (4-1BBL) mRNA was detected by both RT-PCR and Northern blot analysis, and expression of 4-1BBL protein was detected by Western blot analysis of whole cell lysates and by FACS analysis of tumor cells and cell lines. Incubation of tumor cells with a 4-1BB-Ig fusion protein led to the production of IL-8 by the cells, demonstrating that the 4-1BBL is functionally active and signals back into the tumor cells. Furthermore, 4-1BBL expressed on the carcinoma cells functioned as a costimulatory molecule for the production of cytokines (most notably IFN-gamma) in cocultures of T cells and tumor cells. These findings suggest that 4-1BBL expressed on carcinoma cells may significantly influence the outcome of a T cell-tumor cell interaction.

Promotion of activated human B cell apoptosis and inhibition of Ig production by soluble CD95 ligand: CD95-based downregulation of Ig production need not culminate in activated B cell death

CD95/CD95L interactions are vital to normal lymphoid homeostasis and in the protection against autoimmunity. To directly assess the effects of CD95L on activated B cell survival and Ig responses, purified human peripheral blood B cells, activated in vitro with SAC + rIL2, were incubated with a soluble CD95L fusion protein (fp) and assayed for apoptosis and IgG/IgM production. CD95L fp reproducibly increased apoptosis of these activated B cells and inhibited their Ig production. However, CD95L fp-mediated effects on activated B cell survival could be uncoupled from those on Ig production in that a soluble CD40L fp was incapable of reversing CD95L fp-mediated downregulation of Ig responses despite inhibiting CD95L fp-mediated apoptosis. Moreover, despite the specific caspase-8 inhibitor z-IETD-fmk substantially protecting transformed CL-01 B cells from CD95L fp-mediated apoptosis and permitting their ongoing proliferation, caspase-8 inhibition had no protective effects on CD95L fp-mediated inhibition of constitutive IgM production by CL-01 B cells. Collectively, these results point to a CD95-based downregulatory pathway in activated B cells that need not necessarily culminate in their death.

Soluble Fas ligand induces epithelial cell apoptosis in humans with acute lung injury (ARDS)

The goals of this study were to determine whether the Fas-dependent apoptosis pathway is active in the lungs of patients with the acute respiratory distress syndrome (ARDS), and whether this pathway can contribute to lung epithelial injury. We found that soluble Fas ligand (sFasL) is present in bronchoalveolar lavage (BAL) fluid of patients before and after the onset of ARDS. The BAL concentration of sFasL at the onset of ARDS was significantly higher in patients who died. BAL from patients with ARDS induced apoptosis of distal lung epithelial cells, which express Fas, and this effect was inhibited by blocking the Fas/FasL system using three different strategies: anti-FasL mAb, anti-Fas mAb, and a Fas-Ig fusion protein. In contrast, BAL from patients at risk for ARDS had no effect on distal lung epithelial cell apoptosis. These data indicate that sFasL is released in the airspaces of patients with acute lung injury and suggest that activation of the Fas/FasL system contributes to the severe epithelial damage that occurs in ARDS. These data provide the first evidence that FasL can be released as a biologically active, death-inducing mediator capable of inducing apoptosis of cells of the distal pulmonary epithelium during acute lung injury.

Superantigen-driven, CD8+ T cell-mediated down-regulation: CD95 (Fas)-dependent down-regulation of human Ig responses despite CD95-independent killing of activated B cells

Staphylococcal superantigens, including staphylococcal enterotoxin B (SEB), promote vigorous T cell-dependent Ig responses at low dose (0.01 ng/ml). In contrast, more mitogenic high dose SEB (100 ng/ml) profoundly inhibits the Ig responses. To assess the contribution of CD8+ T cells to this inhibition, high dose SEB-dependent killing of activated B cells and down-regulation of Ig responses were determined. Rapid killing (4 h) of activated B cells was effected by high dose SEB-activated CD8+ T cells (CD8*), but not by high-dose SEB-activated CD4+ T cells (CD4*), and required the presence of high dose SEB during the cytotoxicity assay. This killing was abrogated by chelation of extracellular calcium or by treatment with concanamycin A but was only modestly affected by treatment with brefeldin A, suggesting a perforin-based pathway of killing. Despite their widely disparate abilities to rapidly kill activated B cells, CD8* and CD4* demonstrated similar quantitative abilities to effect high dose SEB-dependent down-regulation of Ig responses. Antagonist anti-CD95 mAb substantially reversed high dose SEB-dependent downregulation effected by CD8* but had no appreciable effects on high dose SEB-dependent killing of activated B cells. These observations strongly suggest that the small fraction of activated B cells that secrete Ig are selectively sensitive to CD95-based killing but resistant to CD95-independent killing. This finding may help explain why clinical autoimmunity associated with increased titers of autoantibodies is a predominant feature of defects in CD95 or CD95 ligand.

CD95 (Fas)-based, superantigen-dependent, CD4+ T cell-mediated down-regulation of human in vitro immunoglobulin responses

Naturally occurring microbial superantigens (SAg) have been implicated in several human idiopathic disorders, and a compelling argument for the role of SAg in autoantibody-associated disorders, such as systemic lupus erythematosus, has been proposed. To test the effects of SAg on human in vitro Ig responses, CD4+ T cell + B cell cultures were stimulated with graded doses of staphylococcal enterotoxin B (SEB). Ig-secreting cell (IgSC) responses were very weak in CD4+ T cell + B cell cultures stimulated with SEB at the optimal mitogenic concentration (high dose SEB; 100 ng/ml) but were strong in parallel cultures stimulated with low dose SEB (0.01 ng/ml). High dose SEB actually enhanced B cell differentiation in the presence of CD4+ T cell soluble helper factors as long as the B cells were prevented from physically contacting the CD4+ T cells. However, when cell-cell contact between CD4+ T cells and B cells was permitted, high dose, but not low dose, SEB promoted increased CD4+ T cell-mediated B cell apoptosis with resulting decreases in viable CD20+ B cells and IgSC. High dose, but not low dose, SEB triggered increased levels of soluble CD95 ligand, and down-regulation of IgSC responses and incremental apoptosis of activated B cells were prevented by antagonist anti-CD95 mAb. This strongly suggests that CD4+ T cell-mediated CD95-based killing of activated B cells plays a major role in controlling SEB-driven IgSC responses. Defects in SAg-based down-regulation may contribute to autoimmune disorders such as SLE.

Analysis of the ligand binding site in Fas (CD95) by site-directed mutagenesis and comparison with TNFR and CD40

Fas and its ligand (FasL) are members of the tumor necrosis factor receptor (TNFR) and tumor necrosis factor (TNF) superfamilies, respectively. Fas-FasL interactions trigger controlled cell death (apoptosis) in the immune system and thus play a key role in the regulation of immune responses. Structural details of the Fas-Fas ligand interaction are currently unknown. Previously, six Fas residues were identified by mutagenesis as important for ligand binding. We have now extended our mutagenesis analysis and identified additional residues which contribute to the Fas-FasL interaction. Candidate and control residues were selected based on a molecular model of the Fas extracellular region. Although residues in all three extracellular domains were identified to contribute to binding, the Fas-FasL interaction is centered on the second TNFR-like domain. Important residues were compared to critical positions in TNFR and CD40, another member of the TNFR family.

Agonistic activity of a CD40-specific single-chain Fv constructed from the variable regions of mAb G28-5

A single-chain Fv (sFv) was expressed from the variable regions of the CD40-specific mAb G28-5. The molecule bound CD40 with a high affinity (2.2 nM) and was a monomer in solution. Surprisingly, G28-5 sFv was a potent CD40 agonist that rapidly crosslinked CD40 on the cell surface but did not crosslink CD40-Ig in solution. G28-5 sFv was a more potent agonist than G28-5 IgG and was able to stimulate CD40 responses by B cells and monocytes. G28-5 IgG partially blocked, whereas G28-5 sFv augmented CD40 responses during stimulation with natural ligand (gp39-CD8 fusion protein). These results indicate that the functional activity of ligands built from the binding site of G28-5 is highly dependent upon the size and physical properties of the molecule both in solution and on the cell surfaces.

HLA-DR-triggered inhibition of hemopoiesis involves Fas/Fas ligand interactions and is prevented by c-kit ligand

The function of MHC class II (HLA-DR) Ags in hemopoiesis is not well defined. Here we investigated the effect of anti-HLA-DR mAb H81.9 on human marrow cells. mAb H81.9 inhibited colony formation from purified CD34+ marrow cells in long term culture-initiating cell assays. Inhibition was prevented, however, if c-kit ligand (stem cell factor (SCF)) was added to cultures concurrently with H81.9. DNA histograms from cultured untreated marrow mononuclear cells showed 2+/-1.2% apoptotic nuclei, whereas 14.1+/-5.4% were apoptotic after 12-h exposure to mAb H81.9. The apoptotic peak was reduced to 1.2+/-0.8% when SCF was added to cultures concurrently with mAb H81.9. The addition of Fas-Ig, a fusion protein that neutralizes Fas ligand (Fas-L), also prevented mAb H81.9-induced apoptosis. As determined by terminal deoxynucleotidyl transferase assays, agonistic anti-Fas mAb also induced apoptosis (in 13+/-4% of cells), and combined treatment with anti-Fas mAb and H81.9 was additive (27% apoptotic nuclei). The extent of apoptosis induced by anti-Fas mAb was significantly reduced by SCF. After H81.9 exposure, Fas was up-regulated on CD34+ cells, and Fas-L expression was 2.5-fold higher than in controls or CD34- cells, particularly within a small cell window with low orthogonal scatter (lymphocyte gate). These findings show that HLA-DR-mediated signals inhibit hemopoiesis in human marrow by a mechanism involving Fas/Fas-L-dependent signals that are blocked by c-kit ligand. These data suggest a possible role for MHC class II molecules in the regulation of hemopoiesis.

HLA-DR-triggered inhibition of hemopoiesis involves Fas/Fas ligand interactions and is prevented by c-kit ligand

The function of MHC class II (HLA-DR) Ags in hemopoiesis is not well defined. Here we investigated the effect of anti-HLA-DR mAb H81.9 on human marrow cells. mAb H81.9 inhibited colony formation from purified CD34+ marrow cells in long term culture-initiating cell assays. Inhibition was prevented, however, if c-kit ligand (stem cell factor (SCF)) was added to cultures concurrently with H81.9. DNA histograms from cultured untreated marrow mononuclear cells showed 2+/-1.2% apoptotic nuclei, whereas 14.1+/-5.4% were apoptotic after 12-h exposure to mAb H81.9. The apoptotic peak was reduced to 1.2+/-0.8% when SCF was added to cultures concurrently with mAb H81.9. The addition of Fas-Ig, a fusion protein that neutralizes Fas ligand (Fas-L), also prevented mAb H81.9-induced apoptosis. As determined by terminal deoxynucleotidyl transferase assays, agonistic anti-Fas mAb also induced apoptosis (in 13+/-4% of cells), and combined treatment with anti-Fas mAb and H81.9 was additive (27% apoptotic nuclei). The extent of apoptosis induced by anti-Fas mAb was significantly reduced by SCF. After H81.9 exposure, Fas was up-regulated on CD34+ cells, and Fas-L expression was 2.5-fold higher than in controls or CD34- cells, particularly within a small cell window with low orthogonal scatter (lymphocyte gate). These findings show that HLA-DR-mediated signals inhibit hemopoiesis in human marrow by a mechanism involving Fas/Fas-L-dependent signals that are blocked by c-kit ligand. These data suggest a possible role for MHC class II molecules in the regulation of hemopoiesis.

Human monocytic cells contain high levels of intracellular Fas ligand: rapid release following cellular activation

Human monocytes express both Fas and Fas ligand (FasL) on the cell surface, and the interaction of these molecules induces spontaneous apoptosis. In this report we present a study of monocytic cells by FACS and confocal microscopy using anti-FasL Abs that reveals high levels of preformed FasL within the intracellular compartment. Further analysis by immunoblotting of cell cytoplasmic proteins confirmed the presence of a 37-kDa protein recognized by anti-FasL Abs. Stimulation of the monocytic cells with immune complexes, PHA, or superantigen gave rise to the rapid release of soluble FasL from within the cells. The presence of high levels of FasL within human monocytes suggests that, upon stimulation, the cells can rapidly translocate intracellular FasL to the cell surface and release it into the extracellular milieu. These findings indicate a novel mechanism for monocytes to respond rapidly to environmental changes, resulting in the release of active, soluble FasL.

Human monocytic cells contain high levels of intracellular Fas ligand: rapid release following cellular activation

Human monocytes express both Fas and Fas ligand (FasL) on the cell surface, and the interaction of these molecules induces spontaneous apoptosis. In this report we present a study of monocytic cells by FACS and confocal microscopy using anti-FasL Abs that reveals high levels of preformed FasL within the intracellular compartment. Further analysis by immunoblotting of cell cytoplasmic proteins confirmed the presence of a 37-kDa protein recognized by anti-FasL Abs. Stimulation of the monocytic cells with immune complexes, PHA, or superantigen gave rise to the rapid release of soluble FasL from within the cells. The presence of high levels of FasL within human monocytes suggests that, upon stimulation, the cells can rapidly translocate intracellular FasL to the cell surface and release it into the extracellular milieu. These findings indicate a novel mechanism for monocytes to respond rapidly to environmental changes, resulting in the release of active, soluble FasL.

Differential induction of apoptosis by Fas-Fas ligand interactions in human monocytes and macrophages

Human monocytes undergo spontaneous apoptosis upon culture in vitro; removal of serum from the media dramatically increases the rate of this process. Monocyte apoptosis can be significantly abrogated by the addition of growth factors or proinflammatory mediators. We have evaluated the role of the endogenous Fas-Fas ligand (FasL) interaction in the induction of this spontaneous apoptosis and found that a Fas-immunoglobulin (Ig) fusion protein, an antagonistic anti-Fas monoclonal antibody and a rabbit anti-FasL antibody all greatly reduced the onset of apoptosis. The results indicate that spontaneous death of monocytes is mediated via an autocrine or paracrine pathway. Treatment of the cells with growth factors or cytokines that prevented spontaneous apoptosis had no major effects on the expression of Fas or FasL. Additionally, monocyte-derived macrophages were found to express both Fas and FasL but did not undergo spontaneous apoptosis and were not sensitive to stimulation by an agonistic anti-Fas IgM. These results indicate that protective mechanisms in these cells exist at a site downstream of the receptor-ligand interaction.

Identification of amino acid residues important for ligand binding to Fas

The interaction of Fas (CD95), a member of the tumor necrosis factor receptor (TNFR) family, and its ligand (FasL) triggers programmed cell death (apoptosis) and is involved in the regulation of immune responses. Although the Fas-FasL interaction is conserved across species barriers, little is currently known about the molecular details of this interaction. Our aim was to identify residues in Fas that are important for ligand binding. With the aid of a Fas molecular model, candidate amino acid residues were selected in the Fas extracellular domain 2 (D2) and D3 and subjected to serine-scanning mutagenesis to produce mutant Fas molecules in the form of Ig fusion proteins. The effects of these mutations on FasL binding was examined by measuring the ability of these proteins to inhibit FasL-mediated apoptosis of Jurkat cells and bind FasL in ELISA and BIAcore assays. Mutation of two amino acids, R86 and R87 (D2), to serine totally abolished the ability of Fas to interact with its ligand, whereas mutants K84S, L90S, E93S (D2), or H126S (D3) showed reduced binding compared with wild-type Fas. Two mutants (K78S and H95S) bound FasL comparably to wild type. Therefore, the binding of FasL involves residues in two domains that correspond to positions critical for ligand binding in other family members (TNFR and CD40) but are conserved between murine and human Fas.

Molecular cloning, mapping to human chromosome 1 q21-q23, and cell binding characteristics of Spalpha, a new member of the scavenger receptor cysteine-rich (SRCR) family of proteins

CD5 and CD6, two type I cell surface antigens predominantly expressed by T cells and a subset of B cells, have been shown to function as accessory molecules capable of modulating T cell activation. Here we report the cloning of a cDNA encoding Spalpha, a secreted protein that is highly homologous to CD5 and CD6. Spalpha has the same domain organization as the extracellular region of CD5 and CD6 and is composed of three SRCR (scavenger receptor cysteine rich) domains. Chromosomal mapping by fluorescence in situ hybridization and radiation hybrid panel analysis indicated that the gene encoding Spalpha is located on the long arm of human chromosome 1 at q21-q23 within contig WC1.17. RNA transcripts encoding Spalpha were found in human bone marrow, spleen, lymph node, thymus, and fetal liver but not in non-lymphoid tissues. Cell binding studies with an Spalpha immunoglobulin (Spalpha-mIg) fusion protein indicated that Spalpha is capable of binding to peripheral monocytes but not to T or B cells. Spalpha-mIg was also found to bind to the monocyte precursor cell lines K-562 and weakly to THP-1 but not to U937. Spalpha-mIg also bound to the B cell line Raji and weakly to the T cell line HUT-78. These findings indicate that Spalpha, a novel secreted protein produced in lymphoid tissues, may regulate monocyte activation, function, and/or survival.

Co-ligation of the antigen and Fc receptors gives rise to the selective modulation of intracellular signaling in B cells. Regulation of the association of phosphatidylinositol 3-kinase and inositol 5'-phosphatase with the antigen receptor complex

Cross-linking of the Fc receptor (FcR) to surface immunoglobulin (sIg) on B cells inhibits the influx of extracellular calcium and abrogates the proliferative signal. The mechanism by which this occurs is not well understood. In this report we show that co-cross-linking the FcR to the antigen receptor gives rise to very selective modulation of signal transduction in B cells. Co-cross-linking sIg and the FcR enhanced the phosphorylation of the FcR, the adapter protein, Shc, and the inositol 5'-phosphatase Ship. Furthermore, phosphorylation of the FcR induced its association with Ship. Cross-linking of the FcR and sIg decreased the tyrosine phosphorylation of CD19, which led to a reduction in the association of phosphatidylinositol 3-kinase. In addition, the phosphorylation of several other proteins of 73, 39, and 34 kDa was reduced. Activation of the cells with either F(ab')2 or intact anti-IgG induced very similar changes in levels of tyrosine phosphorylation of most other proteins, and no differences in the activation of several protein kinases were observed. These results indicate that the inhibitory signal that is transmitted through the FcR is not mediated by a global shutdown of tyrosine phosphorylation but is, rather, a selective mechanism involving localized changes in the interactions of adapter proteins and the enzymes Ship and phosphatidylinositol 3-kinase with the antigen receptor complex.

The relationship between human epidermal growth-like factor receptor expression and cellular transformation in NIH3T3 cells

A collection of cell lines expressing each human epidermal growth factor receptor (HER) family member alone or in all pairwise combinations in a clone of NIH3T3 cells (3T3-7d) devoid of detectable epidermal growth factor receptor family members has been generated. Transformation, as measured by growth in soft agar, occurred only in the presence of appropriate ligand and only in cells expressing two different HER family members. Transfection of oncogenic neu (Tneu), conferred ligand-independent transformation only in cells which co-expressed HER1, HER3, or HER4, but not when expressed alone or with HER2. Cell lines were also tested for their ability to form tumors in animals. None of the cell lines expressing single HER family members was able to form tumors in animals with the exception of HER1, which was weakly tumorigenic. Although unable to form tumors when expressed alone, HER2 was tumorigenic when expressed with HER1 or HER3, but not HER4. Of all complexes analyzed, cells expressing HER1 + HER2 were the most aggressive. The relationship between HER1 activation, intracellular calcium fluxes, and phospholipase Cgamma1 activation is well established. We found that activation of HER1 was required for the induction of a calcium flux and the phosphorylation of phospholipase Cgamma1. These activities were independent of, and unaffected by, the co-expression of any other family member. Further, heregulin stimulation of all cell lines including those containing HER1 did not demonstrate any effect on intracellular calcium levels or phospholipase Cgamma1 phosphorylation. This demonstrates that heregulin induced cellular transformation by activating HER3- and HER4-containing complexes does not require the activation of either phospholipase Cgamma1 or the mobilization of intracellular calcium.

Cytokine activation sensitizes human monocytic and endothelial cells to the cytotoxic effects of an anti-CD40 immunotoxin

G28-5 sFv-PE40 is a single-chain immunotoxin that is cytotoxic toward malignant B cells expressing CD40. Human monocytes, which also express cell surface CD40, were found to be insensitive to the immunotoxin. Activation of the monocytic cell line THP-1, or freshly isolated peripheral blood monocytes with IFN-gamma, but not IL-3, GM-CSF, IL-6, or TNF-alpha, greatly sensitized the cells toward G28-5 sFv-PE40, lowering the EC50 value from > 10 micrograms/ml to 80 ng/ml. This sensitization could not be explained simply by the two- to threefold increase in cell surface CD40 expression induced by IFN-gamma since TNF-alpha or the combination of TNF-alpha and IL-6 gave similar increases in CD40 expression but did not sensitize the cells to the immunotoxin. Internalization of G28-5 sFv-PE40 after IFN-gamma activation was also increased threefold, reflective of the increase in CD40 expression. IFN-gamma-treated but not -untreated THP-1 cells produced IL-6 and TNF-alpha following incubation with G28-5 sFv-PE40, indicating an association between CD40 signaling, which induces cytokine production, and sensitivity to the immunotoxin. HUVECs also express CD40 but were found to be insensitive to the anti-CD40 immunotoxin. A combination of IFN-gamma and TNF-alpha, but neither cytokine alone, sensitized the endothelial cells to G28-5 sFv-PE40. These data show that activation with cytokines can sensitize monocytes and endothelial cells to an immunotoxin targeted to CD40, most likely by altering the trafficking and/or processing of the immunotoxin after receptor-mediated internalization.

Cytokine activation sensitizes human monocytic and endothelial cells to the cytotoxic effects of an anti-CD40 immunotoxin

G28-5 sFv-PE40 is a single-chain immunotoxin that is cytotoxic toward malignant B cells expressing CD40. Human monocytes, which also express cell surface CD40, were found to be insensitive to the immunotoxin. Activation of the monocytic cell line THP-1, or freshly isolated peripheral blood monocytes with IFN-gamma, but not IL-3, GM-CSF, IL-6, or TNF-alpha, greatly sensitized the cells toward G28-5 sFv-PE40, lowering the EC50 value from > 10 micrograms/ml to 80 ng/ml. This sensitization could not be explained simply by the two- to threefold increase in cell surface CD40 expression induced by IFN-gamma since TNF-alpha or the combination of TNF-alpha and IL-6 gave similar increases in CD40 expression but did not sensitize the cells to the immunotoxin. Internalization of G28-5 sFv-PE40 after IFN-gamma activation was also increased threefold, reflective of the increase in CD40 expression. IFN-gamma-treated but not -untreated THP-1 cells produced IL-6 and TNF-alpha following incubation with G28-5 sFv-PE40, indicating an association between CD40 signaling, which induces cytokine production, and sensitivity to the immunotoxin. HUVECs also express CD40 but were found to be insensitive to the anti-CD40 immunotoxin. A combination of IFN-gamma and TNF-alpha, but neither cytokine alone, sensitized the endothelial cells to G28-5 sFv-PE40. These data show that activation with cytokines can sensitize monocytes and endothelial cells to an immunotoxin targeted to CD40, most likely by altering the trafficking and/or processing of the immunotoxin after receptor-mediated internalization.

Differential expression of Fas (CD95) and Fas ligand on normal human phagocytes: implications for the regulation of apoptosis in neutrophils

Human neutrophils, monocytes, and eosinophils are known to undergo apoptotic cell death. The Fas/Fas ligand pathway has been implicated as an important cellular pathway mediating apoptosis in diverse cell types. We conducted studies to examine the importance of the Fas/FasL system in normal human phagocytes. Although Fas expression was detected on neutrophils, monocytes, and eosinophils, constitutive expression of FasL was restricted to neutrophils. The three types of phagocytes demonstrated differential sensitivity to Fas-induced apoptosis. Only neutrophils were highly susceptible to rapid apoptosis in vitro after stimulation with activating anti-Fas IgM (mAb CH-11). Fas-mediated neutrophil apoptosis was suppressed by incubation with G-CSF, GM-CSF, IFN-gamma, TNF-alpha, or dexamethasone, as well as the selective tyrosine kinase inhibitors, herbimycin A and genistein. Spontaneous neutrophil death in vitro was partially suppressed by Fas-Ig fusion protein or antagonistic anti-Fas IgG1 (mAb ZB4). In coculture experiments, neutrophils released a soluble factor inducing death in Fas-susceptible Jurkat cells via a mechanism sensitive to the presence of Fas-Ig or anti-Fas IgG1. Immunoblot analysis using specific anti-human FasL IgG1 (mAb No. 33) identified a 37-kD protein in lysates of freshly isolated neutrophils and a 30-kD protein in the culture supernatant of neutrophils maintained in vitro. Our results suggest that mature neutrophils may be irrevocably committed to autocrine death by virtue of their constitutive coexpression of cell-surface Fas and FasL via a mechanism that is sensitive to proinflammatory cytokines, glucocorticoids, and inhibitors of tyrosine kinase activity. Furthermore, neutrophils can serve as a source of soluble FasL, which may function in a paracrine pathway to mediate cell death.

Activation of human monocytes through CD40 induces matrix metalloproteinases

The activation of monocytes/macrophages to secrete pro-inflammatory cytokines and matrix metalloproteinases (MMPs) is critically important in the development of chronic inflammatory diseases. However, the consequence of interactions between activated T cells and monocytes in these inflammatory processes is not well understood. In this study we have investigated the induction of MMPs in human monocytes by activated T cells. We show that fixed cells and the cell membranes from a T cell line, BMS-2, that expresses high levels of the CD40 ligand gp39 (also called TRAP, TBAM, or CD40L) stimulate both the expression of mRNA and the production of MMPs by human monocytic cells. Activation of monocytes by the human T cells could be significantly inhibited by a F(ab')2 fragment of a neutralizing Ab specific for human gp39, but not by an Ab that recognizes murine gp39. Furthermore, recombinant soluble gp39 (sgp39) alone induced marked increases in the levels of a 92-kDa metalloproteinase (gelatinase) in both the human monocytic cell line, THP-1, and peripheral human monocytes, and induction was blocked by the anti-human gp39 Ab. Pretreatment with IFN-gamma significantly enhanced gp39 induction of MMPs in THP-1 cells but not in peripheral monocytes. Up-regulation of mRNA for the 92-kDa MMP by gp39 could be detected within 6 h of stimulation and was maximal 24 h after treatment. MMP enzymatic activity was detectable in the culture medium 12 to 18 h following stimulation of the cells and remained high through 48 h. These results suggest the interaction of T cells with monocytes/macrophages via the gp39-CD40 counter receptors may be significant in development or maintenance of chronic inflammatory lesions.

Activation of human monocytes through CD40 induces matrix metalloproteinases

The activation of monocytes/macrophages to secrete pro-inflammatory cytokines and matrix metalloproteinases (MMPs) is critically important in the development of chronic inflammatory diseases. However, the consequence of interactions between activated T cells and monocytes in these inflammatory processes is not well understood. In this study we have investigated the induction of MMPs in human monocytes by activated T cells. We show that fixed cells and the cell membranes from a T cell line, BMS-2, that expresses high levels of the CD40 ligand gp39 (also called TRAP, TBAM, or CD40L) stimulate both the expression of mRNA and the production of MMPs by human monocytic cells. Activation of monocytes by the human T cells could be significantly inhibited by a F(ab')2 fragment of a neutralizing Ab specific for human gp39, but not by an Ab that recognizes murine gp39. Furthermore, recombinant soluble gp39 (sgp39) alone induced marked increases in the levels of a 92-kDa metalloproteinase (gelatinase) in both the human monocytic cell line, THP-1, and peripheral human monocytes, and induction was blocked by the anti-human gp39 Ab. Pretreatment with IFN-gamma significantly enhanced gp39 induction of MMPs in THP-1 cells but not in peripheral monocytes. Up-regulation of mRNA for the 92-kDa MMP by gp39 could be detected within 6 h of stimulation and was maximal 24 h after treatment. MMP enzymatic activity was detectable in the culture medium 12 to 18 h following stimulation of the cells and remained high through 48 h. These results suggest the interaction of T cells with monocytes/macrophages via the gp39-CD40 counter receptors may be significant in development or maintenance of chronic inflammatory lesions.

Stimulation of CD40 with purified soluble gp39 induces proinflammatory responses in human monocytes

CD40 is a glycoprotein of about 50 kDa that plays a crucial role in B cell growth and differentiation. It is found on the surface of B cells, follicular dendritic cells, monocytes, and some endothelial, epithelial, and carcinoma cells. Engagement of CD40 with anti-CD40 mAbs, gp39 expressed on the cell surface or soluble forms of gp39, primes B cells to efficiently respond to subsequent stimulatory signals leading to B cell proliferation, differentiation, and isotype switching. Peripheral monocytes also express CD40 on the cell surface and expression in increased following treatment with IFN-gamma. Using a soluble murine CD8/human gp39 fusion protein (sgp39) we have found that CD40 plays a crucial role in the regulation of monocyte function. Stimulation of human peripheral monocytes with sgp39 induced homotypic aggregation and significantly increased the expression of several cell-surface proteins including CD54, MHC class II, CD86, and CD40. Soluble gp39 also dramatically enhanced monocyte survival, preventing the onset of apoptosis that normally occurs upon withdrawal of serum. Finally, in the absence of any costimulatory molecules, sgp39 stimulated monocytes to produce TNF-alpha, IL-1 beta, IL-6, and IL-8. These results suggest that ligation of CD40 on human monocytes induces phenotypic changes that would be expected to influence T cell activation by the monocyte and also to enhance or prolong inflammatory responses.

Stimulation of CD40 with purified soluble gp39 induces proinflammatory responses in human monocytes

CD40 is a glycoprotein of about 50 kDa that plays a crucial role in B cell growth and differentiation. It is found on the surface of B cells, follicular dendritic cells, monocytes, and some endothelial, epithelial, and carcinoma cells. Engagement of CD40 with anti-CD40 mAbs, gp39 expressed on the cell surface or soluble forms of gp39, primes B cells to efficiently respond to subsequent stimulatory signals leading to B cell proliferation, differentiation, and isotype switching. Peripheral monocytes also express CD40 on the cell surface and expression in increased following treatment with IFN-gamma. Using a soluble murine CD8/human gp39 fusion protein (sgp39) we have found that CD40 plays a crucial role in the regulation of monocyte function. Stimulation of human peripheral monocytes with sgp39 induced homotypic aggregation and significantly increased the expression of several cell-surface proteins including CD54, MHC class II, CD86, and CD40. Soluble gp39 also dramatically enhanced monocyte survival, preventing the onset of apoptosis that normally occurs upon withdrawal of serum. Finally, in the absence of any costimulatory molecules, sgp39 stimulated monocytes to produce TNF-alpha, IL-1 beta, IL-6, and IL-8. These results suggest that ligation of CD40 on human monocytes induces phenotypic changes that would be expected to influence T cell activation by the monocyte and also to enhance or prolong inflammatory responses.

Immune complexes of LDL induce atherogenic responses in human monocytic cells

The ability of immune complexes of LDL or acetylated LDL (acLDL), together with antibodies to LDL, to induce a proatherogenic phenotype in human monocytic cells has been explored. Treatment of THP-1 monocytic cells or peripheral human monocytes with LDL immune complexes containing intact anti-LDL markedly enhanced the ability of these cells to subsequently bind and take up LDL, whereas aggregated LDL or LDL immune complexes prepared with F(ab')2 fragments of anti-LDL had no significant effect. Activation of THP-1 cells with intact LDL immune complexes also stimulated mRNA expression for the scavenger receptor. Additionally, activation of THP-1 cells with insoluble immune complexes of LDL or LDL stimulated generation of reactive oxygen intermediates that, in turn, could oxidize exogenous LDL. These results indicate that the binding of lipoprotein immune complexes to Fc receptors on monocytic cells activates a series of responses that could accelerate the initiation or progression of atherosclerosis.

Expression of functional CD40 by vascular endothelial cells

The interaction between activated vascular endothelium and T cells has been shown to play an important role in the recruitment and activation of T cells at sites of inflammation. Here we report the expression of CD40 by vascular endothelial cells and its regulation by inflammatory agents. Using the soluble recombinant CD40 ligand, sgp39, we show that the interaction of CD40 with its ligand can lead to endothelial cell activation, which in turn leads to leukocyte adhesion. This adhesion is partly mediated by the expression of E-selectin. In addition to E-selectin expression, sgp39 induces the expression of intercellular adhesion molecule 1 and augments the tumor necrosis factor alpha-induced expression of vascular cell adhesion molecule 1. The effects of sgp39 on endothelial cells can be blocked with anti-gp39 monoclonal antibody (mAb), anti-CD40 mAb, or soluble CD40. Staining of tissues from healthy human skin using anti-CD40 mAb showed very weak expression of CD40 by the endothelium, while skin involved in inflammatory disease showed marked upregulation of CD40 expression. These studies suggest that interactions between cell surface proteins expressed by activated T cells with their receptors on vascular endothelium can stimulate the vasculature at sites of inflammation and may be involved in normal inflammatory responses and in inflammatory disease.

Effect of interferon-gamma on antigen processing in human monocytes

The effect of interferon-gamma (IFN-gamma) on the ability of human monocytic cells to process exogenous (major histocompability complex class II) antigens was investigated. The processing (i.e. protein degradation) of antigens that were internalized via Fc gamma receptor (Fc gamma R) was followed for various times after treatment of cells with IFN-gamma. THP-1 cells that had been treated with IFN-gamma for 4 h degraded antigen, internalized as an immune complex, at an enhanced rate. After 24 h of IFN-gamma treatment the rate of processing was similar to untreated cells. Unexpectedly, in cells which had been treated for 48-72 h there was a significant decrease in the rate of processing of the exogenous antigen. These effects were not due to changes in the rate of internalization of immune complex. The inhibition of the rate of processing was independent of the type of antigen, was dependent on the dose of IFN-gamma, and also occurred with normal human peripheral monocytes. Analysis of the degraded peptides by high-pressure liquid chromatography indicated that some of the peptides generated in the IFN-gamma-treated cells were both quantitatively and qualitatively different from the peptides generated in untreated cells. These data suggest that IFN-gamma modulates the way in which antigens, internalized through Fc receptors as immune complexes, are processed. Additionally, the results imply that decreases in the rate of antigen processing may lead to more efficient antigen presentation.

Activation of p34cdc2 coincident with taxol-induced apoptosis

Toxicity elicited by the antitumor compound taxol has been linked to irreversible tubulin polymerization, cell cycle block at mitosis, and cell death from apoptosis. We have used pulsed drug exposure of synchronized populations to identify two points, one in transition from G0 to G1 and the other at G2/M of cell cycle, that are most sensitive to taxol-induced cell killing. By analyzing these lesions separately, we have differentiated events related to mitotic block from those that may contribute to apoptosis. The taxol lesion forms rapidly and stably in transition or mitotic cells, because secondary washes to remove residual drug will decrease cytotoxicity except for cells in these populations. Both G2/M cells and G0/G1 transition cells synchronously initiated apoptotic DNA fragmentation within 20 h of pulsed taxol treatment, indicating that a sustained mitotic block is not requisite to initiate cell death. Apoptosis was inhibited by cyclohexamide and by 2-aminopurine and sodium orthovanadate; thus, cell cycle progression appeared requisite for cell death. Taxol treatment of G0/G1 or G2/M cells clearly leads to a block of mitosis followed by a perturbation of tyrosine phosphoprotein regulation; however, protein tyrosine phosphorylation correlated with mitotic block rather than time after drug exposure. Conversely, p34cdc2 kinase activation does not occur at mitotic block but rather 20 h after drug exposure and coincident with DNA fragmentation. Taken together, these results suggest that mitotic block may not be a sufficient signal for taxol-induced apoptosis and that the taxol lesion initiates apoptosis via a phosphoregulation pathway possibly involving the p34cdc2 kinase.

CD45-mediated regulation of extracellular calcium influx in a CD4-transfected human T cell line

Transfection of a CD4- Jurkat leukemic T cell line with the human wild-type CD4 gene resulted in the constitutive mobilization of calcium by these cells. The altered calcium phenotype was dependent on expression of a completely functional CD4 molecule on the cell surface. Transfectants receiving the vector alone or those in which a mutated CD4 gene lacking a functional Lck binding region failed to generate a constitutive calcium response. In addition, CD4 wild-type transfectants over time lost CD4 expression. These CD4- revertants failed to constitutively mobilize calcium. Treatment of CD4 wild-type transfected cells with either anti-CD45 mAb, EGTA, or PMA rapidly restored the cells to basal levels of intracellular calcium. Analysis of CD45 cross-linking on CD4+ and CD4- normal Jurkat lines demonstrated that CD4 expression was required for CD45-mediated inhibition of TCR induced calcium responses. CD45-mediated inhibition affected the duration of the response rather than its magnitude. These results, taken together with the observations obtained with CD4 transfected Jurkats suggested that influx of extracellular calcium was the predominant reason for the elevated levels of calcium within the cell. In support of this hypothesis, we could find no evidence of phosphorylated phospholipase C gamma 1 or constitutive inositol 1,4,5 trisphosphate generation in the CD4 wild-type transfected cells, yet both were detectable after anti-CD3 mAb-induced activation. Immunokinase assays of Lck and Fyn precipitated from untreated or anti-CD45-treated wild-type transfectants demonstrated that CD45 cross-linking dephosphorylated Lck and reduced its capacity to phosphorylate enolase. In contrast, neither Fyn autophosphorylation nor its activity was affected by CD45 cross-linking.

CD45-mediated regulation of extracellular calcium influx in a CD4-transfected human T cell line

Transfection of a CD4- Jurkat leukemic T cell line with the human wild-type CD4 gene resulted in the constitutive mobilization of calcium by these cells. The altered calcium phenotype was dependent on expression of a completely functional CD4 molecule on the cell surface. Transfectants receiving the vector alone or those in which a mutated CD4 gene lacking a functional Lck binding region failed to generate a constitutive calcium response. In addition, CD4 wild-type transfectants over time lost CD4 expression. These CD4- revertants failed to constitutively mobilize calcium. Treatment of CD4 wild-type transfected cells with either anti-CD45 mAb, EGTA, or PMA rapidly restored the cells to basal levels of intracellular calcium. Analysis of CD45 cross-linking on CD4+ and CD4- normal Jurkat lines demonstrated that CD4 expression was required for CD45-mediated inhibition of TCR induced calcium responses. CD45-mediated inhibition affected the duration of the response rather than its magnitude. These results, taken together with the observations obtained with CD4 transfected Jurkats suggested that influx of extracellular calcium was the predominant reason for the elevated levels of calcium within the cell. In support of this hypothesis, we could find no evidence of phosphorylated phospholipase C gamma 1 or constitutive inositol 1,4,5 trisphosphate generation in the CD4 wild-type transfected cells, yet both were detectable after anti-CD3 mAb-induced activation. Immunokinase assays of Lck and Fyn precipitated from untreated or anti-CD45-treated wild-type transfectants demonstrated that CD45 cross-linking dephosphorylated Lck and reduced its capacity to phosphorylate enolase. In contrast, neither Fyn autophosphorylation nor its activity was affected by CD45 cross-linking.

Cross-linking of Fc gamma receptor to surface immunoglobulin on B cells provides an inhibitory signal that closes the plasma membrane calcium channel

Stimulation of B lymphocytes by the cross-linking of surface Ig (sIg) with an F(ab')2 antibody fragment leads to the rapid activation of several tyrosine kinases. This gives rise to the activation of phospholipase C gamma (PLC gamma) and the generation of inositol phosphates. These, in turn, lead to a prolonged elevation of intracellular Ca2+ ([Ca2+]i) consisting of a rapid release of Ca2+ from intracellular stores and a sustained influx of extracellular Ca2+. In contrast, co-cross-linking sIg to Fc gamma receptor (Fc gamma RII) with intact anti-sIg induces a much more transient increase in [Ca2+]i. Stimulation of the murine B cell lymphoma, A20, with F(ab')2 anti-sIgG leads to the production of high levels of IL-2, while co-cross-linking of sIgG with Fc gamma RII blocks this response. In studies reported here, we show that co-cross-linking of Fc gamma RII with sIg prevents the influx of extracellular Ca2+ without significantly affecting the tyrosine phosphorylation of substrates including PLC gamma 1, PLC gamma 2, and Syk or the mobilization of Ca2+ from intracellular stores. In cells that had been previously activated with F(ab')2 anti-IgG, co-cross-linking of sIg to Fc gamma RII rapidly abrogated the influx of extracellular Ca2+ by closing the plasma membrane Ca2+ channel. Additionally, even 2-3 h after stimulation of the cells with F(ab')2 fragment, addition of intact anti-IgG to the cells, or removal of extracellular Ca2+, markedly inhibited (> 90%) IL-2 production. These results indicate that co-cross-linking sIg with Fc gamma RII both prevented the opening of and actively closed the Ca2+ channel, and, through this mechanism, Fc gamma RII was able to control production of IL-2. Overall, since influx of extracellular Ca2+ has been found to be necessary for the proliferation and differentiation of B cells, Fc gamma RII may play a critical role in controlling these responses by regulating the opening of the Ca2+ channel.

Cross-linking of Fc gamma receptor I (Fc gamma RI) and receptor II (Fc gamma RII) on monocytic cells activates a signal transduction pathway common to both Fc receptors that involves the stimulation of p72 Syk protein tyrosine kinase

Stimulation of the human monocytic cell line THP-1 by cross-linking either Fc gamma receptor I (Fc gamma RI) or Fc gamma receptor II (Fc gamma RII) gave rise to the rapid phosphorylation of multiple intracellular proteins. The pattern of proteins that were phosphorylated appeared to be identical. Analysis of these proteins by specific immunoprecipitation indicated that stimulation through either receptor did indeed give rise to phosphorylation of the same set of proteins. These included: Fc gamma RII, phospholipase C (PLC) gamma 1, PLC gamma 2, Vav, GAP, and a protein that co-precipitated with the Fc gamma receptors and migrated with a molecular weight of about 70,000. Co-cross-linking an F(ab')2 anti-CD45 monoclonal antibody together with monoclonal antibodies to either of the Fc gamma receptors inhibited phosphorylation of all these proteins. Analysis of the tyrosine kinases in the cells revealed that both receptors stimulated the phosphorylation and activation of a kinase recognized by antibodies to Syk. Furthermore, the Syk kinase became associated with the Fc gamma RII following receptor cross-linking. These data indicate that although the two Fc gamma receptors have different cytoplasmic tails, they are coupled to the same signal transduction cascade that is regulated by CD45 and involves the activation of Syk.

Stimulation of tyrosine phosphorylation and calcium mobilization by Fc gamma receptor cross-linking. Regulation by the phosphotyrosine phosphatase CD45

The binding and subsequent cross-linking of murine IgG2a or human IgG to the Fc gamma R on the monocytic cell line THP-1 induced a rapid, dose-dependent increase in tyrosine phosphorylation of several proteins (a doublet centered around 110 kDa, and bands at 80, 60, and 52 kDa) and smaller increases in other proteins. This phosphorylation was accompanied by an increase in intracellular free Ca2+. The signaling required the cross-linking of the IgG, either through a biotin-avidin complex or with a F(ab')2 second antibody. Cross-linking of an F(ab')2 fragment of mAb 32.2 to Fc gamma RI (CD64) or an Fab fragment of mAb IV.3 to Fc gamma RII (CDw32) gave similar results to those observed with intact murine IgG2a or human IgG. Cross-linking of a F(ab')2 fragment of mAb 3G8 to Fc gamma RIII (CD16) had very little effect. Increases in both tyrosine phosphorylation and intracellular free Ca2+ were significantly reduced in a dose-dependent manner upon treatment of THP-1 cells with the tyrosine kinase inhibitors herbimycin-A, genistein, or erbstatin. Additionally, there was a marked inhibition of both Ca2+ mobilization and tyrosine phosphorylation when a F(ab')2 fragment of a mAb (T191) to the protein tyrosine phosphatase CD45, was co-cross-linked with either Hu-IgG, Mu-IgG2a, F(ab')2 anti-Fc gamma RI, or Fab anti-Fc gamma RII. Taken together these results suggest that signaling through Fc gamma RI (CD64) and Fc gamma RII (CDw32) in the monocytic leukemia cell line THP-1 gives rise to rapid tyrosine phosphorylation of several proteins followed by an increase in intracellular calcium. In addition, CD45 is able to inhibit the intracellular signaling when it is brought into close proximity to the Fc gamma R. This suggests that this transmembrane tyrosine phosphatase may regulate the stimulation of the cells through the Fc gamma R.

Stimulation of tyrosine phosphorylation and calcium mobilization by Fc gamma receptor cross-linking. Regulation by the phosphotyrosine phosphatase CD45

The binding and subsequent cross-linking of murine IgG2a or human IgG to the Fc gamma R on the monocytic cell line THP-1 induced a rapid, dose-dependent increase in tyrosine phosphorylation of several proteins (a doublet centered around 110 kDa, and bands at 80, 60, and 52 kDa) and smaller increases in other proteins. This phosphorylation was accompanied by an increase in intracellular free Ca2+. The signaling required the cross-linking of the IgG, either through a biotin-avidin complex or with a F(ab')2 second antibody. Cross-linking of an F(ab')2 fragment of mAb 32.2 to Fc gamma RI (CD64) or an Fab fragment of mAb IV.3 to Fc gamma RII (CDw32) gave similar results to those observed with intact murine IgG2a or human IgG. Cross-linking of a F(ab')2 fragment of mAb 3G8 to Fc gamma RIII (CD16) had very little effect. Increases in both tyrosine phosphorylation and intracellular free Ca2+ were significantly reduced in a dose-dependent manner upon treatment of THP-1 cells with the tyrosine kinase inhibitors herbimycin-A, genistein, or erbstatin. Additionally, there was a marked inhibition of both Ca2+ mobilization and tyrosine phosphorylation when a F(ab')2 fragment of a mAb (T191) to the protein tyrosine phosphatase CD45, was co-cross-linked with either Hu-IgG, Mu-IgG2a, F(ab')2 anti-Fc gamma RI, or Fab anti-Fc gamma RII. Taken together these results suggest that signaling through Fc gamma RI (CD64) and Fc gamma RII (CDw32) in the monocytic leukemia cell line THP-1 gives rise to rapid tyrosine phosphorylation of several proteins followed by an increase in intracellular calcium. In addition, CD45 is able to inhibit the intracellular signaling when it is brought into close proximity to the Fc gamma R. This suggests that this transmembrane tyrosine phosphatase may regulate the stimulation of the cells through the Fc gamma R.

Differential activation of phosphotyrosine protein phosphatase activity in a murine T cell hybridoma by monoclonal antibodies to CD45

The effect of two anti-CD45 (T200, LCA, Ly5) antibodies on the activation of the murine T-cell hybridoma 13.13 has been evaluated. These studies have been carried out in a system that did not require cross-linking or coclustering of antibodies. Activation of 13.13 cells with the anti-CD3 monoclonal antibody, 145.2C11, gave rise to rapid increases in intracellular calcium and interleukin-2 production. Additionally, within 1 min, phosphorylation on tyrosine of four major proteins of about 130,000, 110,000, 80,000, and 37,000 daltons could be seen. Pretreatment of the cells with the anti-CD45 mAb M1/89.18.7.HK markedly inhibited all three biological responses, while an alternate anti-CD45 antibody, M1/9.3.4.HL.2, had little effect. The two antibodies bound to CD45 with similar affinities, and no differences in the lateral mobility of antibody-CD45 complexes in the cell membrane were observed. The inhibition of activation of the cells by M1/89.18.7.HK was abrogated significantly both by the phosphotyrosine protein phosphatase inhibitor orthovanadate and by excess M1/9.3.4.HL.2. If M1/89.18.7.HK was added to the 13.13 cells after they had already been activated with anti-CD3, it very effectively stimulated dephosphorylation of substrates that had been phosphorylated on tyrosines prior to adding the anti-CD45 antibody. These results indicate that the phosphotyrosine protein phosphatase activity of CD45 is critical to its biological function and that bivalent (i.e. uncross-linked) anti-CD45 antibodies can give rise to markedly different responses. One of the antibodies, M1/89.18.7.HK, appears to behave much like a receptor ligand and is able to activate the enzymatic activity associated with the CD45 transmembrane protein.

Macrophage-derived factors increase low density lipoprotein uptake and receptor number in cultured human liver cells

Recent evidence suggests the possibility that macrophages can influence lipoprotein metabolism. Therefore we investigated the ability of cultured macrophages to alter low density lipoprotein (LDL) uptake in a human liver cell line (HepG2). Conditioned media from phlogogenic-induced mouse peritoneal macrophages or from a human macrophage cell line stimulated with endotoxin increased HepG2 LDL uptake by as much as 60-70%. The increase was due, in part, to a significant macrophage-induced 40% increase in the number of LDL receptors per cell. Although macrophage conditioned media inhibited HepG2 cholesterol synthesis, the LDL receptor up-regulation did not appear to be due to the effects on cholesterol synthesis. The LDL receptor stimulatory activity was sensitive to proteolysis and heat. Its molecular mass was approximately 20 kDa based on gel filtration. Several macrophage secretory proteins were tested in HepG2 cultures for LDL uptake stimulation. Of these, oncostatin M (approximately 18 kDa by gel filtration) gave the strongest response. The rank order for LDL uptake stimulation was oncostatin M much greater than interleukin 6 = interleukin 1 = transforming growth factor-beta 1. A neutralizing antibody directed against oncostatin M inhibited the ability of conditioned media to up-regulate LDL receptors by 85%. Thus, our results indicate that macrophages can secrete several proteins that up-regulate LDL receptors in HepG2 cells and that most of the up-regulatory activity in macrophage conditioned media appears to be due to oncostatin M.

Physical associations between CD45 and CD4 or CD8 occur as late activation events in antigen receptor-stimulated human T cells

Activation of human PBL T cells with solid phase anti-CD3 mAb or during the course of an MLR response gives rise to the association of CD4 or CD8 molecules with the protein tyrosine phosphatase, CD45, on the cell surface. This paired association of cell-surface molecules occurs late in the activation cycle and appears to be dependent upon Ti-CD3-mediated signaling because mitogen-driven activation does not induce formation of the complex. Maximal association occurred 72 to 96 h after exposure to anti-CD3 mAb on both CD4+ and CD8+ T cells. In contrast, association between CD8 and CD45 during an MLR response did not occur until day 6 of a MLR whereas CD4-CD45 association was detected by 72 h of culture. The kinetics of association between CD4 or CD8 and CD45 was measured by fluorescence resonance energy transfer and confirmed by immunoprecipitation of dithiobis succinimidylpropionate or disuccinimidyl suberate cross-linked 125I-labeled resting or activated T cells. The molecules that co-precipitated with either CD4 or CD8 and had an apparent kDa of 180 to 205 could be immunodepleted with anti-CD45 mAb. Furthermore, CD4 or CD8 immunoprecipitates from 96-h activated T cells contained significant levels of protein tyrosine phosphatase activity whereas corresponding immunoprecipitates from resting or recently activated T cells showed little protein tyrosine phosphatase activity. This association may allow CD45 to engage and dephosphorylate lck or another CD4- or CD8-associated substrate in order to reset the receptor complex to receive a new set of stimuli. Our observations suggest that synergistic signaling provided as a consequence of CD4 or CD8 association with the TCR after antigenic stimulation may develop on a different temporal scale than that observed after soluble anti-CD4+ anti-CD3 heteroconjugate antibody cross-linking.

Physical associations between CD45 and CD4 or CD8 occur as late activation events in antigen receptor-stimulated human T cells

Activation of human PBL T cells with solid phase anti-CD3 mAb or during the course of an MLR response gives rise to the association of CD4 or CD8 molecules with the protein tyrosine phosphatase, CD45, on the cell surface. This paired association of cell-surface molecules occurs late in the activation cycle and appears to be dependent upon Ti-CD3-mediated signaling because mitogen-driven activation does not induce formation of the complex. Maximal association occurred 72 to 96 h after exposure to anti-CD3 mAb on both CD4+ and CD8+ T cells. In contrast, association between CD8 and CD45 during an MLR response did not occur until day 6 of a MLR whereas CD4-CD45 association was detected by 72 h of culture. The kinetics of association between CD4 or CD8 and CD45 was measured by fluorescence resonance energy transfer and confirmed by immunoprecipitation of dithiobis succinimidylpropionate or disuccinimidyl suberate cross-linked 125I-labeled resting or activated T cells. The molecules that co-precipitated with either CD4 or CD8 and had an apparent kDa of 180 to 205 could be immunodepleted with anti-CD45 mAb. Furthermore, CD4 or CD8 immunoprecipitates from 96-h activated T cells contained significant levels of protein tyrosine phosphatase activity whereas corresponding immunoprecipitates from resting or recently activated T cells showed little protein tyrosine phosphatase activity. This association may allow CD45 to engage and dephosphorylate lck or another CD4- or CD8-associated substrate in order to reset the receptor complex to receive a new set of stimuli. Our observations suggest that synergistic signaling provided as a consequence of CD4 or CD8 association with the TCR after antigenic stimulation may develop on a different temporal scale than that observed after soluble anti-CD4+ anti-CD3 heteroconjugate antibody cross-linking.