Morphology and lytic mechanisms of interleukin 3-dependent natural cytotoxic cells: tumor necrosis factor as a possible mediator

Populations of interleukin 3 (IL 3)-dependent cells can be derived from mouse bone marrow that display natural cytotoxicity (NC) against Wehi-164 target cells but do not display natural killing against YAC-1 cells. These bone marrow-derived NC cells cultured up to 2 mo in IL 3 do not contain rearranged T cell receptor beta-chain genes. They appear to be mast-like cells by electron microscopy and contain heterogeneous type granules. The molecules that mediate NC appear to be contained in these granules and are preformed because protein synthesis inhibitors have no effect on the capacity of IL 3-dependent NC cells to lyse Wehi-164 target cells. In addition to the IL 3-dependent bone marrow-derived cells, the basophilic leukemia cells, RBL-1, but not P815 mastocytoma cells were found to mediate NC against Wehi-164 cells. Both bone marrow-derived NC and RBL-1 cells can lyse L929 cells in 18 hr, suggesting that the putative NC mediator may be related to lymphotoxin/tumor necrosis factor (TNF). Recombinant human TNF displayed identical properties as NC cells; both entities possessed the same target cell specificity and had similar kinetics of target cell killing. The use of polyclonal rabbit antimouse TNF antibody blocked the actions of NC cells. Thus we believe that the mediation of NC is through the actions of a TNF-like molecule.     

Evidence for two mechanisms by which tumor necrosis factor kills cells

Tumor necrosis factor (TNF) can inhibit the differentiation of preadipocytes to adipocytes and will revert differentiated adipocytes to the preadipocyte state. TNF is not toxic to either adipocytes or preadipocytes when used alone but is highly toxic to these cells when used in conjunction with cycloheximide, yielding virtually 100% killing within 4-6 h of treatment. A cell line (TA1 R-6) was isolated which is resistant to the combined toxic effects of TNF and cycloheximide. This cell line is stable and, unlike the parental cell line, does not morphologically differentiate to adipocytes or express adipocyte-specific mRNAs. It has a more transformed appearance and growth pattern and, while resistant to the toxic effects of TNF and cycloheximide in a 6-h assay, has become sensitive to cytotoxicity induced by TNF used alone in a 3-day assay. The adipocyte differentiation-inducing agents, dexamethasone and indomethacin, block the cytotoxicity induced by TNF alone in the TA1 R-6 line but do not block the rapid cytotoxicity of TNF and cycloheximide in the parental line. These results provide both genetic and pharmacologic evidence that there are at least two distinct or overlapping pathways by which TNF mediates its effects.     

Effects of stress on lysability of tumor targets by cytotoxic T cells and tumor necrosis factor

The effects of stress on four tumor cell lines are analyzed in view of the possibility that stress protects tumor cells against immune attack. We show that stress causes resistance to CTL and TNF in two cell lines. Induction of resistance is time dependent and reversible and not due to failure of killer cells to interact with stressed targets. It is shown that stress induces stress proteins concomitant with induction of resistance to killer cells and TNF. Moreover experiments are presented suggesting that resistance to either immune effector is due to independent mechanisms. The conclusion that stress can induce mechanisms in targets that interfere with the action of TNF as well as with target lysis following a lethal hit by CTL is discussed.     

Dexamethasone inhibits the cytotoxic activity of tumor necrosis factor

Effect of dexamethasone (DEX) on the cytotoxic activity of tumor necrosis factor (TNF) was examined using murine fibroblast cell line (L929 cells). DEX protected cells from the cytotoxic action of TNF. Protection of cytotoxic action was apparent when cells were pre-treated with DEX for 12h and no protection was observed in the presence of cycloheximide. These results suggested that de novo synthesis of new proteins was required for DEX-mediated protection. Moreover, prolonged simultaneous treatment with TNF and DEX resulted in the enhancement of cell growth, suggesting that TNF acted as a growth factor when cells were protected from the cytotoxic action of TNF. These results suggested that the signal transduction system for fibroblast growth enhancing and cytotoxic action of TNF were different from each other and that the interaction between TNF and glucocorticoids may play a modulating role in some inflammatory processes in vivo.     

Differentiation-inducing and cytotoxic effects of tumor necrosis factor and interferon-gamma in myeloblastic ML-1 cells

The effects of the tumor necrosis factor (TNF), and a second pleiotropic cytokine interferon-gamma (IFN), were examined in a line of human myeloblastic leukemia cells (ML-1). By itself, TNF causes ML-1 to differentiate along the monocytic pathway. The cells exhibit an increase in Fc receptors and acquire the morphological characteristics of maturing phenotype. They remain viable and continue to proliferate (at greater than or equal to 50% of the control growth rate) even with 10(2)-10(4) units/ml TNF. IFN alone has similar effects, causing an increase in Fc receptors but little cytotoxicity. In contrast to either cytokine alone, the combination of TNF plus IFN causes a cessation of proliferation and extensive cell death. Cytotoxicity occurs in a synergistic fashion; it requires the simultaneous presence of both cytokines, occurring with concurrent but not sequential exposure. These different responses, differentiation (TNF alone) and cytotoxicity (TNF + IFN), occur with a similar range of doses (approximately 10(2)-10(4) units/ml) and in a similar time frame (beginning on day 2). In other cell types, IFN can augment either the differentiation-inducing or the cytotoxic effect of TNF. In ML-1, the combined application of TNF plus IFN results in a shift from differentiation to cytotoxicity.     

Distinct tumor cell membrane constituents activate human monocytes for tumor necrosis factor synthesis

Several lines of evidence point to an activation mechanism of monocytes/macrophages by tumor cells. In this study we present data for distinct surface structures on K562 and Jurkat cells to directly induce TNF-mRNA expression and TNF production by human peripheral blood monocytes. Northern analysis showed that incubation of monocytes with either K562 or Jurkat cells led to a significant increase in TNF-mRNA expression. In addition, enhanced TNF production was detected in supernatants of monocyte cultures activated by Jurkat cells. Not only viable tumor cells but also metabolically inactivated tumor cells, cytoblasts, and membrane preparations from Jurkat and K562 cells induced TNF-mRNA expression. We identified two different membrane protein fractions with relative molecular mass of 32 to 38 kDa for Jurkat cells and 46 to 54 kDa for K562 cells that were responsible for monocyte activation.     

Natural cytotoxic activity in a cloned natural killer cell line is mediated by tumor necrosis factor

The interleukin-2-dependent mouse natural killer (NK) cell line NKB61A2 concomitantly exhibits NK and natural cytotoxic (NC) activities. This was determined by the cells' ability to lyse both the NK-sensitive YAC-1 lymphoma and the NC-sensitive WEHI-164 fibrosarcoma cell lines in a 4- and 18-hour 51Cr release assay, respectively. Cell-free supernatant from NKB61A2 cells grown in culture for 48 h had substantial lytic activity against WEHI-164. The mouse mast cell line PT18-A17 and the rat basophilic leukemia cell line RBL-2H3, which both express NC activity, also produced a soluble factor during culture which lysed WEHI-164 cells. This activity was increased in the basophilic/mast cells by crossbridging the surface IgE receptors. Similar results were obtained by triggering the basophilic NC cells with the calcium ionophore ionomycin and the tumor promoter phorbol-12-myristate-13-acetate (PMA). Such triggering of NKB61A2 cells, however, did not significantly increase their NC activity. Interestingly, both ionomycin and PMA had an inhibitory effect on the NK activity of NKB61A2. Recently it has been found that tumor necrosis factor (TNF) is a major mediator of NC activity. To determine if the soluble factor responsible for the NC activity of the NK clone was related to TNF, a rabbit polyclonal antiserum to mouse TNF was tested against the cell-free culture medium of NKB61A2, PT18-A17, RBL-2H3 and murine recombinant TNF (Mu-rTNF). The lytic activity of the culture medium from all these cells and the Mu-rTNF control was abrogated by this antibody. These data suggest that the murine cell line NKB61A2 has both NK and NC activities and that the NC activity is due to a factor immunologically similar to TNF. In addition, the enhancement of NC activity in the NK cell line is apparently under control by a separate pathway, different from that in the basophilic cells.     

Augmentation of the generation of cytotoxic T lymphocytes against syngeneic tumor cells by recombinant human tumor necrosis factor

In order to clarify the effect of recombinant human tumor necrosis factor (rHu-TNF) on the antitumor T cell immune response, we examined the effect of rHu-TNF on the generation of cytotoxic T cells (CTL) against syngeneic tumor cells. Spleen cells from X5563 plasmacytoma-transplanted mice were stimulated in vitro with mitomycin C-treated X5563 cells in the presence or absence of rHu-TNF. The generation of CTL was augmented in a dose-dependent manner by the addition of rHu-TNF. The augmenting effect of rHu-TNF was more marked when indomethacin was added to the culture. The augmenting effect was observed only when rHu-TNF was added at the early stage of the generation of CTL. The cell surface phenotype of CTL generated was L3T4- and Lyt2+. The augmentation was shown not only by the chromium-51 release assay but also by the Winn assay. As to the specificity, the augmentation of CTL generation was observed by the addition of rHu-TNF when responder-primed spleen cells were stimulated with the tumor cells in vitro. On the other hand, augmentation was not observed when responder spleen cells were not stimulated with the tumor cells in vitro, or when responder spleen cells were obtained from normal mice. The CTL generated was not cytotoxic against other tumor cells of the same haplotype. Thus, rHu-TNF augmented the generation of CTL against syngeneic tumor cells in an antigen-specific manner. The in vivo effect of rHu-TNF was examined by administering rHu-TNF into X5563-bearing mice. The spleen cells of rHu-TNF-injected mice generated a much higher CTL activity against X5563 cells in vitro than did the spleen cells of uninjected mice. From these results, a possibility can be considered that in some cases, rHu-TNF may exert its antitumor activity by stimulating the immune system.     

Signalling pathway of tumor necrosis factor in normal and tumor cells

Summary Several aspects of the activity and effects of tumor necrosis factor (TNF) were investigated to gain further insight into its cytotoxic mechanism. The relation between number of TNF receptors and TNF susceptibility of both tumor cells and normal cells was studied, utilizing a specific binding assay. Among the tumor cells, a fairly close correlation (r=0.855) was observed between receptor number and sensitivity to TNF. No cytotoxic effect by TNF was observed on any of the normal cells tested, even though TNF receptors were shown to be present, and cell proliferation was apparently stimulated by TNF in some cases. TNF internalization and intracellular distribution were studied by pulse-labelling and Percoll density gradient centrifugation. In L-M (murine tumorigenic fibroblasts, highly sensitive to TNF cytotoxicity) cells and HEL (human embryonic lung cells, non-sensitive to TNF cytotoxicity) cells, receptor-bound ¹²⁵I-labelled recombinant human TNF was rapidly internalized and delivered to lysosomes within 15–30 min, and this was followed by degradation and release into the culture medium. The presence of either a cytoskeletal disrupting agent or a lysosomotropic agent was observed to inhibit the cytotoxic effect of TNF, thus also indicating that TNF internalization, followed by delivery to lysosomes, is essential in the cytolytic mechanism of TNF.As observed by [³H]uridine incorporation, TNF did not affect RNA synthesis in L-R cells (TNF-resistant cell lines derived from L-M cells) and HEL cells, but markedly stimulated (by 3.5 times) RNA synthesis in L-M cells.     

Cell lines cultured at high density are resistant to lysis by tumor necrosis factor and natural cytotoxic cells

It has been suggested that natural cytotoxic (NC) cell activity and tumor necrosis factor (TNF), the molecular mediator of NC activity, are capable of protecting individuals against the progression of incipient tumors or could be useful in cancer therapy regimens. Much of this speculation arises as a result of in vitro studies, on a variety of tumor cells, demonstrating the cytolytic and cytostatic properties of NC and TNF activities. Here, evidence is presented showing that certain mouse fibroblast cell lines, generally considered sensitive to NC and TNF lysis, are quite resistant to these lytic activities when cultured at high cell density. Although a soluble factor that renders these same target cells resistant to NC and TNF lysis has been described, no such factor is involved in this high density-induced resistance. Rather, it appears that cell to cell contact of the targets is critical. Moreover, the induced resistance to NC and TNF lysis does not result from loss of either NC recognition determinants or TNF receptors by the target cells, but is the consequence of increased expression of a protein synthesis-dependent resistance mechanism. These observations raise the issue of the in vivo phenotype of cells characterized in vitro as sensitive to NC and TNF lysis. It is entirely possible that certain cells which are considered sensitive to NC and TNF activities are, in fact, resistant to these cytolytic activities when growing as tumors (i.e., at high cell density). Should this be the so, NC and TNF cytolytic activities may not function in vivo or may function only via some indirect means.     

Recombinant mouse tumor necrosis factor expressed in mammalian cells: effect of glycosylation on cytotoxic activity.

A mouse tumor necrosis factor-alpha (TNF) expression vector, pTNFNeo, was constructed by inserting a 1.3 kb cDNA coding for a full structural region of mouse TNF into an expression plasmid BCMGSNeo. COS7 cells were transfected with the pTNFNeo and a G418-resistant transfectant, BK-2, which stably secreted lytic activity to L929 cells was cloned. The lytic activity in the BK-2 culture spent medium reached up to 6000 U/ml, and was completely and specifically inhibited with antiserum to mouse TNF. Gel filtration chromatography and Western blot analysis indicated that the recombinant TNF in the medium existed in associated forms composed of a mixture of 22 kDa and 17.5 kDa components. Glycopeptidase F digestion indicated that the 22 kDa species was an N-glycosylated form of the 17.5 kDa species. Specific activities of the 22 kDa and the 17.5 kDa species isolated were 6.9 x 10(5) U/mg and 8.1 x 10(6) U/mg, respectively, suggesting that carbohydrate moiety impaired the lytic activity.     

Interleukin 1 and tumor necrosis factor activate common multiple protein kinases in human fibroblasts.

High resolution two-dimensional gel electrophoresis was used to analyze the signal transduction pathways of tumor necrosis factor (TNF-alpha) and interleukin 1 (IL-1 alpha and -beta) in human fibroblasts. Approximately 450 discrete radioactive spots were electrophoretically resolved from cytosolic extracts of cells prelabeled with 32P. At least 63 of these polypeptides exhibited significant and concordant phosphorylation or dephosphorylation in response to TNF or IL-1, despite the fact that different receptors are involved. Most of these changes concerned serine/threonine residues although enhanced tyrosine phosphorylation of several polypeptides was also observed. Phosphorylation patterns induced by a number of other agonists were compared with the patterns induced by IL-1 and TNF. These included activators of protein kinases C and A, bradykinin (a stimulator of inositol phospholipid hydrolysis), epidermal growth factor, heatshock, and mellitin (an activator of phospholipase A2). Although each of these agonists induced changes resulting in a distinct pattern of protein phosphorylation, none of these patterns had significant homology with that induced by IL-1 and TNF. Other assays were performed to verify the involvement of specific kinases. Collectively, these data indicate that IL-1 and TNF activate multiple protein kinases viz. a kinase(s) which activates microtubule-associated protein 2 (MAP-2) kinase, a kinase that phosphorylates the cap-binding protein, and a possibly novel serine/threonine protein kinase.     

Differences in the tumor necrosis factor-alpha-mediated lysis by fixed natural cytotoxic cells and fixed cytotoxic macrophages

TNF-alpha has been shown to be associated with macrophage cell membranes in such a way as to retain cytolytic activity despite fixation of the macrophage effector cells with paraformaldehyde. In this paper we report that, similar to cytotoxic macrophages, natural cytotoxic (NC) cells also use cell-associated TNF to lyse sensitive target cells. However, in contrast to fixed cytotoxic macrophages, NC cells do not retain cytolytic activity after fixation with paraformaldehyde. Additionally, the cytolytic activity of paraformaldehyde-fixed NC cells is not increased by incubation with LPS or by incubation with rTNF before fixation. Western blot analysis indicates that, unlike macrophages, NC cells use a smaller (17 kDa) constitutively active form of TNF. These results indicate that, although both macrophages and NC cells use effector cell-associated TNF to mediate lysis of sensitive targets, the way in which TNF is associated with these two types of effector cells must be different.     

Interleukin-1 alpha and tumor necrosis factor-alpha protect cells against natural killer cell-mediated cytotoxicity and natural killer cytotoxic factor

The protective effects of interferons (IFNs) against NK cell-mediated cytotoxicity (NK-CMC) is well established. We report here that both recombinant tumor necrosis factor-alpha (TNF-alpha) and recombinant interleukin-1 alpha (IL-1 alpha) can also protect some adherent target cells (e.g., the amniotic cells WISH and the cervical epithelial carcinoma cells HeLa-229) from NK-CMC in a dose-dependent manner. Like in the case of IFNs, the level of conjugate formation between target and effector cells (nonadherent peripheral blood lymphocytes) is not affected by pretreatment of the target cells with either TNF-alpha or IL-1 alpha. However, while the main effect of IFNs is to reduce the ability of target cells to stimulate the release of NK cytotoxic factor (NKCF) from effector cells, TNF-alpha and IL-1 alpha do not affect this process but rather reduce the target cell sensitivity to the lytic effect of NKCF. Therefore TNF-alpha and IL-1 alpha induce resistance to NK-CMC by a mechanism that differs from the one attributed to IFNs. The protective effect of TNF-alpha and IL-1 alpha is not mediated by the induction of IFN-beta 2/IL-6.     

Signaling mechanisms in tumor necrosis factor alpha-induced death of microvascular endothelial cells of the corpus luteum

The microvasculature of the corpus luteum (CL), which comprises greater than 50% of the total number of cells in the CL, is thought to be the first structure to undergo degeneration via apoptosis during luteolysis. These studies compared the apoptotic potential of various cytokines (tumor necrosis factor α, TNFα; interferon gamma, IFNγ; soluble Fas ligand, sFasL), a FAS activating antibody (FasAb), and the luteolytic hormone prostaglandin F_2α (PGF_2α) on CL-derived endothelial (CLENDO) cells. Neither sFasL, FasAb nor PGF_2α had any effect on CLENDO cell viability. Utilizing morphological and biochemical parameters it was evident that TNFα and IFNγ initiated apoptosis in long-term cultures. However, TNFα was the most potent stimulus for CLENDO cell apoptosis at early time points. Unlike many other studies described in non-reproductive cell types, TNFα induced apoptosis of CLENDO cells occurs in the absence of inhibitors of protein synthesis. TNFα-induced death is typically associated with acute activation of distinct intracellular signaling pathways (e.g. MAPK and sphingomyelin pathways). Treatment with TNFα for 5–30 min activated MAPKs (ERK, p38, and JNK), and increased ceramide accumulation. Ceramide, a product of sphingomyelin hydrolysis, can serve as an upstream activator of members of the MAPK family independently in numerous cell types, and is a well-established pro-apoptotic second messenger. Like TNFα, treatment of CLENDO cells with exogenous ceramide significantly induced endothelial apoptosis. Ceramide also activated the JNK pathway, but had no effect on ERK and p38 MAPKs. Pretreatment of CLENDO cells with glutathione (GSH), an intracellular reducing agent and known inhibitor of reactive oxygen species (ROS) or TNFα-induced apoptosis, significantly attenuated TNFα-induced apoptosis. It is hypothesized that TNFα kills CLENDO cells through elevation of reactive oxygen species, and intracellular signals that promote apoptosis.     

CD40 induces apoptosis in carcinoma cells through activation of cytotoxic ligands of the tumor necrosis factor superfamily

CD40, a tumor necrosis factor (TNF) receptor (TNFR) family member, conveys signals regulating diverse cellular responses, ranging from proliferation and differentiation to growth suppression and cell death. The ability of CD40 to mediate apoptosis in carcinoma cells is intriguing given the fact that the CD40 cytoplasmic C terminus lacks a death domain homology with the cytotoxic members of the TNFR superfamily, such as Fas, TNFR1, and TNF-related apoptosis-inducing ligand (TRAIL) receptors. In this study, we have probed the mechanism by which CD40 transduces death signals. Using a trimeric recombinant soluble CD40 ligand to activate CD40, we have found that this phenomenon critically depends on the membrane proximal domain (amino acids 216 to 239) but not the TNFR-associated factor-interacting PXQXT motif in the CD40 cytoplasmic tail. CD40-mediated cytotoxicity is blocked by caspase inhibitors, such as zVAD-fmk and crmA, and involves activation of caspase 8 and caspase 3. Interestingly, CD40 ligation was found to induce functional Fas ligand, TRAIL (Apo-2L) and TNF in apoptosis-susceptible carcinoma cells and to up-regulate expression of Fas. These findings identify a novel proapoptotic mechanism which is induced by CD40 in carcinoma cells and depends on the endogenous production of cytotoxic cytokines and autocrine or paracrine induction of cell death.     

Involvement of prostaglandin-producing pathway in the cytotoxic action of tumor necrosis factor.

To elucidate the cytotoxic mechanism of tumor necrosis factor (TNF), we isolated TNF-resistant sublines of L929 cells. As compared with L929 cells, TNF-resistant cells retained similar number and affinity of TNF-binding sites, and showed a similar growth rate. TNF stimulated arachidonate release from L929 cells, while no stimulation was observed at all in TNF-resistant cells tested. The cytotoxic action of TNF on L929 cells was inhibited by indomethacin, suggesting that prostaglandin may be involved in the action. Therefore, TNF-stimulated prostaglandin production was examined in L929 and TNF-resistant sublines. The amount of PGE2 produced by L929 cells was increased more than 5-fold after the addition of TNF, whereas the amount of PGE2 did not change in the resistant sublines following addition of the factor. TNF-stimulated arachidonate release and PGE2 production were reversed by islet-activating protein (IAP)-treatment of L929 cells. These results suggest that arachidonate release and subsequent prostaglandin production are important for the cytotoxic action of TNF and that these processes are mediated by GTP-binding protein (G protein) that is coupled to the TNF-receptor.     

Targeting of tumor necrosis factor to tumor cells: secretion by myeloma cells of a genetically engineered antibody-tumor necrosis factor hybrid molecule

The construction, synthesis and secretion of a genetically engineered antibody-cytokine fusion molecule is described. To target tumor necrosis factor (TNF) to tumor cells, recombinant antibody techniques were used to produce a Fab-like antibody-TNF conjugate. At the gene level, the heavy chain gene of an antitransferrin receptor antibody was linked to a synthetic TNF gene encoding human TNF. Transfection of the heavy chain-TNF gene into a myeloma derived cell line which was producing the light chain of the same antibody, allowed the isolation of a cell line secreting a fusion protein of the expected molecular weight and composition. The culture supernatant of the cell line contained TNF cytotoxic activity towards murine L929 cells and human MCF-7 cells. Cytotoxicity towards the human cancer cells was inhibited by an excess of the original antitransferrin receptor antibody, indicating that the antibody-TNF molecules are targeted to the transferrin receptor rich tumor cells. Since the antibody genes used are chimeric (i.e. composed of mouse variable and human constant regions) and since DNA encoding human TNF was used, the hybrid protein is an example of a humanized immunotoxin-like molecule. These results illustrate the possibilities of antibody engineering technology to create and produce improved agents for cancer therapy. Furthermore, they demonstrate for the first time the ability of myeloma cells to secrete an antibody-cytokine chimeric molecule.     

Liposomes can function as targets for natural killer cytotoxic factor but not for tumor necrosis factor

NK cells exert their lytic action through the release of NK cytotoxic factors (NKCF) after stimulation by the bound target cell. NKCF may be related to granule-derived perforin/cytolysin on one hand and to the pleiotropic cytokine TNF on the other hand. In the present study, we show that NKCF can also lyse artificial lipid vesicles, as had been reported previously for cytotoxic granules and cytolysin. The lysis of large unilamellar vesicles was monitored by measuring the release of the encapsulated fluorescent dye carboxyfluorescein. NKCF-induced lysis was only observed with liposomes composed of a complex mixture of lipids including acidic phospholipids. No lysis could be demonstrated if the liposomes contained phosphatidylcholine as the only phospholipid, suggesting some kind of lipid specificity for the action of NKCF. A remarkable finding was that neither recombinant nor natural TNF were able to lyse large unilamellar vesicles, irrespective of their lipid composition, indicating different ways of interaction of NKCF and TNF with artificial (and presumably also biological) membranes.