Bioactive human recombinant tumor necrosis factor-alpha: an unstable dimer?

Homogeneous expansion of human T-regulatory cells via tumor necrosis factor receptor 2

T-regulatory cells (T(regs)) are a rare lymphocyte subtype that shows promise for treating infectious disease, allergy, graft-versus-host disease, autoimmunity, and asthma. Clinical applications of T(regs) have not been fully realized because standard methods of expansion ex vivo produce heterogeneous progeny consisting of mixed populations of CD4 + T cells. Heterogeneous progeny are risky for human clinical trials and face significant regulatory hurdles. With the goal of producing homogeneous T(regs), we developed a novel expansion protocol targeting tumor necrosis factor receptors (TNFR) on T(regs). In in vitro studies, a TNFR2 agonist was found superior to standard methods in proliferating human T(regs) into a phenotypically homogeneous population consisting of 14 cell surface markers. The TNFR2 agonist-expanded T(regs) also were functionally superior in suppressing a key T(reg) target cell, cytotoxic T-lymphocytes. Targeting the TNFR2 receptor during ex vivo expansion is a new means for producing homogeneous and potent human T(regs) for clinical opportunities.

Targeted Bioactivity of Membrane-Anchored TNF by an Antibody-Derived TNF Fusion Protein

We describe the generation and characterization of a fusion protein consisting of a humanized anti-fibroblast-activating protein (anti-FAP) Ab and human TNF replacing the IgG1 CH2/CH3 Fc domain. The construct was generated by recombinant DNA technology and preserved its IgG1-derived dimeric structure with the TNF molecule linked as a dimer. Expression in CHO cells was optimized in serum-free medium under GMP conditions to achieve production levels up to 15 mg/liter. Recognition of the FAP Ag by the construct was as good as that by the parental anti-FAP Ab. TNF signaling was induce able via both TNF receptor types. When acting in solution, the Ab-linked TNF dimer exhibited a 10- to 20-fold lower activity compared with recombinant trimeric TNF. However, after binding to FAP-expressing cells, immobilized anti-FAP-TNF dimer was equivalent to membrane-anchored TNF with regard to bioactivity. Amplification of TNF-related pathways by mimicking the membrane-integrated TNF signaling was detectable in various systems, such as apoptosis induction or tissue factor production. The difference in TNF receptor type 1 and 2 signaling by the anti-FAP-TNF construct correlated well with its Ag-bound or -soluble status. Translating the approach into a xenograft animal model (BALB/c nu/nu mice), we demonstrated low toxicity with measurable antitumor efficacy for the TNF fusion protein after i.v. application. Immunohistochemical analysis of tumor sections showed restricted TNF-mediated macrophage recruitment to the targeted tissue in a time- and dose-dependent manner. These data warrant transfer of the anti-FAP-TNF immunocytokine into clinical trials for the treatment of FAP-positive tumors.

Thiolated recombinant human tumor necrosis factor-alpha protects against Plasmodium berghei K173-induced experimental cerebral malaria in mice

The introduction of reactive thiol groups in recombinant human tumor necrosis factor (TNF) alpha (rhTNF-alpha) by the reagent succinimidyl-S-acetylthioacetate resulted in the formation of a chemically stabilized rhTNF-alpha trimer (rhTNFalpha-AT; as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis). rhTNFalpha-AT showed a substantially enhanced protective efficacy against the development of experimental murine cerebral malaria (ECM) after intravenous injection compared to the protective efficacy of nonmodified rhTNF-alpha. Administration of thiolated rhTNF-alpha with protected thiol groups (rhTNFalpha-ATA; no stabilized trimers in vitro) exhibited the same protective efficacy against ECM, while in vitro bioactivity was reduced. Parasitemia was significantly suppressed in rhTNF-treated mice that were protected against ECM but not in treated mice that developed ECM. Protection against ECM was not related to increased concentrations in plasma of soluble TNF receptor 1 and 2 directly after injection or at the moment of development of ECM in nontreated mice. The results indicate that thiolation of rhTNF-alpha leads to the formation of stable trimers with increased potential in vivo.

Abdominal surgery reduces the ability of rat spleen cells to synthesize and secrete active tumour necrosis factor-alpha (TNF-alpha) by a multilevel regulation

We have previously shown that abdominal surgery (explorative laparotomy) reduces the ability of lipopolysaccharide (LPS)-triggered spleen macrophages to secrete TNF-alpha. In this study we characterize possible mechanisms which could be responsible for the reduction in splenic production of TNF-alpha. Post-operative and control (unoperated) rat splenocytes or enriched splenic macrophages were cultured with LPS. Steady-state levels of TNF-alpha mRNA were determined by Northern and slot blot analyses, and validated by quantitative reverse transcriptase-polymerase chain reaction (RT-PCR). The amount of TNF-alpha protein was measured by Western blot analysis, and its biological activity was determined by the fibroblast L-929 cytotoxicity assay. Surgery induced a 12-fold inhibition in TNF-alpha activity (P < 0.02), caused up to two-fold reduction in the accumulation of TNF-alpha mRNA (P < 0.01), and suppressed TNF-alpha protein maturation into its 17-kD form in cellular extracts. Post-surgical spleen supernatants revealed mainly a band of a lower molecular weight (14 kD). Our data suggest a multilevel regulation of post-operative inhibition of TNF-alpha response to LPS, at the accumulation of mRNA, translational and secretory levels. We also suggest that the reduced bioactivity could be partially caused by a proteolytic cleavage of TNF-alpha. Since TNF-alpha is an important participant in immune responses, its reduced production and activity may be a central mechanism of post-operative immunosuppression.

The potential biological and clinical significance of the soluble tumor necrosis factor receptors

The role of TNF receptors (TNF-Rs) is not limited to signal transduction but includes extracellular regulatory functions affecting systemic TNF bioavailability. This review summarizes the regulation of TNF-R shedding and its kinetics, the complex interaction between the soluble receptors and their ligand in vitro and in vivo, and the potential diagnostic, prognostic and therapeutic value of the soluble receptors in malignant, inflammatory, infectious and autoimmune disorders.

Effect of cytokines on sperm motility and ionophore-stimulated acrosome reaction

The effects of soluble products from leukocytes (WBC) on sperm motility and calcium ionophore-stimulated acrosome reaction (AR) were examined. Supernatants of mononuclear WBC, isolated from peripheral blood and stimulated by the lectins Phytolacca americana (pokeweed mitogen) or concanavalin A, caused a weak but significant inhibition of progressive sperm motility. The recombinant cytokine interferon-gamma (IFN-gamma) in high concentrations inhibited motility of sperm from 5 of 8 donors tested. The recombinant cytokines tumor necrosis factor-alpha (TNF-alpha) and interleukin-8 (IL-8) did not show any effect on sperm motility. Reactive oxygen species (ROS) produced by stimulation of polymorphonuclear WBC with a phorbol ester (PMA) tended to inhibit sperm motility. Neither supernatants from lectin-stimulated mononuclear WBC, IFN-gamma, TNF-alpha, IL-8, nor ROS showed any significant effect on the ionophore-challenged AR.

Homologous ELISA for detection of oligomeric human TNF: properties of the assay

In order to quantify oligomeric human tumor necrosis factor-alpha (TNF), we have developed a sensitive homologous enzyme-linked immunosorbent assay (Hm-ELISA) using the same monoclonal antibody (MoAb) for both solid and liquid phase. Different anti-TNF MoAb have been compared in terms of their efficacy in the Hm-ELISA, affinity, neutralization capacity and epitope specificity. The data suggest, that effectiveness in the Hm-ELISA may represent a novel characteristic of MoAb. Of the MoAbs tested, 5 N was capable of recognizing oligomeric TNF in the Hm-ELISA with a detection limit of 15 pg/ml. Furthermore, using Hm-ELISA against human TNF, interleukin-8 (IL-8) and lymphotoxin, we have demonstrated that these cytokines are oligomeric in physiological solutions, but are converted into monomeric forms in the presence of the non-ionic detergent Tween 20. High salt buffer was employed to abrogate a nonspecific false positive reaction in the Hm-ELISA found in nearly half of the plasma samples obtained from healthy subjects. Finally, a good correlation between the Hm-ELISA and the L929 bioassay was observed for natural and recombinant TNF measured in human plasma.

Enhanced production of tumour necrosis factor alpha (TNF-alpha) by its precursor on the cell surface of primed THP-1 cells

To clarify the biological significance of tumour necrosis factor alpha (TNF-alpha) precursor, we analysed its expression at the primed and triggered stages using human monocyte-like cell line THP-1. To prime them, THP-1 cells were treated with either recombinant human interferon gamma (rIFN-gamma) or recombinant human tumour necrosis factor alpha (rTNF-alpha). At the primed stage, transient accumulation of TNF-alpha, mRNA and a small amount of 26-KDa TNF-alpha precursor was observed, and the precursor molecule was located on the cell surface. Following treatment of the primed cells with bacterial lipopolysaccharide (LPS), augmentation of transcription of TNF-alpha mRNA and production of a larger amount of TNF-alpha precursor were observed followed by secretion of a larger amount of mature TNF-alpha (17-KDa) than secreted by the unprimed cells (triggered stage). This suggests that with priming THP-1 cells might be changed to a stage where they are ready for production of a larger amount of TNF-alpha at the triggered stage. When either primed or unprimed THP-1 cells were pretreated with anti-TNF-alpha antibody, augmentation of TNF-alpha production by primed THP-1 cells was specifically suppressed, suggesting that TNF-alpha precursor itself may play an important role in the enhancement of TNF-alpha production by the primed macrophages after treatment with LPS.

Endogenous production of tumor necrosis factor by primary cultures of murine calvarial cells: influence on IL-6 production and osteoclast development

We have previously shown that tumor necrosis factor (TNF) and interleukin-1 (IL-1) acted synergistically to stimulate the production of IL-6 by bone marrow stromal and osteoblastic cells; and that an antibody to IL-6 inhibited TNF-induced osteoclast development in murine calvarial cell cultures. Prompted by this evidence, we have now examined whether TNF and/or IL-1 are produced by murine calvarial cells, and whether these cytokines are involved in IL-6 production and osteoclast formation. When cultured under basal conditions, calvarial cells produced TNF and IL-6, and were able to form bone resorbing osteoclasts. A neutralizing antibody against TNF suppressed both basal IL-6 production and the formation of bone resorbing osteoclasts. The anti-TNF antibody also inhibited IL-6 production in response to exogenous IL-1 or parathyroid hormone (PTH). In contrast, a neutralizing anti-IL-1 receptor antibody had no effect on basal, TNF- or PTH-stimulated IL-6 production. These findings suggest that TNF, but not IL-1, is produced by murine bone cells and that endogenous TNF induces the IL-6 production, osteoclast formation, and bone resorption exhibited by these cultures under basal conditions. Furthermore, bone cell-derived TNF amplifies the stimulatory effect of exogenous IL-1 or PTH on IL-6 production by calvarial cells.

Binding of cytokines to pharmaceutically prepared human immunoglobulin

Pharmaceutically prepared IgG, pooled from sera of over 2,000 normal individuals, contained both monomeric and dimeric IgG. Each type of IgG bound 125I-labeled interleukin (IL)-1 alpha, IL-1 beta, IL-6, and tumor necrosis factor (TNF)-alpha. Increased binding to IgG was observed if 125I-IL-1 beta was denatured by heating to 39 degrees C. However, the binding of both nondenatured and denatured 125I-IL-1 beta was not inhibited by unlabeled IL-1 beta. In contrast, binding of 125I-IL-1 alpha, 125I-IL-6, and 125I-TNF alpha was inhibited by the corresponding unlabeled cytokine. Papain-digestion of IgG abolished binding of 125I-TNF alpha but failed to influence the displaceable binding of 125I-IL-1 alpha and 125I-IL-6. 125I-TNF alpha was a mixture of trimeric and monomeric forms, the latter being the predominant form at lower concentrations. The apparent saturability of 125I-TNF alpha was explained by a higher nonspecific binding of monomeric than of trimeric 125I-TNF alpha to IgG. The amounts of cytokine antibodies in IgG preparations would contribute approximately 2 micrograms anti-IL-1 alpha IgG and 1 microgram anti-IL-6 IgG per kg body wt during high dose immune globulin therapy. In conclusion, pharmaceutical preparations of human IgG contain specific and neutralizing, high affinity antibodies against IL-1 alpha and IL-6, but not against TNF alpha or IL-1 beta. There are significant methodological pitfalls that hamper detection of IgG autoantibodies against cytokines.

Kinetic analysis of TNF-alpha oligomer-monomer transition by surface plasmon resonance and immunochemical methods

In this work we have studied the kinetic parameters of oligomeric tumour necrosis factor alpha (TNF-alpha) dissociation using biospecific interaction analysis (BIA), based on surface plasmon resonance (SPR) of TNF-alpha immobilized on a sensor chip, and by an ELISA technique able to detect TNF-alpha oligomers in solution. Validation studies, carried out with sensor chips bearing TNF-alpha oligomers or bovine albumin monomers, verified that: (a) TNF-alpha can be immobilized in the oligomeric form onto sensor chips; (b) the covalent linkage between TNF-alpha and sensor chips is stable under the experimental conditions; (c) TNF-alpha monomers are present on the sensor chips after dissociation; (d) immobilization and dissociation rate constant (kdiss) measurements are reproducible. The kdiss of recombinant TNF-alpha, measured under non denaturing conditions at pH 7.4 by BIA and ELISA were in good agreement, being 0.92 x 10(-5)/s and 1.1 x 10(-5)/s respectively (corresponding to a half life of about 20.9 h and 17.5 h, respectively). The dissociation rate was found to be significantly affected by the presence of detergents and by the pH of the solution, suggesting that TNF-alpha, at low concentrations, exists in solution with different molecular forms depending on the time of storage and buffer composition. Real-time BIA is rapid and does not require particular antibodies or reagents. Thus, the stability of the quaternary structure of natural or recombinant TNF-alpha from human or animal species as well as that of other oligomeric cytokines can probably be studied using this method.

Tumor necrosis factor-alpha modifies adhesion properties of rat islet B cells

The characteristic three-dimensional cell type organization of islets of Langerhans is perturbed in animal models of diabetes, suggesting that it may be important for islet function. Rat islet cells in culture are able to form aggregates with an architecture similar to native islets (pseudoislets), thus providing a good model to study the molecular basis of islet architecture and its role in islet function. Sorted islet B cells and non-B cells were permanently labeled with two different fluorescent dyes (DiO and DiI), mixed, and allowed to form aggregates during a 5-d culture in the presence or absence of TNF-alpha (100 U/ml), a cytokine suggested to be implicated in the early physiological events leading to insulin-dependent diabetes mellitus. Confocal microscopy of aggregates revealed that TNF-alpha reversibly perturbs the typical segregation between B and non-B cells. Insulin secretion, was altered in the disorganized aggregates, and returned towards normal when pseudoislets had regained their typical architecture. The homotypic adhesion properties of sorted B and non-B cells cultured for 20 h in the presence or absence of TNF-alpha were studied in a short term aggregation assay. TNF-alpha induced a significant rise in Ca(2+)-independent adhesion of B cells (from 24 +/- 1.1% to 44.3 +/- 1.2%; n = 4, P < 0.001). These findings raise the possibility that the increased expression of Ca(2+)-independent adhesion molecules on B cells leads to altered islet architecture, which might be a factor in the perturbation of islet function induced by TNF-alpha.

Expression and characterization of bioactive recombinant ovine TNF-alpha: some species specificity in cytotoxic response to TNF

We have expressed and partially purified recombinant ovine tumour necrosis factor alpha (rovTNF-alpha) using a yeast Ty, virus like particle, expression system. RovTNF-alpha is at least as active as recombinant human TNF-alpha (rhTNF-alpha) in two different bio-assays performed on ovine material, whilst approximately 1000-fold more rovrTNF-alpha than rhTNF-alpha is required to induce the same level of cytotoxicity in TNF-sensitive murine cell lines L929 and WEHI 164 clone 13. When cytotoxic assays are performed on the porcine TNF sensitive cell line PK(15)-1512, rovTNF-alpha shows about 2 logs greater activity than on murine cells, whilst rhTNF-alpha is about 1 log more active. A monoclonal antibody, raised against rovTNF-alpha, has been used to demonstrate the presence of nanogram amounts of an appropriately sized glycoprotein to be native ovine TNF-alpha in supernants of LPS stimulated ovine alveolar macrophages. These samples show no detectable cytotoxicity to L929 cells, although they show activity attributable to TNF-alpha (through neutralization by a polyclonal antiserum raised to rovTNF-alpha) in an assay on ovine material. The relative lack of activity on murine cells helps to explain previous reports of inability to assay native ovine TNF-alpha using these cells, in spite of their routine use to assay TNF-alpha from several other species. The sequence features in ovine TNF-alpha which might reduce its affinity for the murine TNF type 1 receptor are discussed.

Biochemical characterization of the extracellular domain of the 75-kilodalton tumor necrosis factor receptor

An expression plasmid encoding the extracellular domain of the 75-kDa human tumor necrosis factor (TNF) type 2 receptor (TNF-R2) was constructed and used to generate a stable cell line secreting soluble TNF-R2 (sTNF-R2). Purified sTNF-R2 was resolved by SDS-PAGE into one band of approximate M(r) 43,000, consistent with a molecular weight of 36,000 +/- 4800 obtained by sedimentation equilibrium analysis. The apparent molecular weight observed by gel filtration chromatography was approximately 136,000. Glycosylation analysis revealed that Asn-149 is fully glycosylated, while Asn-171 is incompletely glycosylated (approximately 50%), and that a proline-, serine-, and threonine-rich region (residues 175-234) contains O-linked carbohydrate structures. Scatchard analysis of [125I]TNF-alpha and [125I]TNF-beta binding to sTNF-R2 gave dissociation constants (Kd) of 0.3 and 0.75 nM, respectively, comparable to those observed for intact cell-surface TNF-R2. The sTNF-R2 was found to block the cytotoxicity of both TNF-alpha and TNF-beta in a murine L-M cell assay. The sizes of the sTNF-R2.TNF-alpha and sTNF-R2.TNF-beta complexes determined by gel filtration chromatography were approximately 322 and 204 kDa, respectively. The stoichiometry of the sTNF-R2.TNF-alpha and sTNF-R2.TNF-beta complexes were examined by size-exclusion chromatography, sedimentation equilibrium, and cross-linking. The data from these studies suggest that at least two molecules of sTNF-R2 can bind to a single TNF-alpha or TNF-beta trimer.

Internalization, lysosomal degradation and new synthesis of surface membrane CD4 in phorbol ester-activated T-lymphocytes and U-937 cells

Protein kinase C activating phorbol esters downregulated membrane CD4 by endocytosis in U-937 and human T-cells. Half-time for internalization (approximately 15 min at 50 ng/ml PMA) was determined by FACS. CD4-bound 125I-labeled anti-CD4 mAb was rapidly degraded in PMA-activated cells, whereas degradation was low in resting cells. Endocytosis and/or degradation of anti-CD4 mAb was suppressed by H7, and by inhibitors of membrane traffic (Monensin) and lysosome function (methylamine, chloroquine). Immunocytochemistry localized CD4 to the surface of unstimulated T-cells. Upon PMA stimulation occasional labeling was seen in endosomes but whole cell CD4 decreased dramatically. However, methylamine-treated PMA blasts showed accumulation of CD4 in lysosomes and accordingly, pulse-chase experiments in biolabeled cell cultures suggested a manifest reduction of CD4 half-life in response to PMA. Despite their low surface CD4 density, PMA blasts exhibited uptake and accelerated degradation of anti-CD4 mAb. Also, inhibitors of protein synthesis enhanced the PMA-induced downregulation, and membrane CD4 reappeared on fully activated as well as unstimulated cells treated with trypsin. Ongoing CD4 synthesis in activated cells was further evidenced by metabolic labeling and Northern blot analysis demonstrating unaltered or slightly increased CD4 protein and mRNA levels resulting from PMA. Our findings demonstrate that phorbol esters downregulate the cellular CD4 pool by endocytosis and subsequent lysosomal degradation of membrane CD4. Transport of CD4 to the cell surface and CD4 synthesis is unaffected by activation.

Oligomeric tumour necrosis factor alpha slowly converts into inactive forms at bioactive levels

The stability of oligomeric human tumour necrasis factor alpha (TNF) at bioactive levels has been studied by two immunoenzymatic assays: one able to specifically detect oligomeric and not monomeric TNF (O-e.l.i.s.a.) and the other able to detect both forms (OM-e.l.i.s.a.). The selectivity of O-e.l.i.s.a. and OM-e.l.i.s.a. for oligomeric and monomeric TNF was demonstrated with isolated forms prepared by partial dissociation of recombinant TNF with 10% (v/v) dimethyl sulphoxide and gel-filtration h.p.l.c. Evidence for instability of oligomeric TNF were obtained in physiological buffers, as well as in serum and cell-culture supernatants, as a function of TNF concentration. In particular, only a half of the TNF antigen was recovered in the oligomeric form after 72 h incubation (37 degrees C) at 0.12 nM, whereas no apparent dissociation was detected at 4 nM. The structural changes observed at picomolar concentrations were rapidly reversed by raising the concentration of TNF to about 2 nM by ultrafiltration, suggesting that subunit dissociation and reassociation reactions occur in the picomolar and nanomolar range respectively. The cytolytic activity of L-M cells correlates with oligomeric-TNF levels after incubation at picomolar concentrations. Moreover, isolated oligomeric TNF was cytotoxic towards L-M cells, whereas monomeric TNF was virtually inactive. In conclusion, the results suggest that bioactive oligomeric TNF is unstable at picomolar levels and slowly converts into inactive monomers, supporting the hypothesis that quaternary-structure changes in TNF may contribute to the fine regulation of TNF cytotoxicity.

Identification and characterization of urokinase receptors in natural killer cells and T-cell-derived lymphokine activated killer cells

Fluorescence-activated cell scanning analysis of human blood cells revealed novel urokinase receptors in large granular lymphocytes and a small subset of T-cells (CD3+). Culturing of T-cells with interleukin-2 to generate CD3+ lymphokine-activated killer cells caused a large increase in urokinase binding, suggesting that the urokinase receptor is an activation antigen. The receptor in lymphocytes was similar to that in monocytes with regard to size, affinity and ligand specificity, but did not mediate degradation of urokinase-inhibitor complexes. It is suggested that lymphocyte-bound pro-urokinase is activated, e.g. by the human T-cell-specific serine proteinase, HuTSP-1, and thereby starts a cascade of plasminogen activation important for extravasation of the cells.

Characterization of a recombinant extracellular domain of the type 1 tumor necrosis factor receptor: evidence for tumor necrosis factor-alpha induced receptor aggregation

An expression plasmid encoding the extracellular portion of the human tumor necrosis factor (TNF) type 1 receptor (TNF-R1) was constructed and used to generate a stable cell line secreting soluble TNF-R1 (sTNF-R1). The sTNF-R1 was purified, and its biochemical properties and its interactions with human TNF-alpha were examined. SDS-PAGE resolved the purified sTNF-R1 into three bands of approximate Mr 24,200, 28,200, and 32,800. Sedimentation equilibrium analysis gave a molecular weight of 25,000 for sTNF-R1 whereas the molecular weight obtained by gel filtration chromatography was approximately 55,000-60,000. Scatchard analysis of [125I]TNF-alpha binding to sTNF-R1 revealed high-affinity binding (Kd = 93 pM), comparable to that observed for the intact receptor on whole cells. Competitive binding experiments showed that sTNF-R1 has a 50-60-fold higher affinity for TNF-alpha than for TNF-beta, in contrast to the equal affinities of TNF-alpha and TNF-beta for the full-length TNF-R1 transiently expressed in mammalian cells. The sTNF-R1 was found to block the cytotoxicity of TNF-alpha and TNF-beta on a murine L-M cell assay. The sizes of the sTNF-R1.TNF-alpha complex determined by gel filtration chromatography and sedimentation equilibrium were approximately 141 and 115 kDa, respectively. The stoichiometry of the complex was examined by Scatchard analysis, size-exclusion chromatography, HPLC separation, amino acid composition, sequence analysis, and sedimentation equilibrium. The data from these studies suggest that at least two molecules of sTNF-R1 can bind to a single TNF-alpha trimer.(ABSTRACT TRUNCATED AT 250 WORDS)

Stabilization of the bioactivity of tumor necrosis factor by its soluble receptors

The receptors for tumor necrosis factor (TNF) exist in cell-associated as well as soluble forms, both binding specifically to TNF. Since the soluble forms of TNF receptors (sTNF-Rs) can compete with the cell-associated TNF receptors for TNF, it was suggested that they function as inhibitors of TNF activity; at high concentrations, the sTNF-Rs indeed inhibit TNF effects. However, we report here that in the presence of low concentrations of the sTNF-Rs, effects of TNF whose induction depend on prolonged treatment with this cytokine are augmented, reflecting an attenuation by the sTNF-Rs of spontaneous TNF activity decay. Evidence that this stabilization of TNF activity by the sTNF-Rs follows from stabilization of TNF structure within the complexes that TNF forms with the sTNF-Rs is presented here, suggesting that the sTNF-Rs can affect TNF activity not only by interfering with its binding to cells but also by stabilizing its structure and preserving its activity, thus augmenting some of its effects.

Anti-interleukin-6 antibodies in normal human serum

High-avidity IgG antibodies to the cytokine interleukin-6 (IL-6) were found in sera of apparently healthy adult individuals. These antibodies specifically interfered with an ELISA (enzyme-linked immunosorbent assay) for IL-6 in which specific polyclonal rabbit antibodies to human recombinant IL-6 (rIL-6) were used. Furthermore, using precipitation of 125I-rIL-6 with rabbit antibodies to human immunoglobulins (Ig), the sera of 7 out of 68 Danish blood donors were found to contain specific antibodies in substantial amounts. Judged by ELISA interference, gel filtration of sera incubated with 125I-rIL-6 and second antibody precipitation of 125I-rIL-6, IgG seemed to be the dominant IL-6 binding protein in these normal sera. Using specific antibodies to human Ig light chains, it was found that the anti-IL-6 antibodies were of polyclonal origin. Moreover, there are at least two epitopes on the IL-6 molecule, because more than one IgG bound to some IL-6 molecules at the same time. The anti-IL-6 antibodies did not cross-react with a number of other human recombinant-derived and native cytokines. The antibodies recognized native as well as rIL-6, but preferentially monomeric IL-6.

Surface membrane CD4 turnover in phorbol ester stimulated T-lymphocytes. Evidence of degradation and increased synthesis

Down-regulation of surface membrane CD4 (smCD4) in phorbol ester stimulated T-cells resulted from internalization. Internalization (T1/2 = 15 min at 50 ng PMA/ml) was followed by degradation of CD4-bound antibodies. Degradation in unstimulated T-cells was comparatively insignificant. Release of degradation products was PMA dose-dependent and could be inhibited by methylamine. Uptake and degradation continued after maximal down-regulation of surface membrane CD4, and methylamine did not inhibit reappearance of smCD4 antigens. Metabolic labelling of T-cells further showed that ongoing synthesis rather than recycling contributed to an accelerated smCD4 turnover in activated cells.

Human tumour necrosis factors-alpha and -beta: differences in their structure, expression and biological properties

Tumour necrosis factor-alpha (TNF-alpha) and lymphotoxin (LT or TNF-beta) are cytokines, best known for their cytotoxic or cytostatic effects on some tumour cells. They are structurally related, compete for a common receptor, and are potent inducers of similar biological responses. TNF-alpha and LT appear to have distinct three-dimensional structures because they differ greatly in their sensitivity to various proteases and chemical agents, and antibodies raised against one cytokine do not cross-react with the other cytokine. The closely linked TNF-alpha and LT genes are independently regulated since many cell types produce only TNF-alpha or LT. Expression of the TNF-alpha gene can be controlled either at the transcriptional or at the post-transcriptional level. In some cell types, TNF-alpha and LT induce qualitatively or quantitatively different biological responses, and LT can antagonize the action of TNF-alpha. The disparate biological activities of TNF-alpha and LT may be related to their different interactions with a common receptor. It is possible that TNF-alpha and LT have different physiological roles.

Cellular targets and receptors for interleukin-6. I. In vivo and in vitro uptake of IL-6 in liver and hepatocytes

Interleukin-6 (IL-6) is a potent stimulator of the hepatic synthesis of acute-phase proteins. 125I-labelled IL-6 disappeared from the blood of rats with an overall half-time of about 1.5 min; 41% of the injected tracer dose was recovered in the liver by 15 min. The clearance was biphasic. The simultaneous injection of tracer and an excess of unlabelled IL-6 eliminated the initial rapid phase, and reduced the hepatic uptake to 14%. Light microscopic autoradiography showed 5% of the grains over non-hepatocytes, and 80% over hepatocytes, accumulating in areas around the bile canaliculi. Thereafter, degradation products accumulated in the bile. At 4 degrees C, isolated rat hepatocytes bound IL-6 with an apparent Kd of 39 pmol l-1 to a uniform class of 4500 receptors per cell with an apparent molar mass of 115-120 kg mol-1. The HepG2 human hepatocellular cell line bound IL-6 with an apparent Kd of 21 pmol l-1 to a uniform class of 1200 receptors per cell with an apparent molar mass of 155-160 kg mol-1. At 37 degrees C, both cell types endocytosed the bound ligand slowly, and degradation products appeared in the medium after a relatively long lag period (40 min in hepatocytes and 1 h in HepG2 cells).