Specific binding of 125I-human interferon-gamma to high affinity receptors on human fibroblasts

Temperature-sensitive polymer-conjugated IFN-? induces the expression of IDO mRNA and activity by fibroblasts populated in collagen gel (FPCG)

Indoleamine 2,3-dioxygenase (IDO) is an intracellular tryptophan-catabolizing enzyme possessing various immunosuppressive properties. Here, we report the use of this enzyme to suppress the proliferation of peripheral blood mononuclear cells (PBMC) co-cultured with IDO-expressing fibroblasts of an allogeneic skin substitute in vitro. Fetal foreskin fibroblasts populated within collagen gel (FPCG) were treated with interferon-gamma (IFN-gamma) conjugated with a temperature-sensitive polymer to induce the expression of IDO mRNA and protein. SDS-PAGE showed successful conjugation of IFN-gamma with the temperature-sensitive polymer. Expression of IDO mRNA was evaluated by Northern analysis. IDO enzyme activity was evaluated by the measurement of kynurenine levels. The results of Northern blot analysis showed an induction of IDO mRNA expression when treated with polymer-conjugated IFN-gamma. Kynurenine levels, as a measure of IDO bioactivity, were significantly higher in IFN-gamma-treated fibroblasts than in controls (P < 0.001). In a lasting effect experiment, the expression of IDO mRNA in FPCG treated with polymer-conjugated IFN-gamma was significantly longer than in those treated with free (non-conjugated) IFN-gamma (P < 0.001). IFN-gamma radiolabeling showed a prolonged retention of IFN-gamma within collagen gel in its polymer-conjugated form, compared to its free form. Presence of IDO protein in FPCG was demonstrated by Western analysis even 16 days after removal of the conditioned medium (containing released IFN-gamma). To demonstrate the immunosuppressive effects of IDO on the proliferation of PBMC, IDO-expressing FPCG treated with polymer-conjugated IFN-gamma were co-cultured with PBMC for a period of 5 days. The results showed a significant reduction in proliferation of PBMC co-cultured with IFN-gamma-treated IDO-expressing fibroblasts, compared to those co-cultured with non-IDO-expressing fibroblasts (P < 0.001). The addition of an IDO inhibitor (1-methyl-D-tryptophan) reversed the suppressive effects of IDO on PBMC proliferation. In conclusion, IDO expression in FPCG suppresses the proliferation of immune cells in vitro. The use of a temperature-sensitive polymer further prolongs the effect of IFN-gamma on the expression of IDO. Therefore, modulating IDO levels in situ might be an alternative for prolonging the survival of skin allografts.

Proliferation of peripheral blood mononuclear cells is suppressed by the indoleamine 2,3-dioxygenase expression of interferon-gamma-treated skin cells in a co-culture system

Indoleamine 2,3-dioxygenase (IDO) is an intracellular tryptophan-oxidizing enzyme possessing various immunosuppressive characteristics. In this study, we report the possible use of this enzyme in an allogenic skin substitute to suppress the proliferation of immune cells. Human fetal skin fibroblasts and keratinocytes were treated with the cytokine interferon-gamma to induce expression of IDO mRNA and protein. IDO enzyme activity was evaluated by measurement of kynurenine levels in the interferon-gamma-treated and -untreated cells. Results of Northern analysis showed a dose-dependent response in expression of IDO mRNA to the various concentrations of interferon-gamma used. Northern blot analysis also showed a time-dependent expression of IDO in response to different durations of interferon-gamma treatment. The level of kynurenine measured, as the bioactivity of IDO enzyme, was significantly higher in the interferon-gamma-treated fibroblasts and keratinocytes compared to those of controls (p < 0.001). To illustrate the immunosuppressive effects of IDO on immune cell proliferation, IDO-expressing fibroblasts were cocultured with human peripheral blood mononuclear cells for a period of 5 days. Results of 3H-thymidine incorporation assays showed a significant reduction in proliferation of the mononuclear cells cocultured with IDO-expressing skin cells compared to monocytes cocultured with control (non-IDO-expressing) skin cells (p < 0.001). Furthermore, addition of the IDO-inhibitor (1-methyl-D-tryptophan) significantly reversed the immunosuppressive effects of IDO on monocyte proliferation (p < 0.001). In conclusion, suppression of peripheral blood mononuclear cell proliferation due to interferon-gamma-induced IDO-expression in allogenic human skin cells might shed new light on developing a nonrejectable allogenic skin substitute.

Immune cell proliferation is suppressed by the interferon-gamma-induced indoleamine 2,3-dioxygenase expression of fibroblasts populated in collagen gel (FPCG)

Indoleamine 2,3-dioxygenase (IDO), a tryptophan-catabolizing enzyme, is an intracellular enzyme possessing various immunosuppressive properties. Here, we report the possible use of this enzyme to suppress proliferation of immune cells cocultured with IDO-expressing fibroblasts of an allogenic skin substitute. Fetal skin fibroblasts embedded within bovine collagen were treated with cytokine interferon-gamma (IFN-gamma) to induce expression of IDO mRNA and protein. Expression of IDO mRNA was evaluated by Northern analysis. IDO enzyme activity was evaluated by measurement of kynurenine and tryptophan levels in the IFN-gamma untreated and treated fibroblasts. The results of Northern analysis showed a dose-dependent increase in expression of IDO mRNA in response to various concentrations of IFN-gamma used. The levels of kynurenine and tryptophan measured, as the bioactivity of IDO, were significantly different in the IFN-gamma treated fibroblasts, compared to those of controls (P < 0.001). In a lasting effect experiment, the expression of IDO mRNA was gradually reduced to an undetectable level within 32 h of IFN-gamma removal. The results of Western blot analysis, however, revealed a significantly longer (192 h) lasting effect of IFN-gamma on IDO protein level, relative to that of mRNA expression. To demonstrate immunosuppressive effects of IDO on proliferation of immune cells, IDO-expressing fibroblasts were cocultured with peripheral blood mononuclear cells (PBMC) for a period of 5 days. The results of (3)H-thymidine incorporation showed a significant reduction in proliferation of PBMC when cocultured with IDO-expressing fibroblasts, compared to those cocultured with non-IDO-expressing fibroblasts (P < 0.001). Furthermore, addition of IDO-inhibitor (1-methyl-d-tryptophan) reversed the suppressive effects of IDO on PBMC proliferation in a dose-dependant fashion. To test the viability of immune cells cocultured with IDO-expressing fibroblasts, FACS analysis of the PI stained PBMC was conducted and no significant difference was found between these cells and the controls. In another set of experiments, we showed that migration rate and subsequent proliferation of IDO-expressing fibroblasts are also the same as those of control cells. In conclusion, IDO-expressing allogenic fibroblasts embedded within collagen gel suppress the proliferation of allogenic immune cells, while they still remain viable in this IDO-induced tryptophan-deficient culture environment.

Evidence of molten globule like state(s) of interferon gamma in acidic and sodium perchlorate solutions

Recombinant porcine interferon gamma (IFN gamma) at neutral pH is characterized by a tryptophan (Trp) fluorescence maximum around 343 nm and a rigid conformation, evidenced from tryptophan polarization values. Guanidine HCl shifts the protein emission spectra further to the red and decreases the fluorescence polarization values, indicating denatured IFN gamma in these solutions. In acidic solutions (3 < pH < 4), the emission spectra show a blue shift and lower tryptophan polarization. The midpoint of transition of these fluorescence properties occurs around pH 3.5-3.6. The protein in NaClO4 solution at neutral pH is similarly characterized by a blue shift in the tryptophan fluorescence maxima and low polarization values. The extent of quenching of tryptophan fluorescence by acrylamide is less in acid and in NaClO4 solutions of IFN gamma compared to its native form. This indicates a lower accessibility of the tryptophan in the altered conformation of the protein. The emission spectra of IFN gamma in NaClO4 solution shows a decrease in the tryptophan fluorescence intensity with simultaneous shift of the emission spectra over time. The presence of two conformational forms of IFN gamma in perchlorate solution is evidenced from an isofluorescent point at 315 nm. The change in the conformational state in perchlorate solution is characterized by first order kinetics. The dye anilinonaphthalene sulfonic acid does not bind either to the native IFN gamma or to its denatured form. However, the dye binds to the acid form of IFN gamma, as well as when the protein is present in NaClO4 solution at neutral pH. These observations, together with the results from literature that IFN gamma retains its secondary structure in acid solution to a considerable degree, would suggest that the protein exists as a molten globule-like state in acidic solution. Similarities of the protein fluorescence and 1-anilino-8-naphthalene-sulfonic-acid (ANS) binding properties of the protein in NaClO4 and acid solutions indicate that IFN gamma also exists in a molten globule-like state in perchlorate solution at neutral pH.

Hemin inhibits the interferon-beta-induced antiviral state in established cell lines

Hemin and other metalloporphyrins are known as very versatile compounds in nature, because they are able to carry out numerous functions in a free state or in association with specific proteins. When Friend murine erythroleukemia cells are treated with IFN-beta plus 100 microM hemin, the antiviral state is not observed, whereas the antiviral effect of IFN-gamma is unaffected by hemin treatment. This inhibitory effect of hemin is not restricted to erythroid cells. In fact, it is also observed in murine L929 and in human cell lines treated with IFN-beta. Neither trivalent iron in other forms nor hemin analogs (such as protoporphyrin IX or Sn(2+)-protoporphyrine IX) mimic this effect. Conversely, Co(3+)-protoporphyrin IX was as effective as hemin. At the transcriptional level, results obtained by run-on assays on nuclei from IFN-treated cells indicate that hemin does not completely inhibit IFN-beta induction of 2-5A synthetase gene(s) at 6 h of treatment but abolishes it at 24 h. In addition, hemin is able to inhibit the accumulation of IFN-induced 2-5A synthetase mRNAs. Experiments carried out to investigate the hemin effect on the early steps of the IFN signaling pathway indicate that hemin interferes with the ability of type I IFN to bind to its receptor, probably by a direct action on the IFN molecule.

Modulation of murine and human interferon-gamma receptor expression by their ligands or phorbol ester

IFN-gamma receptor expression on murine leukaemic L1210-cells has been studied. With the help of a transfected cell-line expressing the heterologous human receptor it was possible to discern receptor-specific properties like internalization from those regulating their expression on the surface. Recombinant IFN-gamma binds specifically to its homologous receptor at 4 degrees C and is rapidly internalized at physiologic temperatures. For this effect to occur, ligand binding to its receptor at 37 degrees C is necessary and sufficient. This notion is confirmed since a reduction in the number of heterologous human IFN-gamma receptors on the murine cell surface occurred exclusively after treatment with human IFN-gamma. Even weak doses of ligand, insufficient to occupy all receptors, led to a pronounced disappearance of binding sites. However, both receptors are simultaneously up-regulated in the presence of TPA, indicating a separate pathway which is not species-specific. Our findings imply that similar elements of the intracellular signal transduction machinery are involved in the control of MuIFN-gamma and HuIFN-gamma receptor expression. The results indicate also that factors involved in binding, internalization, and regulation of receptor gene expression are not species-specific.

Interferon gamma and tumour necrosis factor alpha induce a synergistic antiproliferative response in human Ewing's sarcoma cells in vitro

Three human cell lines derived from Ewing's sarcoma (RM-82, VH-64, and WE-68) were investigated to establish the influence of recombinant human interferon gamma (rhIFN gamma) and tumour necrosis factor alpha (rhTNF alpha) on cell proliferation and survival and to characterize IFN gamma and TNF alpha receptor expression. Incorporation of [3H]thymidine into cells was inhibited by rhIFN gamma after 24 h of incubation. Half-maximal inhibition was observed with 10-80 U/ml rhIFN gamma. A maximal effect (50%-70% inhibition of cell proliferation) was achieved by treatment of cells with 250 U/ml rhIFN gamma. The influence of rhTNF alpha on proliferation was found to differ among cell lines and varied with the concentration and the duration of exposure of cells to this cytokine. In WE-68 and VH-64 cells [3H]thymidine incorporation was not affected by rhTNF alpha up to 2000 U/ml after 96 h of incubation, whereas in RM-82 cells the incorporation was inhibited by 35% after 48 h of incubation with 100 U/ml rhTNF alpha. However, all cell lines showed a synergistic antiproliferative response to the combination of rhIFN gamma and rhTNF alpha after 24 h of incubation. The human recombinant cytokines interleukin(IL)-1 alpha, IL-1 beta, IL-2, IL-3, IL-4, IL-6 and granulocyte/macrophage-colony-stimulating factor, tested alone and in combination with rhIFN gamma and rhTNF alpha, had no influence on cell proliferation. Binding studies in the cell lines with 125I-rhIFN gamma revealed a dissociation constant (Kd) of 160-306 pM and approximately 8000-13,500 receptors/cell. Binding experiments with 125I-rhTNF alpha indicated 430-1250 receptors/cell with Kd ranging from 13 pM to 162 pM. These data indicate that, among various cytokines, only IFN and TNF alpha are capable of potently reducing Ewing's sarcoma cell growth in vitro. Our data suggest that IFN alone or in combination with TNF alpha may be useful in the design of novel strategies in Ewing's sarcoma therapy.

Transcription termination/polyadenylation occurs at multiple sites in the human type I interferon receptor gene

Based on the previously reported sequence, we isolated an independent cDNA clone encoding a binding component of the human type I interferon receptor (IFN-R). This cDNA is identical to the published sequence except that it lacks 62 bases of 5' untranslated sequence and terminates at the first of two potential polyadenylation sites. In Northern blot analyses of poly(A)+RNAs from both IFN-sensitive and IFN-resistant Daudi cells, this cloned cDNA hybridized to a predominant mRNA of 2.4 kb, as well as to mRNAs of 1.8, 4.8, and 5.6 kb, and occasionally 6.9 kb. These various transcripts, which were also observed at similar levels in Raji B cells and two T-cell lines, Jurkat and MOLT-4, were detected after high-stringency washes, and by alternate probes corresponding to subfragments of the cDNA. In contrast, only the 4.8- and 5.6-kb transcripts hybridized to a polymerase chain reaction (PCR)-derived probe that corresponded to genomic sequences immediately down-stream from the second polyadenylation site. These results indicate that the latter transcripts arise from the same gene as the predominant 2.4-kb mRNA due to incomplete transcription termination at either of the known polyadenylation sites. Finally, Northern blot analysis of total RNAs revealed the presence of the predominant 2.4-kb type I IFN-R transcript in numerous tissues from second trimester human fetuses, suggesting that the type I IFN-R gene is constitutively expressed in multiple cell types.

Definition of receptor binding domains in interferon-alpha

Earlier studies from this laboratory had identified three regions in interferon-alpha (IFN-alpha) that influence the active conformation of the molecule. These domains are associated with the amino acid residues 10-35, 78-107, and 123-166. In this report, we define these domains more accurately by identifying their critical clusters of amino acids. Using a panel of IFN-alpha 2a variants in antiviral, growth inhibitory, and receptor binding studies, we are able to show that these three domains, defined by residues 29-35, 78-95, and 123-140, are likely located on the surface of the molecule, with domains 29-35 and 123-140 in close spatial proximity. We conclude that the 29-35 and 123-140 domains are responsible for IFN-alpha receptor binding interactions and constitute receptor recognition sites in IFN-alpha. Extrapolating from our biological activity data, in the context of a number of predictive algorithms that provide insights into the hydrophobicity/hydrophilicity, surface probability, and flexibility of amino acid clusters, we infer that the residues 29-35 influence the active configuration of IFN-alpha most significantly. This region likely represents a loop structure that is relatively rigid in configuration. The carboxy-terminally located strategic domain, 123-140, is comprised of two clusters of amino acid residues, one that forms part of a rigid alpha-helix, the other a more flexible loop structure. Similarly, the 78-95 domain comprises a portion of an alpha-helical structure that is followed by a loop structure. Close examination of the amino acid sequences in all three regions among the different species of IFN-alpha s and human IFN-beta indicate that the 29-35 and 123-140 domains are most highly conserved, yet some variance is apparent in the 78-95 domain. We propose that the 78-95 region influences species specificity among the murine and human IFN-alpha s and determines the differential specificity of action between human IFN-alpha and human IFN-beta.

Epidermal growth factor and transforming growth factor-alpha decrease gamma interferon receptors and induction of intercellular adhesion molecule (ICAM-1) on cultured keratinocytes

The link between the epidermal keratinocytes of the skin and the activated T lymphocytes of the immune system is mediated by a variety of cytokines, including gamma interferon (IFN-gamma). We studied the influence of keratinocyte mitogens such as transforming growth factor-alpha (TGF-alpha), epidermal growth factor (EGF), and somatomedin-C (SM-C) on the ligand binding of 32P-labeled IFN-gamma to cultured keratinocytes derived from normal appearing adult human skin. Keratinocytes placed in a medium devoid of mitogens become growth arrested, and these quiescent cells expressed 2.4 times (28,900 versus 12,200 sites/cell) as many high affinity IFN-gamma receptors (Kd = 0.22 nM) compared to keratinocytes which were actively growing in medium containing TGF-alpha (25 ng/ml) or EGF (10 ng/ml). The reduction in IFN-gamma receptor sites by TGF-alpha/EGF was mitogen specific, as adding SM-C (500 ng/ml) did not have any effect on ligand binding, although it similarly stimulated keratinocyte growth. The reduction in IFN-gamma receptors was time dependent, occurring primarily after 24-48 hours of change in tissue culture conditions. The reduction in the number of high affinity IFN-gamma receptors by TGF-alpha/EGF had immunobiological consequences, because quiescent keratinocytes in basal medium had an increased expression of HLA-DR and intercellular adhesion molecule-1 (ICAM-1) induced by IFN-gamma, compared to actively growing TGF-alpha/EGF treated keratinocytes. These results suggest that rapidly proliferating keratinocytes exposed to TGF-alpha/EGF but not SM-C are capable of altering their response to IFN-gamma by decreasing their number of cell surface high affinity receptors for IFN-gamma.

Demonstration of the presence of a specific interferon-gamma receptor on murine astrocyte cell surface

Interferon gamma (IFN-gamma) is a pleiotropic lymphokine produced by T-lymphocytes which acts as a soluble mediator in immunological reactions. In addition to several immune target cells, such as monocytes and macrophages, it acts on the principal glial population, the astrocytes, inducing Ia antigen expression. We have developed a binding assay for 125I-labeled recombinant murine IFN-gamma, and show that, using this assay, IFN-gamma interacts with a single specific receptor on the murine astrocyte cell membrane. The binding is specific and saturable and it takes place with a Kd = 1.64 x 10(-9) M, with 11,100 receptor molecules per astrocytic cell. The binding shows, as for macrophages, species specificity. Using an immune assay including rabbit antibodies to IFN-gamma and 125I-labeled protein A, we have demonstrated an internalization of the ligand. This is an energy-dependent process, as around 50% of the bound IFN-gamma is endocytosed after 4 h at 37 degrees C when cultures are maintained in complete culture medium.

Posttranscriptional regulation of beta interferon expression in erythroid Friend cells treated with gamma interferon

Treatment of Friend erythroleukemia cells (FLC) with gamma interferon (IFN-gamma) in the presence of anti-IFN-beta antibodies reduces the effectiveness of the antiviral state and the induction of 2'-5'-oligoadenylate synthetase activity, indicating that the antiviral activity of IFN-gamma in FLC is in part mediated by the production of IFN-beta. Accordingly, IFN-gamma induces a less pronounced antiviral state in FLC resistant to IFN-alpha/beta than in wild-type cells. Moreover, while results of run-on assays indicate that both IFN-alpha and -beta genes are constitutively transcribed in these cells, FLC treatment with IFN-gamma induces only IFN-beta mRNA accumulation. These results indicate that posttranscriptional mechanisms are involved in the regulation of IFN-beta and -alpha expression by IFN-gamma. The low amounts of the induced IFN-beta synergize with IFN-gamma in mounting the potent antiviral effect.

Molecular and cellular mechanisms of receptor-mediated endocytosis

In general, receptors are involved in pathways of endocytosis, either constitutive or ligand induced. These receptors cluster in clathrin-coated pits, enter the cell via clathrin-coated vesicles, pass through an acidified endosome in which the receptors and ligands are sorted, and then either recycle to the cell surface, become stored intracellularly, or are degraded in lysosomes. The internalization pathways serve a variety of functions, such as nutrient uptake, removal of activated proteins, clearance of macromolecules, opportunistic entry of certain viruses and toxins, dissociation and degradation of ligand, and receptor-level regulation. Many receptors follow more than one intracellular pathway, depending on the cell type, receptor concentration, type of ligand, ligand valency, and ligand concentration. Although endocytosis is common to all nucleated eukaryotic cells, the factors that regulate these receptor-mediated endocytic pathways are not fully understood. Defective receptors that are not capable of undergoing normal endocytosis can lead to certain disease states, as in the case of familial hypercholesteremia (FH). This review has three objectives: (i) to describe the different routes that receptors and ligands follow after internaliation; (ii) to describe the potential mechanisms which regulate the initiation and subsequent sorting of receptors and ligands so they reach their final destination; and (iii) to describe the potential functions of receptor-mediated endocytosis.

Complement-mediated inactivation of interferon-gamma in ELISA systems

The recovery of a predetermined amount of interferon-gamma (IFN-gamma) added to normal serum was studied in two independent sandwich ELISA systems specific for rat and human IFN-gamma. In both assays the ELISA activity was rapidly lost in fresh but not in heat-inactivated (30', 56 degrees C) serum. Ninety percent of the initial activity had disappeared within 30 minutes upon incubation at 37 degrees C. Serum-mediated inhibition was not species-specific as the ELISA activity of rat IFN-gamma diminished equally well in rat and human sera. Inhibition was critically dependent on the isotype of the solid-phase monoclonal antibody (mAb) used in the ELISA systems. IgG1 and IgG2a mAbs efficiently inhibited the ELISA activity of IFN-gamma, whereas an IgA mAb was ineffective. The inhibition was not influenced by a wide variety of anti-proteolytic agents but was effectively blocked by anti-complementary substances or treatments directed to the first (C1) and third (C3) component of complement. Our results indicate that activation of the classical pathway of complement (CPC) and the concomitant covalent binding of C3 to the IFN-gamma molecule play a major role in the inhibitory process. It is concluded that reduction of the ELISA activity is attributable to diminished accessibility of the detector antibody for the IFN-gamma protein as a consequence of C3 binding.

Monoclonal antibodies against the human interferon-gamma receptor(s)

Enriched human interferon-gamma (HuIFN-gamma) receptor preparations were obtained by affinity chromatography of non-ionic detergent solubilized COLO 205 cell membranes on immobilized recombinant HuIFN-gamma. The active fractions, identified by a competition ELISA, were used as the immunogen in a BALB/c mouse. Fusion of its splenocytes with myeloma cells yielded several hybrids secreting antibodies that inhibit the antiviral activity of HuIFN-gamma; the two most active ones were selected for further characterization. This blocking activity was restricted to both the human species and the gamma type of IFN. Affinity purification of cell membrane extracts on the immobilized monoclonal antibodies resulted in the visualization of a major protein band with an Mr of 90,000, which is in good agreement with the results obtained by other authors [Aguet M. and Merlin G. (1987) J. exp. Med. 165, 988-999; Novick D., Orchansky P., Revel M. and Rubinstein M. (1987) J. biol. Chem. 262, 8483-8487; Sheehan K. C. F., Calderon J. and Schreiber R. D. (1988) J. Immun. 140, 4231-4237].

Growth inhibitory effects of interferon-beta but not interferon-alpha on human glioma cells: correlation of receptor binding, 2',5'-oligoadenylate synthetase and protein kinase activity

The antiproliferative effects of human recombinant interferon-alpha (rIFN-alpha A) and interferon-beta (rIFN-beta ser) were assessed in vitro against seven human glioma cell lines. Further analysis of one of these lines (EFC-2) in response to rIFN-alpha A demonstrated a minimum growth inhibition by day 6 of treatment, whereas a 50% inhibition of cell growth was observed with a dose of 50 U/ml of IFN-beta ser. No significant growth inhibition was seen by rIFN-alpha A at doses up to 500 U/ml. Addition of rIFN-alpha A to rIFN-beta ser-treated EFC-2 cells neither suppressed nor augmented the antiproliferative response to IFN-beta ser. The binding of 125I-labeled rIFN-alpha A or 125I-labeled rIFN-beta ser to EFC-2 cells was inhibited competitively by increasing concentrations of either unlabeled rIFN-alpha A or rIFN-beta ser. This suggests that the cellular receptors for both rIFN-alpha A and rIFN-beta ser appear to be intact and appear to bind both agents equally. Furthermore, incubation of EFC-2 cells for 72 h with either rIFN-alpha A or rIFN-beta ser resulted in an increase in 2',5'-oligoadenylate (2-5A) synthetase activity 5-fold with rIFN-alpha A and 50-fold with rIFN-beta ser. Similarly, the 68-kD IFN-induced protein kinase was induced substantially with rIFN-beta ser but only slightly induced with rIFN-alpha A treatment. These results suggest that EFC-2 human glioma cells demonstrate a differential sensitivity in terms of growth inhibition to rIFN-beta ser and to rIFN-alpha A which appears to correlate with a differential induction of both intracellular 2-5A synthetase and protein kinase activity. These results cannot be explained solely on the basis of surface receptor binding of rIFN-alpha A and rIFN-beta ser. These data do suggest that, for human glioma cells in culture, type I IFN receptors may display a subtle architectural variation that allows equivalent binding of both IFN-alpha and IFN-beta ser, but allows an enhanced signal transduction and biological effect only after binding a specific IFN subtype.

Molecular characterization of the human interferon-gamma receptor: analysis of polymorphism and glycosylation

Different molecular masses have been assigned to the human interferon-gamma receptor (HuIFN-gamma-R) by several authors. After extensive purification from Raji cells, this receptor was shown in a previous work to consist of two major protein species with molecular masses of 92 kD and 50 kD, as revealed by SDS-PAGE. We show here that the 50-kD band is most probably a degradation product of the 92-kD band due to a trypsin-like protease active during the purification process. The native protein of Raji cells seems, therefore, to have a molecular mass of 92 kD. The same molecular mass was found with Colo 205 cells (derived from a colon carcinoma). However, in conditions where degradation does not occur, the HuIFN-gamma-R shows a certain polymorphism: in IM-9 cells, another B-cell line, two bands exist with molecular masses of 95 kD and 85 kD, and in Wish cells, an amnion-derived cell line, one (or two) band(s) can be detected around 87 kD. This polymorphism is due at least in part to a variable extent of N-glycosylation from line to line and also within the same line, since after tunicamycin treatment of the Raji, IM-9, and Wish cells, very similar bands are obtained with a molecular mass of 72 kD.

Characterization of human interferon-gamma receptor purified from placenta

Membranes prepared from human placenta were used for characterization of the receptor for human interferon-gamma (HuIFN-gamma) after large-scale purification. HuIFN-gamma linked covalently to Affigel-10 was used for the purification of the receptor from octylglucoside-solubilized placental membranes. Radiolabeled IFN-gamma [32P]HuIFN-gamma, was used in binding and cross-linking studies to detect the receptor at different stages of the purification. From binding assays it was calculated that an average placenta contained 90-120 micrograms of receptor with a Kd value of 1.3 x 10(-9) M. Thus, human placenta is a rich and convenient source of receptor for IFN- gamma. When purified receptor was cross-linked to [32P]HuIFN-gamma, a variety of cross-linked bands were detected dependent on the preparation conditions. The use of protease inhibitors in the course of processing prevented degradation of the 90-kD intact receptor, showing that the lower-molecular-weight products detected in previous studies are degradation products of the receptor. Furthermore, a 20-kD fragment of the receptor was found to be active in binding HuIFN-gamma.

Crystallization and preliminary X-ray diffraction studies of bovine recombinant immune interferon

Reproducible conditions have been established for the crystallization of recombinant bovine immune interferon. Two cystalline forms of this protein were obtained. A tetragonal form, space group P422, with unit cell dimensions a = b = 59.0 A and c = 125.7 A and an orthorhombic form, space group P2(1)2(1)2(1), with unit cell dimensions a = 42.80 A, b = 79.90 A and c = 85.64 A were obtained under similar crystallization conditions. The orthorhombic form diffracts to 2.6 A resolution, contains a single interferon dimer in the asymmetric unit of structure and is suitable for X-ray diffraction analysis.

A monoclonal antibody against the ligand binding site of the receptor for mouse interferon-gamma

A rat monoclonal IgG2a antibody, GR-20, has been produced against the receptor for mouse interferon-gamma (MuIFN-gamma). Comparison of competitive binding studies performed with either 125I-labeled GR-20 or recombinant (r) MuIFN-gamma, as well as a variety of other studies, suggested that the epitope recognized by the monoclonal antibody (mAb) is in the domain of the receptor that binds ligand. The binding of GR-20 to cells of the monomyelocytic line WEHI-3 was of high affinity (1-2 nM). Approximately 20,000 binding sites were found per cell, a value that is in close agreement with the number of MuIFN-gamma receptors quantified on cells of the same type by ligand binding studies. The mAb also bound to a variety of other mouse cells, suggesting that the same epitope is shared by receptors for MuIFN-gamma, regardless of cell type. The epitope was not detected on two human cell types that were tested, while cells of a rat cell line shown to be minimally responsive to rMuIFN-gamma gave equivocal binding results when they were interacted with GR-20. Binding of the mAb to the receptor did not mimic the effects of ligand. In fact, the opposite was true: binding blocked the induction of three biological effects of MuIFN-gamma, including priming of macrophages for tumor cell killing, upregulation of the expression of class II major histocompatibility antigens (Ia) on the same cell type, and induction of antiviral activity in L cells. Following intravenous injection, initial removal of GR-20 was precipitous, followed after 1 h by a phase which was more gradual, resulting in 5-10% of biologically active mAb remaining in the circulation after 24 h. Such retention should make this mAb useful in a variety of studies in vivo.

Involvement of an inducible factor in interferon-gamma-mediated accumulation of HLA gene transcripts

Earlier studies with a cDNA clone (C5-4) complementary to an interferon (IFN)-gamma-inducible mRNA showed that in human fibroblasts (FS-4), IFN-gamma induced the transcription of the cognate gene, but it required new protein synthesis (Caplen and Gupta, J. Biol. Chem. 263, 332-339, 1988). To determine whether such a strategy is used for the regulation of other cellular genes by IFN-gamma, the regulation of the HLA class I and class II genes and another cellular gene for which a cDNA clone was isolated (C13) was studied. The results indicate that: (i) HLA-B (class I) and C13 gene expression was transcriptionally activated by IFN-gamma and IFN-alpha 2, and it did not require new protein synthesis. (ii) In contrast, the transcription of the HLA-DR alpha was activated by IFN-gamma (and not by IFN-alpha 2), but the accumulation of -DR alpha gene transcripts was strongly inhibited by cycloheximide or anisomycin, which indicated that there was a requirement for some newly synthesized protein factor(s) in this process, apparently at a step subsequent to transcriptional activation. We obtained evidence indicating that the putative protein factor(s) required is actually induced by IFN-gamma. (iii) IFN-gamma-induced transcription of the HLA-B gene was not inhibited by anisomycin or cycloheximide, but the accumulation of HLA-B transcripts plateaued sooner. This latter effect was not due to any toxicity of these inhibitors because it was observed if cycloheximide was added together with IFN-gamma, but not if it was added a few hours later. Furthermore, if cycloheximide was added 24 h after IFN-gamma, it actually caused a superinduction of HLA-B transcripts. The results suggest that some newly synthesized protein factor(s) may be required also for maximal accumulation of HLA-B gene transcripts following treatment with IFN-gamma. The results indicate a dual regulation of HLA class I and class II genes by IFN-gamma, and involvement of multiple mechanisms in the regulation of cellular gene expression by IFN-gamma.

Interferon-induced early changes in nuclear protein interactions with the interferon consensus sequence

A number of genes are transcriptionally regulated by interferons via a cis acting regulatory element (interferon consensus sequence). After incubation with HeLa cell nuclear extracts, a synthetic 29 mers probe representing the consensus was retarded as a single band whose amount increased after the addition of IFN to the cells. DNase I footprinting showed that a region of the non coding strand was protected differentially upon interferon treatment. After nuclear protein blotting and probing with the oligonucleotide, two proteins (114 and 50 kDa) of the desired specificity were revealed. Their absolute and relative amounts do not appear to be dependent of the IFN treatment.

Demonstration and partial characterization of the interferon-gamma receptor on human B lymphocytes

The expression of interferon-gamma (IFN-gamma) receptors on normal human B cells and four B cell lines was studied. Recombinant human IFN-gamma was labeled with [gamma-32P]ATP using the catalytic subunit of a cAMP-dependent protein kinase. All four B cell lines, although differing in their responsiveness to IFN-gamma, were found to express high-affinity receptors (1,000-11,000 receptors/cell). Normal unactivated B lymphocytes were also found to express constitutively high-affinity receptors, approximately 1,400 receptors per cell with an estimated affinity of 295 pM. Activation of the normal B cells in vitro with the polyclonal B cell activator, Staphylococcus aureus Cowan strain I (SAC), resulted in a slight decline in receptor number and a more pronounced fall in receptor density. One of the B cell lines and unactivated normal B cells were shown to internalize labeled IFN-gamma rapidly. Chemical cross-linking of 32P-IFN-gamma to the CB B cell line and to freshly isolated B lymphocytes revealed one major cross-linked receptor-ligand complex which had an estimated molecular weight of approximately 110 kilodaltons. This complex corresponded to a 93 kD receptor cross-linked to recombinant IFN-gamma. Our data indicate that normal B lymphocytes constitutively express an approximately 93 kD IFN-gamma receptor which is similar to the receptor present on Epstein-Barr virus-transformed B cell lines.

The cellular receptor of the alpha-beta interferons

This is a selective review of recent trends in research on the cellular receptor for the alpha-beta interferons. It deals mainly with work published in the last three years (1985-88), and therefore mainly with receptors for the human interferons. The binding characteristics of several human alpha interferons are examined, and the importance of in vitro experimental models for establishing the relationship between receptor binding and the cellular response is emphasized.

Monoclonal antibodies to the human interferon-gamma receptor: blocking of the biological activities of interferon-gamma and purification of the receptor

Monoclonal antibodies against the human interferon-gamma (IFN-gamma) receptor were developed by injecting mice with a preparation of receptor that was purified from solubilized placental membranes by ligand affinity chromatography. Three antibodies were identified by their ability to block the binding of 125I-labeled IFN-gamma to its receptor on HeLa cells at 4 degrees C. One of these antibodies blocked several biological activities of IFN-gamma, including its antiviral activity, its ability to induce HLA-DR surface antigens, and its ability to protect cells from NK cell-mediated cytotoxicity. This antibody exhibited higher binding capacity to cells at 37 degrees C and was significantly less displaceable by an excess of IFN-gamma as compared with the other two antibodies. Immunoaffinity chromatography of solubilized crude placental membrane preparation yielded a purified receptor that exhibited a molecular weight of 88,000. The purified receptor retained its ability to bind 125I-labeled IFN-gamma in solution.

Divergent effects of interferons on the growth of human benign prostatic hyperplasia cells in primary culture

Epithelial cells from human benign prostatic hyperplasia tissues were grown in primary cultures for up to 21 days and the effects of interferons on the growth of the cells were investigated. Interferon-alpha (Wellferon) showed growth inhibition both in the presence and in the absence of 3 x 10(-10)M testosterone propionate (TP) whereas interferon-gamma stimulated growth in a dose dependent manner under similar conditions. Interferon-beta had little effect on growth at the dose levels used in the study. The growth inhibition by interferon-alpha can be induced after stimulation of growth is achieved either with TP or with interferon-gamma. Implications of these findings for clinical use of these lymphokines is discussed.

Combined treatment with Pseudomonas aeruginosa toxin and interferon on mouse and human cells

The combined biological effect of Pseudomonas toxin and beta-interferon on mammalian cells was studied on two cell lines. The first was a virus-producing clone derived from NIH/3T3 mouse fibroblasts transformed by Moloney murine sarcoma virus. The second was a clone derived from human amnion cells. The parameters examined were either retrovirus release from the mouse cells or the rate of protein synthesis in both cell lines. When applied together with Pseudomonas toxin, interferon inhibits virus release even at a Pseudomonas toxin concentration that by itself does not exhibit any biological effect on NIH/3T3 cells. This enhancement phenomenon is both Pseudomonas toxin and interferon dose-dependent. Likewise, the combined treatment of either mouse or human cells with Pseudomonas toxin and the appropriate species-specific interferon, inhibits protein synthesis to a much greater extent than either of these agents alone. The kinetics of the inhibition of virus release is different from that seen with protein synthesis indicating that the enhancement phenomenon observed on virus release is not a result of the inhibition of total cellular protein synthesis. Interferon potentiates the effect of Pseudomonas toxin in a species-specific manner, thus suggesting that this process does not occur at the level of cell receptors but is a consequence of a subsequent intracellular event. It is concluded that the enhancement phenomenon does not reflect a direct interaction between interferon and Pseudomonas toxin, since Pseudomonas incubated together with interferon retained its normal biological activity as indicated by the ability of the toxin molecule to transfer the adenine diphosphoribose (ADP-ribose) moiety of nicotinamide-adenine dinucleotide (NAD) onto elongation factor 2 (EF-2).

Interferons: biological and clinical effects

Interferons play a key role in the immune system as biological response modifiers. Interferons alpha, beta, and gamma have been characterized, their nucleotide sequences defined, and the proteins produced by recombinant DNA technology. The myriad actions of interferons include enhancement of natural killer cell activity and antigen expression, induction of varied proteins, activation of macrophages, and antiviral, antiproliferative and antitumor effects. Clinical trials have demonstrated efficacy of interferons in some malignancies and ongoing studies are investigating results of combinations with other biological response modifiers and cytotoxic agents.

The cellular internalization of recombinant gamma interferon differs from that of natural interferon gamma

Purified natural and recombinant murine gamma interferons (MuIFN-gamma) bind at 4 degrees C to cultured L929 mouse fibroblasts with comparable receptor-binding affinity (Kd = 9 x 10(-10) M). Both 125I-labeled MuIFNs are rapidly internalized by cells at 37 degrees C, although recombinant IFN is internalized somewhat more slowly than natural IFN (t1/2 = 90 sec and 45 sec, respectively). Immunoelectronmicroscopy showed that the majority of bound recombinant MuIFN-gamma was located on the plasma membrane outside of coated areas, whereas natural interferon was found mainly in coated pits. At 37 degrees C most of the recombinant molecules entered the cytoplasm in pinocytotic vesicles, while natural interferon was internalized by the specific mechanism of receptor-mediated endocytosis [1]. However, nearly equal amounts of immunocytochemically detectable molecules of both IFNs were found in the cell nucleus within 2-3 min incubation at 37 degrees C. Thus, the process of translocation of the recombinant IFN-gamma appears to differ from that of the natural product.

Molecular cloning and expression of the human interferon-gamma receptor

A cDNA encoding the human interferon-gamma receptor was isolated from a lambda gt11 expression library using a polyclonal antireceptor antiserum. The gene for this receptor was identified in a cosmid library and transfected into mouse cells. The human interferon-gamma receptor expressed in mouse cells displayed the same binding properties as in human cells. However, transfected cells were not sensitive to human IFN-gamma, suggesting the need for species-specific cofactors in receptor function. As inferred from the cDNA sequence, the human interferon-gamma receptor shows no similarities to known proteins and represents a novel transmembrane receptor. It is most likely the product of a single mRNA and a gene located on chromosome 6q.

Immune interferon receptor: chemical and enzymatic sensitivity

Human immune interferon-gamma (HuIFN-gamma) labeled with 32P was used to study the structure of IFN-gamma receptor. When [32P]HuIFN-gamma was bound and crosslinked to IFN-gamma the receptor of human cells with a bifunctional crosslinker disuccinimidyl suberate (DSS), a single diffused 32P-labeled band corresponding to the IFN-gamma.receptor complex was visualized by SDS-polyacrylamide gel electrophoresis and autoradiography. The size of the [32P]-HuIFN-gamma.receptor complex was about 100-120 kD. Separation of crosslinked complex in reducing and nonreducing gels showed no size differences, suggesting the absence of interchain disulfide linkage. However, binding and formation of the crosslinked IFN-gamma. receptor complex on cells was diminished in the presence of the disulfide reducing agent dithiothreitol (DTT). The reduction was DTT-dose-dependent, suggesting that intramolecular disulfides of the receptor are important for binding. Also, [32P]HuIFN-gamma did not bind if cells were pretreated with and then washed free of DTT, suggesting an irreversible reduction of intrachain disulfide bonds, presumably of the receptor. [32P]HuIFN-gamma also specifically binds to human placental membranes. Each placenta has about 170 ng of IFN-gamma receptors. Covalent attachment of [32P]HuIFN-gamma to placental plasma membranes via DSS produced 2 crosslinked complexes with the molecular sizes of 100-120 kD and 60-70 kD. The IFN-gamma.receptor complex of placental membranes was solubilized with NP-40 after DSS treatment and partially purified with immobilized antibody to the carboxyl terminus of IFN-gamma. Treatment of the receptor complex with trypsin and papain was used to demonstrate its differential proteolytic sensitivity.

Expression of melanocyte stimulating hormone receptors correlates with mammalian pigmentation, and can be modulated by interferons

The relationship between melanogenesis and the expression of melanocyte stimulating hormone (MSH) receptors on the surface of melanocytes was examined using sublines generated from the melanotic JB/MS melanoma. JB/MS cells were propagated in long term culture to allow for phenotypic drift in their characteristics of differentiation, and then were cloned; the cloned cells ranged from well differentiated and pigmented to undifferentiated and amelanotic. Spontaneous and MSH-induced melanogenesis in these different lines was measured and correlated with the number of MSH receptors expressed. After 6 months of in vitro culture, the ability of the cells to respond to MSH was significantly reduced, as were the number of MSH receptors expressed; the cells had reduced pigmentation and were relatively undifferentiated histologically. Subsequently, clonally-derived pigmented cells were found to have numbers of surface MSH receptors (approximately 60,000 per cell) and levels of melanogenic activity similar to the original JB/MS cell line. However, an amelanotic clone had an even more dramatically reduced level of pigmentation which correlated with a further decrease in the expression of MSH receptors (less than 1,000 per cell) and the production of a potent melanogenic inhibitor. We also examined the responses of these various sublines to alpha, beta, and gamma-interferons and found significant heterogeneity in their abilities to respond to these cytokines. This study clearly shows that there is a direct correlation between melanogenesis and the expression of MSH receptors on the surface of melanocytes, and that melanogenic inhibitors may be critically involved in the regulation of mammalian pigmentation.

Interferon gamma is active on human lymphoblastoid Namalva cells without inducing an antiviral state

We demonstrate the presence of high affinity receptors specific for interferon-gamma (IFN-gamma) in human lymphoblastoid Namalva cells. The presence of these receptors, whose binding affinity and cross-linking characteristics were not distinguishable from those of the corresponding receptors in sensitive cells, was not consistent with the lack of responsiveness of Namalva cells to IFN-gamma as regards growth inhibition, induction of 2'-5' oligoadenylate synthetase activity and inhibition of virus multiplication. Nevertheless, IFN-gamma enhanced the expression of two genes, HLA class II and c-myc. Although the mechanism of these IFN-gamma-mediated modifications is not understood, these results provide evidence that the IFN-gamma receptors present in Namalva cells are functional.

The interaction of interferon-alpha and -gamma: regulation of (2-5)A synthetase activity

The interaction of IFN-alpha and IFN-gamma on the induction of antiviral, growth inhibitory, and (2-5)A synthetase activities was investigated in T98G and A549 cells. Synergistic or inhibitory effects which were host-cell dependent were seen with both simultaneous and sequential IFN treatments. Interestingly, IFN-gamma 1 pretreatment affected the levels of IFN-alpha-induced (2-5)A synthetase activity differently in the two cell types. In T98G cells, sequential treatment resulted in an overall decrease in induction of enzyme activity that was not observed in A549 cells. Receptor binding assays and measurements of steady-state mRNA levels indicated that this effect of IFN-gamma pretreatment does not occur at the level of either receptor expression or induction of transcription of the low-molecular-weight form of (2-5)A synthetase.

Purification and carbohydrate structure of natural murine interferon-beta

Mouse interferon-beta (Mu-INF-beta) induced in C-243 cells with Newcastle disease virus was purified in four steps including ammonium sulfate fractionation. DEAE-cellulose, monoclonal Mu-IFN-beta antibody affinity and Mono-S cation-exchange chromatographies. Specific activity of the purified Mu-IFN-beta ranged over 1.1-1.4 X 10(9) NIH units/mg protein. This preparation was submitted to pronase digestion and gel on Fractogel TSK HW-40. The permethylated and acetylated glycopeptide fraction was analyzed by chemical-ionization (ammonia) mass spectrometry. The major glycopeptide is composed of Gal, Man, GlcNAc and NeuAc with a molar ratio of 2.0:3.6:3.4:0.5. The GLC pattern of methyl derivatives obtained by methanolysis and acetylation of fully methylated glycopeptide identified 2,3,4,6-tetra-O-methylgalactose; 3,4,6-tri-O-methyl-mannose; 2,3,4- and 2,4,6-tri-O-methylgalactose; 2,4,di-O-methyl mannose and 3,6-di-O-methylglucosamine. These results when compared with data on N-glycans suggest the following structure for the carbohydrate moiety of Mu-INF-beta: (formula; see text).

Regulation of gene expression by interferons: control of H-2 promoter responses

The magnitude of the response to interferons and the requirement for individual elements in the promoter of the H-2Dd gene were shown to be cell-specific and dependent on the type of interferon used. Three DNA sequences in the promoter were found to bind murine nuclear factors. Two of these sequences are in functionally defined enhancer regions and also bind to the transcription factor AP-1. The third sequence is part of the region involved in interferon regulation and is homologous to the enhancer element of the interferon beta gene. A model for interferon regulation of H-2 promoters is discussed.

Recombinant interferon-gamma inhibits the mitogenic effect of platelet-derived growth factor at a level distal to the growth factor receptor

Highly purified preparations of recombinant human interferons (rIFNs)-alpha A, -beta, and -gamma all inhibited platelet-derived growth factor (PDGF)-induced DNA synthesis in normal human dermal fibroblasts, as monitored by incorporation of [3H]-thymidine into trichloroacetic acid (TCA)-insoluble material. rIFN-gamma was the most potent, since it blocked the PDGF response by 50% at about 10 U/ml or 0.3 ng/ml, whereas with rIFN-alpha A and rIFN-beta 4000 U/ml and 600 U/ml, respectively (10 ng/ml in both cases), were required to achieve the same effect. There was a close parallelism between the ability of these rIFNs to inhibit PDGF mitogenic activity and their capacity to inhibit cell proliferation in serum-containing medium. None of the rIFNs inhibited specific binding of 125I-PDGF to fibroblasts, and none interfered with receptor internalization. The mechanism of action of rIFN-gamma was analyzed further. rIFN-gamma did not inhibit uptake of [3H]-thymidine into these cells. However, it shifted if the time point of initiation of DNA synthesis from about 14 h after stimulation with PDGF to about 18 to 21 h and decreased significantly the rate of the DNA synthesis. rIFN-gamma could be added up to 6 h following stimulation with PDGF with no loss of its inhibitory effect. rIFN-gamma also blocked the mitogenic activity of epidermal growth factor and basic fibroblast growth factor. Taken together these results implicate that rIFN-gamma exerts its antimitogenic effect by inhibiting a process that occurs late in the PDGF signaling pathway and onto which the activity pathways of other mitogens converge. In view of the important role PDGF may play in wound-healing and in the pathogenesis of the proliferative lesions of arteriosclerosis, these data point to a possible role IFN-gamma may play as a regulator of these processes in vivo.

Enrichment and initial characterization of the solubilized receptor for mouse gamma interferon

The work reported here constitutes a first step in characterizing the receptor for mouse gamma interferon at the biochemical level. The myelomonocytic cell line, WEHI-3, was the source of starting material. Iodinated recombinant mouse gamma interferon incubated with WEHI-3 cells, as well as membranes prepared from them, bound specifically to a single class of sites with a Kd of 7 x 10(-9)M. Membranes were solubilized with the non-ionic detergent octyl-beta-D-glucopyranoside. As solubilization proceeded, binding activity could be assayed by precipitating the receptor with acetone in the presence of egg phosphatidylcholine liposomes. The Kd of the receptor in association with liposomes was 13 nM. Again here, only a single class of binding activity was found, and specificity for gamma, compared to other interferons, was maintained. This is the first time that the receptor for mouse gamma interferon has been solubilized and recovered in functional form. Further characterization included at least a 200-fold enrichment of binding activity by ligand affinity chromatography, resulting in the identification of a 95 kDa protein as the most likely candidate for either the receptor or a binding subunit thereof.

Effects of retinoids and interferon-gamma on cultured breast cancer cells in comparison with tumor necrosis factor alpha

The combination of retinoic acid or tumor necrosis factor alpha (TNF-alpha) with interferon gamma (IFN-gamma) resulted in a synergistic amplification of the anti-proliferative effect of IFN-gamma on cultured breast cancer cells. Retinoic acid could be replaced by other biologically active retinoids. This synergism was also observed for the induction of 2'-5'-oligoadenylate-synthetase, an enzyme which is not expressed constitutively on BT-20 human breast cancer cells and not inducible by retinoic acid or TNF-alpha alone. However, both substances augmented the IFN-gamma-mediated expression. On the other hand, only TNF-alpha and not retinoic acid was able to increase the IFN-gamma induced expression of HLA-DR on the cell surface. Both cytokines antagonized the IFN-gamma effect on detachability of cultured BT-20 cells. The combinations of retinoic acid with IFN-gamma increased the down-regulation of specific binding sites for 125I-IFN-gamma. On the other hand, IFN-gamma exerted no effect on the concentration of the cytoplasmic binding protein for retinoic acid. Data obtained in this study demonstrate a different pattern of action between retinoic acid and TNF-alpha regarding their synergism in combination with IFN-gamma.

Regulation of cell growth by interferon

Interferons can regulate growth and differentiation in a wide range of cell types. These mechanisms are currently being examined. Interferons inhibit the growth of tumour cells and are thus potential anti-cancer agents. They can also inhibit normal cell growth in vitro, and stimulate tumour cell growth in vitro. They may also be involved in some autoimmune diseases. This review examines the effect of interferons on cell proliferation, function, and growth, focusing primarily on in vitro cell systems.