Alpha and beta interferons and their receptor and their friends and relations

Identification of an adenine-nucleotide-binding site on interferon alpha2

Using 32P-labeled 2-azidoadenosine 5'-triphosphate (2N3ATP) and 8-azidoadenosine 5'-triphosphate (8N3ATP), we have identified a site on human interferon alpha2 (IFN-alpha2) that binds adenine nucleotides. The results from saturation and competition experiments demonstrated the specificity of the nucleotide interaction. Half-maximal saturation of IFN-alpha2 was observed at 10 microM 2N3ATP or 35 microM 8N3ATP. ATP effectively decreased photoinsertion of both photoaffinity analogs of ATP. Photoinsertion of 8N3ATP was enhanced by MgCl2, independent of the ionic strength, and exhibited an optimum pH between 7.0 and 7.5. Immobilized-Al3+ affinity chromatography and HPLC were used to purify the modified peptides from IFN-alpha2 that had been photolabeled with 8N3ATP and digested with trypsin or chymotrypsin. Overlapping-sequence analysis localized the sites of photoinsertion to the region corresponding to Lys121-Tyr135 in the amino acid sequence of IFN-alpha2, which almost perfectly overlaps a nuclear-localization signal (R120KYFQRITLYLKEKKY135).

Elucidation of the basic three-dimensional structure of type I interferons and its functional and evolutionary implications

The scientific and personal backgrounds of the crystallographic elucidation of the three-dimensional structure of murine interferon-beta (Mu-IFN-beta) are described. This structure, elucidated in 1990, is still the only experimentally determined structure for type I IFNs. Model-building studies for various type I IFNs based on the Mu-IFN-beta structure and the arguments on the receptor-binding epitopes appearing since then are reviewed. An updated set of a table and a figure demonstrating a strong correlation between the degree of amino acid sequence variation in various cytokine proteins and that in their cognate receptor proteins is given. The origin of a remarkably larger rate of evolutionary change in amino acid sequences of cytokine proteins despite their physiologic significance is discussed in view of the cytokine network and the neutral theory of evolution.

Post-translational up-regulation of the cell surface-associated alpha component of the human type I interferon receptor during differentiation of peripheral blood monocytes: role in the biological response to type I interferon

Human peripheral blood monocytes cultured in vitro exhibit a greater sensitivity to the antiviral effect of type I interferon (IFN) compared to freshly isolated monocytes. We evaluated the effect of macrophage differentiation on the expression of type I IFN receptors (IFN-R). Binding studies with iodinated IFN-alpha 2 and Scatchard plot analysis revealed that a single class of high-affinity receptors was present in freshly isolated monocytes. Monocyte differentiation to macrophages resulted in a three- to fourfold increase in the number of cell surface receptors with no change in their affinity. Polymerase chain reaction analysis of RNA revealed that comparable levels of mRNA for the IFN-R alpha (IFNAR1) and IFNAR2 components were expressed in freshly isolated monocytes and 7-day cultured macrophages. Likewise, the levels of IFNAR1 protein remained constant over time in culture. Immunofluorescence studies revealed that IFNAR1 was localized in intracellular compartments of freshly isolated monocytes, whereas it was predominantly detected on the cell surface in 7-day cultured macrophages. The increased expression of IFN-R on the plasma membrane of cultured macrophages may, at least in part, account for the increased antiviral effect of type I IFN in these cells. These modifications represent one of the events occurring during monocyte differentiation that may play a role in the regulation of macrophage functions.

Functional subdomains of STAT2 required for preassociation with the alpha interferon receptor and for signaling

Two members of the STAT signal transducer and activator of transcription family, STAT1 and STAT2, are rapidly phosphorylated on tyrosine in response to alpha interferon (IFN-alpha). Previous work showed that in the mutant human cell line U6A, which lacks STAT2 and is completely defective in IFN-alpha signaling, the phosphorylation of STAT1 is very weak, revealing that activation of STAT1 depends on STAT2. We now find that STAT2 binds to the cytoplasmic domain of the IFNAR2c (also known as IFNAR2-2) subunit of the IFN-alpha receptor in extracts of untreated cells. STAT1 also binds but only when STAT2 is present. The activities of chimeric STAT2-STAT1 proteins were assayed in U6A cells to define regions required for IFN-alpha signaling. Previous work showed that a point mutation in the Src homology 2 (SH2) domain prevents STAT2 from binding to phosphotyrosine 466 of the IFNAR1 subunit of the activated receptor. However, we now find that the entire SH2 domain of STAT2 can be replaced by that of STAT1 without loss of function, revealing that other regions of STAT2 are required for its specific interaction with the receptor. A chimeric protein, in which the N-terminal third of STAT2 has replaced the corresponding region of STAT1, did preassociate with the IFNAR2c subunit of the receptor, became phosphorylated when IFN-alpha was added, and supported the phosphorylation of endogenous STAT1. These results are consistent with a model in which STAT2 and STAT1 are prebound to the IFNAR2c subunit of the resting receptor. Upon activation, the IFNAR1 subunit is phosphorylated on Tyr-466, allowing the SH2 domain of STAT2 to bind to it; this is followed by the sequential phosphorylation of STAT2 and STAT1.

Interferons alpha/beta and their receptors: place in the hierarchy of cytokines

Interferons alpha/beta (IFNs-alpha/beta) are the first cytokines to be produced by recombinant DNA technology. They regulate growth and differentiation, affecting cellular communication, signal transduction pathways and immunological control. This review focuses on the relationships between the structure and biological activities of IFNs-alpha/beta induced as a result of specific interactions with different types of polypeptide receptors as well as on the role of glycolipids in the modulation of these activities. The discovery of the primary structure homology of HuIFNs-alpha and thymus hormone-thymosin alpha 1 (TM alpha 1), the experimental finding of the competition between IFN-alpha and TM alpha 1 for common receptors and the reproduction by reHuIFN-alpha 2 of TM alpha 1 immunomodulating activities create the basis of reHuIFN-alpha therapeutics instead of TM alpha 1, and potentiation of vaccines by reHuIFN-alpha. The first successful attempt at grafting of the HuIFN-alpha 2s TM alpha 1-like immunomodulating site to the designed de novo protein albeferon is described. This article also aims at reviewing recent data concerning the structure of other cytokines and their receptors. Their reciprocal structure-function taxonomy is proposed. The place of IFNs-alpha/beta and their receptors in the hierarchy of cytokines is determined.

Anti-tumor activity of synthetic peptide fragments of the human interferon-alpha 2

We have studied an influence of nine overlapping peptides from the region between amino acid residues 124 and 144 of the human interferon (FIN)-alpha 2 molecule on the growth of human lymphoid tumor cell lines in vitro. It was found that several, but not all, synthetic peptides inhibited proliferation of the same cell lines that IFN did. One of peptides, corresponding to the 124-138 amino acid residues of the IFN molecule (124-138) was most active. Using a human-mouse somatic hybrid cell line, we have shown that antiproliferative activity of the peptide 124-138 and IFN depended on the presence of human chromosome 21. Receptor binding studies also demonstrated that the peptide specifically interacted with membrane receptors on hybrid human-mouse cells carrying human chromosome 21, but not on parental mouse cells. Displacement experiments confirm that IFN and the peptide 124-138 compete for the same binding sites. Taken together, the data presented support a hypothesis that the C-terminal part of the IFN molecule contributes to antiproliferative activity possessed by IFN. Synthetic peptides studied in the present work may serve as a tool for studying tumor cell growth regulation by IFN and may be considered as potential nontoxic anti-tumor agents.

Ingested IFN-alpha has biological effects in humans with relapsing-remitting multiple sclerosis

Parenterally administered human recombinant type I interferons (hrIFN) in relapsing-remitting multiple sclerosis (RRMS) decrease relapses and spontaneous in vitro IFN-gamma production, reduce clinical progression, and decrease magnetic resonance imaging (MRI)-defined disease activity and lesions. Parenterally administered type I IFN use is limited by clinical and chemical toxicities, and the induction of antibodies that abrogate their activity in vivo correlated with the loss of clinical benefit. Therefore, we determined whether ingested IFN-alpha was non-toxic and had biological effects in humans. Ingested hrIFN-alpha showed no toxicity in normal volunteers or patients with RRMS at doses ranging from 300 to 100,000 units. In subjects with RRMS, a significant decrease in Con A-mediated proliferation and serum soluble intercellular adhesion molecule-1 (sICAM-1), a surrogate measure for disease activity in MS, was found after ingesting 10,000 and 30,000 units IFN-alpha The RRMS subjects also showed decreased IL-2 secretion after ingesting 10,000 units IFN-alpha and decreased IFN-gamma, TGF-beta and IL-10 production after ingesting 30,000 units IFN-alpha. The decreased secretion of IFN-gamma and IL-2 by ingested IFN-alpha suggests that oral IFN-alpha may cause a functional inhibition of Th J-like T helper cells in RRMS, a potential site of intervention at the level of effector T cells in MS. Our studies support the oral use of human IFN-alpha as a biological response modifier in humans.

A mutant cell line partially responsive to both IFN-alpha and IFN-gamma

A recessive mutant cell line, B7, which is partially responsive to both interferon (IFN)-alpha and IFN-gamma is described. B7 was FACS sorted from a cellular pool, which was obtained from the parental cell line 2C4, after several rounds of mutagenesis. The partial responsiveness to IFN was observed both in terms of expression of cell surface markers (CD2, class I and II HLAs) and mRNA expression of IFN-stimulated genes (9-27; 6-16; 2'-5' OAS; GBP and HLA-DR alpha). A genetic cross with the U4 mutant (JAK1-, a member of the Janus family of nonreceptor tyrosine kinase) did not restore full IFN responsiveness to B7, and JAK1 cDNA transfection into B7 restored the wild phenotype of the cell line, defining B7 as a member of the U4 complementation group. Nevertheless, JAK1 mRNA was not detected in this mutant. Transcriptional regulator complexes such as IRF1/2 (IFN-regulatory factor) and ISGF3-gamma (IFN-stimulated gene factor) were constitutively formed in the B7 mutant and co-migrated with the IFN-induced complexes expressed in the parental cell line 2C4. Thus, this cell line seems to be useful for understanding cis-acting elements governing JAK1 mRNA expression.

Structure-activity of type I interferons

Type I IFNs constitute a family of proteins exhibiting high homology in primary, secondary, and tertiary structures. They interact with the same receptor and transmit signals to cellular nucleus through a similar mechanism, eliciting roughly homogeneous biological activity. Nevertheless, the members of that family, IFN alpha species, IFN beta and IFN omega, due to local differences in the structure sometime show distinct properties. From the reported data it results that even minute changes or differences in the primary sequences could be responsible for a significant variety of biological actions, thus inducing to the hypothesis that Type I IFNs, rather than to be the result of a redundant replication during the evolution play definite roles in the defense of living organisms to foreign agents.

The human type I interferon receptor. Identification of the interferon beta-specific receptor-associated phosphoprotein

We used specific antibodies recognizing the receptor 1 (IFNAR1) and the recently cloned receptor 2.2 (IFNAR2.2) chains of the human type I interferon receptor complex to demonstrate that the interferon beta (IFN-beta)-specific receptor-associated phosphoprotein is IFNAR2.2 and not an unknown or additional receptor component. Immunoprecipitation experiments demonstrated that IFNAR2.2 is present in Daudi cells as a cell surface protein of approximately 90-100 kDa, which is tyrosine-phosphorylated and associated with IFNAR1, upon stimulation of cells with IFN-beta. IFNAR2.2 was not detected associated with IFNAR1 in cells stimulated with IFN-alpha, suggesting differences in receptor interaction between the two type I interferons. Both IFNAR1 and IFNAR2.2 undergo tyrosine phosphorylation upon induction by either IFN-alpha or IFN-beta. Therefore, it is unclear as to why IFNAR2.2 is not detectable in IFNAR1 immunoprecipitates in IFN-beta-treated cells. These data suggest that, although IFN-alpha and IFN-beta may utilize similar receptor chains, they interact with IFNAR1 and IFNAR2.2 in different ways.

Zinc mediated dimer of human interferon-alpha 2b revealed by X-ray crystallography

BACKGROUND

The human alpha-interferon (huIFN-alpha) family displays broad spectrum antiviral, antiproliferative and immunomodulatory activities on a variety of cell types. The diverse biological activities of the IFN-alpha's are conveyed to cells through specific interactions with cell-surface receptors. Despite considerable effort, no crystal structure of a member of this family has yet been reported, because the quality of the protein crystals have been unsuitable for crystallographic studies. Until now, structural models of the IFN-alpha's have been based on the structure of murine IFN-beta (muIFN-beta). These models are likely to be inaccurate, as the amino acid sequence of muIFN-beta differs significantly from the IFN-alpha's at proposed receptor-binding sites. Structural information on a huIFN-alpha subtype would provide an improved basis for modeling the structures of the entire IFN-alpha family.

RESULTS

The crystal structure of recombinant human interferon-alpha 2b (huIFN-alpha 2b) has been determined at 2.9 A resolution. HuIFN-alpha 2b exists in the crystal as a noncovalent dimer, which associates in a novel manner. Unlike other structurally characterized cytokines, extensive interactions in the dimer interface are mediated by a zinc ion (Zn2+). The overall fold of huIFN-alpha 2b is most similar to the structure of muIFN-beta. Unique to huIFN-alpha 2b is a 3(10) helix in the AB loop which is held to the core of the molecule by a disulfide bond.

CONCLUSIONS

The structure of huIFN-alpha 2b provides an accurate model for analysis of the > 15 related type 1 interferon molecules. HuIFN-alpha 2b displays considerable structural similarity with muIFN-beta, interleukin-10 and interferon-gamma, which also bind related class 2 cytokine receptors. From these structural comparisons and numerous studies on the effects of mutations on biological activity, we have identified protein surfaces that appear to be important in receptor activation. This study also reveals the potential biological importance of the huIFN-alpha 2b dimer.

Activation of different Stat5 isoforms contributes to cell-type-restricted signaling in response to interferons

Tyrosine phosphorylation and activation of the transcription factor Stat5 occur in response to stimuli like granulocyte-macrophage colony-stimulating factor, interleukin-3, or erythropoietin that stimulate both proliferation and differentiation of hematopoietic cells. It is unclear whether Stat5 is part of a proliferative response or part of the events leading to cellular differentiation. Here we report that agents promoting differentiation but not proliferation of hematopoietic cells, like phorbol ester or both types of interferons (IFNs), activate Stat5 in promonocytic U937 cells. Both IFN types caused tyrosine phosphorylation and DNA binding of predominantly one Stat5 isoform (Stat5a) despite expression of both Stat5a and Stat5b proteins. Monocytic differentiation of U937 cells led to a strong decrease in IFN-gamma-mediated activation of Stat5 but not of Stat1. Transactivation of Stat5-target genes occurred in response to IFN-gamma, which activates both Stat5 and Stat1, but not in response to granulocyte-macrophage colony-stimulating factor, which activates only Stat5. Tyrosine phosphorylation of Stat5 is not generally part of the IFN response. IFN-gamma did not cause Stat5 activation in HeLa cells, despite the expression of both Stat5 isoforms at similar levels. By contrast, IFN-alpha caused tyrosine phosphorylation and DNA binding of exclusively the b isoform of Stat5, and activated Stat5b formed a DNA binding activity previously found in HeLa cells and designated IFN-alpha activation factor 2. Taken together, our results demonstrate that ligand binding of IFN receptors leads to an isoform-specific activation of Stat5 in a restricted number of cell lineages. Moreover, they suggest that Stat5 might be part of the differentiation response of myeloid cells.

Different relative activities of human cell-derived interferon-alpha subtypes: IFN-alpha 8 has very high antiviral potency

Interferon-alpha (IFN-alpha) subtypes were separated by HPLC from the IFN mixtures produced by virus-stimulated human lymphoblastoid cells and leukocytes. Together with preparations of lymphoblastoid IFN and recombinant IFN-beta, these were tested in three human tumor cell lines derived from liver, lung, and neuroblasts. Their relative antiviral activities differed markedly: subtype IFN-alpha 8 was the most potent and IFN-alpha 1 the least. The results were broadly similar in all three cells, with some minor differences. when the same preparations were tested for inhibition of thymidine incorporation, the relative activities were quite different: subtypes IFN-alpha 10, IFN-alpha 17, IFN-alpha 21, and IFN-alpha 5 were now the most active, and IFN-alpha 2 was the least active. IFN-alpha 1 and IFN-alpha 8 had comparable intermediate activity. Thus, the differences in activity were not caused by degradation of some subtypes during their separation. IFN-alpha 8 not only had the greatest antiviral activity but also, like IFN-beta, induced an antiviral state in U1 mutant cell lines, which lack the tyrosine kinase, Tyk2, required for signal transduction by other IFN-alpha subtypes.

Characterization of beta-R1, a gene that is selectively induced by interferon beta (IFN-beta) compared with IFN-alpha

We report preliminary characterization of a gene designated beta-R1, which is selectively expressed in response to interferon beta (IFN-beta) compared with IFN-alpha. In human astrocytoma cells, beta-R1 was induced to an equivalent extent by 10 IU/mL IFN-beta or 2500 IU/mL IFN-alpha2. To address the mechanism of this differential response, we analyzed induction of the beta-R1 gene in fibrosarcoma cells and derivative mutant cells lacking components required for signaling by type I IFNs. beta-R1 was readily induced by IFN-beta in the parental 2fTGH cell line, but not by recombinant IFN-alpha2, IFN-alpha Con1, or a mixture of IFN-alpha subtypes. IFN-alpha8 induced beta-R1 weakly. beta-R1 was not induced by IFN-beta in mutant cell lines U2A, U3A, U4A, and U6A, which lack, respectively, p48, STAT1, JAK1, and STAT2. U5A cells, which lack the Ifnar 2.2 component of the IFN-alpha and -beta receptor, also failed to express beta-R1. U1A cells are partially responsive to IFN-beta and IFN-alpha8 but lacked beta-R1 expression, indicating that TYK2 protein is essential for induction of this gene. Taken together, these results suggest that the expression of beta-R1 in response to type I IFN requires IFN-stimulated gene factor 3 plus an additional component, which is more efficiently formed on induction by IFN-beta compared with IFN-alpha.

Interferon-alpha-dependent activation of Tyk2 requires phosphorylation of positive regulatory tyrosines by another kinase

Tyk2 and JAK1, members of the Janus kinase (JAK) family of protein tyrosine kinases, are required for interferon-alpha/beta binding and signaling. Both enzymes are associated with the interferon-alpha/beta receptor, and upon ligand binding, they undergo tyrosine phosphorylation and catalytic activation in an interdependent manner. To identify residues involved in Tyk2 regulation and to understand the basis of the interdependence of Tyk2 and JAK1, six mutated versions of Tyk2 bearing single or multiple point mutations in the tyrosine kinase domain were studied in a cell line lacking endogenous Tyk2. The Y1054F/Y1055F substitutions in the putative activation loop prevented ligand-dependent activation of Tyk2, without abolishing its catalytic potential. The K930R mutation in the ATP binding site generated a kinase-negative protein, which however, still became phosphorylated upon interferon-alpha treatment. The Y1054F/Y1055F substitutions in this kinase-negative Tyk2 abolished the induced phosphorylation. These results indicate that Tyk2 is activated by phosphorylation on Tyr-1054 and/or Tyr-1055 and that this phosphorylation requires another kinase, most likely JAK1. While the Tyk2 forms mutated on Tyr-1054 and Tyr-1055 or on Lys-930 allowed some inducible gene expression, the combination of the three point mutations totally abolished signaling.

The type-I interferon receptor. The long and short of it

The type-I interferon receptor is a multisubunit receptor of the cytokine receptor superfamily. The production of specific monoclonal antibodies against the receptor and the cloning of different receptor subunits have contributed to understanding the type-I interferon receptor structure and function. The present article analyzes these new advances and the role of the different receptor subunits in type-I interferon signaling.

The biologic activity and molecular characterization of a novel synthetic interferon-alpha species, consensus interferon

Consensus interferon (Infergen) is a wholly synthetic type I interferon (IFN), developed by scanning several interferon-alpha nonallelic subtypes and assigning the most frequently observed amino acid in each position, resulting in a consensus sequence. The antiviral, antiproliferative, NK cell activation activity, cytokine induction, and interferon-stimulated gene-induction activity of consensus interferon has been compared with naturally occurring type I interferons. In all of these comparisons, consensus interferon had a higher activity when compared, on a mass basis, with IFN-alpha 2a and IFN-alpha 2b, although the activity was the same for all of these parameters on an antiviral unit basis. That a synthetic type I interferon could have higher activities than naturally occurring molecules is surprising and may be a result of the higher affinity for the array of type I interferon receptors demonstrated for consensus interferon when compared with IFN-alpha. In contrast, consensus interferon was shown to be an inferior inducer of IL-1 beta when compared with IFN-alpha. These results may reflect differential binding to multiple accessory proteins interacting with a type I interferon receptor. These unique biologic properties may lead to a favorable clinical benefit for consensus interferon when compared with the naturally occurring recombinant molecules. Ongoing clinical trials will ascertain whether consensus interferon can be used in a wide array of disease situations, such as chronic viral infections and certain malignancies.

A type I ovine interferon with limited similarity to IFN-alpha, IFN-omega and IFN-tau: gene structure, biological properties and unusual species specificity

A gene encoding a 195 amino-acid (a.a.) polypeptide with a putative 23 a.a. signal sequence that had about 60% a.a. sequence identity to ovine interferon-omega (OvIFN-omega) and 55% or less identity to BoIFN-tau, OvIFN-tau and all known IFN-alpha and -beta has been identified from an ovine genomic DNA library. Surprisingly, it shared almost complete identity to genes for rabbit IFN-omega within its coding sequence and proximal promoter region, although the two were different in their 3'-ends. This IFN (tentatively termed ovine IFN-omega variant, OvIFN-omegav), purified in recombinant form from E. coli, had normal antiviral activity when tested on sheep fetal tongue and brain cells and rabbit kidney cells, but very low activity towards bovine, goat and human cells. It competed with 125I-labeled BoIFN-tau for binding to IFN receptors on ovine cells. Expression of OvIFN-omegav was not detected by reverse transcription-PCR either in ovine peripheral blood leukocytes infected with Sendai virus, or in any other tissues examined. OvIFN-omegav may represent a previously unrecognized, non-virally inducible type I subtype distinct from IFN-alpha, -beta, -omega and -tau. The presence of a conserved gene in rabbit and sheep could reflect a recent interspecies transfer.

Molecular characterization of an alpha interferon receptor 1 subunit (IFNaR1) domain required for TYK2 binding and signal transduction

Binding of alpha interferon (IFNalpha) to its receptors induces rapid tyrosine phosphorylation of the receptor subunits IFNaR1 and IFNaR2, the TYK2 and JAK1 tyrosine kinases, and the Stat1 and Stat2 transcription factors. Previous studies have demonstrated that TYK2 directly and specifically binds to and tyrosine phosphorylates IFNaR1 in vitro. We now report a detailed analysis of the TYK2 binding domain on the IFNaR1 subunit. First, we used an in vitro binding assay to identify the TYK2 binding motif in IFNaR1 as well as the critical residues within this region. The most striking feature is the importance of a number of hydrophobic and acidic residues. A minor role is also ascribed to a region resembling the proline-rich "box 1" sequence. In addition, mutations which disrupt in vitro binding also disrupt the coimmunoprecipitation of the receptor and TYK2. We also provide direct evidence that the binding region is both necessary and sufficient to activate TYK2 in vivo. Specifically, mutations in the binding domain act in a dominant-negative fashion to inhibit the IFNalpha-induced tyrosine phosphorylation of TYK2 and Stat2. Further, introduction of dimerized glutathione S-transferase-IFNaR1 fusion proteins into permeabilized cells is sufficient to induce phosphorylation of TYK2 and the receptor, confirming the role of the binding domain in IFNalpha signal transduction. These studies provide clues to the sequences determining the specificity of the association between JAK family tyrosine kinases and cytokine receptors as well as the functional role of these kinases in cytokine signal transduction.

Recombinant Interferons beta and gamma have a higher antiviral activity than interferon-alpha in coxsackievirus B3-infected carrier state cultures of human myocardial fibroblasts

We compared the antiviral activities of three recombinant human interferons (IFN-alph2a, IFN-beta, and IFN-gamma) in cultured human myocardial fibroblasts to select a candidate for trial in heart disease induced by cardiotropic enterovirus, e.g., coxsackievirus B3 (CVB3). Cells were exposed to CVB3, and after 7 days, when a persistent infection had developed, IFN was added. Virus yields were measured on alternate days for the next 7 or 16 days, and IFN activity was assessed as the percentage reduction in yield. IFN-gamma and IFN-beta were both highly active and reduced virus yields by 2 log (EC(99)) at concentrations of 23.4 IU/ml (SD = 8.6) and 10.1 IU/ml (SD = 3.2), respectively; with 250 IU/ml of either IFN, no infectious virus was formed. Unexpectedly, IFN-alpha2a (EC(99)> 1250 IU/ml) was at least 120 times less active than IFN-beta; after use for 8 days or more, the minor effects it produced were no longer related to the concentration applied. Despite the pharmacokinetic advantages of IFN-alpha2a, our data suggest that IFN-beta should in preference be evaluated in the clinic.

Placental interferons

Trophectoderm of ruminant conceptuses (embryo and associated membranes) secrete tau interferons (IFN tau) as the pregnancy recognition signal. Secretion of IFN tau on gestational days 12-13 for sheep and gestation days 14-17 for cows and goats is critical for pregnancy recognition. IFN tau acts on uterine epithelium to suppress estrogen receptor and oxytocin receptor gene expression, which prevents uterine release of luteolytic pulses of prostaglandin F2 alpha (PGF). Expression of the progesterone receptor (PR) gene in uterine endometrium is not affected by oIFN tau. Maintenance of progesterone secretion by the corpus luteum (CL) ensures establishment of pregnancy. Pig conceptuses secrete both IFN alpha and IFN gamma between days 15-21 of gestation, but their role(s) in early pregnancy is unknown. Estrogen secreted by pig trophoblast between gestational days 11-13 and 15-25 increases endometrial receptors for prolactin and causes exocrine secretion of PGF into the uterine lumen to prevent luteolysis. Shared cell-signaling mechanisms by IFNs and lactogenic hormones through Janus kinases (JAK) 1 and 2 may provide a common pathway to abrogate luteolytic mechanisms to ensure establishment of pregnancy. The role(s) of IFNs produced by human and rodent placentae is not known.

Cell surface binding characteristics correlate with consensus type I interferon enhanced activity

The binding characteristics of a genetically engineered consensus interferon with unusually high biologic activity were compared to the characteristics of recombinant interferon-alpha 2. Both interferon-alpha 2 and the consensus interferon produced typical biphasic Scatchard plots, indicating multiple independent binding sites. The consensus interferon, which exhibited a biologic potency more than 10-fold greater than all other type I interferons, also exhibited binding site affinities greater than those for IFN-alpha 2b. In addition, a larger number of high, and low-affinity cell surface sites were recognized by the consensus interferon, resulting in equivalent numbers of sites at reduced molar concentrations compared to IFN-a2b. Thus, at any given biologic activity, similar numbers of sites were bound by the consensus interferon and IFN-alpha 2, despite differences in their molar concentrations. No differences in internalization kinetics were identified between the two interferons, indicating that the differences in cell surface binding may be sufficient to produce the differences in biologic activity.

Roles of JAKs in activation of STATs and stimulation of c-fos gene expression by epidermal growth factor

The tyrosine kinase JAK1 and the transcription factors STAT1 and STAT3 are phosphorylated in response to epidermal growth factor (EGF) and other growth factors. We have used EGF receptor-transfected cell lines defective in individual JAKs to assess the roles of these kinases in STAT activation and signal transduction in response to EGF. Although JAK1 is phosphorylated in response to EGF, it is not required for STAT activation or for induction of the c-fos gene. STAT activation in JAK2- and TYK2-defective cells is also normal, and the tyrosine phosphorylation of these two kinases does not increase upon EGF stimulation in wild-type or JAK1-negative cells. In cells transfected with a kinase-negative mutant EGF receptor, there is no STAT activation in response to EGF and c-fos is not induced, showing that the kinase activity of the receptor is required, directly or indirectly, for these two responses. The data do not support a role for any of the three JAK family members tested in STAT activation and are consistent with a JAK-independent pathway in which the intrinsic kinase domain of the EGF receptor is crucial. Furthermore, data from transient transfection experiments in HeLa cells, using c-fos promoters lacking the STAT regulatory element c-sis-inducible element, indicate that this element may play only a minor role in the induction of c-fos by EGF in these cells.

Differential recognition of the type I interferon receptor by interferons tau and alpha is responsible for their disparate cytotoxicities

Interferon tau (IFN tau), originally identified as a pregnancy recognition hormone, is a type I interferon that is related to the various IFN alpha species (IFN alpha s). Ovine IFN tau has antiviral activity similar to that of human IFN alpha A on the Madin-Darby bovine kidney (MDBK) cell line and is equally effective in inhibiting cell proliferation. In this study, IFN tau was found to differ from IFN alpha A in that is was > 30-fold less toxic to MDBK cells at high concentrations. Excess IFN tau did not block the cytotoxicity of IFN alpha A on MDBK cells, suggesting that these two type I IFNs recognize the type I IFN receptor differently on these cells. In direct binding studies, 125I-IFN tau had a Kd of 3.90 x 10(-10) M for receptor on MDBK cells, whereas that of 125I-IFN alpha A was 4.45 x 10(-11) M. Consistent with the higher binding affinity, IFN alpha A was severalfold more effective than IFN tau in competitive binding against 125I-IFN tau to receptor on MDBK cells. Paradoxically, the two IFNs had similar specific antiviral activities on MDBK cells. However, maximal IFN antiviral activity required only fractional occupancy of receptors, whereas toxicity was associated with maximal receptor occupancy. Hence, IFN alpha A, with the higher binding affinity, was more toxic than IFN tau. The IFNs were similar in inducing the specific phosphorylation of the type I receptor-associated tyrosine kinase Tyk2, and the transcription factors Stat1 alpha and Stat2, suggesting that phosphorylation of these signal transduction proteins is not involved in the cellular toxicity associated with type I IFNs. Experiments using synthetic peptides suggest that differences in the interaction at the N terminal of IFN tau and IFN alpha with the type I receptor complex contribute significantly to differences in high-affinity equilibrium binding of these molecules. It is postulated that such a differential recognition of the receptor is responsible for the similar antiviral but different cytotoxic effects of these IFNs. Moreover, these data imply that receptors are "spare'' with respect to certain biological properties, and we speculate that IFNs may induce a concentration-dependent selective association of receptor subunits.

Interferon-beta treatment of human disease

After nearly 20 years in the clinic, the benefits of interferon-beta treatment in specific disease states are being recognized increasingly. Two major clinical studies completed within the past two years have firmly established the use of this drug in the treatment of multiple sclerosis. Its therapeutic activity in several indications, including acute and chronic viral hepatitis, has also been demonstrated; however, the general use of interferon-beta in these settings awaits the results of ongoing studies and the development of practical dosing regimens.

Interaction of the transcriptional activator Stat-2 with the type I interferon receptor

Binding of interferon-alpha (IFN alpha) to the multisubunit type I IFN receptor (IFNR) induces activation of the Tyk-2 and Jak-1 kinases and tyrosine phosphorylation of multiple signaling elements, including the Stat proteins that form the ISGF3 alpha complex. Although Jak kinases are required for IFN alpha-dependent activation of Stats, the mechanisms by which Stats interact with these kinases are not known. We report that Stat-2 associates with beta s subunit of the type I IFN receptor in an interferon-dependent manner. This association is rapid, occurring within 1 min of interferon treatment of cells, and is inducible by various type I (alpha, beta, omega) but not type II (gamma) IFNs. The kinetics of Stat-2-IFNR association are similar to the kinetics of phosphorylation of Stat-2, suggesting that during its binding to the type I IFNR, Stat 2 acts as a substrate for interferon-dependent tyrosine kinase activity. These findings support the hypothesis that the type I IFNR acts as an adaptor, linking Stat proteins to Jak kinases. Interaction of Stat-2 with the beta s subunit of the type I IFNR may be a critical signaling event, required for the formation of the ISGF3 alpha complex and downstream transcription of interferon-stimulated genes.

Mutant U5A cells are complemented by an interferon-alpha beta receptor subunit generated by alternative processing of a new member of a cytokine receptor gene cluster

The cellular receptor for the alpha/beta interferons contains at least two components that interact with interferon. The ifnar1 component is well characterized and a putative ifnar2 cDNA has recently been identified. We have cloned the gene for ifnar2 and show that it produces four different transcripts encoding three different polypeptides that are generated by exon skipping, alternative splicing and differential use of polyadenylation sites. One polypeptide is likely to be secreted and two are transmembrane proteins with identical extracellular and transmembrane domains but divergent cytoplasmic tails of 67 and 251 amino acids. A mutant cell line U5A, completely defective in IFN-alpha beta binding and response, has been isolated and characterized. Expression in U5A cells of the polypeptide with the long cytoplasmic domain reconstitutes a functional receptor that restores normal interferon binding, activation of the JAK/STAT signal transduction pathway, interferon-inducible gene expression and antiviral response. The IFNAR2 gene maps at 0.5 kb from the CRFB4 gene, establishing that together IFNAR2, CRFB4, IFNAR1 and AF1 form a cluster of class II cytokine receptor genes on human chromosome 21.

An Alu cassette in the cytoplasmic domain of an interferon receptor subunit

All the cloned subunits of interferon receptors (IFNRs) belong to the type II cytokine receptor family (CRF2). Although three members of CRF2 encoded on human chromosome 21 share a 50 amino acid cytoplasmic homology domain (the IRH2 domain), a fourth subunit, the second cloned chain of the type I IFNR (IFNIR-2), contains a juxtamembrane 20 amino acid stretch of high similarity to the IRH2 domain that stops abruptly. Comparison of the membrane-distal portion of the IFNIR-2 cytoplasmic domain with sequence databases revealed a very high similarity to Alu repeat sequences. We provide evidence that all but 18 amino acids of the predicted cytoplasmic domain of the IFNIR-2 chain are encoded by an Alu cassette in its antisense orientation. Incorporation of an Alu cassette into the receptor chain is proposed to occur by a splicing mechanism. All previous well-characterized examples of insertion of an antisense Alu cassette into an open reading frame have involved alternative splicing. Thus, we predict the existence of an alternatively spliced product of the IFNIR-2 chain with a substantially different cytoplasmic domain.

Antiviral activity of autocrine interferon-beta requires the presence of a functional interferon type I receptor

We and others have previously observed that the antiviral effects of autocrine interferon (IFN)-alpha/beta activity cannot be abolished by neutralizing antibodies, even when present to a large excess. This raises the possibility that the major part of autocrine activity is triggered intracellularly, possibly bypassing the transmembrane IFN-alpha/beta receptor. To examine this possibility, cells derived from IFN-alpha/beta Ro/o knockout mice lacking a functional IFN-alpha/beta receptor were stably transformed with pHMB-KbMuIFN beta or pMFG-MuIFN beta plasmids encoding a constitutively expressed murine IFN-beta gene. Four different clones were isolated and examined for resistance to a retrovirus, MFG-LacZ, and to Semliki Forest virus. Despite the production of autocrine IFN-beta at levels inducing high antiviral resistance in control cells, none of the clones displayed antiviral resistance. Thus, despite its failure to be neutralized by potent antiserum, the antiviral activity of autocrine IFN-beta takes place via the transmembrane IFN-alpha/beta receptor, and no additional pathway is involved.