Showing papers on "Interferon published in 1977"

Functional and morphologic characterization of human T cells continuously grown in vitro.

Abstract: Long-term growth (now over 13 months) of thymus-derived lymphocytes from numerous normal human bone marrow and peripheral blood cell samples was accomplished by using a factor present in media obtained from mitogen-stimulated human peripheral blood lymphocytes. This long-term growth could neither be initiated nor maintained by mitogens alone. All cell cultures were greater than 90% E rosette-positive, whereas the tests for B cell markers, surface IgG and IgM, and EAC rosette were routinely negative. There was no evidence for the presence of granulocytes, monocytes, and their precursors in these cultures. The E rosette-positive cells were then tested to see if they had T cell functions. PHA, Con A, and pokeweed mitogens stimulated lymphproliferative responses in these cultures comparable to those of fresh peripheral blood cells. These proliferating cells were also able to release cell mediators, such as interferon and colony-stimulating activity. Further evidence for the T lymphocyte nature of these cultured cells was obtained from one-way mixed leukocyte cultures in which these cells responded to but were unable to stimulate allogeneic cells. The functional and morphologic characteristics of these cultured cells show that these cells are T cells that grow continuously in vitro.

Synthesis of low molecular weight inhibitor of protein synthesis with enzyme from interferon-treated cells

Abstract: PROTEIN synthesis in cell-free systems from mouse L cells pretreated with the antiviral agent interferon1 shows an enhanced sensitivity to inhibition by double-stranded RNA (dsRNA) (refs 2–4). The inhibition of protein synthesis is dependent on incubation with ATP as well as dsRNA and we and others have reported a dsRNA-dependent protein kinase activity(s) in extracts from interferon-treated cells5–8. The possible involvement of a viral dsRNA-mediated inhibition of protein synthesis in the sequence of events following virus infection in intact, interferon-treated cells has been discussed previously2,4. The situation with extracts from interferon-treated cells5–8 is reminiscent of that in rabbit reticulocyte lysates in which phosphorylation of an initiation factor by a protein kinase has been implicated in the inhibition of protein synthesis in a variety of conditions including the presence of dsRNA9–12. In the interferon-treated L-cell system, however, in addition to the kinase, a heat-stable, low molecular weight inhibitor (LMW inhibitor) of protein synthesis is formed on incubation with ATP and dsRNA (ref. 6). It remained possible from our previous work6, that the LMW inhibitor might be a small phosphorylated peptide product of the dsRNA-dependent kinase. We show here that this is not the case. In addition, we show that the enzyme responsible for the synthesis of the LMW inhibitor will bind to a column of dsRNA attached to a solid support. It can be eluted in high salt but is relatively unstable in this form. We have used the highly-purified enzyme in its stable column-bound state to synthesise and radioactively label the LMW inhibitor. This synthesis, the partial purification of the inhibitor and its activity in the inhibition of protein synthesis in cell-free systems from L cells and rabbit reticulocytes, are reported here. A more detailed characterisation of the inhibitor is described in the accompanying paper13.

Human Interferon: Mass Production in a Newly Established Cell Line, MG-63

Abstract: MG-63 cells, a line derived from an osteosarcoma, produced high yields of interferon after superinduction with polyinosinic acid.polycytidylic acid, cycloheximide, and actinomycin D. Advantages of MG-63 cells over diploid fibroblasts as a substrate are: no requirement for aging between confluency and induction, no requirement for priming, and 3.7-fold higher yields per square centimeter of culture surface. Physicochemically and biologically, MG-63 cell interferon resembles fibroblast rather than leukocyte interferon.

Interferon: an inducer of macrophage activation by polyanions.

Abstract: Purified mouse fibroblast interferon (IF) directly rendered resting macrophages tumoricidal. The physicochemical properties and species specificity of the stimulatory agent fall within the present definition of IF. Since a number of polyanions induce macrophage IF, the antitumor and antimicrobial activities may result from the ability of newly released IF to modify macrophage activity.

Enhancement of IgE-mediated histamine release from human basophils by viruses: role of interferon.

Abstract: Human leukocytes maintained in culture are induced to release histamine when exposed to ragweed antigen E or anti-IgE. Leukocyte cultures incubated with virus (i.e. HSV-1, Influenza A, and Adeno-1) but not exposed to ragweed antigen E or anti-IgE fail to release histamine. If, however, leukocyte cultures are first exposed to virus and then to ragweed antigen E or anti-IgE, significant enhancement of histamine release occurs. Both infectious and inactivated virus enhance histamine release and the degree of enhancement is related to the concentration of virus and the length of the incubation. Tissue culture fluid harvested 8 h after exposure of leukocytes to virus contains a soluble factor which is capable of enhancing histamine release when added to fresh leukocyte cultures. This factor has all the properties of interferon including species specificity and cannot be dissociated from the antiviral activity of interferon. Moreover, both known inducers of interferon (poly I:poly C) and standard preparations of interferon are capable of enhancing histamine release. The enhancement of histamine release by interferon represents a new biological role for interferon.

Cellular immunity and herpesvirus infections in cardiac-transplant patients.

Abstract: We observed severe infection with herpes simplex virus in cardiac-transplant patients despite their high serum antibody levels to this virus. Therefore, we sought to correlate clinical susceptibility to two herpesvirus (simplex and zoster) infections with specific cellular immunity, assessed by the transformation and interferon responses of peripheral blood mononuclear cells to heat-inactivated antigens. Transformation and interferon response to herps simplex virus was maximally depressed immediately after transplantation, the time when severe and prolonged infection with herps simplex virus occurred. Six months to six years after transplantation, both clinical susceptibility and cellular immunity to herpes simplex virus were normal. Herpes zoster infections were more frequent than normal at all times after cardiac transplantation; depressed or absent cellular responses to the varicella zoster virus paralleled that susceptibility. In these patients the risk of severe herpesvirus infections correlated with depressed cellular immune responses to the specific viral agent involved.

Defective interfering particles with covalently linked [+/-]RNA induce interferon

Abstract: Defective interfering (DI) particles of vesicular stomatitis virus which contain covalently linked complementary [+]message and [-]anti-message RNA as a single-stranded ribonucleoprotein complex within the particle, are extremely efficient inducers of interferon. A single particle can induce a quantum yield of interferon. A single molecule of double-stranded RNA presumed to form, at least in part, on entry into the cell is thought to induce interferon synthesis. Conventional [-]RNA DI particles with the same polypeptide composition as [+/-]RNA DI particles fail to induce interferon.

Antiviral activity of interferons.

Abstract: INTRODUCTION.............................................................. 543 ESTABLISHMENT OF THE ANTIVIRAL STATE ........... .................... 544 Interferon Binding ................... ............................. 544 Development of Antiviral Activity................................. 546 LOCUS OF THE INTERFERON-INDUCED INHIBITION OF VIRUS GROWTH. . 547 Evidence that Interferon Treatment Inhibits Virus Uncoating ....... ........... 547 Evidence that Interferon Treatment Inhibits Transcription of the Viral Genome. 547 Evidence that Interferon Treatment Inhibits Viral Protein Synthesis ..... ...... 550 Observations in virus-infected cells ........................... 551 Observations in cell-free systems ....... .................... 552 Evidence that Interferon Treatment Inhibits Terminal Events in the Replication Cycle of Murine Leukemia Viruses ........ ................. 557 INTERFERON TREATMENT IS INEFFECTIVE IN SYSTEMS IN WHICH THE SV40 GENOME IS INTEGRATED INTO AN INTERFERON-RESISTANT VIRUS OR A HOST GENOME ........................... 559 DISCUSSION ............................. 560 LITERATURE CITED........................... 562

Nature of inhibitor of cell-free protein synthesis formed in response to interferon and double-stranded RNA.

Abstract: PROTEIN synthesis in cell-free systems from mouse L-cells pretreated with the antiviral agent interferon1 shows an enhanced sensitivity to inhibition by double-stranded RNA (dsRNA) (refs 2–4). A dsRNA-dependent protein kinase5–7 and a heat-stable, low-molecular-weight inhibitor (LMW inhibitor) of protein synthesis which is formed in such systems on incubation with ATP and dsRNA (ref. 7), seem likely to be involved. We have shown8 that the LMW inhibitor can be conveniently synthesised using a highly-purified enzyme fraction from interferon-treated cells bound to dsRNA on an inert support. Here we report its partial characterisation.

Stimulation of prostaglandin E production in cultured human fibroblasts by poly(I)-poly(C) and human interferon.

Abstract: PROSTAGLANDINS (PGs) have emerged as one of the principal mediators of the inflammatory process1–3. Elevation of tissue concentration of PGs was noted following inflammation3,4, or treatment with various hormones5,6, cyclic nucleotides (ref. 7 and U.Z., B. Strulovici and H.R.L., in preparation), serum8, drugs8,9 and antigen10. The beneficial effects of anti-inflammatory drugs have been correlated with their ability to suppress PG production either by curtailing precursor availability (glucocorticosteroids11,12) or by inhibiting PG synthetase activity (non-steroidal drugs1). Viral infections are associated with interferon production13 and with inflammation14. Little is known about the possible involvement of interferon in the inflammatory reaction, though it has been suggested that interferon may have anti-inflammatory properties15. It has been shown that an interferon inducer, double-stranded polyinosinate·polycytidylate [poly(I)·poly(C)], as well as human interferon, stimulate hyaluronic acid production by human synovial fibroblasts in culture16. Overproduction of hyaluronic acid is known to be associated with the inflammatory process16; the effect is prevented by cortisol or indomethacin16. We report here the stimulation by poly(I)·poly(C) and by human interferon of prostaglandin E (PGE) production by human synovial and foreskin fibroblasts in culture. Cortisol inhibited this effect.

Cytotoxic Effects of Interferon in Vitro on Granulocytic Progenitor Cells

Abstract: We have utilized in vitro marrow culture techniques to evaluate the cytotoxicity for granulocytic progenitor cells of two highly purified human leukocyte interferon preparations. Concentration- and time-related decrements in granulocytic colony-forming capacity in agar occurred with human and mouse marrow. Although mouse marrow cells were less sensitive than were human cells, these data indicate lack of strict species specificity for the cell growth-inhibitory effects of interferon. Similar cytotoxicity was noted for normal and leukemic human clonogenic cells exposed to interferon for prolonged periods. The decrease in the proportion of granulocytic progenitor cells in DNA synthesis, which occurred at high concentrations, and the diminution by interferon of the cytotoxicity caused by cytosine arabinoside demonstrate that interferon decreases DNA synthesis of granulocytic progenitor cells. The lack of enhanced cytotoxicity for rapidly proliferating mouse post-endotoxin marrow cells indicates that interferon is not a cell cycle-stage-specific drug. These data seem useful for evaluating the suppressive effects of interferon on granulopoiesis and for devising clinical trials with this agent.

Induction and decay of human fibroblast interferon mRNA

Abstract: Polyadenylylated interferon mRNA, obtained from induced human fibroblasts, was quantitatively assayed by synthesis of biologically active human interferon in Xenopus laevis oocytes. The assay for interferon mRNA was used to distinguish between various hypotheses relating to interferon induction and biosynthesis. The data demonstrate that on induction with poly(I-poly(C) human fibroblasts accumulate interferon mRNA for 1-1.5 hr, after which time the mRNA is rapidly degraded with a half-life (t 1/2) of 18 min. Treatment of cells with cycloheximide prolongs the period of accumulation to 3 hr and decreases the rate of mRNA inactivation (t 1/2 = 49 min). Treatment with actinomycin D decreases the rate of inactivation still further (t 1/2 = 68 min). A comparison of cellular interferon synthesis with the relative amounts of interferon m RNA after simple induction or inductionin the presence of the inhibitors (superinduction) indicated a general correlation. Thus, on induction, the genes for interferon are activated to produce a transcript for a short time. The superinducing treatments prolong the period of accumulation and decrease the rate of degradation of this transcript.

Nonspecific Protection of Mice against Influenza Virus Infection by Local or Systemic Immunization with Bacille Calmette-Guerin

Abstract: Bacille Calmette-Guerin (BCG) has been used effectively to protect nonspecifically against bacterial infections and neoplasms, probably by enhancement of cell-mediated immunity. It has been suggested that cell-mediated immunity plays a role in the host defense against certain viral infections. In recent in vitro studies, macrophages from animals sensitized by BCG were more effective in lowering the titer of influenza virus than were macrophages from control animals. The purpose of this study was to investigate the in vivo effectiveness of nonspecific immune stimulation with BCG on influenza virus infection in mice. Immunization with BCG resulted in significant protection of mice. Also, the local (nasal) route of immunization was more effective than the systemic (intraperitoneal) route against the intranasal inoculum of virus, a finding which suggests that important role of local immunity, i.e., either earlier stimulation of secretory antibody or nonspecific cell-mediated immunity. The time course of the resistance ot infection suggests that interferon was not the protective mechanism.

Effect of human leukocyte interferon on the growth of human osteosarcoma cells in tissue culture.

Abstract: Nine osteosarcoma cell lines, originally developed from six osteosarcoma tumours in five patients, and two cell lines of non-tumour origin (glia and fibro-blast) were grown in vitro in the presence of human leukocyte interferon (L-IF). L-IF exerted a dose-dependent inhibition of growth in all these lines. The inhibitory activity displayed characteristics typical of interferons. Inhibition of cell growth occurred at a much lower L-IF concentration for the osteosarcoma than for the non-tumour-derived lines. Inhibition of tumour cell growth was observed at concentrations obtained in the serum of osteosarcoma patients treated with interferon.

Inhibition by anti-interferon serum of lymphocytic choriomeningitis virus disease in suckling mice.

Abstract: Inoculation of newborn mice with lymphocytic chloriomeningitis (LCM) virus resulted in decreased weight gain, liver cell necrosis, and death. Injection of potent sheep immunoglobulin against mouse interferon markedly inhibited these manifestations of LCM virus disease despite the fact that these treated mice had 100-fold more LCM virus in their serum. We conclude that interferon induced by LCM virus is responsible in large part for the syndrome of growth inhibition, liver cell necrosis, and death observed in LCM virus-infected suckling mice.

Immune interferon I. Production by lymphokine‐activated murine macrophages

Abstract: Unfractionated murine spleen cells produce immune interferon (type II) upon stimulation with antigen or mitogen. When spleen cells were passed over glass bead columns, interferon production decreased whereas the mitotic response to the stimulants drastically increased. When these cells were further purified over nylon wool columns, interferon production was totally abolished whereas thymidine incorporation in stimulated cultures was invariably high. Interferon production by nylon wool column-purified lymphocytes could be restored with macrophages grown from bone marrow cultures or spleen cells but not with macrophages from proteose peptone-induced peritoneal exudate cells. It was also found that pure macrophage cultures from spleens of BCG-immunized mice consistently produced interferon activity without any further stimulation. When culture supernatants of activated T lymphocytes, which did not contain any interferon activity, were transferred to macrophage cultures from different sources and incubated for 45 h, interferon activity could be detected in supernatants of macrophage cultures from bone marrow and spleen but not in those from proteose-induced peritoneal exudate cells. It is concluded that certain macrophage populations can be induced to produce interferon activity whereas others are refractory to this induction which appears to be linked to their differentiation state.

Differential Effects of Interferon on the Expression of H-2K, H-2D, and Ia Antigens on Mouse Lymphocytes

Abstract: Lindahl and co-workers (1, 2) have reported that mouse interferon enhances the expression of H-2 antigens on the surface of murine leukemia L 1210 cells (H-2 d ) and normal mouse thymocytes and splenic lymphocytes from DBA/2 (H-2 d ) or C57 BL/6 (H-2 b ) mice (3) and presented evidence that interferon was the responsible factor (1, 3). Furthermore, inoculation of DBA/2 mice with potent partially purified mouse interferon preparations (or viral or nonviral inducers of endogenous interferon) was also accompanied by a marked increase in the expression of H-2 antigens on the surface of thymocytes and splenic lymphocytes (4). The availability of restricted antisera against determinants controlled by subregions of the H-2 complex (Table I) has permitted us to show that interferon enhances the expression of H-2K and H-2D antigens, while affecting only slightly, or not at all the expression of Ia antigens on mouse lymphocytes.

Successful prophylaxis against rabies in mice and Rhesus monkeys: the interferon system and vaccine.

Abstract: Addition of interferon to ineffective rabies virus vaccines by the local injection of either exogenous interferon or a potent interferon inducer (a complex of polyriboinosinic-polyribocytidylic acid containing poly-L-lysine and carboxymethylcellulose) into the footpads of mice previously challenged with rabies virus dramatically reduced the mortality rate. A significant reduction in mortality rate was also noted when the interferon system was administered to rhesus monkeys, but only when treatment was given 6 hr after challenge with rabies virus. Since the monkeys were given an overwhelming challenge of virus, the treatment had to be given quickly to obtain results comparable to those in mice.

Relative ability of mitogens to stimulate production of interferon by lymphoid cells and to induce suppression of the in vitro immune response.

Abstract: SummarySeveral T cell mitogens, con-canavalin A (Con A), phytohemagglutinin P (PHA-P), and staphylococcal enterotoxin A (SEA), were compared for their ability to inhibit the in vitro antibody response and to stimulate the production of mitogen (antigen) -type interferon in mouse spleen cell cultures. It was found that the ability to inhibit the plaque-forming cell (PFC) response to sheep red blood cells was proportional to the ability of these mitogens to induce interferon in the cultures. SEA was the most effective inhibitor of the PFC response and the best inducer of mitogen-type interferon, followed by Con A, with PHA-P being the least effective. The data suggest that SEA would be the most suitable inducer of mitogen-type interferon in quantity as a prerequisite to purification and characterization of the molecule. The data are supportive of previous studies suggesting a role for mitogen (antigen) -type interferon in regulation of the immune response via suppressor T cells.We wish to thank Ms. Phyllis ...

Is interferon tissue specific?- Effect of human leukocyte and fibroblast interferons on the growth of lymphoblastoid and osteosarcoma cell lines.

Abstract: The growth inhibitory effect of human leukocyte and fibroblast interferons was tested in vitro. The effect of fibroblast interferon was more pronounced on osteosarcoma cells and the effect of leukocyte interferon was more pronounced on lymphoid cells. This suggests that the capacity of interferon to inhibit cell growth is, in some measure, tissue specific.

Interferon suppresses the transition of quiescent 3T3 cells to a growing state.

Abstract: PAUCKER, Cantell and Henle reported that crude mouse interferon preparations depressed multiplication of suspended L cells1. Since then evidence has accumulated indicating that interferon molecules which inhibit multiplication of virus also inhibit the growth of cells in culture2–9. Interferon was shown to suppress incorporation of 3H-thymidine into cellular DNA in various systems—for example, release from a double thymidine block in synchronised L929 cells10; activation of mouse spleen lymphocytes by phytohaemagglutinin (PHA) or allogeneic lymphocytes11; and in steady-state growth of cultured mouse leukaemia L 1210 cells12 in a chemostat. However, there has been no direct evidence to distinguish between the effect of interferon on the rate of initiation of DNA synthesis (entry into the S phase) and that on the rate of DNA synthesis in the S phase, and it is uncertain which step of the cell cycle is interfered by interferon. It is well known that most cells in cultures of confluent or serum starved mouse 3T3 stay in the early G1 or GO stage of the cell cycle, and an addition of serum to these quiescent cells stimulates their growth, leading to DNA synthesis and cell division13,14. We describe here a suppressive effect of interferon on the transition from the quiescent to the growing state in BALB/c 3T3 cells. The data indicate that interferon suppresses the initiation of DNA synthesis.

Tumour cell lines induce interferon in human lymphocytes.

Abstract: VIRAL infection of cells induces synthesis and release of interferon which renders uninfected cells resistant to subsequent virus infection. Interferon production can also be stimulated by non-viral agents, such as microorganisms1, substances of microbial origin and synthetic polymers2. Moreover, interferon can be stimulated in lymphocytes by mitogenic lectins3, antilymphocyte sera4, viral antigens5–7, PPD8 and exposure to allogeneic cells9. An antitumour effect of interferon and interferon inducers has been demonstrated in vivo10–12. It has been postulated that interferon may act by directly inhibiting tumour cell growth13 and by stimulating host defence mechanisms14–16. We report here that certain human cell lines are able to induce high levels of interferon on contact with human lymphocytes.

Cellular origin of interferon induced by bacterial lipopolysaccharide.

Abstract: Bacterial lipopolysaccharide (LPS) induces interferons with different properties in mouse macrophages and B lymphocytes. Macrophage interferon is labile at 56 degrees C and is neutralized by anti-mouse fibroblast interferon at a dilution of 1:6,142. B cell interferon is more heat stable and is neutralized by the same antiserum only at a dilution of 1:276. Serum obtained early (1 h) after an intravenous injection of 100 mug of LPS resembled macrophage interferon, whereas serum obtained at later times resembled more and more B cell interferon. The diverse cellular origin of LPS-induced interferon may explain the broad hyporesponsiveness produced by LPS in animals.

Defense Mechanisms Against Primary Influenza Virus Infection in Mice: I. The Roles of Interferon and Neutralizing Antibodies and Thymus Dependence of Interferon and Antibody Production

Abstract: To investigate the defensive roles and production of interferon and antibodies, C3H/He mice were subjected to various immunosuppressive treatments and infected with influenza virus. In infected normal control mice the pattern of pulmonary viral growth can be divided into three phases. The first phase is characterized by an exponential increase of virus titer, the second by a rapid decrease, and the third by a moderate decrease. At the time of transition from the first phase to the second in pulmonary virus growth, interferon could be detected in the tracheobronchial washings of infected mice, but neutralizing antibodies could not. In infected B cell-deprived mice and infected anti-µ-treated mice, the transition from the first phase to the second occurred without any detectable antibody production, and interferon could be induced in the early stage of infection. However, the pulmonary virus in these mice increased again exponentially until the death of the mice. In infected T cell-deprived mice which could not induce interferon, but produced IgM-neutralizing antibodies, the second phase was not observed after the first phase, but a transient plateau phase could be demonstrated, and then the pulmonary virus increased again exponentially until the death of the mice. In anti-γ-treated infected mice, pulmonary virus growth and production of interferon and neutralizing antibody were almost similar to those of infected normal control mice except for the absence of IgG neutralizing antibody production. Although anti-α-treated infected mice produced interferon and no IgA antibody, the transition from the first exponential increase of pulmonary virus to the second rapid decrease was seen, but then the virus increased exponentially again until the death of the mice. These results suggest that interferon plays an important role in the transition from the first phase to the second, and that T cells are required for interferon induction in mice infected with influenza virus. These data also suggest that IgA antibodies play an important role in the inhibition of virus propagation in the lungs after the disappearance of interferon. Moreover, infected T cell-deprived mice could produce only IgM neutralizing antibodies, but not IgG and IgA antibodies. Therefore, T cells are required for the production of IgG and IgA antibodies and eventually for defense functions in mice infected primarily with influenza virus.

Synthesis of human interferon by Xenopus laevis oocytes: two structural genes for interferons in human cells

Abstract: Human fibroblasts and leukocytes produce interferons which may be distinguished by their antigenic and species specificity as well as by their molecular weight distributions. To elucidate the basis for these differences, we isolated mRNA from induced human fibroblasts and lymphoblastoid (Namalva) cells and studied the products of translation in Xenopus laevis oocytes. The mRNA from the respective cells yielded translation products, in oocytes, that were characteristic of the cells from which the mRNA was derived. We conclude that human cells contain at least two structural genes for interferon, coding for polypeptides differing in primary sequence. Fibroblasts synthesize a single species of interferon; lymphoblastoid cells synthesize two species, the fibroblast and leukocyte types.

Interferon, Double‐Stranded RNA and RNA Degradation: Characteristics of an Endonuclease Activity

Abstract: Extracts from interferon-treated Ehrlich ascites tumor cells differ in various biochemical characteristics from extracts from control cells. Thus, as reported earlier, double-stranded RNA (dsRNA) promotes the phosphorylation by ATP of at least two proteins in extracts from interferon-treated cells, but not, or to only a lesser extent, in extracts from control cells. Moreover reovirus mRNAs are degraded faster in reaction mixtures containing extracts from interferon-treated cells than in those containing extracts from control cells but only if the reaction mixtures are supplemented with dsRNA and ATP. The faster RNA degradation in extracts from interferon-treated cells is due to enhanced endonuclease action. We designated the agent(s) catalyzing this as endonucleaseINT. There is some enhancement of nuclease activity by dsRNA and ATP also in extracts from control cells. The extent of this enhancement is, however, much smaller than in extracts from interferon-treated cells. We report now that the promotion of mRNA cleavage in extracts from interferon-treated cells by dsRNA and ATP is apparently not a consequence of a possible impairment in the attachment of the mRNA to ribosomes since (a) the protein synthesis inhibitors (sparsomycin and edeine) do not affect the degradation and (b) dsRNA and ATP enhance RNA degradation also in the 200000 × g supernatant fraction from extracts of interferon-treated cells and this fraction is essentially free of ribosomes. (The ribosome concentration in our 200000 × g supernatant fraction is less than 1% of that in the 30000 × g extract.) GTP or the ATP analogs AMP(CH2)PP or AMP-P(CH2)P do not substitute for ATP and dsDNA or DNA · RNA hybrids do not substitute for dsRNA in activating endonucleaseINT. The optimal concentration of ATP in activating endonucleaseINT is 1 mM or higher. The addition of dsRNA and ATP to extracts from interferon-treated cells affects the degradation of some RNAs much more than that of others. Thus it promotes the degradation of reovirus mRNA from the large size class strongly, the degradation of those from the medium size class to an intermediate extent and that of those from the small size class the least. Furthermore it enhances the degradation of mRNA from either control or interferon-treated Ehrlich ascites cells more than that of Ehrlich ascites cell ribosomal RNA or mouse globin mRNA. Reovirus dsRNA is not cleaved by endonucleaseINT. The partially purified mouse interferon preparation (specific activity 8 × 108 NIH reference standard units/mg protein) used in several experiments was free of nuclease activity under our experimental conditions. Moreover the addition to extracts from control cells (or extracts from interferon-treated cells) of this interferon preparation together with dsRNA and ATP did not enhance nuclease activity.