INTERFERON. A REVIEW AND ANALYSIS OF RECENT OBSERVATIONS

Biologic effects of interferons

If one were to review articles on IFN published between 1957 and 1967 it would become apparent that virtually none of the tenets held then are still valid today. Whereas IFN was long considered to be a specific antiviral substance without any effect on normal cellular metabolism, we accept today that it affects normal cell division and many specialized cellular functions. In this respect it is not unique; IFN is a prototype of a family of similar substances now called cytokines that all appear to function as regulatory molecules. It was held that the production of IFN constituted a specific response to a viral infection. Today we believe that IFN is an integral part of a cytokine network and that they and other cytokines may be produced constitutively at low levels. These substances exert multiple effects on virtually all cells. They play an important role in host defense against viral and parasitic infections, and in the resistance to experimental tumors. IFN can be shown to exert effects on the immune system and on lymphocyte circulation. Lastly, because of the multiplicity of their biologic effects, they may even contribute to the pathogenesis of certain diseases. Thus, when large amounts of IFN are administered or induced in newborn mice they can cause liver, kidney, and pulmonary disease. The field of IFN and cytokine research continues to expand and there is an increasing number of therapeutic applications. Twenty years from now, scientists and clinicians may be surprised that we understood so little of how IFN act and how inadequately we used them to treat disease.

Repression of interferon action: induced dedifferentiation of embryonic cells

Cells cultured from young (6-day) chicken embryos differ from those of older (13-day) embryos in having a greater susceptibility to infection by certain viruses and a considerably lesser sensitivity to the action of interferon. These circumstances parallel those observed in the intact embryo. The addition of a small percentage of cells from young embryos alters the response of cells cultured from older embryos by increasing viral susceptibility sevenfold and decreasing sensitivity to interferon 25-fold. We postulate that a repressor which inhibits the expression of interferon in older embryonic cells is elaborated by cells from young embryos.

Inhibition of protein synthesis in noninfected L cells by partially purified interferon preparations

Partially purified interferon preparations, obtained from L-cell monolayers infected with Newcastle disease virus (NDV), were shown to inhibit protein synthesis in noninfected L cells. The incorporation of several amino acids-(14)C was equally sensitive to the pretreatment of the cells with the interferon preparation. Treatment of L-cell monolayers for 24 hr with 800 units of interferon resulted in a 50% decrease in amino acid incorporation. The degree of inhibition was found to be a function of the interferon concentration and the time of exposure of the cells to the partially purified preparations. No inhibitory effect was detected in medium obtained from noninfected cells and purified in an identical manner. The inhibitory effect was shown to be cell specific in that the partially purified interferon from L cells did not reduce amino acid incorporation in heterospecific cell lines. Heating the interferon preparations at 60 degrees C destroyed their antiviral activity and their ability to inhibit valine-(14)C incorporation in L cells.

Adeno-associated satellite virus interference with the replication of its helper adenovirus

Adeno-associated satellite virus type 4 interferes with the replication of its helper adenovirus. No interferon-like soluble substance could be detected in satellite-infected cultures and other DNA- and RNA-containing viruses were not inhibited by coinfection with satellite virus under conditions which reduced adenovirus yields by more than 90% in monkey cells. Altering the concentration of adenovirus in the presence of constant amounts of satellite resulted in a constant degree of interference over a wide range of adenovirus inocula and suggested that adenovirus concentration was not a significant factor in the observed interference. The interference with adenovirus replication was abolished by pretreating satellite preparations with specific antiserum, ultraviolet light or heating at 80 degrees C for 30 min. This suggested that infectious satellite virus mediated the interference. Satellite virus concentration was found to be a determinant of interference and studies indicated that the amount of interference with adenovirus was directly proportional to the concentration of satellite virus. 8 hr after adenovirus infection, the replication of adenovirus was no longer sensitive to satellite interference. This was true even though the satellite virus was enhanced as effectively as if the cells were infected simultaneously with both viruses. Interference with adenovirus infectivity was accompanied by reduced yields of complement-fixing antigen and of virus particles which suggested that satellite virus interfered with the formation and not the function of adenovirus products. When cells were infected either with adenovirus alone or with adenovirus plus satellite, the same proportion of cells plated as adenovirus infectious centers. However, the number of plaque-forming units of adenovirus formed per cell in the satellite-infected cultures was reduced by approximately 90%, the same magnitude of reduction noted in whole cultures coinfected with satellite and adenovirus. This suggested that all cells infected with the two viruses were producing a reduced quantity of adenovirus.

Intrinsic interference: a new type of viral interference

The hemadsorption-negative plaque test has revealed a new type of viral interference, termed intrinsic interference. Several unrelated types of noncytopathic viruses were shown to induce in infected host cells a state of interference unique in being directed solely against superinfection by Newcastle disease virus (NDV). The NDV-refractory state arises only in those individual cells of a population actually infected by the inducing virus, and presumably results from the action of a protein(s) coded for by the viral genome. Thus, intrinsic interference differs fundamentally from that mediated by an extrinsic protein detectable under conditions favoring resistance to a broad spectrum of viruses and characteristic of interference induced by interferon, the latter being coded for by the cell genome. Intrinsic interference is defined as a viral genome-induced cellular state of resistance to challenge by high multiplicities of NDV, coexistent with a state of susceptibility to a broad spectrum of other viruses, similarly tested at high multiplicities. The capacity to induce intrinsic interference was demonstrated with rubella virus, Sindbis virus (arbovirus, group A), West Nile virus (arbovirus, group B), poliovirus (MEF, type 2), the lactic dehydrogenase virus (Riley's agent), and an unidentified nonhemadsorbing, noncytopathic adventitious virus. A state of intrinsic interference was also observed in the V5 line of mouse cells carrying a murine leukemia virus, probably resulting from some heretofore unsuspected contaminating virus. The molecular basis for intrinsic interference is not known, but it appears to involve a step in the NDV growth cycle beyond that of viral attachment, entry, and eclipse.

Rabbit macrophage interferons. II. Some physicochemical properties and estimations of molecular weights

Antiviral factors present in cultures of rabbit peritoneal macrophages or rabbit kidney (RK) cells infected with Newcastle disease virus (NDV) and those in cultures of uninfected macrophages all fulfilled the biological and physicochemical criteria for classification as interferons. Virus-induced macrophage and RK interferons were slightly more stable to heat or acid than "spontaneously produced" or endotoxin-induced macrophage interferon. Interferon activity in serum of NDV-infected rabbits was decidedly more labile than NDV-induced macrophage interferon. However, these differences in lability were too slight to serve as a useful basis for distinguishing one rabbit interferon from another. Rabbit interferons from various sources could be differentiated by filtration through Sephadex G-100 and their molecular weights estimated by comparison with elution profiles of a series of marker proteins of known molecular weight. Each of four different preparations of rabbit interferons was found to contain more than one molecular component. Elution peaks for three NDV-induced interferons were equivalent to the following molecular weights: RK approximately 44,000-45,000 and > 134,000 (variable and < 1% when present); macrophage approximately 37,000, 44,000-45,000, and > 134,000 (variable and <1% when present); and serum approximately 50,000-52,000 and > 134,000 ( approximately 10% and heat labile). NDV-induced serum interferon may also contain another molecular component of mol wt approximately 45,000 represented by a trailing shoulder from the major 51,000 mol wt peak. Endotoxin-induced macrophage interferon proved to be polydisperse. Sephadex filtration of this interferon did not reveal clear and consistent elution patterns, partially owing to its low initial titer and lability. However, variable peaks of biological activity could be detected in Sephadex fractions equivalent to approximate molecular weight values of > 134,000, 72,000-78,000, 33,000-38,000, 28,000-30,000, and possibly a component of 42,000-45,000. A major component of mol wt approximately 37,000 was present in all samples of endotoxin-induced macrophage interferon. The other constituents may be biologically active subunits or polymers. These data indicate that rabbit macrophages produce two primary kinds of interferon: (a) an RK-like component of mol wt approximately 45,000 that is synthesized in greatest amount after viral induction, and (b) a different species of mol wt approximately 37,000 that can also be synthesized in the absence of viral induction. The presence of major interferon constituents of mol wt approximately 51,000 and > 134,000 in rabbit serum after viral induction suggests that macrophages are not the principal interferon-producing cells that respond to intravenous injection of NDV.

Rabbit macrophage interferons. I. Conditions for biosynthesis by virus-infected and uninfected cells

Interferon is produced in cultures of rabbit leukocytes in response to infection with Newcastle disease virus or in the absence of known viral infection. The macrophage appears to be the responsible producing cell. Cultures prepared from sterile peritoneal exudates, which contained about 90% macrophages, are at least as efficient as cultures of rabbit kidney (RK) cells in their capacity to synthesize NDV-induced interferon. Interferon can be detected in the medium by 2 hr after viral infection and the titers usually reach a peak of 10,000 PDD(50)/ml by 4-6 hr. Exposure to actinomycin prior to or shortly after viral induction effectively blocks interferon synthesis by cells of both types. However, macrophages become refractory to actinomycin by 30-60 min compared with 607-120 min for RK cells, a finding which suggests earlier and more rapid transcription of interferon-specific messenger RNA in macrophages. Macrophages harvested from the peritioneal cavity of rabbits injected intravenously with NDV 48 hr previously also exhibit slightly reduced capacity to synthesize interferon, but this tolerant state is less marked than is tolerance to production of circulating interferon in intact rabbits. Interferon is also synthesized by rabbit macrophages not infected with virus but simply incubated at 37 degrees C in medium with or without added bacterial endotoxin. Uninfected polymorphonuclear leukocytes, rabbit kidney and spleen cells produced no detectable interferon under similar conditions of cultivation. No interferon was released by intact macrophages incubated at 4 degrees C or by ultrasonically disrupted macrophages incubated at 37 degrees C. Although interferon titers were found to be higher when uninfected cultures were exposed to 10-100 microg/ml of E. coli lipopolysaccharide, unavailability of suitable pyrogen-free maintenance media precluded answering the question whether macrophages can continually synthesize and release interferon spontaneously. Interferon yields from uninfected macrophages were only 1% or less of the yields from NDV-infected macrophages, but the rates of synthesis were similar under both conditions. Studies with actinomycin and puromycin revealed that sequential transcriptive and translational events are required for de novo interferon synthesis by uninfected cells in a manner similar to virus-induced interferon synthesis. The physical properties and molecular weights of these rabbit interferons are discussed in the following report (12).